The plastic behaviour of sheet metal in biaxial tension testing

This study compares the plastic behaviour of a near isotropic sheet metal at a constant straining rate under conditions of balanced biaxial and uniaxial tension with close attention being paid to the calibration of the apparatus and analysis of deformation. Plastic behaviour consistent with two stages of strain hardening was observed for biaxial tension but not for uniaxial tension. Further investigation to attempt to explain this behaviour by studying the variation of plastic anisotropy under uniaxial tension has revealed a variation in lateral strain ratio throughout the strain range. It is demonstrated that the difference in plastic behaviour observed for uniaxial and biaxial tension can not be attributed to experimental method but rather to the difference in constraint for the testing processes and that the high initial strain hardening rate is prolonged in hydraulic bulging. Following investigation of these phenomena it was concluded that the perceived behaviour of sheet metal under biaxial tension during hydraulic bulging is fundamentally different to that under uniaxial tension.