Hisashi Igaki

Effect of Water Environment and Multiple Surface Cracks on Cyclic Fatigue Behavior of Glass Ceramics.

This paper examines the effects of interaction and coalescence of multiple surface cracks in water environment on fatigue behavior in brittle materials such as glass ceramics. For glass ceramic specimens with the three collinear cracks which introduced the different crack center distances by Vickers indentation, the cyclic fatigue tests in water environment were carried out with four point bending load. It was found that the time to failure for glass ceramics in water environment significantly decreased with the decrease in the crack center distance of multiple surface cracks, and was shorter than that in air. Moreover, from the crack growth parameter for a single crack, the predictions of the time to failure for multiple surface cracks were conducted by taking the interaction and coalescence into consideration. The predictions using the proposed crack growth model agree well with the experimental results.

Fatigue Behavior of Alumina Ceramics in Water Environment.

In the present study, the static and cyclic fatigue behavior of alumina ceramics was investigated under different environments of air and ion exchanged water. Bend specimens, with an indentation induced flaw at the center, were tested under a static and cyclic load applied by four-point bending. The test results have shown that in the case of static fatigue, time-to-failure in water was somewhat lower than that in air, while, in the case of cyclic fatigue, there was no significant difference by environment. The K1-V characteristics determined by double torsion method that utilizes relaxation technique were used to predict time-to-failure under static and cyclic loading of alumina. The predictions in lifetime were significantly longer than the experiments for both environments of air and water. The predictions in lifetime could explain the tendency of lifetimes in static fatigue qualitatively, but the lifetimes in cyclic fatigue were significantly lower than the predictions for both environments of air and water. These facts show that there may be any effect of stress cycling on time-to-failure in alumina.

Effect of hydrostatic pressure on the fracture strength of soda-lime glass. (Investigation with respect to subcritical crack growth)

In order to examine the effect of subcritical crack growth on fracture strength under multiaxial stress by considering subcritical crack growth, soda-lime glass specimens having a controlled surface flaw introduced by Knoop microhardness indentation were fractured by four-point bending under different hydrostatic pressures up to 100 MPa and different stressing rate conditions. For both as-indented specimens and specimens without residual stresses, it was found that the fracture stress evidently increased under hydrostatic pressure of 100 MPa and a high stressing rate condition, as subcritical crack growth was fully restricted under those conditions.