  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.
