Disregard the insidious effects of pitting corrosion at your peril
Pitting corrosion is an extremely localised form of material attack, during which a material surface is perforated by a series of holes (or pits) in the presence of an aqueous solution. Corrosion of this nature is highly destructive, as these pits can rapidly perforate material causing severe damage, while the surrounding material regions remain unaffected. Pitting generally takes months or years to initiate, but once initiated the pits penetrate the material at exponential rates
Ambient temperature creep
This Tech Tip concerns failures that can arise from creep, but at ‘normal’ (ambient) temperature conditions. The well-known condition for any manifestation of creep deformation leading to premature cracking, or ultimate failure, is that temperatures need to exceed about 40% of the material yield strength, for a sustained length of time. These conditions are seldom encountered in normal applications. However, when the material is polymeric, there is a very definite and common case of seemingly low temperature creep.
Beware of the insidious effects of localised corrosion damage in crevices and below gaskets
Crevice corrosion can have highly detrimental consequences due to the localised nature of attack that often goes undetected in-service until final failure, associated with leakage or localised stress concentrations, occurs.
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.