High Propagation Velocity Research Articles

Crack propagation velocities in bi-phase plates under static and dynamic loading

An investigation of the dependence of the crack-propagation velocity in complex bimaterial plates at different loading rates was undertaken. The specimens were fractured under the influence of both static and dynamic loadings and the crack-propagation velocities were detected by high speed photography with the optical method of caustics. The investigation was concentrated in detecting the influence that the different loading rates have on the fracture velocities in both phases of the plates and how this influence interferes—counteracting or superimposing—with the other factors that determine the crack propagation process in bi-material specimens. These factors are the effect of interface, the influence of the mechanical characteristics of each phase on the crack-propagation velocity etc. The results show that for constant and given material characteristics of the bi-phase plate the crack propagation velocity in the first (notched) phase tends to increase with increasing strain rates. The same is valid for the crack propagation velocity in the second phase, but only for the case when fracture occurs under the influence of a dynamic load. A significant discrepancy of the latter statement occurs, however, in the case of fracture under a continuously-increasing static load. In this case the crack-propagation velocity in the second phase reaches some remarkably high values, which are of the order of high strain-rate dynamic crack propagation velocities. In this way, the crack-arrest effect on the crack propagation velocity appears to be more significant in the case of a static loading than it is for the case of dynamic loading.

A study of one special type of mudflow in the French Alps

Summary One of the most dangerous movements in the French Alps is a special type of mudflow sometimes affecting masses of several millions of cubic metres with a high velocity of propagation and large spread if the mudflow is directed by a valley profile. Forecasting the phenomenon is of great practical importance and for this reason it is necessary to understand the mechanism of formation of this type of mudflow. Generally, these mudflows are observed in larger masses of scree in which there is water circulation. The screes originate from rapidly weathered cliffs of rock. The rocks concerned include sedimentary rocks easily transformed mechanically into silt size particles. In order to forecast the occurrence of this type of mudflow, the authors have attempted to explain the mechanism of formation of such mudflows in the bed of the River Ravoire (Savoie). One other example is given, for which the above mentioned mechanism seems to be the cause of the mudflows.

Is there horizontal phase propagation of 5-min oscillations at high velocities?

New observations of the photospheric 5-min oscillations are presented which prove that the physical reality of the very high horizontal phase propagation velocities observed in connection with the oscillations cannot be maintained. Instead, a statistical model is proposed to explain the observed phase relations.

Time-dependent aspects of rock behavior as measured by a servocontrolled hydraulic testing machine

The closed-loop, servocontrolled hydraulic testing machine was used to study the mechanisms of rock failure. It was found that applied load relaxes appreciably as soon as macrocracks initiated at the specimen surfaces. The behaviors of load relaxation were obtained for Berea sandstone, Tennessee marble and Arkose sandstone. Load relaxation was used to establish the residual strength and creep behavior of the fractured specimens of Tennessee marble and Arkose sandstone. The load relaxation was also used to explain the experimental result that the negative slopes of the postfailure curves of all four rock types increased with a decrease in the applied deformation rate. In addition, a fast response visicorder was used to record the rapid fracture of charcoal granite and Berea sandstone. The result indicated that the testing machine was not able to follow the fracture processes due to its slow response and the high crack propagation velocity.

Photoplastic Effect in ZnO

A physical model is proposed for the photoplastic effect in ZnO, i.e., the reversible increase in flow strength upon illumination. The model is based on the assumption of Coulomb interaction between dislocations, which are negatively charged, and photoionized traps, which contain excess positive charge. Negative charge on dislocations has been observed in silicon, germanium, cadmium sulfide, and other semiconductors. The excess positive charge appears when interstitial Zn+ atoms are doubly ionized to Zn2+ under the influence of light. Several measurements were made of the photoplastic effect which confirmed the proposed model. First it was found that the photoplastic effect depended on the deformation rate so that at a high strain rate, 5×10−2 sec−1, the effect disappeared, presumably because of the high propagation velocity of the dislocations, which did not allow sufficient time for the photoionization to grow. At lower strain rates, 5×10−4 sec−1, a repeated yielding occurred, also explained by the model. In a second experiment photoconductivity at high dislocation densities was studied. An analysis of the photoconductivity data made it possible to predict the saturation illumination intensity and the time constant of the photoplastic effect. Agreement with the observed values was found. In a third experiment it was found that the flow stress, with and without illumination, depended on the square root of the density of interstitial zinc atoms.

Shock waves at high densities. — II

We continue the study of shock waves propagating through plasmas in such critical physical conditions as can be of interest in astrophysical problems. In a preceding paper (see reference in the text) we studied the case of very high temperatures. Now, in Part II, we analyse densities ≥1010 g cm−3. We extend our interest only up to 1015 g cm−3, because for higher densities the presence of nuclear forces introduces many difficulties (not yet completely resolved) in the formulation of a general state equation. Between these two limits we find the range of densities that are likely to be present in the interiors of neutron stars and supernovae. We calculate the shock characteristics (for one-dimensional stationary shocks) for the above range of densities modifying the Rankine-Hugoniot conditions and the state equation of the medium to take into account the nucleonic degeneracy, the pair creation, the radiation, the transformation of chemical composition following the beta-equilibrium reactions, and so on. We obtain that weak shock waves can easily take off the plasma nucleonic degeneracy. But strong shock waves (z=P2/P1>104) characterized by very high propagation velocities (v⋍c) must occur in order to increase the temperature up to 109oK; we treated these cases relativistically. Finally we give tables and graphs of the results obtained. Astrophysical applications are only outlined; they will be treated elsewhere.

Some technical applications of ultrasonic location in gases

Radar systems cannot locate targets at close range because of the high velocity of propagation of electromagnetic waves; one solution is to use ultrasonic waves, which are much slower. This entails both investigation of environmental effects, and development of new transmitters and transducers. Successful results have been obtained in Doppler-shift speed measurements of cars and railway trucks, and pulse-echo distance measurements in coal mines and blast furnaces.

Photoelastic study of elastic impact on the edge of a plate

The Hertz impact theory assumes a quasi-static stress distribution adjacent to the point of contact. This assumption has been tested by a plane photoelastic investigation. The impacting bodies were made from glass plate, which is well suited to impact tests because at high rates of loading it is purely elastic up to fracture and shows no relaxation. Its small photoelastic sensitivity is an advantage for the investigation of high local stresses. A disadvantage is its large elastic modulus giving high propagation velocities. An apparatus based on single-shot exposure has been developed allowing us to follow quasi-cinematographically the wave propagation in the impacted plate at time intervals of micro-seconds with an exposure time determined by a spark of 0·6 μsec half-value duration. The fringes at the point of impact could be photographed very sharply with this arrangement because of their low velocities. The isoclinics were also photographed at various instants during the impact time which was between 45 and 100 μsec according to the impacting mass. Comparison of photographs with others taken under static compression of the same bodies in similar conditions confirms the similarity of the stress distribution near the impact or contact point. Hence the impact force can be determined photoelastically by static calibration. The maximum impact forces found in this way for the various impact masses and velocities coincide well with those calculated from the elementary theory. As an additional check, the area of the surface of contact was measured under static and dynamic loading. A comparison with the values calculated from the Hertz theory would be inaccurate because the impact duration was of the same order of magnitude as the natural period of the impinging bodies and hence smaller than the impact duration according to Hertz. The quasi-cinematographical series of photographs shows clearly the vibrations excited in the impacting bodies. The coefficient of restitution, which relates mainly to this part, could be determined from the impact force—time curve and the impulse. The fracture strength of glass proved appreciably higher under impact than under static loading.

Motor Unit Distribution of the Dorsal Longitudinal Flight Muscles In Locusts

ABSTRACT The peripheral pathways of the ‘fast’ motor fibres to the locust dorsal longitudinal flight muscles are described at the single unit level, from electrophysiological and histological studies. This is summarized in a diagram on Pl. 1. Both pterothoracic dorsal longitudinal muscles consist of five anatomically distinct motor units, arranged in layers from dorsal to ventral. Each of the four more ventral units of both muscles receives a motor axon from the segment in front via the recurrent nerve, whereas the uppermost motor unit is innervated in each case from the segment containing the muscle. The motor units are nearly equal both in size and capability for work. Each of the five ‘fast’ motor axons innervates one topographically distinct bundle of muscle fibres. There is no overlap between muscle motor units. Even within a single muscle, motor units are capable of vibrating at independent frequencies. This indicates that the coupling of units which occurs during flight is neither structurally nor functionally rigid. With respect to peripheral features, the motor units within each dorsal longitudinal muscle are designed for fast response which improves the synchronization when the relevant neurons fire simultaneously (large motor axons, 15–25 μ in average diameter with high propagation velocity, 8 m./sec. at flight temperature). It is suggested that a tonic motor output, containing at least three units, which was recorded from mesothoracic nerve IBa, travels to the small lateral dorsal muscles.

Fracture Phenomena in Metal Plates

By comparing data obtained from tensile tests at room temperature on plates containing cracks of various lengths, it has been found that alloys of metal can be classified into three groups. Each of these groups differs distinctly with respect to the effect of cracks on strength reduction, crack propagation and mode of failure. Theory is presented on (i) the mode of failure, (ii) effect of finite width on critical stress and (iii) size effect. Resulting formulae are expressed in simple parameters: nominal stress, critical crack length and material constant. For the two most common material groups, criteria of fast fracture is deter‐mined. In the case of the group characterized, on one extreme, by large strength reduction and high velocity of crack propagation, fracture still remaining shear type, application of Griffith theory of fracture is discussed and circumstances are described leading to this type of instability. Experimental results presented in support of theory given herein, include tests on plates of varying width and of alloys of magnesium, aluminium, titanium and iron.

Conduction in smooth muscle: comparative structural properties.

The individual fibers of smooth muscle of high propagation velocity (e.g. esophagus) are longer and more closely packed than in those of low velocity (e.g. cat intestine). The space between fibers and within bundles calculated from electron micrographs is 4.4% for pig esophagus muscularis mucosae, 9.0–12.5% for taenia coli and cat intestine, 18.2% for the slow dog retractor penis and 39% for the nonconducting carotid artery. Predominant intercellular separation for esophagus and taenia is 700 A°, for cat longitudinal intestinal 1600 A°, dog retractor 3000 A° and carotid artery 10,000 A°. Two types of interfiber ‘bridges’ both crossed by membranes were observed. These may represent relatively low resistance pathways between fibers during conduction or they may serve a mechanical function. Vesicles are abundant beneath cell membranes and are arranged in longitudinal ridges, some 20 rows/cell. Among the myofilaments are found dark bodies which appear to be composed of noncontractile protein.

Experiments for the Determination of Transient Stress and Strain Distributions in Two-Dimensional Problems

Abstract The object of this investigation was to develop a method utilizing photoelasticity for determining transient stress and strain distributions in two-dimensional problems. Numerous investigators have approached this problem using the common photoelastic materials which have a relatively high modulus of elasticity and correspondingly high velocity of wave propagation. However, many difficulties were encountered in photographically recording data, and only a few reliable stress patterns were obtained. In order to avoid these difficulties, a material with a low modulus of elasticity and a correspondingly low velocity of wave propagation was developed. As a result the 8-mm. Fastax camera is capable of recording precise fringe patterns. The material used was a member of the epoxy-resin family, modified to give the desired properties. Both its static and dynamic properties were determined as accurately as possible. It was found that the strain-fringe value of the material is approximately constant, but the modulus of elasticity and stress-fringe value are different for static and dynamic loadings. Preliminary studies were conducted to develop the method using a circular disk under a radially applied concentrated impact load as the model. A simply supported beam under central impact was then studied, and deflection curves obtained were compared with curves theoretically predicted by Saint Venant and Flamant. The comparisons showed good agreement. An analysis of the formation of the fringe pattern for various times after impact also was made.

Styroflex Aluminum-sheathed air-dielectric cable

STYROFLEX coaxial cable was developed to meet the demand for a uniform, low-loss cable which could be supplied in long lengths, without joints. Prior to the advent of styroflex, low attenuation in conjunction with high velocity of propagation were attainable only with rigid line or wave guide in short sections joined together.

The Mechanism of the High Velocity of Propagation of Lightning Discharges

Observations by Schonland and Collens with Boys' camera show that the luminous tip of the leader stroke of the lightning discharge advances with a high velocity sometimes exceeding 3×109 cm per second. From the fact that electrons traveling 109 cm per second produce about 6000 new electrons per cm of actual path and that 6000 new electrons per cm in the direction of the field (i.e., α=6000) requires 4×106 volts per cm shows that to give the electrons a velocity in the direction of the field of 109 cm per second would require a field exceeding 4×106 volts per cm even at the tip of the stroke. The fields, and therefore the electron velocities, are certainly far less than this value. Accordingly the velocity of the tip greatly exceeds the velocity of the electrons in the tip. It is possible to account for this velocity as follows: Calculations based on justified assumptions concerning the conditions known to exist in a lightning stroke show that if the field just ahead of the tip averages 105 volts per cm for about one centimeter, ionization by collision by the electrons present in the gas before the stroke would be great enough to advance the tip at the observed rate. Such a field is found not too high to be consistent with the low conductivity of the leader path. In this way a propagation of the leader stroke of a velocity many times higher than the velocity of the electrons can be attained.