An Improved Approach for Forming Limit Prediction of Strongly Anisotropic Thin-Walled Tube Combining M–K Model and Ductile Fracture Criterion

Accurate prediction of the forming limit at necking of strongly anisotropic thin-walled tubes is an urgent requirement in tube hydroforming. M–K model is one of the most widely used prediction models. However, the failure criterion in M–K model is based on the ratio of major true strain increments in the groove to uniform regions, which sometimes results in overestimation of the limit strains. To address this issue, a combined model of M–K model and ductile fracture criterion (DFC) for tube was proposed in this paper, in which the failure condition of the groove region is determined by the DFC. The characteristics of the M–K + DFC model were analyzed by combining with the DF2012 phenomenological DFC. To validate the M–K + DF2012 model, the limit strains at necking of an AA6061 tube with strong in-plane anisotropy were tested through tube-controllable biaxial loading experiments. The results show that the forming limit curve (FLC) predicted by the M–K + DFC model does not exceed the FLC calculated by the M–K model and the fracture forming limit curve (FFLC) determined by the corresponding DFC. The M–K + DF2012 model provides a reasonable forming limit prediction of the AA6061 tube, and the shortcoming of the M–K model mentioned earlier is overcome. Meanwhile, a reasonable constitutive model considering the in-plane anisotropy is very important in the forming limit prediction of a strongly anisotropic thin-walled tube.