Evaluating Damage and Microcracking Behavior of Granite Using NMR Testing under Different Levels of Unconfined Compression

Abstract

This work discusses the results from tests performed to investigate the damage and microcracking behaviors of granites. The samples, previously loaded to varying predefined stress levels and then unloaded to zero, were tested using a nuclear magnetic resonance (NMR) technique. The variations in the micro-NMR parameters, including the NMR-derived porosity, transverse relaxation time (T2) spectra distribution, and T2 area of the samples, were quantitively analyzed prior to and after the uniaxial loading/unloading tests. The results show that the induced damage in the samples increases with the loading ratio, and the rate of increase is significantly accelerated when the loading ratio is greater than 0.7. An NMR-based damage parameter was introduced to evaluate the degradation process under unconfined compression. Three stages of microcracking evolution were distinguished under the unconfined loading condition according to the variations in the proportion of the micropores and macropores. The NMR technique offers an alternative method to characterize the mesodamage and microcracking of rock under compression and has the advantages of being accurate and nondestructive.