Fracture behavior characterization of AA6082 thin-walled rectangular tubes under low-velocity impact

The quasi-static and drop hammer axial impact tests are conducted for AA6082 thin-walled rectangular tubes. The experimental results indicate that the fracture severity of AA6082 thin-walled tubes under low-velocity impact is strongly influenced by impact velocity, with slower impact velocity resulting in more pronounced damage. To investigate the mechanical behavior underlying this phenomenon, dog-bone along different loading directions, R5, and shear specimens are extracted from the thin-walled tubes and tested under three different strain rates. The results show that AA6082 exhibits weak anisotropy and a negligible strain rate effect on strength, but a positive strain rate effect on fracture displacement. At a strain rate of 0.1/s, the fracture displacements of the dog-bone, shear, and R5 specimens are 27.88%, 29.19%, and 10.12% higher than those at the strain rate of 0.001/s, respectively. Three hardening models, namely the back propagation neural network optimized by ant colony optimization algorithm (ACO-BP) model, Johnson-Cook model, and Lim-Huh model, are calibrated to capture AA6082 strain rate-dependent hardening behavior. Among them, the ACO-BP hardening model outperforms the others with the highest accuracy, particularly in precisely capturing the initial yield stress and post-necking plastic deformation. At a strain rate of 0.001/s, the ACO-BP model achieved a root mean square error of 0.076, which is considerably lower than those of the other two hardening models. For quasi-static axial crushing simulations, three fracture criteria are compared, including constant fracture strain model, Rice-Tracey, and Brozzo model. The Brozzo ductile fracture model has the highest simulation accuracy, with a mean absolute error of 9.5356 between the simulated and experimental load values. Finally, a strain rate-dependent Brozzo fracture criterion is introduced to simulate the dynamic axial crushing behavior under drop hammer impact. Compared with the experimental data, the errors in the simulated peak force and crushing displacement are about 5.51% and 3.47%, respectively. The simulated mechanical response shows excellent agreement with experimental results, which indicates that their effectiveness in predicting strain rate-dependent plastic deformation and fracture behavior of thin-walled structures. These findings contribute to a deeper understanding of the impact-induced fracture behavior of thin-walled tubes and support the development of more reliable predictive models for crash safety applications.