Crack growth onset in thin aluminum sheets under mixed mode I/II loading: A new form of the Equivalent Material Concept

Abstract

Ductile failure of diagonally loaded square plate (DLSP) samples, created from Al 6061-T6 and Al 7075-T6 alloys with small thickness weakened by central cracks, is assessed theoretically and experimentally under combined in-plane loading. To provide different combinations of mode I and mode II loadings in the tests on DLSP, various crack inclination angles are regarded in the aluminum specimens. Then, the load-carrying capacity (LCC) and the fracture initiation angle (FIA) of the DLSP samples are experimentally obtained from the fracture tests. The experimental observations indicate that considerable plastic deformation forms in the vicinity of the crack tip in DLSP specimens, especially in Al 6061-T6, during the tests. Due to the small thickness of theses specimens, they are considered as thin-walled components and hence, plane-stress condition is applied for their analysis. In order to evade elastic-plastic failure predictions, which are time-consuming and complex, a new form of the Equivalent Material Concept (EMC) is proposed and used in the theoretical predictions. To consider the influence of T-stress on the failure predictions, the generalized maximum tangential stress (GMTS) criterion is employed for estimating the combined mode I/II fracture toughness of DLSP specimens. It is revealed that EMC-GMTS mixed criterion can supply very good estimations of fracture toughness under combined mode I/II loadings. Moreover, the performed elastic-plastic finite element analyses (FEA) indicate that the crack growth in the specimens begins by significant yielding around the crack tip, particularly for Al 6061-T6 specimens, confirming well the experimentally observed failure evidences.