The elastic limit is the upper limit of stress a material can handle before permanent deformation. Elasticity allows a material to return to its original shape. Plastic deformation occurs when stress exceeds the elastic limit. The stress-strain curve shows the relationship between stress and strain. Ductile materials have a high amount of plastic deformation before failure, while brittle materials do not have a yield point.
The elastic limit of a material is an important consideration in civil, mechanical, and aerospace engineering and design. The elastic limit, also known as the 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).
The 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 size, after the removal of a load or stress. All materials will deform when a stress or load is applied. Strain is a measure of the amount of strain that occurs when a material is under stress.
Elastic deformation occurs when a material is exposed to low stresses. It will disappear after the stress is removed and the material will return to its original state. Plastic deformation will occur at stresses above the elastic limit. A material that undergoes plastic deformation will not fully recover and return to its original size after stress removal.
This property can be illustrated using the example of a spring. If a weight is hung on 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 has exceeded its elastic limit.
Materials have a measurable relationship between the 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 deformation 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, such as steel and brass, will experience a large amount of plastic deformation before ultimate failure occurs. Brittle materials, such as glass and concrete, will show little or no plastic deformation, and complete failure often occurs soon after a critical stress value has been reached. For this reason, brittle materials typically do not have a yield point.
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