A device for high uniaxial compression of single crystal specimens at low temperatures

A design is described with which elastic and sufficiently uniform compression of single-crystal specimens by ～ 0.4 to 0.5% can be achieved at liquid helium temperatures. The provision of a sliding side support for the specimen, which prevents shear deformations arising in the compression process, makes it possible to achieve deformations almost an order of magnitude greater than the limiting elastic deformations possible for free specimens.     

Width of adiabatic shear bands formed under combined stresses

Abstract

An equation for the half width of an adiabatic shear band formed under combined stresses is derived. The importance of strain rate, stress, temperature rise, and thermal conductivity is described. Confirmation is found for the proposition that shear band width is independent of stress state and this is confirmed by comparing the theory with the experimental results of other workers.

MST/933     

Damage accumulation modeling under uniaxial low cycle fatigue at elevated temperatures

The paper presents a fatigue damage accumulation model, which allows us to predict fatigue life under low cycle uniaxial loadings at elevated temperatures. The structure of the model has been based on the stress–strain curves obtained during the experimental study. The model has been verified experimentally by applying experimental studies carried out on ENAW-2024T3 aluminum alloy and 2Cr–2WVTa steel. Moreover, a comparison between the results of fatigue life prediction using the proposed damage accumulation model was done with the results obtained on the basis of various generally applied models, based on the Manson–Coffin dependency. Furthermore this paper presents the results of experimental studies carried out on the aluminum alloy ENAW 2024 T3 under uniaxial low cycle fatigue loadings in the conditions of elevated temperatures. In the course of the study, material constants and the parameters of the stress–strain curve in the range of low cycle fatigue for four levels of temperatures (20, 100, 200 and 300 °C) were set.     

Development of shear bands in dynamic plane strain compression of depleted uranium and tungsten blocks

We study the initiation and growth of shear bands in prismatic bodies of rectangular cross-section made of either depleted uranium or tungsten and deformed in plane strain compression at a nominal strain-rate of 5000 s⁻¹. It is assumed that defects are distributed symmetrically with respect to the two centroidal axes and each quadrant has up to 300 randomly distributed defects in the form of a weaker material; the flow stress for the weaker material in a quasistatic simple compression test is taken to be 5% lower than that for the original material. It is found that, in the deformed configuration, shear bands in depleted uranium blocks are inclined at approximately 42.5° counterclockwise from the horizontal axis, those in tungsten are inclined at nearly 135°. When shear bands initiate, the total compressive force required to deform the body drops sharply for the uranium blocks but gradually for the tungsten blocks. After a shear band has developed, dead zones form in both uranium and tungsten blocks; the size of the dead zone in the tungsten block is more than that in the uranium block. When the shear modulus for the tungsten is artificially changed so as to equal that for the uranium, the angle of inclination for the shear bands in tungsten blocks changes to that found for the uranium blocks. This suggests that the value of the shear modulus plays a noticeable role in the development of shear bands. We have also studied the effect, on the initiation of shear band, of modeling the defects as either very weak or very strong material.     

Nucleation and anisotropic crystalline growth of polyethylene under shear

The crystallization of polyethylene was observed during shear experiments in isothermal conditions. The nucleation and the crystalline growth rates were measured from a microscopic observation of the growing morphologies. An unusual formation and development of row nuclei was observed throughout the experiment, followed by anisotropic growth. Three main growth rates were measured with respect to the main directions of the process. All these growth rates are enhanced differently by the shear rate. This result is interpreted first by an increase of the equilibrium melting temperature emanating from an entropy loss due to the chain orientation, second the anisotropy of growth is discussed as an effect of chain orientation with respect to the growth front of lamellae and of a local flow different from macroscopic shear-flow. An overall kinetic equation with shear effect has been proposed for this polyethylene and tested in a stimulation of crystallization during injection moulding of a polyethylene disc. The shear effect on the crystallization is necessary to predict the crystallization temperature and the thickness of polymer crystallized during the filling stage.     

Adiabatic shear bands in α-titanium tube under external explosive loading

The radial pressure explosive experiment is successfully used to investigate the formation of adiabatic shear bands (ASBs) in α-titanium (α-Ti) tube under external explosive loading. The ASBs initiate at the inner surface of α-Ti tube, and most of the shear bands are in spiral form along the cross-section of the tube towards counterclockwise direction. Tip of a shear band propagates along the surface of the maximum shear stress. Four patterns of ASBs such as bifurcation, collection, crossing and N-shape are observed. The developed shear bands are the preferred sites for nucleation, growth and coalescence of microvoids. The nucleation of microvoid is caused by the local hot spots and stress concentration in the shear bands. The coalescence of microvoids forms the crack within ASBs, and when the critical crack length is reached catastrophic fracture occurs.     

Synthesis of graphitic carbon particle chains at low temperatures under microwave irradiation

Chains of graphitic carbon particles were formed by microwave irradiation of polyethylene glycol at temperatures between 160 and 220 °C for 40 min, in the absence of a catalyst. Chains were comprised of individual particles ranging in size from 340 to 620 nm; particle size increased with synthesis temperature. The D/G ratio measured by Raman spectroscopy was 0.91, indicative of a mixture of amorphous and graphitic material. SEM, TEM and TGA measurements confirmed this. Our experiments show that the chains are an intermediate product, which when heated further under hydrothermal conditions, produce MWNTs.     

Oxidation of TiC at low temperatures

The isothermal oxidation of TiC powders was carried out at low temperatures of 350–500 °C at oxygen pressures of 3.9, 7.9 and 16 kPa under a static total pressure of 39.5 kPa, achieved by mixing with argon, using an electro-microbalance. The oxidation kinetics are described by the one-dimensional diffusion equation. It was found that oxidation consists of four steps, I (fast step), II (slow step), III (fast step) and IV (slow step), at all the pressures. Two activation energies were obtained in steps II–IV: 125–150 kJ mol^–1 below about 420 °C and 42–71 kJ mol^–1 above that temperature. The low- and high-temperature oxidation mechanisms are discussed in connection with the formation of oxycarbide/titanium suboxides and the crystallization of anatase, followed by the generation of cracks in the grains.     

Deformation of silicon at low temperatures

The dislocation arrangements produced around microhardness indentations made in silicon at room temperature have been studied by transmission electron microscopy. Loops consisting of 30°- and 60°-dislocations are produced and move on the {111} planes. It is suggested that, during indentation, the theoretical shear strength is exceeded locally and that the observed dislocations arise as a result of the accommodation of the displacements due to block slip. On annealing up to 1030° C the loops do not appear to be mobile, rather new loops consisting of edge and screw components are formed which can move large distances.     

Strength of adhesives at low temperatures

The paper reports tests on the shearing strength of a range of adhesives at 77K.     

Hot electrons in metals at low temperatures

Hot electrons in metals at helium temperatures under steady conditions can be produced by passing an electric current of moderate density ( 10⁶ A/cm²) through thin, narrow (~1 m wide) metallic films in good thermal contact with bulk single-crystal dielectric substrates. This paper is concerned with the theory of hot electrons in normal metals at low temperatures (when _D, where is the average electron energy and _D is the Debye temperature). The theory is formulated in terms of realistic electron and phonon dispersion laws, taking into account the experimental possibility of heat removal from the sample. In the case in which the temperature approximation of Kagnov, Lifshitz, and Tanatarov is not satisfied when elastic scattering of electrons is dominant in a steady state electric field, the kinetic equation is derived for the energy-dependent, hot electron distribution function, which determines the associated nonlinear responses. The solution of this equation is discussed for a simple model. It is shown that the experimental information on the electron-phonon interaction in a metal can be obtained in terms of the well-known spectral functions S() ² F() and g() _tr ² F(). This is illustrated by experiments determining the nonlinear field dependence of the resistance, by tunnel experiments, and by critical current hysteresis measurements (for superconducting metals). Theoretical estimates which support the observability of the effects under discussion are presented.