ADVANCED MATERIALS & PROCESSES | NOVEMBER/DECEMBER 2024 13 Residual stress is generally defined as the stress that is retained in equilibrium in a body or material after manufacturing or processing operations or component usage in the absence of any external forces. Residual stresses can be purposefully engineered into components with the goal of extending component perfor- mance beyond the material and structural capability, or they can be unintentionally introduced by the manufacturing or assembly process. In each case, the measurement of residual stress is critical input for process optimization, life predictions, and quality control. An overview of most known origins of residual stresses is listed in Fig. 1[1,2]. Looking at the length-scale in which residual stresses reach equilibrium, the residual stresses in crystalline materials can generally be categorized as: a) type I, also called macro-stresses, where the equilibrium is reached over macroscopic lengths; b) type II or intergranular stresses where the equilibrium is reached over the length of few grains; and c) type III, also called micro-stresses or intragranular stresses, where the equilibrium is reached within a grain over the distance of few atomical planes. Type III stresses are generated by local atomic defects such as vacancies, interstitials, or dislocations. This classification of residual stress types is schematically illustrated in Fig. 2. These definitions are also important to keep in mind when planning measurements, as different techniques interrogate different volumes and length-scales. Therefore, the results are relevant within the context of the interrogated volume[3-5]. When planning residual stress measurements, it is important to keep the application in mind: How will the residual stress data be used? Why do we want to understand the residual stresses that may be present? These questions are fundamental when evaluating applicability of the measurement tools, and when optimizing Fig. 1 — Origin of residual stresses can be generally created by three major processes: manufacturing or assembly processes, microstructure driven, or in-operation process induced. Fig. 2 — Illustration of residual stress/strain types in a polycrystalline material.
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