Strength Measurement of Ceramic Spheres Using a Diametrally Compressed "C-Sphere" Specimen

A "C-sphere" specimen geometry was conceived and developed to measure failure stress of bearing-grade silicon nitride (Si₃N₄) balls caused by tension at the ball's surface. The induced method of fracture also allows for the study of surface-located strength-limiting flaws in ceramic spheres. A slot is machined into the balls to a set depth to produce the C-sphere geometry. A simple, monotonically increasing uniaxial compressive force produces an increasing tensile stress at the C sphere's outer surface that ultimately initiates fracture. The strength is determined using a combination of failure load, C-sphere geometry, and finite element analysis. Additionally, the stress field was used to determine the effective areas and effective volumes of a C-sphere as a function of Weibull modulus. To demonstrate this new specimen, C-sphere flexure strength distributions were determined for three commercially available bearing-grade Si₃N₄ materials (NBD200, SN101C, and TSN-03NH), and differences among their characteristic strengths and Weibull moduli were found.