In the present work, the compatibility relationship on the failure criteria between aluminium and polymer was established, and a mechanics-based model for a three-layered sandwich panel was developed based on the M-K model to predict its Forming Limit Diagram (FLD). A case study for a sandwich panel consisting of face layers from AA5754 aluminium alloy and a core layer from polyvinylidene difluoride (PVDF) was subsequently conducted, suggesting that the loading path of aluminium was linear and independent of the punch radius, while the risk for failure of PVDF increased with a decreasing radius and an increasing strain ratio. Therefore, the developed formability model would be conducive to the safety evaluation on the plastic forming and critical failure of composite sandwich panels.

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Tube hydroforming has been widely applied by the automotive sector to produce hollow parts. As a popular tube hydroforming test method, tube hydro-bulging needs an analytical failure model to analyze the formability of tubular materials. In the present work, a failure prediction model has been developed to predict the bulging height limit (BHL) of the hydro-bulging test. The model utilized Hill’s orthogonal anisotropic model to characterize the tube material, a geometry model to characterize the non-loading path and the M-K model to predict failure. Defects in multiple directions were taken into consideration. The developed model was applied on two tubes of different materials as case studies to verify its validity. It is shown that the developed model is capable of predicting the forming limit or determining the imperfection factor of tubular materials.