YIELD STRESS

An ideal plastic body does not yield until some critical stress (scrit) is reached.Most materials that approach being plastic exhibit elastic properties below this point so this critical stress is at the elastic limit or yield point of the material. Mechanical analog is a sliding mass (where scrit is analogous to frictional resistance). Beyond the yield point a plastic material strains continuously and permanently. Yield strength is the value of stress (called the yield stress) at which a material begins to undergo permanent deformation beyond the elastic limit(the stress beyond which a material begins to undergo permanent ductile deformation). Rupture occurs at the stress (failure stress) at which a material fails under a given set of conditions. Ultimate Strength is the greatest stress a material can support without failure (breaking) under any given condition. Fundamental strength is the greatest stress a material can sustain without continuous deformation (creep) over an essentially unlimited time.