Principal Fracture Research Articles

The relationship between regional stress field, fracture orientation and depth of weathering and implications for groundwater prospecting in crystalline rocks

In a uniform granite gneiss study area in central Zimbabwe, lineaments oriented parallel to the maximum regional compressive stress orientation exhibit the thickest regolith development, while lineaments oriented perpendicular to the maximum compressive stress show the shallowest development of weathered regolith. The principal fracture set orientations were mapped using aerial imagery. The regional stress field, estimated from global stress maps, was used to determine the stresses acting on each principal lineament orientation. Multi-electrode resistivity profiling was carried out across fractures with different orientations to determine their subsurface regolith conditions. The results indicate that the 360 and 060° lineaments, which are sub-parallel to the principal compressive stress orientation (σ1) exhibit maximum development of the regolith, while 130° lineaments perpendicular to σ1 do not exhibit significant regolith development. Since regolith thickness has been positively correlated with groundwater resources, it is suggested that fractures with orientations sub-parallel to the principal compressive stress direction constitute favourable groundwater targets. Knowledge of the regional stress field and fracture set orientations can be used as an effective low cost tool for locating potentially higher yielding boreholes in crystalline rock terrains.

Fracture surface and correlation of buckling force with aspect ratio of C 60 crystalline whiskers

The structure and mechanical properties of crystalline whiskers with an average diameter of 600 nm, composed of C 60 molecules were studied by transmission electron microscopy combined with nanonewton force measurements used in atomic force microscopy. C 60 nanowhiskers with a body-centered tetragonal structure are compressed along their long axis. Young's modulus of the C 60 nanowhiskers was estimated to be 28 ± 5 GPa. The buckling stress was correlated to the aspect ratio of length to diameter of the C 60 nanowhiskers. The (100) surface was the principal fracture surface of the C 60 nanowhiskers.

Fracture mechanism maps in unirradiated and irradiated metals and alloys

This paper presents a methodology for computing a fracture mechanism map in two-dimensional space of tensile stress and temperature using physically-based constitutive equations. Four principal fracture mechanisms were considered: cleavage fracture, low temperature ductile fracture, transgranular creep fracture, and intergranular creep fracture. The methodology was applied to calculate fracture mechanism maps for several selected reactor materials, CuCrZr, 316 type stainless steel, F82H ferritic–martensitic steel, V4Cr4Ti and Mo. The calculated fracture maps are in good agreement with empirical maps obtained from experimental observations. The fracture mechanism maps of unirradiated metals and alloys were modified to include radiation hardening effects on cleavage fracture and high temperature helium embrittlement. Future refinement of fracture mechanism maps is discussed.

Effect of Heat Treatment Temperature on the Fracture Characteristics of Carbon/Carbon Composite Discs

Tensile properties and fracture morphologies of C/C composites used as aircraft brake disc at different heat treatment temperature have been studied. Fracture morphologies characteristics of tensile sample were observed with scanning electron microscope (SEM), and fracture mechanism was analyzed. The test results show that, with increase of heat treatment temperature, tensile strength of C/C composites has a descend trend, whereas, breaking strain has an upward trend, and failure mode changes from brittle fracture into “pseudo-plasticity” fracture. Destructional form includes interface debonding, extraction, break and matrix damage and so on, among which interfacial failure is principal fracture mechanism.

Fractures and drainage in the granite mountainous areaA study from Sierra Nevada, USA

This paper provides an analysis of relationships between drainage patterns and fractures in the part of Sierra Nevada, California, north of the Yosemite Valley. Bedrock is Cretaceous granite and cut by numerous lineaments of various orientation, length and geomorphic expression. We have mapped fractures and drainage lines from aerial photographs, 1:40 000 scale, in four test areas ranging in size from 32.5 to 266 km 2 . Azimuths are shown on rose diagrams for fractures and drainage lines and then visually and statistically compared. The coincidence of drainage and fracture patterns is strong, which implies causal relationships. In plan, the majority of valleys follow fractures even if this locally means a different orientation in respect to the regional slope arising from tectonic tilt of the range. Main streams occupy deeply incised troughs coincident with ‘master fractures’ of regional extent. Among two principal fracture directions, SSW–NNE to SW–NE and WSW–ENE, the former exerts more control on the drainage lines. The presence of a central zone of structural weakness within the major valleys provided significant constraints for the course of glacial erosion and may explain why multiple Pleistocene glaciers did not succeed in transforming valley cross-sections into expected U-shapes.

Fractures and drainage in the granite mountainous area: A study from Sierra Nevada, USA

This paper provides an analysis of relationships between drainage patterns and fractures in the part of Sierra Nevada, California, north of the Yosemite Valley. Bedrock is Cretaceous granite and cut by numerous lineaments of various orientation, length and geomorphic expression. We have mapped fractures and drainage lines from aerial photographs, 1:40 000 scale, in four test areas ranging in size from 32.5 to 266 km 2. Azimuths are shown on rose diagrams for fractures and drainage lines and then visually and statistically compared. The coincidence of drainage and fracture patterns is strong, which implies causal relationships. In plan, the majority of valleys follow fractures even if this locally means a different orientation in respect to the regional slope arising from tectonic tilt of the range. Main streams occupy deeply incised troughs coincident with ‘master fractures’ of regional extent. Among two principal fracture directions, SSW–NNE to SW–NE and WSW–ENE, the former exerts more control on the drainage lines. The presence of a central zone of structural weakness within the major valleys provided significant constraints for the course of glacial erosion and may explain why multiple Pleistocene glaciers did not succeed in transforming valley cross-sections into expected U-shapes.

Inferences on the hydrothermal system beneath the resurgent dome in Long Valley Caldera, east-central California, USA, from recent pumping tests and geochemical sampling

Quaternary volcanic unrest has provided heat for episodic hydrothermal circulation in the Long Valley caldera, including the present-day hydrothermal system, which has been active over the past 40 kyr. The most recent period of crustal unrest in this region of east-central California began around 1980 and has included periods of intense seismicity and ground deformation. Uplift totaling more than 0.7 m has been centered on the caldera’s resurgent dome, and is best modeled by a near-vertical ellipsoidal source centered at depths of 6–7 km. Modeling of both deformation and microgravity data now suggests that (1) there are two inflation sources beneath the caldera, a shallower source 7–10 km beneath the resurgent dome and a deeper source ∼15 km beneath the caldera’s south moat and (2) the shallower source may contain components of magmatic brine and gas. The Long Valley Exploration Well (LVEW), completed in 1998 on the resurgent dome, penetrates to a depth of 3 km directly above this shallower source, but bottoms in a zone of 100°C fluid with zero vertical thermal gradient. Although these results preclude extrapolations of temperatures at depths below 3 km, other information obtained from flow tests and fluid sampling at this well indicates the presence of magmatic volatiles and fault-related permeability within the metamorphic basement rocks underlying the volcanic fill. In this paper, we present recently acquired data from LVEW and compare them with information from other drill holes and thermal springs in Long Valley to delineate the likely flow paths and fluid system properties under the resurgent dome. Additional information from mineralogical assemblages in core obtained from fracture zones in LVEW documents a previous period of more vigorous and energetic fluid circulation beneath the resurgent dome. Although this system apparently died off as a result of mineral deposition and cooling (and/or deepening) of magmatic heat sources, flow testing and tidal analyses of LVEW water level data show that relatively high permeability and strain sensitivity still exist in the steeply dipping principal fracture zone penetrated at a depth of 2.6 km. The hydraulic properties of this zone would allow a pressure change induced at distances of several kilometers below the well to be observable within a matter of days. This indicates that continuous fluid pressure monitoring in the well could provide direct evidence of future intrusions of magma or high-temperature fluids at depths of 5–7 km.

Fracture behavior of AZ91 magnesium alloy

Optical microscopy and scanning electron microscopy (SEM) were used to examine the fracture behavior of AZ91 magnesium alloy ruptured by tensile and impact tests. AZ91 alloy generally revealed features of brittle fracture and cleavage is the principal fracture mode. The fracture behavior for tensile test is different from that for impact test due to different load forms. The fracture morphologies are different in different areas of the impact specimen. The Mg/Mg 17Al 12 interface often acted as the crack initiation source.

A synchrotron study of bladder urolith architecture

The principal aim of this study was to assess a new approach to the characterization of uroliths using synchrotron radiation. To achieve this, a detailed investigation of the crystalline nature of a human bladder urolith has been undertaken. Changes in the phase composition and crystalline mineral nature have been measured from the urolith core center to its outer surface. Data were collected using a microbeam, synchrotron probe, and image plate. Rietveld analysis has enabled us to determine that the unit cell dimensions of the majority phases (anhydrous uric acid and calcium oxalate monohydrate) are significantly greater in the core region but become progressively smaller from the outer to inner regions. The crystallites of both phases are also shown to possess significant radial orientation which varies through the urolith and reaches a maximum at a point of principal fracture. The analysis has also allowed us to study the change in average crystallite morphology; the crystallites of both phases are shown to decrease in size toward the outer parts of the urolith although this is in a nonuniform fashion. Evidence of calcium oxalate dihydrate was also found, but only within the outermost region of the urolith.

Multistage emplacement of the Mount Givens pluton, central Sierra Nevada batholith, California

The ca. 90 Ma Mount Givens pluton is one of the largest granodioritic to granitic intrusions in the Sierra Nevada batholith of California. Emplacement of the pluton occurred during a critical time in the tectonic evolution of the central Sierra Nevada magmatic arc, marked by a transition from regional contraction to dextral transcurrent shear. A model for the emplacement of the intrusion is developed based on detailed mapping of the pluton and its wall rocks and characterization of its internal structure by measurements of the anisotropy of magnetic susceptibility (AMS) at 351 stations. One of the key results of the study is documentation of a strong correlation between petrologic and structural fabrics in the pluton, and determination that these fabrics reflect internal magma chamber dynamics more than regional tectonic strain. The ∼80-km-long, 15–30-km-wide pluton crystallized from a multiphase, three-segment magma chamber marked by a bulbous northern lobe and linear central and southern segments. The pluton is interpreted to be tabular in shape with a thickness of ∼5 km. Most of the space for the pluton was created by piecemeal block downdrop of the magma chamber floor along three principal fracture sets, the most important of which were steeply dipping, northwest-trending fractures formed parallel to the structural grain of the arc, and vertical, north-trending extension fractures formed in response to a component of arc-parallel dextral shear. Some of these fractures acted as magma conduits, episodically filling the pluton as source rocks became depleted in melt. An initial, voluminous intrusive event (stage 1) quickly filled the southern chamber with granodiorite magma, but only partially filled the northern and central chambers. Stage 2 magmatism involved underplating of megacrystic granite in the northern chamber and lateral flow of a large batch of this magma from the northern to the central chamber, the latter delineated by a 20-km-long belt of megacrystic granite containing subhorizontal magnetic lineations that connects the pluton segments. Floor downdrop eventually ceased to be an effective space-making process in the northern lobe, and renewed magmatism (stage 3) led to expansion and doming of the chamber. As the northern lobe cooled, a ring fault ruptured within the viscoelastic stage 1–2 carapace, allowing ring dike intrusion (stage 4) and sinking of a central flap of consolidated material. The temporal and spatial variations in emplacement mechanisms demonstrated for the Mount Givens pluton (i.e., fracture generation, floor downdrop, underplating, inflation, ring diking) suggest that end-member models (e.g., fracture vs. diapir) are oversimplifications of the pluton assembly process.

Fractography and fracture mechanics property assessment of advanced structural ceramics

Fracture characteristics and mechanical properties of advanced structural ceramics produced by advanced technology and developed for high temperature applications are reviewed from the viewpoint of fractography. The main processing routes for preparation of in situ reinforced ceramic matrix composites, nanocomposites, and laminar/layered composites are summarised and their principal microstructure parameters are described. The main observation techniques, levels/steps of observation, and principal fracture mechanics relations are summarised. The relationships between rupture strength, Weibull modulus, and defect distribution, and the possibilities for strength improvement through improved processing, are discussed. Recent results relating to strength and reliability prediction are given. The fractographic appearances of process and bridging zone toughening mechanisms are described and the relationship between fracture toughness, microstructure, and fracture micromechanisms is discussed. Interfaces (macro- and microscale) and residual stresses and their effect on toughness, strength, and reliability of nano- and layered composites are analysed. The influence of loading mode on fracture and mechanisms leading to fatigue degradation in these materials is summarised. Fracture surface characteristics of high temperature crack growth and creep failure are described and the effects of oxidation and corrosive degradation on fracture and strength of these materials are considered. The role of fractography in the development of advanced ceramics and in efforts to improve reliability is described.

The Triplane Fracture

The triplane fracture of the distal tibia consists of several anatomic types, including two to four principal fracture fragments. According to previous calculations, 7% of all ankle fractures in girls and 15% in boys birth to 18 years old who have open growth plates are triplane fractures. In this update of a previous study and literature review of 209 cases (108 boys and 101 girls), the mean ages in boys and girls were 14.8 and 12.8 years, respectively (P < 0.00005). The triplane fracture did not occur in children younger than 10 and not in those older than 16.7 years. Thirty-five percent of the cases were treated without reduction, 30% with closed reduction, and 35% with open reduction. The choice of treatment depended on many variations in the materials reviewed. Excellent results were reported in 79% of 184 cases with follow-up. Neither age, sex, stage, nor treatment had a consistent influence on the frequency of less-than-excellent results. There was a weak tendency to deteriorating results with time to follow-up (r = 0.19, P = 0.016). Inadequate reduction seemed to be an important factor in the few cases with fair or poor results (4.7%), even if this presumption could not be verified statistically.

Analysis of macroscopic fractures in granite in the HDR geothermal well EPS-1, Soultz-sous-Foreˆts, France

An exhaustive analysis of 3000 macroscopic fractures encountered in the geothermal Hot Dry Rock borehole, EPS-1, located inside the Rhine graben (Soultz-sous-Foreˆts, France), was done on a continuous core section over a depth interval from 1420 to 2230 m: 97% of the macroscopic structures were successfully reorientated with a good degree of confidence by comparison between core and acoustic borehole imagery. Detailed structural analysis of the fracture population indicates that fractures are grouped in two principal fractures sets striking N005 and N170 °, and dipping 70 °W and 70 °E, respectively. This average attitude is closely related to the past tectonic rifting activity of the graben during the Tertiary, and is consistent with data obtained from nearby boreholes and from neighbouring crystalline outcrops. Fractures are distributed in clusters of hydrothermally altered and fractured zones. They constitute a complex network of fault strands dominated by N–S trends, except within some of the most fractured depth intervals (1650 m, 2170 m), where an E–W-striking fracture set occurs. The geometry of the pre-existing fracture system strikes in a direction nearly parallel to the maximum horizontal stress. In this favorable situation, hydraulic injections will tend both to reactivate natural fractures at low pressures, and to create a geothermal reservoir.

Analysis of Shear-Wave Polarization in VSP Data: A Tool for Reservoir Development

Summary This paper presents results of a new method of using vertical seismic profile (VSP) data to deduce information on certain rock properties that are important for reservoir development. These properties are the direction of maximum horizontal stress or the orientation of aligned fractures and the variation of these quantities with depth. Such information can be important for secondary recovery, horizontal drilling, and infill drilling. The VSP method involves a shear-wave (S-wave) source at the surface and a downhole three-component (3-C) geophone to record first-arrival S waves along predominantly vertical ray paths. The data are analyzed to determine particle motion directions (polarizations) and relative travel time of the two S waves. In simple cases, the polarization of the faster S wave is oriented parallel to the direction of maximum horizontal compressive stress orthe predominantfracture alignment.The travel-time differences between the S waves contain information on the effects of unequal horizontal stresses. We recorded a number of VSP's for S-wave analysis in California and Texas oil fields. Results from three VSP's are presented here. In nearly every case, we found measurable travel-time differences and also significant variation of the S-wave polarization azimuths with depth. Polarization azimuths, although typically consistent over depth intervals of several hundred feet, tended to change abruptly at various depths. A simple layer-stripping technique made it possible to follow the polarization changes and determine travel-time differences over successive depth intervals. The variations with depth are particularly interesting because they indicate changes of principal stress direction or fracture orientation.

Analysis of subsurface flow and formation anisotropy in a fractured aquitard using transient water level data

A new methodology is proposed to estimate the actual direction of water flow in a fractured aquitard. According to the conceptual model, the flow direction in a fractured aquitard is dictated by the azimuth of the fractures, since almost no flow occurs within the rock matrix, which possesses very low hydraulic conductivity. Thus, while ground water in reality continues to flow along the fractures, calculations based on water level data may incorrectly indicate changes in hydraulic gradients and azimuth of flow. The present methodology is based on simultaneous analysis of hydraulic gradients and flow direction, as calculated from equipotential surfaces. The approach was tested in a fractured chalky aquitard, and the calculated direction of principal fractures was found to be in good agreement with subsurface fracture data obtained from a tunnel excavated in the vicinity of the study area.

Biotite percussion figures in naturally deformed mylonites

Under experimental conditions, characteristic fracture patterns can be produced on cleavage plates on mica by using a blunt tool. If stress is applied rapidly by striking the surface in a controlled way, a pattern known as the “percussion figure” is produced. When the stress is applied by steady pressure on the tool, a different but complementary pattern of fracture is formed. In sum, these induced fractures constitute the “pressure figure”. The orientation of each of these two sets of fractures with respect to the optical axial plane (OAP) of mica is different and therefore diagnostic of the manner in which they are produced. These patterns are distinct from those formed as a result of exsolution of Fe-Ti oxides which are commonly visible in sections of biotite cut parallel to the basal plane (001). A description is given of percussion figures produced by natural deformation in biotites from mylonite belts cutting the Proterozoic metasediments of the Feidong Group in eastern Anhui Province and another from Yunnan Province, China. The principal fracture of the natural percussion figure evidently is parallel to the (OAP) of the biotite and the other two sets are quite distinct as well, thus identifying it really as a percussion figure. Microscopic inclusions of sphene also are located along the crystallographically controlled fracture planes of the percussion figures. The data indicate that high strain rates would be required to form these natural percussion figures and that a special history of deformation must have affected the mylonites in which they occur. It is proposed that the homogeneous deformation of the mylonite in a ductile regime was complicated by strain hardening which led to episodes of abrupt stress itself relief (stick-slip) at rates of strain high enough to induce the formation of percussion figures in the biotites.

The characterization of uniaxial graphite fiber/epoxy composites by mechanical testing and scanning electron microscopy

AbstractThe effect of the constituent material on the mechanical properties and on the overall crack propagation behavior of uniaxial graphite/epoxy composites was studied using two different types of graphite fibers, i.e., PAN precursor and pitch precursor fibers in combination with an epoxy known for its toughness. The tensile properties and interlaminar shear strength (ILSS) of composites with various fiber volume fraction were studied using an Instron Universal Testing Machine. The fracture surface of composites was studied using scanning electron microscopy (SEM). The surprising result from the tensile tests is that the percent elongation of AS4/epoxy composites was much higher than that of individual components, i.e., the epoxy and the fiber. The stress–strain curves of AS4 fiber/epoxy composites show yielding. The yielding is interpreted to result from shear stress concentration due to the fiber breakage. Two causes of failure mode were observed in this study: (1) ductile plastic deformation by the PAN‐based AS4/epoxy composites; (ii) brittle catastrophic failure shown by the pitch‐based VSB‐16/epoxy composites. The tensile strengths and moduli of the composites increased as the volume fraction of fibers increased. The principal fracture features of AS4/epoxy composites under tension were fiber pullout (bundles and single fibers) and very rough fracture surface due to the pullout of fiber bundles with epoxy, whereas VSB‐16/epoxy composites under tensile and bending loads showed only tensile (or compressive) failure. The composites of epoxy/PAN (polyacrylonitrile)‐based fibers (AS4) showed greater ILSS than those of the pitch‐based fiber (VSB‐16)/epoxy. The most characteristic features of the interlaminar shear failure surface of AS4/epoxy composites are delamination and formation of hackles.

Cryptic Structural Trends Revealed by Cenozoic Sedimentation on the Malta-Sicily Shelf: ABSTRACT

Fault control of sedimentation patterns affects upper Miocene carbonate deposition. Fault trends, northeasterly or northwesterly, appear to exert major controls on sedimentation and represent the principal fractures seen on geologic maps of the islands. Evidence, however, shows a regionally persistent north-south facies alignment apparent throughout the Cenozoic. A reactivated Mesozoic basement fracture pattern is thought to control this alignment. Probable mesozoic sedimentation controls and facies distributions on the Malta shelf thus influence trap generation and hydrocarbon migration.

Thermal stress fracture of refractory lining components: Part I. Thermoelastic analysis

This paper presents a mathematical model and corresponding stress analysis suitable for the development of theoretical criteria useful for the design and selection of refractory components of linings of high-temperature furnaces and metallurgical process vessels. Refractory lining components, for which the maximum principal tensile stress fracture criterion is assumed valid, are simulated using a two-dimensional constant heating rate thermoelastic model. Dimensional analysis and the finite element numerical method are utilized to develop a general solution for the maximum principal tensile stress in a rectangular shape of arbitrary length and width in terms of thermal and mechanical properties and heating and cooling rate. Implications of the general stress solution with regard to fracture behavior and design recommendations are discussed with reference to results previously reported in the literature.

Thermal stress fracture of refractory lining components: Part II. Safe heating and cooling rates

The solution for the maximum principal tensile stress of a two-dimensional constant heating rate thermoelastic model has been used to develop a resistance-to-fracture-initiation parameter useful for the design and selection of refractory components that accounts for thermal and mechanical properties, geometry, and temperature range, as well as distinguishes between the heating and cooling cases. The parameter, namely safe heating or cooling rate, is defined as the maximum rate at which a rectangular shape can be heated or cooled through a specified temperature range without causing fracture, which is taken to be governed by the maximum principal tensile stress fracture criterion. Computation of the parameter is easily accomplished using a graphical technique. Tabulated values are used to plot dimensionless relationships that give the combinations of variables that both satisfy the fracture criterion and produce a specified value of hot face temperature. Good agreement was found between model predictions of safe heating rate and experimental values reported in the literature.