Abstract
This experimental paper comprises the results of acoustic emission (AE),
microscopic and ultrasonic measurements of samples subjected to slowly
increasing compressive stress. On the basis of conducted measurements the
successive stages of the material structural degradation have been
recognized. The objects of study were samples made of C 120 aluminous
porcelain. The investigated material has found at present the application
in the fabrication of technical elements like overhead power line
insulators. In the case of such objects, not only high mechanical
strength, but especially elevated durability as well as operational
reliability are required. The expected "life time" of net insulators
during exploitation is about 40 years. The analysis of obtained
mechanoacoustic dependences pointed out a complex mechanism of degradation
of the material. Microscopic investigation of samples, which were stressed
to different levels of load, enabled to specify the development of gradual
growth of microcracks and successive crushing out of elements of the
structure. These processes appear to be similar to the ageing processes
occurring in the material during long period of exploitation under a
working load. Three stages of the structure degradation were
distinguished. 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 the destructtion of samples.
Its range is relatively narrow and reveals the AE activity of high energy.
The effectiveness of dispersive and fibrous reinforcement of modern
aluminous porcelain C 120 type has been described. Structural
strengthening by corundum grains and mullite needle shaped crystals
improves mechanical parameters and distinguishes this material from
typical aluminosilicate ceramics. The presented results enable drawing up
the conclusions concerning the resistance of investigated material to the
ageing degradation process development during long term operation.
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