The purpose of this study is to investigate a nondestructive method for predicting the fatigue limit of spheroidal graphite cast iron using high resolution X-ray CT. Axial load fatigue test specimens were cut out from a large spheroidal graphite cast iron equivalent to FCD 350, and graphite and defects in the material were detected using high resolution X-ray CT for all specimens. Fatigue limit was estimated from the graphite and defect sizes using the fatigue limit estimation formula based on the four-parameter method.
Axial load fatigue test was performed in accordance with JIS (Japanese Industrial Standards). Repetition frequency was 17Hz, stress ratio was R ＝ －1, and number of cycles during the test was 1.0 × 107. The specimen used was JIS type 1 of 8.00mm in diameter. Fracture origins were observed in all fatigue fracture surfaces using a scanning electron microscope (SEM) in order to compare the results between the defects observed by X-ray CT and the fracture origins observed in the fatigue test.
The fatigue limit estimated by the defect with the largest volume detected by X-ray CT was 5% lower than the experimental fatigue limit of 125MPa, which is considered safe estimation. However, in the fatigue test, the fracture origin was not necessarily the defect with the largest volume. Therefore, the fatigue limit was estimated by the average defect size when the cumulative distribution function of ten defects with the largest volume of each test piece was F ＝ 50%. The result was 11% larger than the experimental fatigue limit, which is considered a dangerous estimation. These results indicate that estimation of fatigue limit using a nondestructive method is feasible.
