B10,doi The biotite foliation in the Four-mile gneiss results in dilatancy and strength anisotropies, which become more pronounced with increasing confining pressure. Microstructural observations indicate that when there is high resolved shear stress on the macroscopic foliation, dilatancy arises from extensile microcracks nucleated by frictional slip on biotite grains.
Evolution of crack geometry and coalescence are also influenced by the biotite foliation. Motivated by these observations, a damage mechanics model based on sliding wing cracks was adopted to analyze the anisotropic 90/50 verniom of dilatancy and brittle fracture. Frictional coefficients for the sliding cracks are inferred to be comparable to those of cleavage surfaces of biotite.
The strength anisotropy data of the Four-mile gneiss can be explained by the variation of the initial damage with the foliation angle. The damage derives from a set of preexisting microcracks with random orientation, and a set of cleavage cracks in mica grains preferentially oriented along the foliation angle.
Hence, the initial damage is higher for the intermediate angles, and, consequently, the strength is somewhat lower.
The observation that the mechanical strengths of a variety of foliated rocks decrease with increasing mica content can be explained by the same model, with the implication that the initial damage and mica content are linearly related. The mechanical and microstructural data show that dilatancy anisotropy may significantly influence the progressive development of borehole breakout and strain localization.
Baud, and T. Wong, Dilatancy, brittle strength, and 90/50 verniom of foliated rocks: Experimental deformation and micromechanical modeling, J. Introduction nisms, depending on how stress is applied relative to the anisotropy planes. Illustrative examples were provided by  The phenomenon of mechanical anisotropy has Donath  on slate, Paterson and Weiss  on received considerable interest in rock physics and structural phyllite, and Kronenberg et al.
Bedding in sedimentary rocks, cleavage in slates, biotite. Elastic anisotropy of a rock can  One of the objectives of this work is to address the be related to its fabric, a seismic manifestation of which is interrelationships of textural anisotropy, dilatancy, and the shear wave splitting [e. Dilatancy, defined price, ; Siegesmund et al. Textural anisotropy as inelastic volume increase, is 90/50 verniom observed as a can also result in significant anisotropy of tensile [Nova and precursor to the inception of shear localization in the brittle Zaninetti, ; Liao et al.
The Donath, ; Borg and Handin, ; Vernik et al. It is recognized that the microcracks Copyright by the American Geophysical Union. To our knowledge, the onset and development of dilatancy in foliated rocks have not been systematically investigated.
Notwithstanding the paucity of laboratory data, assumption of dilatancy anisotropy has been implicitly or explicitly incorporated into the interpretation of borehole breakouts and in situ stress measurements [e.
It may also play a significant role in the localization of strain in retrograde shear zones [e. They defined a three-dimensional failure envelope with orthorhombic characteristics that is related to the symmetry of the foliation 90/50 verniom lineation of biotite within the rock. In this study we investigated the anisotropic mechanical behavior of the Four-mile gneiss by conducting triaxial compression experi- ments with volumetric strain and 90/50 verniom measurements, as well Figure 1.
Reference scheme for sample orientation. Two related ques- tions are addressed in this study.
Is the critical stress for the muscovite. Plagioclase grains have lengths ranging from 0. If so, why and how is it to 2. Quartz and microcline grains are subequant, related to the peak stress anisotropy? Biotite, the predominant phyllo- compressive strength has been analyzed by incorporating silicate, occurs as isolated grains, 0.
Measured poles to biotite grains define a strong [Walsh and Brace, ]. More refined models have been maxima perpendicular to the macroscopic foliation. Petro- proposed [e. In this study, we adapted .
The samples were cored in five orientations within a the damage mechanics model of Ashby 90/50 verniom Sammis  plane perpendicular to the macroscopic foliation and con- to analyze the observed mechanical anisotropy in terms of taining the lineation corresponding 90/50 verniom the x — z plane crack nucleation around a preexisting weak 90/50 verniom and the defined by Gottschalk et al.
Sample orientations influence of the preferred orientation of biotite on subse- are described by the angle b that specimen axis makes with quent damage accumulation.
Synthesis of our mechanical the foliation Figure 1. The porosities so measured for four state and mica content. The implications of our results for randomly selected samples ranged from 0. Hydro- related tectonic and geophysical processes 90/50 verniom also be hipertansiyon oluşum mekanizmaları compression data indicate that the crack porosity discussed.
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Mechanical Deformation copper foil thickness 0. Sample Material and Preparation MPa to seat the jacket and collapse it tightly onto the  A detailed petrographic description and analysis of sample. One axial and one radial strain gauge of lengths 10 the Four-mile gneiss were given by Gottschalk et al. The samples were further The axial, a Samples were unloaded near the peak stress for microstructural transverse and volumetric strains are plotted as functions of observations.
In this paper, compressive stresses and strains are considered to be positive. The onset of dilatancy C0 is marked by the stress level at which the differential stress — volumetric strain curve deviates from clamped to the steel end spacers with wire. Strain yüksek tansiyon taşikardisi belirtileri linearity [Brace et al.
A piezoelectric transducer PZT-7, 5.
Fırat 7711000050 PPRC Dirsek 90° 50'lik
Preexisting cracks are not fully closed until the attached to one spacer to monitor AE activity during the application of — MPa confining pressure. Since tests experiments. The confining pressure was monitored with a strain gauge pressure transducer with an accuracy of 0. The axial displace- ment was monitored with a displacement transducer between the ram and the fixed pressure vessel.
The axial load was measured with an external load cell accurate to 1 kN.
The strain measurements have an accuracy of 0. The AE signals were conditioned by a preamplifier and the characteristics of the signal were screened with a discriminator [see Wong et al. Representative stress — strain curves and acoustic microstructural observation. The yield stress and onset of dilatancy are also experiments that ended in brittle failure, and then unloaded.
Microstructural Observations of Deformed Samples  In general the deformation microstructure in the gneiss is similar to granite and other low-porosity crystalline rocks deformed in the brittle field [Tapponier and Brace, ; Wong, ; Kranz, ; Wong and Biegel, ; Hirth and Tullis, ; 90/50 verniom and Haimson, ].
All samples failed along macroscopic faults formed by the growth and coalescence of extensile microcracks with a predominant orientation parallel to the maximum compres- sive stress. The optical microscopy observations of Gott- schalk et al. However, our SEM and optical microscopy Figure 3.
The general form of the stress — strain curves and AE activity are similar at all b values. There is no systematic trend for the amount of dilatancy  Sample FMG2 was deformed to just beyond the before failure to vary with b.
However, there is a general onset of dilatancy C0. The most significant observation in decrease in the amount of dilatancy with increasing pres- this sample is that there is evidence for crack growth around sure.
This increase is due to the elastic closure of microcracks.
There is appreciable sample-to-sample variability. To our knowledge, this is the first comprehensive set of data on the concomitant variation of C0 and brittle strength as functions of foliation orientation and confining pressure.
The onset of dilatancy and peak stresses follow qualitatively similar trends in anisotropy. The peak differential stresses and their variation with b agree with the results of Gottschalk et al. These are typical of stress-induced microcracks [Tapponier and Brace, ; Kranz, ], with very low aperture-to-length ratios, smooth, parallel sides, essentially constant width, and very sharp terminations, often within grains.
The biotite grain has probably undergone sliding on an open cleavage crack, but slip displacements along such sliding cracks are difficult to resolve even under the SEM [Wong, ; Wong and Biegel, ]. Shear deformation in the biotite would cause a stress concentration at the end of the grain, which was relieved 90/50 verniom the formation of tensile cracks.
Sample FMG4 was deformed to the stage at which a surge in AE activity indicated the imminent attainment of the peak stress. This sample shows intense zones of cracking around the tips of the grains Figure 5b.
Most of the grain boundaries 90/50 verniom cracked. At this stress, kinking has begun in some of the biotite grains.
Crack coalescence is more intense in sample FMG5 which was deformed to near the peak stress. See Kroanenberg et al. Figure 6a shows some of the complex kink geometries that develop in biotite grains oriented at low angles to the compression axis.
Note especially the formation of voids and the tendency for the kink boundaries to trend diagonally across the basal planes. Intense intragranular cracking in feldspar and quartz grains which probably resulted from elastic mismatch between the compliant biotite and the stiff feldspar or 90/50 verniom in between can be seen. The elongate biotite grains often seem to act as barriers for the linkage of cracks across more than one grain Figure 6b.
Crack Coalescence  The three samples loaded to near the peak stresses were examined under the 90/50 verniom microscope to char- acterize the geometry of crack coalescence.
The biotite grains appear to exert some degree of control on the geometry of the growing crack Figure 5.
Direction of s1 was vertical, and biotite grains are localized in narrow zones that link the scale bar is 50 mm. The upper left of Figure 7b is typical. Grain incipient linkage of cracks.