Cyclic fatigue-crack propagation in sapphire in air and simulated physiological environments

Single-crystal aluminas are being considered for use in the manufacture of prosthetic heart valves. To characterize such materials for biomedical appl ication, subcritical crack growth by stress corrosion (static fatigue) and by cyclic fatigue has been examined in sapphire along (1 (1) over bar 00) p lanes in 24 degrees C humid air and 37 degrees C Ringer's solution (the lat ter as a simulated physiological environment). The relationships between cr ack-propagation rates and the linear-elastic stress intensity have been det ermined for the first time in sapphire for both modes of subcritical cracki ng. It was found that growth rates were significantly faster at a given str ess intensity in the Ringer's solution compared to the humid air environmen t. Mechanistically, a true cyclic fatigue effect was not found in sapphire as experimentally measured cyclic fatigue-crack growth rates could be close ly predicted simply by integrating the static fatigue-crack growth data ove r the cyclic lending cycle. (C) 2000 John Wiley & Sons, Inc.