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In order to predict fatigue crack propagation at very long life a piezoelectric fatigue machine was built in our laboratory. This device is able to fail specimen at 1010 cycles and to determine thresholds up to 109 mm/cycle. The originality of this machine resides in the possibility to apply simultaneously to a specimen a constant tension effect and longitudinal ultrasonic vibratory effort with adjustable amplitude. That is to say, the R ratio can be varied from -1 to 0.9.
It appears that the fatigue thresholds are about the same in conventional fatigue and in resonant fatigue if the computation of the stress intensity factor K is correct. But there is a very large difference between the endurance limits at 106 cycles and 109 cycles. It means about 10% for steels and 30% for aluminium and nickel-based alloys.
To improve the relation between thresholds and fatigue limits, the gigacycle fatigue is studied in a Ni alloy manufactured by powder metallurgy with two sizes of inclusions. The testing temperature is 450°C. The first conclusion is that there is not any infinite fatigue limit until 109 cycles. The second conclusion is a possible correlation between the fatigue limit and the threshold at R ratio = 0. This relation is much more improbable for R = -1 because an incubation phenomenon exists around an inclusion or a porosity loaded at low stress level.
gigacycle fatigue, threshold, prediction
Professor, CNAM — ITMA, Paris,