Failure Prediction of Bonded Joints of Composite Engineering Structures

Abstract

Abstract First part of the paper presents brief review of fracture mechanics applications to progressive failure prediction in adhesive bonded joints under mechanical loading end environmental effects (temperature and moisture) typical for the aircraft and civil engineering structures. Existing concepts of critical energy release rates and critical stress intensity factors for fracture modes I and II are evaluated in the context of failure and durability of adhesive bonded joints. The challenging problem of reliable experimental fracture toughness characterization of bonded laminated composites is emphasized. Second part of the paper presents novel development of 3-D variational analysis aimed at predicting crack propagation in adhesive composite bonded joints exposed to mechanical loading combined with variable temperature and moisture. A general 3-D variational thermo-hygro-mechanical approach is developed, based on the principle of minimum total potential energy. A brick-type composite mosaic model is allowing to incorporate the material property variation in all three coordinate directions. The displacement field in each of the bricks is approximated in terms of triple series with Bernstein basis functions of an arbitrary degree used in all the coordinate directions. In the case of no pre-existing cracks, all of the bricks are perfectly bonded through the displacement continuity conditions. If there is a 2-D surface crack perpendicular to one of the coordinate axes (the crack can be arbitrarily located inside the structure, including any part of the interfaces or bondlines), then the respective displacement continuity conditions are released along the corresponding surface area. A progressive failure analysis approach is then developed using the energy release rates evaluated for various possible scenarios of a cohesive, adhesive or interlaminar crack propagation. Several cracks of the same type or different types can be analyzed simultaneously. Numerical examples illustrate application of the developed analysis approach to double-lap adhesive bonded joints with cross-ply laminated composite adherends under some typical thermomechanical loading cases. The moisture effect is incorporated through the time-dependent critical energy release rates for Modes I and II. Several pre-existing cracks are introduced (separately or conjointly), and their propagation is followed using the computed strain energy release rates around the crack tip. Importantly, each of the distinct failure modes (adhesive, cohesive or interlaminar) is characterized by its specific critical strain energy release rate sensitive to the penetrating moisture. Numerical results and their comparison with available experimental data show that the developed analysis approach is capable of predicting the paths of dominating failure modes in adhesively bonded laminated composite joints.