Experimental and Numerical Characterisation of Fatigue Damage in Z-pinned Composite Structures

  • Felix Warzok

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)


A thorough micro-mechanical characterisation of the mode I and mode II fatigue behaviour of Z-pins was conducted. Z-pin failure during mode I fatigue loading depends on Z-pin misalignment, fatigue amplitude and embedded Z-pin length. No failure was observed for misalignments below 5° for a nominal embedded Z-pin length of 8~si{millimeter}. Reductions of the post-fatigue pull-out force and energy dissipation depend on fatigue amplitude and range between 0% and over 80%. SEM-analysis of fatigued Z-pins show that large amplitudes lead to a significant loss of fibres and a reduction of Z-pin diameter, by over 10%.
For mode II only small fatigue displacements could be applied without Z-pin failure. The behaviour was found to be dominated by the largest fatigue displacement experienced. No influence of the applied number of fatigue cycles could be distinguished. The quasi-static post-fatigue peak load and failure displacement show small reductions below 10%. SEM images show the same failure mechanisms, longitudinal splitting and tensile fibre failure, in in fatigued and non-fatigued specimens.
The findings from the micro mechanical study were transferred into a phenomenological model and validated with bespoke meso-scale Double Cantilever Beam (DCB) and End Loaded Split (ELS) tests. For mode I, the specimens confirmed the micro-mechanical observations, leading to excellent agreement between simulation and experiment (differences between 5% and 10%).
ELS fatigue behaviour not exceeding the Z-pin mode II fatigue failure displacement could be predicted very well (differences < 10%). For higher displacements failure occurred gradually and predicting accurate curve shapes was not feasible. In a laminate without fibres oriented in 0° direction Z-pins could completely suppress displacement between the ELS-cantilever arms until catastrophic Z-pin failure occurred. The absence of shear displacement prevented the applicability of the proposed modelling solution.
Date of Award14 Nov 2017
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorStephen R Hallett (Supervisor) & Giuliano Allegri (Supervisor)

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