Strength Measurement of Ceramic Spheres Using a Diametrally Compressed “C‐Sphere” Specimen

Abstract

A “C‐sphere” specimen geometry was conceived and developed to measure failure stress of bearing‐grade silicon nitride (Si 3 N 4 ) 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 Si 3 N 4 materials (NBD200, SN101C, and TSN‐03NH), and differences among their characteristic strengths and Weibull moduli were found.