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.
Residual stress often contributes to premature failure and there are a number of ways of measuring these stresses. Such techniques are generally complex and delicate (such as X ray diffraction, neutron diffraction, or hole drilling techniques) and should not be undertaken lightly as they are complex, expensive, and require a high skill level. The justification to pursue the quantitative residual stress measurement route can be assisted by other tell-tale signs such as premature corrosion. It is well known that plastic deformation, without subsequent stress relief heat treatment, can lead to built-in residual stresses in components, which make the high stress areas more susceptible to corrosion. Typical examples include the preferential corrosion (or rusting) at U-bends in reinforcing steel and the head/point of steel wire nails, indicative of the higher residual stress in these deformed areas, and the consequent initiation of rusting.
Whether one undertakes complex (and expensive) quantitative assessment measurements of residual stress (to establish whether this is relevant to the cause of failure) can be pre-empted relatively simply by intelligent preliminary sectioning (which is a substantially cheaper option). There are many examples where the effects of residual stress, not previously suspected of contributing to the failure stress were manifested by simple oblique sectioning of the (rectangular) component. With ‘in built’ residual stresses with magnitude in excess of 50 to 100MPa, the tapered cut section readily deforms as the local stress redistributes, which can lead to the thin (tapered) section peeling away, (under net local tensile stress field), leading to a wider saw cut width, on the one hand. Alternatively, a closing up of the saw cut, (under net local compressive residual stress fields) leads to a narrower saw cut width, and jamming of the blade.
Inbuilt residual stresses in tube/pipe often manifest as the tube is worn unsymmetrically around its circumference, leading to locally different tube wall thicknesses, and ultimately bending of the pipe as the stresses and wall thicknesses become non-symmetric. Rather than complex and expensive strain gauging and hole drilling, accurate measurement of residual stress in individual pipes can be obtained in short sections of pipe by measuring the diameter precisely, before and after introducing a longitudinal section cut through the annulus. Reliable measures of the average residual stress are obtainable for the change in diameter of the pipe section, and correlated well with complex strain gauging hole drilling methods, at a fraction of the cost.
Although these techniques can be very useful they need to be used judiciously, the limitations of each technique need to be understood before they are applied. For example although austenitic stainless steels (such as 304 and 316) are non-magnetic, ferritic, martensitic and duplex stainless steels are magnetic. This could be useful as it would allow austenitic stainless steels to be identified but it would also allow martensitic stainless steels to be classified as normal steel (a trick that could be used by an unscrupulous scrap merchant). Relying on ring splitting techniques for residual stress measurement could result in large magnitude surface residual stresses being missed if these stresses were distributed evenly on the inner and outer surfaces of a tube.
So the message then is to be to be aware of alternative methods of analysing non-standard performance of components and apply these judiciously, before perhaps rushing to the electron microscope or applying equally expensive techniques.
Published in Technical Tips by Origen Engineering Solutions on 1 April 2018