An increase in apparent mode II fracture toughness in Fiber Reinforced Polymers has been found from modified Transverse Crack Tension (mTCT) tests with compressive stresses added orthogonal to the delamination plane. Numerical reproduction of these results has not yet led to a good understanding of the reasons for this behavior. Currently, an analytical approach is used for determining the mode II fracture toughness from an mTCT test. In this research, the validity of the analytical relation is investigated for the case of an added compressive stress. The behavior of the material in the
test is therefore numerically investigated in a qualitative manner. A mesh, using a material model for plasticity and a cohesive law for interface elements to form the crack, which have been proven to give good results, has been build. A refinement on mesh and geometrical dimensions, focused on the pressure zone of the specimen, ensures the degree of detail required to investigate the local behavior. A parameter study on the inlfuence of geometrical and material parameters is performed and linked to results from previous conducted experiments. A standard mTCT test without compressive stress shows a linear load displacement relation until crack initiation and then a constant load at which the crack grows through the specimen. This crack growth load is used for the analytical approach. The results from this research show a change from a constant crack growth load to a linear crack growth load range in the zone of the applied compressive stress. After crack initiation, the crack growth load through the compressed zone has an increasing, nearly linear relation, based on the compressive stress and length of the pressure zone. The analytical approach, in
which the highest reached load on the specimen before failure is used as input, no longer applies for this case. The failure load is now dependent on the length of the pressure zone and pressure magnitude.