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

对于韧性较差的陶瓷材料，断裂韧性的评价非常重要。在大试样单独制备进行韧性测试时，大试样的<K_（IC）>值有时会与目标材料的<K_（IC）>值不同。因此，压痕微破裂（IM）方法变得非常有用，因为不需要大的试样。然而，IM方法包括许多问题，如裂纹扩展的时间依赖性和复杂的相互作用的不同类型的裂纹，在本研究中，声发射（AE）技术应用于识别裂纹类型的测量过程中IM方法。为了明确识别，使用玻璃作为模型材料，因为可以原位观察。在压痕的初始阶段，在压头的接触区域上观察到小的表面裂纹。在载荷增加或减小时，压痕下方几公斤重左右处产生一个放射状的硬币形中间裂纹。在压痕载荷降低过程中，两个硬币形中间裂纹发展为半个硬币形裂纹。就在取出载荷之前，横向型裂纹开始。每一条裂纹都伴随有声发射，中间裂纹的振幅最大。与这些类型的裂纹不同，复合材料中发生了界面脱粘。为了明确剥离的识别，模型样品是由透明塑料作为基体和铅笔芯或碳火作为填充材料制成的。对声发射信号进行波形分析后发现，剥离声发射信号具有幅值小、频率高的特点。经过三年的研究，明确了通过分析声发射信号的波形来识别不同类型的裂纹是可能的。

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.