PSI - Issue 57

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Author name / Structural Integrity Procedia 00 (2019) 000 – 000

Guillaume THOQUENNE et al. / Procedia Structural Integrity 57 (2024) 191–198

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5.2. Tests results and analysis

5.2.1. Macro spalling analysis Four rolling contact fatigue tests have been performed and only one led to a valid result. Others encountered perturbation which have led to invalidate the results, and particularly indents due to passage of particles which made us to improve the oil filtration on the test bench. The valid test result shows a macro spall at about 9 million cycles. The spall depth measurement, performed with a video microscope, gives a value of 0.7 mm depth (Fig. 6.). This depth has been compared to the measured hardness profile of the specimen. The initiation depth of the spalling is located at the end of the transition zone which is at 0.62 mm depth (see Fig. 7.). The methodology developed in the paragraph 4.2 has been applied using the hardness profile measurement maid on the tested specimen. We thus obtained the torsional fatigue limit profile (50% of failure, 10 7 cycles). By comparing this profile to the shear stress profile at the hertzian pressure of 1 355 MPa, we can see that the crack initiation is in the region where the shear stress is close to the fatigue limit of the material (see Fig. 8). It has to be noted that the Fig. 8 shows a fatigue resistance curve better than the shear stresses applied on the specimen. This is due to the fact that the torsional fatigue limit is estimated for 50% probability of failure (see paragraph 4.2). We can reasonably consider a 5% to 10% dispersion around the mean values (here we consider 7% dispersion). Thus, we can observe that the shear stresses at the transition zone are closer to the material fatigue limit.

Fig. 6. Macro spalling and depth measurement.