Measurement of the intrinsic process zone in rock using acoustic emission

Laboratory specimens with a smooth boundary (no notch) were fabricated from four different rock types (grain sizes from 0.1-10 mm) and were tested in three-point bending. The locations of acoustic emission that occurred around peak load were used to characterize the dimensions of the region of localized microcracking, the so-called intrinsic process zone. The size of this zone was found to vary significantly between the rock types, but was not found to vary significantly for different size beams of the same material, An approximately linear relation was found between the width of the intrinsic process zone and the logarithm of the grain size. An explanation of size effect evolves naturally by considering the intrinsic process zone as a material characteristic. Two different models were used to analyze stress at failure. The first treated the intrinsic process zone as a mathematical crack, an infinite stress raiser, from which a K-IC value was calculated. The second model assumed the intrinsic process zone was a cohesive notch, a finite stress raiser, from which the theoretical tensile strength was defined. The second method seemed somewhat better at predicting size eject on nominal peak stress. (C) 1998. Published by Elsevier Science Ltd.