Showing papers on "Discontinuity (geotechnical engineering) published in 1995"

Impact of drops on solid surfaces : self-similar capillary waves, and splashing as a new type of kinematic discontinuity

Abstract: The impact of drops impinging one by one on a solid surface is studied experimentally and theoretically. The impact process is observed by means of a charge-coupled-device camera, its pictures processed by computer. Low-velocity impact results in spreading and in propagation of capillary waves, whereas at higher velocities splashing (i.e. the emergence of a cloud of small secondary droplets, absent in the former case) sets in. Capillary waves are studied in some detail in separate experiments. The dynamics of the extension of liquid lamellae produced by an impact in the case of splashing is recorded. The secondary-droplet size distributions and the total volume of these droplets are measured, and the splashing threshold is found as a function of the impact parameters.The pattern of the capillary waves is predicted to be self-similar. The calculated wave profile agrees well with the experimental data. It is shown theoretically that the splashing threshold corresponds to the onset of a velocity discontinuity propagating over the liquid layer on the wall. This discontinuity shows several aspects of a shock. In an incompressible liquid such a discontinuity can only exist in the presence of a sink at its front. The latter results in the emergence of a circular crown-like sheet virtually normal to the wall and propagating with the discontinuity. It is predicted theoretically and recorded in the experiment. The crown is unstable owing to the formation of cusps at the free rim at its top edge, which results in the splashing effect. The onset velocity of splashing and the rate of propagation of the kinematic discontinuity are calculated and the theoretical results agree fairly well with the experimental data. The structure of the discontinuity is shown to match the outer solution.

The Effect of H2O on the 410-Kilometer Seismic Discontinuity.

Abstract: The 410-kilometer seismic discontinuity is generally considered to be caused by a phase transformation of the main constituent of the upper mantle, olivine, α-(Mg,Fe) 2 SiO 4 , to β-(Mg,Fe) 2 SiO 4 . Recent data show that H 2 O dissolves in olivine and other nominally anhydrous mantle minerals and that the partitioning of H 2 O between olivine and β-(Mg,Fe) 2 SiO 4 is about 1:10. Such behavior strongly affects the region over which the α to β phase transformation occurs and hence the seismic discontinuity that results. The observed width of the discontinuity constrains the maximum H 2 O content of upper mantle olivine to about 200 parts per million by weight.

Shear fracture patterns and connectivity at geometric complexities along strike‐slip faults

Abstract: Shear fracture patterns and connectivity at geometric complexities such as echelon fault steps and fault bends along strike-slip faults are investigated by simulating shear fracture propagation paths using a displacement discontinuity boundary element method together with a maximum distortional strain energy density criterion. It is shown that shear fracture patterns and connectivity depend on the specific geometric configurations, the applied stress orientations and the coefficient of friction. For a given initial configuration of the major faults, the subsequent patterns of the associated shear fractures and their linkage can be predicted if the remote state of stress and the coefficient of friction are known. The analyses of fault geometries commonly encountered in nature have important implications for evaluating the effect of faults on fluid flow and earthquake hazards associated with the geometric complexities along strike-slip faults in the upper crust.

Stochastic dynamics of hysteretic media

Abstract: Classic plasticity theory regards the yielding condition as discontinuity between elastic and plastic phases. This discontinuity leads obvious negative consequences in the mathematical features of the algorithm one uses in solving solid and structural mechanics problems.

Lateral variations in mantle velocity structure and discontinuities determined from P, PP, S, SS, and SS — SdS travel time residuals

Abstract: On the basis of P, PP, S, SS arrival times and SS - S410S, SS - S660S differential times, we construct models of mantle P and S velocity structure and boundary topography of the 410-km and 660-km discontinuities. Events from the catalog of the International Seismological Centre (ISC) are relocated relative to the International Association of Seismology and Physics of the Earth's Interior 1991 (IASP91) velocity model using both P and S arrival times. The arrival times are corrected for ellipticity and the PP and SS residuals are corrected for the topography at the bounce point. The cap-averaged PP - P and SS - S differential time residuals, plotted at the PP and SS surface reflection points, form broad coherent patterns. The geographic distribution of the cap averaged residuals agrees quite well with PP - P and SS-S differential time residuals derived from long period Global Digital Seismograph Network (GDSN) data. A robust lp inversion scheme is used to infer global mantle structure. Synthetic tests indicate that for regions well sampled by SS - S410S and SS - S660S differential times, the velocity estimates are not seriously contaminated by the topography of the 410- and 660-km discontinuities. However, estimates of boundary deflections may be influenced by extensive P and S velocity variations of 3% or greater. We find the 410-km discontinuity to be depressed by as much as 24 km beneath North America. Conversely, the discontinuity is deflected upward underneath Eurasia. In some regions the topography of the 660-km discontinuity is quite distinct from that of the 410-km discontinuity, but the two appear to be positively correlated. A series of depressions are found at several intersections of the 660-km discontinuity with known subduction zones. The elevated topography in the 410-km discontinuity beneath Europe is underlain by a trough in the 660-km discontinuity. A number of subduction zones are characterized by a thinning of the transition zone. Negative P and S velocity anomalies, underlying back-arc basins and tectonically active continental regions, encircle the Pacific. Where they are resolved, the stable continental cratons are systematically positive velocity features that extend below 200 km. With the inclusion of PP and SS travel time residuals we are better able to constrain midmantle structure. Most notably, in the depth range 35–660 km beneath the Northwest Pacific we observe high P velocity. Where they are resolved, mid-ocean ridges are most clearly imaged as low velocity features in the S model. The northern portion of the Mid-Atlantic Ridge is underlain by negative S velocity anomalies. In the Pacific, the East Pacific Rise is an extensive low S velocity anomaly.

Direct evidence for the undulation of the 660‐km discontinuity beneath Tonga: Comparison of Japan and California array data

Abstract: Short-period seismograms of Tonga deep earthquakes recorded by Japanese and Californian seismic networks are stacked to identify the S-P converted wave associated with the 660-km discontinuity. The travel-time difference between this S-P converted wave and the direct P wave is used to constrain the depth of the 660-km discontinuity. Analysis of a total of 29 events produced a detailed topographical map of the discontinuity beneath the Tonga subduction zone. Two events which exhibit clear S-P conversions in both Japan and California data are selected to show directly the depth variations of the 660-km discontinuity adjacent to the subducting slab. The S-P conversion points on the ray paths to Japan are observed to be approximately 10 to 30 km deeper than the conversion points on those to California, which represents direct evidence for a slab-induced depression of the 660-km discontinuity.

Midnight velocity shear zone and the concept of Harang discontinuity

Abstract: The authors address the question of the origin and meaning of the Harang discontinuity. They consider mappings of the ionosphere, auroral zones, and magnetosphere. They argue for a relation between the velocity shear zone in the auroral ionosphere and the Harang discontinuity. However because of the upward directed field-aligned currents it is very hard to trace field lines well enough to show mappings of these zones. They argue however for the behavior of auroral arcs in relation to the appearance of substorm events to provide strong support for this relationship.

Surface rupture at a fault bend: the 28 June 1992 Landers, California, earthquake

Abstract: Mapping of surface rupture associated with the 28 June 1992 Landers earthquake along a portion of the north-central Emerson fault permits us to characterize the fractures and to interpret them in terms of a contractional bend in the fault trace. The main rupture exhibits right-lateral and reverse slip and is localized in a zone about 50-m wide along the present-day trace of the Emerson fault. The surface deformation within its northeastern block, the hanging wall, is accommodated by a dome structure about 2-km long and 0.5-km wide and a complex assemblage of fractures. The complexity of the fracture pattern around the fault bend results in part from (1) interaction between the main rupture and many shorter subparallel faults, (2) the initiation and propagation of fractures at geometric complexities such as fault segment tips and fault bends, and (3) bedding plane faulting. Both geological data and rupture kinematics indicate that uplift and shortening are associated with the fault bend. The state of stress around the Emerson fault bend is determined for co-seismic conditions (fault slip boundary conditions) and interseismic conditions (the remote applied stress and frictional fault boundary conditions) using both displacement discontinuity boundary element and dislocation methods, respectively. We also investigate a possible fault propagation path at an idealized bend using the maximum distortional strain-energy density criterion. Results of the theoretical analyses provide support for an understanding of the location and the asymmetry of the dome structure and the mechanisms for some of the observed fracture systems in terms of the state of stress and fault propagation paths around the fault bend and other smaller-scale complexities along the fault trace.

Fundamentals of Residual Stresses in Joints Between Dissimilar Materials

Abstract: When a discontinuity in material properties exists across a bonded interface, stresses are generated as a result of any thermal or mechanical loading. These stresses significantly affect strength and failure characteristics and may be large enough to prevent successful fabrication of a reliable joint. The use of an interlayer material to successfully reduce mismatch stresses, thereby preventing joint failure or improving joint strength and reliability, requires knowledge of failure mechanisms and of the effects of interlayer properties on the critical stress components.The origin of residual stresses developed during cooling of a ceramic-metal joint from an elevated fabrication temperature is illustrated qualitatively in Figure 1. Away from edges, the in-plane (parallel to interface) stresses are typically compressive in the ceramic and tensile in the metal. These stresses can cause cracking perpendicular to the interface, leading to spalling or delamination failures. Such failures are frequently observed in thin-film and coating geometries. Where the interface intersects a free edge, large shear and axial (perpendicular to the interface) stresses are generated. The edge stresses are typically tensile within the ceramic and tend to promote crack propagation within the ceramic parallel and adjacent to the interface. This is the most commonly observed failure mode in bonded structural components.

Dynamic Fracture Mechanics: Displacement Discontinuity Method

Abstract: This work describes the formulation and numerical implementation of both two- and three-dimensional indirect boundary element methods (for example, the fictitious load method and the displacement discontinuity method). It also provides an analysis of crack problems in elastostatic and elastodynamic fracture mechanics, and methods for evaluating weight functions for dynamic problems.

Rupture history of the Great Bolivian Earthquake: Slab interaction with the 660‐km discontinuity?

Abstract: Teleseismic body waves of the great Bolivian earthquake of June 9, 1994 are analyzed to determine the focal parameters and rupture history. Broadband seismograms reveal a complex rupture process: A small initial event (Mw 7.2) was followed about 10 s later by a large moment release pulse of about 40 s duration. Focal mechanisms determined for the mainshock indicate normal faulting with one very shallow NE dipping plane. Azimuthal variation in body-wave displacement pulse widths suggest northward rupture. From master event and body-wave inversion, the main moment release is located 25–50 km NE of the initiation point at about 650-km depth with only small depth variations between the initial and main event. This suggests that rupture was N directed on the near-horizontal plane. Because the slab along other parts of the Andean arc at about 600-km depth dips steeply, a sub-horizontal plane may imply shearing perpendicular to slab dip. Downdip compression on a sub-horizontal plane would then imply that the slab does not penetrate the 660-km seismic discontinuity, but rather, is being sheared out to the NE. This interpretation is not unique as other scenarios are also possible. Such an event immediately above the 660-km discontinuity suggests massive deformation above the discontinuity with no smooth slab penetration into the lower mantle.

Three-dimensional finite-element analysis of jointed concrete pavement with discontinuities

Abstract: A research study was conducted using the finite-element code ABAQUS to investigate the effects of pavement discontinuities on the surface deflection response of a jointed plain concrete pavement-subgrade model subjected to a standard falling weight deflectometer load. A significant improvement over the multilayered linear static analysis that does not allow for any discontinuity is shown. A three-dimensional pavement-subgrade finite-element model with appropriate boundary conditions has been developed. Transverse joints with dowel bars are modeled using gap and beam elements for an uncracked section, a section with cracked concrete layer, and a section with cracked concrete and cracked cement-treated base layers.

Probabilistic stability analysis of excavations in jointed rock

Abstract: Excavations in jointed rock may liberate rock blocks that may fall by gravity or slide along the discontinuity. The orientation of discontinuities is one of the major input parameters in the conven...

Enhanced local tomography

Abstract: Local tomography is enhanced to determine the location and value of a discontinuity between a first internal density of an object and a second density of a region within the object. A beam of radiation is directed in a predetermined pattern through the region of the object containing the discontinuity. Relative attenuation data of the beam is determined within the predetermined pattern having a first data component that includes attenuation data through the region. In a first method for evaluating the value of the discontinuity, the relative attenuation data is inputted to a local tomography function ƒ.sub.Λ to define the location S of the density discontinuity. The asymptotic behavior of ƒ.sub.Λ is determined in a neighborhood of S, and the value for the discontinuity is estimated from the asymptotic behavior of ƒ.sub.Λ. In a second method for evaluating the value of the discontinuity, a gradient value for a mollified local tomography function ∇ƒ.sub.Λe (xij) is determined along the discontinuity; and the value of the jump of the density across the discontinuity curve (or surface) S is estimated from the gradient values.

Perforated thermoplastic sheet in which the perforation craters have sidewalls that present converging portions and diverging portions, a fibrous composite plane material including such a sheet, and methods of manufacture

Abstract: The perforated sheet, in particular for sanitary articles, and made of a substance that is impervious to liquid, includes a multiplicity of craters each having a base in the plane of the outside face, an apex, and a sidewall that slopes between the base and the apex. The peripheral shape of the base includes at least one zone of discontinuity such that on going round the periphery of each crater, its sidewall presents non-uniform inclination with angles of inclination relative to the perpendicular to the plane of the outside face of the sheet that vary between positive values and negative values, said angle being negative at least in the zone of discontinuity. The peripheral shape of the base is preferably the result of partially overlapping a plurality of curved geometrical shapes, in particular circles and ellipses, and the zone of discontinuity corresponds to the junction between superposed shapes. The composite plane material includes said sheet covered in thermoplastic fibers on at least its outside face, which fibers are bonded to one another and to the sheet.

A simple fracture mechanics model for mixed-mode failure in concrete

Abstract: A simple fracture mechanics model is presented based on the fictitious crack concept where the crack initiation is controlled by a failure surface in the normal and shear stress space. The postfailure behaviour is controlled by a softening isotropic kinematic rule based on the effective stress-crack opening deformation. The model in its simplest configuration needs only four parameters, the uniaxial tensile and compressive strengths, the value of fracture energy ( G 1 ) and a constant controlling the isotropic softening the yield surf ace. The model has been implemented in the inner softening band approach where a strong discontinuity is included in the element formulation.

Stochastic Modeling of Small-Scale, Anisotropic Structures in the Continental Upper Mantle,

Abstract: : New methods for the analysis of three-component seismograms have been applied to data from both continental and oceanic regions. Polarization anisotropy, manifested as the splitting of surface, guided, and shear body waves, has been observed in all regions, but no significant azimuthal anisotropy has been detected. This is expected if the local orientation of olivine crystals in the upper mantle is incoherent on the scale of the path lengths used in these experiments (Delta = 35 deg - 50 deg). From a detailed study of trans-Australia paths, we have shown that the shear-wave anisotropy in the western Australia craton averages 3-4% in the uppermost mantle, but it terminates abruptly at a Lehmann (L) discontinuity near a depth of 250 km. Our physical interpretation is that the L discontinuity beneath the ancient continental cratons marks the transition from an anisotropic mechanical boundary layer to a more isotropic region of a thick continental tectosphere. Above L, the temperatures have evidently remained cold enough to freeze-in the small-scale anisotropic structures and tectonic fabrics generated in episodes of orogenic compression associated with tectospheric stabilization. Below L, such structures were either never generated or were annealed out subsequent to their formation. These hypotheses about subcontinental structure have considerable bearing on a diverse set of geological and geodynamical problems, ranging from formation of the cratons to kimberlite vulcanism.

Transport of energy across a discontinuity by fluid loading

Abstract: Short‐circuiting effects due to fluid loading have been investigated for a plane flexural wave obliquely incident upon a line discontinuity in an elastic plate. General expressions for the transmission and reflection coefficients have been derived and exact analytical formulas are given for the required Green’s functions. The transmission coefficient resulting from the fluid loading has been evaluated and found to have a strong, step‐function‐like angular dependence vanishing at large angles of incidence. In addition, the near‐field pressure and velocity fields have been numerically computed to directly examine the energetics of the transmission process. Transmission due to the fluid has been found to arise from propagating acoustic fields that must carry energy a distance of the order of a flexural wavelength. Accordingly, for angles greater than the critical angle, sin θc=k0/ks, transmission due to the fluid is strongly suppressed due to phase cancellation along the line discontinuity, thus explaining t...

Uniform asymptotic solution for the diffraction by a discontinuity in curvature

Abstract: An asymptotic solution for the electromagnetic diffraction by a discontinuity in curvature in a perfectly conducting surface or in an impedance surface is developed The solution is uniform through the reflection transition region It is of the uat type and gives the field in the transition region up to the order k-3/2 included where k is the wave number It is continuous and all its derivatives in the direction normal to the transition direction are continuous The solution is established for a curvature discontinuity along a generatrix of a two dimensional surface illuminated by a plane wave Its efficiency is illustrated by numerical examples