Observation of crack propagation in non-metals and fracture mechanism

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The propagation velocity of crack can be measured with high resolution with a optical system because when a light beam incidents obliquely on a very flat crack plane in a transparent solid, it can be totally reflected at the crack plane. The objective of this study is to understand the fracture mechanism in solid. The optical method with high-speed response has already been established, then by using the grant we planed to observe the shape of the crack tip together, too. With this apparatus the crack propagation mainly in NaCl single crystals was measured and the change of the shape of the crack tip was observed with 0.1 musec resolution.In the first year, we divided the parallel laser beam into several long and narrow ones and constructed the optical system by using a multi-channel wave memory to measure the change of the intensity of every light beam at the same time. In the last year, the number of the channels was improved from 4 to 8, although it was very difficult to eliminate the interference among the channels because the photo sensors with high frequency and the amplitudes are set up in narrow space each other. By the observation of crack extension in NaCl single crystals, we made the following clear. Crack extension is not uniformly both spatially and temporally. That is ; the initiation of crack propagation occurs locally and proceeds through its expansion. In some cases, one part of the crack may begin to propagate very late.The crack velocity often reaches the maximum value (about 1000m/sec) immediately after its initiation of propagation and crack can also propagate, on the other hand, at the lower speed at the beginning and at the upper later. For the experiment of the observation of the crack propagation in semi-conductor that is one of the objective of this plan, the measurements could not be run with adequate resolution because of lack of uniformity of the laser beam.