A New Test Method for Mode II Fatigue Crack Growth in Hard Materials

Flaking type failure in rolling contact fatigue is usually attributed to the fatigue induced by cyclic subsurface shearing stress caused by contact loading. We suppose that this type of subsurface crack growth is essentially a mode II fatigue crack growth under the condition in which tensile mode growth is suppressed due to the compression stress field arising from the contact stress. Although the newly developed apparatus is, as the former apparatus, based on the principle that the static KI and the compressive stress parallel to the pre-crack are superimposed to mode II loading, we employ a new mechanism for mode II load application. In the new apparatus, a direct loading system is employed instead of the four-points-shear loading system used in the former apparatus. A new device for the application of compressive stress parallel to the pre-crack is also developed. Due to these alterations, mode II cyclic loading tests for hard steels have become possible at arbitrary stress ratios, including fully reversed loading (R=-1) which is the case of rolling contact fatigue. The tests were made on bearing steel SUJ2 and 0.75% carbon steel. The hardness of these two steels was around HRC60. The test results showed that stable mode II fatigue crack growth occurred in the region of ΔKII=3-10MPa√m, and that ΔKII-threshold values for mode II fatigue crack growth of these two steels were estimated to be around 3MPa√m. The apparent effect of load ratio was not observed in the data of R=0 and R=-1.