New fractographic aspects of natural and artificial fractures in chalks from the Upper Galilee, Israel, and experimental fracture in Perspex

The fractographies of an artificial fracture that was formed by a blast and a natural fracture cutting chalks that display mirror planes and hackled fringes are similar. In both fractures the mirror and frringe are separated by clear boundaries, and the locations of the maximum hackle intensity and maximum φ angle (produced between the mirror plane and the fringe) coincide with the positions where segments of the mirror boundaries that have different curvatures meet. On both the natural and artificial fractures, radial hackles associated with the different boundary segments focus on distinct origins. The φ angle varies from 0°to 30°± 5°on the natural joint and from 30°± 8°to 50°± 8°on the artificial fracture. The coinciding positions of hackle intensity and φ maxima observed in the two fractures match well with bifurcating fractures induced by experimental Hertzian fracture in Perspex. These are further supported by a geometric mismatch analysis. The present results demonstrate that the K(I) increases, which occur in positions where fractures of different ellipticities meet, enhance hackle intensity, crack-branching and the value of φ. (C) 2000 Elsevier Science Ltd. All rights reserved.