The failure analysis of adhesively bonded joints

A new model to predict the failure load of adhesively bonded joints is proposed. Anextensive finite element analysis and experimental study of single and double lap jointswith aluminium and composite adherends is first undertaken to validate the model.Emphasis is then placed on understanding the behaviour and predicting the failure loadofbondedjoints with thin postbuckling composite skins.The design of structural adhesively bonded joints is complicated by the presence ofsingularities at the ends of the joint and the lack of suitable failure criteria. Literaturereviews indicate that bonded joint failure typically occurs after a damage zone at the endof the joint reaches a critical size. For bonded joints suffering cohesive failure thedamage zones form in the adhesive. For bonded composite joints suffering inter/intralaminarfailure the damage zones form in the adherends. Based on these findings adamage zone model using a critical damage zone size and predominantly strain basedfailure criteria is proposed to predict the failure load of adhesively bonded joints. Thedamage zone model correctly predicts the joint failure locus and appears, up to a point,to be insensitive to finite element analysis mesh refinement. Results from experimentaltesting of various graphite/epoxy composite and aluminum lap joints are obtained andcompared with finite element analysis. Good correlation of experimental andnumerically predicted joint failure loads are achieved.While the damage zone model can be classed as a scientific approach which explains thephysical process of failure, a modified damage zone model is also developed whichallows the model to be implemented in a practical engineering analysis environment. Anumber of failure criteria are evaluated in conjunction with the modified damage zonemodel.Using the knowledge and experience gained from the research into the analysis ofadhesively bonded joints, failure of skin-to-rib/stiffener joints in postbuckling structuresis then investigated both experimentally and numerically. Good correlation ofnumerically predicted joint behaviour and experimental test results is obtained for bothdirect compressive buckling and shear buckling skins. This correlation shows that thepostbuckling behaviour of stiffened panels can be accurately modelled using the finiteelement method. Bonded joint failure predictions for these joints are made with thedamage zone model to validate it for more complex structures. The results from thisinvestigation show that bonded joints that form hard points in postbuckling structurescan induce premature failure.It is concluded that the damage zone model can be successfully applied across a broadrange of aluminium and composite joint configurations and loading conditions. Toimplement the damage zone model for this broad range of joints, an adhesive peel strainfailure criterion is shown to be consistently accurate. Because this criterion is applied tothe adhesive, detailed modelling of composite plies is not required and therefore it canbe implemented relatively simply using either finite element analysis or basic onedimensionalshear lag algorithms