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Chapter 6:
Quantitative Evaluation of Residual Stresses
The stress field that exists in the bulk of some materials without the application of external loads or other stress sources is known as “residual stress.” Many service failures of structural or mechanical components are caused by the combination of residual stress fields in the material and mechanical stresses produced by applied loads. One of the most famous examples of the effect of residual stresses is the Liberty Bell in Philadelphia, Pennsylvania. The bell cracked some years after its manufacturing, probably due to residual stresses generated by the casting process. Compressive stresses are sometimes introduced deliberately, e.g., the shot peening used to improve fatigue resistance. Furthermore, in natural or artificial multiphase materials, residual stresses can arise from differences in thermal expansion, yield stress, or stiffness. In the case of the Liberty Bell, manufacturers can avoid this kind of failure by understanding the casting process and its influence on the introduction of residual stresses in cast parts. Moreover, accurate knowledge of the levels of introduced compressive stresses is important to ensure the stress resistance of the material or the part. Consequently, accurate residual stress measurement becomes a valuable task not only when the structure integrity must be evaluated but also for the development of new materials or fabrication procedures or for the design of mechanical parts. Although recent advances in finite-element-based analyses have improved predictions about residual stress distributions, it is essential to accurately assess the history of the structure of the raw material from which the mechanical part is created, which can be done in a few experimental cases. For this reason, current experimental methods cannot be fully replaced to determine the magnitude and principal direction of residual stresses, not only in raw materials but also in components under operating conditions.
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