Paper detail

A critical assessment on Kassapoglou's statistical model for composites fatigue

Kassapoglou recently proposed a model for fatigue of composite materials which seems to suggest that fatigue SN curve can be entirely predicted on the basis of the statistical distribution of static strengths. The original abstract writes "Expressions for the cycles to failure as a function of R ratio are derived. These expressions do not require any curve fitting and do not involve any experimentally determined parameters. The fatigue predictions do not require any fatigue tests for calibration". These surprisingly ambitious claims and attractive results deserve careful scrutiny. We contend that the results seem to be due to a number of approximations and incorrect derivations, and one particular misleading assumption, which make the model not conform to a fatigue testing in a given specimen with resulting SN curve distribution. The quantitative agreement of some predictions (the scatter of distribution of fatigue lives being close to the mode value found in typical composites of aeronautical interest in the large Navy database) should not motivate any enthusiasm. It is believed that a proper statistical treatment of the fatigue process should not make wear-out constants disappear, and hence the SN curves would depend on them, and not just on scatter of static data. These serious concerns explain the large discrepancies found by 3 independent studies which tried to apply Kassapoglou's model to composite fatigue data, and to other well known results.