FCP Report No. 107

Small Crack Growth in Biaxial Fatigue

by

Dennis W. Worthem
Department of Mechanical and Industrial Engineering

Abstract

Crack-growth rate studies were performed for thin-walled tubes of Inconel 718 under biaxial, strain-controlled fatigue loading at room temperature. Loading modes included axial, torsion, and combined tension-torsion where Δγ / Δε = √3 for effective strain amplitudes of 1.0% and 0.5% at strain ratios of Re = -1, and Re = 0. From replicas of the surface, it was observed that from initiation through the “small crack” region (up to 1 mm) cracks initiated and propagated on or near planes of maximum shear amplitude. This indicated the presence of combined Mode-I and Mode-II loadings. On these replicas the cracks were measured tip to tip. The crack-growth rate was calculated from these measurements and plotted versus combined stress-intensity factor.

For nearly all loading cases the “small crack” effect is observed, i.e. a decrease or small increase in the crack growth rates versus combined stress intensity factor as compared to the “long crack” behavior where the rate increases much faster. Additionally an increasing Mode-I loading compared to Mode-II loading decreases the life of the specimen. Crack linking does not affect the overall crack growth behavior. The differences in life and behavior are probably explained by the different crack-growth micro-mechanisms. Possible crack-growth micro-mechanisms are described in the literature. The crack-growth data presented in this paper contains evidence for their presence. Recommendations for future study include tests to observe these mechanisms.

The results presented here are for a Mode-I loading that was at most 60% of the Mode-II loading.

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