MATERIALS AND WELDING BVN TM MW1 – Non-Destructive Testing

MATERIALS AND WELDING BVN TM MW1 – Non-Destructive Testing
Head Office – Department Technology Equipment - V0 – January 2009

CONTENT 1 - General principles 2- Visual testing 3- Liquid penetrant testing 4- Magnetic particle testing 5- Radiographic testing 6- Ultrasonics testing

1 - General principles

NON-DESTRUCTIVE TESTING – General principles
The field of Nondestructive Testing (NDT) is a very broad, interdisciplinary field that plays a critical role in verifying that structural components and systems are built in compliance with the applicable standards. NDT technicians and engineers define and implement tests that can detect and eventually locate and size material imperfections. These tests are performed in a manner that does not affect the future usefulness of the object or material. In other words, NDT allows parts and materials to be tested without damaging them. NDT provides an excellent balance between quality control and cost- effectiveness. Generally speaking, NDT applies to industrial inspections. Technologies used in NDT are similar on the principles to those used in the medical field. NDT is a matter of qualified specialists. Surveyors are not these specialists but are to be able to communicate with these specialists.

NDT Methods The number of NDT methods that can be used to inspect components and make measurements is large and continues to grow. Researchers continue to find new ways of applying physics and other scientific disciplines to develop NDT methods. However, there are six common NDT methods that are used widely in industry. These methods are : Surface method such as visual inspection, liquid penetrant testing, magnetic particle testing, electromagnetic or eddy current testing, Volumetric method such as radiography, and ultrasonic testing. Each method is limited by its physical principles and may be complemented by an other method

QUALIFICATION OF PERSONNEL INVOLVED IN NDT

QUALIFICATION OF PERSONNEL INVOLVED IN NDT Operators are generally to be qualified according to a nationally recognised scheme with a grade equivalent to level II qualification of ISO 9712, EN473 or SNT-TC-1A Personnel responsible for the preparation of NDE procedures should be qualified to a grade equivalent to level III of the same schemes Operators qualified to level I may be engaged in the test under the supervision of personnel qualified to level II or III Qualification and certification scheme of EN 473 applies to NDE techniques listed below; To be noted that visual examination is part of such schemes

NON-DESTRUCTIVE TESTING REQUIREMENTS Each piece tested Batch of pieces tested using one piece of the batch PRODUCT(S) during/from manufacturing process TESTED AREA TEST RESULT Criteria as per Rules or standard agreed Type of testing Extent of testing

Extent and criteria for Non-Destructive Testing : According to recognised standard proposed by the manufacturer (EN standards, ASME, API, JIS etc.) According to the requirement of the Society’s Rules as applicable (piping systems etc.) In absence of specific standard proposed by the manufacturer, as a minimum, following IACS recommendations may be placed at the disposal of the manufacturer and are acceptable : IACS Recommendation 68 IACS Recommendation 69

MATERIALS AND TESTING – Principles of testing
THE TESTING REQUIREMENTS ARE GIVEN BY THE RULES APPLICABLE TO EACH TYPE OF PRODUCT Example of steel castings :

THE TESTING REQUIREMENTS ARE GIVEN BY THE RULES APPLICABLE TO EACH TYPE OF PRODUCT Example of rolled steel plates :

2 – VISUAL TESTING

NON-DESTRUCTIVE TESTING – Visual Testing
Most extensively used method which is very practical and economical. Examination is conducted on clean surfaces (on finished welds free from paint). Equipment used by visual inspector ; Vernier, Gauges, comparator (typically for casting surface) etc. Before welding starts; Joint preparation (parent metal condition, edge preparation, groove angles, root opening, alignment…) During welding; Visible imperfections, cleaning… After welding; Visible imperfections, weld dimensions…

3 – LIQUID PENETRANT TESTING

NON-DESTRUCTIVE TESTING – Liquid Penetrant Testing
The method is based on the ability of a penetrating liquid (known as penetrant) to be drawn into a discontinuity. After removal of the excess penetrant, a developer such as a fine powder is added and the penetrant is drawn out the discontinuity by capillary action. Only imperfections which break the surface can be detected Procedure shall detail the calibration equipment, surface preparation, cleaning and drying prior to testing, temperature range, type of penetrant, cleaner and developer used, penetrant application and removal, penetration time, developer application and development time, lighting conditions during examination and identification system of welds examined Red color penetrant and white developer are usually used for welds examination

Liquid Penetrant Testing may detect imperfections not visible with the unaided eyes LPT produces an indication that is much larger and easier for the eye to detect than the physical imperfection itself. Many imperfections are so small or narrow that they are undetectable by the unaided eyes. Due to the physical features of the eye, there is a threshold below which objects cannot be resolved. This threshold of visual acuity is around inch for a person with a perfect vision. The indication is highly contrasted, typically red dye with white developer under normal light. When a fluorescent penetrant testing is performed, the penetrant materials are formulated to be highly visible under ultraviolet light in darkened area.

Red dye with white developer under normal light

Fluorescent dye with developer under ultraviolet light

Surface Preparation: One of the most critical steps of a liquid penetrant inspection is the surface preparation. The surface must be free of oil, grease, water, paint or other contaminants that may prevent penetrant from entering flaws.

Penetrant Application: Once the surface has been thoroughly cleaned and dried, the penetrant material is applied by spraying, brushing, or immersing the part in a penetrant bath.

Penetrant Dwell: : The penetrant is left on the surface for a sufficient time to allow as much penetrant as possible to be drawn from or to seep into an imperfection. Penetrant dwell time is the total time that the penetrant is in contact with the part surface. Dwell times are usually recommended by the penetrant producers or required by the procedure being followed. The times vary depending on the application, penetrant materials used, the material, the form of the material being inspected, temperature of part examined, and the type of defect being inspected for. Minimum dwell times typically range from 5 to 60 minutes. Generally, there is no harm in using a longer penetrant dwell time as long as the penetrant is not allowed to dry.

Excess Penetrant Removal: This is a delicate part of the technique because the excess penetrant must be removed from the surface of the sample while not removed from the imperfection. Depending on the penetrant system used, this step may involve cleaning with a solvent, direct rinsing with water, or first treating the part with an emulsifier and then rinsing with water.

Developer Application: A thin layer of developer is then applied to the sample to draw penetrant trapped in the physical imperfections back to the surface where it will be visible. Developers come in a variety of forms that may be applied by dusting (dry powdered), dipping, or spraying (wet developers)

Indication Development: The developer is allowed to stand on the part surface for a period of time sufficient to permit the extraction of the trapped penetrant out of the surface imperfections. This development time is stated by the procedure (typically a minimum of 10 minutes). Inspection: Inspection is then performed under appropriate lighting to detect indications that may be present. Clean Surface: The final step in the process is to thoroughly clean the part surface to remove the developer from the parts that were found to be acceptable.

Common Uses of Liquid Penetrant Inspection Liquid penetrant inspection (PT) is one of the most widely used nondestructive evaluation (NDE) methods. Its popularity can be attributed to two main factors: its relative ease of use and its flexibility. PT can be used to inspect almost any material provided that its surface is not extremely rough or porous. As mentioned above, one of the major limitations of liquid penetrant testing is that imperfections must be opened to the surface.

Primary Advantages The method has high sensitivity to small surface discontinuities. The method has few material limitations, i.e. metallic and nonmetallic, magnetic and nonmagnetic, and conductive and nonconductive materials may be tested. Large areas and large volumes of parts/materials can be tested rapidly and at low cost. Parts with complex geometric shapes are routinely tested. Indications are produced directly on the surface of the part and constitute a visual representation of the indication. Aerosol spray cans make penetrant materials very portable. Penetrant materials and associated equipment are relatively inexpensive.

Primary Disadvantages Only surface breaking defects can be detected. Only materials with a relatively nonporous surface can be inspected. Precleaning is critical since contaminants can mask defects. Metal smearing from machining, grinding, and grit or vapor blasting must be removed prior to LPI. The inspector must have direct access to the surface being inspected. Surface finish and roughness can affect inspection sensitivity. Multiple process operations must be performed and controlled. Post cleaning of acceptable parts or materials is required. Chemical handling and proper disposal is required.

4 – MAGNETIC PARTICLE TESTING

NON-DESTRUCTIVE TESTING – Magnetic Particle Testing
The material is magnetized using an electromagnetic magnet (Yoke method) or using the magnetic field produced from an electric current (Prods method) If the magnetized material is cracked, a local magnetic field is created across the gap (flux leakage). This local field can attract magnetic particle either in a dry powder or in a liquid suspension. Imperfections which break the surface and subsurface imperfections can be detected Procedure shall detail the surface preparation, magnetizing equipment, calibration methods, detection media and application, viewing conditions and identification system of welds examined

Basic Principal: In theory, magnetic particle inspection (MT) is a relatively simple concept. It can be considered as a combination of two nondestructive testing methods: magnetic flux leakage testing and visual testing. Consider the case of a bar magnet. It has a magnetic field in and around the magnet. Any place that a magnetic line of force exits or enters the magnet is called a pole. A pole where a magnetic line of force exits the magnet is called a north pole and a pole where a line of force enters the magnet is called a south pole.

Basic Principal: The first step in a magnetic particle inspection is to magnetize the component that is to be inspected. If any defects on or near the surface are present, the defects will create a leakage field. After the component has been magnetized, iron particles, either in a dry or wet suspended form, are applied to the surface of the magnetized part. The particles will be attracted and cluster at the flux leakage fields, thus forming a visible indication that the inspector can detect.

Magnetic Field Characteristics Magnetic Field In and Around a Bar Magnet A magnetic field is a change in energy within a volume of space. The magnetic field surrounding a bar magnet can be seen in the magnetograph below. A magnetograph can be created by placing a piece of paper over a magnet and sprinkling the paper with iron filings. The particles align themselves with the lines of magnetic force produced by the magnet. The magnetic lines of force show where the magnetic field exits the material at one pole and reenters the material at another pole along the length of the magnet. It can be seen in the magnetograph that there are poles all along the length of the magnet but that the poles are concentrated at the ends of the magnet. The area where the exit poles are concentrated is called the magnet's north pole and the area where the entrance poles are concentrated is called the magnet's south pole.

Magnetic Field Characteristics Magnetic Fields in and around Horseshoe and Ring Magnets Magnets come in a variety of shapes and one of the more common is the horseshoe (U) magnet. The horseshoe magnet has north and south poles just like a bar magnet but the magnet is curved so the poles lie in the same plane. The magnetic lines of force flow from pole to pole just like in the bar magnet. However, since the poles are located closer together and a more direct path exists for the lines of flux to travel between the poles, the magnetic field is concentrated between the poles.

Magnetic Field Characteristics Magnetic Fields in and around Horseshoe and Ring Magnets If a bar magnet was placed across the end of a horseshoe magnet or if a magnet was formed in the shape of a ring, the lines of magnetic force would not even need to enter the air. The value of such a magnet where the magnetic field is completely contained with the material probably has limited use. However, it is important to understand that the magnetic field can flow in loop within a material.

Magnetic Field Orientation and Flaw Detectability To properly inspect a component for imperfections, it is important to understand that the orientation between the magnetic lines of force and the flaw is very important. There are two general types of magnetic fields that can be established within a component. A longitudinal magnetic field has magnetic lines of force that run parallel to the long axis of the part. Longitudinal magnetization of a component can be accomplished using the longitudinal field set up by a coil or solenoid. It can also be accomplished using permanent magnets or electromagnets. A circular magnetic field has magnetic lines of force that run circumferentially around the perimeter of a part. A circular magnetic field is induced in an article by either passing current through the component or by passing current through a conductor surrounded by the component.

Magnetic Field Orientation and Flaw Detectability The type of magnetic field established is determined by the method used to magnetize the specimen. Being able to magnetize the part in two directions is important because the best detection of imperfections occurs when the lines of magnetic force are established at right angles to the longest dimension of the defect. This orientation creates the largest disruption of the magnetic field within the part and the greatest flux leakage at the surface of the part. As can be seen in the image below, if the magnetic field is parallel to the defect, the field will see little disruption and no flux leakage field will be produced..

Magnetic Field Orientation and Flaw Detectability An orientation of 45 to 90 degrees between the magnetic field and the defect is necessary to form an indication. Since defects may occur in various and unknown directions, each part is normally magnetized in two directions at right angles to each other. If the component below is considered, it is known that passing current through the part from end to end will establish a circular magnetic field that will be 90 degrees to the direction of the current. Therefore, defects that have a significant dimension in the direction of the current (longitudinal defects) should be detectable. Alternately, transverse-type defects will not be detectable with circular magnetization. ..

Magnetization Using Direct Induction (Direct Magnetization) With direct magnetization, current is passed directly through the component. Recall that whenever current flows, a magnetic field is produced. Using the right-hand rule, which was introduced earlier, it is known that the magnetic lines of flux form normal to the direction of the current and form a circular field in and around the conductor. When using the direct magnetization method, care must be taken to ensure that good electrical contact is established and maintained between the test equipment and the test component. Improper contact can result in arcing that may damage the component. It is also possible to overheat components in areas of high resistance such as the contact points and in areas of small cross-sectional area.

Magnetization Using Indirect Induction (Indirect Magnetization) Indirect magnetization is accomplished by using a strong external magnetic field to establish a magnetic field within the component. As with direct magnetization, there are several ways that indirect magnetization can be accomplished. The use of permanent magnets is a low cost method of establishing a magnetic field. However, their use is limited due to lack of control of the field strength and the difficulty of placing and removing strong permanent magnets from the component.

Magnetization Using Indirect Induction (Indirect Magnetization) Electromagnets in the form of an adjustable horseshoe magnet (called a yoke) eliminate the problems associated with permanent magnets and are used extensively in industry. Electromagnets only exhibit a magnetic flux when electric current is flowing around the soft iron core. When the magnet is placed on the component, a magnetic field is established between the north and south poles of the magnet. The use of coils and solenoids is a third method of indirect magnetization. When the length of a component is several times larger than its diameter, a longitudinal magnetic field can be established in the component. The component is placed longitudinally in the concentrated magnetic field that fills the center of a coil or solenoid. This magnetization technique is often referred to as a "coil shot.".

Magnetic Particle Examination – Two directions check To properly inspect a part for cracks or other defects, it is important to become familiar with the different types of magnetic fields and the equipment used to generate them. As discussed previously, one of the primary requirements for detecting a defect in a ferromagnetic material is that the magnetic field induced in the part must intercept the defect at a 45 to 90 degree angle. Flaws that are normal (90 degrees) to the magnetic field will produce the strongest indications because they disrupt more of the magnet flux. Therefore, for proper inspection of a component, it is important to be able to establish a magnetic field in at least two directions. A variety of equipment exists to establish the magnetic field for MPI. One way to classify equipment is based on its portability. Some equipment is designed to be portable so that inspections can be made in the field and some is designed to be stationary for ease of inspection in the laboratory or manufacturing facility. Portable equipment will be discussed first.

Portable Magnetizing Equipment for Magnetic Particle Examination Electromagnets Today, most of the equipment used to create the magnetic field used in MT is based on electromagnetism. That is, using an electrical current to produce the magnetic field. An electromagnetic yoke is a very common piece of equipment that is used to establish a magnetic field. It is basically made by wrapping an electrical coil around a piece of soft ferromagnetic steel. A switch is included in the electrical circuit so that the current and, therefore, the magnetic field can be turned on and off. They can be powered with alternating current from a wall socket or by direct current from a battery pack. This type of magnet generates a very strong magnetic field in a local area where the poles of the magnet touch the part being inspected. Some yokes can lift weights in excess of 40 pounds..

Lights for Magnetic Particle Examination Magnetic particle inspection can be performed using particles that are highly visible under white light conditions or particles that are highly visible under ultraviolet light conditions. When an inspection is being performed using the visible color contrast particles, no special lighting is required as long as the area of inspection is well lit. A light intensity of at least 1000 lux is recommended when visible particles are used, but a variety of light sources can be used. When fluorescent particles are used, special ultraviolet light must be used. Fluorescence is defined as the property of emitting radiation as a result of and during exposure to radiation. Particles used in fluorescent magnetic particle inspections are coated with a material that produces light in the visible spectrum when exposed to near-ultraviolet light. This "particle glow" provides high contrast indications on the component anywhere particles collect. Particles that fluoresce yellow-green are most common because this color matches the peak sensitivity of the human eye under dark conditions. However, particles that fluoresce red, blue, yellow, and green colors are available.

Ultraviolet Light Ultraviolet light or "black light" is light in the 1,000 to 4,000 Angstroms (100 to 400nm) wavelength range in the electromagnetic spectrum. Spot lights, on the other hand, provide concentrated energy that can be directed to the area of inspection. A spot light will generate a six inch diameter circle of high intensity light when held fifteen inches from the inspection surface. One hundred watt mercury vapor lights are most commonly used, but higher wattages are available

Quantitative Quality Indicator (QQI) The Quantitative Quality Indicator (QQI) or Artificial Flaw Standard is often the preferred method of assuring proper field direction and adequate field strength. The use of a QQI is also the only practical way of ensuring balanced field intensity and direction in multiple-direction magnetization equipment. QQIs are often used in conjunction with a Gauss meter to establish the inspection procedure for a particular component. They are used with the wet method only, and like other flux sharing devices, can only be used with continuous magnetization. The QQI is a thin strip of either or inch thick AISI 1005 steel. A photoetch process is used to inscribe a specific pattern, such as concentric circles or a plus sign. QQIs are nominally 3/4 inch square, but miniature shims are also available. QQIs must be in intimate contact with the part being evaluated. This is accomplished by placing the shim on a part etched side down, and taping or gluing it to the surface. The component is then magnetized and particles applied. When the field strength is adequate, the particles will adhere over the engraved pattern and provide information about the field direction. When a multidirectional technique is used, a balance of the fields is noted when all areas of the QQI produce indications.

Quantitative Quality Indicator (QQI)

Pie Gage The pie gage is a disk of highly permeable material divided into four, six, or eight sections by nonferromagnetic material. The divisions serve as artificial defects that radiate out in different directions from the center. The diameter of the gage is 3/4 to 1 inch. The divisions between the low carbon steel pie sections are to be no greater than 1/32 inch. The sections are furnace brazed and copper plated. The gage is placed on the test piece copper side up and the test piece is magnetized. After particles are applied and the excess removed, the indications provide the inspector the orientation of the magnetic field. The principal application is on flat surfaces such as weldments or steel castings where dry powder is used with a yoke or prods. The pie gage is not recommended for precision parts with complex shapes, for wet-method applications, or for proving field magnitude. The gage should be demagnetized between readings. Several of the main advantages of the pie gage are that it is easy to use and it can be used indefinitely without deterioration. The pie gage has several disadvantages, which include: it retains some residual magnetism so indications will prevail after removal of the source of magnetization, it can only be used in relatively flat areas, and it cannot be reliably used for determination of balanced fields in multidirectional magnetization.

Pie Gauge

The particles that are used for magnetic particle inspection are a key ingredient as they form the indications that alert the inspector to defects. Dry Magnetic Particles Dry magnetic particles can typically be purchased in red, black, gray, yellow and several other colors so that a high level of contrast between the particles and the part being inspected can be achieved. The size of the magnetic particles is also very important. Dry magnetic particle products are produced to include a range of particle sizes. The fine particles are around 50 mm (0.002 inch) in size, and are about three times smaller in diameter and more than 20 times lighter than the coarse particles (150 mm or inch). This make them more sensitive to the leakage fields from very small discontinuities. However, dry testing particles cannot be made exclusively of the fine particles. Coarser particles are needed to bridge large discontinuities and to reduce the powder's dusty nature. Additionally, small particles easily adhere to surface contamination, such as remnant dirt or moisture, and get trapped in surface roughness features. It should also be recognized that finer particles will be more easily blown away by the wind; therefore, windy conditions can reduce the sensitivity of an inspection. Also, reclaiming the dry particles is not recommended because the small particles are less likely to be recaptured and the "once used" mix will result in less sensitive inspections.

Wet Magnetic Particles The wet magnetic particle testing method is generally more sensitive than the dry because the suspension provides the particles with more mobility and makes it possible for smaller particles to be used since dust and adherence to surface contamination is reduced or eliminated. The wet method also makes it easy to apply the particles uniformly to a relatively large area. Wet method magnetic particles products differ from dry powder products in a number of ways. One way is that both visible and fluorescent particles are available. Most nonfluorescent particles are ferromagnetic iron oxides, which are either black or brown in color. Fluorescent particles are coated with pigments that fluoresce when exposed to ultraviolet light. Particles that fluoresce green-yellow are most common to take advantage of the peak color sensitivity of the eye but other fluorescent colors are also available. The particles used with the wet method are smaller in size than those used in the dry method for the reasons mentioned above. The particles are typically 10 µm and smaller and the synthetic iron oxides have particle diameters around 0.1 µm.

Dry and Wet Magnetic Particles

Dry Particle Examination Dry particle inspection is well suited for the inspections conducted on rough surfaces. When an electromagnetic yoke is used, the AC or half wave DC current creates a pulsating magnetic field that provides mobility to the powder. The primary applications for dry powders are unground welds and rough as-cast surfaces. Dry particle inspection is also used to detect shallow subsurface cracks. Dry particles with half wave DC is the best approach when inspecting for lack of root penetration in welds of thin materials. Half wave DC with prods and dry particles is commonly used when inspecting large castings for hot tears and cracks.

Steps in performing an inspection using dry particles Prepare the part surface - the surface should be relatively clean but this is not as critical as it is with liquid penetrant inspection. The surface must be free of grease, oil or other moisture that could keep particles from moving freely. A thin layer of paint, rust or scale will reduce test sensitivity but can sometimes be left in place with adequate results. Any loose dirt, paint, rust or scale must be removed. Apply the magnetizing force - Use permanent magnets, an electromagnetic yoke, prods, a coil or other means to establish the necessary magnetic flux. Dust on the dry magnetic particles - Dust on a light layer of magnetic particles. Gently blow off the excess powder - With the magnetizing force still applied, remove the excess powder from the surface with a few gentle puffs of dry air. The force of the air needs to be strong enough to remove the excess particles but not strong enough to dislodge particles held by a magnetic flux leakage field. Terminate the magnetizing force - If the magnetic flux is being generated with an electromagnet or an electromagnetic field, the magnetizing force should be terminated. If permanent magnets are being used, they can be left in place. Inspect for indications - Look for areas where the magnetic particles are clustered.

Wet Suspension Examination Wet suspension magnetic particle inspection, more commonly known as wet magnetic particle inspection, involves applying the particles while they are suspended in a liquid carrier. Wet magnetic particle inspection is most commonly performed using a stationary, wet, horizontal inspection unit but suspensions are also available in spray cans for use with an electromagnetic yoke. A wet inspection has several advantages over a dry inspection. First, all of the surfaces of the component can be quickly and easily covered with a relatively uniform layer of particles. Second, the liquid carrier provides mobility to the particles for an extended period of time, which allows enough particles to float to small leakage fields to form a visible indication. Therefore, wet inspection is considered best for detecting very small discontinuities on smooth surfaces. On rough surfaces, however, the particles (which are much smaller in wet suspensions) can settle in the surface valleys and lose mobility, rendering them less effective than dry powders under these conditions.

Steps in performing an inspection using wet suspensions Prepare the part surface - Just as is required with dry particle inspections, the surface should be relatively clean. The surface must be free of grease, oil and other moisture that could prevent the suspension from wetting the surface and preventing the particles from moving freely. A thin layer of paint, rust or scale will reduce test sensitivity, but can sometimes be left in place with adequate results. Any loose dirt, paint, rust or scale must be removed. Apply the suspension - The suspension is gently sprayed or flowed over the surface of the part. Usually, the stream of suspension is diverted from the part just before the magnetizing field is applied. Apply the magnetizing force - The magnetizing force should be applied immediately after applying the suspension of magnetic particles. When using a wet horizontal inspection unit, the current is applied in two or three short busts (1/2 second) which helps to improve particle mobility. Inspect for indications - Look for areas where the magnetic particles are clustered. Surface discontinuities will produce a sharp indication. The indications from subsurface flaws will be less defined and lose definition as depth increases.

Steps in performing an inspection using wet suspensions

Light Requirements When Using Visible Particles Magnetic particle inspections that use visible particles can be conducted using natural or artificial lighting. Inspector must constantly stay aware of the lighting conditions and make adjustments when needed. To improve the uniformity of lighting from one inspection to the next, the use of artificial lighting is recommended. Artificial lighting should be white whenever possible and white flood or halogen lamps are most commonly used. The light intensity is required to be 100 foot-candles at the surface being inspected. It is advisable to choose a white light wattage that will provide sufficient light, but avoid excessive reflected light that could distract from the inspection.

Light Requirements When Using Fluorescent Particles - Ultraviolet Lighting When performing a magnetic particle inspection using fluorescent particles, the condition of the ultraviolet light and the ambient white light must be monitored. Standards and procedures require verification of lens condition and light intensity. Black lights should never be used with a cracked filter as the output of white light and harmful black light will be increased. The cleanliness of the filter should be checked visually and cleaned as necessary before warming-up the light.

Indication of a crack in a saw blade & Indication of cracks in a weldment

Indication of cracks originating at a fastener hole & Before and after inspection pictures of cracks emanating from a hole

Cracks in a drive shaft. & Indication of cracks in a bearing

Indication of a crack in a in a crane hook & Indication of cracks at a sharp radius

5 – RADIOGRAPHIC TESTING

NON-DESTRUCTIVE TESTING – Radiographic Testing
A source of ionizing radiation (X or Gamma) is placed on one side of the component and a radiographic film, enclosed within a lightproof container, is placed opposite the source. After a calculated period of time, the source is removed and the film is chemically processed. Particularly good at detecting volumetric flaws such as voids, porosity and solid inclusions Procedure shall detail the type of radiation source, test arrangement and films overlapping, type and position of image quality indicators (IQI), image quality, film system and intensifying screens used if any, exposure conditions, scattered radiation control, film processing, film density and identification system of welds examined

Nature of Penetrating Radiation

X-ray Generators The major components of an X-ray generator are the tube, the high voltage generator, the control console, and the cooling system. X-rays are generated by directing a stream of high speed electrons at a target material such as tungsten, which has a high atomic number. When the electrons are slowed or stopped by the interaction with the atomic particles of the target, X-radiation is produced. This is accomplished in an X-ray tube such as the one shown here. The X-ray tube is one of the components of an X-ray generator.

Radio Isotope (Gamma) Sources Manmade radioactive sources are produced by introducing an extra neutron to atoms of the source material. As the material rids itself of the neutron, energy is released in the form of gamma rays. Two of the more common industrial gamma-ray sources for industrial radiography are iridium-192 and cobalt-60. In comparison to an X- ray generator, cobalt-60 produces energies comparable to a MeV X-ray system and iridium-192 to a 460 keV X-ray system. These high energies make it possible to penetrate thick materials with a relatively short exposure time. This and the fact that sources are very portable are the main reasons that gamma sources are widely used for field radiography. Of course, the disadvantage of a radioactive source is that it can never be turned off and safely managing the source is a constant responsibility. Physical size of isotope materials varies between manufacturers, but generally an isotope material is a pellet that measures 1.5 mm x 1.5 mm. ,

Radio Isotope (Gamma) Sources The source capsule and the pigtail is housed in a shielding device referred to as a exposure device or camera. Depleted uranium is often used as a shielding material for sources. The exposure device for iridium-192 and cobalt-60 sources will contain 45 pounds and 500 pounds of shielding materials, respectively. Cobalt cameras are often fixed to a trailer and transported to and from inspection sites. When the source is not being used to make an exposure, it is locked inside the exposure device.

Radio Isotope (Gamma) Sources To make a radiographic exposure, a crank-out mechanism and a guide tube are attached to opposite ends of the exposure device. The guide tube often has a collimator at the end to shield the radiation except in the direction necessary to make the exposure. The crank-out cable is stretched as far as possible to put as much distance as possible between the exposure device and the radiographer. To make the exposure, the radiographer quickly cranks the source out of the exposure device and into position in the collimator at the end of the guide tube. At the end of the exposure time, the source is cranked back into the exposure device. There is a series of safety procedures, which include several radiation surveys, that must be accomplished when making an exposure with a gamma source.

Geometric Unsharpness Geometric unsharpness refers to the loss of definition that is the result of geometric factors of the radiographic equipment and setup. It occurs because the radiation does not originate from a single point but rather over an area. Consider the images below which show two sources of different sizes, the paths of the radiation from each edge of the source to each edge of the feature of the sample, the locations where this radiation will expose the film and the density profile across the film. In the first image, the radiation originates at a very small source. Since all of the radiation originates from basically the same point, very little geometric unsharpness is produced in the image. In the second image, the source size is larger and the different paths that the rays of radiation can take from their point of origin in the source causes the edges of the notch to be less defined.

Geometric Unsharpness The three factors controlling unsharpness are source size, source to object distance, and object to detector distance. The source size is obtained by referencing manufacturers specifications for a given X-ray or gamma ray source. Industrial x-ray tubes often have focal spot sizes of 1.5 mm squared but microfocus systems have spot sizes in the 30 micron range. As the source size decreases, the geometric unsharpness also decreases. For a given size source, the unsharpness can also be decreased by increasing the source to object distance, but this comes with a reduction in radiation intensity.

Geometric Unsharpness The object to detector distance is usually kept as small as possible to help minimize unsharpness. However, there are situations, such as when using geometric enlargement, when the object is separated from the detector, which will reduce the definition. The applet below allow the geometric unsharpness to be visualized as the source size, source to object distance, and source to detector distance are varied. The area of varying density at the edge of a feature that results due to geometric factors is called the penumbra. The penumbra is the gray area seen in the applet.

Kilovoltage X-ray generators come in a large variety of sizes and configurations. There are stationary units that are intended for use in lab or production environments and portable systems that can be easily moved to the job site. Systems are available in a wide range of energy levels. When inspecting large steel or heavy metal components, systems capable of producing millions of electron volts may be necessary to penetrate the full thickness of the material. Alternately, small, lightweight components may only require a system capable of producing only a few tens of kilovolts. Focal Spot Size Another important consideration is the focal spot size of the tube since this factors into the geometric unsharpness of the image produced. Generally, the smaller the spot size the better. But as the electron stream is focused to a smaller area, the power of the tube must be reduced to prevent overheating at the tube anode. Therefore, the focal spot size becomes a tradeoff of resolving capability and power.

Image Considerations The usual objective in radiography is to produce an image showing the highest amount of detail possible. This requires careful control of a number of different variables that can affect image quality. Radiographic sensitivity is a measure of the quality of an image in terms of the smallest detail or discontinuity that may be detected. Radiographic sensitivity is dependant on the combined effects of two independent sets of variables. One set of variables affects the contrast and the other set of variables affects the definition of the image.

Since radiographic contrast and definition are not dependent upon the same set of factors, it is possible to produce radiographs with the following qualities: Low contrast and poor definition High contrast and poor definition Low contrast and good definition High contrast and good definition

Radiography Image Quality Radiographic contrast is the degree of density difference between two areas on a radiograph. Contrast makes it easier to distinguish features of interest, such as defects, from the surrounding area. The image to the right shows two radiographs of the same stepwedge. The upper radiograph has a high level of contrast and the lower radiograph has a lower level of contrast. While they are both imaging the same change in thickness, the high contrast image uses a larger change in radiographic density to show this change. In each of the two radiographs, there is a small circle, which is of equal density in both radiographs. It is much easier to see in the high contrast radiograph.

Radiographic Contrast Radiographic contrast describes the differences in photographic density in a radiograph. The contrast between different parts of the image is what forms the image and the greater the contrast, the more visible features become. Radiographic contrast has two main contributors: subject contrast and detector (film) contrast.

Subject Contrast Subject contrast is the ratio of radiation intensities transmitted through different areas of the component being evaluated. It is dependant on the absorption differences in the component, the wavelength of the primary radiation, and intensity and distribution of secondary radiation due to scattering. The absorption differences within the subject will affect the level of contrast in a radiograph. The larger the difference in thickness or density between two areas of the subject, the larger the difference in radiographic density or contrast. However, x-rays using a low kilovoltage will generally result in a radiograph with high contrast. This occurs because low energy radiation is more easily attenuated. Therefore, the ratio of photons that are transmitted through a thick and thin area will be greater with low energy radiation. This in turn will result in the film being exposed to a greater and lesser degree in the two areas.

Film Contrast Film contrast refers to density differences that result due to the type of film used, how it was exposed, and how it was processed. Since there are other detectors besides film, this could be called detector contrast, but the focus here will be on film. Exposing a film to produce higher film densities will generally increase the contrast in the radiograph.

Film Contrast A typical film characteristic curve, which shows how a film responds to different amounts of radiation exposure, is shown to the right. From the shape of the curves, it can be seen that when the film has not seen many photon interactions (which will result in a low film density) the slope of the curve is low. In this region of the curve, it takes a large change in exposure to produce a small change in film density. Therefore, the sensitivity of the film is relatively low. It can be seen that changing the log of the relative exposure from 0.75 to 1.4 only changes the film density from 0.20 to about However, at film densities above 2.0, the slope of the characteristic curve for most films is at its maximum. In this region of the curve, a relatively small change in exposure will result in a relatively large change in film density. For example, changing the log of relative exposure from 2.4 to 2.6 would change the film density from to Therefore, the sensitivity of the film is high in this region of the curve. In general, the highest overall film density that can be conveniently viewed or digitized will have the highest level of contrast and contain the most useful information.

Radiography Image Quality Radiographic definition (Sharpness) is the abruptness of change in going from one area of a given radiographic density to another. Like contrast, definition also makes it easier to see features of interest, such as defects, but in a totally different way. In the image to the right, the upper radiograph has a high level of definition and the lower radiograph has a lower level of definition. In the high definition radiograph it can be seen that a change in the thickness of the stepwedge translates to an abrupt change in radiographic density. It can be seen that the details, particularly the small circle, are much easier to see in the high definition radiograph. It can be said that the detail portrayed in the radiograph is equivalent to the physical change present in the stepwedge. In other words, a faithful visual reproduction of the stepwedge was produced. In the lower image, the radiographic setup did not produce a faithful visual reproduction. The edge line between the steps is blurred. This is evidenced by the gradual transition between the high and low density areas on the radiograph.

Definition (Sharpness) As mentioned previously, radiographic definition is the abruptness of change from one density to another. Geometric factors of the equipment and the radiographic setup, and film and screen factors both have an effect on definition. Geometric factors include the size of the area of origin of the radiation, the source-to-detector (film) distance, the specimen-to-detector (film) distance, movement of the source, specimen or detector during exposure, the angle between the source and some feature and the abruptness of change in specimen thickness or density.

Radiographic definition Geometric Factors The effect of source size, source-to-film distance and the specimen-to- detector distance were change the radiographic definition. But briefly, to produce the highest level of definition, the focal-spot or source size should be as close to a point source as possible, the source-to-detector distance should be a great as practical, and the specimen-to-detector distance should be a small as practical. This is shown graphically in the images below.

Radiographic definition Film and Screen Factors - The last set of factors concern the film and the use of fluorescent screens. A fine grain film is capable of producing an image with a higher level of definition than is a coarse grain film. Wavelength of the radiation will influence apparent graininess. As the wavelength shortens and penetration increases, the apparent graininess of the film will increase. Also, increased development of the film will increase the apparent graininess of the radiograph. The use of fluorescent screens also results in lower definition. The reason that fluorescent screens are sometimes used is because incident radiation causes them to give off light that helps to expose the film. However, the light they produce spreads in all directions, exposing the film in adjacent areas, as well as in the areas which are in direct contact with the incident radiation. Fluorescent screens also produce screen mottle on radiographs. Screen mottle is associated with the statistical variation in the numbers of photons that interact with the screen from one area to the next.

Radiographic Density Photographic, radiographic or film density is a measure of the degree of film darkening. Technically it should be called "transmitted density" when associated with transparent-base film since it is a measure of the light transmitted through the film. Density is a logarithmic unit that describes a ratio of two measurements. Specifically, it is the log of the intensity of light incident on the film (I0) to the intensity of light transmitted through the film (It) . Similar to the decibel, using the log of the ratio allows ratios of various sizes to be described using easy to work with numbers. The following table shows the relationship between the amount of transmitted light and the calculated film density.

Radiographic Density From this table, it can be seen that a density reading of 2.0 is the result of only one percent of the incident light making it through the film. At a density of 4.0 only 0.01% of transmitted light reaches the far side of the film. Industrial codes and standards typically require a radiograph to have a density between 2.0 and 4.0 for acceptable viewing with common film viewers. Above 4.0, extremely bright viewing lights is necessary for evaluation. Contrast within a film increases with increasing density, so in general the higher the density the better. When radiographs will be digitized, densities above 4.0 are often used since digitization systems can capture and redisplay for easy viewing information from densities up to 6.0.

Radiographic Density Film density is measured with a densitometer. A densitometer simply has a photoelectric sensor that measures the amount of light transmitted through a piece of film. The film is placed between the light source and the sensor and a density reading is produced by the instrument

Controlling Radiographic Quality One of the methods of controlling the quality of a radiograph is through the use of image quality indicators (IQIs). IQIs, which are also referred to as penetrameters, provide a means of visually informing the film interpreter of the contrast sensitivity and definition of the radiograph. The IQI indicates that a specified amount of change in material thickness will be detectable in the radiograph, and that the radiograph has a certain level of definition so that the density changes are not lost due to unsharpness. Without such a reference point, consistency and quality could not be maintained and defects could go undetected. Image quality indicators take many shapes and forms due to the various codes or standards that invoke their use. In the United States, two IQI styles are prevalent: the placard, or hole-type and the wire IQI. IQIs comes in a variety of material types so that one with radiation absorption characteristics similar to the material being radiographed can be used.

Wire IQIs ASTM Standard E747 covers the radiographic examination of materials using wire IQIs to control image quality. Wire IQIs consist of a set of six wires arranged in order of increasing diameter and encapsulated between two sheets of clear plastic. E747 specifies four wire IQI sets, which control the wire diameters. The set letter (A, B, C or D) is shown in the lower right corner of the IQI. The number in the lower left corner indicates the material group. The same image quality levels and expressions (i.e. 2-2T) used for hole-type IQIs are typically also used for wire IQIs. JIS Standard wire IQI consist of a set of seven wires arranged.

Placement of Wire IQIs IQIs should be placed on the source side of the part over a section with a material thickness equivalent to the region of interest. If this is not possible, the IQI may be placed on a block of similar material and thickness to the region of interest. When a block is used, the IQI should be the same distance from the film as it would be if placed directly on the part in the region of interest. The IQI should also be placed slightly away from the edge of the part so that at least three of its edges are visible in the radiograph. When film side IQI are used a lead letter “F” should be placed beside the IQI to indicate the positioning.

Radiographs should be reviewed for film quality prior to interpreting the image for possible defects. Radiographs should be checked for identification, density and sensitivity and also for the presence of artefacts that may interfere with the assessment. Where film quality is unacceptable the area of weld covered by the film should be re- radiographed..

Identification - Manufacturers may have a method of radiographic identification which is linked to a quality system but the following is a guide to the normal requirements for details appearing on the radiograph. The identification should include the manufacturer’s symbol, the component/item/weld number as appropriate, the location within the weld (such as location markers 1 to 2, B to C etc) and the date radiography was carried out. The identification details usually appear in the image but sometimes a system of "flashing" the details on to the film before exposure is used. In all cases location markers which indicate the diagnostic length (extent of the weld on the film to be examined) must appear as radiographic images. The repair status of the weld should also be shown, usually by markers R1(repair), R2(second repair) etc. Identification details must not encroach on the weld area of interest - the length of weld and heat affected zone between the length markers.

Radiographs (developed film exposed to x-ray or gamma radiation) are generally viewed on a light-box. However, it is becoming increasingly common to digitize radiographs and view them on a high resolution monitor. Proper viewing conditions are very important when interpreting a radiograph. The viewing conditions can enhance or degrade the subtle details of radiographs. Before beginning the evaluation of a radiograph, the viewing equipment and area should be considered. The area should be clean and free of distracting materials. Magnifying aids, masking aids, and film markers should be close at hand. Thin cotton gloves should be available and worn to prevent fingerprints on the radiograph. Ambient light levels should be low. Ambient light levels of less than 2 fc are often recommended, but subdued lighting (rather than total darkness) is preferable in the viewing room. The brightness of the surroundings should be about the same as the area of interest in the radiograph. Room illumination must be arranged so that there are no reflections from the surface of the film under examination. Film viewers should be clean and in good working condition.

The radiographic process should be performed in accordance with a written procedure or code, or as required by contractual documents. The required documents should be available in the viewing area and referenced as necessary when evaluating components. Radiographic film quality and acceptability, as required by the procedure, should first be determined. It should be verified that the radiograph was produced to the correct density on the required film type, and that it contains the correct identification information. It should also be verified that the proper image quality indicator was used and that the required sensitivity level was met. Next, the radiograph should be checked to ensure that it does not contain processing and handling artifacts that could mask discontinuities or other details of interest. The technician should develop a standard process for evaluating the radiographs so that details are not overlooked. Once a radiograph passes these initial checks, it is ready for interpretation. Radiographic film interpretation is an acquired skill combining visual acuity with knowledge of materials, manufacturing processes, and their associated discontinuities. If the component is inspected while in service, an understanding of applied loads and history of the component is helpful. A process for viewing radiographs (e.g. left to right, top to bottom, etc.) is helpful and will prevent overlooking an area on the radiograph. This process is often developed over time and individualized. The mind as well as the eyes need to occasionally rest when interpreting radiographs

In addition to producing high quality radiographs, the radiographer must also be skilled in radiographic interpretation. Interpretation of radiographs takes place in three basic steps: (1) detection, (2) interpretation, and (3) evaluation. All of these steps make use of the radiographer's visual acuity. Visual acuity is the ability to resolve a spatial pattern in an image. The ability of an individual to detect discontinuities in radiography is also affected by the lighting condition in the place of viewing, and the experience level for recognizing various features in the image. Discontinuities: Discontinuities are interruptions in the typical structure of a material. These interruptions may occur in the base metal, weld material or "heat affected" zones. Discontinuities, which do not meet the requirements of the codes or specifications used to invoke and control an inspection, are referred to as defects.

RADIOGRAPHS ARE TO BE INTERPRETED BY QUALIFIED PERSONNEL OF THE MANUFACTURER OR ITS SUBCONTRACTOR

NON-DESTRUCTIVE TESTING – Radiographic Testing - Welds
Cold Laps Cold lap is a condition where the weld filler metal does not properly fuse with the base metal or the previous weld pass material (interpass cold lap). The arc does not melt the base metal sufficiently and causes the slightly molten puddle to flow into the base material without bonding

Porosity Porosity is the result of gas entrapment in the solidifying metal. Porosity can take many shapes on a radiograph but often appears as dark round or irregular spots or specks appearing singularly, in clusters, or in rows. Sometimes, porosity is elongated and may appear to have a tail. This is the result of gas attempting to escape while the metal is still in a liquid state and is called wormhole porosity. All porosity is a void in the material and it will have a higher radiographic density than the surrounding area.

Cluster porosity Cluster porosity is caused when flux coated electrodes are contaminated with moisture. The moisture turns into a gas when heated and becomes trapped in the weld during the welding process. Cluster porosity appear just like regular porosity in the radiograph but the indications will be grouped close together.

Slag inclusions Slag inclusions are nonmetallic solid material entrapped in weld metal or between weld and base metal. In a radiograph, dark, jagged asymmetrical shapes within the weld or along the weld joint areas are indicative of slag inclusions.

Incomplete penetration (IP) Incomplete penetration (IP) or lack of penetration (LOP) occurs when the weld metal fails to penetrate the joint. It is one of the most objectionable weld discontinuities. Lack of penetration allows a natural stress riser from which a crack may propagate. The appearance on a radiograph is a dark area with well-defined, straight edges that follows the land or root face down the center of the weldment.

Internal concavity Internal concavity or suck back is a condition where the weld metal has contracted as it cools and has been drawn up into the root of the weld. On a radiograph it looks similar to a lack of penetration but the line has irregular edges and it is often quite wide in the center of the weld image

Internal or root undercut Internal or root undercut is an erosion of the base metal next to the root of the weld. In the radiographic image it appears as a dark irregular line offset from the centerline of the weldment. Undercutting is not as straight edged as LOP because it does not follow a ground edge.

Incomplete fusion (IF) Incomplete fusion (IF) is a condition where the weld filler metal does not properly fuse with the base metal. Appearance on radiograph: usually appears as a dark line or lines oriented in the direction of the weld seam along the weld preparation or joining area

External or crown undercut External or crown undercut is an erosion of the base metal next to the crown of the weld. In the radiograph, it appears as a dark irregular line along the outside edge of the weld area.

Offset or mismatch Offset or mismatch are terms associated with a condition where two pieces being welded together are not properly aligned. The radiographic image shows a noticeable difference in density between the two pieces. The difference in density is caused by the difference in material thickness. The dark, straight line is caused by the failure of the weld metal to fuse with the land area.

Inadequate weld reinforcement Inadequate weld reinforcement is an area of a weld where the thickness of weld metal deposited is less than the thickness of the base material. It is very easy to determine by radiograph if the weld has inadequate reinforcement, because the image density in the area of suspected inadequacy will be higher (darker) than the image density of the surrounding base material.

Excess weld reinforcement Excess weld reinforcement is an area of a weld that has weld metal added in excess of that specified by engineering drawings and codes. The appearance on a radiograph is a localized, lighter area in the weld. A visual inspection will easily determine if the weld reinforcement is in excess of that specified by the engineering requirements.

Cracks Cracks can be detected in a radiograph only when they are propagating in a direction that produces a change in thickness that is parallel to the x-ray beam. Cracks will appear as jagged and often very faint irregular lines. Cracks can sometimes appear as "tails" on inclusions or porosity.

Tungsten inclusions Tungsten inclusions. Tungsten is a brittle and inherently dense material used in the electrode in tungsten inert gas welding. If improper welding procedures are used, tungsten may be entrapped in the weld. Radiographically, tungsten is more dense than aluminum or steel, therefore it shows up as a lighter area with a distinct outline on the radiograph.

Oxide inclusions Oxide inclusions are usually visible on the surface of material being welded (especially aluminum). Oxide inclusions are less dense than the surrounding material and, therefore, appear as dark irregularly shaped discontinuities in the radiograph.

NON-DESTRUCTIVE TESTING – Radiographic Testing - Welds - Discontinuities in Gas Metal Arc Welds (GMAW) Welds Burn-Through Burn-Through results when too much heat causes excessive weld metal to penetrate the weld zone. Often lumps of metal sag through the weld, creating a thick globular condition on the back of the weld. These globs of metal are referred to as icicles. On a radiograph, burn-through appears as dark spots, which are often surrounded by light globular areas (icicles).

NON-DESTRUCTIVE TESTING – Radiographic Testing - Castings
Gas porosity or blow holes Gas porosity or blow holes are caused by accumulated gas or air which is trapped by the metal. These discontinuities are usually smooth-walled rounded cavities of a spherical, elongated or flattened shape. If the sprue is not high enough to provide the necessary heat transfer needed to force the gas or air out of the mold, the gas or air will be trapped as the molten metal begins to solidify. Blows can also be caused by sand that is too fine, too wet, or by sand that has a low permeability so that gas cannot escape. Too high a moisture content in the sand makes it difficult to carry the excessive volumes of water vapor away from the casting. Another cause of blows can be attributed to using green ladles, rusty or damp chills and chaplets.

Sand inclusions and dross Sand inclusions and dross are nonmetallic oxides, which appear on the radiograph as irregular, dark blotches. These come from disintegrated portions of mold or core walls and/or from oxides (formed in the melt) which have not been skimmed off prior to the introduction of the metal into the mold gates. Careful control of the melt, proper holding time in the ladle and skimming of the melt during pouring will minimize or obviate this source of trouble.

Shrinkage Shrinkage is a form of discontinuity that appears as dark spots on the radiograph. Shrinkage assumes various forms, but in all cases it occurs because molten metal shrinks as it solidifies, in all portions of the final casting. Shrinkage is avoided by making sure that the volume of the casting is adequately fed by risers which sacrificially retain the shrinkage. Shrinkage in its various forms can be recognized by a number of characteristics on radiographs. There are at least four types of shrinkage: (1) cavity; (2) dendritic; (3) filamentary; and (4) sponge types. Some documents designate these types by numbers, without actual names, to avoid possible misunderstanding.

Cavity Shrinkage Cavity shrinkage appears as areas with distinct jagged boundaries. It may be produced when metal solidifies between two original streams of melt coming from opposite directions to join a common front. Cavity shrinkage usually occurs at a time when the melt has almost reached solidification temperature and there is no source of supplementary liquid to feed possible cavities.

Dendritic shrinkage & Filamentary shrinkage Dendritic shrinkage is a distribution of very fine lines or small elongated cavities that may vary in density and are usually unconnected. Filamentary shrinkage usually occurs as a continuous structure of connected lines or branches of variable length, width and density, or occasionally as a network.

Sponge shrinkage Sponge shrinkage shows itself as areas of lacy texture with diffuse outlines, generally toward the mid-thickness of heavier casting sections. Sponge shrinkage may be dendritic or filamentary shrinkage. Filamentary sponge shrinkage appears more blurred because it is projected through the relatively thick coating between the discontinuities and the film surface.

Cold shuts Cold shuts generally appear on or near a surface of cast metal as a result of two streams of liquid meeting and failing to unite. They may appear on a radiograph as cracks or seams with smooth or rounded edges

Inclusions Inclusions are nonmetallic materials in an otherwise solid metallic matrix. They may be less or more dense than the matrix alloy and will appear on the radiograph, respectively, as darker or lighter indications. The latter type is more common in light metal castings.

6 – ULTRASONICS TESTING

NON-DESTRUCTIVE TESTING – Ultrasonics testing
Ultrasonics Testing (UT) Ultrasonics use transmission of high-frequency sound waves into a material to detect imperfections or to locate changes in material properties. The most commonly used ultrasonic testing technique is pulse echo, wherein sound is introduced into a test object and reflections (echoes) are returned to a receiver from internal imperfections or from the part's geometrical surfaces.

Basic Principles of Ultrasonic Testing Ultrasonic Testing (UT) uses high frequency sound energy to conduct examinations and make measurements. Ultrasonic inspection can be used for flaw detection/evaluation, dimensional measurements, material characterization, and more. A typical UT inspection system consists of several functional units, such as the pulser/receiver, transducer, and display devices. A pulser/receiver is an electronic device that can produce high voltage electrical pulses. Driven by the pulser, the transducer generates high frequency ultrasonic energy. The sound energy is introduced and propagates through the materials in the form of waves. When there is a discontinuity (such as a crack) in the wave path, part of the energy will be reflected back from the flaw surface. The reflected wave signal is transformed into an electrical signal by the transducer and is displayed on a screen. In the applet below, the reflected signal strength is displayed versus the time from signal generation to when a echo was received. Signal travel time can be directly related to the distance that the signal traveled. From the signal, information about the reflector location, size, orientation and other features can sometimes be gained.

Ultrasonic Inspection is a very useful and versatile NDT method. Some of the advantages of ultrasonic inspection that are often cited include: The depth of penetration for flaw detection or measurement is superior to other NDT methods. Only single-sided access is needed when the pulse-echo technique is used. It allows to determine reflector position and estimating size and shape. Minimal part preparation is required. Electronic equipment provides instantaneous results. Detailed images can be produced with automated systems. It has other uses, such as thickness measurement, in addition to flaw detection.

As with all NDT methods, ultrasonic inspection also has its limitations, which include: Surface must be accessible to transmit ultrasound. Skill and training is more extensive than with some other methods. It normally requires a coupling medium to promote the transfer of sound energy into the test specimen. Materials that are rough, irregular in shape, very small, exceptionally thin or not homogeneous are difficult to inspect. Cast iron and other coarse grained materials are difficult to inspect due to low sound transmission and high signal noise. Linear defects oriented parallel to the sound beam may go undetected. Reference standards are required for both equipment calibration and the characterization of flaws .

Wave Propagation Ultrasonic testing is based on time-varying deformations or vibrations in materials, which is generally referred to as acoustics. All material substances are comprised of atoms, which may be forced into vibrational motion about their equilibrium positions. Many different patterns of vibrational motion exist at the atomic level, however, most are irrelevant to acoustics and ultrasonic testing. Acoustics is focused on particles that contain many atoms that move in unison to produce a mechanical wave. When a material is not stressed in tension or compression beyond its elastic limit, its individual particles perform elastic oscillations. When the particles of a medium are displaced from their equilibrium positions, internal (electrostatic) restoration forces arise. It is these elastic restoring forces between particles, combined with inertia of the particles, that leads to the oscillatory motions of the medium. In solids, sound waves can propagate in four principle modes that are based on the way the particles oscillate. Sound can propagate as longitudinal waves, shear waves, surface waves, and in thin materials as plate waves. Longitudinal and shear waves are the two modes of propagation most widely used in ultrasonic testing.

Longitudinal and shear Wave Propagation: In longitudinal waves, the oscillations occur in the longitudinal direction or the direction of wave propagation. Since compressional and dilational forces are active in these waves, they are also called pressure or compressional waves. They are also sometimes called density waves because their particle density fluctuates as they move. Compression waves can be generated in liquids, as well as solids because the energy travels through the atomic structure by a series of comparison and expansion (rarefaction) movements. In the transverse or shear wave, the particles oscillate at a right angle or transverse to the direction of propagation. Shear waves require an acoustically solid material for effective propagation, and therefore, are not effectively propagated in materials such as liquids or gasses. Shear waves are relatively weak when compared to longitudinal waves. In fact, shear waves are usually generated in materials using some of the energy from longitudinal waves..

Longitudinal and shear Wave Propagation:

Modes of Sound Wave Propagation In air, sound travels by the compression and rarefaction of air molecules in the direction of travel. However, in solids, molecules can support vibrations in other directions, hence, a number of different types of sound waves are possible. Waves can be characterized in space by oscillatory patterns that are capable of maintaining their shape and propagating in a stable manner. The propagation of waves is often described in terms of what are called “wave modes.” As mentioned previously, longitudinal and transverse (shear) waves are most often used in ultrasonic inspection. However, at surfaces and interfaces, various types of elliptical or complex vibrations of the particles make other waves possible.

Modes of Sound Wave Propagation Longitudinal wave Transverse (Share) wave Surface (Rayleigh) Wave Plate (Lamb) wave

Properties of Acoustic Plane Wave Among the properties of waves propagating in isotropic solid materials are wavelength, frequency, and velocity. The wavelength is directly proportional to the velocity of the wave and inversely proportional to the frequency of the wave. This relationship is shown by the following equation. In ultrasonic testing, the shorter wavelength resulting from an increase in frequency will usually provide for the detection of smaller discontinuities. This will be discussed more in following sections.

Wavelength and Defect Detection In ultrasonic testing, the inspector must make a decision about the frequency of the transducer that will be used. As we noted that changing the frequency when the sound velocity is fixed will result in a change in the wavelength of the sound. The wavelength of the ultrasound used has a significant effect on the probability of detecting a discontinuity. A general rule is that a discontinuity must be larger than one-half the wavelength to stand a reasonable chance of being detected. Sensitivity and resolution are two terms that are often used in ultrasonic inspection to describe a technique's ability to locate flaws. Sensitivity is the ability to locate small discontinuities. Sensitivity generally increases with higher frequency (shorter wavelengths). Resolution is the ability of the system to locate discontinuities that are close together within the material or located near the part surface. Resolution also generally increases as the frequency increases.

Wavelength and Defect Detection The wave frequency can also affect the capability of an inspection in adverse ways. Therefore, selecting the optimal inspection frequency often involves maintaining a balance between the favorable and unfavorable results of the selection. Before selecting an inspection frequency, the material's grain structure and thickness, and the discontinuity's type, size, and probable location should be considered. As frequency increases, sound tends to scatter from large or course grain structure and from small imperfections within a material. Cast materials often have coarse grains and other sound scatters that require lower frequencies to be used for evaluations of these products. Wrought and forged products with directional and refined grain structure can usually be inspected with higher frequency transducers.

Properties of material affect its speed of sound Sound does travel at different speeds in different materials. This is because the mass of the atomic particles and the spring constants are different for different materials. The mass of the particles is related to the density of the material, and the spring constant is related to the elastic constants of a material. The general relationship between the speed of sound in a solid and its density and elastic constants is given by the following equation: Examples of approximate compressional sound velocities in materials are: Aluminum cm/microsecond, 1020 steel cm/microsecond, Cast iron cm/microsecond. Examples of approximate shear sound velocities in materials are: Aluminum cm/microsecond, 1020 steel cm/microsecond, Cast iron cm/microsecond.

Attenuation of Sound Waves When sound travels through a medium, its intensity diminishes with distance. In idealized materials, sound pressure (signal amplitude) is only reduced by the spreading of the wave. Natural materials, however, all produce an effect which further weakens the sound. This further weakening results from scattering and absorption. Scattering is the reflection of the sound in directions other than its original direction of propagation. Absorption is the conversion of the sound energy to other forms of energy. The combined effect of scattering and absorption is called attenuation. Ultrasonic attenuation is the decay rate of the wave as it propagates through material. Attenuation of sound within a material itself is often not of intrinsic interest. However, natural properties and loading conditions can be related to attenuation. Attenuation often serves as a measurement tool that leads to the formation of theories to explain physical or chemical phenomenon that decreases the ultrasonic intensity.

Acoustic Impedance Sound travels through materials under the influence of sound pressure. Because molecules or atoms of a solid are bound elastically to one another, the excess pressure results in a wave propagating through the solid. The acoustic impedance (Z) of a material is defined as the product of its density (p) and acoustic velocity (V). Z = pV Acoustic impedance is important in: the determination of acoustic transmission and reflection at the boundary of two materials having different acoustic impedances. the design of ultrasonic transducers. assessing absorption of sound in a medium.

Reflection and Transmission Coefficients (Pressure) Ultrasonic waves are reflected at boundaries where there is a difference in acoustic impedances (Z) of the materials on each side of the boundary. (See preceding page for more information on acoustic impedance.) This difference in Z is commonly referred to as the impedance mismatch. The greater the impedance mismatch, the greater the percentage of energy that will be reflected at the interface or boundary between one medium and another. The fraction of the incident wave intensity that is refracted can be derived because particle velocity and local particle pressures must be continuous across the boundary. When the acoustic impedances of the materials on both sides of the boundary are known, the fraction of the incident wave intensity that is reflected can be calculated with the equation below. The value produced is known as the reflection coefficient. Multiplying the reflection coefficient by 100 yields the amount of energy reflected as a percentage of the original energy.

Reflection and Transmission Coefficients (Pressure) Note that the reflection and transmission coefficients are often expressed in decibels (dB) to allow for large changes in signal strength to be more easily compared. To convert the intensity or power of the wave to dB units, take the log of the reflection or transmission coefficient and multiply this value times 10. However, 20 is the multiplier used in the applet since the power of sound is not measured directly in ultrasonic testing. The transducers produce a voltage that is approximately proportionally to the sound pressure. The power carried by a traveling wave is proportional to the square of the pressure amplitude. Therefore, to estimate the signal amplitude change, the log of the reflection or transmission coefficient is multiplied by 20. If reflection and transmission at interfaces is followed through the component, only a small percentage of the original energy makes it back to the transducer, even when loss by attenuation is ignored. For example, consider an immersion inspection of a steel block. The sound energy leaves the transducer, travels through the water, encounters the front surface of the steel, encounters the back surface of the steel and reflects back through the front surface on its way back to the transducer. At the water steel interface (front surface), 12% of the energy is transmitted. At the back surface, 88% of the 12% that made it through the front surface is reflected. This is 10.6% of the intensity of the initial incident wave. As the wave exits the part back through the front surface, only 12% of 10.6 or 1.3% of the original energy is transmitted back to the transducer

Refraction and Snell's Law When an ultrasounic wave passes through an interface between two materials at an oblique angle, and the materials have different indices of refraction, both reflected and refracted waves are produced. This also occurs with light, which is why objects seen across an interface appear to be shifted relative to where they really are. For example, if you look straight down at an object at the bottom of a glass of water, it looks closer than it really is. A good way to visualize how light and sound refract is to shine a flashlight into a bowl of slightly cloudy water noting the refraction angle with respect to the incident angle. Refraction takes place at an interface due to the different velocities of the acoustic waves within the two materials. The velocity of sound in each material is determined by the material properties (elastic modulus and density) for that material. In the animation below, a series of plane waves are shown traveling in one material and entering a second material that has a higher acoustic velocity. Therefore, when the wave encounters the interface between these two materials, the portion of the wave in the second material is moving faster than the portion of the wave in the first material. It can be seen that this causes the wave to bend.

Refraction and Snell's Law Note that in the diagram, there is a reflected longitudinal wave (VL1' ) shown. This wave is reflected at the same angle as the incident wave because the two waves are traveling in the same material, and hence have the same velocities. This reflected wave is unimportant in our explanation of Snell's Law, but it should be remembered that some of the wave energy is reflected at the interface. In the applet below, only the incident and refracted longitudinal waves are shown. The angle of either wave can be adjusted by clicking and dragging the mouse in the region of the arrows. Values for the angles or acoustic velocities can also be entered in the dialog boxes so the that applet can be used as a Snell's Law calculator.

Refraction and Snell's Law When a longitudinal wave moves from a slower to a faster material, there is an incident angle that makes the angle of refraction for the wave 90o. This is know as the first critical angle. The first critical angle can be found from Snell's law by putting in an angle of 90° for the angle of the refracted ray. At the critical angle of incidence, much of the acoustic energy is in the form of an inhomogeneous compression wave, which travels along the interface and decays exponentially with depth from the interface. This wave is sometimes referred to as a "creep wave." Because of their inhomogeneous nature and the fact that they decay rapidly, creep waves are not used as extensively as Rayleigh surface waves in NDT. However, creep waves are sometimes more useful than Rayleigh waves because they suffer less from surface irregularities and coarse material microstructure due to their longer wavelengths

Signal-to-Noise Ratio The detection of a defect involves many factors other than the relationship of wavelength and flaw size. For example, the amount of sound that reflects from a defect is also dependent on the acoustic impedance mismatch between the flaw and the surrounding material. A void is generally a better reflector than a metallic inclusion because the impedance mismatch is greater between air and metal than between two metals. Often, the surrounding material has competing reflections. Microstructure grains in metals and the aggregate of concrete are a couple of examples. A good measure of detectability of a flaw is its signal-to-noise ratio (S/N). The signal-to-noise ratio is a measure of how the signal from the defect compares to other background reflections (categorized as "noise"). A signal-to-noise ratio of 3 to 1 is often required as a minimum.

Piezoelectric Transducers The conversion of electrical pulses to mechanical vibrations and the conversion of returned mechanical vibrations back into electrical energy is the basis for ultrasonic testing. The active element is the heart of the transducer as it converts the electrical energy to acoustic energy, and vice versa. The active element is basically a piece of polarized material (i.e. some parts of the molecule are positively charged, while other parts of the molecule are negatively charged) with electrodes attached to two of its opposite faces. When an electric field is applied across the material, the polarized molecules will align themselves with the electric field, resulting in induced dipoles within the molecular or crystal structure of the material. This alignment of molecules will cause the material to change dimensions. This phenomenon is known as electrostriction. In addition, a permanently- polarized material such as quartz (SiO2) or barium titanate (BaTiO3) will produce an electric field when the material changes dimensions as a result of an imposed mechanical force. This phenomenon is known as the piezoelectric effect.

Piezoelectric Transducers

Characteristics of Piezoelectric Transducers The transducer is a very important part of the ultrasonic instrumentation system. It incorporates a piezoelectric element, which converts electrical signals into mechanical vibrations (transmit mode) and mechanical vibrations into electrical signals (receive mode). Many factors, including material, mechanical and electrical construction, and the external mechanical and electrical load conditions, influence the behavior of a transducer. Mechanical construction includes parameters such as the radiation surface area, mechanical damping, housing, connector type and other variables of physical construction. The piezoelectric element is cut to 1/2 the desired wavelength. To get as much energy out of the transducer as possible, an impedance matching is placed between the active element and the face of the transducer. Optimal impedance matching is achieved by sizing the matching layer so that its thickness is 1/4 of the desired wavelength. This keeps waves that were reflected within the matching layer in phase when they exit the layer. The backing material supporting the crystal has a great influence on the damping characteristics of a transducer.

Characteristics of Piezoelectric Transducers A cut away of a typical contact transducer is shown below:

Transducer Efficiency, Bandwidth and Frequency A transducer that performs well in one application will not always produce the desired results in a different application. For example, sensitivity to small defects is proportional to the product of the efficiency of the transducer as a transmitter and a receiver. Resolution, the ability to locate defects near the surface or in close proximity in the material, requires a highly damped transducer. The frequency noted on a transducer is the central or center frequency and depends primarily on the backing material. Highly damped transducers will respond to frequencies above and below the central frequency. The broad frequency range provides a transducer with high resolving power. Less damped transducers will exhibit a narrower frequency range and poorer resolving power, but greater penetration. The central frequency will also define the capabilities of a transducer. Lower frequencies (0.5MHz- 2.25MHz) provide greater energy and penetration in a material, while high frequency crystals (15.0MHz-25.0MHz) provide reduced penetration but greater sensitivity to small discontinuities.

Radiated Fields of Ultrasonic Transducers The sound that emanates from a piezoelectric transducer does not originate from a point, but instead originates from most of the surface of the piezoelectric element. Round transducers are often referred to as piston source transducers because the sound field resembles a cylindrical mass in front of the transducer. The sound field from a typical piezoelectric transducer is shown below. The intensity of the sound is indicated by color, with lighter colors indicating higher intensity. These are sometimes also referred to as diffraction effects. This wave interference leads to extensive fluctuations in the sound intensity near the source and is known as the near field. Because of acoustic variations within a near field, it can be extremely difficult to accurately evaluate flaws in materials when they are positioned within this area.

Radiated Fields of Ultrasonic Transducers The pressure waves combine to form a relatively uniform front at the end of the near field. The area beyond the near field where the ultrasonic beam is more uniform is called the far field. In the far field, the beam spreads out in a pattern originating from the center of the transducer. The transition between the near field and the far field occurs at a distance, N, and is sometimes referred to as the "natural focus" of a flat (or unfocused) transducer. The near/far field distance, N, is significant because amplitude variations that characterize the near field change to a smoothly declining amplitude at this point. The area just beyond the near field is where the sound wave is well behaved and at its maximum strength. Therefore, optimal detection results will be obtained when flaws occur in this area.

Transducer Types It is important to choose transducers that have the desired frequency, bandwidth, and focusing to optimize inspection capability. Most often the transducer is chosen either to enhance the sensitivity or resolution of the system. Transducers are classified into groups according to the application. Contact transducers and Immersion transducers Dual element transducers and single transducers Delay and normal line transducers Angle beam and straight beam transducers

Couplant A couplant is a material (usually liquid) that facilitates the transmission of ultrasonic energy from the transducer into the test specimen. Couplant is generally necessary because the acoustic impedance mismatch between air and solids (i.e. such as the test specimen) is large. Therefore, nearly all of the energy is reflected and very little is transmitted into the test material. The couplant displaces the air and makes it possible to get more sound energy into the test specimen so that a usable ultrasonic signal can be obtained. In contact ultrasonic testing a thin film of oil, glycerin or water is generally used between the transducer and the test surface. When scanning over the part or making precise measurements, an immersion technique is often used. In immersion ultrasonic testing both the transducer and the part are immersed in the couplant, which is typically water. This method of coupling makes it easier to maintain consistent coupling while moving and manipulating the transducer and/or the part.

Data Presentation Ultrasonic data can be collected and displayed in a number of different formats. The three most common formats are know in the NDT world as A- scan, B-scan and C-scan presentations. Each presentation mode provides a different way of looking at and evaluating the region of material being inspected. Modern computerized ultrasonic scanning systems can display data in all three presentation forms simultaneously.

Crack Tip Diffraction When the geometry of the part is relatively uncomplicated and the orientation of a flaw is well known, the length (a) of a crack can be determined by a technique known as tip diffraction. One common application of the tip diffraction technique is to determine the length of a crack originating from on the backside of a flat plate as shown below. In this case, when an angle beam transducer is scanned over the area of the flaw, the principle echo comes from the base of the crack to locate the position of the flaw (Image 1). A second, much weaker echo comes from the tip of the crack and since the distance traveled by the ultrasound is less, the second signal appears earlier in time on the scope (Image 2).

Attenuation Measurements Ultrasonic wave propagation is influenced by the microstructure of the material through which it propagates. The velocity of the ultrasonic waves is influenced by the elastic moduli and the density of the material, which in turn are mainly governed by the amount of various phases present and the damage in the material. Ultrasonic attenuation, which is the sum of the absorption and the scattering, is mainly dependent upon the damping capacity and scattering from the grain boundary in the material. However, to fully characterize the attenuation required knowledge of a large number of thermo-physical parameters that in practice are hard to quantify.

Attenuation Measurements Relative measurements such as the change of attenuation and simple qualitative tests are easier to make than absolute measure. Relative attenuation measurements can be made by examining the exponential decay of multiple back surface reflections. However, significant variations in microstructural characteristics and mechanical properties often produce only a relatively small change in wave velocity and attenuation. Absolute measurements of attenuation are very difficult to obtain because the echo amplitude depends on factors in addition to amplitude. The most common method used to get quantitative results is to use an ultrasonic source and detector transducer separated by a known distance. By varying the separation distance, the attenuation can be measured from the changes in the amplitude.

Calibration Methods Calibration refers to the act of evaluating and adjusting the precision and accuracy of measurement equipment. In ultrasonic testing, several forms of calibration must occur. First, the electronics of the equipment must be calibrated to ensure that they are performing as designed. This operation is usually performed by the equipment manufacturer. It is also usually necessary for the operator to perform a "user calibration" of the equipment. This user calibration is necessary because most ultrasonic equipment can be reconfigured for use in a large variety of applications. The user must "calibrate" the system, which includes the equipment settings, the transducer, and the test setup, to validate that the desired level of precision and accuracy are achieved. The term calibration standard is usually only used when an absolute value is measured.

Calibration Methods In ultrasonic testing, there is also a need for reference standards. Reference standards are used to establish a general level of consistency in measurements and to help interpret and quantify the information contained in the received signal. Reference standards are used to validate that the equipment and the setup provide similar results from one day to the next and that similar results are produced by different systems. Reference standards also help the inspector to estimate the size of flaws. In a pulse- echo type setup, signal strength depends on both the size of the flaw and the distance between the flaw and the transducer. The inspector can use a reference standard with an artificially induced flaw of known size and at approximately the same distance away for the transducer to produce a signal. By comparing the signal from the reference standard to that received from the actual flaw, the inspector can estimate the flaw size. Some of these specimens are shown in the figure below.

Introduction to the Common Standards Calibration and reference standards for ultrasonic testing come in many shapes and sizes. The type of standard used is dependent on the NDE application and the form and shape of the object being evaluated. The material of the reference standard should be the same as the material being inspected and the artificially induced flaw should closely resemble that of the actual flaw. This second requirement is a major limitation of most standard reference samples. Most use drilled holes and notches that do not closely represent real flaws. In most cases the artificially induced defects in reference standards are better reflectors of sound energy (due to their flatter and smoother surfaces) and produce indications that are larger than those that a similar sized flaw would produce. Producing more "realistic" defects is cost prohibitive in most cases and, therefore, the inspector can only make an estimate of the flaw size. Computer programs that allow the inspector to create computer simulated models of the part and flaw may one day lessen this limitation.

IIW Type Mini Standard IIW type blocks are used to calibrate instruments for both angle beam and normal incident inspections. Some of their uses include setting metal- distance and sensitivity settings, determining the sound exit point and refracted angle of angle beam transducers, and evaluating depth resolution of normal beam inspection setups. Instructions on using the IIW type blocks can be found in the annex of American Society for Testing and Materials Standard E164, Standard Practice for Ultrasonic Contact Examination of Weldments.

The Miniature Angle-Beam or ROMPAS Calibration Block Standard The miniature angle-beam is a calibration block that was designed for the US Air Force for use in the field for instrument calibration. The block is much smaller and lighter than the IIW block but performs many of the same functions. The miniature angle-beam block can be used to check the beam angle and exit point of the transducer. The block can also be used to make metal-distance and sensitivity calibrations for both angle and normal-beam inspection setups.

Distance/Area-Amplitude Blocks Standard Distance/area amplitude correction blocks typically are purchased as a ten- block set, as shown above. Aluminum sets are manufactured per the requirements of ASTM E127 and steel sets per ASTM E428. Sets can also be purchased in titanium. Each block contains a single flat-bottomed, plugged hole. The hole sizes and metal path distances are as follows: 3/64" at 3" 5/64" at 1/8", 1/4", 1/2", 3/4", 11/2", 3", and 6" 8/64" at 3" and 6"

Distance Amplitude Correction (DAC) Acoustic signals from the same reflecting surface will have different amplitudes at different distances from the transducer. Distance amplitude correction (DAC) provides a means of establishing a graphic ‘reference level sensitivity’ as a function of sweep distance on the A-scan display. The use of DAC allows signals reflected from similar discontinuities to be evaluated where signal attenuation as a function of depth has been correlated. Most often DAC will allow for loss in amplitude over material depth (time), graphically on the A-scan display but can also be done electronically by certain instruments. Because near field length and beam spread vary according to transducer size and frequency, and materials vary in attenuation and velocity, a DAC curve must be established for each different situation. DAC may be employed in both longitudinal and shear modes of operation as well as either contact or immersion inspection techniques.

Distance Amplitude Correction (DAC) A distance amplitude correction curve is constructed from the peak amplitude responses from reflectors of equal area at different distances in the same material. A-scan echoes are displayed at their non-electronically compensated height and the peak amplitude of each signal is marked on the flaw detector screen or, preferably, on a transparent plastic sheet attached to the screen. Reference standards which incorporate side drilled holes (SDH), flat bottom holes (FBH), or notches whereby the reflectors are located at varying depths are commonly used. It is important to recognize that regardless of the type of reflector used, the size and shape of the reflector must be constant.

Weldments (Welded Joints) The most commonly occurring defects in welded joints are porosity, slag inclusions, lack of side-wall fusion, lack of inter-run fusion, lack of root penetration, undercutting, and longitudinal or transverse cracks. With the exception of single gas pores all the defects listed are usually well detectable by ultrasonics. Most applications are on low-alloy construction quality steels, however, welds in aluminum can also be tested. Ultrasonic flaw detection has long been the preferred method for nondestructive testing in welding applications. This safe, accurate, and simple technique has pushed ultrasonics to the forefront of inspection technology. Ultrasonic weld inspections are typically performed using a straight beam transducer in conjunction with an angle beam transducer and wedge. A straight beam transducer, producing a longitudinal wave at normal incidence into the test piece, is first used to locate any laminations in or near the heat-affected zone. This is important because an angle beam transducer may not be able to provide a return signal from a laminar flaw.

Weldments (Welded Joints) The second step in the inspection involves using an angle beam transducer to inspect the actual weld. Angle beam transducers use the principles of refraction and mode conversion to produce refracted shear or longitudinal waves in the test material. [Note: Many AWS inspections are performed using refracted shear waves. However, material having a large grain structure, such as stainless steel may require refracted longitudinal waves for successful inspections.] This inspection may include the root, sidewall, crown, and heat-affected zones of a weld. The process involves scanning the surface of the material around the weldment with the transducer. This refracted sound wave will bounce off a reflector (discontinuity) in the path of the sound beam. With proper angle beam techniques, echoes returned from the weld zone may allow the operator to determine the location and type of discontinuity. .

Weldments (Welded Joints) To determine the proper scanning area for the weld, the inspector must first calculate the location of the sound beam in the test material. Using the refracted angle, beam index point and material thickness, the V-path and skip distance of the sound beam is found. Once they have been calculated, the inspector can identify the transducer locations on the surface of the material corresponding to the crown, sidewall, and root of the weld.

Angle Beam Transducer T F   斜角探头试件

Surface beam transducer

MATERIALS AND WELDING BVN TM MW1 – Non-Destructive Testing

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MATERIALS AND WELDING BVN TM MW1 – Non-Destructive Testing

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