Delamination cracking in advanced aluminum-lithium alloys - Experimental and computational studies

Delamination cracking in advanced aluminum-lithium (Al-Li) alloys plays a dominant role in the fracture process. With the introduction of these materials into components of aerospace structures, a quantitative understanding of the interplay between delamination cracking and macroscopic fracture must be established as a precursor to reliable design and defect assessment. Delamination cracking represents a complex fracture mechanism with the formation of transverse cracks initially on the order of the grain size. In this work, interrupted fracture toughness tests of C(T) specimens, followed by incremental polishing, reveal the locations, sizes and shapes of delamination cracks and extensions of the primary macrocrack. These observations suggest that delamination crack sizes scale with loading of the primary crack front expressed in terms of J / σ₀. Using a 3-D, small-scale yielding framework for Mode I loading, a companion finite element study quantifies the effects of prescribed delamination cracks on local loading along the macroscopic (primary) crack and ahead of the delamination cracks. An isotropic hardening model with an anisotropic yield surface describes the constitutive behavior for the 2099-T87 Al-Li alloy plate examined in this study. The computational results characterize the plastic zone size, the variation of local J ahead of the macrocrack front and the stress state that serves to drive growth of the macrocrack and delamination crack. The computational studies provide new, quantitative insights on the observed increase in toughness that has been observed during fracture experiments caused by delamination cracks that divide the primary crack front.