The influence of static Mode I and static Mode II loading on Mode I fatigue crack growth has been studied with 12 weight % chromium steel.
Static Mode I was superimposed as a tensile load on fatigue-loading with mean stress equal to zero, thus resulting in R-values of - 1, - 0.33, 0, +0.25, and +0.5. For these values, fatigue-crack growth curves were determined in the threshold regime with crack growth rates typically between 3 × 10-13 and 5 × 10-9 m/cycle.
In addition, a constant static Mode II load was superimposed on the same above-mentioned Mode I load. The resulting crack growth rates were found to vary systematically with crack length and load ratio. For R-ratios ⩽ 0 enhanced fatigue crack growth rates were observed at crack lengths up to a defined value. After a certain crack length was attained, slightly reduced crack growth rates were detected. For high R-ratios, long as well as short fatigue cracks grew slower than under pure Mode I fatigue loading. The results are discussed in light of the crack closure phenomenon and the fracture morphology, which was studied in a scanning electron microscope (SEM).
In the Appendix, a basic procedure for determining stress-intensity values for high-frequency (21 kHz) fatigue loading is given in comparison with conventional loading.