In this study, acoustic emissions (AE) were monitored during a Mode I fracture toughness test on a large cracked chevron notched Brazilian disc (CCNBD). The fracture toughness KIC was calculated to be 1.55 MPa. (m)1/2 for this sample. A rapid increase in AE event rate occurs six seconds prior to failure; AE source locations initially show a linear feature extending from the notch tip, the locations then spread laterally to the edge of the sample causing macroscopic failure . AE source mechanism analysis, based on first motion polarity response, demonstrated events were predominantly of tensile type. Microstructural analysis was conducted through optical examination, demonstrating micro-fracture density increases from a background density of 4 cm/cm2 to 11.5 cm/cm2 in the fracture process zone (FPZ). AE locations were used to infer the width of the process zone, which was found to be variable and extended up to 10 mm on either side of the macro fracture plane. In the fracture process zone in front of the arrested macro fracture tip, micro-fracture density was found to be equal to 44 cm/cm2. The width of the FPZ based on the distribution of AEs is almost equal to the optically determined width. The micro-fracture density increases exponentially from the far-field towards the macro-fracture plane. This study demonstrates that the FPZ can be studied and defined using AE.
Brittle fracture processes are taking place at the grain scale and have a profound influence on the mechanical properties of rocks. Being a natural byproduct of micro-fracture growth and brittle fracturing, Acoustic Emissions (AE) have proven to be a good diagnostic tool for the better understanding of fracture processes in rocks at various scales. Valuable work has been done in laboratory AE studies, to understand fracture processes in compressional tests [1-5]. Laboratory experiments of rock fracture were performed by controlling axial stress to maintain a constant AE rate, permitting quasi-static propagation of shear fracture (Mode II), [1]. The development of fast acquisition systems enabled AE experiments to show fracture development under constant stress loading, thought to better approximate the low strain condition in the Earth [5]. Through monitoring of AE hypocenters and based on knowledge of the focal mechanism type, [5 & 6], it has been shown that tensile cracking was dominant only in the area in front of the fault (process zone). Shear became the major/dominant mode of cracking in a damaged zone behind the process zone. Based on the nucleation model [7 & 8], it is revealed that the earthquake generation process can be considered as a transition process from a quasi-static (very low velocity of cm/s to m/s) nucleation to a dynamic rupture (velocity of Rayleigh wave in rock), [9 & 10]. The fracture process zone (FPZ) in rocks is defined as the region affected by micro-cracking and frictional slip surrounding the visible crack tip propagating under stress, [11 & 12]. The width of the FPZ is defined as the longest distance between visible cracks on either side of the macrofracture and/or fault and its length is defined as the length between the fault tip and the crack with the greatest distance in front of the fault tip.