Delamination-driven failure processes in two-dimensional composite structures: Delamination growth, crack kinking and nonlinear buckling

Delamination is an important mode of failure in laminated and sandwich composites. This study describes a cohesive layer model which has been successfully employed to predict the initiation and track the growth of delamination. A significant feature of the present model is that it can be used for geometrically nonlinear problems as it is formulated in terms of appropriate stresses and strains. A finite element approach which could account for the contact between delaminated surfaces as well as the progressive failure of the cohesive layer was employed to study several test cases. As a preliminary, examples of a double cantilever and a compressed beam specimens were studied in detail to identify the role of the key parameters of the model, viz. the thickness of the cohesive layer and the strength and stiffness of the cohesive layer material. It is found that the model is fairly robust and is not sensitive to changes in parameters other than the critical strain energy release rates in the opening and shearing modes respectively. This was followed by an investigation of delamination growth in columns and rings made of laminated composite material as well as sandwich columns. A dynamic analysis incorporating appropriate damping with a sufficiently slow rate of application of load was implemented to closely simulate quasi-static loading. Experimental results are found to corroborate the accuracy of the model. In laminated composites, matrix cracking was found to have a significant effect in the advanced stages of loading history and this has been accounted for by the implementation of a micro-mechanical model installed in the material in conjunction with the cohesive layer model placed along the potential delamination. Better correlation with experimental results was thus achieved.; It was observed in experiments that the interfacial crack in sandwich structures may not remain at the interface and tends to kink into the core. A kinking model which is based on identifying the crack planes and treating the cracking material as “cohesive” has been developed. The model was able to capture the delamination kinking into the core, directional changes in the crack growth, the return of the crack to the facesheet-core interface, and finally the interfacial delamination propagation. Experimental results are found to corroborate the accuracy of the model.; A feature of the compressive behavior of the structures carrying delamination is delamination buckling. Delamination buckling can interact adversely with other characteristic forms of buckling of the structure, such as the face sheet wrinkling and overall buckling in sandwich columns. It is shown that such interactions can lead to premature collapse. It is also shown that it is important to account for the contact between the delaminated surfaces in the analysis...