STRUCTURAL ALLOYS are commonly subjected to a variety of thermal and thermomechanical loads. If the stresses in a component develop under thermal cycling without external loading, the term thermal fatigue (TF) or thermal stress fatigue is used. This process can be caused by steep temperature gradients in a component or across a section and can occur in a perfectly homogeneous isotropic material. For example, when the surface is heated it is constrained by the cooler material beneath the surface, and thus the surface undergoes compressive stresses. Upon cooling, the deformation is in the reverse direction, and tensile stresses could develop. Under heat/cool cycles, the surface will undergo TF damage. Examples of TF are encountered in railroad wheels subjected to brake-shoe action, which generates temperature gradients and, consequently, internal stresses (Ref, ). On the other hand, TF can develop even under conditions of uniform specimen temperature, instead caused by internal constraints such as different grain orientations at the microlevel or anisotropy of the thermal expansion coefficient of certain crystals (noncubic). Internal strains and stresses can be of sufficiently high magnitude to cause growth, distortion, and surface irregularities in the material (Ref). Consequently, thermal cycling results in damage and deterioration of the microstructure. This behavior has been observed in pure metals such as uranium, tin, and cadmium-base alloys and in duplex steels with ferritic/martensitic microstructures. The term thermomechanical fatigue (TMF) describes fatigue under simultaneous changes in temperature and mechanical strain (Ref, ). Mechanical strain is defined by subtracting the thermal strain from net strain, which should be uniform in a specimen. The mechanical strain arises from external constraints or externally applied loading. For example, if a specimen is held between two rigid walls and subjected to thermal cycling (and is not permitted to expand), it undergoes “external” compressive mechanical strain. Examples of TMF can be found in pressure vessels and piping; in the electric power industry, where structures experience pressure loadings and thermal transients with temperature gradients in the thickness direction; and in the aeronautical industry, where turbine blades and turbine disks undergo temperature gradients superimposed on stresses due to rotation. In the railroad application discussed earlier, when external loading due to rail/wheel contact is considered, then the material undergoes the more general case of TMF. The temperature rise on the surfaces of cylinders and pistons in automotive engines combined with applied cylinder pressures also represents TMF. Based on the mechanical strain range, the results of TF and TMF tests should correlate well. A distinction must be drawn between isothermal high-temperature fatigue as cyclic straining under constant nominal temperature conditions versus TMF. As such, isothermal fatigue (IF) can be considered a special case of TMF. In most the deformation and fatigue damage under TMF cannot be predicted based on IF information. Therefore, TMF experiments have been considered in studies of both stress-strain representation and damage evolution.
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Originally Posted by Michael2
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