Measuring the Mechanical Properties of Aluminum Sheets and Their Resistance Spot Welds at Large Strains Using Digital Image Correlation Coupled with a Modified Shear Test

The constitutive behavior of aluminum alloy sheet and their resistance spot welds at large strains is critical for light weight vehicle design analysis and life prediction. However, data from uniaxial tensile tests are usually limited to small strains or by material instability. A novel technique was developed using digital image correlation coupled with a modified shear test to directly measure the stress - strain curves of aluminum alloy sheet at large strains. The modified shear sample prevents end rotation of the shear zone as compared to the ASTM B831 test. The results show that the effective stress - effective strain curves from shear tests match those obtained by uniaxial tension, but only by incorporating material anisotropy using the Barlat-Lian yield function. For the first time, the technique was applied to aluminum resistance spot welds to determine both the shear strength and stress-strain curves of spot welds at large strains. The results show that for non heat treatable AA5754 spot welds, there was a slight change in yield strength but significant change in work hardening rate whereas for heat treatable AA6022-T4 spot welds, there was a significant decrease in both yield strength and work hardening rate. Technique details and experimental validation will be discussed.