Fracture criterion and forming pressure design for superplastic bulging

The flow localization factor (FLF), which enables quantitative description of the localization process of unstable plastic flow, was proposed in the authors’ previous work. In this paper, the relation between the FLF and superplastic forming limits is further investigated via finite element simulations and experiments on the bulging of superplastic Ti–6Al–4V sheets at 900 °C. With the insight from these studies, a superplastic fracture criterion, which is in terms of an integral form of the FLF, is proposed. Fracturing is explained as the result when the flow localization accumulated throughout the entire forming process achieves a critical value. Satisfactory results are obtained for the verifications of the fracture criterion under various forming conditions. Finally, pressure design guidelines for superplastic bulging based on the proposed fracture criterion are recommended. Designers can adopt the most appropriate forming path according to the requirements of each specific case.