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**Source: **STP26676S

A general approach to prediction is discussed which would treat crack growth as a continuous stochastic process. Each different crack description and length would be a state in this process, with residual strength failure constituting the terminal state.

The specific theory necessary for crack growth prediction is discussed. The effective stress intensity variable developed by Walker for crack growth under tension-tension cycling is extended to compression-tension cycling. The use of this variable with constant amplitude data for prediction of crack growth under spectrum loading is described. A means of calculating the conditional probability of residual strength failure (given the predicted crack growth history) is outlined.

The prediction method is tested on a controlled set of laboratory data from large center cracked 7075-T76 aluminum panels. Constant amplitude data are used to predict crack growth under a randomized flight-by-flight load spectrum typical of a location on an aircraft wing. Agreement is found to be good despite evidence of mechanism differences between the constant amplitude and variable amplitude crack growth processes.

**Keywords:**

failure, fatigue (materials), cracking (fracturing), crack propagation, toughness, cyclic loads, cyclic variations, stresses, residual stress, mathematical prediction, stochastic processes, frequency distribution, probability, aircraft, aircraft panels, structural members, aluminum, evaluation

**Author Information:**

Brussat, TR *Stress engineer, Science and Engineering, Lockheed-California Co., Burbank, Calif.*

**Committee/Subcommittee:** E08.05

**DOI:** 10.1520/STP26676S