Fatigue Threshold Re-Visited
Fatigue is one of the most common contributors to engineering and plant failures, particularly in engineering systems that experience some form of repetitive or cyclic loading. This fatigue phenomenon has been known for over a hundred years, and now, with the discipline of Fracture Mechanics, it is generally fairly well understood. However fatigue still accounts for over 80% of unexpected engineering failures, and despite efforts to understand fatigue and mitigate against it, its occurrence is frequent and ubiquitous. Perhaps one of the reasons for this is the general misunderstanding of fatigue threshold and its effect on fatigue life. To an extent, this was discussed in our August 2017 Tech Tip (available on our website), but this present article extends this and addresses a different issue referring to threshold dependence itself.
Complementary shear stress failures
So often in engineering we express concerns about whether a component is ‘strong enough’ to do the task for which it has been designed. But ‘strength’ is not always the critical parameter that needs to be considered, as our recent Tech Tips have highlighted failures are exacerbated by, for example, corrosion, cyclic loading (fatigue), temperature and/or material microstructure. There is also the question of whether ‘strength’ is the appropriate parameter to consider when toughness or resistance to fatigue crack initiation may be more appropriate, or when loading direction should specifically be taken into account. Frequently our perception of strength in materials refers to tensile strength, and indeed this is vital, but may not always be the limiting factor in a failure and in some cases both compressive strength and resistance to shear should be assessed. Although often discounted, components can and indeed do fail in shear.
Hole Drilling Techniques for Measuring Residual Stress
As referred to in a recent Technical Tip on ‘perceptions’ and easy observation of features which assist in Failure Analysis, one of the phenomena encountered is that related to Residual Stress. For current purposes, residual stress may be regarded as that stress that remains in a component or structure when it is in the unloaded condition. It may also be regarded as that stress that is intrinsic in the component, due to its fabrication or loading history, and is essentially in self-equilibrium. Familiar examples in everyday life include distortion of components after welding, toughened glass (as used in vehicle windscreens and windows), pre-stressed concrete/reinforcing bars in civil structures, and carburised and nitrided gear surfaces, which have an induced compressive stress surface layer to resist fatigue crack initiation. Although residual stress induced by carburising and shot peening are typically beneficial, generally residual stresses arising from fabrication are deleterious to component performance, mostly through premature fatigue initiation or stress corrosion cracking (SCC).
Perception and Simple Indicators
Quite frequently our Technical Tips detail complex techniques for determining the origins of failure, as such techniques are often essential to obtain the correct information and interpretation. However, there is also a place for greater perception and the use of simple indicators at the early stages of a failure analysis, to glean as much as possible from seemingly straight forward observations, before moving on to more complex analysis, and some of these are addressed here. These include simple methods of composition discrimination such as magnetic response of a part in stainless steels (to distinguish between austenitic and martensitic microstructures). Similarly in a steel component that has come into contact with/abraded against another component or foreign body, there will often be not only abrasion damage, but also local heating of the surface. The severity of this wear induced abrasion can be inferred from the temper colours that characterise steels heated to different temperatures. Such colours are clear demarcations of temperature exposure and are indicative of temperature gradients, the degree of damage and severity of the loading and give clues to potential material changes below the surface.
Basics of Heat Treatment
The purpose of heat treatment is to cause desired changes in the structure of a metallic material, thereby affecting its properties. Heat treatment can be employed to change the properties of most metals and alloys, with ferrous alloys typically undergoing the most dramatic changes.
An unusual example of Cavitation Damage
An interesting and frequently encountered aspect of damage in pumps and impellers is that of damage caused by cavitation. Cavitation refers to damage caused in a fluid due to flow and pressure changes that result in substantial local pressure changes and the formation, and subsequent collapse, of ‘bubbles’ or voids.
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.