2D4: Integrity of Materials & Components Properties of Materials Failure Modes and Prevention Failure Prevention Strategies Non-destructive Testing

Forces on Materials Tensile Force Compressive Force Shear Force

Bending Tension Compression

Effects of Loading Provided that load is not excessive, material itself will return to its original size when load is removed (material behaving in “elastic” manner At increased stresses deformation becomes permanent - material is said to be “plastic”

Main Modes of Failure Ductile failure Brittle failure Metal fatigue Buckling Corrosion Wear Creep

Ductile Failure Material moves into plastic region and loses its original shape There is a reduction in cross-sectional area which increases the stress (stress is proportional to area) Failure occurs at the place of reduced cross-sectional area Temperature is a major factor The higher the temperature, the more ductile materials become i.e. they can elongate more before failure

Brittle Failure Occur very suddenly and without warning Occur because the structure of a material does not slip, either owing to the material structure or there is insufficient time due to the intensity of the load Small cracks spread through the material so quickly that massive failure is produced Speed of failure often results in some energy in material being released as sound, giving brittle failure a characteristic “crack”

Brittle Failure Note that in other failure modes, the actual failure may be by brittle mode, but this will only be part of the sequence of failure Some factors that promote brittle failure: Low temperature Impact or Snatch Loading e.g. lifting equipment Residual tensile stress e.g. pre-tensioned beams Inherently brittle material e.g. glass and ceramics In appearance, a brittle failure shows no sign of any deformation - the parts could be fitted back together. The surface will be bright and there may be chevron markings across the failed surface”

Metal Fatigue Most common type of failure where conditions producing mechanical vibration occur Fluctuating stress conditions produced by vibration (e.g. aircraft wings going up and down during flight) can cause formation of a crack that propagates through the material Crack reduces the area of material resisting the stresses until the remaining material can no longer resist the stresses and fails Failure is generally a brittle failure as it is the less ductile materials that are used to resist cyclic loading

Metal Fatigue Initial crack always starts from the surface and penetrates into body of material Surface blemishes e.g. machining marks and foreign body inclusions are likely candidates to set fatigue failure into action, as are holes for bolts, rivets, inspection hatches etc. The final area of metal in place when failure occurs is clearly identified by its colour - the surface of the crack will be dull due to the effects of air and moisture being in contact and setting up mild corrosion, while the newly failed surface will show signs of new, clean brittle failure Metal fatigue cracks can be readily detected by NDT Surface defects should be removed where possible by polishing

Buckling When a compressive force is put into a rod, beam or bar the force is resisted by the material As force increases, the material distorts, preventing a straight transfer of stress through the material Failure is due to tensile forces on the extreme causing ductile failure which propagates through the material and leads to catastrophic collapse Can be better resisted by a ductile material than a brittle material due to ability to resist tensile forces introduced by the “bowing” on the outside surface Can be limited by installing intermediate supports to limit movement from the straight

Corrosion Affects only metals Requires the presence of an electrolyte (normally water), potential differences between metals to allow flow of current Chemical change in which body of metal loses atoms - when repeated millions of times, loss of material can be observed as corrosion

Corrosion Rate of corrosion depends on: Can be observed by: Stress in material Strength of electrolyte (pH value) Environment and exposure (heat accelerates corrosion) Reactivity of metal Metal impurities Can be observed by: Localised pitting in surface of material Overall thinning of material Can be detected by discoloration or NDT methods

Corrosion Corrosion removes material, so there is less component to carry the stress it was designed to take Protection: Control environment Selection of corrosion resistant material Protect metal with coating Sacrificial anode of magnesium, aluminium or zinc which is dissolved instead of the component

Wear Can be produced by: Scuffing Abrasion Pitting Lack of lubrication between moving parts. Movement of parts causes friction Abrasion Occurs when small components of foreign material which is harder than component get between moving parts e.g. sand particles - surface then becomes scored Pitting Combination of above Particles produced during scuffing become detached and hardened, becoming the equivalent of foreign particles and cause abrasion

Creep When a material is under stress near to its elastic limit, it undergoes a process of plastic deformation known as creep Extent to which creep acts is dependant on: Time (creep is a slow process) Temperature (creep accelerated in high temperatures) Not a true mode of failure as failure is often either brittle or ductile

Preventing Failure in Design Failure Mode and Effect Analysis (FMEA) Incorporation of Safety Factors Correct selection of materials Removal of points of weakness Remove sharp edges Increase amount of material where slots or holes are needed

Testing and Quality Assurance Checks at each stage of a process Management process (usually based on ISO9000) to ensure the checks are carried out and recorded Records are important in the event of failure

Preventing Failure in Use Use within manufacturers stated parameters Specific maintenance e.g. lubrication Non-destructive testing

Failure Investigation Techniques Collection of samples Fault Tree Analysis Accident Investigation: Gather samples of failed material Look closely at failed surfaces Record information Lab analysis may be necessary

NDT Test integrity of material without destroying components Types of NDT: Visual inspection Penetrant inspection Radiography Ultrasonic testing Thermal imaging

Visual Inspection Use naked eye, microscope or magnifying glass Requires good light source Surface may require cleaning Cheap and easy but applies to surface defects only

Penetrant Inspection Uses dye penetrant to highlight defects for visual inspection Only relates to surface defects Easy to use off-site as dyes are in aerosol cans

Radiography Gamma or X-rays are passed through material and onto a strip of film Radiation triggers reaction in film which, when developed, shows where material is sound and where defects exist Gives permanent record Radiation hazard present

Ultrasonic Testing Uses generator transmitting ultrasound waves into material and detecting them when reflected Equipment usually hand-held Results need to be interpreted by skilled operatives Can detect defects within the material Can be used on metallic and non-metallic objects

Thermal Imaging Cameras that can detect heat and show small variations onto colour screen Main use it to determine if a part is running hot I.e. lacking lubrication or rubbing where it isn’t supposed to Cameras are small Only provides a snapshot Comparison between pictures is difficult by eye, but computer can be used

Previous Exam Questions Explain the purpose of non-destructive testing (NDT) on items of plant (2 marks) Other than visual inspection, outline the principles, benefits and limitations of two NDT techniques (8 marks) A serious accident occurred when a threaded steel rod in the braking mechanism of a hoist failed. An investigation revealed a fatigue failure of the rod. Describe the mechanisms and characteristics of fatigue failure (6) Outline the factors that may contribute to such a failure (6) Describe the measures that could have been taken which might have prevented the failure (8 marks)