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Dynamic fatigue or constant stress-rate testing is a convenient method for generating SCG (slow crack growth) data on ceramics and glasses. The tests are simple, inexpensive, and accomplished quickly. However, the use of dynamic fatigue testing to generate slow crack growth data for SCG parameter estimation assumes that the test specimen lifetime is dominated by a single region of the SCG curve. This assumption breaks down when excessive stressing rates are used and significant portions of the crack growth occurs in all three regions of the SCG curve. The resultant averaging of data representing all three regions of the SCG curve results in an error in the estimated SCG parameters n and D, and such data may not be accurate for predicting the lifetime of industrial components.
Dynamic fatigue tests covering five orders of magnitude well below the inert strength were performed on an alumina. The tests were performed using ASTM C 1161 “B” flexure specimens in accordance with ASTM Test Method for Determination of Slow Crack Growth Parameters of Advanced Ceramics by Constant Stress Rate Testing (C 1368-97). Statistical analysis of the experimental results indicates a statistically significant effect of stress rate on the SCG parameter n for stress rates above several MPa/s. The experimental results are in agreement with analytical results of Sudreau et al.  that show significant effects for rates greater than approximately one MPa/s. Recommendations for dynamic fatigue testing are given with regard to the current procedures in ASTM C 1368-97.
dynamic fatigue, slow crack growth, ceramics, strength, life prediction, stress rate, stress corrosion, reliability
Materials Research Engineer, NASA Glenn Research Center, Cleveland, OH
Professor, University of Washington, Seattle, WA