An Approach on Lifetime Prediction for Ceramics under Elevated Temperature with Static Fatigue

Abstract

Stress—Time—Probability (STP) diagrams for sintered Si₃N₄ at 1200°C were studied with static fatigue experiment in three—point bending specimen. Microcracks were introduced before the tests by Knoop indentation at the center of the specimen tension side in order to reduce the scatter in fatigue data.

Three-point bending tests and static fatigue experiments were performed on sintered Si₃N₄ specimens to give Weibull’s parameters and SCG exponent seperately. It can be deduced from the curve of Pr verse (σ_f-σ_u),in which σ_u is threshold value of Weibull distribution, that the average strength meets the following equation:

$${{\bar{\sigma }}_{f}} = {{\sigma }_{u}} + MO{{R}_{0}}\Gamma \left( {1 + \frac{1}{m}} \right)$$

By using different σ_u, to carry on linear regression of lnin[1/(1—P_f)] verses ln(σ_f—σ_u),we can get different values of ${{\bar{\sigma }}_{f}}$. Comparing them with the measured value enable us to make out σ_u value.

STP diagrams were established for sintered Si₃N₄ ceramics at 1200°C at different fracture probabilities.