Abstract
The applicability of both prediction methods for low-cycle fatigue life of powder superalloy based on the Manson-Coffin equation and damage mechanics were addressed. Both fatigue life prediction models were evaluated by low-cycle fatigue experimental data of powder superalloy FGH96 with non-destructive standard parts and those with inclusions. Due to the characteristics of high strength and low plasticity of powder superalloy FGH96, errors in calculating the plastic strain amplitude deviate severely the prediction outcomes when using Manson-Coffin method. Meanwhile, by introducing the damage variable which characterizes the material damage, the damage evolution equation can be built by fitting the experimental data of standard parts and also applied to powder superalloy specimens containing inclusion. It is indispensable to accurately calculate the damage characterization parameter through finite element analysis in local stress concentration around the inclusion. The applicability of the prediction model was verified by the test life cycles of experimental specimens with different types and sizes of inclusions subsequently. Testing and simulation work showed much better prediction accuracies globally for the damage mechanics approach.
Highlights
Since the emergence of powder superalloys in the 1970s, the material revolution of aeroengine turbine disks has never stopped
China is catching up in this field, and has produced first- and second-generation powder superalloys represented by FGH95 and FGH96, and is overcoming the technical difficulties associated with third-generation powder superalloys [1,2,3]
When the Manson-Coffin equation is applied for the fatigue life prediction, it obviously cannot take this kind of micro defect into account, nor can it reflect the fatigue crack initiation information of powder superalloy containing inclusions [9]
Summary
Since the emergence of powder superalloys in the 1970s, the material revolution of aeroengine turbine disks has never stopped. When the Manson-Coffin equation is applied for the fatigue life prediction, it obviously cannot take this kind of micro defect into account, nor can it reflect the fatigue crack initiation information of powder superalloy containing inclusions [9]. The powder superalloy FGH96 was revealed through analysis to be very sensitive to surface defects and the inclusion involving its location and size has an important influence on the FGH96 superalloy’s low-cycle fatigue lifetime. The applicable analysis of both the Manson-Coffin model and model based on damage mechanics was respectively performed for low-cycle fatigue crack initiation life prediction of powder superalloy containing various types of inclusions and the applicability was assessed by comparison with experimental data. Summing up the experimental data, it was found that there was a linear relationship between the plastic strain amplitude and the fatigue life to cyclic failure in the double logarithmic coordinate system. The material strain-life relationship can be obtained by Formula (1), which is called the Manson-Coffin equation:
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