Value your tyres
Accurate modelling of material properties is vital in numerical analyses where changes in material properties can have a significant effect on the results of numerical analyses such as those employed in the modelling of automotive tyres.
Temperature effects on mechanical property performance.
In today’s ever more stringent economic climate, reliable material performance is becoming even more important, as premature failures are not readily tolerated, especially when they can be anticipated and avoided. Sometimes such failures are related to the temperature behaviour of materials, and this month’s Technical Tip refers to two examples that illustrate such temperature related failures.
Overlook stress concentrations at your peril
The effect of stress concentrating features such as notches, fillets, grooves and threads, has been known for many years and has been documented in many texts. However they are often overlooked and cause failure by locally increasing stresses and aiding fatigue crack imitation.
Preserve your fracture surfaces or lose information
When a component fractures, the atomically clean fracture surfaces contain many features indicative of how and why the fracture occurred. Preservation of these fracture surfaces is one of the most important aspects of any failure investigation and precautions should be taken to provide the failure analyst with a sample in the best possible condition for accurate assessment of the failure scenario.
Diffraction based residual stress analysis
Diffraction techniques, using both X-rays and neutrons, enables residual stress profiles of varying depths to be developed for the component in question. Although the cost of these processes and difficulties associated with their application generally limit these techniques to scientific applications, they are viable in certain industrial applications.
Micro-CT scanning is a novel alternative to conventional non-destructive testing
The micro computational tomography (CT) scanning of components is becoming an increasingly attractive option for the non-destructive testing of components both prior to and during service.
Laser shock peening is a novel technique for introducing beneficial residual stresses into components
The controlled introduction of compressive residual stresses into the surface layers of components can effectively delay the onset of crack initiation and slow down crack growth rates. Laser shock peening is one such technique of introducing these residual stresses.
Beware of the insidious effects of creep
The combined effects of high stress and temperature and resulting failure by creep mechanisms is well known - but is often overlooked in relatively low temperature environments.
Understand risk and reap the benefits of improved safety and reduced operating cost
The use of risk assessments has become commonplace within industry and has proved successful in reducing injury within industry. The same process can be used to reduce business risk and to focus resources where they can be most effective from a business perspective.
Be aware of the potential for metal dusting in high temperature environments or risk severe material degradation!
Although limited to specific environments and temperature ranges, 'metal dusting' can lead to rapid degradation/thinning of material operating in high temperature environments.
Select equipment conservatively and prevent unexpected failure
In our line of business, we are exposed to a wide range of very interesting failures. We also encounter certain types of failures repeatedly, as end users are unaware of the effect of certain load conditions on the performance of their component or equipment. These types of failures are particularly concerning since the fundamental understanding is well known, and is well taught at universities and colleges, but design mistakes and/or misinformed plant selection still occurs. A typical example of this concerns the understanding - or lack of it - of repeated loading and fatigue conditions. It is well known that fatigue failure accounts for between 75 to 80% of all failures, and technically the effect of cyclic stress level, stress concentrations (notches, holes, cracks etc.) is well understood, yet failures still occur surprisingly often.
Environmentally Induced Fatigue Failure
Although the effect of flaws and stresses is often considered by designers the effect of environment is sometimes overlooked. In adverse environments such as those experienced in the chemical, marine and biomedical fields the operating environment may cause failure from flaws which would otherwise not propagate.