Comparison between direct and indirect measurement methods for bulge tests

摘要

To reinforce the standardization of the bulge test measuring procedures by comparison of two different bulge forming measurement methods. Two different bulge forming measurement methods are used simultaneously in order to reinforce the standardization of the bulge test measuring procedures. An indirect method, Digital Image Correlation (DIC), is compared with a direct one, ultrasound pulse-echo method. The main conclusion is that the DIC system is a valid indirect measurement method to study bi-axial sheet metal forming. The constant pressure bulge test method was used and it yielded positive results for comparing the direct and indirect method (considering thickness measurement of the bulge dome), as an important research implication is that the touch less measurement method could be applied to other sheet metal forming processes. Tension tests are used as a standard accepted procedure to determine material parameter values for characterizing the forming sheet behaviour. However, by using a tension test, only a limited strain range can be considered for determining the true stress - true strain curve. Based on this limitation, the bulge test is used to achieve a much larger strain range under bi-axial loading conditions. An indirect method, Digital Image Correlation (DIC), is compared with a direct one, ultrasound pulse-echo method, in situ, real time and on the same specimen.