Cyclic fatigue‐crack propagation, stress‐corrosion, and fracture‐toughness behavior in pyrolytic carbon‐coated graphite for prosthetic heart valve applications

Fracture‐mechanics tests were performed to characterize the cyclic fatigue, stress‐corrosion cracking, and fracture‐toughness behavior of a pyrolytic 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 Ic ) between 1.1 and 1.9 MPa √ m , and subcritical stress‐corrosion crack velocities (d a /d t ) which were a function of the stress intensity K raised to the 74th power (over the range ∼10 −9 to over 10 −5 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 (d a /d N ) were seen to be a function of the 19th power of the stress‐intensity range Δ K (over the range ∼10 −11 to over 10 −8 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 carbon‐coated graphite is reasoned to be a vital consideration in the design and life‐prediction procedures of prosthetic devices manufactured from this material.