Short Stress Pulses Research Articles

Experimental investigation of longitudinal pulse propagation in unidirectional composite rods

Values of longitudinal moduli of unidirectional fibre-reinforced composites have been determined experimentally under conditions of short stress pulse loading. The velocity of propagation of a short pulse, produced by the detonation of a small amount of lead azide, was measured in a number of rod specimens having various fractions of fibre reinforcement. It is shown that the response of the reinforced specimens, as far as wave speed is concerned, is practically the same as the response of a homogeneous material. The pulse speed, c O , is determined by the simple formula c O = (E ∗/ϱ) 1 2 , where E ∗ and ϱ are effective longitudinal modulus and density of the composite, based on the ‘rule of mixture’. Most of the tests were conducted with glass/epoxy and glass/polyester specimens. Some tests were conducted with aluminium/epoxy specimens, made of a single aluminium rod embedded in an epoxy matrix. It was found that even when the pulse length was not much longer than the ‘fibre’ diameter, the simple approach for calculating wave speed, mentioned above, was still applicable.

Determination of dislocation damping in copper single crystals with stress pulse method

Dislocation damping in copper single crystals was determined by the stress pulse method. For this purpose very short stress pulses were used, maximum length 30 μs, by which loading conditions of a metallic matrix in the frontal region of a propagating brittle crack were simulated. At loading by these very short stress waves, strain rates\(\dot \gamma \) above 104 s−1 are achieved, however, the strain rate during loading is not constant. Therefore a technique was elaborated that enables to compare the measured results of mechanical characteristics with very short stress pulses (\(\dot \gamma \) ≠ ≠ const.) with results obtained on a device with a constant strain rate of the specimen. With the use of the dependence τ = τ(\(\dot \gamma \)) the existence of damping mechanisms of a viscous character was proved and the damping coefficientB determined. These results agree with theoretical assumptions in [10,20] about the probable mechanical behaviour in the region of the propagating brittle crack.

A Model for Stress Wave Propagation in Composite Materials

A theoretical model is developed to describe stress wave propaga tion in plate laminate composites. The dispersive effects in the com posite are accounted for by direct analogy with a viscosity effect in the model. The model gives excellent predictions of steady and tran sient wave shapes and velocities, and of the attenuation of short stress pulses. It averages out only the fine structure which results from in dividual reverberations between the plates of the composite. The success of the theoretical model for plate laminate composites should provide a basis for good semi-empirical modeling of the stress wave propagation characteristics of a wide variety of composite materials.

Plastic deformation at high strain rates

In the present paper a new method, an original testing device and techniques for the study of plastic deformation in materials at high strain rates up to 2×104 s−1 achieved with the use of very short stress pulses (length within 10–20 μs) are described. The suggested method of yieldpoint determination respects the effects of “delayed plastic deformation” in the yield point at loading by very short intensive stress pulses. The investigations were carried out in polycrystalline ARMCO-iron and low-alloyed steel, and besides on the specimens irradiated by neutrons with integral dose 1·35×1019 ncm−2. The experimental results obtained are interpreted from the point of view of the present knowledge of the process of plastic deformation in bcc metals.

Low-Stress Shock and Release Wave Behavior of Porous Carbon

Shock-reverberation techniques and transmitted-wave experiments were used to determine multiple shock states and release adiabats from shock-induced states for a 0.68-g/cm3 graphite foam and a 1.37 g/cm3 carbon felt. These experiments indicated that the samples were not compacted to solid density in the 1–3 μsec duration of the stress pulse, even though the peak pressures were well in excess of the quasistatic yield strength of the materials. For the very low-density graphite foam, release adiabats centered at initial shock states between 0.7 and 3.2 kbar were found to be indistinguishable from the principal Hugoniot; whereas for the more dense material, release adiabats centered between 0 and 25 kbar were substantially different from the principal Hugoniot. This seemingly anomalous behavior of the graphite foam is likely due to a separation of the solid portions of the sample from the gauge. A prediction of the distance at which a short stress pulse is overtaken by trailing relief waves is given for the carbon felt.

Use of Thick Transducers to Generate Short-Duration Stress Pulses in Thin Specimens

It is shown how the pulse-echo method for determining sound velocities and acoustic attenuation can be applied to thin specimens by generating short stress pulses with thick transducers. The method is described in conjunction with its use in the observation of phase transformations in which both transit-time data and attenuation measurements are used to determine the mechanism and kinetics of the transformation. Results are shown for the β → α a phase transformation in high-purity plutonium.

Dislocation Sources in Crystals

Lithium fluoride crystals were subjected to short stress pulses (1–10 μsec in length) in order to study dislocation nucleation in them. It was found that several heterogeneities can lead to dislocation nucleation: (a) cleavage steps, (b) dislocation loops, (c) glide bands, (d) inclusions or ``dirt'' particles, (e) precipitates (as grown), and (f) radiation-induced precipitates. Also, it was found that annealing treatments at moderate temperatures (200–600°C) followed by ``rapid'' cooling (>5°C/hr) made segments of previously pinned dislocations become mobile. These mobile segments multiplied as they moved through the crystals. Several treatments were tried that were ineffective in producing dislocation nucleation. These included: temperature changes, surface wetting, simultaneous irradiation and stressing, reflections from free surfaces, and diagonal bending. Carefully cleaned glass spheres were pressed into contact with dislocation-free regions of a LiF crystal. In this way, shear stresses that were 100 times greater than the normal yield stress of the crystal (∼G/85) were reached without causing homogeneous dislocation nucleation. It is concluded that small foreign heterogeneities cause most of the dislocation nucleation in real crystals. Homogeneous nucleation occurs only at high stresses (∼G/30). Relatively little dislocation multiplication results from classical Frank-Read sources (almost none in LiF). Most dislocation multiplication occurs as a prior dislocation moves through a crystal.