The influence of crack tip plasticity in the growth of small fatigue cracks
In order to rationalize observed differences in the growth behavior of large and small cracks, local crack tip opening micromechanics have been characterized for both crack size regimes in a high strength aluminum alloy. It is found that crack tip opening displacement, crack tip opening load, and crack opening mode all differ widely for large and small cracks at equivalent cyclic stress intensities (..delta..K). High crack tip opening displacements and relatively low, approximately constant, crack opening loads for microcracks account both for their rapid rate of growth relative to large cracks and the absence of a microcrack threshold stress intensity. Crack tip plastic zone sizes also were measured, and it was found that the ratio of plastic zone size to crack length for small cracks is about 1.0, while for large cracks the same ratio is <<1. Simple empirical corrections to ..delta..K are found inadequate to correlate the growth of large and small cracks. It is concluded that for small cracks, linear elastic fracture mechanics similitude does not apply, and that an alternative crack driving force must be formulated.
- Research Organization:
- Southwest Research Inst., San Antonio, TX
- OSTI ID:
- 5111392
- Journal Information:
- Metall. Trans., A; (United States), Vol. 15A:8
- Country of Publication:
- United States
- Language:
- English
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