Strength of a C-Sphere flexure specimen

A “C–Sphere” flexure strength specimen geometry was conceived and developed to measure a relevant strength of bearing–grade Si3N4 balls and to relate that to surface–located strength–limiting flaws and to ultimately link those flaw populations to rolling contact fatigue performance. A slot was machined into the balls to a set depth to produce the C–sphere geometry. C–sphere specimens were then diametrally compressed to produce a monotonically increasing flexure or hoop tensile stress at their surface that caused their fracture. The strength was determined using the combination of failure load, C–sphere geometry, and FEA, and the stress field was used to determine C–sphere effective areas and effective volumes as a function of Weibull modulus. A description of the specimen and the aforementioned analysis are provided and a comparison of C–sphere flexure strength distributions of two bearing grade SijN4 materials (NBD200 and SN101C) is given.