A zone of distributed microcracking is often suggested to accompany tensile macrocrack propagation in rocks and ceramics. The microcracking is said to be largely responsible for (1) high values of fracture energy (2) increasing resistance to fracture with crack extension and (3) the dependence of fracture mechanics data on the experimental setup. In the present paper, the material breakdown processes in imperfectly elastic Westerly granite are investigated using ultrasonic wave probing and in situ microscopy during mode I fracture experiments. These observations are compared with an in situ reflection/transmission microscopy investigation of mode I fracture in a near-ideal elastic polycrystalline alumina (Al2O3). As defined by the spatial distribution of longitudinal and surface attenuation in wedge-loaded double-cantilever beam specimens of Westerly granite, the fracture processes zone is elongate in the direction of fracture propagation (15--40 mm long by 1--2 grain dimensions wide; grain size 0.75 mm). As revealed by in situ reflection microscopy, the ultrasonic wave energy is partially transmitted through the developing fracture surfaces via two sources of crack interface traction: (1) remnant islands of unfractured material left behind the advancing fracture front and (2) geometrical interlocking of the microstructurally rough fracture surfaces. A similar zone of crack flank tractions is found in the alumina (greater than 2000 μm long; grain size 20--100 μm). No evidence of a diffuse kidney-shaped cloud of microcracking distributed ahead of the main fracture tip (predicted by many fracture models) was found in either material.

Instead, interface-localized microcracking was observed to operate at positions where the tractions, or restraining forces, are transmitted across the nascent fracture surfaces. Crack flank tractions shield the main crack tip from high levels of stress and are relieved by friction-induced microcracking and microcrack rupture of intact-material bridges. As a consequence of the crack history dependence of the crack tip shielding, it is proposed that, even under small-scale inelastic deformation conditions appropriate to linear elastic fracture mechanics, neither R-curve behavior nor applied-KI/subcritical crack velocity relationships are intrinsic properties of these and similar materials. ¿ American Geophysical Union