13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Mode I Fatigue Crack Growth Behaviour in a Welded Cruciform Joint under Biaxial Stresses Xiang Zhang1,*, Haiying Zhang2, Rui Bao3 1 Department of Aerospace Engineering, School of Engineering, Cranfield University, Bedford, MK43 0AL, UK 2 Aircraft Strength Research Institute, Xi’an, 710065, P.R. China 3 Institute of Solid Mechanics, School of Aeronautic Science & Engineering, Beihang University, Beijing, 100191, P.R. China * Corresponding author: xiang.zhang@cranfield.ac.uk Abstract Biaxial load fatigue crack growth tests were performed for a cruciform joint made of aluminium-lithium alloy 2198-T8 containing a butt weld joint fabricated by the friction stir welding process. In all tests crack propagated in parallel with the weld joint. Two material rolling directions were studied in relation to the welding and crack growth direction, showing that the material rolling direction affects the crack growth path. Specimens welded orthogonally to the rolling direction exhibit shorter fatigue life than specimens welded parallel to the rolling direction. A simple method is then developed for predicting crack growth rate. FEM is used to calculate the stress intensity factors and T stress, which are subsequently used to predict the crack propagation rates and trajectory. Influence of applied stress biaxiality on stress intensity factors, T stress, crack trajectory and growth rates have been analysed. Out-of-plane bending caused by the specimen geometry is also modelled. Predicted and test measured crack growth lives are in reasonably good agreement. Reasons for discrepancy are discussed. Keywords: Biaxial stresses, weld joint, crack propagation, fatigue, finite element method, aluminium alloy 1. Introduction Trend in aircraft structural design and manufacture has been moving towards more integral structures by extrusion, machining and welding rather than the traditional mechanical fastening. Fatigue crack growth behaviour under biaxial stresses is complex even without weld joints. Welding will add additional challenge in the analysis due to the process induced residual stress and changes in weld metal microstructure and mechanical properties. Fatigue crack growth behaviour in aluminium welded joints has been investigated by experiments [e.g. 1-2], analysis [3], or modelling [4-7]. These research efforts were carried out under the uniaxial load cases. Current state-of-the-art in predicting crack growth rates is to use empirical crack growth laws and incorporate the effective stress intensity factor ratio. Biaxial load fatigue crack growth behaviour (without weld) has been investigated [8-16] for various applied biaxial load ratios (k = y/ x). In summary, qualitatively similar results have been reported for a number of steel and aluminium alloys that crack growth rate is higher at k=-1 (pure shear), than k=0 (uniaxial) followed by k=1. Difference in the growth rates in different biaxial stress ratios depends also on the magnitude of applied stress. In terms of crack growth trajectory, crack was kept straight at k≤1 [10-11]. Crack growth deviation was found to be dependent on the KI/T ratio [13-15]. The maximum tangential stress criterion (function of KI and KII) is widely used for crack turning analysis [17]. The criterion is implemented in some FE codes, e.g. ABAQUS and FRANC2D/3D. The influence of weld joint (in terms of welding residual stress and microstructure change) on crack
RkJQdWJsaXNoZXIy MjM0NDE=