The area between Points I and 2 is called the elastic region. If stress is removed, the material will return to its original length.

Point 2 is the proportional limit (PL) or elastic limit, and Point 3 is the yield strength (YS) or yield point.

The area between Points 2 and 5 is known as the plastic region because the material will not return to its original length.

Point 4 is the point of ultimate strength and Point 5 is the fracture point at which failure of the material occurs.

Figure 3 is a stress-strain curve typical of a ductile material where the strength is small, and the plastic region is great. The material will bear more strain (deformation) before fracture.

Figure 4 is a stress-strain curve typical of a brittle material where the plastic region is small and the strength of the material is high.

The tensile test supplies three descriptive facts about a material. These are the stress at which observable plastic deformation or "yielding" begins; the ultimate tensile strength or maximum intensity of load that can be carried in tension; and the percent elongation or strain (the amount the material will stretch) and the accompanying percent reduction of the cross-sectional area caused by stretching. The rupture or fracture point can also be determined.

Figure 4 Typical Brittle Material Stress-Strain Curve

Summary

The important information in this chapter is summarized below.

Stress-Strain Relationship Summary

Bulk Modulus

The Bulk Modulus of Elasticity is the elastic response to hydrostatic pressure and equilateral tension, or the volumetric response to hydrostatic pressure and equilateral tension. It is also the property of a material that determines the elastic response to the application of stress.

Fracture point is the point where the material fractures due to plastic deformation.

Ductile material will deform (elongate) more than brittle material, shown in the figures within the text. The stress-strain curves discussed in this chapter for ductile and brittle demonstrated how each material would react to stress and strain.

Figures 3 and 4 illustrate the specific points for ductile and brittle material, respectively. Hooke's Law applies between Points 1 and 2. Elastic region is between Points 1 and 2. Plastic region is between Points 2 and 5.