Metal particles constraint in glass matrix composites and its impact on fracture toughness enhancement

Experimental values of the fracture toughness obtained for borosilicate glass reinforced by vanadium or molybdenum particles fall far behind to the theoretical expectations according to the crack bridging view. A theoretical traction/separation law accounting of the stress triaxiality developed in particles due to limited debonding from the matrix was used together with a weight-function-based approach to study the contribution of such tractions on the magnitude of the stress intensity at the crack tip in chevron-notched specimen. The results calculated from the experimentally found maximum load provide a very short bridging zone of the order of particle size. Yet the corresponding restraining stress intensity twice exceeds the applied stress intensity factor. It is concluded that the crack bridging mechanism (extrinsic toughening) does not describe the observed toughening satisfactory. It is suggested that the change of the toughening mechanism from crack bridging to crack trapping (intrinsic toughening) can explain the experimental observations. This change closely relates to the extent of particle/matrix debonding which depends both on internal factors like debonding toughness, particle size and/or volume fraction, and temperature.