Solid Rupture Research Articles

Shear band analysis and shear moduli calibration

Strain localization is a well known phenomenon, generally associated with plastic deformation and rupture in solids, especially in geomaterials. In this process, deformation is observed to concentrate in narrow zones called shear bands. This phenomenon has been studied extensively in the last 20 years by different researchers, experimentally, theoretically and numerically. A criterion for the onset of localization can be predicted solely on the basis of the constitutive law of the material, using the so-called shear band analysis. This criterion gives the critical orientation, and the critical stress state and strain for a given loading history. An important point, already stressed by Vardoulakis in 1980, is that in particular, out-of-axes shear moduli play a central role in the criterion. These are the moduli involved in the response to a deviatoric stress increment with principal axes oriented at 45° from total stress principal axes. Out-of-axes shear moduli are difficult parameters to calibrate; common tests, with fixed principal stress and strain directions, do not provide any information on these moduli, as long as they remain homogeneous. Still, real civil engineering and environmental problems are definitely not simple axisymmetric triaxial tests; practical modeling involves complex stress paths, and need complex parameters to be calibrated. Only special tests, like compression–torsion on hollow cylinder tests, or even more complex tests can be used for shear moduli calibration. However, shear band initiation in homogeneous, fixed-axes tests does activate out-of-axes shear. Hence, it is natural that shear band analysis makes shear moduli enter into the analysis. Then, a typical inverse analysis approach can be used here: experimental observation of strain localization in triaxial tests can be used together with a proper shear band analysis for the model considered, in order to determine out-of-axes shear moduli. This approach has been used for a stiff marl in the framework of a calibration study on a set of triaxial tests. The steps of the method are presented, and the bifurcation surface in the stress space is exhibited.

Oriented rupture of solids by highly viscous fluid

1. Fluids of varying viscosity may be used for oriented rupture of artificial and natural solids, including rock. 2. The use of highly viscous fluids such as modeling clay differs from the use of low-viscosity fluids such as water in the following major respects: a) the fissure front propagates some distance ahead of the modeling-clay front; as a result, higher fluid pressure and rupture force are required in this case; b) in all directions from the center of the borehole and the initiating slot, the motion of the modeling clay occur uniformly, forming practically ideal circular symmetry; this is an important distinguishing feature, which is true of both the fluid and the fissure fronts; c) in the plane normal to the slot surface, from the borehole axis to the propagation front of the modeling clay, the aperture of the fissure along any radius is described by an ellipse, and the modeling-clay body takes the form of an ellipsoid of revolution. 3. These features of oriented-fissure development under the action of pressurized viscous fluid not only represent valuable increase in our knowledge, but also provide the basis for further experiments, improvement of the OFR method and corresponding equipment, and the development of fundamentally new and nontraditional geotechnologies.

Methods of controllable internal rupture in solids and liquids for obtaining atomically-clean surfaces and investigating their adhesion

Controllable internal rupture (CIR) methods for obtaining atomically-clean solid surfaces and closed ultrahigh vacuum (UHV) space are described. A number of modifications of CIR methods have been developed. Their performance, characteristics and applications for investigation of adhesion are considered. CIR methods are based on the creation of virgin atomically-clean rupture surfaces in the bulk of solids during deformation of specially manufactured heterogeneous systems. For all modifications of the method, the samples have been manufactured in the form of vacuum-tight capsules and the investigation procedure has been carried out in atmospheric conditions. CIR methods, in addition to well-known UHV methods, offer a broadening of the research opportunity and simplification of the experimental equipment as well as offering maintenance of surface cleanliness for a long time (up to some months).