Mode I interlaminar crack propagation in continuous glass fiber/polypropylene composites: temperature and molding condition dependence

The mode I interlaminar fracture toughness of unidirectional continuous glass fiber/polypropylene composites above (23°C) and below (-40°C) the glass transition temperature of the PP matrix was investigated. Three molding conditions, leading to different levels of fiber dispersion and matrix microstructure, were studied. Fracture toughness testing performed employing double-cantilever beam (DCB) specimens showed that the molding conditions strongly influenced the fracture toughness of the composites studied. Similar values of fracture toughness were obtained at the two test temperatures investigated (23 and -40°C). Fractographic observations revealed that crack propagation differed significantly for each of the molding conditions and test temperatures studied. Depending on the fiber dispersion and matrix microstructure resulting from the different molding conditions, crack propagation occurred either at the fiber-matrix interface or in the matrix interspherulitic regions. A transition from stable crack propagation behavior at 23°C to unstable crack propagation behavior at -40°C was noted only when the crack propagated at the fiber–matrix interface. This effect was attributed to the ductile–brittle transition of the amorphous PP phase, more concentrated at the fiber-matrix interface, at the glass transition temperature near 0°C.