Main Page | See live article | Alphabetical index

Tensile strength

Tensile strength is an important concept in engineering, especially in the fields of material science and mechanical engineering.

The tensile strength of a material is the maximum amount of tensile stress that can be applied to it before it ceases to be elastic. If too much force is applied the material will break or become plastic, i.e., once the force exertion is stopped the material won't go back to its initial shape.

The tensile strength where the material becomes plastic is called yield tensile strength. This is the point where the deformation (strain) of the material is unrecovered, and the work produced by external forces is not stored as elastic energy but it is transformed to heat and energy for growing of cracks. This is a remarkable point for the engineering properties of the material since it may loose the loading capacity or it undergoes large deformations. On the figure below this point is in between the elastic and the plastic region.

The ultimate tensile strength (UTS) of a material is the limit stress where the material cracks grows so that the material continuity is loosen and it breaks to 2 pieces with sudden release of the elastic energy stored (energy is released as noise and/or heat and/or more cracks e.g. for brittle materials). This is an unwanted stress point for the engineering applications. This point is the fracture marked on the figure below.

Tensile strength is measured in units of force per unit area. In the SI system, the unit is newton per square metre (N/m² or Pa - Pascal). The U.S customary unit is pounds per square inch (psi).

The breaking strength of a rope is specified in units of force, such as newtons, without specifying the cross-sectional area of the rope. This is often loosely called tensile strength, but this not a strictly correct use of the term.

For ductile materials such as steel, copper, aluminum and such cases of soils, tensile strength has magnitudes as the compressive strength (usualy the compressive strength is greater even for perfect crystalic metals). On the opposite, for brittle materials such as rock, concrete, cast iron, glass, etc. tensile strenght has negligible magnitude comparing to the compressive strength and it is assumed zero for most engineering applications.

Tensile strength can be measured for liquids as well as solids. For example, when a tree draws water from its roots to its upper leaves by transpiration, the column of water is pulled upwards from the top by capillary action, and this force is transmitted down the column by its tensile strength. Air pressure from below also plays a small part in a tree's ability to draw up water, but this alone would only be sufficient to push the column of water to a height of about ten metres, and trees can grow much higher than that. (See also cavitation, which can be thought of as the consequence of water being "pulled too hard".)

Some typical tensile strengths of some materials:

MaterialYield strength (MPa)Ultimate strength (MPa)
Structural Steel ASTM-A36250400
Steel High strengh alloy ASTM A-514690760
Stainless Steel AISI 302 - Cold-rolled520860
Cast Iron 4.5% C, ASTM A-48-170
Alluminum Alloy 2014-T6400455
Copper 99.9% Cu70220
Titanium Alloy (6% Al, 4% V)830900
Nylon, type 6/64575

External links