A hard nut to crack
One of the most ubiquitous measurements in mechanical engineering is that of hardness, nominally a convenient method of estimating the strength of a metal component. Hardness measurements are quick and easy to undertake and indeed do give a measure of the strength of the metal, but in the light of the previous ‘Tech Tip' on Work Hardening it is worth putting this in context. In addition, this Tech Tip also explores the terms ‘work hardening’ and ‘hardness’, in an effort to clarify the matter.
The tensile test of material strength is probably one of the most common means of characterising a material’s strength, although similar tests in compression are also ubiquitous. The stress strain curve of a typical tensile stress strain test for a ductile metal is well known, initially exhibiting a conventional linear elastic section, then as strain is increased, there is increased non linearity and a rising curve, leading ultimately to a maximum, consistent with necking in the material’ and then a fairly rapid drop off in stress followed by rupture or fracture. It is useful, especially for engineering design purposes, to record the limit of non-linearity as the ‘yield point’ (or if there is no distinct transition) the ‘proof stress’ (at a specific strain, typically 0.2%). Beyond this yield point, as the strain increases the stress also increases, but generally more slowly, and this behaviour is known as work hardening.
Tensile versus Bending Strength
In engineering stress analysis, the question is often asked ‘What is the component’s strength?’, and in particular, ’What is the value of the material strength’? For the case of steels and most metals, the stress-strain behaviour is well known and the concepts of ‘Yield’ and ‘Ultimate Tensile Strength’ (UTS) are well understood and widely utilised. However, a topic still attracting attention is that of the value of ‘strength’ and its dependence on the method of measurement: in particular, whether the value is determined from direct tensile testing or from bending tests, often referred to as ‘modulus of rupture’ (or MOR) tests. This is the kernel of this month’s Technical Tip.
Additive manufacturing - a design-driven manufacturing process
The last decade has seen drastic improvements into the capabilities of the freeform fabrication machines used for additive manufacturing (AM) with particular emphasis in the mechanical properties and density of AM produced metallic components, enabling AM to become an established manufacturing methodology.
The successful application of materials in engineering structures and mechanical systems depends on good design and the efficient use of materials properties in relation to their strength and yield properties. The common approach is to use results from a standard tensile test, and design to values of measured yield stress and yield strain, but this effectively assumes that there is a single principal stress and does not accurately account for complex stress fields or shear stresses.
Fracture dependence on modes of loading - Modes I, II and III
In some engineering failures, the detail of collapse frequently is related to cracking, through overload, stress corrosion cracking (SCC) or fatigue, for example. The mode of fracture can reveal a great deal about the stress state at the time of loading and fracture, and there are characteristic features that can be recognised which assist in the determination of how the cracking developed and failure took place. This Technical Tip discusses these and their characteristics, and how useful this can be to the failure analysis investigator.
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.