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The influence of aging and grain morphology on the fracture behavior of an Al-2.6Li-0.09Zr alloy was investigated by varying the extrusion parameters that affect grain structure; namely, extrusion ratio and product geometry. Fracture specimens were partially fractured and serially sectioned to characterize crack progression into the plastic zone and to assess the interaction between the crack front and the grain structure. Fracture in the higher toughness, underaged condition is characterized by crack tunneling and grain boundary microcracking in a zone surrounding the continuous crack surface. The fracture process for the overaged material is distinguished by large, intergranular delamination cracks perpendicular to the main fracture surface and extending deep into the plastic zone. This delamination cracking phenomenon was investigated using linear, two-dimensional finite-element simulations of the delaminations at the crack tip. The results indicate that when a delamination crack is longer than one half of the section thickness, the stress intensity factor (KI) for the delamination crack is independent of crack length. Furthermore, a section with multiple delamination cracks has a higher load carrying capacity than a section with only one crack, assuming that failure is KI dependent. This delamination formation reduces through-thickness constraint and hastens development of plane-stress conditions. The effect of this delamination cracking on the toughness of aluminum-lithium (Al-Li-X) alloys is in accordance with the functional form of the Bilby, Cottrell, and Swinden model developed for plane-stress fracture and defined by Kc ∞ (B1Eϕf) where B1 is delamination spacing, E is Young's modulus, and ϕf is the flow stress.
fracture behavior, fracture toughness, delamination, aluminum-lithium alloys, grain morphology, delamination toughening, plane-stress fracture, thickness effect, finite-element modeling, fracture mechanics, fatigue (materials)
Research associate, University of Bristol, Bristol,
Professor, School of Mechanical Engineering, Purdue University, West Lafayette, IN
Professor, School of Materials Engineering, Georgia Institute of Technology, Atlanta, GA