Sheet bulge testing

Uniaxial tension tests are accepted worldwide as a standardized testing procedure that is usually carried out to determine material parameter values to characterize the forming behavior of sheet materials. Due to the uniaxial loading condition present during a tension test, only a limited strain range is considered in the determination of the true stress-true strain curve used in numerical simulation to describe the material's strain hardening. To extend the validity of the hardening curve, the bulge test was introduced, since a larger strain range is achieved under biaxial loading conditions. Although the bulge test has been an acknowledged testing method for several years, no unanimous agreement exists about the precise testing conditions and the evaluation method used. As a consequence, test results for the same material have varied considerably from test laboratory to test laboratory, and previous applications of the test have rather been limited. To achieve greater confidence in bulge testing, there is a great need to standardize the test conditions, measuring procedures, and test evaluation algorithms. This paper describes the usage and standardization of the hydraulic bulge test.