BMFB 4283 NDT & FAILURE ANALYSIS Lectures for Week 4 Prof. Qumrul Ahsan, PhD Department of Engineering Materials Faculty of Manufacturing Engineering

Issues to address 4.0 Ultrasound 4.1 Introduction 4.2 Theory   4.1 Introduction 4.2 Theory 4.3 Equipment 4.4 Inspection Principles 4.5 Applications

Ultrasonic Testing This presentation was developed to provide students in industrial technology programs, such as welding, an introduction to ultrasonic testing. The material by itself is not intended to train individuals to perform NDT functions but rather to acquaint individuals with the NDT equipment and methods that they are likely to encounter in industry. More information has been included than might necessarily be required for a general introduction to the subject as some instructors have requested at least 60 minutes of material. Instructors can modify the presentation to meet their needs by simply hiding slides in the “slide sorter” view of PowerPoint.” This presentation is one of eight developed by the Collaboration for NDT Education. The topics covered by the other presentations are: Introduction to Nondestructive Testing Visual Inspection Penetrant Testing Magnetic Particle Testing Radiographic Testing Eddy Current Testing Welder Certification All rights are reserved by the authors and the presentation cannot be copied or distributed except by the Collaboration for NDT Education. A free copy of the presentations can be requested by contacting the Collaboration at NDT-ed@cnde.iastate.edu.

Introduction This module presents an introduction to the NDT method of ultrasonic testing. Ultrasonic testing uses high frequency sound energy to conduct examinations and make measurements. Ultrasonic examinations can be conducted on a wide variety of material forms including castings, forgings, welds, and composites. A considerable amount of information about the part being examined can be collected, such as the presence of discontinuities, part or coating thickness; and acoustical properties can often be correlated to certain properties of the material.

Ultrasound – An Introduction Ultrasonic Testing (UT) uses high frequency sound (ultrasound) energy to conduct examinations and make measurements. Ultrasonic inspection can be used for flaw detection/ evaluation, dimensional measurements, material characterization, and more.

Basic Principles of Sound Sound is produced by a vibrating body and travels in the form of a wave. Sound waves travel through materials by vibrating the particles that make up the material. The pitch of the sound is determined by the frequency of the wave (vibrations or cycles completed in a certain period of time). Ultrasound is sound with a pitch too high to be detected by the human ear.

Basic Principles of Sound (cont.) The measurement of sound waves from crest to crest determines its wavelength (λ). The time it takes a sound wave to travel a distance of one complete wavelength is the same amount of time it takes the source to execute one complete vibration. The sound wavelength is inversely proportional to its frequency. (λ = 1/f) Several wave modes of vibration are used in ultrasonic inspection. The most common are longitudinal, shear, and Rayleigh (surface) waves.

direction of osscillation Longitudinal wave a wave formed by individual particles oscillating in the direction of propagation propagated through solids, liquids and gases This wave is the most easily generated and detected. Almost all of the sound energy used in UT originates as longitudinal sound and then may be converted to the other modes for special applications. direction of osscillation direction of propagation wave length

Transverse wave particle motion is transverse or at an angle to the direction of propagation only in solids because distance between molecules is so great in liquid and gases This wave is the most easily generated and detected speed about half of longitudinal direction of osscillation direction of propagation wave length

Surface wave Rayleigh wave elliptical particle motion penetration approx. one wavelength velocity of approx. 90 percent of shear wave Reflection of Rayleigh waves from cracks on the surface or from sub-surface discontinuities may be seen on the CRT

Lamb wave (Plate wave) generated in a relatively thin solid substance whose thickness is about one wavelength consist of a mixture of zig-zag reflected longitudinal and transverse detect surface and sub-surface discontinuities

Stoneley (Leaky Rayleigh Waves) Summary of Waves The table below summarizes many, but not all, of the wave modes possible in solids. Wave Types in Solids Particle Vibrations Longitudinal Parallel to wave direction Transverse (Shear) Perpendicular to wave direction Surface - Rayleigh Elliptical orbit - symmetrical mode Plate Wave - Lamb Component perpendicular to surface (extensional wave) Plate Wave - Love Parallel to plane layer, perpendicular to wave direction Stoneley (Leaky Rayleigh Waves) Wave guided along interface Sezawa Antisymmetric mode

Frequency, Wavelength and Velocity Frequency is the number of oscillation in one sec Period is the time is to make on oscillation t = 1/f Wavelength expressed as  is given as the distance between two successive crests in the waveform, this distance varies with frequency and velocity The velocity of sound propagation varies from one material to another. It depends on the elastic property and density of the material.

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 and the impedance restrict movement of velocity. The acoustic impedance (Z) of a material is defined as the product of its density () and acoustic velocity (v). Z = Driving pressure/velocity of particle = P/v or V 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.

Features of Ultrasound Ultrasonic waves are very similar to light waves in that they can be reflected, refracted, and focused. Reflection and refraction occurs when sound waves interact with interfaces of differing acoustic properties. In solid materials, the vibrational energy can be split into different wave modes when the wave encounters an interface at an angle other than 90 degrees. Ultrasonic reflections from the presence of discontinuities or geometric features enables detection and location. The velocity of sound in a given material is constant and can only be altered by a change in the mode of energy.

BEHAVIOR OF SOUND WAVES Reflection Refraction Diffraction Mode conversion Attenuation

Reflection at Normal Incidence When an ultrasonic wave incidents normal to an interface between two materials and the materials have different acoustic impedance , both reflected and transmitted waves are produced. The 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

Reflection at Normal Incidence The fraction of the incident wave intensity that is reflected can be calculated with the equation below and known as the reflection coefficient  Since the amount of reflected energy plus the transmitted energy must equal the total amount of incident energy, the transmission coefficient is calculated by simply subtracting the reflection coefficient from one.  T = 1 - R = 4Z1Z2 / [Z2+ Z1]2

Reflection at Oblique Incidence When an ultrasonic 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. The reflected energy follows the same laws as light, i.e. the angle of incidence is equal to the angle of reflection

Refraction at Oblique Incidence Refraction takes place at an interface due to the different velocities of the acoustic waves within the two materials. The refraction can be calculated by Snell’s Law as shown in the following equation. Where: VL1 is the longitudinal wave velocity in material 1. VL2 is the longitudinal wave velocity in material 2.

MODE CONVERSION Ultrasonic energy when reflected, may change from one waveform to another (compressional to shear, shear to surface, etc) This mode change is accompanied by the appropriate change in velocity.

refracted transverse wave Critical Angles L medium 1 medium 2 reflected wave refracted wave incident wave T L perspex steel reflected wave refracted waves incident wave  = 1 = 27.5°° (first critical angle) T = 33.3 L = 90° T L perspex steel reflected wave refracted transverse wave incident wave  = 36.4° T = 45° L T O surface wave perspex steel reflected wave incident wave  = 2 = 57° ( second critical angle) T = 90°

Ranges for incident waves 27,5° L 57° perspex 90° steel T 33,3°

DIFFRACTION Diffraction is the apparent bending of sound waves around the tips of a narrow reflector. Diffuse reflection or scatter occurs at these positions resulting in a small amount of energy being bent around the defect

ATTENUATION Attenuation is the loss intensity of the ultrasonic beam as it passes through a material and is dependent on the physical properties of the material. Two factors affect the sound attenuation: Absorption Cause by the interaction of the particles as they vibrate during the passage of sound waves The movement of particles cause friction, and dissipated as heat As the frequency increases, the absorption greater due to raid movement Scatter Cause by grain boundaries, porosity, non metallic inclusions, etc Larger grain size greater scattering Coarse grain will be more attenuative than fine grain material Rough surface also cause attenuation

Attenuation Attenuation can be represented as a decaying exponential. The amplitude change of a decaying plane wave can be expressed as: A0 is the unattenuated amplitude of the propagating wave at some location. The amplitude A is the reduced amplitude after the wave has traveled a distance z from that initial location. The quantity  is the attenuation coefficient of the wave traveling in the z-direction. The dimensions of are nepers/length, where a neper is a dimensionless quantity. Attenuation can also measured in terms of decibels (dB) dB = 20 log (I0/I)

Ultrasound Generation Ultrasound is generated with a transducer. A piezoelectric element in the transducer converts electrical energy into mechanical vibrations (sound), and vice versa. The transducer is capable of both transmitting and receiving sound energy.

Sound beam Since the ultrasound originates from a number of points along the transducer face, the ultrasound intensity along the beam is affected by constructive and destructive wave interference . 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. Where: N = Near Field Length or Transition from Near Field to Far Field D = Diameter of the Transducer F = Frequency of the Transducer l = Wavelength, V= Velocity of Sound in the Material

Sound beam The area beyond the near field where the ultrasonic beam is more uniform is called the far field. 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. 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. Where: N = Near Field Length or Transition from Near Field to Far Field D = Diameter of the Transducer F = Frequency of the Transducer l = Wavelength, V= Velocity of Sound in the Material

Sound beam  = angle of divergence near field far field acoustical axis (central beam) N = near field length  = angle of divergence Far zone is beyond near zone. The beam diverges resulting in decay in sound intensity as the distance from the crystal is increased. In the far zone, large and small reflectors follow different laws: Large reflectors: follow inverse law- the amplitude is inversely proportional to the distance Small reflectors: follow inverse square law- the amplitude is inversely proportional to square of the distance.

Beam Divergence/Beam Spread the energy in the beam does not remain in a cylinder, but instead spreads out as it propagates through the material. The phenomenon is usually referred to as beam spread but is sometimes also referred to as beam divergence or ultrasonic diffraction. beam spread is twice the beam divergence. Where: θ = Beam divergence angle from centerline to point where signal is at half strength.   V = Sound velocity in the material. (inch/sec or cm/sec)1 a = Radius of the transducer. (inch or cm)1 F = Frequency of the transducer. (cycles/second)

Dead zone dead zone The dead zone is the ringing time of the crystal and is minimized by the damping medium behind the crystal. It is not possible to detect defect in this zone. 2 4 6 8 10

Principle of Ultrasonic Inspection Ultrasonic waves are introduced into a material where they travel in a straight line and at a constant speed until they encounter a surface. At surface interfaces some of the wave energy is reflected and some is transmitted. The amount of reflected or transmitted energy can be detected and provides information about the size of the reflector. The travel time of the sound can be measured and this provides information on the distance that the sound has traveled.

Principle of transit time measurement finish signal (echo) transmitter Stop- watch start signal (pulse) probe transit time measurement sound transit path work piece

Equipment Equipment for ultrasonic testing is very diversified. Proper selection is important to insure accurate inspection data as desired for specific applications. In general, there are three basic components that comprise an ultrasonic test system: - Instrumentation - Transducers - Calibration Standards

Transducers Transducers are manufactured in a variety of forms, shapes and sizes for varying applications. Transducers are categorized in a number of ways which include: - Contact or immersion - Single or dual element - Normal or angle beam In selecting a transducer for a given application, it is important to choose the desired frequency, bandwidth, size, and in some cases focusing which optimizes the inspection capabilities.

Chac: of Transducers Some transducers are specially fabricated to be more efficient transmitters and others to be more efficient receivers. 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. 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.

Chac: of Transducers 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. High frequency transducers, when used with the proper instrumentation, can improve flaw resolution and thickness measurement capabilities dramatically. Broadband transducers with frequencies up to 150 MHz are commercially available. Transducers are constructed to withstand some abuse, but they should be handled carefully. Misuse, such as dropping, can cause cracking of the wear plate, element, or the backing material. Damage to a transducer is often noted on the A-scan presentation as an enlargement of the initial pulse.

Contact Transducers Contact transducers are designed to withstand rigorous use, and usually have a wear plate on the bottom surface to protect the piezoelectric element from contact with the surface of the test article. Many incorporate ergonomic designs for ease of grip while scanning along the surface.

Probe construction - Contact Transducers The cork separator and corrugations in the Perspex reduce the cross-talk/chatter between crystals The probe are used mainly for testing thin sections (thickness measurement) and to detect near surface defect

Contact Transducers (cont.) A way to improve near surface resolution with a single element transducer is through the use of a delay line. Delay line transducers have a plastic piece that is a sound path that provides a time delay between the sound generation and reception of reflected energy. Interchangeable pieces make it possible to configure the transducer with insulating wear caps or flexible membranes that conform to rough surfaces. Common applications include thickness gauging and high temperature measurements.

Straight beam probe housing socket matching- element damping- block protecting face (probe delay) crystal workpiece Sound pulse

Probe construction - Contact Transducers Contact transducers are available with two piezoelectric crystals in one housing. These transducers are called dual element transducers. One crystal acts as a transmitter, the other as a receiver. This arrangement improves near surface resolution because the second transducer does not need to complete a transmit function before listening for echoes. Dual elements are commonly employed in thickness gauging of thin materials.

TR-probe / dual crystal probe transmitter socket receiver socket acoustical barrier damping blocks crystal delay

Probe delay with TR-probes 2 4 6 8 10 IP BE work piece TR-probe

Cross talk at high gain IP BE flaw echo cross talk echo TR-probe flaw 2 4 6 8 10 TR-probe flaw

Transducers (cont.) Angle beam transducers incorporate wedges to introduce a refracted shear wave into a material. The incident wedge angle is used with the material velocity to determine the desired refracted shear wave according to Snell’s Law) Transducers can use fixed or variable wedge angles. Common application is in weld examination.

Angle beam probe damping blocks housing socket perspex wedge (probe delay) crystal workpiece Sound pulse

Transducers (cont.) Immersion transducers are designed to transmit sound whereby the transducer and test specimen are immersed in a liquid coupling medium (usually water). Immersion transducers are manufactured with planar, cylindrical or spherical acoustic lenses (focusing lens).

Selection of Probe Frequency

Instrumentation Test equipment can be classified in a number of different ways, this may include portable or stationary, contact or immersion, manual or automated. Further classification of instruments commonly divides them into four general categories: D-meters, Flaw detectors, Industrial and special application.

Pulser- Receivers Ultrasonic pulser-receivers can be used for flaw detection and thickness gauging in a wide variety of metals, plastics, ceramics, and composites. The pulser section of the instrument generates short, large amplitude electric pulses of controlled energy, which are converted into short ultrasonic pulses when applied to an ultrasonic transducer. Control functions associated with the pulser circuit include: Pulse length or damping (The amount of time the pulse is applied to the transducer.) Pulse energy (Typical pulser circuits will apply from 100 volts to 800 volts to a transducer.)

Pulser- Receivers In the receiver section the voltage signals produced by the transducer, which represent the received ultrasonic pulses, are amplified. The amplified radio frequency (RF) signal is available as an output for display or capture for signal processing. Control functions associated with the receiver circuit include : Signal rectification (The RF signal can be viewed as positive half wave, negative half wave or full wave.) Filtering to shape and smooth return signals Gain, or signal amplification Reject control 2 4 6 8 10 work piece probe sound wave starts at crystal light point transmitter transmission pulse

Instrumentation (cont.) D-meters or digital thickness gauge instruments provide the user with a digital (numeric) readout. They are designed primarily for corrosion/erosion inspection applications. Some instruments provide the user with both a digital readout and a display of the signal. A distinct advantage of these units is that they allow the user to evaluate the signal to ensure that the digital measurements are of the desired features.

Instrumentation (cont.) Flaw detectors are instruments designed primarily for the inspection of components for defects. However, the signal can be evaluated to obtain other information such as material thickness values. Both analog and digital display. Offer the user options of gating horizontal sweep and amplitude threshold.

Instrumentation (cont.) Industrial flaw detection instruments, provide users with more options than standard flaw detectors. May be modulated units allowing users to tailor the instrument for their specific needs. Generally not as portable as standard flaw detectors.

Instrumentation (cont.) Immersion ultrasonic scanning systems are used for automated data acquisition and imaging. They integrate an immersion tank, ultrasonic instrumentation, a scanning bridge, and computer controls. The signal strength and/or the time-of-flight of the signal is measured for every point in the scan plan. The value of the data is plotted using colors or shades of gray to produce detailed images of the surface or internal features of a component.

Data Presentation Information from ultrasonic testing can be presented in a number of differing formats. Three of the more common formats include: A-scan B-scan C-scan These three formats will be discussed in the next few slides.

Data Presentation - A-scan Time Signal Amplitude A-scan presentation displays the amount of received ultrasonic energy as a function of time. Relative discontinuity size can be estimated by comparing the signal amplitude to that from a known reflector. Reflector depth can be determined by the position of the signal on the horizontal sweep.

Data Presentation - B-scan B-scan presentations display a profile view (cross-sectional) of a test specimen. Only the reflector depth in the cross-section and the linear dimensions can be determined. A limitation to this display technique is that reflectors may be masked by larger reflectors near the surface.

Data Presentation - C-scan The C-scan presentation displays a plan type view of the test specimen and discontinuities. C-scan presentations are produced with an automated data acquisition system, such as in immersion scanning. Use of A-scan in conjunction with C-scan is necessary when depth determination is desired. The C-scan presentation provides an image of the features that reflect and scatter the sound within and on the surfaces of the test piece.

Images of a Quarter Produced With an Ultrasonic Immersion Scanning System Gray scale image produced using the sound reflected from the front surface of the coin Gray scale image produced using the sound reflected from the back surface of the coin (inspected from “heads” side)

COUPLING Water, glycerine, oils, petroleum grease, silicon grease, wall paper paste, etc. Water with wetting agent suitable for smooth surface Grease/heavy oil for hot, vertical or rough surface

Test Techniques Ultrasonic testing is a very versatile inspection method, and inspections can be accomplished in a number of different ways. Ultrasonic inspection techniques are commonly divided into three primary classifications. Pulse-echo and Through Transmission (Relates to whether reflected or transmitted energy is used) Normal Beam and Angle Beam (Relates to the angle that the sound energy enters the test article) Contact and Immersion (Relates to the method of coupling the transducer to the test article)

Test Techniques – Pulse-Echo (cont.) Digital display showing signal generated from sound reflecting off back surface. Digital display showing the presence of a reflector midway through material, with lower amplitude back surface reflector. The pulse-echo technique allows testing when access to only one side of the material is possible, and it allows the location of reflectors to be precisely determined.

Test Techniques – Through-Transmission Digital display showing received sound through material thickness. Digital display showing loss of received signal due to presence of a discontinuity in the sound field.

Test Techniques – Normal and Angle Beam In normal beam testing, the sound beam is introduced into the test article at 90 degree to the surface. In angle beam testing, the sound beam is introduced into the test article at some angle other than 90. The choice between normal and angle beam inspection usually depends on two considerations: The orientation of the feature of interest – the sound should be directed to produce the largest reflection from the feature. Obstructions on the surface of the part that must be worked around.

NORMAL BEAM TECHNIQUE Thickness Measurement Single Crystal Probe normally for wall thickness more than 30mm single backwall multiple backwall - eliminate paint thickness Twin crystal (TR) Probe normally for thickness less than 30mm be careful with cross talk echo Velocity correction Specimen thickness = indicated thick X Velocity in specimen Velocity in cal. block

ANGLE BEAM TECHNIQUE Angle Beam Transducers and wedges are typically used to introduce a refracted shear wave into the test material. An angled sound path allows the sound beam to come in from the side, thereby improving detect-ability of flaws in and around welded areas.

Test Techniques – Contact Vs Immersion To get useful levels of sound energy into a material, the air between the transducer and the test article must be removed. This is referred to as coupling. In contact testing (shown on the previous slides) a couplant such as water, oil or a gel is applied between the transducer and the part. In immersion testing, the part and the transducer are place in a water bath. This arrangement allows better movement of the transducer while maintaining consistent coupling. With immersion testing, an echo from the front surface of the part is seen in the signal but otherwise signal interpretation is the same for the two techniques. 1 2 Defect 2 4 6 8 10 FWE BWE 1 IP 2 4 6 8 10 FWE BWE DE IP IP = Initial Pulse FWE = Front Wall Echo DE = Defect Echo BWE = Back Wall Echo

Calibration Standards Calibration is a operation of configuring the ultrasonic test equipment to known values. This provides the inspector with a means of comparing test signals to known measurements. Calibration standards come in a wide variety of material types, and configurations due to the diversity of inspection applications. Calibration standards are typically manufactured from materials of the same acoustic properties as those of the test articles. To produce reliable and reproducible test results

Calibration Standards (cont.) Thickness calibration standards may be flat or curved for pipe and tubing applications, consisting of simple variations in material thickness. ASTM Distance/Area Amplitude Distance/Area Amplitude standards utilize flat bottom holes or side drilled holes to establish known reflector size with changes in sound path form the entry surface. NAVSHIPS

Calibration Standards (cont.) There are also calibration standards for use in angle beam inspections when flaws are not parallel to entry surface. These standards utilized side drilled holes, notches, and geometric configuration to establish time distance and amplitude relationships. IIW DSC DC Rhompas SC ASME Pipe Sec. XI

Qualification Standards Qualification standards differ from calibration standards in that their use is for purposes of varying proper equipment operation and qualification of equipment use for specific codes and standards. DC-dB Accuracy AWS Resolution IOW Beam Profile

Standard Test Blocks To perform Calibration block? Reference block? Equipment characteristic verification Range/Time base calibration Reference level or sensitivity setting Calibration block? Reference block?

EQUIPMENT CHARACTERISTIC Horizontal linearity Screen height linearity Amplitude control linearity Resolution Maximum penetrating power Pulse length

CALIBRATION-Normal probe Determine test range Min. TR at least = block thickness Calibration echoes – 1st and 2nd BWE

CALIBRATION-Angle probe Use 100mm curvature of V1 or 25mm/50mm curvature of V2 block Range – min. 100mm for V1 Range – 100mm/125mm for V2

Beam profile Use IOW or ASME block

CALIBRATION Probe index Probe angle

Range calibration 2 4 6 8 10 div. 50 100 mm steel, L

Calibration block 1 with angle beam probes 5 10 15 70° 100 mm 60° 45°

1st echo from circular section 2 4 6 8 10 5 10 15 100 mm

Echo sequence from 100 mm radius 2 4 6 8 10 5 10 15 100 mm 200 mm 300 mm

25 mm radius of calibration block 2 this wave will be absorbed ! s = 25 mm s = 100 mm s = 175 mm etc. 1 2 3

50 mm radius of calibration block 2 s = 50 mm s = 125 mm s = 200 mm etc. 1 2 3

100 mm range calibration on V2 5 10 15 2 4 6 8 10 100 mm steel

SENSITIVITY SETTING 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

Inspection Applications Some of the applications for which ultrasonic testing may be employed include: Flaw detection (cracks, inclusions, porosity, etc.) Erosion & corrosion thickness gauging Assessment of bond integrity in adhesively joined and brazed components Estimation of void content in composites and plastics Measurement of case hardening depth in steels Estimation of grain size in metals On the following slides are examples of some common applications of ultrasonic inspection.

Thickness Gauging Ultrasonic thickness gauging is routinely utilized in the petrochemical and utility industries to determine various degrees of corrosion/erosion. Applications include piping systems, storage and containment facilities, and pressure vessels.

Principle, sound wave in the workpiece transmitter sound wave work piece probe 2 4 6 8 10

Principle, sound pulse at the back wall transmitter work piece probe 2 4 6 8 10

Principle, sound pulse at the coupling surface 2 4 6 8 10 work piece probe transmitter

Principle, echo display and 2nd run 2 4 6 8 10 work piece probe back wall echo transmitter

Tandem technique (top) 1 T 10 15 5 10 15 5 R

Tandem technique (middle) 2 T 10 15 5 10 15 5 R

Tandem technique (bottom) 3 T R 5 10 15 5 10 15

Probe delay electrical zero (initial pulse) mechanical zero (surface) 2 4 6 8 10 electrical zero (initial pulse) mechanical zero (surface) crystal probe protecting face

Probe delay electical zero (initial pulse) probe 2 4 6 8 10 2 4 6 8 10 delay (wedge) mechanical zero (surface) sound wave work piece

Flaw location and echo display 2 4 6 8 10 work piece probe back wall echo flaw flaw echo

Flaw location and echo display work piece probe back wall echo flaw echo 2 4 6 8 10

Flaw location and echo display work piece probe back wall echo flaw echo 2 4 6 8 10

Flaw location and echo display work piece probe back wall echo flaw echo covered by initial pulse 2 4 6 8 10

Flaw location and echo display work piece probe back wall echo: without with flaw 2 4 6 8 10

Flaw location and echo display work piece probe flaw echo sequence 2 4 6 8 10

Angle beam probe with both wave types 10 20 30 40 L T possible flaw locations 2 4 6 8 10

Flaw Detection - Delaminations Contact, pulse-echo inspection for delaminations on 36” rolled beam. Signal showing multiple back surface echoes in an unflawed area. Additional echoes indicate delaminations in the member.

Flaw Detection in Welds One of the most widely used methods of inspecting weldments is ultrasonic inspection. Full penetration groove welds lend themselves readily to angle beam shear wave examination.

Flaw detection

Flaw detection 10 20 30 40

Flaw detection 10 20 30 40

Bad flaw orientation

Improper flaw location crack

Improper flaw orientation 10 20 30 40

Perfect flaw orientation 10 20 30 40

Flaw detectability with improper flaw orientation reflected sound waves 5 10 15 sound beam flat defect

Flaw distance s

Near surface detectability with angle beam pobes 10 20

Angle reflection 10 20 30 40 crack

Angle reflection

Vertical, near surface flaw 10 20 30 40

Flaw location s = k•R s s = sound path k = scale factor 2 4 6 8 10 s = sound path k = scale factor R = screen reading s = k•R 15 10 5 s work piece reflector

Flaw location with angle beam probes Sound entry point projection point "flaw triangle" a ß d s a = s•sin ß d = s•cos ß ß = probe angle s = sound path a = surface distance d = depth ß flaw location

Flaw location with angle beam probes a = surface distance a' = reduced surface distance x = x-value = distance: index point - front edge of probe a x a' 15 10 5 s d reflector work piece

Flaw location with an angle beam probe on a plate 15 10 5 T s d = apparent depth apparent flaw location

Flaw location with an angle beam probe on a plate d' = apparent depth d = real depth T = work piece thickness a = s • sin d‘ = s • cos d = 2T - d a 15 10 5 d real flaw location T s d' apparent flaw location

Large defects parallel to the scanning surface

Scanning the edge of the defect Flaw echo drops to 50% of its maximum value

Determination of the defect area probe position with echo amplitude reduced to 50 % delamination "half value" positions

Flaw sizes and echo amplitudes IP BE R IP BE R IP BE R 2 4 6 8 10 2 4 6 8 10 2 4 6 8 10

Flaw distances and echo amplitudes IP R BE 2 4 6 8 10 IP R BE 2 4 6 8 10 IP R BE 2 4 6 8 10

Distance amplitude curves on the CRT screen B 4 S 6 mm BE F 3 mm 4 mm 500 100 200 300 400

Defect evaluation by comparison - 1 instrument gain: G = 34 dB IP F BE 80 % F 2 4 6 8 10

Defect evaluation by comparison - 2 2 4 6 8 10 IP BE R instrument gain: 34 dB

Defect evaluation by comparison - 3 IP RE BE + 8 dB 2 4 6 8 10 instrument gain: 42 dB

Distance amplitude curve (DAC) Echo 1 2 3 4 Position 1 2 3 4 10 20 30 40 10 20 30 40 10 20 30 40 10 20 30 40 2 4 6 8 10

DAC and TCG DAC time corrected gain 2 4 6 8 10 2 4 6 8 10

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Advantages of Ultrasonic Testing Sensitive to both surface and subsurface discontinuities. Depth of penetration for flaw detection or measurement is superior to other methods. Only single-sided access is needed when pulse-echo technique is used. High accuracy in determining reflector position and estimating size and shape. Minimal part preparation required. Electronic equipment provides instantaneous results. Detailed images can be produced with automated systems. Has other uses such as thickness measurements, in addition to flaw detection.

Limitations of Ultrasonic Testing Surface must be accessible to transmit ultrasound. Skill and training is more extensive than with some other methods. Normally requires a coupling medium to promote transfer of sound energy into 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 characterization of flaws.