APPARENT ENHANCED FATIGUE RESISTANCE UNDER CYCLIC TENSILE LOADING FORA HIPED SILICON-NITRIDE

Cyclic tensile loading tests of a commercial HIPed silicon nitride at elevated temperatures have indicated apparent ''enhanced'' fatigue res istance compared to static tensile loading tests under similar test co nditions. At 1150-degrees-C, stress rupture results plotted as maximum stress versus time to failure did not show significant differences in failure behavior between static, dynamic, or cyclic loading condition s, with all failures originating from preexisting defects (slow crack growth failures). At 1260-degrees-C, the stress rupture results showed pronounced differences between static, dynamic, and cyclic loading co nditions. Failures at low static stresses (<175 MPa) originated from e nvironmentally assisted (oxidation) and generalized creep damage, whil e failures at similar times but much greater (up to 2x) cyclic stresse s originated from preexisting defects (slow crack growth failures). At 1370-degrees-C, stress rupture results did not show as pronounced dif ferences between static, dynamic, and cyclic loading conditions, with most failures originating from environmentally assisted (oxidation) an d generalized creep damage.