Stress field

About: Stress field is a research topic. Over the lifetime, 11926 publications have been published within this topic receiving 226417 citations.

Model validation of a 3d simulation of dislocation dynamics: discretization and line tension effects

Abstract: Some fundamental aspects of 3D computer modelling of plastic flow are presented together with the results of a detailed study of related approximations, such as the discretization of space and of dislocation line curvature. More specifically, the stress field generated by different discretizations of dislocation loops and the critical stress for dislocation multiplication through the Frank-Read mechanism, are compared to the predictions of the elastic theory of dislocations in the isotropic approximation. Although crude in appearance, the approximation adopted in these simulations to describe the curvature does not drastically affect the behaviour. Moreover, it leads to critical stress values for a pinned dislocation segment in excellent agreement with previous computations.

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.

General Theory of Elastodynamic Source Fields

Abstract: The most important and interesting source of elastic radiation in geophysics is an earthquake, or tectonic source, because the radiation field from such an energy source provides information on the largely unknown stress field within the earth. The actual mechanisms or processes of material failure undoubtedly can be described parametrically by the radiation field in terms of rupture velocity, rupture geometry, and the initial and residual stress within the region of failure. Accurate estimates of stress and the parameters of failure are therefore of particular significance in any description of the physical state of the material and would not be unrelated to the larger-scale dynamical processes taking place within the earth. A number of methods and theories are presently used in estimating some of these parameters. The present study is intended to extend the dynamical theory of tectonic sources in order to provide a more complete description of earthquakes in terms of these basic parameters of rupture, including prestress. No assumptions are made concerning the nature of equivalent forces at the source or of their time dependence. The theory predicts the spatial and temporal form of the radiation field in terms of the initial prestress field and the basic rupture parameters. These predictions follow from the recognition that an earthquake is a relaxation source and that such a phenomenon is described analytically as an initial-value problem. Consequently, such a source satisfies the conservation of energy and linear and angular momentum conditions required for a spontaneous source. The radiation field is produced by the continuous reduction of stored potential strain energy in the elastic medium surrounding a growing rupture zone, where it is assumed that the rupture, or at least a part of the total rupture zone, has a well-defined boundary at a given time to which boundary conditions are applicable. The compatibility of this geometrically sharp, time-varying boundary condition with probable failure processes in the earth is examined and judged to be good. Analytical expressions for the radiation field from an arbitrary source of elastic radiation are given, and within the framework of this formulation the properties of a spontaneous tectonic source are contrasted with ‘applied force’ sources and their special properties, as well as with some of the field observations of earthquake radiation fields. These considerations demonstrate the need for a more general and complete description of tectonic sources in order to explain all the observations and, more fundamentally, to deduce more precisely the nature of the physical processes of failure in the earth. It is concluded that a relaxation theory will provide the flexibility required to describe the characteristics of the observed radiation field and will also provide estimates of rupture parameters bearing on the processes of failure and the state of the material. A complete development of the dynamical relaxation theory for tectonic sources, including considerations of the total energy release and the final equilibrium field, constitutes the main result of this study, providing explicit expressions for the dynamic and static displacement and energy fields. It is shown that the radiation field will generally have a frequency-dependent shape, dependent to first order on the ratio of rupture length to radiation wavelength. A relatively simple example is considered, and the radiation pattern and displacement and energy spectra computed. The radiation pattern is contrasted with the pattern from a more complicated rupture geometry and serves to demonstrate the pattern-shape dependence on frequency, rupture geometry, and prestress. The energy and displacement spectra are also found to have maxima and minima, the number being dependent on the rupture geometry and their location and spacing being dependent on rupture length and velocity.

A short-time diffusion correlation for hydrogen-induced crack growth kinetics

Abstract: Analysis of hydrogen-stress field interactions have led to kinetic criteria for slow crack growth. Using both elastic and plastic stress fields under opening-mode loading, criteria for stage I, II, III growth are developed in terms of the pressure tensor gradient at the crack tip. It is proposed that stage I (stress-intensity dependent) growth kinetics are predominantly controlled by the elastic stress field while stage II (nearly stress-intensity independent) kinetics are controlled by the plastic stress field. Measurements of slow crack growth in cathodically-charged AISI 4340 steel verify the overall aspects of the correlation. Detailed measurement and analysis of the increase in crack-tip radius with increasing applied stress intensity have led to a proposed decrease in crack growth rate during stage II growth. Some experimental evidence corroborates this later hypothesis and is consistent with long range diffusional flow of hydrogen as the controlling mechanism for crack growth kinetics.

Longitudinal vibrations of nano-rods by stress-driven integral elasticity

Abstract: In elastic continuous structures defined on bounded domains, Eringen strain-driven integral model leads to ill-posed elastostatic problems since the constitutive stress field, got by convoluting th...