Abstract
In order to investigate effects of BN content and load variation on the fatigue crack growth behavior of Si3N4-BN composite ceramics (SNB), fatigue crack growth tests under constant amplitude loading and repeated two-step load sequences were carried out using compact type (CT) specimens. Crack length and macroscopic crack closure were measured using the unloading elastic compliance method. The maximum stress intensity factor for the same fatigue crack growth rate decreases with the increase of the BN content. This decrease with the increase of the BN content is resulted from the decline of the fracture toughness. Fatigue crack growth rate strongly depends on the maximum stress intensity factor, and is also influenced by the load amplitude. As for the load amplitude dependency of the crack growth rate, SNB40 is the most remarkable in comparison with SNB20 and SNB30. Regardless of the BN content, fatigue crack growth rate under repeated two-step loading was found to be accelerated in comparison with that under constant amplitude loading. The acceleration was due to the breaking of interlocking under high level loading. The acceleration of the crack growth rate of SNB40 is the most remarkable because the crack growth rate of SNB40 is strongly dependent on the load amplitude. The fatigue crack growth acceleration behavior under repeated two-step loading could not be explained by the equivalent stress intensity factor range ΔKeq in consideration of both of Kmax and ΔK. However, the equivalent effective stress intensity factor range ΔKeff, eq in consideration of both of Kmax and ΔKeff could account for the acceleration behavior qualitatively but not quantitatively.
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