Predicting Fatigue Crack Growth Life in Z-fibre Reinforced Composites

  • Suchao Tang

    Student thesis: Doctoral ThesisDoctor of Philosophy


    In the past three decades, the quest to produce lightweight structures has led to an increasing use of fibre reinforced polymer composite materials. A long-standing problem with composite laminates is their low delamination resistance and poor impact damage tolerance due to the lack of through-thickness reinforcement. Z-fibre (or known as Z-pinning) is an effective and simple method of increasing the delamination resistance and impact damage tolerance of composite laminates. Although the prediction methods of z-pinned structures under static load have been well developed in literature, few papers can be found that address the prediction of z-pinned structural response under cyclic loading.

    The aim of this PhD project is to develop a methodology for accurately predicting the fatigue life of pin reinforced laminates, beyond the current state of the art. In order to achieve this aim, work has been undertaken to meet the following objectives:

    • To model existing laminates (unpinned and pinned), to understand modelling techniques such as the virtual crack closure technique (VCCT) and the cohesive zone method (CZM), and to validate models against examples in literature;
    • To develop a fatigue degradation law to account for the reduction of pins’ bridging force under cyclic loading;
    • To implement the degradation law to the prediction framework and predict the fatigue crack growth rate and life of pin-reinforced composite laminates;
    • To conduct experimental tests to validate the accuracy of the proposed prediction methodology.

    The scope of this work is limited to pin-reinforced laminate and simple joint subjected to the mode I loading. A fracture mechanics based approach is proposed to predict fatigue crack growth life of z-pinned joints, which uses the finite element method (to deliver the crack tip strain energy release rate) in conjunction with a crack growth rate law. The strain energy release rate is evaluated by the VCCT method in finite element analyses, and cohesive elements are used at discrete pin locations to represent the z-pin bridging forces. The cohesive parameters within the FE model are degraded with the cycle numbers to account for the property degradation and bridging force reduction during fatigue loading.

    A degradation law is proposed to describe the process of a z-pin debonding and frictional pull-out from laminate under the mode I fatigue loading, which is based on the damage mechanics approach with empirical fitting parameters.

    As demonstration examples, the fatigue crack growth life of a pin-reinforced double cantilever beam (DCB) and a pin-reinforced “top-hat” joint have been predicted by the proposed methodology. The predicted results are validated by experimental test results; both the crack length vs. fatigue life curve and fatigue crack growth rate vs. crack length curve are found to be in good agreement.
    Date of Award2019
    Original languageEnglish
    Awarding Institution
    • Coventry University

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