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

The orientation dependence of long fatigue crack propagation was investigated for an aluminum-lithium (Al-Li) alloy (1420) at a stress ratio of 0.1. The methods used were three-point bending, compact tension and in situ pulsating fatigue in the scanning electron microscope (SEM) chamber. The in situ fatigue observations were emphasized on crack path morphology during fatigue. The extrinsic toughening was analyzed for different plane orientations based on the fracture mechanics. It was indicated that, Al-Li alloy 1420, a multi-layer microstructure, can be regarded as an in situ laminated composite. 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 laminated structure with the increase of the closure force. The crack propagation arresting is observed at the range of ΔK 3 and 4 and 6 and 9 MPa m( 1/2 ) for the L-T and T-S orientations, respectively. The delamination toughening and 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.