The Measurement of Residual Stresses Using Neutron Diffraction

Abstract

Residual stresses are common in engineering materials. They are elastic stresses that exist in the absence of external forces and are produced through the differential action of plastic flow, thermal contraction, and/or changes in volume created by phase transformations. Differential plastic flow can occur during the forming of a part. For example, the grinding of a surface will plastically extend a thin surface layer relative to the underlying material through frictional forces. This tends to throw the near-surface region into compression, which is balanced by a tensile stress through the bulk of the part. In such a situation, the requirements of force equilibrium lead to a rather high level of compressive stress in the near-surface region and a low level of tensile stress through the bulk. Differential thermal contraction often occurs during the nonuniform cooling of a large part, in the vicinity of welds, and between the matrix and reinforcement phases of a composite. Differential volume changes occur during the precipitation of second phases, i.e., the atomic volume of the precipitating phase generally differs from that of the host matrix. If it is larger, the second phase is placed in compression, with the matrix in tension, and vice versa. The residual stress states discussed in the previous paragraph are of two basic types: macrostress and microstress. Residual macrostresses are long-range relative to the scale of the microstructure––that is, they extend continuously across a part. Residual stresses arising from forming and joining are of this type. They generally vary with position and are extensive in nature. A plate with compressive residual stresses on the flat surfaces will deflect if the compressive region on one surface is removed. Destructive stress measurement techniques such as hole drilling or strain gauging and sectioning can be used to determine residual macrostresses.