Prediction of fatigue thresholds in adhesively bonded joints using damage mechanics and fracture mechanics

The prediction of fatigue threshold in composite adhesively bonded joints using continuum damage mechanics (CDM) and fracture mechanics (FM) approaches has been investigated. Two joint types were considered in this study: double lap (DL) and lap strap (LS) joints. The substrates, which were made of uni-directional (UD) or multi-directional (MD) composite laminates, were bonded together using an epoxy film adhesive. The joints were tested under fatigue loading with a load amplitude ratio of 0.1 at various test temperatures. Damage evolution laws were derived using thermodynamics principles. The number of cycles to failure was then expressed in terms of the stresses in the adhesive layer and material constants. The stresses were calculated from non-linear finite element analyses, considering both geometrical and material non-linearities. The damage laws generated for the UD/DL joint data were then used to predict the fatigue crack initiation thresholds for the MD/DL, UD/LS, and MD/LS joints. The FM approach uses the crack closure integral method to compute the strain energy release rate at the threshold load (Gth) from the results of geometrical non-linear finite element analysis. The Gth value for an inherent crack size at the centre of the bond-line in the UD/LS joint is used as the failure criterion in order to predict the fatigue threshold for the MD/LS, UD/DL, and MD/DL joints. It was found that the predictions using CDM were slightly more accurate than those obtained using the FM approach. In general, when predicting the fatigue thresholds of the LS joints using the DL joints data, or vice versa, good agreement was obtained between the measured and predicted thresholds at ambient and low temperatures, but poor agreement was seen at the high test temperature. This was attributed to the deleterious effect of creep, which was greater in the DL joints than in the LS joints.