Simple and accurate fracture toughness testing methods for pyrolytic carbon/graphite composites used in heart-valve prostheses.

The fracture toughness is a critical material property for the pyrolytic carbon materials used in mechanical heart-valve prostheses; however, making accurate toughness measurements has traditionally been problematic due to difficulties in fatigue precracking specimens. In this work, a simple, effective, and reliable precracking method is presented where a sharp precrack is "popped in" from a razor micronotch, which allows significant savings of time and materials relative to fatigue precracking methods. It is further shown that equivalent results may be obtained using razor micronotched specimens directly without precracking, provided the notch is sufficiently sharp. Indeed, mean toughness values of 1.46+/-0.13 and 1.35+/-0.09 MPa radicalm were obtained for the pyrolytic carbon-coated graphite materials, using precracked and razor micronotched specimens, respectively. The difference between these mean values proved to be statistically insignificant, and these values are in general agreement with published fracture toughness results obtained using fatigue precracked specimens.