Investigation into the shear stress, localization and fracture behaviour of DP600 and AA5182-O sheet metal alloys under elevated strain rates

- Strain rate effects on shear behaviour of DP600 and AA5182-O alloy sheets are presented.
- At high shear strains, both materials exhibited negative strain rate sensitivity due to thermal softening.
- A methodology was developed to obtain the uniaxial tensile work hardening response to large strain levels using shear test results.
- Microscopic measurements based on grain boundary rotation revealed sharper strain localization and higher peak strains at fracture under high strain rate conditions than was measured using DIC measurements.

Shear tests were performed at strain rates ranging from quasi-static (0.01 s−1) to elevated rates (600 s−1) considering DP600 steel and AA5182-O aluminum alloy sheet at room temperature. The shear specimen due to Piers et al. [J. Peirs, P. Verleysen, J. Degrieck, Experimental Mechanics, 52 (7), pp. 729-741, 2012] was scaled (reduced in size) to perform high strain rate shear testing. In situ digital image correlation (DIC) techniques were employed to measure the strains in the experiments and methods are proposed for characterizing the local strain within shear bands. Using the DIC strain measurements along with finite-strain theory and the logarithmic objective stress rate, a simple methodology was developed to obtain the work hardening response to large strain levels using only shear and tensile experiments. At lower strains, the DP600 shows positive rate sensitivity while the AA5182 exhibited limited sensitivity as strain rate increases. At equivalent strains greater than approximately 20%, the DP600 and AA5182 alloys demonstrated a reduced work hardening rate at elevated strain rates. For both alloys, the strain to localization (using the Zener-Holloman criterion) and subsequent fracture strain, measured using the DIC technique, decreased with strain rate in shear loading, but increased under uniaxial tensile loading. Microscopic assessment of fractured DP600 specimens was also performed using measurement of grain boundary rotation to determine local strains at fracture corresponding to length scales below the DIC measurements. The local strains at final failure are much higher and reveal an increase in the shear fracture strain with strain rate, in contrast to the trends based on the DIC analysis.