Materials and Structure

Although engineering materials are normally used in their elastic range it is necessary to expose them to higher stresses if they are to be shaped into components. It is also necessary to know the limit of elastic behavior when designing a component that will be exposed to high stress during use. The stress-strain diagram opposite shows the response of a material to a uniaxial tensile stress over the full range to material fracture. This test was conducted by controlling the strain-rate and measuring the stress needed to get to a given strain. Both stress and strain are measured in engineering units which use the initial dimensions of the sample to normalize the applied forces and the extensions produced. Critical points on the curve are indicated with red circles.

From: Ashby and Jones, "Engineering Materials,"
Pergamon (1986)

The range of elastic deformation for this ductile sample is short and the "Yield Stress," s_y, marks the transition from elastic to plastic behavior. This point is hard to locate and a stress that produces a measurable plastic strain is the normally tabulated quantity. This "Offset yield stress" or "Proof stress,"s_0.001, corresponds to a plastic deformation of 0.1%. When plastically deforming the sample reduces in cross-section in a uniform manner up to the "Tensile Stress," s_TS, at which point the deformation becomes localized and the material reduces in area in one location, forming a "neck." Deformation continues in the neck region until the "Final fracture," which is a "Ductile Fracture."