A. R. Rosenfield

Abstract: A method is suggested for representing the change with time of the distribution of moving dislocations within a linear array under non-equilibrium conditions. The discrete dislocations are smeared into a continuous distribution of infinitesimal dislocations, and it is assumed that the stress/velocity relations for both kinds of dislocations are identical. The results are discussed in terms of previous discrete calculations.

Abstract: A very simple, empirical criterion for fracture initiation in brittle rock is described. The criterion is particularly applicable in areas of low confining stress. Such areas occur around underground excavations, and the criterion is therefore suitable for predicting the extent of fracturing around these excavations for support design purposes. Two examples of the application of the criterion are described. These are firstly to sidewall slabbing in a haulage and secondly to spalling from the face of a bored tunnel.

Abstract: Theoretical analysis and experimental results are presented to describe the influence of pore pressure on tensile fracture initiation and propagation direction. Fracture is influenced by both pore pressure magnitude on a local scale around the crack tip and by the orientation and distribution of pore pressure gradients on a global scale. Simple fracture initiation tests conducted on cylindrical limestone and sandstone rock samples are first described which support an effective stress law for tensile fracture initiation in very porous rocks. In other experiments, controlled fractures are propagated both mechanically and hydraulically across sandstone slabs subjected to non-uniform pore pressure fields. The fractures are shown to propagate towards regions of higher local pore pressure. Finallt, the influence of pressure gradient induced body forces is demonstrated by generating fractures connecting two pore pressure source points. The experimental results are supported by poroelasticity and fracture mechanics analysis of the process, and may be applied to influence hydrailic fracture orientation in the field.

Abstract: Summary One answer to the question posed in the title is that we will have more accurate data for arrival times of SH waves, because the rotational component around the vertical axis is sensitive to SH waves although not to P-SV waves. Importantly, there is another answer related to seismic sources, which will be discussed in this paper. Generally, not only dislocations commonly used in earthquake models but also other kind of defects could contribute to producing seismic waves. In particular, rotational strains at earthquake sources directly generate rotational components in seismic waves. Employing the geometrical theory of defects, we obtain a general expression for the rotational motion of seismic waves as a function of the parameters of source defects. Using this expression, together with one for translational motion, we can estimate the rotational strain tensor and the spatial variation of slip velocity in the source area of earthquakes. These quantities will be large at the edges of a fault plane due to spatially rapid changes of slip on the fault and/or a formation of tensile fractures.

Abstract: A recently proposed model for a continuum with microstructure is further substantiated by identifying the microstructure with dislocations. In particular, the continuum is viewed as a superimposed state composed of a perfect lattice state, an immobile dislocation state, and a mobile dislocation state. It is assumed that each state evolves continuously in space-time and transitions from one state to another take place spontaneously according to the balance laws of effective mass and momentum. When the constitutive equations are subjected to the requirements of invariance, familiar statements from dislocation dynamics are deduced. When plastic strain and yield are identified in terms of the parameters characterizing the dislocation states, familiar flow rules and yield surfaces are produced. The capability of the model to predict not only Tresca and Von-Mises plastic behavior but also phenomena such as kinematic hardening, different responses in tension and compression, latent hardening, and the Bauschinger effect, is shown. Finally, the appropriateness of our equations to model creep, cyclic plasticity, and fatigue, is illustrated.