Apparent Enhanced Fatigue Resistance under Cyclic Tensile Loading for a HIPed Silicon Nitride

Abstract

Cyclic tensile loading tests of a commercial HIPed silicon nitride at elevated temperatures have indicated apparent “enhanced” fatigue resistance compared to static tensile loading tests under similar test conditions. At 1150°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 conditions, with all failures originating from preexisting defects (slow crack growth failures). At 1260°C, the stress rupture results showed pronounced differences between static, dynamic, and cyclic loading conditions. Failures at low static stresses (<175 MPa) originated from environmentally assisted (oxidation) and generalized creep damage, while failures at similar times but much greater (up to 2 x) cyclic stresses originated from preexisting defects (slow crack growth failures). At 1370°C, stress rupture results did not show as pronounced differences between static, dynamic, and cyclic loading conditions, with most failures originating from environmentally assisted (oxidation) and generalized creep damage.