Chapter 8 - Yield Strength, Tensile Strength, and Ductility

Abstract

All solids have an elastic limit beyond which something happens. A totally brittle solid may fracture suddenly (e.g., glass). Most engineering materials do something different; they deform plastically or change their shapes in a permanent way. It is important to know when, and how, they do this—to design structures to withstand normal service loads without any permanent deformation, and rolling mills, sheet presses and forging machinery that will be strong enough to deform the materials that are formed. To study this, prepared samples are carefully pulled in a tensile testing machine, or they are compressed in a compression machine, and the stress required to produce a given strain is recorded. This chapter starts with a discussion on linear and nonlinear elasticity. Rubbers are exceptional in behaving elastically to high strains; almost all materials, when strained by more than about 0.001 (0.1%), do something irreversible: and most engineering materials deform plastically to change their shape permanently. This section discusses load–extension curves for nonelastic (plastic) behavior, followed by a discussion on true stress–strain curves for plastic flow. Next this chapter focuses on plastic work and tensile testing. This is followed by the discussion on data for yield strength, tensile strength, and tensile ductility. This chapter concludes with a note on the hardness test.