1986 Volume 35 Issue 390 Pages 241-247
In order to establish a life prediction method for gas turbuine blades, fatigue tests using intermittent strain waveforms which simulated the service loading of gas turbine blades were carried out for SUS 316 stainless steel and two Ni-base superalloys (IN 738 LC and Inco. 713 LC) at elevated temperatures. The fatigue behavior and fatigue strength in these tests had been reported in the previous papers by the present authors.
In this paper, several methods of damage evaluation using intermittent strain waveforms were examined on the basis of a linear cumulative damage concept, and the following conclusion was obtained;
The failure life (Nes) in the intermittent strain cycling can be predicted by the equation below, which takes the acceleration of crack propagation rate due to the superposed small strain cycling into consideration.
Nes·(1/Nh+η·n2/N2)=1
where Nh and N2 are the fatigue lives observed under trapezoidal and constant strain waves respectively. η is the ratio of crack propagation rate under superposed small strain cycling to that under constant strain amplitude cycling, which is represented by the following equations.
R≤0;η=2α/(1-R)α R>0;η=2α·(1+R)/(1-R)
where α is the Paris's exponent and R is the stress ratio calculated by using the cyclic stress-strain relationship.
