Mechanical mechanism of specimen size effect on impact toughness of Al 6061: Experiment and simulation

Impact toughness has a strong size effect, which restricts the accurate evaluation of material toughness and safety of structural design. However, our understanding of the internal mechanical mechanism that influences the size effect remains incomplete. To fill this gap, this paper adopts the method of combining experiment, theory, and simulation. From the point of view of the dependence of plastic and fracture behavior of material on stress state, the internal mechanical mechanism of the size effect of impact toughness (α_k) was studied. Firstly, the Charpy impact tests of different sizes were carried out on Al 6061. α_k was divided into cracking initiation toughness (α_ini) and cracking growth toughness (α_gro). The variation of α_ini and α_gro with specimen size was analyzed. Then, through a series of tests, the influence of stress state on the plastic and fracture behavior of Al 6061 was studied. To simulate the whole process of the Charpy impact test, an uncoupled 3D fracture model was proposed. Subsequently, a stress state dependent yield function, modified Voce hardening law, and the proposed fracture model were used to simulate the Charpy impact tests. The predicted results were in good agreement with the tests. Finally, the internal mechanical mechanisms of the size effects of α_ini and α_gro were analyzed. The results show that: (1) The α_ini and α_gro have significant size effects, and there are significant differences between them. (2) The internal mechanical mechanism of the size effect of α_ini was revealed; that is, the dependence of the plastic behavior of the material on the stress state is the internal root cause of the dependence of α_ini on the specimen size. (3) The internal mechanical mechanism of the size effect of α_gro was revealed; that is, the dependence of the plastic behavior of the material on the stress state and the fracture behavior of the material under ultra-high stress triaxiality is the internal fundamental reason affecting the size effect of α_gro. (4) This study reveals the internal complex and intimate relationships between plastic behavior, fracture behavior, toughness behavior, stress state, and the size effect, providing a valuable reference for accurate and comprehensive understanding, evaluation, and selection of metal materials.