Stress measures how hard the internal material of a part is being pushed or pulled, defined as force divided by the cross-sectional area it acts on.
Units
In US units, stress is psi (pounds per square inch); in metric, pascals — usually MPa (megapascals, N/mm²) for engineering parts.
Worked example
A steel rod with a 0.5 in² cross-section carries a 5,000 lb pull: σ = 5,000 ÷ 0.5 = 10,000 psi. In metric, a 10,000 N load on a 50 mm² area gives 10,000 ÷ 50 = 200 MPa.
Types of stress
The basic formula above is axial (normal) stress — straight tension or compression. Shear stress is force acting parallel to the area (like a bolt being sheared). Bending stress varies across a beam’s cross-section. Compare the calculated stress to the material’s yield strength; the ratio of yield strength to working stress is the factor of safety.
Frequently asked questions
What is the stress formula? Stress equals force divided by area.
What units is stress in? psi in US units, MPa (N/mm²) in metric.
Stress vs strain? Stress is force per area; strain is the resulting deformation per unit length.