Effects Of Stress Waves Research Articles

Control of Guideway Friction by Stress Wave (Effect of a single impact on the reduction in friction).

In general, when a slider is about to be moved, friction is always at the maximum. This phenomenon has made it difficult to locate a slider at a definite position with submicron accuracy. The authors attempted to challenge this widely accepted concept in this paper. The basic idea is to strike a guideway from the side with a small steel ball, so that stress waves are generated in the body of the guideway. The stress waves induce slight movement of the asperities on the surface of the guideway. The movement will lead to the breakage of metal-to-metal cohesion and will cause considerable decrease in friction. The idea was tested using a steel block with a hardened surface for a guideway and another smaller block for a slider. The results obtained are as follows. (1)Even a small ball can induce a considerable change in the relative slding speed of metal-to-metal contact regions between the guideway and the slider. The relative movement is strongly dependent on the kinetic energy of the ball. (2) The above movement causes a great reduction in the friction coefficient. When a ball of 25.4mm diameter is dropped from a height of 80mm, the coefficient of friction drops from 0.37 to 0.05 when the slider speed is 1 × 10-2mm/sec. (3) Though the low friction level is sustained for only 0.02sec, this period is long enough for a slider to start moving. (4) The lower the slider speed, the greater the reduction in friction.

Strain measurements inside solids subjected to lithotripter pulses

To study the direct stress wave effects on the fragmentation of kidney stones or gallstones during lithotripsy, it is important to know the evolution of the stress state inside concretions subjected to lithotripter pulses. Knowledge of these stress fields can be used, along with measurements of the mechanical properties of the stone, to predict locations and modes of failure. A technique of implanting monocrystalline silicon strain gauges within plaster concretions to obtain information about these internal stress fields was developed. Concretions of simple geometries, i.e., disks and spheres, were chosen for this initial study so that the reflections from the boundaries can be more easily identified and compared with theoretical models. Experimental results were obtained for both spherically diverging and focused shock wave sources. Theoretical models based on geometrical acoustics were developed for a spherical wave front incident on a disk and on a sphere. Predictions from these models match well with the experimental results. In addition, the caustic surfaces for a spherically diverging wave incident on a sphere are present. [Work supported by NIH Grant No. DK39796.]

Effect of stress waves on laminated composite plates

Several types of waves in a layered composite material were examined. In order to obtain explicit approximate solutions, a perturbation method was used. This approach views the nonhomogeneous layering as a perturbation on a homogeneous but anisotropic solid. For propagation normal to the direction of layering, it was necessary to include the second-order perturbation terms to get qualitative agreement. Also considered were waves which travel parallel to the laminae. The material in these waves follows an elliptical path. The orientation of which depends on the position in the material. All these waves are also examined numerically, and there is excellent agreement between the perturbation expansions and the numerical results.

Reduction Effect of Impact Stress Wave Propagating in Porous Medium.

The subchondral bones and the articular cartilages constituting the joint parts in biological hard tissues have shock absorbing capabilities as bones strike each other at the joints. In order to clarify this phenomenon, acrylic plates of a porous medium of different diameters were modeled to represent these articular cartilages. In the experiment, when these models are impacted with different impact velocity, the reduction effect of shock waves which pass through the pores is measured using strain gages. It is found from the results of these experiments that when shock waves pass through the pores of a model, shock-absorbing effects, especially as regards maximum stress and duration times, appear. Moreover, the same results are obtained by applying the one- dimensional wave theory and the dynamic finite-element method to these models.

Stress wave effects in a finite element analysis of an impulsively loaded articular joint.

A dynamic contact finite element formulation was used to study transient stresses in the impulsively loaded rabbit knee, an established experimental model of mechanically induced osteoarthrosis. The computations were used to test the hypothesis that stress wave propagation and reflection, from juxtarticular interfaces of material property discontinuity, could be responsible for markedly increased levels of transient local cartilage stress. The finite element results demonstrated intuitively credible stress wave propagation and interfacial reflection phenomena. However, the magnitude of these waves was not nearly large enough to appreciably alter the quasi-static stress distributions otherwise prevailing. Thus, local stress wave reflection from interfaces of modulus discontinuity (for example the cartilage/subchondral plate) probably does not contribute appreciably to the heightened tissue sensitivity to impulsive loading experimentally observed in this animal model.

Analysis of damage in stress wave loaded unidirectional composites

Previous experimental work on tensile stress-strain curves at strain rates of quasi-static, intermediate and impact loading for unidirectional composites revealed different failure modes for both carbon fibre reinforced composites and glass fibre reinforced composites. Based on the shear-lag model, the stress wave effect is related to material response at the microstructural level, focusing on the transient fibre stresses and matrix shear stresses at the notch tip and in front of the matrix split. Qualitative correlations with the test results are attempted.

The role of stress waves in thoracic visceral injury from blast loading: Modification of stress transmission by foams and high-density materials

Materials have been applied to the thoracic wall of anaesthetised experimental animals exposed to blast overpressure to investigate the coupling of direct stress waves into the thorax and the relative contribution of compressive stress waves and gross thoracic compression to lung injury. The ultimate purpose of the work is to develop effective personal protection from the primary effects of blast overpressure—efficient protection can only be achieved if the injury mechanism is identified and characterized. Foam materials acted as acoustic couplers and resulted in a significant augmentation of the visceral injury; decoupling and elimination of injury were achieved by application of a high acoustic impedance layer on top of the foam. In vitro experiments studying stress wave transmission from air through various layers into an anechoic water chamber showed a significant increase in power transmitted by the foams, principally at high frequencies. Material such as copper or resin bonded Kevlar incorporated as a facing upon the foam achieved substantial decoupling at high frequencies—low frequency transmission was largely unaffected. An acoustic transmission model replicated the coupling of the blast waves into the anechoic water chamber. The studies suggest that direct transmission of stress waves plays a dominant role in lung parenchymal injury from blast loading and that gross thoracic compression is not the primary injury mechanism. Acoustic decoupling principles may therefore be employed to reduce the direct stress coupled into the body and thus reduce the severity of lung injury—the most simple decoupler is a high acoustic impedance material as a facing upon a foam, but decoupling layers may be optimized using acoustic transmission models. Conventional impacts producing high body wall velocities will also lead to stress wave generation and transmission—stress wave effects may dominate the visceral response to the impact with direct compression and shear contributing little to the aetiology of the injury.

Laser shock-induced mechanical and microstructural modification of welded maraging steel

The effect of laser-induced high-intensity stress waves on the hardness, fatigue resistance, and microstructure in the heat affected zone of welded 18 Ni(250) maraging steel was investigated. Laser-shock processing increased the hardness and fatigue strength of the weldments. Some melting of the surface was involved during laser-shock hardening which produced the reverted austenite phase. Microscopic analyses showed an increased dislocation density in the laser-shocked area.

An investigation of ball-on-ball impact

The Bureau of Mines, U.S. Department of the Interior, has used a shock accelerometer to measure deceleration in ball-on-ball impact. The accelerometer signal was analyzed by a fast-Fourier-transform (FFT) technique, and signal components were identified from the accelerometer characteristics, solid mechanics, and finite-element modal and spectral analysis. Following frequency-domain digital filtering to remove extraneous frequencies, the remaining signal was reconstituted by an inverse FFT. Supporting Hertzian quasistatic and elastic and elastic-plastic dynamic finite-element calculations were carried out. All calculations predicted similar peak decelerations and contact times. The former agreed well with experiment. However, stress-wave effects make it difficult to use deceleration measurements to define contact times, even at an impact velocity of 8 ms−1. Such an impact must be analyzed as dynamic rather than static.

Impedance characteristics of transducers and reciprocity calibration

This paper investigates impedance characteristics of electrodynamic-type and piezoelectric-type transducers. The inherent relation between mechanical impedance of a mass load and electrical impedance of the transducing element of transducers is revealed. The substance of reciprocity calibration is just to utilize the impedance inversive property of both types of transducers so that one can determine the mechanical quantities (force and velocity) by measuring electrical quantities, which can be measured conveniently and accurately. According to the theory, for high-frequency calibration, mass values in the reciprocity calibration formulas should be replaced by their equivalent masses rather than static mass values, because the mechanical impedances of mass loads will be remarkably changed due to the stress wave effect. Making the correction can improve calibration accuracy by about one-half order of magnitude.

Generalized thermoelasticity for an infinite body with a spherical hole.

The present paper is concerned with a one dimensional generalized thermoelastic problem based on Lord and Shulman's theory. This theory is for dynamic coupled thermoelasticity which includes the time needed for acceleration of a thermal wave, and is based on the modified Fourier's law of heat conduction and the Duhamel-Neumann relation. A generalized thermoelastic problem in an infinite body with a spherical hole is analyzed by means of the Laplace transform technique. The boundary condition is that a constant heat flux is flowing into the infinite body and the displacement at a spherical hole is constrained. Because the effects of thermal and stress waves may well be short-lived, the inverse Laplace transforms are obtained in an approximate method which is valid for small time values.

Propagation of the front of fast cracks in hard polymers

3. V. V. Moskvitin, Plasticity under Alternating Loads [in Russian], Moscow State Univ. (1965). 4. V. V. Moskvitin, Cyclic Stresses in Structural Elements [in Russian], Nauka, Moscow (1981). 5. R. M. Shneiderovich, Strength during Static and Pulsating Loading [in Russian], Mashinos- troenie, Moscow (1968). 6. N. G. Bychkov, A. N. Petukhov, and I. V. Puehkov, "Certain features of the kinetics of deformation of structural materials during cyclic elastoplastic deformation," Probl. Prochn., No. ii, 7-11 (1986). 7. I. A. Birger, "Deterministic and statistical models of endurance," in: Problems of the Keliability of Aircraft [in Russian], Mashinostroenie, Moscow (1985), pp. 105-150. 8. I. A. Birger, "Prediction of safe life under low-cycle fatigue," Probl. Prochn., No. 10, 39-44 (1985). 9. Yu. M. Temis, "Evaluating plasticity and creep in engineering calculations," in: Analy- tical and Numerical Methods of Solving Boundary-Value Problems of Plasticity and Visco- elasticity [in Russian], UNTs AN SSSR, Sverdlovsk (1986), pp. 100-106. i0. A. P. Gusenkov, Strength under Isothermal and Nonisothermal Low-Cycle Loading [in Rus- sian], Nauka, Moscow (1978). PROPAGATION OF THE FRONT OF FAST CRACKS IN HARD POLYMERS Yu. A. Kostandov and S. I. Fedorkin UDC 620.117.5:539.219.2 Experimental investigations of the dynamic failure of linear and cross-linked polymers such as polymethyl metacrylate (PMMA) and cured epoxy resins showed that in specimens with more stored elastic energy (this corresponds to higher nominal static stresses) the crack accelerates more rapidly to its limit speed [1-3]. The mean rate of crack growth increases from zero, and the time of acceleration usually amounts to some tens of microseconds. Various solutions in determining the limit crack growth rate obtained within the frame- work of the theory of elasticity practically lead to the same results which are confirmed by some experimental data. There exists nevertheless information contradicting the assertion that the limit crack growth rate is dependent solely on the elastic constants of the material. It is assumed that the limit crack growth rate is also dependent on the nature of the energy supply [i]. In connection with that the dependence of the limit growth rate of fast cracks, propagating under conditions of pulsed effects, on the parameters of the process of failure requires thorough investigation. Numerous experimental data testify to the jumpwise nature of the propagation of the crack tip in dynamic failure of materials. Some researchers [i, 5] ascribe this to the effect of stress waves reflected from the boundaries of the specimen. The same effect is apparently also the cause of the j umpwise motion of a crack in splitting cyrstals, which is accompanied by the transition of the crack into neighboring planes [i, 6]. That is why only some publications [2, 3, 7] contain data on the jumpwise nature of crack propagation not connected with the interaction of the crack with the boundaries of the speci- men. In the present work we investigated the nature and speed of propagation of the front of cracks initiated by the detonation of a conductor by a current pulse [8] in specimens of materials ED-2OMA, ED-20MTGFA (epoxy resins cured by maleic and methyl tetrahydrophthaleic anhydrides) and PMMA that are free of static stresses or are prestressed. The specimens were rectangular plates 4 mm thick; halfway along their length there was a shaped notch with radius of curvature 25 mm and radius at the tip 0.05 mm. Into the notch we placed a strip of tin plate which detonated when capacitors charged to high voltage were discharge on it. The dimensions of the plates were chosen such that the incidence of elastic waves reflected from the boundaries of the specimen into the zone of registration was pre- vented. M. V. Frunze Simferopol University, Simferopol. Translated from Problemy Prochnosti, No. 9, pp. 23-26, September, 1988. Original article submitted November 24, 1986. 1156 0039-2316/88/2009-1156512.50 9 1989 Plenum Publishing Corporation

Wave effect correction to reciprocity calibration results of vibration primary standards

The paper reveals the stress wave effect (SWE) on reciprocity calibration results of vibration primary standards and derives exact correct formulas of mechanical standards according to reciprocity theory and stress wave theory. The paper further expounds that both the electrodynamic reciprocity method (ERM) and the piezoelectric reciprocity method (PRM) need this correction at higher frequencies and shows that taking this measure can eliminate a system error of 10−2 to 10−3 orders of magnitude.

Method of evaluating the effectiveness of networks of borehole-charge detonations

1. A method of computing the failure and microcracking zones formed on detonating borehole charges in rock ore has been developed within the framework of the kinetic theory of the strength of solids on the basis of a proposed failure criterion. 2. The effectiveness of selecting a delay interval of the order of 1 msec for charges of radius R=0.125 m is indicated in an example of the computation of the failure zones that developed during the detonation of two borehole charges. In this case, the mutual effect of stress waves on the formation of zones of increased microcracking and failure can actually be ensured in the region between charges, and the formation of significant zones with a low microcrack concentration can also be avoided.

Motion of a granular material in a tall vibrating system

The vertical motion of particles is considered for a tall vibrating system; a study is made of the effects of stress waves on the distance between the bottom of the vessel and the bottom of the bed.

The Mechanics of Rock Failure Due to Water Jet Impingement

Abstract The initiation of fracture in a rock mass subjected to the impingement of a continuous water jet has been studied. The jet is assumed to place a quasistatic pressure loading on the surface of the rock, which is treated as a saturated, porous-elastic, isotropic, and homogeneous half-space. While this pressure loading is held constant, the impinging water flows through the rock according to Darcy's law and pressurizes the fluid in the pores. The pore pressure distribution couples with the stress field due to the surface loading to produce an effective stress field, which can start tensile fracturing directly under the load. At various time intervals after initial impingement, the effective-stress field is computed using finite element methods and the results, together with the Griffith criterion for tensile failure, produce the loci of the zones of fracture initiation. The behavior of these zones is displayed as a function of the two jet parameters - pressure and nozzle diameter - and the five rock properties: Young's modulus, Poisson's ratio, tensile strength, porosity and permeability, and time. To experimentally verify that pore pressure plays an important role in the mechanism of rock fracture due to jet impingement, thin sheets of copper (0.001 to 0.005 in.) were placed between a continuous jet (up to 20,000 psi) and the surface of a block of Indiana limestone. The purpose of the copper sheet was to allow the pressure of the jet to be transmitted to the rock, but to prevent water from entering the pore structure. Using pressure substantially greater than the threshold pressure of pressure substantially greater than the threshold pressure of limestone (3,500 psi) where penetration always occurred in the absence of the copper sheet, placement of the sheet was sufficient to prevent any visible damage from occurring to the rock surface, provided the jet did not penetrate the copper first. provided the jet did not penetrate the copper first Introduction The method by which a water jet penetrates and fractures a rock mass is highly complicated and poorly understood. This is mainly because the rock is subjected during the impact to several separate processes, each of which can cause failure. Failure can result from the effects of dynamic stress waves, static pressure loading and erosion. The degree of failure caused by each mechanism is, of course, dependent on the rock properties and jet parameters. parameters. In the first few microseconds of impingement, a subsonic jet pressure on the rock surface reaches the so-called "water hammer" pressure on the rock surface reaches the so-called "water hammer" pressure of pvv(c) and then drops to the nozzle stagnation pressure pressure of pvv(c) and then drops to the nozzle stagnation pressure of approximately 1/2 pv2. (p = fluid density, v = jet velocity, and v(c) = velocity of compression waves in the liquid.) During this initial period of impact, large-amplitude compressive waves are caused to emanate from the point of impingement. Upon reflection off a free surface, these waves become tensile and can cause spalling failures. This mode of failure is usually important with pulsed jet impingement. For continuous jets the spalling effects are small and will be neglected for this study. During the impingement process, the water of the jet flows into the accessible pore space of the rock mass. Since in a continuous jetting process the jet applies a quasi-static pressure loading to the rock surface, the water in the pores is pressurized while the surrounding rock mass is simultaneously stressed. The intent of this paper is to describe the role played by this static pressure loading coupled with the pore-pressure distribution, or pressure loading coupled with the pore-pressure distribution, or the "effective stress," in the first moments of penetration. In studying the process, we will take into account the influence of jet parameters and rock properties. In the course of the impingement process, the jet pressure loading is constantly being redistributed over the crater as it is formed. During this progressive removal of material, erosion is also contributing. The process of erosion is in itself highly complex, so no attempt will be made to characterize it here. EFFECTS OF STATIC PRESSURE DISTRIBUTION-ZERO PORE PRESSURE It has been shown by Leach and Walker that a water jet emanating from the nozzle depicted in Fig. 1 applies a quasi-scatic pressure loading to the surface upon which it is impinging. SPEJ P. 10

Tensile Fragmentation of Glass Fibers

When a virgin glass fiber fractures by static tension, it does not break into two or three pieces but shatteres as a cloud of dust, and disappears. The dynamic process of this fragmentation was observed by means of high-speed photography at a rate of 4×105 frames/sec. It was found that the rupture was initially produced at a cross-section of the fiber as in ordinary tensile tests and then fragmentation began to propagate from the section towards the fixed ends of the fiber at a constant speed (177±5 m/sec). Attempts to interpret the results as the effects of nondispersive stress waves propagating in continuous media seemed unsuccessful. It was, however, pointed out that the observed fragmentation velocity was in close agreement with the value of the fission velocity predicted by the wave-mechanical theory of Fitzgerald.

Stress Waves at Rigid Right Angle Joint

This investigation includes both analytical and experimental studies of the effect of a rigid right angle joint on the transmission of flexural and longitudinal stress waves. The results of this study are useful in understanding the dynamic response of structures subjected to high rate loadings. Analysis of this problem involves simultaneous solution of the effects of longitudinal and flexural stress waves. Solutions are obtained numerically using the method of characteristics. The excellent agreement between analysis and experiment substantiates the analysis developed in this study.

Ultrasonic Treatment of Specimens in the Electron Microscope

A method and technical details are described for direct observation of effects of high amplitude ultrasonic stress waves on dislocations in metal crystals. The technique combines, for the first time, ultrasonic treatment at high acoustic energy densities (up to 109 ergs/cm3 at frequencies between 30 and 100 kc), observation of damping of stress waves, and electron microscopy. Results from this in situ technique allow fundamental refinement of present theories on ultrasonic activation of dislocation mechanisms.

Stress-wave effects in the design of long bars and stepped shafts

This paper presents some general criteria for determining when longitudinal impact stresses in long bars need to be approached by the more exact stress-wave propagation theory rather than approximate engineering theories and makes comparison of results. The analysis is done by a logical “shifting procedure” by steps which avoids mathematical difficulties and enables a computer to be used directly when specific numerical calculations are needed. Special design considerations for cross-section changes and stepped bars are discussed.