Tensile Stress

Tensile stress σ is the force applied per unit cross-sectional area: σ = F/A, where F is the tensile force and A is the cross-sectional area of the sample.Stress has the same units as pressure: pascals (Pa), where 1 Pa = 1 N m^-2. The stresses in engineering materials are often large, so MPa (10⁶ Pa) and GPa (10⁹ Pa) are commonly used.Using stress rather than force allows samples of different thickness to be compared. A thick wire needs a larger force than a thin wire of the same material to reach the same stress.For a wire of diameter d, the cross-sectional area is A = πd²/4. The diameter should be measured with a micrometer at several points along the wire and a mean taken, and it must be converted into metres before calculating the area.For example, a force of 50 N on a wire of cross-sectional area 1.0 × 10^-7 m² produces a tensile stress of 50 ÷ (1.0 × 10^-7) = 5.0 × 10⁸ Pa.