The index of hackle raggedness on joint fringes

In a previous laborious study we showed that considerably more new area is formed in hackle sections than in en echelon sections per given nominal area of fringe. In the present study we resort to an alternative quantitative method that would enable a relatively rapid correlation between the fracture mode of secondary fractures on joint fringes and their change in fracture morphology. We introduce a set of seven criteria that enable a geologist to place every fringe on the 'index of hackle raggedness' (IHR) along a scale between 1 and 10. This method is applied to the 'Mrákotin joint set' from a granite quarry in the Bohemian Massif of the Czech Republic. The results show an IHR range between 1.4 ± 0.7 and 6.8 ± 0.7. The gap between the top value of the scale, 10, and the value 6.8 probably suggests that the joints in this study did not form under the most dynamic conditions. Experimental results carried out on polymers show that dynamic fracture is better correlated with the dynamic stress intensity and additional fracture mechanics parameters than with fracture velocity. The demonstration that en echelon segmentation may be obtained at maximum fracture velocity, if mechanical constraints are imposed on the system by crystal anisotropy, possibly suggests that similar results may be obtained under other constraints that need to be elucidated.