ABSTRACT
If we list the ways that stress can affect a material, we find that it can cause strain, which can be elastic deformation for low stress levels, and plastic deformation at higher stress levels. Failure can be caused by a single application of stress, when it exceeds the tensile strength, whereby the material is effectively stretched until it fails. Failure can be caused by repeated application of stress (cyclic stress) at levels below the tensile strength through a process known as
fatigue
. Fatigue itself can be subdivided into two categories, known as low-cycle fatigue (for stresses that exceed the yield strength) and high-cycle fatigue (for stresses that do not exceed the yield strength). Failure can also occur as a result of the application of stress at elevated temperature. This latter process is known as
creep
and amounts to a plastic flow of the material, which can often be slow at first (primary creep), then almost reduces to zero rate of strain over the “plateau region” (secondary creep), but finally increases in rate again as the material approaches failure (tertiary creep).
