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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 3 1 Residual stresses are not all the same. They are often thought of as ei- ther surface related (special processing of surfaces to create beneficial com- pressive stresses) or bulk (often coming from thermal processing). In addition to these major categories of residual stress, there are also various classes, in- cluding Type-1, Type-2, and Type-3 re- sidual stresses (Fig. 3). TYPES OF RESIDUAL STRESS Type-1 residual stress is the most common. It is macroscopic in nature and represents average stress under lo- cal conditions. Note that residual stress- es are volumetric stresses and must be described by a complete set of stress tensors. Although residual stresses are often described as a single value, this is not entirely correct and assumptions about the other orientations of stress must be made. If the entire stress state is not known, it is often best to conduct measurements and/or simulations to determine the magnitude and orienta- tions of the residual stress state instead of simply assuming a single unidirec- tional stress state. (a) boundaries. Figure 4 shows an example of a warm-worked nickel-base superal- loy microstructure exhibiting substan- tial sub-grain distortion and rotation. This has the potential of containing Type-3 residual stresses. To take full advantage of their ca- pabilities andminimize any detrimental effects to product performance, residu- al stresses require continuous atten- tion and research. Key elements of this endeavor include measurement and simulation. These techniques go hand- in-hand to define, optimize, control, and understand residual stresses. MEASUREMENT TECHNIQUES Several methods are available for measuring residual stress. Figure 5 shows potential measurement tech- niques, applicable length scales, and potential sources of residual stress. Note that not all methods are equally capable of measuring the specific re- sidual stresses for any scenario. Each method has its strengths and weak- nesses. It is often beneficial to use more than one technique to gain a more com- plete picture of the actual stress state. Modeling and simulation of resid- ual stress has made significant progress and is now able to make predictions at Type-2 residual stresses are more complicated in that they describe grain-level stresses. It is well known that metallic materials have grain-lev- el property differences based on grain orientation. Systematic changes in modulus and yield strength occur as a function of crystal orientation relative to applied stresses. As a polycrystalline material is stressed, some grains will yield, while others are only elastically loaded. This is similar to the local strain- ing of material discussed earlier, but on the grain level. The schematic in Fig. 3 shows a cluster of grains where the ori- entation of adjacent grains are differ- ent. After a specific loading cycle, some of the grains plastically yield and oth- ers do not, resulting in a grain-level re- sidual stress profile that is not smooth, but discretized in regions based on the stresses within each grain. Similar to Type-2, Type-3 residu- al stresses are caused by variations in strain within a grain as a result of either localized phase transformation, poly- gonization due to some level of anneal- ing or nonuniform strain within grains, or strain localization near or at grain (b) (c) (d) Fig. 3 — Volume of a polycrystalline alloy (a), where the sectioning plan provides reference for a surface residual stress profile produced by shot peening (b), which is an example of Type-1 residual stresses that occur over a large length scale. A sample of internal grains (c) exhibits different crystallographic orientations, resulting in varying levels of residual stress due to the anisotropic behavior of each specific grain, which is an example of Type-2 residual stresses occurring over grain size length scales. Fig. 4 — Photomicrograph of nickel-base superalloy Waspaloy. This orientation imaging micrograph (electron backscatter diffraction pattern image) shows clear straining and polygonization within the deformed grains. Dark spots near grain and prior twin boundaries are a result of high localized strain and crystal orientation unable to be determined.