ACOUSTIC-EMISSION, MICROSTRUCTURE, AND DAMAGE MODEL OF DRY AND WET SANDSTONE STRESSED TO FAILURE

Twenty-three uniaxial compression tests were performed on dry and wet Flechtingen sandstone from Germany. Compressive strength of wet core i s 60% of the strength of dry core. Before fracture, the transverse P w ave speed drops by 13% and the pulse amplitude by 22% for wet and 37% for dry cores. Accumulated strain energy doubles for dry core. Acousti c emissions (AE) are detected with 10 sensors for 19 cores. AE activit y starts at 84% of the fracture strength of wet cores (55 MPa) and at 91% of the strength of dry cores (87 MPa). The ratio of located to rec orded AE is 0.37 for dry and 0.13 for fully wet cores. AE hypocenter p atterns document the development of two opposite fracture cones. The n egative slope of cumulative AE-amplitude frequency distribution drops by 50% before failure in dry cores. The slope of the wet core drops an d recovers. Energy discrimination of AE detected by a broadband sensor resolves different stages of damage and captures the onset of the dil atant throughgoing macrofracture. Using the analogy to visible light m icrofracturing events are separated into high-energy short pulses (blu e AE) and low-energy pulses with long duration times (red AE). Blue AE are explained by intragranular grain breakage, red AE by multiple sti ck slip on crack planes or grain boundaries. Deformed cores show highl y fractured calcite cement and mostly intact quartz grains. The stocha stic damage model for brittle composites developed highlights that mic rofracturing of the sandstone is controlled by the amount and distribu tion of the weak mineral (calcite).