Volume 20, Issue 3, 01 March 1949
Index of content:
20(1949); http://dx.doi.org/10.1063/1.1698348View Description Hide Description
The first part of the paper (Sections I to V) is concerned with the general technique used in the discussion of stress‐strain laws for inviscid elastic‐plastic materials with work hardening. It is postulated that when the ``mechanical state'' of such a material is known a given infinitesimal change of stress produces a uniquely defined infinitesimal change of strain. Once the assumption is made as to which variables determine the mechanical state, the conditions of continuity, uniqueness, irreversibility, and consistency can be used to extract rather far‐reaching information regarding the structure of the stress‐strain law. The procedure is applied to the case where the mechanical state is determined by the components of stress and permanent strain.
The second part of the paper (Sections VI to VIII) is concerned with various problems of plastic equilibrium. Structural stability in the plastic range is discussed and the difficulties arising in the formulation of stability problems for nonconservative systems are pointed out. Statically determinate problems in the Saint‐Venant‐Mises theory of plasticity are reviewed with special reference to discontinuous solutions. Finally, the ``shake‐down'' problem is discussed for an elastic‐plastic structure under the action of loads, each of which varies in a random manner between given extreme values.
20(1949); http://dx.doi.org/10.1063/1.1698349View Description Hide Description
Approximate expressions are obtained for the potential distribution, maximum current density, and beam spread of tubular electron beams of finite thickness. Comparisons are made with previously published results for very thin tubular beams and solid cylindrical beams.
20(1949); http://dx.doi.org/10.1063/1.1698350View Description Hide Description
A practical mathematical solution is presented, which separates the direct and reflected components of microwave radiation when a single pair of plane waves is received. Very promising operational applications are noted when it is desired to determine elevation angles precisely, continuously, and instantaneously. The method is well suited to the measurement of very small elevation angles. The results of sample tests of the solution warrant further study in this direction.
20(1949); http://dx.doi.org/10.1063/1.1698351View Description Hide Description
A method of calculating stresses and deflections for beams whose materialcreeps is presented in this paper. Complete tension creep test data at constant temperature are used to define creepcharacteristics of the material. Then by using Bernoulli's hypothesis of plane sections and the techniques developed earlier for interpretation of the relaxation creep tests, a method of beam analysis is shown. Stresses and deflections may be calculated for any desired time interval. This includes the time prior to the occurrence of the steady state creep. The latter aspect appears to have been ignored by others.
20(1949); http://dx.doi.org/10.1063/1.1698352View Description Hide Description
The metal‐strip delay medium is shown to be analogous to a transmission‐line low pass filter. A filter analysis gives the image phase constant and image admittance of the equivalent filter section. The index of refraction of the medium is obtained from the former and the reflection coefficient from the latter. It is shown that the physical parameters may be chosen so that the strip medium is perfectly matched to space at a particular frequency. Formulas are given which hold for normal incidence even when the strips are close together. The same method may be applied to delay media consisting of thin metal obstacles of any shape arranged uniformly in parallel planes. Examples are circular disks, ellipses, and rectangles. Certain general conclusions are drawn for this class of media.
20(1949); http://dx.doi.org/10.1063/1.1698353View Description Hide Description
The response characteristics of a number of inorganic phosphors have been investigated in the extreme ultraviolet region (∼170A) for the purpose of selecting a luminescent screen suitable for the measurement of relative intensities in conjunction with photo‐multipliers. Preliminary tests indicate that in this spectral region luminescence is excited rather strongly in the tungstates. Sample screens of CaWO4 have been studied photographically in order to determine to what extent such screens give rise to a faithful reproduction of the intensity distribution of the exciting radiation. Qualitative calculations show that the luminescent intensity excited by weak soft x‐ray radiation is above the noise level of the conventional photo‐multiplier.
20(1949); http://dx.doi.org/10.1063/1.1698354View Description Hide Description
General design curves for use of lead networks on log‐db plots are given. A variety of parallel and bridge T networks along with d.c. networks are evaluated for use in a.c. servo mechanisms. The restrictions existing on some networks are enumerated, and suggestions on general design procedure are made.
20(1949); http://dx.doi.org/10.1063/1.1698355View Description Hide Description
A study of the activation energy associated with the viscous intercrystalline slip shows that the conventional theories of grain boundary, e.g., the intercrystalline amorphous cement theory and the abrupt transitional theory, are both untenable. A grain boundary model is described in which the transition region at the boundary is considered as consisting of numerous disordered groups of atoms or diffused holes. The intercrystalline slip occurs through the atomic rearrangement by thermal agitation within each ``disordered group'' by a shear process involving as units of flow only a few atoms. This grain boundary model and slip mechanism are consistent with experimental facts and furnish, furthermore, a unified viewpoint as to the mechanism of the viscous intercrystalline slip, the volume diffusion in metals, and the constant rate creep of metal crystals under small stress. Further experiments are described concerning the influence of previous deformation and impurities on grain boundaryviscosity. It has been found that the grain boundaryviscosity is lower in a specimen subjected to a heavier deformation prior to its recrystallization. A very small amount of impurities was found to be able to block partially or completely the grain boundary slip in aluminum,iron, and copper. These observations are readily understood on the basis of the proposed grain boundary model and slip mechanism.
20(1949); http://dx.doi.org/10.1063/1.1698357View Description Hide Description
After a brief review of various applications of ultrasonic techniques, considerable attention is paid to the established methods employed for making sound waves visible. It is shown that these methods are primarily modifications of the technique of ``schlieren photography'' originally developed by Foucault and used frequently for photographing phenomena (e.g., air flow analysis) in which refractive index gradients are set up. The advantages and disadvantages of the various techniques are discussed in some detail and it is shown that a technique employed by Willard is particularly well adapted to the visualization of ultrasonic waves. A modification of this method is discussed, the experimental apparatus is described and typical photographs of ultrasonic wave patterns are shown. In particular, the close analogy existing between light waves and ultrasonic waves with respect to the wave phenomena of refraction, diffraction, and interference is demonstrated in a number of photographs.
20(1949); http://dx.doi.org/10.1063/1.1698358View Description Hide Description
20(1949); http://dx.doi.org/10.1063/1.1698359View Description Hide Description