PSI - Issue 13

Akira Maenosono et al. / Procedia Structural Integrity 13 (2018) 694–699 Akira Maenosono et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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and BCC crystalline alloys. In FCC and BCC alloys, the presence of multiple slip systems enables shear deformation on planes where the shear stress is nearly maximum. Therefore, the driving force for fatigue crack growth shows a relatively low crystallographic orientation dependence. In contrast, there is only a basal plane in each grain of titanium alloys, which gives a variety of driving forces for crack growth on the basal plane. Hence, when a fatigue crack preferentially propagates along the basal plane, the constraint of crack tip deformation causes a scatter of driving force for crack growth on the basal plane, which thereby increases the degree of the scatter in the fatigue life of the titanium alloys. In addition, as mentioned regarding Fig. 2, some portions of the fatigue cracks do not completely develop along the basal plane, particularly when the angle between the notch alignment direction and loading direction is below 20°. In other words, other crack growth mechanisms, such mode I crack growth and crack coalescence, can also be activated, as minor mechanisms. The activation of the minor crack growth mechanism can cause scatter of fatigue life. Moreover, the fatigue crack growth behavior alters when the crack tip meets and passes across a prior β grain boundary, which can also cause scatter. These points causing scatter in fatigue life will be an issue for future investigation. . Conclusions We investigated microstructurally small fatigue crack growth behaviors in a fully laminated Ti – 6Al – 4V alloy in terms of crystallography and crack tip deformation. The dominant crack growth mechanism was crack growth on the basal plane via mode II crack tip deformation. Therefore, the shear stress on the basal plane at the crack tip is the primary factor controlling the small fatigue crack growth behavior. However, other crack growth mechanisms are also present, e.g., crack growth via mode I crack tip deformation and crack coalescence with preexisting damage. Investigation of the extra mechanisms will be future work. Furthermore, the mode II crack growth mechanism is not reproducible, because the growth behavior is strongly affected by the presence of grain boundaries. These factors cause the significant scatter in fatigue crack growth rates and associated fatigue life. Acknowledgments This work was financially supported by the Cross-ministerial Strategic Innovation Promotion Program (Structural Materials for Innovation). Nalla, R. K., Ritchie, R. O., Boyce, B. L., Campbell, J. P., & Peters, J. O. (2002). Influence of microstructure on high-cycle fatigue of Ti-6Al-4V: bimodal vs. lamellar structures. Metallurgical and Materials Transactions A, 33(3), 899-918. Ritchie, R. O., & Lankford, J. (1986). Small fatigue cracks: a statement of the problem and potential solutions. Materials Science and Engineering, 84, 11-16. Omura, T., Koyama, M., Hamano, Y., Tsuzaki, K. & Noguchi, H. (2017) Generalized evaluation method for determining transition crack length for microstructurally small to microstructurally large fatigue crack growth: Experimental definition facilitation, and validation. International Journal of Fatigue, 95, 38-44. Chowdhury, P., & Sehitoglu, H. (2016). Mechanisms of fatigue crack growth – a critical digest of theoretical developments. Fatigue & Fracture of Engineering Materials & Structures, 39(6), 652-674. Koyama, M., Li, H., Hamano, Y., Sawaguchi, T., Tsuzaki, K., & Noguchi, H. (2017) Mechanical-probabilistic evaluation of size effect of fatigue life using data obtained from single smooth specimen: An example using Fe-30Mn-4Si-2Al seismic damper alloy. Engineering Failure Analysis, 72, 34-47. Koyama, M., Yamanouchi K., Wang, Q., Ri, S., Tanaka, Y., Hamano, Y., Yamasaki, S., Mitsuhara, M., Ohkubo M., Noguchi, H., & Tsuzaki., K. (2017). Multiscale in situ deformation experiments: A sequential process from strain localization to failure in a laminated Ti-6Al-4V alloy. Materials Characterization, 128, 217-225. Bantounas, I., Dye, D., & Lindley, T. C., 2009. The effect of grain orientation on fracture morphology during high-cycle fatigue of Ti – 6Al – 4V. Acta materialia, 57(12), 3584-3595. Bridier, F., Villechaise, P., & Mendez, J. (2008). Slip and fatigue crack formation processes in an α/β titanium alloy in rel ation to crystallographic texture on different scales. Acta Materialia, 56(15), 3951-3962. References