Cyclic fatigue-crack propagation, stress-corrosion, and fracture-toughness behavior in pyrolytic carbon-coated graphite for prosthetic heart valve applications
✍ Scribed by Ritchie, R. O. ;Dauskardt, R. H. ;Yu, Weikang ;Brendzel, A. M.
- Publisher
- John Wiley and Sons
- Year
- 1990
- Tongue
- English
- Weight
- 1002 KB
- Volume
- 24
- Category
- Article
- ISSN
- 0021-9304
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✦ Synopsis
Fracture-mechanics tests were performed to characterize the cyclic fatigue, stresscorrosion cracking, and fracture-toughness behavior of a pyrolyk carbon-coated graphite composite material used in the manufacture of cardiac valve prostheses. Testing was carried out using compact tension C(T) samples containing "atomically" sharp precracks, both in room-temperature air and principally in a simulated physiological environment of 37°C Ringer's lactate solution. Under sustained (monotonic) loads, the composite exhibited resistancecurve behavior, with a fracture toughness (K,J between 1.1 and 1.9 M P a f i , and subcritical stress-corrosion crack velocities (da/dt) which were a function of the stress intensity K raised to the 74th power (over the range to over m/s). More importantly, contrary to common perception, under cyclic loading conditions the composite was found to display true (cyclic) fatigue failure in both environments; fatigue-crack growth rates (da /dN) were seen to be a function of the 19th power of the stress-intensity range AK (over the range to over lo-* m/cycle). As subcritical crack velocities under cyclic loading were found to be many orders of magnitude faster than those measured under equivalent monotonic loads and to occur at typically 45% lower stress-intensity levels, cyclic fatigue in pyrolytic carboncoated graphite is reasoned to be a vital consideration in t h e design and lifeprediction procedures of prosthetic devices manufactured from this material.