Showing papers on "Stress field published in 1980"

State of stress in the conterminous United States

Abstract: stress provinces indicate that these transitions can be abrupt, occurring over <75 km in places. In the western United States, a region of active tectonism characterized by high levels of seismicity and generally high heat flow, the stress pattern is complex, but numerous stress provinces can be well delineated. Despite relative tectonic quiescence in the eastern and central United States, a major variation in principal stress orientation is apparent between the Atlantic Coast and midcontinent areas. Most of the eastern United States is marked by predominantly compressional tectonism (combined thrust and strike slip faulting), whereas much of the region west of the southern Great Plains is characterized by predominantly extensional tectonism (combined normal and strike slip faulting). Deformation along the San Andreas fault and in parts of the Sierra Nevada is nearly pure strike slip. Exceptions to this general pattern include areas of compressional tectonics in the western United States (the Pacific Northwest, the Colorado Plateau interior, and the Big Bend segment of the San Andreas fault) and the normal growth faulting along the Gulf Coastal Plain. Sources of stress are constrained not only by the orientation and relative magnitude of the stresses within a given province but also by the manner of transition of the stress field from one province to another. Much of the modem pattern of stress in the western United States can be attributed to present transform motion and residual thermal and dynamic effects of Tertiary subduction along the western edge of the North American plate. Abrupt stress transitions around actively extending regions in the western United States probably reflect shallow sources of stress and anomalously thin lithosphere. Large areas characterized by a uniform stress field in the central and eastern United States suggest broad scale plate tectonic forces. In the midcontinent region, both ridge push and asthenospheric viscous drag resistance to lithospheric motion can explain the NE-SW compression in the cold, thick lithosphere of the craton, although drag-induced stress directions (resistance to absolute plate motion) correlate better with the data than do the ridge push directions. Asthenospheric counterflow models do not apply in this region, as predicted stress orientations are about 90 o off. A region of compression oriented approximately perpendicular to the continental margin and Appalachian fold belt is defined by the stress data along the Atlantic Coast. This NW-SE compression is in direct contrast with previous models predicting extension perpendicular to passive continental margins due to lateral density contrasts at the continental-oceanic crust interface. Ridge push forces, while capable of producing a component of compression across the coastal area, do not explain the observed orientation of the stress. Two speculative mechanisms are suggested to explain the observed orientations: (1) rotation of stress (or strain) due to anisotropic Appalachian basement structure and (2) flexural effects associated with erosion and isostatic rebound of the Appalachians.

REVIEW—Transformation Toughening in Ceramics: Martensitic Transformations in Crack-Tip Stress Fields

Abstract: The toughness of ceramics can be substantially enhanced by inducing a martensitic transformation in the stress field of the dominant crack. The specific characteristics of martensitic transformations pertinent to this toughening process are examined in this paper. A generalized model of the toughening is developed, which indicates the specific roles of the transformation parameters on the expected magnitude of the toughening. Finally, the implications of the analysis for the development of high-toughness ceramics are discussed.

Tensile Cracks in Creeping Solids

Abstract: The aim of the paper is to answer the question: which loading parameter determines the stress and strain fields near a crack tip, and thereby the growth of the crack, under creep conditions? As candidates for relevant loading parameters, the stress intensity factor K I , the path-independent integral C*, and the net section stress σ n e t have been proposed in the literature. The answer, which is attempted in this paper, is based on the time-dependent stress analysis of a stationary crack in Mode I tension. The material behavior is modeled as elastic-nonlinear viscous, where the nonlinear term describes power law creep. At the time t = 0, load is applied to the cracked specimen, and in the first instant the stress distribution is elastic. Subsequently, creep deformation relaxes the initial stress concentration at the crack tip, and creep strains develop rapidly near the crack tip. These processes may be analytically described by self-similar solutions for short times t. An important result of the analysis is that small-scale yielding may be defined. In creep problems, this means that elastic strains dominate almost everywhere except in a small "creep zone" which grows around the crack tip. If crack growth ensues while the creep zone is still small compared with the crack length and the specimen size, the stress intensity factor governs crack growth behavior. If, however, the calculated creep zone becomes larger than the specimen size, the stresses become finally time-independent and the elastic strain rates can be neglected. In this limiting case, the stress field is the same as in the fully-plastic limit of power law hardening plasticity that has been treated in the literature. The loading parameter that determines the near tip fields uniquely is then the path-independent integral C*. It should be emphasized that K 1 and C* characterize opposite limiting cases. Which case applies in a given situation can be decided by comparing the creep zone size with the specimen size and the crack length. Criteria for small-scale yielding are worked out in several alternative forms. Besides several methods of estimating the creep zone size, a convenient expression for a characteristic time is derived also, which characterizes the transition from small-scale yielding to extensive creep of the whole specimen.

Oceanic intraplate earthquakes: Implications for local and regional intraplate stress

Abstract: Focal mechanisms of intraplate earthquakes provide the only means at present by which to characterize the long-wavelength tectonic stress field in oceanic lithosphere. Stress orientations inferred from focal mechanisms may not accurately reflect the state of stress in the epicentral area, however, or the measured stresses may be dominated by local rather than regional sources. To establish a data set with which to study these possibilities, a comprehensive catalog of 159 oceanic intraplate earthquakes has been compiled for events since 1963 with mb 4.7 or larger. Focal mechanisms are available for approximately one quarter of the events, and several new mechanisms are presented here. For a representative subset of this catalog (83 events), the bathymetry and tectonic history of the epicentral areas have been assembled, and the earthquakes have been rated according to their association with (1) a preexisting fault zone, which might decouple the P axis of the focal mechanism from the true orientation of maximum compressive stress, and (2) large bathymetric relief, which might be a source of large local stresses. Oceanic intraplate earthquakes are commonly found in association with zones of previous weakness (usually fracture zones), but they do not show any particular association with large bathymetric features. In the central Indian Ocean there are enough focal mechanisms available to establish a well-defined NW-SE orientation for P axes and presumably for the direction of greatest compressive stress. The consistency of the P axes of these widely varying mechanisms in the presence of the Ninetyeast Ridge, a site of major intraplate deformation and large bathymetric relief, is remarkable. A possible explanation is that in the presence of a large number of preexisting faults with a range of orientations, slip occurs on those faults which have large resolved shear stresses from the regional stress field. In such an instance the P axis of focal mechanisms will tend to show a consistent alignment with the true direction of maximum stress.

Microfracture Development in Compacting Sediments: Relation to Hydrocarbon-Maturation Kinetics

Abstract: Hydrocarbon production zones for source rocks in various basins throughout the world are at depths ranging from less than 3,000 m to greater than 5,000 m. Theoretical calculations completely independent of these observations indicate a comparable range of depths for the onset of microfractures in overpressured sedimentary beds undergoing burial. The basis for these calculations is a nonisothermal model of sedimentation which includes fluid flow as a pressure-dissipating mechanism, and water expansion and gravitational loading as pressure-producing mechanisms. A microfracture criterion is incorporated from experimental observations as a critical fluid pressure with respect to the least principal stress in a regional stress field. The theoretical results indicate that, for constant rate of sediment burial, the depth to the onset of microfractures is fixed for a given geothermal gradient and regional stress distribution. The onset depth is increased when lateral expansion of a depositional basin is restricted by the pressure of the surrounding rock, and decreased when the degree of horizontal restraint is lessened. Once initiated, the average rate of propagation of the microfracture zone keeps pace with the rate of sediment burial, at least for significant parts of geologic time. For a given geothermal gradient and regional stress distribution, maturation possibilities vary widely prior to microfracture development, depending chiefly on the burial rate and kinetic factors.

The correlation of fracture transitions

Abstract: A theory of fracture transitions consistent with fracture mechanics is outlined. Transitions occur when a certain characteristic length attains a critical value: in linear elastic fracture mechanics this characteristic length is that of an inhomogeneous stress field, as for instance the radius of a notch. The critical length is proportional to a material factor E Gamma /Y2 (or (Kc/Y)2) where E, Gamma , Kc and Y are Young's modulus, fracture surface energy, critical stress intensity factor and yield stress in a uniaxial test. To a first approximation the constant of proportionality is characteristic of the test only, and ranges from approximately 102 in Hertzian indentation to approximately 10-1 in notched bar fracture. Two types of transition are considered: at the boundary between elastic and plastic-elastic conditions, when the yield criterion is just exceeded, and for fully plastic-elastic conditions when the plastic zone attains some limiting size imposed by the test. In the first case the characteristic length of the stress field is that of a specimen dimension, in the second the length of the plastic zone. The analysis therefore relates fracture transitions in different tests and materials to each other, and is illustrated by experimental data on indentation of polymethylmethacrylate and notched bar tests of silicon iron. A new fracture diagram is proposed.

Faulting patterns in north-central Nevada and strength of the crust

Abstract: NNE normal fault trends characterize much of the northern Basin and Range province. These faults make sharp bends to NNW and ENE trends in north-central Nevada in the vicinity of a mid-Miocene rift characterized by a zone of diabase dike swarms, graben-filling flows, and a coinciding aeromagnetic anomaly. Despite a roughly 45° change in the least principal stress direction since mid-Miocene time, pre-existing NNW- and ENE-trending faults in the vicinity of the rift accommodated the extension whereas regionally, major crustal blocks were faulted along a NNE trend, approximately perpendicular to the modern least principal stress direction. An assumed uniform regional stress field (derived from geologic and geophysical indicators of the modern principal stress field) and the observed oblique slip on the pre-existing faults were combined in an analysis utilizing an empirically derived frictional sliding law and the Coulomb failure criterion. This analysis constrained the ratio of the least principal stress to the greatest principal stress (S3/S1) as well as the inherent shear strength of intact crustal rocks, τc. While both parameters, S3/S1 and τC, are functions of unknowns including pore pressure and the cohesion (frictional strength) of the pre-existing faults, reasonable assumptions about these parameters lead to τc estimates that agree well with values obtained from laboratory experiments simulating crustal conditions. At a depth of 10 km, the analysis indicates that the minimum inherent shear strength of intact crustal rocks must range between 150–450 bars for zero pore pressure and 150–350 bars for hydrostatic pore pressure, whereas the corresponding maximum shear stresses at 10-km depth are 970–1200 bars for zero pore pressure and 640–770 bars for hydrostatic pore pressure.

Fatigue crack growth in polymers

Abstract: A number of fatigue crack propagation laws applied in the study of polymers is described. Consideration of the stress field distribution at the crack tip leads to the application of fracture mechanics. It is shown that a simplified relationship of the form da/dN =Fλα, whereφ is a function ofKIC,Kmax,Kmin andKTH appears to be a convenient expression for cyclic crack growth. The effect of mean stress is more complicated than that in the field of metals, the compressive component of cyclic stress may delay the crack growth. Cyclic tests in tension performed on PMMA and PVC are dependent on ΔK and its mean value,K m . The threshold value,KTH, is also influenced byK m but a more complicated behaviour due to strain rate effects may be observed. Other differences, such as the position of upper and lower transition points and growth rate changes with frequence, are noted. The effect of biaxial cyclic loading of PMMA and PVC plates is compared and some differences highlighted. The results available so far indicate little effect of the crack curving on its growth. However, it is shown that, while the increasing biaxiality can substantially retard the crack growth in PMMA, no such effect was recorded in PVC. Finally, it is shown that at very high stress levels (region III), the cyclic crack growth consists of two propagation modes, namely, a pure cyclic propagation, together with slow growth. At lower stress levels, slow growth disappears and the crack propagates in pure fatigue (region II). In region I, the propagation is very slow, without the usual correspondence between cycles and striations. The results recently obtained on glass reinforced plastics (GRP) are also presented and differences highlighted.

Structure, tectonics, and stress field of the Coso Range, Inyo County, California

Abstract: The tectonics of the Coso Range has been described as having arcuate and ring faults both suggesting the presence of a circumscribed subsidence bowl or calderalike feature. New information suggests the Coso Range is situated in an area of transition between the stress of the right slip San Andreas fault-plate interaction and the extensional tectonics of the Basin and Range. Arcuate faults in the Coso Range are interpreted to have been produced by the regional stress field rather than to have been of volcanogenic origin. Focal mechanisms of small-magnitude earthquakes support the stress directions indicated by local fault patterns. Fumeroles in the area are primarily associated with oblique slip faults rather than with arcuate or ring faults. The geothermal reservior is therefore much different from that of a caldera or subsidence bowl, and the overall geothermal potential is probably less than earlier estimates.

Nonlocal Stress Field at Griffith Crack.

Abstract: : By means of nonlocal elasticity theory, the Griffith crack problem is solved to determine the state of stress in a plate weakened by a line crack. The maximum stress is found to be finite and occurs away from the crack tip. Cohesive stress is estimated and compared with the calculations based on atomic lattice theory. (Author)

Crustal Stresses Inferred from Fault-Plane Solutions of Earthquakes and Neotectonic Deformation in Switzerland

Abstract: P-axes inferred from fault-plane solutions of 23 recent earthquakes in Switzerland and adjacent areas reveal a NNW — SSE (E Switzerland) to NW — SE and WNW — ESE (W Switzerland) orientation of maximum horizontal compressive stress in the upper crust. Nearly the same orientation of maximum compression is indicated by in situ stress measurements. The orientation of maximum horizontal compressive stress corresponds well with the orientation of maximum horizontal crustal shortening as derived from a kinematic analysis of Neogene and Quaternary structural features. Evidently the stress field which causes the present seismicity is very similar in its orientation to the stress field of the last 5 to 10 million years which produced the neotectonic deformation. In several cases of earthquakes in the Helvetic Alps, the Lake of Constance area and in the Jura Mountains a distinct relation between the parameters of the fault-plane solution and neotectonic deformation is shown up.

Creep crack propagation by cavitation near crack tips

Abstract: A model in which cracks propagate by nucleation, growth, and coalescence with cavities in front of the crack tip is analysed in detail. The applied stress field is assumed to be elastic or elastic–plastic, and the relaxation of stresses from the growth of cavitation damage is included in the analysis. The spacing of the cavities, the threshold stress for the nucleation of cavities, and the self-diffusion coefficients are the important material parameters. The model results in a threshold stress intensity and a K I n dependence of steady-state crack propagation rate when K I > K th. The stress distribution in front of the crack during the transient and the steady states is calculated. The size of the ‘damage zone’ varies from about 10 to 100 cavity spacings. Since the typical cavity spacing in engineering materials is about 1 μm, the damage zone is expected to be a few grain diameters in length.

Compressive Strength and Damage Mechanisms in Ceramic Materials. I. Temperature-Strain Rate Dependence of Compressive Strength and Damage Mechanism in Aluminum Oxide. II. Threshold Microfracture during Elastic-Plastic Indentation of Ceramics.

Abstract: : The results of compression tests of Al2O3 performed over a wide range in temperature and strain rate are interpreted in terms of dominant damage mechanisms. It is shown that compressive failure in Al2O3 is caused by one of three different mechanisms, each based on tensile (Mode I) growth of predominantly axial microcracks, and each characteristic of a specific temperature-strain rate regime. The concepts developed should be applicable to other strong ceramics. In addition, indentation experiments were carried out for a variety of ceramics. It is found that the threshold for microfracture during elastic-plastic indentation corresponds to radial, rather than subsurface median, crack formation. This is contrary to the fundamental assumption of existing models for threshold crack nucleation by sharp indenters or particles; the results indicate the need to modify the stress field calculations used in these models. (Author)

On the stress singularity at an interface crack

Abstract: The plane elastostatic stress field near an interface crack is considered and a model developed for the interface crack tip which is free from the inter-penetration of the crack flanks present in some earlier investigations, yet permits propagation in a crack opening mode along or near the interface. The basis of the model lies in taking, as the configuration to which the loads are applied, a crack tip which subtends an opening angle. By means of an asymptotic eigen-analysis, the main features of this departure from the norm in LEFM are identified and some examples presented which demonstrates its potential usefulness.

Steady-state thermal stresses caused by an imperfectly conducting penny-shaped crack in an elastic solid

Abstract: A solution is given for the stress field in a large solid containing an imperfectly conducting, penny-shaped crack. The heat flux between the crack faces is assumed to be proportional to the local temperature difference. It is shown that the shear stress on the crack plane depends only upon the average temperature of one crack surface. Numerical results are obtained for the case in which the crack obstructs an otherwise uniform heat flux.

Tectonic stresses in France: In situ measurements with a flat jack

Abstract: In situ stress measurements in thick limestone layers of the Parisian Basin and in neighboring regions yield a remarkably homogeneous stress field. The flat jack method has been used. Comparison with over-coring gave good agreement for the azimuth of the principal axes. In the Jurassic of Burgundy and Poitou as well as in a Carboniferous outcrop near the English Channel, maximum compression is found along the NNW-SSE direction.

On the theory of growth faulting

Abstract: A detailed and rigorous geomechanical analysis of the stability of overpressured, gently sloping, sediment layers is presented that underlies the Multi-Unit Delta Model described in Part 1 (Crans, Mandl and Haremboure, Journ. Petrol. Geol., 2, 3, 1980). That delta model explains and permits quantitative reproduction of main features associated with growth faulting. Starting from the equilibrium equations, the Coulomb-Mohr yield criterion and the proper initial and boundary conditions, the elastic and plastic stress fields in the sloping, overpressured layer are derived. The plastic stress field is calculated on the grid generated by the "characteristics" of the hyperbolic partial differential equation for the plastic stress state. These characteristics, being called in stress analyses "slip lines", are potential faults. In the case considered, a parameter equation is derived for one set of slip lines, (potential growth faults), which may simplify into cycloids under special conditions. Once the plastic stress field has been generated, the plastic deformation of the layer can be calculated by introducing the proper boundary conditions to the flow rules or plastic "velocity equations" being discussed extensively. To complete the rheological description, the behaviour of the sediment layer is described by attributing also thixotropic properties to the sediment. Although the case discussed is a very specific one, it illustrates how structural geological phenomena can be modelled on the computer in an appropriate geomechanical way. Such a numerical computer model shows the unique relation between plastic stress state and fault pattern, and the non-unique relation between plastic stress state and deformation pattern, being typical for the theory of plasticity.

A mathematical simulation for the pattern of seismic transference in north china

Abstract: Paper describes a certain process of solving a space-time inversion problem. A certain North China region is treated as a grillage of tectonic structures composed of 24 major faults and as an elastic-ideally plastic body in plane strain at a depth of 15 km. It is subjected to uniform stress along the boundaries. By means of the finite element method, the stress field is calculated and the zones of seismic risk are delineated in this region. By reducing the coefficient of friction in a fault from static to kinetic, stress and strain energy may be released there. Five major earthquakes in the last 12 years in this region are simulated in this manner. By comparing the stress fields before and after the stress release, one obtains the change in strain energy, fault offsets, areas of seismic risk etc. They are compared to data from actual measurements. If they don't agree well, tectonic framework, parameters of mechanical properties and external stress field are adjusted until they do.The results show that the present method can reproduce in essence the pattern of seismic transference in the last 12 years and may give some idea about the zones of seismic risk in the future. They also suggest that, after an earthquake, there are narrow sectors in the left-frontal part along the slippage directions that become safer than before, while in the remaining parts the danger of shear fracture increases.

An earthquake sequence in the Sierra Nevada-Great Basin boundary zone: Diamond valley

Abstract: The Diamond Valley, California, earthquakes of September 1978 occurred near the southern termination of the north-striking, east-dipping Genoa Fault, a major normal fault exhibiting cumulative Holocene offsets of up to 10 meters along the eastern margin of the Carson Range. Master-event location of the 14 largest events ( ML ≧ 3.0), using two close-in temporary stations for control, revealed a tight cluster 2 km in extent. P -wave first motions for the main shock ( ML = 5.0) resolve a strike-slip mechanism with an east-west axis of minimum compressive stress. Faulting (right-lateral) was assigned to the southeast-striking plane on the basis of aftershock migration in that direction. This style of faulting partially accommodates the regional stress field in zones separating left-stepping normal faults of the Sierra Nevada-Great Basin boundary zone. Seismic moments, Wood-Anderson magnitudes, and stress drops were computed for aftershocks using close-in digital seismograms; stress drops were higher than those found by Douglas and Ryall (1972) for aftershocks of the 1954 Fairview Peak earthquake some 130 km to the east. One identifiable characteristic of this sequence is that the ratio of P -to S -wave spectral corner frequencies is considerably greater (2.5) than unity.

Focal mechanism and tectonic stress field of coastal southeast china

Abstract: The analysis of focal mechanism for 70 events in the SB coastal China indicates that the principal compression axes of the recent stress field is characterized by a fan-shaped distribution. The nature of focal dislocation of the east portion of the coastal region is quite different from that of the West.Based on the stress field data of Southeast Asia from earthquake focal mechanism analysis, it seems clear that the stress field, as well as its character, may reasonably be explained by the movement of the Pacific and the Indian Ocean plates relative to the Eurasian Plate.

Surface dislocation model of a dislocation in a two-phase medium

Abstract: The surface dislocation method developed earlier for solving the free surface boundary problem is now extended to the two-phase interface boundary problem wherein a lattice dislocation is situated in one of the phases. The interface is planar where two semi-infinite half spaces of different elastic properties are joined. The interface consists of four surface arrays of dislocations, two in each phase, so that the continuity of two stress components and two displacement components is maintained. The continuous distribution of dislocations is employed to arrive at the distribution function representing the surface arrays. The Airy stress functions for the two phases are derived and shown to give the same result as that obtained earlier by other methods. The distortions involved across the interface are represented in terms of simple surface arrays to show the advantage of the surface dislocation model. The stress field around the dislocation in the two-phase medium is plotted and the effect of the shear modulus of the second phase and of Poisson's ratio discussed. The advantages of applying the surface dislocation model either by the continuous distribution method or the discrete dislocation method are indicated.

Assumed Stress Finite Element Analysis of Through-Cracks in Angle-Ply Laminates

Abstract: In the present paper a new assumed stress finite element method, based on a complementary energy method, is developed for the analysis of cracks in angle-ply laminates. In this procedure, the fully three-dimensional stress state (including transverse normal and shear stresses) is accounted for; the mixed-mode stress and strain singularities, whose intensities vary within each layer near the crack front, are built into the formulation a priori; the interlayer traction reciprocity conditions are satisfied a priori; and the individual cross-sectional rotations of each layer are allowed; thus resulting in a highly efficient and cost-effective computational scheme for practical application to fracture studies of laminates. Results obtained from the present procedure, for the case of an uncracked laminate under bending and for the case of a laminate with a through-thickness crack under far-field tension, their comparison with other available data, and pertinent discussion, are presented. N accurate three-dimensional stress analysis of angle-ply laminates with cracks and/or holes, as opposed to the use of simpler "classical laminates plate theories," is often times mandatory to understand 1) the complicated feature of the often-observed non-self-similar crack growth in sym- metric angle-ply laminates; 2) the subcritical damage in the form of matrix crazing, splitting, and delamination that is observed to precede final failure in a laminate; and 3) to more clearly understand the hole-size effects in a laminate. Quasi-three-dimensional analyses of cracked angle-ply laminates, with the assumption of 1) zero transverse normal stress in the laminate, 2) perfect bonding between lamina, and 3) each laminate being treated as a homogeneous anisotropic medium, were recently reported by Wang et al. ! The procedures in Ref. 1 -do not account, a prior, for the mixed- mode stress and strain singularities near the crack front, and hence involve expensive computations using very fine finite element meshes of conventional, polynomial-based elements. From these very-fine-mesh finite element solutions, even though one may obtain high stress-gradient solutions in the limit, it is often inconvenient to extract the results for mixed- mode stress intensity factors near the crack front. Also, the finite element that is used in Ref. 1 is the multilayer assumed- stress hybrid element originally developed by Mau et al.2 for the analysis of uncracked laminates. In the procedure of Ref. 2, a stress field is assumed independently in each layer and interlayer traction reciprocity conditions are enforced through the method of Lagrange multipliers, which necessarily complicates the formulation and results in expensive com- putations. Also, since the stresses are independently assumed in each layer, the computational procedure in Refs. 1 and 2 become prohibitively expensive for a large number of layers. Finally, it is noted that the effects of transverse normal stress a33 (x3 being the thickness-coordinate of the laminate) are ignored in Refs. 1 and 2.