Tensile strength

The tensile strength of a material is the maximum amount of tensile stress that it can be subjected to before it breaks.

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

Once past the elastic limit, the material will not relax to its initial shape after the force is removed. See Hooke's law and modulus of elasticity. 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 will lead to contraction (see Poisson), cracks and ultimately failure of the construction. Clearly, this is a remarkable point for the engineering properties of the material since here the construction may lose its loading capacity or undergo large deformations. On the stress-strain curve below this point is in between the elastic and the plastic region.

The ultimate tensile strength (UTS) of a material is the limit stress at which the material actually breaks, with sudden release of the stored elastic energy (released as noise and/or heat and/or more cracks e.g. for brittle materials). This point is the fracture marked X on the curve below.

For steel, the elastic limit is at about 0,2% and the breaking point is at 25% of the total (relative) extension (ε = ΔL/L - the figure is not to scale.) In steel constructions, the maximum allowable tensile stress at any point in the construction is 2/3 of the yield strength (or 0,2% deformation stress in metals or alloys without clearly defined yield stress). This comes down to a safety factor of 1.5.

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 (or 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.

In brittle materials such as rock, concrete, cast iron, glass or soil, tensile strength is negligible compared 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:

Single-walled carbon nanotubes have the highest tensile strength of any material yet measured, with the highest single measurement of a nanotube being 63 GPa. As of 2004, however, no macroscopic object constructed using a nanotube-based material has had a tensile strength remotely approaching this figure, or substantially exceeding that of high-strength materials like kevlar.

See also

• Vertical strength

External links

• Tensile Strength Test (http://matse1.mse.uiuc.edu/~tw/metals/e.html)
• January 2003 sci.physics thread on water tensile strength and trees (http://www.lns.cornell.edu/spr/2003-01/msg0047472.html) and stuff
• Theory re the discrepancy in static vs dynamic measurements of water's tensile strength (http://stacks.iop.org/0022-3727/20/1080)