• cohesive onset delamination

This thesis work focuses on the study of a damage model to be applied to cohesive elements for mode II fatigue delamination for unidirectional laminates (UD HTA/913C). Composite aircraft structures are subjected to a damage mechanism called delamination. The latter can be caused by the presence of hygrothermal loads, assembly problems, maintenance, delaminations created during production. Extensive literature shows that delamination mainly occurs in the opening (Mode I) and shear (Mode II) modes. Once the ENF (End-Notched Flexure) specimen was defined, for the study of mode II delamination, a two-dimensional finite element model was developed through the implementation of a local damage law of the cohesive elements in the Abaqus program. The constitutive behavior of the cohesives is implemented through an external subroutine written in Fortran language (UMAT), which allows to numerically simulate the experimental tests. The first phase of the experimental test predicts a precracking, in which the simulated cohesive elements respect the constitutive law of traction-separation defined by the program. For the fatigue phase, on the other hand, the constitutive behavior of the cohesive element is described by the subroutine. The following work has allowed us to verify the implementation of the UMAT, in the Abaqus environment, to adequately simulate the degradation of cohesive elements in fatigue-stressed composites in the presence of delamination at each load level imposed in the experimental tests.