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A procedure is described for drastically reducing the computation time in calculating crack growth for variable-amplitude fatigue loading when the loading sequence is periodic. By the proposed procedure, the crack growth, r, per loading period is approximated as a smooth function and its reciprocal is integrated, rather than summing crack growth cycle by cycle. The savings in computation time result since only a few pointwise values of r must be computed to generate an accurate interpolation function for numerical integration. Further time savings can be achieved by selecting the stress intensity coefficient (stress intensity divided by load) as the argument of r. Once r has been obtained as a function of stress intensity coefficient for a given material, environment, and loading sequence, it applies to any configuration of cracked structure. Any of a broad range of prediction models (such as “retardation models”) may be used in conjunction with the proposed procedure. Agreement with results obtained using cycle by cycle summation is demonstrated, for two retardation models.
fatigue (materials), crack propagation, cyclic loads, mathematical prediction
Research specialist, Fatigue and Fracture Mechanics, Science and Engineering, Lockheed-California Company, Burbank, Calif.