Abstract:

The objective of this paper is to study the effects of three dimensional constraints on fatigue crack growth under aircraft spectrum loading. A modified strip yielding model is proposed for stress states other than plane stress condition and used to evaluate the three dimensional constraint factors associated with crack growth. Then the constraint factors, describing the transition from plane stress to plane strain dominated crack growth, are used to correlate crack growth data under constant amplitude loading and to calculate crack growth life under standard and typical aircraft spectrum loading and block spectrum loading. The NASA’s life prediction code FASTRAN II is modified by incorporating the constraint factors. The modified code is then applied to calculate crack growth in several finite thick sheets of aluminum alloy materials and high strength steel under TWIST spectrum loading, synthesized mission spectra of fighter and aerotransport and a block spectrum. The modified code can make prediction only based on one set of crack growth data under constant amplitude loading. In general, the predictions are within 16% of the test results under TWIST and within 33% under all of the spectra. The results demonstrated that the present method and the modified code can be used in engineering practice with higher accuracy.

Key words: three dimensional constraints, fatigue crack growth, spectrum loading, life prediction.