yield

Evidence of plastic deformation in structural materials.  Also known as plastic flow or creep.

yield point

The first stress in a material, usually less than the maximum attainable stress, at which an increase in strain occurs without an increase in stress.  Only certain metals-those which exhibit a localized, heterogeneous type of transition from elastic to plastic deformation-produce a yield point.  If there is a decrease in stress after yielding, a distinction can be made between upper and lower yield points.  The load at which a sudden drop in the flow curve occurs is called the upper yield point.  The constant load shown on the flow curve is the lower yield point.

yield point elongation

The extension associated with discontinuous yield which occurs at approximately constant load following the onset of plastic flow.  It is associated with the propagation of Luder lines or bands” (Automotive Steel Partnership, 1991, p. 18).

yield strength

The stress at which a material exhibits a specified deviation from proportionality of stress and strain.  An offset of 0.2% is used for many metals.  Compare with tensile strength. yield stress.  The stress level of highly ductile materials, such as structural steels, at which large strains take place without further increase in stress. (See Figure Y1)

yield stress

“A stress at which a steel exhibits the first measurable permanent plastic deformation” (Automotive Steel Partnership, 1991, p. 19). The level of stress when plastic flow begins during a uniaxial tensile test. (See Figure Y2)

young’s modulus or elastic modulus

The stress at which a material initially exhibits permanent plastic deformation in a tensile test. Figure Y3 labels yield strength on a stress/strain curve of a tensile test.