Experimental Studies on Three-dimensional Stress Fields near Fast Propagating Crack Tips by means of Pulses Holographic Microscopy

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A fast propagating crack often appears when a plate of brittle material is broken by external tensile force. The crack is of the opening mode and propagates at a speed of several hundred m/s. The stress field around the crack tip satisfies the plane stress condition in the region a certain distance away from the tip. Namely, the stress field is twoーdimensional there. The stress field, however, shows threeーdimensional (3-d) nature in the vicinity of the crack tip. Fracture of brittle materials actually occurs in the 3-d stress field, therefore, it is important to make the structure of the 3-d field clear. Accordingly, we carried out the experimental studies which were focused on the three point described below.(1) How far does the 3-d field spread from the fast propagating crack tip?(2) What relation is between the surface singularity of the 3-d field and the crack front edge angle of the fast propagating crack?(3) What shape does the fast propagating crack have in the 3-d field and in … More the specimen?As a result, we obtained the following conclusions.(1) The 3-d stress field spreads as far as about half of the specimen thickness from the crack tip, in the direction of 72 degrees from the propagation direction of the crack. The result is same as that on a stationary crack which was shown by Shimizu and Shimada (1978), Shimada and Sasaki (1983), Shimizu et al. (1985) and Rosakis and Ravi-Chandar (1986). It can, consequently, be said that the extent of the 3-d stress field is independent of crack speed. And it was also shown that the extent of the 3-d field is independent of specimen thickness.(2) On a specimen surface, the crack opening displacement (COD) of the fast propagating crack is proportional to square root of the distance r from the crack tip, even in the 3-d stress field. It means that the surface singularity of the 3-d field is 1/<square root>r. The result is in agreement with the theoretical result on a slowly propagating crack given by Bazant and Estenssoro (1979). The singularity of 1/<square root>r is, hence, independent of crack speed. The crack front edge angle is not a right angle but obtuse. In case that crack speed is relatively lower, the crack front edge angle is not at right angle but obtuse. In case that crack speed is relatively lower, the crack front edge angle is in agreement with the numerical result on a slowly propagating crack given by Bazant and Estenssoro (1979). But the crack front edge angle decreases and approaches a right angle, as the crack speed increases. It can, thus, be said that the crack front edge angle varies in order to preserve the surface singularity to be 1/<square root>r, when the crack speed changes.(3) Crack opening displacement is proportional to the square root of the distance from the crack front edge, not only on a specimen surface but also on the mid-plane in a plate specimen. And the value of the dynamic stress intensity factor on the mid-plane is nearly same as that on a specimen surface. Less