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

The prediction of fatigue threshold in composite adhesively bonded joints u sing continuum damage mechanics (CDM) and fracture mechanics (FM) approache s has been investigated. Two joint types were considered in this study: dou ble lap (DL) and lap strap (LS) joints. The substrates, which were made of uni-directional (UD) or multi-directional (MD) composite laminates, were bo nded together using an epoxy film adhesive. The joints were tested under fa tigue loading with a load amplitude ratio of 0.1 at various test temperatur es. 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 fr om non-linear finite element analyses, considering both geometrical and mat erial non-linearities. The damage laws generated for the UD/DL joint data w ere 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 integr al method to compute the strain energy release rate at the threshold load ( Gth) from the results of geometrical non-linear finite element analysis. Th e Gth value for an inherent crack size at the centre of the bond-line in th e UD/LS joint is used as the failure criterion in order to predict the fati gue 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 usin g the FM approach. In general, when predicting the fatigue thresholds of th e LS joints using the DL joints data, or vice versa, good agreement was obt ained between the measured and predicted thresholds at ambient and low temp eratures, but poor agreement was seen at the high test temperature. This wa s attributed to the deleterious effect of creep, which was greater in the D L joints than in the LS joints.