Initiation and growth of fatigue cracks in sheets with U-shaped notches in the first and mixed modes of fracture

Abstract

This study presents the effects of the geometry of the notches on the fatigue life, crack initiation, and growth in mixed mode and first mode of the fracture. For this purpose, some samples were tested with U-shaped notches with different radii and depths and various orientations with respect to the loading directions under fatigue loads. Using the fatigue crack growth rates, the Paris law coefficients were obtained for the used material in different conditions. It was shown that these coefficients are independent of the geometry of the samples. Fatigue crack growth behaviors in the mixed mode tests were also in good agreement with the result of the numerical simulations. Predictions of the maximum tangential stress criterion and numerical simulation for the position of the crack initiation on the notch root were compared with the tested observations, which showed good agreement. The fatigue life of the test samples was compared with the analytical results provided by the Manson-Coffin law. It was shown that a 25% decrease in notch depth increases the fatigue life by 3 times, also, an increase of 0.5 mm in the notch radius increases the fatigue life by 40%. Finally, the fracture surface of the samples was checked using an optical microscope. This study showed that the fracture surfaces have one or two lateral shear lips and plane stress conditions were established.   REFERENCES [1] X.-L. Zheng, Modelling fatigue crack initiation life, Int. J. Fatigue, 15 (1993) 461-466, https://doi.org/10.1016/0142-1123(93)90257-Q. [2] M. De Freitas, L. Reis, B. Li, Evaluation of small crack growth models for notched specimen under axial/torsional fatigue loading, Facta universitatis-series: Mechanics, Automatic Control and Robotics, 3 (2003) 657-669, [3] H. Zhang, A. Fatemi, Short fatigue crack growth from a blunt notch in plate specimens, Int. J. 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