Fatigue is a key factor for durable designs of composite structures. Delamination is the most important damage mechanisms for these materials. In many composite components delaminations grow under a varying mode mix that depends on the crack length. It is important to develop methods that can characterise subcritical, mixed-mode growth in fatigue delamination. The objective of this investigation is the characterisation of variable mixed-mode fatigue delamination in composite laminates. A mixed-mode fatigue delamination model is proposed. Opposite to literature mixed-mode fatigue delamination models, the proposed model assumes a non-monotonic variation of the propagation parameters with the mode mix. The mixed-mode end load split (MMELS) test, in which the propagation mode varies with the crack extent, is analysed. Two theoretical approaches present in the literature are considered. However, the resulting expressions are not equivalent and a more accurate alternative analysis is carried out based on the virtual crack closure technique. Significant findings are found for materials characterisation.

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