Numerical Simulation of the Fatigue Damage Growth in Unidirectional Composites based on Fibre-Matrix Debonding

  • Seidel, Alexander (Technical University of Munich)
  • Sørensen, Bent F (Technical University of Denmark)
  • Drechsler, Klaus (Technical University of Munich)

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The structural design of composite parts with respect to their fatigue behaviour is usually carried out utilising empirical design approaches. Therefore, macroscopic properties are determined experimentally by testing coupon specimens. Nevertheless, this method is time-consuming and usually underestimates the actual fatigue capabilities as side effects (like failure initiating in the tab region) lead to a shorter observed fatigue life than the actual fatigue life of the material itself. As described by e.g. Jespersen and Mikkelsen [1], fatigue in unidirectional composites manifests as a progression of adjacent fibre breaks starting at an initial point (usually near backing bundles). Additionally, it is also known that broken fibres introduce a fibre-matrix debonding which grows cyclically and creates a stress field at the debond crack tip. Sørensen et al. [2] suggested that this stress field can cause a fibre break in an adjacent fibre and thus drive the microscopic fatigue damage. The present work builds upon the analytical model of Sørensen et al. to describe the progressive fatigue failure and extends it to a finite element simulation. That way, certain simplifications made in the analytical model can be overcome (e.g. elastic to plastic matrix, hexagonal packing to statistical fibre distribution) and the process can be studied in more detail. The approach simulates the progressive fibre-matrix debonding of a single fibre during cyclic loading. The intact interface is modelled by inserting a layer of cohesive elements between the fibre and the matrix. The debond growth and with it the cycle-dependent shear stress in the debonded region is included by replacing failed cohesive elements in the debonded region by cohesive elements with an adapted material model to represent a slipping contact. Additionally, the model of a single fibre can be extended to a multiple-fibre configuration… [1] Kristine M. Jespersen and Lars P. Mikkelsen. Three dimensional fatigue damage evolution in noncrimp glass fibre fabric based composites used for wind turbine blades. Composites Science and Technology, 153:261–272, 2017. [2] Bent F. Sørensen, Stergios Goutianos, Lars P. Mikkelsen, and Søren Fæster. Fatigue damage growth and fatigue life of unidirectional composites. Composites Science and Technology, 211:108656, 2021.