抄録
In a previous paper, it was shown that a crack, with the initial length longer than a critical one, can propagate in an elastic medium with the crack velocity which approaches asymptotically to the S wave velocity in the medium. The next problem is how to stop the crack propagation thus acceralated. A way to do this is to assume the pre-stress in the medium is limited to a region, from-xm to+xm, say, where x is in the direction of crack propagation. This problem is studied in the present paper and we get the following results. (1) crack does not stop at once even when it passes through x=xm (Fig. 4, 5, 6). (2) Final crack length cs is about twice as long as xm (Fig. 7). (3) The longer the length xm, the larger the mean velocity of the crack (Fig. 8).