Changes in the nanomechanical properties of the constituent minerals in the ductile fauske marble and the brittle kuru granite during progressive failure

The nanoscale elastic moduli and hardness of the constituent minerals of the Class II Kuru granite and the Class I Fauske marble were experimentally investigated. The aims were to correlate the microcrack patterns with the nanomechanical properties of the minerals, and to help understand the important roles of the nanomechanical properties of the minerals in brittle and ductile behaviors. Cylindrical rock specimens were uniaxially loaded to various stress levels in both the pre- and post-peak stages. The specimens were then unloaded to zero, and two thin sections –– one parallel with and the other perpendicular to the loading direction –– were prepared from each specimen. Nano-indentation tests were conducted on the thin sections to measure the elastic moduli and hardness of the major constituent minerals in the rocks. The test results showed that both the elastic moduli and hardness of the minerals abruptly decreased when the applied stress was above 80 % of the uniaxial compressive strength of the rock in the pre-peak stage and also in the entire post-peak stage. At the same time, the values of the two properties became more scattered with increasing damage to the minerals. The number of intragranular cracks was significantly less in the harder quartz and microcline than in the softer calcite. The abundant intragranular cracks in the calcite dissipated most of the strain energy in the Class I marble, such that the rock was not burstable after failure. A small number of intragranular cracks were created in the quartz and microcline in the Class II granite, such that most of the strain energy in the minerals was released to eject rock after failure. Intragranular cracking is thus a key factor in determining whether a rock is burst-prone or not.