On Cohesive Modelling of Carbon/Epoxy Composites - Delamination and Fibre Compressive Failure
Doctoral thesis, 2015
Carbon Fibre Reinforced Polymers (CFRP) are widely used in engineering applications where weight saving and high mechanical performance are key factors. However, an inherent weakness of laminated CFRP:s is their relatively low resistance to delamination. The first part of this thesis is devoted to methods to extract cohesive laws associated with delamination. The method is based on fracture mechanical tests and measurement of the displacement field close to the crack tip. Pure mode cohesive laws are determined by an optimization procedure involving finite element (FE) simulations. An initiation based formulation allows for a straight forward determination of the mixed mode cohesive law. FE simulations show that the fracture loads and local displacements are in good agreement with the experiments.
The second part of the thesis is concerned with mixed mode cohesive modelling under small scale yielding conditions. Under such loading conditions, a robust cohesive model should conform to the predictions of linear elastic fracture mechanics. Both isotropic and orthotropic adjacent continuums under plane stress or plane strain are treated. By analytical derivations, it is concluded that two conditions are sufficient for mixed mode cohesive laws to achieve this property. These design rules address the choice of initial cohesive stiffnesses and the formulation of the softening response during mixed mode cohesive separations. Validating FE-simulations where SSY conditions are imposed on a circular domain support the results. It is also demonstrated that this minimizes the discrepancy to LEFM predictions for fracture mechanical specimens where boundary effects influence the elastic fields close to the crack tip.
The third part of the thesis focuses on a failure type entirely different from delamination: longitudinal compressive failure. In a CFRP, almost all the load is carried by the fibres. With compressive strengths along the fibre direction substantially lower than the tensile strength, prediction of longitudinal compressive failure plays a key role in the design of structural CFRP components. The dominating failure mode due to longitudinal compression is kink-band formation. The objective in the third part is to extract the cohesive law associated with kink-band formation. Equilibrium of configurational forces is used for this purpose. Identified configurational forces are continuously measured by monitoring the displacements field on the specimen's lateral surface. The kink-band is formed in the intended region of the specimen and the evaluation shows a peak stress and fracture energy in the anticipated ranges.
Cohesive zone model