Energy dissipation in mode II fatigue crack 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 delaminations caused by in-plane shear. ENF quasi-static and fatigue tests were performed and the results show that damage propagates ahead of the crack tip in a process zone. Acoustic Emission was used to verify that the process zone dissipates energy which should be accounted for when characterizing mode II delamination growth. The extent of the process zone in an ENF specimen cannot be measured by the means of visual observations made from the side of the specimen. Therefore, the definition of a crack tip is not recommended in mode II delamination studies. Instead, an effective crack length that includes the damaged zone ahead of the crack tip should be defined. More studies are necessary to understand and quantify fracture in mode II delamination growth before developing methods to assess it using fracture mechanics.