The amount of longitudinal stress a material can sustain when in tension, measured in units of force per unit area. Tensile strength is commonly expressed as megapascals (MPa) or pounds per square inch (psi) of original cross section. The amount of stress that a material can withstand before it yields and plastically deforms is referred to as the tensile yield strength. The maximum stress that a material can withstand before eventually failing is called the ultimate tensile strength. Tensile strength is measured by placing a standard test piece in the jaws of a tensile machine, gradually separating the jaws, and measuring the stretching force necessary to break the test piece.