Local Shear Planes Research Articles

Forced shear deformation behaviors of annealed pure titanium under quasi-static and dynamic loading

The mechanical behavior and microstructure characteristics of pure titanium with hat-shaped specimens compressed under quasi-static and dynamic loading were investigated. Results show that higher yield stress and peak stress are observed in dynamic specimen as compared to quasi-static specimen. Microstructure characterization reveals that a wide shear localization region is formed in quasi-static specimen, mainly composed of narrow elongated structures identified as resulting from the multiple generations of twins and subsequently subdivision and elongation along the shear direction, while an ASB is appeared in dynamic specimen, consisting of ultrafine equiaxed grains regarded as the result of rotational dynamic recrystallization of subgrains. The RDR mechanism is verified by thermodynamic and kinetic calculations. Three types of primary twins {101‾2}, {112‾2} and {112‾1} and multiple generations of twins are observed in both specimens. The proportion of {112‾1} extension twins increases significantly in dynamic specimen and {101‾1} contraction twins are detected within the ASB. The microhardness of shear localization region in quasi-static specimen and ASB in dynamic specimen is higher than that of other regions due to the strain hardening and ultrafine-grained strengthening, respectively. Microtexture analysis reveals that the strong microtexture induced by twinning that <0001> direction of the grains is ∼35° away from the local shear plane normal forms in the shear localization region of quasi-static specimen, while different microtexture that the <112‾0> direction and the {101‾0} plane of the grains parallel to the local shear direction and shear plane respectively, appears in the ASB of dynamic specimen.

Improved technical guide from physical model tests for TDR landslide monitoring

Time-domain reflectometry (TDR) can help observe the initiation and evolution of localized shear planes in both rock and soil slopes, and has temporal and spatial resolutions suitable for landslide monitoring. An improved and standardized technical guide for TDR cable installation and data interpretation is needed to utilize it better. TDR technique involves sending an electromagnetic pulse into a coaxial cable grouted in a pre-drilled borehole, and capturing the reflected signal from cable deformity, which is triggered by the localized shear deformation in the underground. This study developed a large direct shear box for testing TDR responses in various cable-grout-ground assemblies, in order to reliably model the cable-grout-ground interaction at the localized shear. Different combinations of cable type, grout condition, soil type, and shear bandwidth were tested using the new physical model to re-examine TDR's response to ground deformation in sliding mode. The results revealed some misconceptions in previous studies that used cable-grout assemblies without considering the entire interaction with the ground. The implications of the experimental results are carefully considered and new technical recommendations are provided for cable installation and data interpretation to foster the improved use of TDR. A new data reduction method that involves data filtering, differential waveform, and three-sigma rule is proposed to support the recommended installation setup for robust early detection of a localized shear plane.

Mechanical properties and failure of ECAE processed Mg97Y2Zn1 at different strain rates

As-cast Mg97Y2Zn1 billets with a two-phase microstructure were processed with four passes of equal channel angular extrusion (ECAE) using different routes (A, BC, and C) at 350 °C to refine the microstructure. The duplex microstructure of this material consists of a Mg-rich alloy matrix with a long-periodic-stacking-order (LPSO) reinforcement phase. Mechanical properties were evaluated under both quasi-static and dynamic loading conditions. It was found that routes A and BC were more effective in producing a Mg alloy with nearly isotropic properties. For route C, the specimens more easily fractured due to pre-existing localized shear planes induced by ECAE. Microstructural observations revealed that the post-processed average matrix grain size could be as fine as ∼1 μm. Uniaxial compressive loading and micro-hardness measurements verified the enhancement of strength, and it was noticed that the increase of strength comes primarily from grain refinement of the Mg matrix. In the composite, we also focused on elucidating the deformation mechanism of the LPSO phase. It is believed that localized deformation is responsible for the final failure of the ECAE processed Mg97Y2Zn1 alloy under both quasi-static and high strain rate loading. The grain morphology and distribution after deformation indicated that the shear localization resulted from the rotation and rearrangement of grains. Moreover, cracks were found to propagate along the grain boundaries leading to final failure.

Adiabatic shear deformation behaviors of cold-rolled copper under different impact loading directions

Adiabatic shear deformation behaviors in cold-rolled copper under different impact loading directions were systemically investigated in this paper. Split Hopkinson Pressure Bar (SHPB) system was used for the dynamic test. The microstructure and microtexture of the received shear localization regions were investigated by optical microscopy (OM), electron backscatter diffraction (EBSD) technique and microhardness tests. Three hat-shaped specimens were cut from cold-rolled copper sheet with the direction with an angle of 0° (RD sample), 45° (RD-45° sample) and 90° (TD sample) from the rolling direction within the rolling plane. The stress-strain curves indicate that the mechanical properties of the cold-rolled copper sheet show regularity in the work hardening dominated stage and that the shear stress of TD sample is the highest in that of the those three. OM results show that the morphology of adiabatic shear band in three samples shows differences that their deformation processes fall into two categories, pure relative shearing in RD sample and relative shearing with rotation in RD-45° and TD sample. EBSD results reveal that ultrafine grains with high-angle-boundaries are found within the adiabatic shear bands in three samples. The microhardness measurements show that the microhardness of shear region in three samples are significant higher than that of other regions. However, the size of grains within the shear band in RD sample are much larger than that in the other two samples. Microtexture analysis reveals that the grain orientations inside the shear band in three samples show similar characteristics that the <110> direction tends to parallel to the local shear direction and the {111} and {100} planes tend to parallel to the local shear plane. The calculated results show that rotational dynamic recrystallization can take place in the shear band.

Coalescing microstructure and fabric transitions with AMS data in deformed limestone: Implications on deformation kinematics

The microstructures of the heterogeneously deformed limestones of the Kolhan Group, Singhbhum Craton (SC), eastern India are investigated to study deformation mechanism, grain size and shape preferred orientation (SPO) development. The texture of the deformed rocks is analyzed by Electron Back-Scattered Diffraction (EBSD) in order to determine crystallographic preferred orientation (CPO) development in calcite grains. The c-axis pattern in protomylonite and mylonite define girdle patterns developed perpendicular to local shear planes showing monoclinic symmetry. The latter is interpreted in terms of a top-to-south shear, which is consistent with the observed microstructural and fabric data from the deformed rocks. The Anisotropy of Magnetic Susceptibility (AMS) studies are performed on 22 heterogeneously strained limestone specimens to document strain variation within the Kolhan rocks. The field fabric, SPO data measured from calcite and mica grains (κcal and κmi) and calcite CPO data are compared with observed variation in AMS fabric intensity (Pj) across the Kolhan basin. Integration of these data sets help to establish presence of large internal thrust planes within the Kolhan basin, which form a part of the regional foreland vergent thrust system developed along the northern margin of the SC.

Thermally induced failure mechanism transition and its correlation with short-range order evolution in metallic glasses

The effect of temperature on the short-range order (SRO) structures, deformation mechanisms and failure modes of metallic glasses (MGs) is of fundamental importance for their practical applications. However, due to lack of direct structural information at the atomistic level from experiments and the absence of previous molecular dynamics (MD) simulations to reproduce experimental observations over a wide range of temperature, this issue has not been well understood. Here, by carefully constructing the atomistic models of Cu64Zr36 and Fe80W20 MGs, we are able to reproduce the major deformation modes observed experimentally, i.e. single shear banding (SB) at low temperatures, multiple shear-bandings at intermediate temperatures and homogeneous plastic flow at elevated temperatures. By examining the evolution of SRO, we find that the different failure modes exhibit distinctively different full-icosahedron (FI) evolution pathways at different temperatures. Specifically, at low temperatures, the FI concentration first deceases to a minimum, then recovers slightly, and finally comes to a plateau; at intermediate temperatures, it first decreases and then reaches a plateau; while at elevated temperatures, it decreases simply monotonically. These different pathways arise from the dynamic competition between the destruction and recovering of FI clusters. We further show that the local softening caused by the destructions of FI clusters is crucial for the formation of localized shear planes and further shear bands. Since our simulations exhibit the same trend for both MG systems, it is expected that these findings may be generic for a wide range of MGs.

Adiabatic shear localization in pure titanium deformed by dynamic loading: Microstructure and microtexture characteristic

The adiabatic shear localization characteristic of annealed pure titanium was studied by means of hat-shaped specimen and Split Hopkinson Pressure Bar system. The microstructure and microtexture of the received shear localization regions were investigated by optical microscopy (OM), electron backscatter diffraction (EBSD) technique, transmission electron microscopy (TEM) and microhardness tests. The results show that microstructure within the adiabatic shear band consists of ultrafine grains with well defined high-angle-boundaries. In the vicinity of the shear band, grains are elongated toward the shear direction, and deformation twins including {101_2}<1_011> and {112_2}<112_3_> are frequently observed. Microhardness tests reveal that the value of Vickers hardness in the shear band is the higher than all of the outside regions due to the strengthening effect of the ultrafine grains. Microtexture analysis reveals that a stable orientation, in which the <112_0> direction tends to align with the local shear direction and the {101_0} plane tends to parallel to the local shear plane, develops in the shear localization region. The calculated results show that the dynamic recrystallization (DRX) can take place in the process of deformation on the basis of the rotational dynamic recrystallization mechanism.

Shear banding in commercial pure titanium deformed by dynamic compression

A cylindrical hexagonal-close-packed Ti sample was pre-deformed by dynamic compression to produce coarse-grained and ultrafine-grained structures in different parts of the sample followed by further dynamic compression to failure, making it possible to explore the effect of stored strain and grain boundary energy on shear banding in the material. A long shear band that formed during the final compression process passed through a complete diagonal of the sample. Electron backscattered diffraction was used to systematically investigate the shear-banding-induced structural evolution. Results show that the original stored energy in the matrix plays a significant role in the competition between deformation-induced grain refinement and grain growth, which determines the final average grain size in a shear band. Shear banding leads to grain reorientation such that one close-packed 〈112¯0〉 direction and one 〈101¯0〉 direction in most grains are parallel to the local shear direction and the normal direction to the local shear plane, respectively. The grain orientation in the shear band favours prismatic 〈a〉 slip, while the texture in the matrix, which is a stable compression texture, benefits the basal 〈a〉 slip. The results advance our understanding of the shear banding behaviour in heterogeneous deformation conditions and also the overall mechanical behaviour of materials under dynamic compression.

Particle simulation of the failure process of brittle rock under triaxial compression

In order to investigate the failure process of brittle rock under triaxial compression through both experimental and numerical approaches, the particle simulation method was used in numerical simulations and the simulated results were compared with those of the experiment. The numerical simulation results, such as fracture propagation, microcrack distribution, stress-strain response, and damage patterns, were discussed in detail. The simulated results under various confining pressures (0–60 MPa) are in good agreement with the experimental results. The simulated results reveal that rock failure is caused by axial splitting under uniaxial compression. As the confining pressure increases, rock failure occurs in a few localized shear planes and the rock mechanical behavior is changed from brittle to ductile. Consequently, the peak failure strength, microcrack numbers, and the shear plane angle increase, but the ratio of tensile to shear microcracks decreases. The damage formation during the compression simulations indicates that the particle simulation method can produce similar behaviors as those observed through laboratory compression tests.

Atomic mechanism of shear localization during indentation of a nanostructured metal

Shear localization is an important mode of deformation in nanocrystalline metals. However, it is very difficult to verify the existence of local shear planes in nanocrystalline metals experimentally. Sharp indentation techniques may provide novel opportunities to investigate the effect of shear localization at different length scales, but the relationship between indentation response and atomic-level shear band formation has not been fully addressed. This paper describes an effort to provide direct insight on the mechanism of shear localization during indentation of nanocrystalline metals from atomistic simulations. Molecular statics is performed with the quasi-continuum method to simulate the indentation of single crystal and nanocrystalline Al with a sharp cylindrical probe. In the nanocrystalline regime, two grain sizes are investigated, 5 nm and 10 nm. We find that the indentation of nanocrystalline metals is characterized by serrated plastic flow. This effect seems to be independent of the grain size. Serration in nanocrystalline metals is found to be associated with the formation of shear bands by sliding of aligned interfaces and intragranular slip, which results in deformation twinning.

Development of schistosity by dissolution–crystallization in a Californian serpentinite gouge

To understand the behaviour and deformation mechanisms of serpentinites in the seismogenic zone we study the deformation macro- and microstructures of serpentinites along the Santa Ynez Fault in the San Andreas System. At the outcrop scale, deformation is localized in a gouge zone that shows three different structures: (1) micrometric undeformed fragments (clasts) of the previously serpentinized peridotite, (2) localized shear planes (Y and R) and (3) a penetrative schistosity (S). Observations under SEM and TEM reveal that the schistosity corresponds to serpentine fibres, parallel to each other, and whose orientation varies as they wrap around clasts. TEM micro-textures indicate that these long fibres result from continuous syntectonic growth rather than from reorientation of pre-existing fibres implying a slow transfer process that occurs at short distances. We propose a dissolution–diffusion–crystallization process for the formation of the schistosity that corresponds to a low strain-rate creeping process of deformation that can be effective in aseismic fault segments.

Brittle to Plastic Transition in the Dynamic Mechanical Behavior of Partially Saturated Granular Materials

The effect of liquid viscosity, surface tension and strain rate on the deformation behavior of partially saturated granular material was studied over a ten order of magnitude range of capillary number (the ratio of viscous to capillary forces). Glass spheres of average size 35 microns were used to make pellets of 35% porosity and 70% liquid saturation. As the capillary number increased, the failure mode changed from brittle cracking to ductile plastic flow. This change coincided with the transition from strain-rate independent flow stress to strain-rate dependent flow stress noted previously [Iveson, S. M., Beathe, J. A., and Page, N. W., 2002, “The Dynamic Strength of Partially Saturated Powder Compacts: The Effect of Liquid Properties,” Powder Technol., 127, pp. 149–161]. This change in failure mode is somewhat counter-intuitive, because it is the opposite of that observed for fully saturated slurries and pastes, which usually change from plastic to brittle with increasing strain rate. A model is proposed which predicts the functional dependence of flow stress on capillary number and also explains why the flow behavior changes. When capillary forces dominate, the material behaves like a dry powder: Strain occurs in localized shear planes resulting in brittle failure. However, when viscous forces dominate, the material behaves like a liquid: Shear strain becomes distributed over a finite shear zone, the size of which increases with strain rate. This results in less strain in each individual layer of material, which promotes plastic deformation without the formation of cracks. This model also explains why the power-law dependency of stress on strain rate was significantly less than the value of 1.0 that might have been expected given that the interstitial liquids used were Newtonian.

Crenulation-slip development in a Caledonian shear zone in NW Ireland: evidence for a multi-stage movement history

Abstract In Scotland and Ireland, a Laurentian passive margin sequence, the Dalradian Supergroup, was deformed during the c. 470–460 Ma Grampian orogeny, resulting in the formation of crustal-scale recumbent nappes. In Ireland, this passive margin sequence is in general bounded to the SE by the Fair Head-Clew Bay Line (FHCBL), a segment of a major lineament within the Caledonides. Adjacent to the FHCBL, Dalradian metasediments in two separate inliers have undergone post-Grampian strike-slip movement, with the initially flat-lying Grampian nappe fabric acting as a décollement-like slip surface in both cases. As the orientation of these foliation slip surfaces was oblique to the local shear plane in both inliers, displacement along these pre-existing foliation surfaces was also accompanied by crenulation slip. However, the crenulation-slip morphologies produced imply the opposite sense of movement in the two inliers. 40 Ar- 39 Ar dating of muscovite defining the crenulation-slip surfaces indicates that post-Grampian dextral displacement took place along the FHCBL at 448 ± 3 Ma. A subsequent phase of sinistral movement along the FHCBL took place at c. 400 Ma, based on previously published Rb-Sr muscovite ages for synkinematic pegmatites. The kinematic information obtained from crenulationslip morphologies combined with geochronology can thus be used to constrain the reactivation history of a major crustal-scale shear zone.

Dimples on nanocrystalline fracture surfaces as evidence for shear plane formation.

Tensile experiments of fully dense nanocrystalline structures with a mean grain size of less than 100 nanometers demonstrate a considerable increase in hardness but a remarkable drop in elongation-to-failure, indicating brittle behavior. However, dimple structures are often observed at the fracture surface, indicating some type of ductile fracture mechanism. Guided by large-scale atomistic simulations, we propose that these dimple structures result from local shear planes formed around clustered grains that, because of their particular misorientation, cannot participate in the grain boundary accommodation processes necessary to sustain plastic deformation. This raises the expectation that general high-angle grain boundaries are necessary for good ductility.

Spatial variability in the flow of a valley glacier: Deformation of a large array of boreholes

Measurements of the deformation of a dense array of boreholes in Worthington Glacier, Alaska, show that the glacier moves with generally bed‐parallel motion. Strain in the 200 m deep valley glacier is constant near the surface but follows a nonlinear vertical profile below a depth of about 120 m. By a depth of 180 m, the octahedral strain rate reaches 0.35 yr−1. The three‐dimensional velocity field shows spatial complexity with significant deviations from plane strain, despite relatively simple valley geometry in the vicinity of the 6×106 m3 study volume. No evidence was found for time‐varying deformation or movement along localized shear planes. Observations were made by repeatedly measuring the long‐axis geometry of 31 closely spaced boreholes over a 70 day period, and three additional holes after 1 full year of deformation. The holes were spaced 15 to 30 m apart. Installation and measurement of such a large number of boreholes required the development of a semiautomated hot water drilling system that creates straight and vertical boreholes with uniform walls. The equipment and procedures enables borehole profiles to be measured without the use of hole casing. Inclinometry measurements collected in the holes were processed, analyzed for error, and visualized as a fully three‐dimensional data set. The new methods offer unique insight into small‐scale spatial and temporal variations in the pattern of flow in a valley glacier.

How useful are ‘millipede’ and other similar porphyroblast microstructures for determining synmetamorphic deformation histories?

ABSTRACT Oppositely concave microfolds (OCMs) in and adjacent to porphyroblasts can be classified into five nongenetic types. Type 1 OCMs are found in sections through porphyroblasts with spiral‐shaped inclusion trails cut parallel to the spiral axes, and commonly show closed foliation loops. Type 2 OCMs, commonly referred to as ‘millipede’ microstructure, are highly symmetrical, the foliation folded into OCMs being approximately perpendicular to the overprinting foliation. Type 3 OCMs are similar to Type 2, but are asymmetrical, the foliation folded into OCMs being variably oblique to the overprinting foliation. Type 4 OCMs are highly asymmetrical, only one foliation is present, and this foliation is parallel to the local shear plane. Type 5 OCMs result from porphyroblast growth over a microfold interference pattern.Types 1 and 2 are commonly interpreted as indicating highly noncoaxial and highly coaxial bulk deformation paths, respectively, during porphyroblast growth. However, theoretically they can form by any deformation path intermediate between bulk coaxial shortening and bulk simple shearing. Given particular initial foliation orientation and timing of porphyroblast growth, Type 3 OCMs can also form during these intermediate deformation paths, and are commonly found in the same rocks as Type 2 OCMs. Type 4 OCMs may indicate highly noncoaxial deformation during porphyroblast growth, but may be difficult to distinguish from Type 3 OCMs. Thus, Types 1–3 (and possibly 4) reflect the finite strain state, giving no information about the rotational component of the deformation(s) responsible for their formation. Furthermore, there is a lack of unequivocal independent evidence for the degree of noncoaxiality of deformation (s) during the growth of porphyroblasts containing OCMs. Type 2 OCMs that occur independently of porphyroblasts or other rigid objects might indicate highly coaxial bulk shortening, but there is a lack of supporting physical or computer modelling.It is possible that microstructures in the matrix around OCMs formed during highly noncoaxial and highly coaxial deformation histories might have specific characteristics that allow them to be distinguished from one another. However, determining degrees of noncoaxiality from rock fabrics is a major, longstanding problem in structural geology.

The effect of block rotations on the global seismic moment tensor and the patterns of seismic P and T axes

Distributed brittle deformation of the Earth's crust involving block rotations is comparable to the deformation of a granular material, with fault blocks acting like the grains. The deformation of a granular material is not adequately described using classical continuum mechanics because the individual grains within the material rotate in a manner that is not uniquely determined by the large‐scale average deformation. Thus a theoretical link has not existed between the kinematics of deformation involving block rotation and the associated effects on the seismic moment tensor and focal mechanism solutions. We establish this link using micropolar continuum theory (Eringen, 1964, 1966 a, b; Eringen and Suhubi, 1964) and the analysis of the effect of block rotations on fault slickenline patterns by Twiss et al. (1991). This theory takes into account two separate scales of motion: a large‐scale average motion of the material, the macromotion, composed of a macrodeformation rate (i.e., a macrostrain rate) and a macrospin, and a local motion, the microspin, that describes the average rotation rate of grains in the material. The micropolar kinematic theory allows us to predict the orientations of coseismic slip directions V on local shear planes of any orientation in a large‐scale shear zone. We define a local and a global asymmetric micropolar seismic moment tensor in terms of these slip directions. For a restricted kinematic model, the theory shows that two scalar parameters, D and W, determine the symmetry of the global micropolar seismic moment tensor and the pattern of seismic P (shortening) and T (lengthening) axes. The deformation rate parameter D is defined in terms of the principal values of the deformation rate tensor . The deformation is transtensional (constrictional) if 0 ≤ D ≤ 0.5, plane strain if D = 0.5, and transpressional (flattening) if 0.5 &lt; D ≤ 1. The net vorticity parameter W is a normalized value of the difference between microspin and macrospin . It is an objective variable. W = 0 implies that the global micropolar seismic moment tensor is symmetric and that P and T axis patterns have orthorhombic or higher symmetry. W ≠ implies that the global micropolar seismic moment tensor is asymmetric and that P and T axis patterns have monoclinic symmetry. The antisymmetric part of the global micropolar seismic moment tensor is associated with the net vorticity that characterizes the deformation. W has different values for different models of rigid block rotation and thus could serve to identify the rotation mechanism.

Theory of slickenline patterns based on the velocity gradient tensor and microrotation

Brittle fault zones commonly are characterized by a multitude of lineated shear planes having a wide distribution of orientations. In our model, we assume the faulted rock comprises an aggregate of rigid blocks whose surfaces are the shear planes. We define two independent scales of motion to distinguish the local rigid rotation of the blocks with their shear planes (the “micromotion”) from the average deformation of the material on a macroscopic scale (the “macromotion”). The macromotion is defined by the macrovelocity gradient tensor, which is separated into symmetric and antisymmetric parts, the deformation rate and the macrospin tensors respectively; the micromotion is defined by a microspin tensor. On any shear plane, slickenlines form parallel to the direction of the maximum rate of shear, which we assume is determined by the direction of the maximum component of the macrovelocity gradient tensor tangent to the shear plane and by a component of the microspin in the shear plane. For a restricted kinematic model, patterns of slickenline orientations are defined on a uniform distribution of shear plane orientations by the values of D and W. D is a ratio of differences in the principal values of the deformation rate tensor, and W is a ratio of the net spin to the maximum rate of macroshear. We distinguish between instantaneous slickenline patterns and finite-deformation patterns. The two are different if the shear planes rotate relative to the principal deformation rate axes, in which case finite-deformation slickenlines form curved or crossing lineations. If W = 0, the instantaneous slickenline patterns have orthorhombic symmetry (0 < D < 1), or higher symmetry ( D = 1 or 0). If W ≠ 0, the instantaneous slickenline patterns have monoclinic symmetry. For pure shear in an isotropic body, instantaneous slickenline patterns have orthorhombic symmetry ( W = 0, D = 0.5) whereas for pure shear accommodated by shear on a planar anisotropy ( W = −1, D = 0.5), the pattern is monoclinic. For simple shear with the rigid rotation of local shear planes defined by the shear-induced spin ( W = 0, D = 0.5), the instantaneous slickenline pattern is orthorhombic but the finite-deformation pattern is monoclinic. For simple shear with no shear plane rotation ( W = −1, D = 0.5), the instantaneous pattern is monoclinic. Special cases of our hypothesis are equivalent to the hypothesis that slickenlines are parallel to the direction of maximum resolved shear stress (e.g. Angelier, 1979, 1984). The slickenline patterns predicted for this case are always of orthorhombic or higher symmetry, and are equivalent to our instantaneous patterns for which W = 0 and to our finite-deformation patterns for which, in addition, there is no microspin.

Curved slickenfibers: a new brittle shear sense indicator with application to a sheared serpentinite

Brittle fault zones are commonly characterized by penetrative fracturing of the rock to form an aggregate of blocks, so that the rock becomes, in essence, like a granular material composed of rigid ‘grains’. The surfaces of the blocks, or ‘grains’, have a wide distribution of orientations, and shearing of the blocks past one another on these planes accommodates the large-scale motion, i.e. the macromotion, in the fault zone. During a macromotion that is a non-coaxial deformation, the rigid blocks and their surfaces may undergo a progressive rigid rotation that is distinct from the macromotion and is described by the microspin. If a macroscopic non-coaxial deformation has monoclinic symmetry, this symmetry should be reflected in the sense of rigid rotation of the blocks and their surfaces. On the surfaces of the blocks, which are local shear planes, the history of displacement may be recorded by mineral fibers that grow progessively with displacement. Because of the rigid rotation of the local shear planes, slickenfiber lineations commonly are curved. The sense of curvature from the youngest to the oldest part of the fiber, looking down the normal to the local shear plane, is different for those planes whose normals are on opposite sides of the unique monoclinic symmetry plane for the macroscopic shearing. The intersection of the symmetry plane with the plane of the fault zone defines the macroscopic slip direction, and the sense of rigid rotation of the local shear planes determined from the lineations defines the shear sense. Application of this technique to the disrupted and sheared margins of the Feather River Peridotite in the northern Sierra Nevada of California indicates that late deformation involved dextral-normal oblique slip along the Melones fault zone.

Changes of soil water suction, conductivity and dry strength during deformation of wet undisturbed samples

Deformation (without failure along local shear-planes) of wet clayey soil during tillage and traffic operations can cause significant undesirable changes in soil physical properties, even though it is not generally accompanied by severe compaction. Field conditions were simulated by deforming wet, dense, unsaturated, cylindrical field samples in quick (or undrained) triaxial tests. In these tests, soil samples were compressed to half their initial height, which was accompanied by large lateral expansion. Soil water suction (at constant water content) typically increased from 5 to 30 kPa and air and water transmission characteristics dramatically decreased with deformation. The soil water suction increase occurred at a nearly constant degree of saturation, which may explain the often observed phenomenon of soil adhering to tool and tyre surfaces. Deformation doubled tensile strength after drying. Deformability was characterized by a coefficient of viscosity and a threshold shear stress, typical values being 7·10 6 mPa.s and 6·10 7 mPa, respectively.