Recovery Strain of Sheet Metals during Unloading in the Tension Test and Its Application to Roll-forming

Abstract

The recovery strain during unloading in the tension test has been measured by strain gauge in the range from 0 to 3% tensile prestrain for sheet metals including 18-8 stainless steel, aluminum, aluminum alloys, copper, 7:3 Brass, zinc and titanium in the annealed and temper rolled state.
The relation between the recovery strain and tensile prestrain (or prestress) and the influence of temper rolling on this relation are discussed.
The recovery strain ε_R increases with increasing tensile prestrain in all metals tested. The ratio of ε_R to elastic strain σ_F⁄E estimated by prestress σ_F divided by Young’s modulus E is greater than unity and amounts to about 2.6 in zinc, 1.6 in 7:3 Brass, 1.4 in 18-8 stainless steel and 1.2 in copper, titanium, aluminum and aluminum alloys. By temper rolling ε_R increases in all metals tested, while recovery-strain-to-elastic-strain ratio tends to decrease in copper, aluminum and 7:3 Brass, but increases in 18-8 stainless steel.
The following relation is found to exist for all metals between ε_R and elastic strain σ_F⁄E:
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\ oindentwhere k and σ₀ are constants depending on the material and its process history.
Finally ε_R is found to have good correlation with the shape of product by cold roll-forming.