Probability and scatter in cumulative fatigue damage.

A study of the problems in aircraft design caused by scatter in fatigue life. A simple example comparing design for static loads with design for static loads with lesign for cyclic loads is given to provide a measure of the magnitude of the effect of fatigue scatter. Another example illustrates that different distributions may greatly affect estimates of failure probability and life, but that design effects may be far less pronounced, possibly even insignificant. Confidence levels are discussed in order to help clarify those concepts especially relevant to fatigue work; it is shown that the use of confidence levels can benefit by application of acceptance-inspection-plan techniques. Also discussed is the currently favored use of prescribed, constant scatter factors, an approach not well founded on present concepts of cumulative damage. The Memorandum presents a straightforward approach to probability and scatter in spectrum loading consistent with cumlative-damage theories, using constant-amplitude data and incorporating the suggested determination of confidence levels.