Orientation effects in creep
Creep in materials refers to the gradual strain extension of that material under sustained load over extended time periods. In metals, creep is of particular concern because it typically only manifests over long time periods, for example ten to twenty years, and at temperatures which are in excess of approximately 40% of the melting point. However at higher temperatures or high stress levels the period in which damage manifests can reduce significantly.
Orientation effects in metals can result in unexpected consequences.
Although it is normally assumed that engineering materials are isotropic this assumption is often not valid.
Remember corrosion exacerbates the potential for fatigue failure
Fatigue of engineering materials and their degradation through micro-cracking, as a result of cyclic loading, is extremely common in materials, especially metals and alloys, accounting for up to eighty percent of all structural failures. Real world engineering structures are subjected to a range of environmental conditions, which can and usually do exacerbate the fatigue circumstances and accelerate the fatigue crack advancement process.
The pros and cons of galvanic coupling
Cathodic protection is a method of reducing the rate of corrosion damage to a metal surface by supplying it with electrons from an external source, effectively forcing it to become the cathodic (passive) element of a galvanic cell.
Don't discount small critters and biological growths.
Microbial Induced Corrosion can cause rapid localised attack and degradation of many metals including stainless steels.
Beware of highly stressed components working in a corrosive environment
The combined influences of tensile stress and a corrosive environment can lead to catastrophic failure of susceptible materials by stress corrosion cracking mechanisms (SCC). Often these failures occur after relatively short periods in operation without warning, but with proper understanding and care, SCC can be avoided.
Don’t preclude DIC
Digital Image Correlation (DIC) is a non-contact optical technique for the analysis of surface displacement fields of a specimen during deformation/loading. This information can be used for subsequent analysis of the surface strain and has the potential to allow for continual in-service monitoring of components.
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.