Effects of loading mode on mechanical properties of high strength steel Q690 and their application in coupon test

Abstract

The coupon test methods specified in the current specifications do not satisfactorily address the strain rate effect on mechanical properties of materials, which may cause some deviations in the results. Therefore, there is a demand for alternative loading modes of coupon test. In this paper, effects of loading mode on the mechanical properties of high strength steel (HSS) Q690 were tested, considering monotonic tensile coupon test, tensile coupon test with displacement-controlled holds, and tensile coupon test with strain-controlled holds. Parameters such as pre-strain, loading rate, hold times, and hold duration were considered. Results show that the yield strength, ultimate strength, and ultimate strain of steel Q690 were hardly affected by the stress relaxation induced by holds, while increase with the increase of loading rate. The largest difference of the ultimate strength induced by loading mode was 6.2 MPa which is less than 1% of the ultimate strength of the material, and that of the ultimate strain was 5.8E-3. The stress drop during stress relaxation increases with the increase of pre-strain and strain rate. The normalized stress drop at the pre-strain of 0.04 could be 37.4% larger than that at the pre-strain of 0.01. The normalized stress drop of specimen with strain rate of 0.001/s could be 27.7% larger than that with strain rate of 0.0001/s. Tensile coupon test with two displacement-controlled holds is proposed to obtain the static mechanical properties of HSS Q690. Based on the test results, the values of parameters of the Cowper-Symonds model were calibrated to consider the effect of strain rate on the dynamic yield strength and ultimate strength, respectively, of steel Q690.