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Crack growth behavior, fracture mechanism, and microstructural evolution of G115 steel under creep–fatigue loading conditions

Zhengxin Tang,H. Jing,Lianyong Xu,D. Chi,Lei Zhao,Yongdian Han,Bo Xiao,Yu Zhang,Hai-zhou Li,Wu Dongquan

Published 2019 in International Journal of Mechanical Sciences

ABSTRACT

Abstract Creep–fatigue crack growth (CFCG) behavior, fracture mechanism, and microstructural evolution of G115 steel under different hold times and initial stress intensity factor ranges (ΔKin) were investigated. The crack propagation durations increased significantly with decreasing ΔKin and increasing dwell time. In da/dN vs. ΔK plots, a “hook” was formed, and it entered the fast crack growth zone at approximately Δ K = 36 MPa m , which corresponds to σref = σ0.2 under plane stress. In (da/dt)avg vs. (Ct)avg plots, data for a specific hold time in a wide load range exhibited a unique relationship. It appears that the parameter D0 in this relationship is affected by hold time and constraint, while φ is independent of these factors. Scanning electron microscopy analyses revealed that with increasing hold time (60–600 s), the fracture mode changed from transgranular-dominated to intergranular-dominated. Meanwhile, the fracture mode changed from intergranular-dominated to predominantly transgranular with high ductility when ΔKin increased (17–29  MPa m ). According to electron backscatter diffraction observations, many substructured and recrystallized grains formed after CFCG and the deformed grains further decreased as ΔKin increased. The increased crack resistance with decreased deformation grains may combine with the ductility fracture effect to decrease the CFCG rates under high load levels. Additionally, the local misorientation angle reduced significantly after CFCG, and the differences among various load levels were limited.

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