Internal pressure

Abstract: MY attention was first directed to these phenomena by the occurrence of curves like that reproduced in Fig. 11. In the course of experiments on vaso-dilator reflexes I sometimes observed what looked like a reflex of this kind, in a limb of which the nerves had been divided; the figure _ reproduced shows the effect of a fall of arterial pressure produced by exI citing the central end of the depressor nerve on the volume of the hind leg of the rabbit, the sciatic and the nerves accompanying the femoral artery having been cut. It will be noticed that as long as the fall of blood-pressure lasts there is a passive diminution of volume in the limb, but that as soon as the previous height of blood-pressure is attained, on cessation of the excitation, there is a considerable expansion of the limb lasting for some time. Such a curve would usually be explained by stating that Fig. 1. Effect of depressor excitation on there was present all through the volume of enervated leg. Upper curve blood-pressure, next below it volume of excitation a relaxration of the vessels, leg, upper of two chronographs-period but that it was prevented from showof excitation of depressor nerve, lower ing itself in an actual expansion of one-time in 10 sec. intervals. the limb by the simultaneous fall of blood-pressure; although it was

Abstract: A sealed reactor fuel can when subjected to sufficiently high thermal stresses in the presence of an internal pressure will yield plastically. A simple model of the can is used to show that the plastic strains so produced may cause ratchetting or plastic cycling as the temperature gradient across the can wall cycles because of startup and shutdown of the reactor. On the assumption that creep is negligible, approximate criteria are derived for the onset of ratchetting and plastic cycling, simple expressions are obtained for the plastic strains incurred by each cycle, and failure of the can due to the above mechanisms is discussed both for work-hardening and non-work-hardening material. Consideration is then given to the effect of stress relaxation due to creep when the mean temperature of the can is sufficiently high to cause complete relaxation of the thermal stress while the reactor is at power, creep being ignored while the reactor is shut down. Under these conditions, it is found that the crite...

Abstract: The unit-cell parameters of quartz, SiO2, have been determined by single-crystal diffraction at 22 pressures to a maximum pressure of 8.9 GPa (at room temperature) with an average precision of 1 part in 9000. Pressure was determined by the measurement of the unit-cell volume of CaF2 fluorite included in the diamond-anvil pressure cell. The variation of quartz unit-cell parameters with pressure is described by: a −4.91300 (11) = −0.0468 (2) P + 0.00256 (7) P2 − 0.000094 (6) P3, c − 5.40482 (17) = − 0.03851 (2) P + 0.00305 (7) P2 − 0.000121 (6) P3, where P is in GPa and the cell parameters are in angstroms. The volume–pressure data of quartz are described by a Birch–Murnaghan third-order equation of state with parameters V0 = 112.981 (2) a3, KT0 = 37.12 (9) GPa and K′ = 5.99 (4). Refinement of K′′ in a fourth-order equation of state yielded a value not significantly different from the value implied by the third-order equation. The use of oriented quartz single crystals is proposed as an improved internal pressure standard for high-pressure single-crystal diffraction experiments in diamond-anvil cells. A measurement precision of 1 part in 10 000 in the volume of quartz leads to a precision in pressure measurement of 0.009 GPa at 9 GPa.

Abstract: [1] We have determined the postspinel transformation boundary in Mg2SiO4 by combining quench technique with in situ pressure measurements, using multiple internal pressure standards including Au, MgO, and Pt. The experimentally determined boundary is in general agreement with previous in situ measurements in which the Au scale of Anderson et al. [1989] was used to calculate pressure: Using this pressure scale, it occurs at significantly lower pressures compared to that corresponding to the 660-km seismic discontinuity. In this study, we also report new experimental data on the transformation boundary determined using MgO as an internal standard. The results show that the transition boundary is located at pressures close to the 660-km discontinuity using the MgO pressure scale of Speziale et al. [2001] and can be represented by a linear equation, P(GPa) = 25.12 − 0.0013T(°C). The Clapeyron slope for the postspinel transition boundary is precisely determined and is significantly less negative than previous estimates. Our results, based on the MgO pressure scale, support the conventional hypothesis that the postspinel transformation is responsible for the observed 660-km seismic discontinuity.

Abstract: 1. The determination of the distribution of stress in the neighbourhood of a crack in an elastic body is of importance in the discussion of certain properties of the solid state. The theory of cracks in a two-dimensional elastic medium was first developed by Griffith1 who succeeded in solving the equations of elastic equilibrium in two dimensions for a space bounded by two concentric coaxial ellipses; by considering the inner ellipse to be of zero eccentricity and by assuming that the major axis of the outer ellipse was very large Griffith then derived the solution corresponding to a very thin crack in the interor of an infinite elastic solid. Because of the nature of the coordinate system employed by Griffith the expressions he derives for the components of stress in the vicinity of the crack do not lend themselves easily to computation. An alternative method of determining the distribution of stress in the neighbourhood of a Griffith crack was given recently by one of us2 making use of a complex stressfunction stated by Westergaard.3 This method suffers from the disadvantage that the Westergaard stress-function refers only \"to the case in which the Griffith crack is opened under the action of a uniform internal pressure; the stress-function corresponding to a variable internal pressure does not appear to be known. In the present note we discuss the distribution of stress in the neighbourhood of a Griffith crack which is subject to an internal pressure, which may vary along the length of the crack, by considering the corresponding boundary value problem for a semi-infinite two-dimensional medium. The analysis is the exact analogue of that for the three-dimensional \"circular\" cracks developed in the previous paper2 except that now we employ a Fourier cosine transform method in place of the Hankel transform method used there. A method is given for determining the shape of the crack resulting from the application of a variable internal pressure to a very thin crevice in the interior of an elastic solid, and for determining the distribution of stress throughout the solid. The converse problem of determining the distribution of pressure necessary to open a crevice to a crack of prescribed shape is also considered. As an example of the use of the method the expressions for the components of stress, due to the opening of a crack under a uniform pressure, are derived and are found to be in agreement with those found in the earlier paper.2 2. We consider the distribution of stress in the interior of an infinite two-dimensional elastic medium when a very thin internal crack — c^y^c, * = 0 is opened under the action of a pressure which may be considered to vary in magnitude along the length of the crack. For simplicity we shall consider the symmetrical case in which the applied pressure is a function of |y| but the analysis may easily be extended to the