Introduction to residual stress
Residual stresses are those stresses which are 'locked' into components and structures without the application of any other external load. These stresses are usually caused by manufacturing processes. Most manufacturing processes impart residual stresses into components. The extent of residual stress application depends on the severity of the process. Effects can vary from 'near-surface' (caused by machining, grinding, etc) to the setting up of fundamental stress systems throughout the component or structure (caused by casting, forging, welding, heat-treatment, etc).
Residual stresses can have negative effects on the performance of structures. For example, tensile stresses within un-heat-treated welds can lead to premature cracking when combined with applied alternating service loads. An example of the positive effects of residual stresses is the application of shot-peening to produce compressive near-surface stresses to increase the component fatigue limit. Accordingly, in order to assess the likely performance of a component, a knowledge of the residual stresses levels within that component is of fundamental importance.
Measurement of residual stresses cannot be carried out using conventional stress analysis procedures since the sensor measures only change in stresses (caused by applied loads). Such sensors are insensitive to the residual stresses accumulated during the history of the component.
Several methods exist for the calculation of residual stresses from strain changes brought about by removing material from the component. In its simplest form, a strain gauge is mounted on the component surface; the component is then sectioned to isolate the strain gauge while the gauge output is monitored – a destructive procedure.
Drilling of a small, shallow hole next to a strain gauge can, in many cases, be considered a ‘semi-destructive process’. The measured relaxed strains are decoded to evaluate the distribution of stresses away from the surface. This is the basis of the target strain gauge / centre hole drilling method used by Stresscraft Ltd.
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