The effect of stress anisotropy on the brittle failure of granite is investigated under uniaxial compression. Non‐standard asymmetric compression tests are performed on cores of Aue granite (diameter 52 mm, length 100 mm), in which 20 per cent of the core top surface remains unloaded. The edge of the asymmetric steel loading plate acts as a stress concentrator, from where a shear rupture is initiated. The propagation of the fracture‐related process zone from top to bottom of the core is mapped by microcrack‐induced acoustic emissions. Compared to standard uniaxial tests with symmetric loading, in the asymmetric tests both a greater quantity and more localized distributions of emission event hypocentres are observed. The maximum event density doubles for asymmetric (20 events per 10−6 m3) compared to symmetric tests. The cluster correlation coefficient, a measure of strain localization in the faulting process, reaches 0.15 for symmetric and 0.30 for asymmetric tests. The clustering of events, however, is found post‐failure only. Three different amplitudes are used to determine b‐values discussed as a possible failure precursor. Focal amplitudes determined at a 10 mm source distance and maximum amplitudes measured at eight piezoceramic sensors lead to b‐values that drop before rock failure. First‐pulse amplitudes automatically picked from emission wavelets show no anomaly. First‐motion polarity statistics of amplitudes indicate that a shear‐crack‐type radiation pattern is responsible for 70 per cent of the failure of granite, irrespective of stress boundary conditions. For type‐S events with an equal percentage of dilatational and compressional first motions, focal mechanisms are determined by fitting measured first‐pulse amplitudes to an assumed double‐couple radiation pattern. While hypocentres of large type‐S events align parallel to the later fracture plane, their fault plane solutions show no coherent pattern. Spatial views of fracture planes reconstructed from X‐ray computed tomograms reveal local small‐scale changes in fracture plane orientation. Nodal planes from average fault plane solutions of the microscopic acoustic emission events coincide with the overall orientation of the macroscopic fracture plane azimuth (strike angle) determined from thin sections and tomograms.

Abstract. In uniaxial compression tests performed on Aue granite cores (diameter 50 mm, length 100 mm), a steel loading plate was used to induce the formation of a discrete shear fracture. A zone of distributed microcracks surrounds the tip of the propagating fracture. This process zone is imaged by locating acoustic emission events using 12 piezoceramic sensors attached to the samples. Propagation velocity of the process zone is varied by using the rate of acoustic emissions to control the applied axial force. The resulting velocities range from 2 mm/s in displacement-controlled tests to 2 lam/s in tests controlled by acoustic emission rate. Wave velocities and amplitudes are monitored during fault formation. P waves transmitted through the approaching process zone show a drop in amplitude of 26 dB, and ultrasonic velocities are reduced by 10%. The width of the process zone is -9 times the grain diameter inferred from acoustic data but is only 2 times the grain size from optical crack inspection. The process zone of fast propagating fractures is wider than for slow ones. The density of microcracks and acoustic emissions increases approaching the main fracture. Shear displacement scales linearly with fracture length. Fault plane solutions from acoustic events show similar orientation of nodal planes on both sides of the shear fracture. The ratio of the process zone width to the fault length in Aue granite ranges from 0.01 to 0.1 inferred from crack data and acoustic emissions, respectively. The fracture surface energy is estimated from microstructure analysis to be -2 J. A lower bound estimate for the energy dissipated by acoustic events is 0.1 J.

A study of the wear behavior of several classes of potential cage m~terials for ai~craft gas turbine bearings was conducted, using a special wear-testing machine to approximate the projected service conditions of such bearings.It was found that: 1. Several heterogeneous alloys containing silver as a soft, low shear strength phase showed superior wear properties to presently used silicon-containing nickel-copper alloy ("S" monel) and iron-silicon bronze, even when silver-plated. The latter alloys were used as reference cage materials.. . . . The hard, load-supporting phase showing the most promise was a nickel base soith. additives of silicon or a silicide.. . 3. The materials showing the best wear properties were most efficiently fabTlca~ed by powder metallurgy techniques, utilizing the infiltration method for adding the siluer,

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