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The small-crack anomaly, where small cracks tend to grow either faster or slower than large cracks when compared on the basis of linear-elastic stress-intensity factors, has been shown to be significant for some materials and loading conditions. Conventional linear-elastic analyses of small cracks in homogeneous bodies are considered inadequate because of microstructural influences not accounted for in the stress-intensity factor and because of thenonlinear stress-strain behavior at notches and in the crack-front region. In this paper, plasticity effects and crack-closure transients are reviewed and investigated.
This paper presents a review of some common small-crack test specimens, the underlying causes of the small-crack effect, and the fracture-mechanics parameters that have been used to correlate or predict their growth behavior. Although microstructural features are important in the initiation and growth of small cracks, this review concentrates on continuum mechanics concepts and on the nonlinear behavior of small cracks. The paper reviews some stress-intensity factor solutions for small-crack test specimens and develops some simple elastic-plastic J integral and cyclic J integral expressions that include the influence of crack closure. These parameters were applied to small-crack growth data on two aluminum alloys, and a fatigue life prediction methodology is demonstrated. For these materials, the crack-closure transient from the plastic wake was found to be the major factor in causing the small-crack effect. Plasticity effects on small-crack growth rates were found to be small in the near threshold region, in that the elastic stress-intensity factor range and the equivalent value from the cyclic J integral gave nearly the same value.
cracks, elasticity, plasticity, stress-intensity factor, J, integral, crack opening displacement, surface crack, crack closure, crack propagation, fatigue (material), microstructure
Senior scientist, NASA Langley Research Center, Hampton, VA