Stiffness

A measure of a material's stiffness, defined as the ratio of stress to strain in the elastic region of loading. A higher modulus means less deflection under a given load; for plastics and composites it is typically measured in tension to ISO 527.

The maximum tensile stress a material can withstand before failure, determined by pulling a specimen to rupture in a tensile test. It is one of the most widely quoted mechanical properties on datasheets for metals, plastics, laminates and membranes.

Sudden sideways instability of a slender or thin-walled element under compressive load, occurring at stresses that can be far below the material's strength. Resistance to buckling depends on stiffness and geometry, which is one reason sandwich construction is used to stabilise thin composite skins.

A material's strength divided by its density, commonly called the strength-to-weight ratio. High specific strength is the principal reason fibre-reinforced composites displace metals in weight-critical aerospace, motorsport and marine structures.