Xu Zhang

Abstract:

Strain gradient plasticity is a successful framework to explain the size effect of mechanical behavior in microscale and nanoscale. Most of the components of Micro-Electro-Mechanical System (MEMS) system are often subjected to a cyclic loading, thus, fully understanding and modeling the cyclic deformation of material in microscale and nanoscale is urgent and important. Here, we proposed a thermodynamically consistent strain gradient cyclic plasticity model. The cyclic response with isotropic and kinematic hardening is predicted. Specifically, the isotropic hardening is supposed to be related with the dissipative part of plastic work of the material, while the stored energy in the plastic deformed material is supposed to contribute to the kinematic hardening. The evolution of back-stress during cyclic response is an extension of classical Armstrong-Frederick model through the inclusion of plastic strain gradient. The new established strain gradient cyclic plasticity model could predict the size effect in twisting of wires. Then, the abnormal Bauchinger effect observed experimentally, i.e. torque-twist curve shows reverse plasticity at positive torque, can be captured using the new established strain gradient plasticity model. Furthermore, the closed expression of the cyclical torsion behavior of thin wires is obtained, and the cyclic hardening response is then systematically investigated.