Microcracking Behavior of Gabbro During Monotonic and Cyclic Loading

Microcracks are an integral part of intact rocks that can develop when the rock is exposed to static or dynamic loads. The loading may not fail the rock. It, however, may cause permanent damage to the sample by expanding the existing microcracks or developing new microcracks. Investigating the development of microcracks in intact rocks is significant to investigate the damage evolution, which is not visible in macro-scale. In this study, the evolution of microcracks in gabbro is investigated under static and dynamic loading. Gabbro samples were subjected to monotonic and cyclic loading until a crack stress threshold (i.e., crack closure, crack initiation, crack damage). After each test, microcracking properties of the sample such as Linear Microcrack Density (LMD), crack length and crack types were defined using fluorescence thin section and combining polarized and fluorescence light optical microscopy. The results demonstrated that samples that are subjected to cyclic loading experience microcracks propagation earlier than those subjected to monotonic loading. In addition, for the samples under the same stress level, higher LMD was observed for the samples that were subjected to cyclic loading. This trend becomes more significant at higher stress levels. Except for the samples that are loaded until the peak strength, the length of the microcracks is higher for those that are subjected to cyclic loading. The trend of LMD changes during cyclic loading followed the sample displacement changes trend. In general, microcracks are propagated usually followed the weakness planes; however, in samples that are failed during cyclic loading, new microcracks formed which do not follow the weakness planes and show a random orientation. In a gabbro sample, it was observed that microcracks first propagate in biotite and then in plagioclase, amphibole, and pyroxene. Pyroxene and amphibole showed more resistance to the generation of new microcracks. This was partially due to high stiffness of minerals and partly due to alteration. Plagioclase offers the highest LMD among the gabbro component minerals. It was because it has two cleavage weakness planes and has a lower elastic modulus in comparison to the amphibole and pyroxene.

Automated summary

The study shows that samples subjected to cyclic loading exhibit higher Linear Microcrack Density (LMD) at the same stress level, as well as longer microcrack lengths. Microcracks usually propagate along weakness planes, but in samples failed during cyclic loading, new randomly oriented microcracks form. In a gabbro sample, microcracks first propagate in biotite and then in plagioclase, amphibole, and pyroxene, with pyroxene and amphibole showing more resistance to new microcrack generation.