Size effect on springback behavior due to plastic strain gradient hardening in microbending process of pure aluminum foils

In order to investigate the springback behavior of pure aluminum foils, scaled microbending experiments with different thicknesses ranging from 25 μm to 500 μm were carried out. In the experiments, it is observed that springback angle increases with decreasing foil thickness, which indicates obvious presence of size effects, and is attributed to plastic strain gradient hardening. So a constitutive model, taking into accounts both plastic strain and plastic strain gradient hardening, is proposed to analyze the microbending process of thin foils. The model is based on the relationship between yield stress and dislocation density, and is adopted to predict the springback angle after microbending both by analytical expressions and by FE simulation. It is confirmed that the predictions by using the proposed hardening model agree well with the experimental data, while those by using conventional elastoplastic model cannot capture such size effects.