Tensile strength is the ability of a material to withstand tensile stresses (those which tend to lengthen it). The tensile strength of a substance is defined as the smallest stress (force divided by unit cross-sectional area) required to break it.

Tensile strength is tested by stretching. A metal extends overall at first, but later expansion concentrates around the point of fracture. Curves A and B (see graph) show typical extensions. Curve A, for mild steel, remains linear to its elastic limit. If the load is released early, the metal returns to its original length. Curve B is typical of softer metals. A range of relative strengths of metals is also shown.