Strength is an important property of every material as it determines whether the material can be used in certain applications and the size of the corresponding structure. It is defined as the resistance to failure by loading (such as tension, compression or torsion). The greater this resistance, the stronger the material.
Strength differs from material to material, but it is also dependent on the type of loading and the time period over which the load is applied as well as on temperature; generally speaking, it decreases with increasing temperature. Depending on the type of load, one differentiates between bending, fatigue, torsional, compression and tensile strength. The tensile strength is the most important parameter and it is determined on test bars, the shape and dimensions of which are standardised, in a tensile-testing machine; it is the maximum tensile stress that the test piece is just capable of withstanding before rupture and it is expressed in units of newtons per square millimetre of cross section (N/mm2) or megapascals (MPa).
The strength of most pure metals is low. It can be increased, however, by alloying, forming (especially cold working) or heat treatment. The tensile strength of unalloyed aluminium, Al99,8, is about 100 N/mm2, that of alloys with magnesium about 240 N/mm2 and that of alloys with copper, magnesium and zinc about 500 N/mm2. Aluminium alloys have moderate tensile strength and fatigue strength (the maximum alternating stress that can be applied repeatedly without failure or permanent deformation after a given number of cycles, typically 100 million); it is these properties, usually in combination with others, that make them suitable for many applications in the fields of mechanical engineering, aircraft construction and shipbuilding as well as road and rail vehicles.