Towards the fundamentals of mode II fatigue delamination growth

The use of laminated composite materials in primary structures is still limited by the occurrence of in-service delaminations. Considering that interlaminar shear is one of the predominant loads experienced by composite structures, understanding the damage mechanisms involved in mode II delaminations is crucial for the development of a damage tolerance philosophy. Therefore, this work examines whether the energy dissipated in the process zone ahead of the crack tip should be accounted for when assessing fatigue in-plane shear delaminations. ENF tests were performed and the results showed that damage propagates ahead of the main crack tip in a process zone, which dissipates energy that should be accounted for when characterizing mode II delamination growth. It is not possible to measure appropriately the extent of the process zone in an ENF specimen from observations made from its side, and the definition of a crack tip is not advisable in mode II delamination studies. Instead, an effective crack length, which includes the damaged zone ahead of the main crack tip, should be defined. More studies are necessary in order to understand and define appropriately the extent of the process zone in mode II delaminations.