MECHANICAL TESTING

True Stress / True Strain

Whereas engineering stress is based on the original dimensions of a specimen, the true stress is based on an instantaneous measurement of the cross-sectional area. True stress-true strain curves are often called flow curves, which represent plastic flow of the material. The flow curve is often used to determine two parameters characteristic of the material, the strain hardening exponent and the coefficient of the strength of the material. These values are often need to be determined for any type of cold formation of the material.

For ductile metals in tension, the material becomes unstable and begins to neck after a particular load is applied. The engineering stress-strain curve will indicate that less stress is needed to further deform the material, when in actuality, the necked area requires more stress for further deformation. This increase in stress is known as strain hardening. The true stress of a material far exceeds the engineering stress and continues to increase until the point of failure.

Index of Mechanical Testing