Effect of Strain Rate on Tensile Properties

Static tensile tests are often performed at very low strain rate [10 ^–6 mm/mm (sec)] and variations permissible in conventional testing machines are generally so limited that they do not influence the static properties. But machines capable of varying strain rate by as much as 100,000 time have been developed and tensile tests in such machines have revealed that tensile properties are quite susceptible to changes under increasing strain rates.

In general it is observed that yield strength and ultimate tensile strength increase with increasing strain rates, percentage elongation seems to first increase with strain rate but then decreases of remains constant. One important influence of high strain rate is that for such material as low-carbon steels which do not show yield point in normal tensile tests, yield point appears. Further, yield strength out of all static properties is not affected by strain rate. Figures 22 and 23 show how these properties are influenced by strain rate.

The effect of strain rate on mechanical properties becomes more pronounced at elevated temperatures. The load applied at strain rate higher than 10 mm/mm (sec) is classified as impact load, and under impact loads ductility of a material has tendency to decrease.

                           Figure: Stress-Strain Diagram for Mild Steel at different Strain Rates

                                 Figure: Effect of Strain Rate on σY, σu and % Elongation