Showing papers on "Hydrostatic stress published in 1968"

Effects of high hydrostatic pressure on the mechanical behavior of a crystalline polymer–polyoxymethylene

Abstract: The true stress-true strain behavior of polyoxymethylene, n(-CH2O), as an example of a bulk semi-crystalline polymer, has been investigated for constant hydrostatic environmental pressures from 1 atmosphere to 8 kilobars with the principal objectives of elucidating the factors controlling flow and fracture. Experiments were conducted in uniaxial tension at room temperature and constant strain rate. The tensile observations were supplemented by measurements of bulk compressibility and stress relaxation behavior at pressure. In contrast with metals and inorganic compounds, the modulus, yield stress and fracture stress of POM increase strongly with pressure by a factor of approximately three at 8 kilobars. The modulus increase is shown from the stress relaxation measurements to be associated with a pressure-induced increase in the β-transition temperature which points to the potential usefulness of the concept of pressure-temperature super-position of mechanical behavior. The characteristics of the pressure dependence of the yield stress demonstrate that yield criteria based on continum mechanics considerations, including the Mohr or Coulomb-Navier criterion, are not valid for general deformation (non-plane strain) conditions in this polymer. The concept of a critical volume change determining the initiation of yielding is suggested to be applicable to semi-crystalline polymers. Comparison with analogous changes in yield stress with temperature points to an increasing contribution to the control of yielding by the initially disordered regions with increasing pressure or decreasing temperature. The fracture behavior observed at pressure eliminates the concepts of a critical stress as a fracture criterion for POM and of a simple reduction in normal stress at points of stress concentration as the principal effect of the applied pressure on fracture.

Stress-generated ice crystals in a nearly isothermal two-phase system

Abstract: The use of the two-phase system ice–water to illustrate the effects on equilibrium of both hydrostatic and non-hydrostatic stresses has been a recurrent theme in the history of thermodynamic theory. The effects of hydrostatic pressure on the melting point of ice are firmly established by theory and experiment. Those of non-hydrostatic stress are still a subject of debate today; several theorists have predicted ice re-crystallization under such stress, but the magnitude of any slight melting-point depression is not known with certainty. The recrystallization of ice caused by local variations in hydrostatic stress was predicted and experimentally confirmed over a century ago. Cavities deep within temperate glaciers provide a suitable environment for the occurrence of this latter phenomenon. A water-filled cavity intersected by a tunnel in nearly stagnant ice of the Blue Glacier, Washington State, U.S.A., was lined with large and unusual single ice crystals which apparently owe their origin to the effects of hydrostatic stress. Even the minute differences in pressure melting point around this cavity are adequate to remove the heat of fusion as ice forms within it. There is evidence that interstitial movement of melt water in the surrounding ice also contributes to the heat and mass transfer. The form of these crystals indicates that they grew into slightly supercooled water. It is suggested that this growth pattern is sustained by the existence of oriented stresses at the cavity walls.

Stress Tensor of a Finite Dislocation

Abstract: Expressions for the stress field of a dislocation segment are given in a general dyadic form, without reference to a particular coordinate system. They are derived from the formulae of Peach and Koehler, and limited to the case of a linear, isotropic, infinite continuum. The stress tensor due to a finite length of dislocation is expressed in the form c σ ∞ + σ ', which in the extreme case reduces to σ ∞ , the well-known result of an infinitely long dislocation. The stress field of an arbitrary dislocation network can be deduced by the simple addition of the stress field of each component segment, as a direct application of the present study.

Stability of the undistorted states of an isotropic elastic body

Abstract: Incremental stability criteria appropriate for compressible and incompressible, general isotropic elastic solids are used to study the stability of all undistorted states of an arbitrary body subject to dead loads corresponding to a uniform hydrostatic stress. General conditions sufficient for stability of all such hydrostatic stress states are derived. The results indicate a geometrical dependence for the apparent shear and bulk moduli. The relation of the main results (4.10) and (4.21) to uniqueness theorems in the theory of small deformations superimposed on large, to broad plausibility restrictions imposed on the elasticities so as to assure physically reasonable material behavior, and to the propagation of elastic waves is also outlined; these relations constitute only specific application of certain general theorems already in the literature. Finally, it is shown that the principal results coincide with estimates that may be gotten by aid of Holden's inequality.