Lifetime Prediction for Fatigue Damage in Bonded Joints

A procedure is outlined and programmed in software in order to predict the fatigue threshold in composite adhesively bonded joints. Two different joint configurations are considered in this study, namely, Double Lap (DL) and Lap Strap (LS). The substrates are made of either uni-directional (UD) or multi-directional (MD) composite laminates and bonded together using an epoxy film adhesive. The theory of continuum damage mechanics has been used to develop damage evolution laws for UD/DL joints at different temperatures. The number of cycles to failure was then expressed in terms of the stresses in the adhesive layer and material constants (damage parameters). The stresses were calculated from non-linear finite element analyses. In order to calculate the damage parameters, a system of equations, in which the number of unknowns is less than the number of equations, has been derived using a combination of experimental data, finite element results and the damage law. The system of equations has been solved using the concept of pseudo-inverse by solving a singular value decomposition problem. Software has been developed to automatically calculate the damage parameters and produce the required load-number of cycle to failure curves. From those curves, the fatigue crack initiation thresholds were predicted for several joints, e.g. MD/DL, UD/LS and MD/LS joints. Good predictions were found.