Property Information

The strength of a material is its resistance to failure by permanent deformation (usually by yielding).
A strongmaterial requires high loads to permanently deform (or break) it - not to be confused with a stiff material, which requires high loads to elastically deform it.
For metals, polymers, woods and composites, "strength" on the selection charts refers to loading in tension (as failure is by yielding).
For brittle materials (ceramics), failure in tension is by fracture, and the "tensile strength" is very variable. The "strength" on the selection charts is then "compressive strength" (which requires a much higher load).
Specific strength is strength divided by density.

Many engineering components are designed to avoid failure by yield or fracture (cranes, bikes, most parts of cars, pressure vessels).
In structural applications, brittle materials are nearly always used in compression (e.g. brick, stone and concrete for bridges and buildings).
In transport applications (e.g, aeroplanes, racing bikes) high strength is needed at low weight. In these cases materials with a large "specific strength" are best.

Tensile testing is used to find many important material properties. The compression test is similar but uses a stocky specimen to prevent bending.

Two measures of strength are defined - yield strength and ultimate tensile strength. Strength on the selection charts means yield strength.

Strength is measured by applied stress, which is equal to force/area. The units of stress are N/m² or Pascals (1 Pa = 1 N/m² ; 1 MPa = 1 N/mm²

Specific strength is strength/density - it is mostly just used for comparing materials, so the units are not important.