13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- where, p1, p2 and p3 are material related parameters, f = (T+th) -1 is the frequency of the load waveform and T is the load period of the baseline triangular waveform. It can be seen that there are two extreme cases. On one extreme, η approaches zero, indicating that there is almost no interaction between creep and fatigue. For the other extreme case in which η approaches 1, it implies that the interaction reaches the most significant. Accordingly, taking D(ΔK)q as the basic term, (da/dN) interaction can thereby be expressed as following: interaction d ( ) ( ) d q a D K N h = D (6) where Kmax = ΔK / (1 - R), D and q are material related parameters. 3. Crack growth experiment on FGH97 The material in this study is a nickel based PM superalloy, named FGH97, taken from a turbine disk. It was strengthened by the formation of γ′ precipitates based on Ni3Al and received both the solid-solution-strengthening and age-hardening treatment. The nominal chemical composition is as follows (wt%): C 0.02-0.06, Cr 8.0-10, Mo 3.5-4.2, W 5.2-5.9, Al 4.8-5.3, Ti 1.6-2.0, Co 15.0-16.5, Nb 2.4-2.8, Hf 0.1-0.4, Mg ≤ 0.02, Zr ≤ 0.015, B ≤ 0.015, Ce ≤ 0.010 and the balance nickel[7]. Compact tension (CT) specimens were employed, where width W = 50 mm and initial crack length a0 = 22.5 mm. More details can be found in Ref.[6]. The baseline loading cycle is a triangular wave of load ratio R = 0.05. Different dwell time th = 90s, 450s and 1500s at the maximum load are superimposed to the baseline loading cycle to obtain a trapezoidal creep-fatigue loading condition, see Fig. 1. The baseline triangular fatigue loading cycle can be reasonably regarded as th = 0s reasonably. A typical service temperature of 750°C was conducted. (a) (b) Figure 1. Schematic of loading waveforms (a) Triangular; (b) Trapezoidal
RkJQdWJsaXNoZXIy MjM0NDE=