David E. Dunn

Experimental sliding friction and cataclasis of foliated rocks

Abstract Forty-five degree sawcuts were made in 80 cores of gneiss and mylonite such that foliation-to-sawcut angles of 0, 15, 75, 90, and 135° were produced. Triaxial compression sliding friction tests at confining pressures of 140 and 500 bars indicate that the coefficient of sliding friction is influenced by variation of foliation to sliding surface angles. In clean surface tests there is reactivation of micaceous foliation planes causing interlocking steps to form on the sliding surface. A 1-mm thick layer of gouge inserted between sliding surfaces prevents the interlocking of steps and lowers the coefficient of sliding friction. Variation in grain size exerts no systematic influence on the coefficient, nor does quartz or mica abundance; however, the incidence of stick-slip increases with quartz content at 140 bars confining pressure. Stick-slip was the dominant behavior for all compositions and orientations at 500 bars and a strain rate of 10−4/sec. Stick-slip as an earthquake mechanism probably depends on composition only at very shallow depths. Grooves developed on the sliding surfaces probably are modified Riedel shear fractures. Glass-like material produced in both the clean-surface and gouge-included tests suggests the local development of very high temperatures during sliding. Sliding-induced cataclasis occurs with very small displacements and the width of the cataclastic zone depends on grain size.

Wide Field Of View Varifocal Near-Eye Display Using See-Through Deformable Membrane Mirrors

Accommodative depth cues, a wide field of view, and ever-higher resolutions all present major hardware design challenges for near-eye displays. Optimizing a design to overcome one of these challenges typically leads to a trade-off in the others. We tackle this problem by introducing an all-in-one solution — a new wide field of view, gaze-tracked near-eye display for augmented reality applications. The key component of our solution is the use of a single see-through, varifocal deformable membrane mirror for each eye reflecting a display. They are controlled by airtight cavities and change the effective focal power to present a virtual image at a target depth plane which is determined by the gaze tracker. The benefits of using the membranes include wide field of view (100° diagonal) and fast depth switching (from 20 cm to infinity within 300 ms). Our subjective experiment verifies the prototype and demonstrates its potential benefits for near-eye see-through displays.

Superposed Deformation and Polymetamorphism, Brevard Zone, South Carolina

Textural studies and detailed field mapping in the Brevard–Poor Mountain belt in northwestern South Carolina indicate that this zone experienced at least two periods of polyphase deformation and metamorphism. The first deformation, F 1 , produced sheared-out isoclinal folds. Progressive metamorphism, M 1a , in the almandine-amphibolite facies accompanied F 1 , as did dislocation metamorphism, M 1b , and retrogressive metamorphism, M 1c , produced by the attenuation and shearing out (tectonic slides) of the isoclines. A second deformation, F 2 , nearly coplanar and coaxial with F 1 , also was accompanied by polyphase metamorphism, M 2 . The first phase of this metamorphism, M 2a , was progressive, attaining the upper greenschist or greenschist-amphibolite facies. Dislocation metamorphism, M 2b , and retrogressive metamorphism, M 2c , occurred during the waning stage of F 2 deformation. Tentatively, we correlate F 1 and M 1 with the Taconic orogeny and F 2 and M 2 with the Acadian orogeny. All aspects of F 1 , M 1 are more intense than F 2 , M 2 , indicating that Ordovician tectonism was the most important diastrophism affecting this portion of the Brevard9s development. A third retrogressive metamorphism, M 3 (?), is proposed but the evidence is not conclusive. The distribution and structural relations of this third phase of retrogressive metamorphism suggest that it is probably mainly related to late Paleozoic faulting.

Effect of anisotropy on the coefficient of sliding friction in Schistose rocks

Abstract Foliated schist samples were sawcut at 45° to th cylinder axis and tested at 500 bars confining pressure, 25°C, and 10−4sec−1 shortening rate, to examine the effect of anisotropy on the coefficient of sliding friction (μ). Cylinders were cored from sample blocks in a variety of orientations to produce sawcut-to-foliation (STF) angles ranging from 0° to 180° in 15° increments. The variation of μ with foliation orientation is sinusoidal. Maximum values of μ occur when the foliation is parallel to principal stress planes ( STF = 30°−45° and 105°−120°; τ = 0 ), and minimum values of μ correspond to planes of maximum resolved shear stress ( STF = 75° and 165°, τ = τ ). Sliding-surface damage and gouge development vary inversely with μ. Surface damage results from the interaction of asperities with the sawcut surface, and failure and cataclasis of asperities produces gouge. Precursor events visible on force-time records, and partial loading tests terminated before megascopic sliding, indicate that surface damage and gouge are generated before megascopic sliding on the sawcut. Apparently, high resolved shear stress produces slip along the foliation prior to sliding on the sawcut. Foliation slip generates gouge at the sawcut interface; and the effect of the gouge is to lower μ.

Mesoscopic Fabric and Structural History of Brevard Zone and Adjacent Rocks, North Carolina

Structural fabric along and across the Brevard zone in North Carolina suggests that Phase I isoclinal folds overturned northwestward, mylonitic rocks along the zone, and Phase II open folds with northeast-trending axes all are compatible with a structural history involving compressive straining perpendicular to the length of the zone. The northeast-trending mineral lineation associated with mylonitic rocks occurs parallel to the direction of maximum extension within the mylonitic foliation surface, and no fabric elements observed require major northeast-southwest transport along the zone. The Brevard belt appears to have developed initially as an isoclinal synform, and reverse-slip displacement along the zone occurred as isoclinal folds overturned northwestward. Consistency of Phase I and Phase II structural fabric along and across the zone suggests that the belt does not mark a major structural discontinuity, and may indicate that the Blue Ridge thrust sheet does not root in the Brevard belt.

Membrane AR: varifocal, wide field of view augmented reality display from deformable membranes

Accommodative depth cues, a wide field of view, and ever-higher resolutions present major design challenges for near-eye displays. Optimizing a design to overcome one of them typically leads to a trade-off in the others. We tackle this problem by introducing an all-in-one solution - a novel display for augmented reality. The key components of our solution are two see-through, varifocal deformable membrane mirrors reflecting a display. They are controlled by airtight cavities and change the effective focal power to present a virtual image at a target depth plane. The benefits of the membranes include a wide field of view and fast depth switching.

Mitigating vergence-accommodation conflict for near-eye displays via deformable beamsplitters

Deformable beamsplitters have been shown as a means of creating a wide field of view, varifocal, optical see- through, augmented reality display. Current systems suffer from degraded optical quality at far focus and are tethered to large air compressors or pneumatic devices which prevent small, self-contained systems. We present an analysis on the shape of the curved beamsplitter as it deforms to different focal depths. Our design also demonstrates a step forward in reducing the form factor of the overall system.