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A Novel Proof Test for Silicon Nitride Balls


  • C. Landis—contributing editor

  • This work was funded by the Mission Oriented Investigation and Experimentation (MOIE) program of The Aerospace Corporation under Contract Number FA8802-09-C-0001, administered by U.S.A.F. Space and Missile Center.

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A novel proof test has been developed for the silicon nitride balls used in hybrid bearings. The ball is compressed diametrally between two hemispherical conforming dies, which causes the ball's equator to bulge, thereby generating a tensile hoop stress that propagates naturally occurring flaws. The flaw propagation is detected by an acoustic emission transducer. In practice, the ball is repeatedly loaded to the desired proof stress, unloaded, rotated by a small angle, and reloaded until the entire ball has successively passed through the equator. The finite-element technique is used to calculate the principal stresses in the ball under the applied load. For simplicity, the preexisting flaw is modeled as a half-penny crack. The principal stresses are combined with the crack's stress intensity factor to calculate the proof test's threshold for flaw detection based on two accepted theories of mixed-mode fracture. The tested balls had preexisting naturally occurring flaws in the range of 100–300 μm. It is also shown that an angular rotation as large as 30° worked well in practice.