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Elastic Limit Testing

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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. The determination of the elastic limit of a material involves measuring the greatest stress that can be applied to the material under test without causing permanent deformation.

When a material is exposed to low stress elastic strain occurs. It will disappear after the stress is removed, and the material will return to its original shape, or dimensions. 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 shape or dimensions after the stress is removed. When a material is subjected to a load beyond the elastic limit, permanent deformation will occur. This is the lowest stress at which permanent deformation can be measured. A manual load-unload procedure is required, and the accuracy is critically dependent on equipment and operator skill. For elastomers, such as rubber, the elastic limit is much larger than the proportionality limit. Also, precise strain measurements have shown that plastic strain begins at low stresses

We can use the example of a spring to illustrate this property. If a weight is attached to one end of a spring, with the opposite end fixed, the spring will extend. If a small amount of weight is now added and then removed, the spring will return to its original length. If excessive 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 gone over the elastic limit.

The elastic limit of a material is an important consideration in civil, mechanical, and aerospace engineering and design. Elastic limit, also known 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).


TEST STANDARDS:

• ASTM D638 - 10 Standard Test Method for Tensile Properties of Plastics
• ASTM D1043 - 10 Standard Test Method for Stiffness Properties of Plastics as a Function of Temperature by Means of a Torsion Test
• ASTM C203 - 05a(2012) Standard Test Methods for Breaking Load and Flexural Properties of Block-Type Thermal Insulation