The drive to produce lightweight vehicles with improved fuel economy has resulted in an increased interest in utilization of aluminum alloys for automotive body structures due to their higher strength-to-weight ratio. To support the utilization of aluminum alloys in automobile structures, their dynamic behavior must be considered under crash conditions, although traditionally, the strain rate sensitivity of aluminum alloys has been considered to be low. In this work, three aluminum sheet alloys, AA5754, AA5182 and AA6111, which are prime candidates for replacing mild steel in automobile structures, are tested in tension at quasi-static and high strain rates.

In order to characterize the constitutive response of AA5754, AA5182 and AA6111 at high strain rates, tensile experiments were carried out at strain rates between 600 s⁻¹ and 1500 s⁻¹, and at temperatures between ambient and 300°C, using a tensile split Hopkinson bar (TSHB) apparatus. As part of this research, the apparatus was modified in order to provide an improved means of gripping the sheet specimens. Quasi-static experiments also were conducted using an Instron machine.

The tensile experiments showed that the rate sensitivity of the flow stress is low for these alloys, with the strain hardening response of AA5754 showing a mild sensitivity to strain rate in the range of strain rates considered. However, increases in strain rate appeared to significantly enhance the ductility of these alloys. Analysis of the stress-strain data demonstrated that the strain at which Considere’s criterion is satisfied increased at rates of high strain. This behavior implies that the onset of necking is delayed under high rate conditions, permitting the material to elongate more prior to localization. Levels of damage in AA5754 and AA5182 were also found to increase with strain rate, presumably due to the elevated strains to failure.

The experimental data was fit to the Johnson-Cook and Zerilli-Armstrong constitutive models for all three alloys. The resulting fits were evaluated by numerically simulating the tensile experiments conducted using a finite element approach. Of the two models, the Zerilli-Armstrong constitutive model was more accurate in predicting the flow stress of these materials at the strain rates and temperatures considered. The finite element simulations using both constitutive models were unable to accurately predict the necking strain of these alloys.