Study on fatigue crack propagation and extrinsic toughening of an Al-Li alloy

The orientation dependence of long fatigue crack propagation was investigat ed for an aluminum-lithium (AI-Li) alloy (1420) at a stress ratio of 0.1. T he methods used were three-point bending, compact tension and in situ pulsa ting fatigue in the scanning electron microscope (SEM) chamber. The in situ fatigue observations were emphasized on crack path morphology during fatig ue. The extrinsic toughening was analyzed for different plane orientations based on the fracture mechanics. It was indicated that, AI-Li alloy 1420, a multi-layer microstructure, can be regarded as an in situ laminated compos ite. The interface of the laminate plays a significant role on the fatigue crack growth. The crack path was governed by the weak interface of the lami nated structure with the increase of the closure force. The crack propagati on arresting is observed at the range of Delta K 3&4 and 6&9 MPa m(1/2) for the L-T and T-S orientations. respectively. The delamination toughening an d arresting toughening are the predominant extrinsic toughening mechanisms for the L-T and T-S orientations. (C) 2000 Elsevier Science S.A. All rights reserved.