Identification of Micro-cracks in Ceramics by AE-Wave Form Analysis.

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Evaluation of fracture toughness is very important for ceramics which has poor toughness. When the large specimen is prepared separatery for toughness testing, the <K_(IC)> value of large specimen is sometimes different from that of objective materials. Therefore the Indentation Microfracture (IM) method becomes very useful, because of the unnecessity of large specimen. However, IM method includes many problems as time dependency of crack extension and complex interaction of different types of crack, etc. In this research, acoustic emission (AE) technique is applied to identify the type of cracks during the measurement by IM method. To make clear the identification, glass was used as a model materials, because of the possibility of in situ observation. At the initial indentation period, small surface cracks were observed on the contact area of the indentor. Radial type penny shape median crack initiated just beneath the indentor around few kgw during the increase of load, or decreasing load. Two penny shaped median cracks developed to half penny shaped cracks during decreasing the indentation load. Just before taking out the load, lateral type cracks initiated. Acoustic emissions were accompanied to every crack and the amplitude of the median crack was the largest one. Different from these types of the cracks, interfacial debonding occurres in the composite materials. To make clear the identification of the debonding, model samples were made from the transparent plastic as a matrix and lead of the pencil or carbon fires as a filler materials. After wave form analysis of the AE signals, small amplitude and high frequency were characterised to that of debonding. After three years research,it was cleared that the identification the different type of cracks is possible by analysing the wave form of the AE signal.