Crack Shapes During Biaxial Fatigue
by
Ted A. Beer
Department of Mechanical and Industrial Engineering
Abstract
Fatigue cracks from Inconel 718 specimens were incrementally sectioned to obtain estimates of crack profiles and aspect ratios. The specimens were tested to failure in strain control under three loading conditions: tension, torsion, and combined tension-torsion. A least squares fit ellipse was fit to the data from the cracks, giving a good estimate of the actual crack profile. For cracks which encountered an obstruction such as a grain boundary, the surface growth stopped, and growth was deeper into the specimen and the crack took on the shape of a rectangle.
Correlations between surface crack length and crack depth were found to be good for cracks with surface lengths in the range of 0.1 to 1.0 mm. Cracks from lower strain specimens have larger aspect ratios than cracks from high strain, low life fatigue specimens. The correlation suggests that for fatigue cracks from a specimen with equal strain amplitudes, tension, torsion, and combined tension-torsion cracks have very similar crack aspect ratios. For Δ ε / 2 = 0.5%, the average aspect ratio given by maximum crack depth divided by surface crack length was 0.22. For Δ ε / 2 = 1.0%, an average aspect ratio of 0.15 was found.
Observations of crack growth characteristics were also made, including crack bifurcation and the Stage I to Stage II transition.
Keywords: Biaxial fatigue, Small cracks, Crack growth, Aspect ratio
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