Osumanu, Ahmed2024-07-242024-07-242024-05https://hdl.handle.net/11299/264267University of Minnesota M.S. thesis. May 2024. Major: Civil Engineering. Advisor: Joseph F. Labuz. 1 computer file (PDF); v, 60 pages.Subjecting rock specimens to increasing mechanical or environmental loads results in irreversible damage at a yield stress state, which may eventually lead to failure. This study used acoustic emission (AE) methods to investigate the yield stress states of Valentine limestone and Dunnville sandstone. By observing AE responses and assuming the Kaiser effect occurs when a stress state contacts the yield surface, a loci of threshold stresses forming a line in the σ1 – σ3) stress plane was developed for the sandstone. Cyclic loading on two subcore specimens of Valentine limestone was performed to study the uniaxial yield stress. In addition, axisymmetric(σ1>σ2=σ3) probing of the damage surface was conducted on both limestone and sandstone specimens using a Hoek-Franklin pressure cell. A modified University of Minnesota plane strain device enabled multiaxial compression tests on rectangular limestone specimens instrumented with AE sensors. Stress, strain, and AE events were recorded and cumulative AE counts versus axial stress plots were used to determine yield stress states. The AE results showed yield stress levels not detected on the stress-strain curves. While AE events during loading were few and inconsistent for the limestone, the AE activity was significant for sandstone specimens. A Mohr-Coulomb yield criterion was fitted to the yield stress data assuming a common vertex for the failure and yield surfaces. Thus, both internal friction angle and cohesion increase during hardening. In the multiaxial tests on the limestone, the Kaiser effect could not be established for the AE onset stress to be compared with the measured in-situ state of stress because of insufficient AE counts.enAcoustic emissionDamage surfaceIn-situ stressKaiser effectYield stressThesis or Dissertation