Determination of forming limit for aluminium alloy sheet eliminating the interferences of through-thickness stress and non-linear strain path

Abstract

The accuracy of the forming limit diagram (FLD) determined through Nakazima test is often influenced by non-linear strain path (NLSP) and through-thickness stress. The influence of these factors on measurement accuracy can be weakened or avoided in Marciniak test. But meanwhile, Marciniak test is difficult in operation and possesses poor repeatability. Therefore, the Nakazima test eliminating the influencing factors of accuracy is still an attractive method due to the easy operability and good repeatability. In the present research, two FLDs of aluminium alloy 6014-T4 that were established through Nakazima and Marciniak tests were compared. In order to obtain an accurate FLD in plane-stress state and linear strain paths via Nakazima test, a modified compensation method was applied in Nakazima test to eliminate the interferences of NLSP and through-thickness stress on measuring forming limits. The ultimate strains measured in Nakazima test were transformed to stress-based forming limits through constitutive models to remove the influence of through-thickness stress. Then, the corrected ultimate stresses were transformed back to the limit strains along linear strain paths. Compensated results of Nakazima test showed a good agreement with the experimental results from Marciniak test, proving the validity of this method. Thus, through the easy-operate Nakazima test, an FLD with comparable accuracy to that based on Marciniak test was established.