STRAIN AMPLITUDE-DEPENDENT HARDENING PROPERTY OF Q235 STEEL FOR METALLIC DAMPERS

Metallic damper has been widely used as a cheap seismic reduction technology in various engineering structures, and it is usually made from steel with low yield strength but high ductility. Under seismic loading, metallic dampers would experience stochastic cyclic loading paths with its strain amplitude varying significantly, and in such case the steel material experiences cyclic hardening and cyclic softening. Experimental studies including constant strain amplitude, ascending strain amplitude, and descending strain amplitude were firstly conducted on steel Q235 to investigate its mechanical performance under various cyclic loading protocols. The largest strain amplitude exceeds 10% of the original value, and the influence of strain amplitude and strain history was extensively scrutinized. The results demonstrate that: the hardening capacity of Q235 steel depends strongly on strain amplitude, and the plastic strain cumulated under larger strain amplitude would increase the hardening capacity of it under smaller strain amplitude tests followed while the influence of deformation history can be ignored in ascending strain amplitude tests. Furthermore, the Chaboche model parameters of Q235 steel under different constant strain amplitudes were calibrated using the optimization toolbox in software MATLAB. The optimized parameters indicate that simply considering the strain amplitude-dependent property of isotropic hardening is enough to describe hysteretic behavior of steel Q235. Using the shear damper as an example, the strain amplitude dependence property of material hardening capacity was considered by programming the subroutine USDFLD upon finite element software ABAQUS. The importance of considering this hardening property in predicting the performance of shear dampers is analyzed. It shows that the strain-amplitude dependent properties of cyclic hardening of Q235 steel have a significant impact on the analysis of damper performance and need to be concerned.