Variable mixed-mode delamination in composite laminates under fatigue conditionstesting & analysis

Abstract

Most of the failures in structural elements in use are a consequence of mechanical fatigue. Therefore, fatigue is a decisive factor in designing durable mechanical elements. In laminated composite materials, the fatigue process involves different damage mechanisms that result in the degradation of the material. One of the most important damage mechanisms is the delamination between plies of the laminate. In aeronautical applications, composite plates are sensitive to impact and delamination occurs readily in composite laminates on impact. Many composite components have curved shapes, tapered thickness and plies with different orientations, which make the delamination grow with a mode mix that depends on the extent of the crack. Thus, delaminations generally grow under varying mode mix. It is therefore important to develop methods that can characterise subcritical, mixed-mode growth in fatigue delamination. The main objective of the present investigation is the characterisation of the variable mixed-mode delamination in composite laminates under fatigue conditions. To this end, a mixed-mode fatigue delamination model is proposed. Oppositely to the mixed-mode fatigue delamination models present in the literature, the proposed model takes into account the non-monotonic variation of the propagation parameters with the mode mix observed in different experimental data. Moreover, the mixed-mode end load split (MMELS) test, which main characteristic is that the propagation mode of the interlaminar crack varies with the crack extent, is analysed. Two theoretical approaches present in the literature are considered. However, the resulting expressions for the MMELS test are not equivalent and the differences between approaches can be up to 50 times. A more accurate alternative analysis of the MMELS test is carried out in the present study for comparison. The alternative analysis is based on the finite element method and the virtual crack closure technique. Significant findings are found for precise materials characterisation using the MMELS test. A MMELS test rig is also designed and built. Different specimens of essentially unidirectional carbon/epoxy laminates are tested for the experimental characterisation of fatigue delamination under varying mode mix. A fractographic analysis is also conducted in some of the delaminated fracture surfaces. The experimental results are compared to the predictions of a proposed model for the fatigue propagation of interlaminar cracks.