Analytical modeling of fracture strain and experimental validation in incremental sheet forming

Prediction of fracture strain in incremental sheet forming (ISF) process is quite essential to evaluate the formability under optimized process design. In the present work, new analytical formulas of stress triaxiality are derived after considering the effects of bending, shearing and cyclic loading based on membrane analysis. These deformation mechanisms, considered as key factors of improved formability, are thoroughly analyzed through the derivation of stress triaxiality. A novel analytical prediction model of fracture strain is further proposed by combining the analytical model of stress triaxiality with a generalized ductile fracture criterion. For the validation of analytical models, a set of experiments for conical part and pyramid part with different sheet materials and process parameters have been carried out. Finite element analysis is also conducted to further validate the prediction accuracy of stress triaxiality and equivalent strain. The effects of deformation mechanisms on fracture strain are evaluated in details by range analyses.