Mode I Interlaminar Fracture Toughness of Unidirectional Carbon Fiber Composites Using a Novel Wedge-Driven Delamination Design

The mode I interlaminar fracture behavior of unidirectional carbon fiber composites with PMR-15 polyimide and PEEK (polyetheretherketone) matrix resins has been studied at various temperatures and testing rates. These materials were evaluated for strain energy release rate, G_I, using a novel wedge-driven delamination (WDD) design. In this WDD configuration, G_I is directly proportional to the measured load necessary to advance the sample onto the wedge. The crack front remains essentially stationary in the laboratory reference frame during steady crack growth, facilitating the monitoring of the fracture process. Furthermore, the crack velocity can be set to any desired value by controlling the machine crosshead speed. Steady crack growth was observed in the polyimide laminates, which exhibited an average fracture toughness of 370 J/m² at 25°C. The measured G_I values were slightly dependent on test rate and showed an increase in toughness at increasing temperatures in the range of 25 to 200°C. The toughening mechanism of bundle pullout seemed to be more dominant at higher temperatures, possibly accounting for the higher toughness values. The PEEK laminates exhibited the stick/slip crack growth behavior typical of ductile matrix composites. Slow growth regions were identified by whitened areas on the fracture surface, while the fast fracture areas remained unwhitened. The strain energy release rates of the composite corresponding to both the onset of fast fracture and the arrest of the crack were measured as a function of temperature and rate. While the arrest G_I values were fairly independent of temperature and rate, the G_I values for instability onset increased with increasing temperature.