Fracture failure modes in lightweight bearings

Rolling contact bearings fail in a wide variety of different modes. A fracture mode of failure becomes a possibility when operating with significant tensile hoop stress in the races. A series of sixteen bearings operated with tensile stresses in the races were analyzed for cracks. Ten of the sixteen bearings generated cracks of critical size resulting in rapid fracture of the races. From these data, an estimate is made of the critical stress intensity factor for AISI 52100 and AISI M-50, the materials currently used in aircraft engine bearings. Flaws are generated in the races apparently due to a combination of tensile hoop stress and the superimposed Hertz stress pattern. An attempt was made to correlate the flaw size and shape to the known stress fields, or components of the stress fields, but currently these have not been completely successful. Using relatively simple, established fracture mechanic techniques, a limiting stress level can be determined for a given flaw size, but experimental data must be relied upon to predict the flaw size, particularly the depth of the flaw. By proper design, the stresses can be maintained at sufficiently low levels to preclude rapid fracture in the outer races. The area of concern arises in engines of the future with increased rotor speeds which will generate high hoop stresses in the bearing inner races. Bearing materials with increased fracture toughness may be required for these applications.