摘要:

The problems of stress analysis for linearly viscoelastic materials are considered. This is the simplest group of materials which exhibit the general stress‐strain characteristics found in polymers and plastics. Three basic aspects are considered: measurement of material properties, determination from these of the operator equations between stress and strain or equivalently of the viscoelastic model, and use of this in the theoretical analysis of stress distributions. Quasi‐static analysis, in which inertia forces are negligible, is treated quite generally. The wave problems which arise when inertia effects are included are restricted to one‐dimensional space variations. A series of typical solutions of these types is discussed.

ISSN:0021-8979    

DOI:10.1063/1.1722464

出版商:AIP    

年代:1956

数据来源: AIP

摘要:

Extensive studies of the viscoelastic properties of polymers undertaken in the author's laboratory by means of the method of stress relaxation are here reviewed. The discussion is divided into four parts: chemical stress relaxation, stress relaxation in amorphous polymers, stress relaxation in crystalline polymers, and stress relaxation in certain natural polymers and polyelectrolytes. Mathematical description of the phenomena are presented in simple form. The relation between structure and viscoelastic properties of polymers are discussed and a rather complete over‐all picture of these phenomena seems to be emerging.

ISSN:0021-8979    

DOI:10.1063/1.1722465

出版商:AIP    

年代:1956

数据来源: AIP

摘要:

The measurement of stress relaxation in concentrated solutions of polymethyl methacrylate has revealed a relation between the spectrum of relaxation times, the weight average molecular weight, and the molecular weight distribution. The spectrum of relaxation times and the essentially identical theories developed by Rouse, Bueche, and Zimm are used to calculate an average relaxation time which is directly proportional to the weight average molecular weight. It is then shown that a first approximation to the distribution of molecular weights may be calculated if one assumes that the friction factor increases with the 1.5 power of the molecular weight. This distribution for selected samples of polymethyl methacrylate is compared with the Schulz distribution in which the parameters were obtained from light scattering and osmotic pressure experiments. The ratiosMz/Mwcalculated from the theory and stress relaxation data on a rough fraction, a whole polymer, and a blend of two whole polymers are found to be 1.3, 1.7, and 1.9, respectively.

ISSN:0021-8979    

DOI:10.1063/1.1722466

出版商:AIP    

年代:1956

数据来源: AIP

摘要:

A theory of the viscosity of raw rubbers is developed on the postulate that raw rubbers, when subjected to continuous shear in the non‐Newtonian region of flow, consist of microscopic rheological units which are semipermanent aggregates of many rubber molecules [M. Mooney and W. E. Wolstenholme, J. Appl. Phys.25, 1098 (1954)]. The theory treats these units as tacky, elastic solids, whose relative movement and slippage constitute the macroscopic flow of the rubber. The resulting theoretical flow curve differs little from that given by H. M. Smallwood [J. Appl. Phys.8, 505 (1937)], but the interpretation of the parameters is radically different. The new theory leads to expressions for the number of temporary point attachments per unit area between two touching rheological units and for the mean life of these attachments.Agreement with experimental data is slightly better than in Smallwood. Two types of observed deviation from the theory can be interpreted as due to two plausible phenomena not included in the mathematical treatment.

ISSN:0021-8979    

DOI:10.1063/1.1722467

出版商:AIP    

年代:1956

数据来源: AIP

摘要:

The scattering of 16‐Mev bremsstrahlung by 3, 6, and 9 in. of iron was measured for small angles using a thallium‐activated sodium‐iodide scintillation counter. The experimental results closely fitted a theoretical curve calculated on the basis of single scattering.

ISSN:0021-8979    

DOI:10.1063/1.1722468

出版商:AIP    

年代:1956

数据来源: AIP

摘要:

Obukhoff's statistical theory of turbulent mixing is proposed as a replacement for the heuristic theories of Gallet and Villars‐Weisskopf, and is applied to the problem of the scattering of radio waves by refractive index fluctuations. In the case of ionospheric scattering, order‐of‐magnitude agreement with the observed scattered power is obtained if the refractive index fluctuations are attributed to electron density fluctuations produced by turbulent mixing in the lower edge of theElayer. In the case of tropospheric scattering, it appears that order‐of‐magnitude agreement with the observed scattered power can be obtained, except during the summer months, by attributing the refractive index fluctuations to temperature fluctuations. During the summer months and at low scattering heights, humidity and its fluctuations are expected to play a prominent role. Experimental and theoretical evidence is cited in favor of perennial fractional‐degree temperature fluctuations in the troposphere. A comparison of the Obukhoff, Villars‐Weisskopf, and Booker‐Gordon models is given, and it is found that the Obukhoff model predicts the most scattered power.

ISSN:0021-8979    

DOI:10.1063/1.1722469

出版商:AIP    

年代:1956

数据来源: AIP

摘要:

Upper‐atmosphere air densities and temperatures have been calculated from the measured trajectory data of a falling sphere. Densities were calculated from the equation for the drag force on the falling sphere, and temperatures were then obtained by using the hydrostatic equation and the equation of state of a perfect gas. The aerodynamic background and instrumentation are described; the method of calculation and the errors are discussed. The results of four rocket flights which carried the experiment are given. Comparison with the average of other rocket measurements indicates agreement with these results.

ISSN:0021-8979    

DOI:10.1063/1.1722470

出版商:AIP    

年代:1956

数据来源: AIP

摘要:

Measurements of neutron irradiation effects upon the Young's modulus and logarithmic decrement of a set of copper single crystals of 99.999% purity have been made at room temperature. Interpretations of the results can best be given in terms of an oscillating dislocation line mechanism in which the oscillations are suppressed by the presence of interstitial atoms, vacancies, or some combination thereof. Comparisons of the results with the predictions of impurity line pinning are given. It is shown that an interpretation in this sense leads to generally accepted values for the dislocation densities and average segment lengths.

ISSN:0021-8979    

DOI:10.1063/1.1722471

出版商:AIP    

年代:1956

数据来源: AIP

摘要:

Ferrites having both high relative density (∼98%) and high dc resistivity (>109ohm‐centimeters) can be prepared by properly sintering nickel copper ferrites which have both an iron deficiency and a small amount of manganese or cobalt present. Magnetic saturation at room temperature shows a linear dependence upon copper content in this system varying from 3150 gauss for NiFe1.9Mn0.02O4±to 2400 gauss for furnace cooled CuFe1.9Mn0.02O4±. Direct current resistivity remains high for mixtures of these two ferrites if 2Cu<Ni. An anomalous break in the curve relating dc resistivity with copper content occurs when the copper to nickel ratio exceeds this 1:2 value.

ISSN:0021-8979    

DOI:10.1063/1.1722472

出版商:AIP    

年代:1956

数据来源: AIP

摘要:

The purpose of these studies was to investigate the source of noise which is generated when thin dielectrics are subjected to relatively low dc voltages as reported by Boyer. It has been suggested by J. H. Mason that this phenomenon may be caused by the ionization in small voids due to poor electrode contact. Tests reported in this paper show that the application of 300 volts dc to dry films having a total resistance between test electrodes of 1.5×10 in. ohms was sufficient to give rise to a steady noise voltage if the electrodes were not in intimate contact. During some of the experimental work a two phase dielectric was subjected to dc voltage up to 1425 volts with no pulses. As expected, when the polarity was reversed, the dc current momentarily increased many‐fold, and then during its decay, noise was measured simultaneously. The noise ceased when the dc current became constant. A similar phenomena has been observed during the decay of the hetero‐change on a polarized dielectric. The test methods used provide a very sensitive method for detecting ionization in extremely small voids.

ISSN:0021-8979    

DOI:10.1063/1.1722473

出版商:AIP    

年代:1956

数据来源: AIP