The CVI SiC/SiC composites develop a network of matrix cracks under load. The density of matrix cracks is enhanced by rather strong interfaces: the crack spacing may be as small as 10-20 pm whereas it is at least 10 times larger in the presence of rather

weak interfaces. Matrix cracking is an alternative mechanism of energy dissipation.

A process zone of diffused matrix microcracks is generated at the notch tip or at the tip of a preexisting main macroscopic crack. Extension of this crack results from the random failures of fiber bundles located within the process zone.35 Due to the pres­ence of a more or less large process zone associated with a jagged crack, a crack length cannot be defined and conventional concepts of fracture mechanics are not appropriate (stress intensity factor) or cannot be easily determined (strain energy release rate, У-integral). Although the validity of the stress inten­sity factor concept to measure fracture toughness is questionable, this is an interesting characteristic for comparing CVI SiC/SiC composites to other materials. Fracture toughness values on the order of 30MPaVm have been measured on single edge notch bending (SENB) test specimens.2, Strain energy

release rates ranging from 3 to 8 KJ m~2 have been determined on CVI SiC/SiC composites, respec­tively, with weak or strong interfaces.35 The cor­responding values of the У-integral ranges from 11 KJ m~ (weak interfaces) to 29 KJ m~ (strong interfaces).35 These values are quite high. The afore­mentioned stress intensity factors are maintained up to at least 1400 ° C.2