Cyclic softening behavior of structural steel with strain range dependence

Abstract

In seismic analysis, the cyclic performance of structural steel members is closely related to the strain range dependence effect. The strain range dependence effect has been extensively investigated from the perspective of hardening during the range expansion. However, the research on the cyclic softening behavior induced by the shrinkage of the loading range is insufficient, which corresponds to the scenario of the descending-magnitude long tail after the peak value in seismic analysis. In this study, eight cyclic coupon tests of Q345B structural steel are conducted to comprehensively investigate the strain range dependence effect experimentally at first. The test results indicate that the stress amplitudes experience loop-wise softening when the strain range drops to a lower level and gradually converge to a stabilized magnitude higher than the reference value obtained from constant-amplitude cases. Moreover, the cyclic softening is only influenced by the historical maximal loading range regardless of the intermediate loading history. For describing the experimental observations, a sophisticated constitutive model for general structural steel is proposed wherein a novel concept of incomplete collapse effect, which possesses a clear geometric interpretation, is specially designed to characterize the cyclic softening phenomena. Considering the future demand in multi-scale analysis, the constitutive model is implemented by inverse-P norm algorithm applicable to all spatial dimensions. A semi-automatic and efficient calibration method is developed to elude manually calibrating numerous entangled material parameters. Finally, the accuracy of the proposed constitutive model is validated by two batches of material-level coupon tests and a member-level experiment of two shear damper specimens, which demonstrates the capabilities of the proposed constitutive model in reasonably predicting the cyclic softening behavior with the strain range dependence as well as other hysteretic properties.