MICROSCOPIC FRACTURE PROCESSES IN A GRANITE

The deformation of a competent, brittle, granitic rock is thought to h ave two main components: elastic and brittle deformation, the latter c aused by axial microcracking. Dynamic fatigue testing of Lac du Bonnet granite would, however, suggest the presence of a third mechanism, co mpaction. Compaction is not the same as elastic crack closure; compact ion entails permanent damage along grain boundaries that are under hig h compression. During compaction, the axial stiffness (elastic modulus ) of the rock increases and the permanent crack volume becomes negativ e (compression). Compaction is active at all stress levels, but it is most noticeable at low stress where its presence is not masked by dila tion caused by axial microcracking.