Tham khảo tài liệu 'gas turbines part 12', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 264 Gas Turbines Fig. 41. Fatigue crack growth at 649oC with 5 min hold time at the maximum load Bain et al. 1988 . Fig. 42. The predicted Fatigue crack growth in Udmet 720 at 649oC with 5 min hold time. Telesman et al. 2008 also observed strong microstructural dependence of the hold time fatigue crack growth rate in the LSHR P M disk superalloy. Using heat treatments to control the grain size at constant but with different cooling rate and subsequent aging treatments to control the y size they related the increased fatigue crack growth rate with the stress relaxation potential possessed in each microstructure with different primary secondary and tertiary y sizes and distribution. This can also be explained qualitatively by Life Prediction of Gas Turbine Materials 265 Eq. 89 stress relaxation would impart a lower grain boundary sliding rate resulting in lesser grain boundary damage during the hold time and hence reducing the overall crack growth rate. Finally it should be pointed out that none of the above phenomena could be sufficiently addressed with Eq. 87 . In general the present model Eq. 89 also presents the effect of creep damage in a multiplication factor for every fatigue crack increment will induce coalescence with the creep damage accumulated ahead of the propagating fatigue crack. Damage tolerance analysis The damage-tolerance philosophy assumes materials or components entering into service have defects in their initial conditions. Then the component life is basically the life of crack propagation starting from an initial flaw as N f f__ f AK 91 where a0 is the initial crack size af is the final crack size at fracture or sometimes a dysfunction crack size reduced by a safety factor and the crack growth rate function f AK can be any of the aforementioned particularly Eq. 79 86 or 89 depending on the operating condition. In the simple case under pure mechanical fatigue condition where the crack growth rate is expressed by the Paris law Eq. 79 the .
