Cyclic deformation characteristics of S355 and S690 steels under different loading protocols

Abstract

Despite of excellent high strength to self-weight ratios of the S690 steels, when compared with the S355 steels, there is a widespread concern regarding the ductility of the S690 steels. It is generally considered that the ductility of the S690 steels is significantly lower than that of the S355 steels – this is the general understandings the authors attempt to investigate. This paper presents an experimental investigation into cyclic deformation characteristics of both S355 and S690 steels through low-cycle high-strain cyclic tests with two different loading protocols. A detailed account of the results of 32 cyclic tests on both the S355 and the S690 funnel-shaped coupons is presented. Effects of four different target strains and two different loading frequencies are also examined in details. For the ranges of loading protocols, strain amplitudes, and frequencies considered, the hysteretic responses of these coupons of the two steels are compared directly in terms of engineering stress–strain curves based on their nominal diameters. Microstructures of the fractured coupons of the two steels are also identified for comparison. Contrary to the general understandings, it is demonstrated that the high strength S690 steels do have a good ductility under both monotonic and cyclic actions. Moreover, depending on specific loading protocols and target strains, the cyclic deformation characteristics of the S690 steels are demonstrated to be superior to those of the S355 steels in terms of the number of cycles completed prior to failure and their corresponding energy dissipation characteristic under various target strains up to ±10.0%. The findings of this experimental investigation highlight the importance of establishing ductility requirements and cyclic deformation characteristics for the high strength S690 steels in accordance with specifically designed cyclic tests rather than relying solely on conventional monotonic tensile tests.