Consider the impact of structural relaxation
Many engineering structures must be designed to operate successfully in hostile thermal environments. Pipe hangers supporting hot and heavy steam lines, aircraft engine blades, and injection molding machine components are just a few examples. When designing such mechanical systems, it is wise to consider the impact that structural relaxation due to creep might have on long-term performance.
Creep is a thermally induced phenomenon that typically occurs in crystalline structures, like metals. Its effects, as observed on a macroscopic level, are caused by the diffusional flow of vacancies and dislocations on a microscopic level within the crystalline structure. These vacancies are point defects, and they tend to favor grain boundaries that are normal, rather than parallel, to the applied stress. Their movement tends to be from regions of high concentration to regions of low concentration. Dislocations in grains are line defects. Their movement tends to be activated by high stresses, although this may also occur at intermediate temperatures. Grain boundary sliding is sometimes considered as a separate mechanism that also contributes to creep deformation. Continue reading