Comparison of Acoustic Emission and Structure Degradation in Compressed Porcelain and Corundum Materials

The paper presents the results of acoustic emission (AE),
microscopic and ultrasonic measurements of the samples subjected to slowly
increasing compressive stress. On the basis of performed measurements, the
successive stages of the materials structural degradation have been recognized.
The object of research and comparison were the samples made of the
electrotechnical materials - aluminous porcelain and corundum ceramics. Both
investigated materials have wide technical application. The analysis of obtained
mechanoacoustic characteristics pointed out a complicated mechanism of
degradation of the materials. Microscopic investigation of samples, which were
stressed to different levels of load, enabled to specify the various course of
the gradual growth of microcracks and successive crushing out of elements of the
structure. These effects appear to be similar to the aging processes occurring
in the materials during long periods of exploitation under working load. For the
two tested materials there were distinguished three stages of the structure
degradation. The preliminary and subcritical ones show low or moderate intensity
of AE signals and considerable variety for the particular samples. The critical
stage directly precedes destruction of the samples. Its range is relatively
narrow and contains AE activity of high energy. The presented results enable to
draw up conclusions concerning the resistance of investigated materials to the
aging degradation processes development during long-term operation. Comparison
of the results of mechanical, acoustic and microscopic studies have revealed
that the differences registered for the strength and characteristics are due to
inhomogeneities of the materials in the semi-macro as well as in the
micro-scale. The effectiveness of dispersive and fibrous reinforcement of
aluminous porcelain was described. Strengthening of porcelain by corundum grains
and mullite needle-shaped crystals improves mechanical parameters and
distinguishes this material from typical aluminosilicate ceramics. The
occurrence of groupings of bigger grains in the structure of the corundum
material represents most probably the intermediate state, leading to the known
effect of the abnormal grain growth (AGG).