PSI - Issue 13

Masataka Aibara et al. / Procedia Structural Integrity 13 (2018) 1148–1153 Masataka Aibara/ Structural Integrity Procedia 00 (2018) 000 – 000

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5. Discussion 5.1 Increasing behavior of stress intensity factor value at each point

k B value was smaller than k A value throughout the analysis. As shown in Fig. 3, the distance from the center of the equilibrium growth part to the local growth part crack tip is longer than the distance to the equilibrium growth part crack tip by b 0 . The distance may be considered equivalent to crack length. In other words, the crack length of the local growth part is longer than that of the equilibrium growth part. The reason for the strange results is considered to be due to the crack shape that the local growth part has a small radius of curvature. In the three-dimensional crack shape the crack tip of the local growth part is more difficult to open than that of the equilibrium growth part under the tensile loading. In addition, point B is farther than point A from the point where distance between crack surfaces is the longest. In other words, the crack tip opening displacement (CTOD) on the local growth part is small. Therefore, k B value was smaller than k A value. As crack 1 grows, the radius of curvature at the local growth part gradually increases, then k B value increases. We analyzed the growth of the fatigue crack for a / b is 0.2, as shown in Fig. 3. For fatigue crack shape which has another a/b , if it is assumed that the longest distance between crack surfaces does not change, k B value would always be smaller than k A value. 5.2 Proposal of concept for predicting fatigue life All the number of cycles during this fatigue crack growth simulation, k B is always smaller than k A . We consider that there exists a force to prevent the local growth part from growing by the equilibrium growth part. And we call the force as “ restoration force. ” The restoration force is considered to be expressed by the difference between k A value and k B value. When the force is strong, the equilibrium growth part will absorb the local growth part with a small number of cycles in the process of crack growth. By the quantification of the force, we will be able to predict the crack shape that the fatigue life is equal to that of crack 1. Acknowledgements This work was financially supported by the Cross-ministerial Strategic Innovation Promotion Program (Structural Materials for Innovation). References Goto, M., 1992. Scatter characteristics of fatigue life and the behavior of small cracks. Fatigue & Fracture of Engineering Materials & Structures. 15, 953-963. Omura, T., Koyama, M., Hamano, Y., Tsuzaki, K., Noguchi, H., 2017. Generalized evaluation method for determining transition crack length for microstructurally large fatigue crack growth: Experimental definition, facilitation, and validation. International Journal of Fatigue. 95, 38-44. Murakami, Y., 1976. A simple procedure for the accurate determination of stress intensity factors by finite element method. Engineering Fracture Mechanics. 8, 643-655.