Material characterization of duplex stainless steel by superplastic free bulging test
Macroscopic superplastic behaviour of metallic or non metallic materials is usually represented by the strain rate sensitivity, and it can be determined by tensile tests at elevated temperature. However, a uniaixal stress state occurs in the test whilst a multiaixal stress state is induced in actual superplastic forming processes. The difference of the stress state might affect the reliability of the strain rate sensitivity obtained from the tension test. In the current study, the material characterization of duplex stainless steel, UNS S31803, has been carried out to determine the strain rate sensitivity throughout superplastic free bulging test under constant pressure. The measured height of bulged plate during the test was transformed into strain rate and stress corresponding to height at each time by theoretical considerations. The strain rate sensitivities were calculated for each pressure level. After calculating the strain rate sensitivity and strength coefficient for each test case, the flow stress equations have been employed in the finite element simulation to verify the experiments. The bulged height obtained from the simulation showed a reasonable agreement with the experimental findings. In the future, if a study on the effect of fillet radius of test tool and microstructure evolution is carried out, it is thought that the reliability of the material characterization by superplastic free bulging test can be enhanced.
Blank optimization for free bulging of Inconel 718 to maximize bulged height at high temperatures
Thickness profiled blank is designed via the optimization technique to maximize the bulged height in free bulging for Inconel 718. The thickness of the blank is described by the Bezier curve by using four control points. The location of each control points is used for the design variables of optimization. The material parameters for the flow stress equation of Inconel 718 are obtained from the free bulging test at constant pressure condition by using a flat blank and verified via numerical analysis. The objective function and constraints for optimization are the maximization of the bulged height within a limited strain range. The equivalent static load method for non-linear static response structural optimization with a move limit scheme is used for optimization. The result of optimization shows 21.95% increased bulged height and uniformly distributed strain after bulging. The free bulging test using blank with optimized profile is conducted to verify the optimization process. The results of bulged height and deformed shape are compared with those from numerical analysis, and the comparison shows good agreement.
Finite plastic deformation of a circular membrane under hydrostatic pressure
Abstract An analysis of the plastic deformation including instability phenomena of a circular membrane subject to one-sided hydrostatic pressure is presented. Equations determining stresses and strains are given for the deformation process of materials with a parabolic stress-strain curve. Numerical solutions have been carried out for some special cases. Comparison is made with theoretical and experimental results obtained by other investigators.