Brittle fracture prediction method considering mixed modes for column-to-beam connections

Abstract

This study entailed an improvement in fracture assessment based on mixed-mode loading. Brittle fractures can be evaluated using the Weibull stress regardless of the differences in plastic constraints. However, the Weibull stress is proposed for cracks subjected to open loading (mode I), and it does not consider the effects of in-plane (mode II) or out-plane shear loadings (mode III). Structural connections under large deformations are subjected to modes I, II, and III at the crack tip. Fracture tests on beam-to-diaphragm connections in buildings have indicated that failure occurs when ductile cracks propagate from the strain concentration region of a weld access hole, where mode II dominates. Additionally, they cannot be appropriately evaluated using the conventional Weibull stress approach. This study investigated the relationship between mode differences and fracture initiation for several specimens modeled as beam-to-diaphragm connections. A mixed-mode ratio with stress intensity factor was used as an index to evaluate the mode differences. The fracture prediction considers the differences in modes by correcting the evaluated values on the basis of the mixed-mode ratio. The correction results shows more reasonable fracture prediction than conventional methods for beam-to-column connections under different modes.