September_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | S E P T E M B E R 2 0 2 0 2 9 *Member of ASM International RESIDUAL STRESS: BOTH FRIEND AND FOE Residual stress exists in all classes of materials and significantly impacts many facets of product design and manufacturing. With the launch of a technical committee on this topic, ASM International is creating a specialized network of volunteers to advance the science of this important phenomenon. David Furrer,* FASM, Pratt & Whitney, East Hartford, Connecticut R esidual stress is a major element of component and system design, manufacture, andapplication, but it represents one of the most compli- cated and lesser known phenomena that can greatly impact product perfor- mance. Residual stresses are relative- ly straightforward to understand from a high level, but as one delves deeper into the science and engineering the topic becomes more challenging. Residual stresses are effectively locked-in stresses within a material that are a result of various prior path-depen- dent processing. As a whole, integration of all volumetric residual stress must equal zero to be considered in equi- librium. If these locked-in stresses are disturbed, such as by machining some volume from the material, an imbal- ance in residual stresses will result and the stresses will re-equilibrate causing the material to geometrically distort. Similarly, material that has locked- in stresses in addition to external loads will result in local stresses that com- prise a combination of internal resid- ual stresses and externally applied stresses. The primary reason for apply- ing various surface treatments to com- pressive residual stresses is to mitigate high externally applied stresses. Com- pressive surface residual stresses must be accompanied by an equal integra- ted amount of tensile stresses to reach equilibrium. STRESS FORMATION Residual stresses are formed in three major ways: thermally induced lo- cal plastic strains within a component, externally applied stresses that provide local plastic strains, and phase trans- formation-induced local plastic strains due to volume fluctuations during phase changes. The first two are closely related, with the only difference being the means by which local plastic strains are induced. Figure 1 shows how resid- ual stresses are formed by thermally in- duced local plastic strains and by phase transformation-induced strains. For the thermally induced pro- cesses, a volume of material is heat- ed to high temperature and allowed to AS A WHOLE, INTEGRATION OF ALL VOLUMETRIC RESIDUAL STRESS MUST EQUAL ZERO TO BE CONSIDERED IN EQUILIBRIUM. Fig. 1 — Mechanisms of generation of bulk residual stresses. Top row depicts steps in the process of thermally induced residual stress formation; bottom row depicts steps in the process for phase transformation-induced residual stress formation [1] .