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Delayed fracture behavior of polycrystalline alumina was investigated
under long-term static and cyclic loading by four- and three- point
flexure tests. The stress-probability-lifetime relationships in both
single- and multiple-crack growth models were obtained with the merging of
the concepts of statistical variations in strength and subcritical crack
growth leading to delayed fracture. Specimens with two different sizes
were prepared to evaluate the size effect on the strength and fatigue life
distribution. Experimental results showed that the size effect of
strength could be predicted by the volume-distributed-flaw Weibull
function. Both single- and multiple-crack growth models provided good
descriptions of static and cyclic strength-probability-lifetime for large
and small specimens and no significant cyclic fatigue effect was detected.
The predictions of fatigue lifetime from small to large specimens with
different flexure modes agreed well with the experimental results due to
the greater scatter of small-specimen data. The existence of larger
grains on the outer edges of small specimens makes a greater scatter of
testing results. However, the same predictions from large to small
specimens underestimated the testing results. This could be explained by
the less scatter of data and the worse microstructure of large specimens.
The frequency effect was investigated with small specimens under several
frequencies. Comparison of experimental results and theoretical
predictions provided further support that no true cyclic fatigue effect
was found in this particular alumina, as the total failure time in cyclic
loading was independent of frequency. In addition, the statistics of
fracture positions in fatigue specimens may explain the slight difference
between single- and multiple-crack growth models adopted in this study. |
