The Effect of Unloading Positions and Times on Sheet Metal’s Stress-Strain Curve

Kunmin Zhao,Wumei Fang,Baotang Wang,Daxin Ren

doi:10.4236/jmmce.2018.63029

Kunmin Zhao, Wumei Fang + Show 2 more

Open Access

https://doi.org/10.4236/jmmce.2018.63029

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Abstract

The loading-unloading-reloading process could affect the tensile deformation of metals with the combined function of stress relaxation and work hardening, which has been reported in multiple experiments. Nevertheless, the effects of different unloading positions and unloading times have not been investigated. In this study, unloading-reloading tests on three materials (AL6061, HSLA and Q195) were conducted. The stress exhibits a rapid rise momentarily upon reloading and stabilizes afterward while the post stress-strain curve deviates up or down from the monotonic tensile curve. The ductility is enhanced by the unloading-reloading process in general. Different unloading positions and unloading times have different degrees of influence on the stretching of these metals. The effect of loading conditions on a medium manganese steel was further studied. The functions to modify the post stress-strain relationship after unloading-reloading were established.

Highlights

Stretching forming as one of the important methods to explore the performance of metal materials has complex plastic forming mechanism
What requires noticing especially is that the total elongations of the experiments for Q195 are obtained by curve fitting according to the trend because of the incomplete datum caused by the interruption of extensometer
The first time unloading at 75% of the total elongation of monotonic loading (TEML) has a maximum increase (2.5%) of total elongation where the increase of total elongation reduced while the unloading times increase

Summary

Introduction

Stretching forming as one of the important methods to explore the performance of metal materials has complex plastic forming mechanism. The mechanism of metal plastic deformation during tensile progress is mainly dislocation movement, and the dislocation moving is too difficult to make the elongation change at low temperature [1] [2]. The loading-unloading cycle progress can affect materials' property during tensile experiments along with stress relaxation and it has great influence on stretch forming. Stress relaxation enhances metal ductility markedly confirmed by the increase of elongation [1] [4] [5] [6] [7]. In the course of stress relaxation, the dislocation density decreases gradually and it will increase again upon reloading to reach the previous state. The elongation will increase as a consequence of less density in the critical regions and the variation of flow stress or hardening behavior due to the addition of dislocation density in critical regions owing to the redistribution [2] [3] [4]

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Conclusion