A novel mechanical test has been developed to measure the fracture toughness of the silicon-nitride balls used in modern hybrid bearings important for many defense, space and industrial applications. The ball is compressed diametrally between two hemispherical conforming dies, which causes the ball’s equator to bulge, generating a tensile hoop stress. Under applied load, a precrack placed at the equator grows. To calculate the ball’s fracture toughness at crack instability, finite element calculations of the applied stress field and an analytical solution for the stress intensity factor are used in the “two point plus semiellipse” method. The new technique appears more accurate than the indentation technique used to measure the toughness of ceramics.

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