PSI - Issue 13

Tomoki MIZOGUCHI et al. / Procedia Structural Integrity 13 (2018) 1071–1075 Tomoki Mizoguchi / Structural Integrity Procedia 00 (2018) 000 – 000

1075

5

4. Case study: example of TRIP-maraging steel First, we note the relationship between the lamellar alignment direction from the loading direction and the nano-roughness. The fraction of the effective crack surface area was plotted against the lamellar alignment direction, as shown in Fig. 7(a). Based on our proposed criterion, the friction effect was significant when the lamellar alignment was similar to the loading direction. In addition, the effect of friction in Fig. 7(b) was relatively small, although the lamellar alignment direction was similar to the loading direction. This could be related to the thickness of the lamellae. Figures 7(b) to (d) show the SE images of the cracks with the lamella aligned parallel to the loading direction. From these images, the effect of friction was observed to be small in the thick lamellar structure regions. In this study, we investigated the relationship between the microstructure and the crack plane friction based on our proposed criterion; therefore, in future, we will examine the relationship among the proposed criterion, actual friction, and associated crack growth rate.

Fig. 7. (a) Relationship between lamellar alignment direction and effect of crack surface friction. (b, c, d) SE images corresponding to (b), (c) and (d) in (a).

5. Conclusions We proposed methods for the quantification of multi-scale crack roughness. (1) Nano-roughness occurs preferentially when a crack propagates across a lamellar structure. (2) When the lamellar alignment direction is similar to the loading direction, the degree of crack deflection decreases with increasing lamellar thickness in the order of tens of micrometers. (3) To demonstrate the relationship between the fatigue crack growth rate and the influence of friction on cracks in TRIP maraging steels, a finer fatigue cycle interval is required for obtaining the SE images.

Acknowledgement

This work was financially supported by JSPS KAKENHI (JP16H06365) and the support is greatly appreciated.

References

Ballarini, R., Plesha, M.E., 1987. The effects of crack surface friction and roughness on crack tip stress fields. International Journal of Fracture 34. 195-207. Gray, G.T., Williams, J.C., Thompson, A.W., 1983. Roughness-Induced Crack Closure: An Explanation for Microstructurally Sensitive Fatigue Crack Growth, Metallurgical Transactions A 14, 421-433. Hamada, S., Suemasu, T., Fukudome, S., Koyama, M., Ueda, M., Noguchi, H., 2018. Roughness-induced stress shielding effect in fatigue crack propagation under Mode II loading, International Journal of Fatigue In press. Koyama, M., Zhang, Z., Wang, M., Ponge, D., Raabe, D., Tsuzaki, K., Noguchi, H., Tasan, C.C., 2017. Bone-like crack resistance in hierarchical metastable nanolaminate steels. Science 355(6329), 1055-1057. Llorca, J., 1992. Roughness-induced crack closure: A numerical study. Fatigue & Fracture of Engineering Materials & Structures 15, 655-669. Millán, J., Sandlöbes, S., Al-Zubi, A., Hickel, T., Choi, P., Neugebauer, J., Ponge, D., Raabe, D., 2014. Designing Heusler nanoprecipitates by elastic misfit stabilization in Fe – Mn maraging steels, Acta Materialia 76, 94-105. Pippan, R., Strobl, G., Kreuzer, H., Motz, C., 2004. Asymmetric crack wake plasticity – a reason for roughness induced crack closure, Acta Materialia 52, 4493 4502. Suresh, S., Ritchie, R.O., 1982. A geometric model for fatigue crack closure induced by fracture surface roughness, Metallurgical Transactions A 13(9), 1627 1631. Wang, M.M., Tasan, C.C., Ponge, D., Dippel, A.C., Raabe, D., 2015. Nanolaminate transformation-induced plasticity – twinning-induced plasticity steel with dynamic strain partitioning and enhanced damage resistance, Acta Materialia 85, 216-228. Wang, M.M., Tasan, C.C., Ponge, D., Kostka, A., Raabe, D. 2014 Smaller is less stable: Size effects on twinning vs. transformation of reverted austenite in TRIP maraging steels, Acta Materialia 79, 268-281. Zhang, Z., Koyama, M., Wang, M.M., Tsuzaki, K., Tasan, C.C., Noguchi, H., 2017. Effects of lamella size and connectivity on fatigue crack resistance of TRIP maraging steel, International Journal of Fatigue 100, 176-186.