Failure of Sandwich Structures with Sub-Interface Damage

by Shipsha, Andrey

This thesis addresses some aspects concerned with fatigue behaviour and damage tolerance of foam core sandwich structures with composite faces. Two different foam cores typically used in aeronautical and marine applications are considered. The focus is on face/core interface cracks (debonds) and sub-interface impact damage in the core; the face sheet is regarded undamaged. The first part of the thesis (Papers A, B and C) is concerned with fatigue behaviour of cellular foams under constant and shedding load-amplitude. Mode I and mode II fatigue crack growth is experimentally investigated. Stress intensity factors at the crack tip are computed using the finite element method and assuming validity of linear elastic fracture mechanics. The fatigue crack propagation is characterised in terms of the Paris’ law equations and the fatigue threshold ?Kth is experimentally evaluated. The obtained crack growth rates are shown to be very high, especially under mode I loading, suggesting that a conventional damage tolerance approach may be impractical for design purposes. Instead, a design based on keeping the applied stress intensity range below the fatigue threshold ?Kth may be recommended. In the next part (Papers D and E) the focus is shifted to analysis of residual static strength and failure mechanisms of sandwich beams with impact damage. An idealised 2D damage configuration is investigated. Two typical failure modes are considered; shear failure of the core and local buckling of the face sheet. Failure mechanisms of impactdamaged beams and the effect of bridging in the peripheral regions of impact damage are thoroughly examined. The properties of the crushed core, underlying the impact damage, are experimentally determined. A parametrised finite element model of impact damage with implemented crushed core and bridging conditions in the peripheral regions of impact damage is developed. A point-stress criterion is used for post-impact fracture analysis. The model shows good agreement with post-impact experimental results. The last paper links the two aforementioned parts of the thesis addressing the fatigue life of impact-damaged sandwich beams under constant load-amplitude. A tentative approach is suggested for estimation of the threshold load levels for sandwich beams with known impact damage size. It is based on a modified point-stress criterion coupled with experimentally measured fatigue threshold stress intensity range ?Kth. The proposed approach shows good correlation with experiments and may be applied for evaluation of maximum allowable load levels during the preliminary design of sandwich structures.