Abstract:

In order to find how the mesostructure character of rock influences the crack propagation, the heterogeneity degree, grain size in limestone, marble and granite gneiss samples were examined respectively by means of scanning electronic microscope(SEM) and polarizing microscope. Then, the three-point bending samples with an pre-existing mode I crack were impacted by drop-weight and the cracking modes of above-mentioned three types of rocks were captured by high speed photography. By contrast, there is largest microcracking area ahead the crack tip in the granite gneiss samples with the highest heterogeneity, so is the lowest dimensionless ultimate crack velocity during the dynamic fracture process. On the contrary, since the lowest heterogeneity of the limestone samples, the smallest microcracking zone and highest dimensionless crack velocity were obtained. In this regard, there should be an internal characteristic length scale that domains the ultimate crack speed in rocks: the larger length scale is, the lower crack velocity limit would be.

Key words: rock, heterogeneity, crack velocity, microcracking propagation zone, internal characteristic length scale