The elastic limit of a material is an important consideration in civil, mechanical, and aerospace engineering and design. Elastic limit, also referred to as yield point, is an upper limit for the stress that can be applied to a material before it permanently deforms. This limit is measured in pounds per square inch (psi) or Newtons per square meter, also known as pascals (Pa).
Elastic limit is a function of the elasticity of a material. Elasticity is the ability of a material to return to its original shape, or dimensions, after a load or stress is removed. All materials will deform when a stress or load is applied. Strain is a measure of the amount of deformation that occurs when a material is under stress.
Elastic strain occurs when a material is exposed to low stress. It will disappear after the stress is removed, and the material will return to its original state. Plastic strain will occur at stresses above the elastic limit. A material that experiences plastic strain will not fully recover and return to its original dimensions after the stress is removed.
This property can be illustrated using the example of a spring. If a weight is hung onto one end of a spring, with the opposite end fixed, it will extend the spring. If a small amount of weight is applied and then removed, the spring will return to its original length. If too much weight is applied to the spring, it will permanently deform and will not return to its original length when the weight is removed. The spring has undergone plastic deformation because the stress caused by the weight exceeded the elastic limit.
Materials have a measurable relationship between applied stress and the resulting strain. This relationship can be plotted in a stress-strain curve. The slope of the stress-strain curve remains constant in the region where elastic strain occurs. The elastic limit is the point where the applied stress causes an onset of permanent deformation, and the slope of the stress-strain curve changes.
Not all materials have an elastic limit. Ductility is a measure of the amount of permanent deformation before complete failure. Ductile materials, like steel and brass, will experience a large amount of plastic deformation before ultimate failure occurs. Brittle materials, like glass and concrete, will show little or no plastic deformation, and complete failure often occurs immediately after a critical value of stress has been reached. For this reason, brittle materials typically do not have a yield point.