Determination of the mode II fatigue delamination toughness of laminated composites

A study was conducted with the aim of determining a criterion for defining the onset of delamination growth for mode II loading using the end-notched flexure (ENF) test, and then using this criterion to develop a mode II toughness versus number of cycles diagram for use in design. Unidirectional graphite/epoxy specimens were fatigue tested in load control at various percentages of the mode II fracture toughness. These tests were stopped whenever a compliance change was observed, at which point the specimens were removed from the test fixture and ultrasonically inspected. The specimens were then returned to the ENF fixture for additional testing, and the process repeated. In this manner, it was determined that a 2% compliance change was necessary for crack advance to occur across a specimen's full width. Additional tests were then conducted, and the 2% compliance change criterion used, to develop a mode II fatigue initiation toughness curve, i.e., a plot showing fatigue initiation toughness versus the number of loading cycles.