Fiber-reinforced polymer (FRP) composites are used in various engineering applications due to their many advantageous properties, but predicting failure remains challenging due to complex failure behavior. The Computational Mechanics group at TU Delft uses an inter-element cohesive zone method, referred to as the Ortiz model, to analyze microscale crack propagation. Although effective for modeling complex failure processes, the method suffers from severe mesh dependency, restricting large-scale simulations. This research implements and investigates the shifted fracture method (SFM) as an alternative to reduce mesh dependency while maintaining computational complexity. The SFM modifies key aspects of the Ortiz model, including an area correction term, crack propagation algorithm, and shifted weak form equations and cohesive zone conditions using Taylor expansions. Three versions of the SFM were tested: (1) full SFM implementation, (2) reduced SFM with only the area correction and crack propagation algorithm, and (3) an extended Ortiz model with only the area correction term. Numerical simulations show that the reduced SFM is robust, mesh-independent, and efficient in basic fracture tests. The area correction term proved critical in reducing mesh dependency. The extended Ortiz model, on the other hand, showed less accurate results in predicting cracks, despite the additions of the area correction term. Furthermore, the maximum principal tensile stress criterion for determining the crack direction was unsuitable for mixed-mode scenarios, resulting in inaccurate paths. Although the reduced SFM shows promising results, it requires further development to handle complex fracture scenarios like FRP composites. Features such as crack initiation, merging, and termination were implemented and showed potential but require further validation. This research concludes that while the reduced SFM is effective for basic testing, further refinements are needed for broader application to FRP composites.

According to experiments the fatigue damage behaviour of fibre reinforced polymers (FRP) experiences a load sequence effect. In this report the ability of the cohesive fatigue damage model of Dávila (2020) to simulate the behaviour after a load amplitude change is investigated. For this a double cantilever beam (DCB) with fictitious material properties is loaded in mode I. Two element layers with each its own cohesive law are used to take into account the different behaviour of the epoxy and the bridging fibres in the wake of the crack. The calculated crack growth rate is compared to the experiments done by Jensen et al. (2021), since the conditions of the simulations are most similar to that of their experiments.
When the base and bridge layer only differ in their quasi-static cohesive law no load sequence effect is seen, since the conditions at the start of the second load block are the same as for the constant amplitude test at the same crack length. By allowing fatigue damage to accumulate only in the base layer and the bridge layer to only experience quasi-static loading, a history effect is seen. However, the high-to-low transition does not give the same transition behaviour as seen in experiments. In addition, the results of the constant amplitude tests are unrealistic.
When the fatigue damage accumulation of the bridge layer is faster than that of the base layer, a transition behaviour comparable, although less profound, to that of experiments is seen. This difference in fatigue damage accumulation is achieved by adjusting the coefficients of the fatigue damage rate function to the values corresponding to another R-ratio. This result may indicate a possible underlying mechanism for the transition behaviour of FRP.