Showing papers on "Slip (materials science) published in 1977"

Deformation of pearlite

Abstract: Pearlite with its lamellae oriented mainly parallel to the longitudinal direction was prepared by Bolling's method of transformation in a steep temperature gradient. The Fe-0.7 pct Mn-0.9 pct C pearlite was drawn into wire and also into strip in dies designed to minimize macroscopically nonuniform deformation. Cross sections of the drawn wires and strip were examined by conventional and high-voltage transmission electron microscopy and were analyzed by quantitative metallography for a) average interlamellar spacing, b) distribution of interlamellar spacings, and c) orientation relationship between the cementite lamellae and the slip systems in the ferrite. The strength of pearlite is proportional to the reciprocal square root of the average interlamellar spacing, and the proportionality constant analogous to the Hall-Petch constant (k) is related to the strength of the cementite lamellae. If the stress for the propagation of slip through the cementite is assumed constant, a Hall-Petch type of equation can be derived for the strengthening of the pearlite against slip in the ferrite by piled-up groups of dislocations. Evidence for the plastic deformability of cementite is presented; sufficiently thin cementite plates were fully plastic. The exponential strain hardening of drawn pearlitic wires and of rolled pearlite is explained in terms of locally inhomogenous deformation revealed by the lack of fragmentation of the lamellae.

Earthquake Stress Drops, Ambient Tectonic Stresses and Stresses That Drive Plate Motions

Abstract: A variety of geophysical observations suggests that the upper portion of the lithosphere, herein referred to as the elastic plate, has long-term material properties and frictional strength significantly greater than the lower lithosphere. If the average frictional stress along the non-ridge margin of the elastic plate is of the order of a kilobar, as suggested by the many observations of the frictional strength of rocks at mid-crustal conditions of pressure and temperature, the only viable mechanism for driving the motion of the elastic plate is a basal shear stress of several tens of bars. Kilobars of tectonic stress are then an ambient, steady condition of the earth’s crust and uppermost mantle. The approximate equality of the basal shear stress and the average crustal earthquake stress drop, the localization of strain release for major plate margin earthquakes, and the rough equivalence of plate margin slip rates and gross plate motion rates suggest that the stress drops of major plate margin earthquakes are controlled by the elastic release of the basal shear stress in the vicinity of the plate margin, despite the existence of kilobars of tectonic stress existing across vertical planes parallel to the plate margin. If the stress differences available to be released at the time of faulting are distributed in a random, white fashion with a mean-square value determined by the average earthquake stress drop, the frequency of occurrence of constant stress drop earthquakes will be proportional to reciprocal faulting area, in accordance with empirically known frequency of occurrence statistics.

High-frequency radiation from crack (stress drop) models of earthquake faulting

Abstract: Summary. We present a theory for the radiation of high-frequency waves by earthquake faults. We model the fault as a planar region in which the stress drops to the kinematic friction during slip. This model is entirely equivalent to a shear crack. For twodimensional fault models we show that the high frequencies originate from the stress and slip velocity concentrations in the vicinity of the fault’s edges. These stress concentrations radiate when the crack expands with accelerated motion. The most efficient generation of high-frequency waves occurs when the rupture velocity changes abruptly. In this case, the displacement spectrum has an u-’ behaviour at high frequencies. The excitation is proportional to the intensity of the stress concentration near the crack tips and to the change in the focusing factor due to rupture velocity. We extend these two-dimensional results to more general three-dimensional fault models in the case when the rupture velocity changes simultaneously on the rupture front. Results are similar to those described for twodimensional faults. We apply the theory to the case of a circular fault that grows at constant velocity and stops suddenly. The present theory is in excellent agreement with a numerical solution of the same problem. Our results provide upper bounds to the high-frequency radiation from more realistic models in which rupture velocity does not change suddenly. The u-’ is the minimum possible decay at high frequencies for any crack model of the source.

Seismic moments of major earthquakes and the average rate of slip in central Asia

Abstract: Seismic moments for 12 major earthquakes (M ≥ 7.6) in central Asia from 1911 to 1967 were calculated from long-period Rayleigh and Love wave spectral densities. With fault lengths estimated from geological field observations of surface faulting, intensity distributions, or master event relocations of aftershocks, the calculated moments place bounds on the average slip and fault widths. The following table summarizes the calculated moments, estimated fault lengths, and inferred possible average displacements.

Fatigue deformation of TiAl base alloys

Abstract: The fatigue behavior of Ti-36.3 wt pct Al and Ti-36.2 wt pct Al-4.65 wt pct Nb alloys was studied in the temperature range room temperature to 900°C. The microstructures of the alloys tested consisted predominantly of γ phase (TiAl) with a small volume fraction of γ phase (Ti3Al) distributed in lamellar form. The alloys were tested to failure in alternate tension-compression fatigue at several constant load amplitudes with zero mean stress. Fracture modes and substructural changes resulting from fatigue deformation were studied by scanning electron microscopy and transmission electron miscroscopy respectively. The ratio of fatigue strength (at 106 cycles) to ultimate tensile strength was found to be in the range 0.5 to 0.8 over the range of temperatures tested. The predominant mode of fracture changed from cleavage type at room temperature to intergranular type at temperatures above 600°C. The fatigue microstructure at low temperatures consisted of a high density of a/3 [111] faults and dislocation debris of predominantly a/2 [110] and a/2 [110] Burger's vectors with no preferential alignment of dislocations. At high temperatures, a dislocation braid structure consisting of all 〈110〉 slip vectors was observed. The changes in fracture behavior with temperature correlated well with changes in dislocation substructure developed during fatigue deformation.

Creep and plasticity of hexagonal polycrystals as related to single crystal slip

Abstract: The role of slip on basal, prismatic and pyramidal systems of hexagonal single crystals in determining inelastic polycrystalline behavior is studied using a uniform strain-rate upper bound and a self-consistent method. Steady power-law creep is considered. Included as a limiting case is rigid-perfectly plastic behavior, for which the upper bound to the yield stress of the polycrystal coincides with the Bishop-Hill bound for these materials. When the resolved shear stress needed to produce a given level of slip on the pyramidal systems is large compared to that on the other systems the upper bound lies well above the self-consistent estimate. Scif-consistent theory indicates that overall inelastic deformation of a polycrystal is possible without pyramidal slip. Implications for hexagonal materials, including ice, are discussed.

Observations on microstructurally sensitive fatigue crack growth in a widmanstätten Ti-6Al-4V alloy

Abstract: Fatigue crack growth rates in commercial purity Ti-6A1-4V can be substantially reduced with a beta anneal from levels associated with the mill anneal, owing primarily to crystallographic crack bifurcation in the Widmanstatten packets. This microstructurally sensitive fatigue crack growth occurs when the reversed plastic zone is less than the packet size and results in a fracture surface with a faceted morphology. It appears from replica and scanning electron microscopic examination that the facets are comprised of three superposed features,viz cleavage-like river-line patterns, very fine striations and traces of slip lines (and slip-band cracks). Limited X-ray evidence suggests a facet orientation some 8 to 10 degrees off the basal plane.

Slip Velocity during Wetting of Solids

Abstract: A slip velocity at the contact line of a liquid drop on a solid surface originates because of the force induced by the gradient of the chemical potential in the liquid along the solid-liquid interface. Einstein's equation is used to relate the slip velocity and the force. The gradient occurs because (a) the interaction potential at the liquid-solid interface near the contact line due to the neighboring molecules differs from that of a semi-infinite liquid and depends upon the distance from the contact line and (b) the chemical potential is pressure dependent.

Steady-state seismic slip – A precise recurrence model

Abstract: Regularity of seismian slip along a 9 km segment of the Calaveras fault zone is believed to result from steady-state loading of a creeping fault to generate stresses on an isolated stuck patch which moves in a stick--slip event in the magnitude range 3 to 4 whenever a critical threshold is reached. The patch behavior can be described by a simple model similar to the spring-driven frictional models used in laboratory simulations of stick--slip. The (M > or = 3) recurrence time for this model is directly proportional to the seismic slip (computed from magnitudes) since the last time the threshold was reached. If the model is correct, an (3 < or =M< or =4) earthquake should occur at 37/sup 0/K 17' +- 2' N, 121/sup 0/ 39' +- 2' W within 48 days of January 1, 1977.

Compressive strength and microplasticity in polycrystalline alumina

Abstract: The compressive strength of polycrystalline alumina at 23° C is found to be strain-rate sensitive, but insensitive to environment. Scanning electron microscopy of specimens loaded to near failure indicates the origin of the strength-strain-rate dependence to be localized plasticity in the form of twinning and, possibly, slip. The interaction of these deformation bands with grain boundaries causes the initiation of microcracks. Higher stresses produce still more twin/slip-nucleated microcracks, which finally coalesce at failure. It is suggested that twinning also may be related to the tensile failure of alumina.

Pulsed laser induced deformation in an Fe-3 Wt Pct Si alloy

Abstract: The plastic deformation produced by laser induced stress waves was investigated on an Fe-3 wt pct Si alloy. The intensity and duration of the stress waves were varied by changing the intensity and pulse length of the high energy pulsed laser beam, and also by using different overlays on the surfaces of the specimens. The resulting differences in the distribution and intensity of the deformation caused by the stress waves within the samples were determined by sectioning the specimens and etching (etch pitting) the transverse sections. The magnitude of the laser shock induced deformation depended on the laser beam power density and the type of surface overlay. A combination transparent plus opaque overlay of fused quartz and lead generated the most plastic deformation. For both the quartz and the quartz plus lead overlays, intermediate laser power densities of about 5×108 w/cm2 caused the most deformation. The shock induced deformation became more uniform as the thickness of the material decreased, and uniform shock hardening, corresponding to about 1 pct tensile strain, was observed in the thinnest specimens (0.02 cm thick). 200 ns laser pulses caused some surface melting, which was not observed for 30 ns pulses, the pulse length used in most of the experiments. Deformation of the Fe-3 wt pct Si alloy occurred by both slip and twinning.

Relative motion of grains during superplastic flow in an Al-9Zn-1 wt.%Mg alloy

Abstract: The topological feature of the relative motion of grains during superplastic flow of an Al-9.3Zn-1.03Mg-0.22 wt.%Zr alloy has been examined by the translation of reference markers in the surface and interior of specimens. Microstructural evidence was obtained for grain-switching events based on the relative motion of a group of four grains. The translation of the grains seemed to occur by sliding and rotation. On the basis of the results of a texture study, accommodation strains required in this process are attributed predominantly to slip. Hence, the role played by slip in deformation at maximum strain-rate sensitivity is particularly significant for this alloy.

A three-dimensional viscoelastic model of a strike slip fault

Abstract: An analytic approximation to the Green's function for the displacements due to a strike slip point source in an elastic layer over a viscoelastic half-space is developed. This approximate Green's function is useful because it can be analytically integrated over the fault surface. Comparison with a numerical integration of the exact solution integral indicates that the approximation is quite good. The approximate Green's function is integrated analytically to obtain the displacements due to a finite rectangular strike slip fault in an elastic layer over a viscoelastic half-space. Ground displacements and angle changes from a model survey net are computed to illustrate the viscoelastic relaxation which follows a fracture in the elastic region.

Deformation mechanisms in experimentally deformed plagioclase feldspars

Abstract: Optical and transmission electron microscopy have been used to study the microstructures in a series of plagioclase feldspars which had been experimentally deformed in compression. The observations show that deformation takes place by three mechanisms: (1) brittle fracture, (2) twinning and (3) slip due to the generation and motion of dislocations. Optical “deformation lamellae” are shown to be due to bunches of microfractures and to walls of tangled dislocations. Twins and fractures are often intimately associated and dislocations are often generated at fracture steps or voids. Moving dislocations apparently always generate a strip of fault in the slip plane. This, together with structural considerations as well as the visibility of dislocations (under various diffracting conditions) and the orientation of the applied stress, has made it possible to determine the slip systems which have operated in the deformed specimens.

Pallets made of synthetic resins

Abstract: A synthetic resin pallet is provided with at least one slip preventing member on the upper and or lower surfaces of the deck board. The slip preventing member is made of relatively soft and flexible material, preferably a polyolefin resin.

Microscopic boundary layer effects and rough sphere rotation

Abstract: A calculation of the rough sphere angular velocity correlation function is presented which takes into account the presence of both the microscopic boundary layer and outer hydrodynamic regions around the test particle. The boundary layer region is approximately accounted for by the slip boundary condition while generalized hydrodynamic equations (frequency dependent transport coefficients plus fluid spin variables) are used to describe the outer region. The approximate calculation of the slip coefficient and the use of the slip coefficient to account for processes occurring in the boundary layer are discussed in the context of kinetic theory results. Since a description of both regions is incorporated, the present theory contains both microscopic and collective effects. The structure of the result is compared in some detail to a recent renormalized kinetic theory calculation on the same system and various levels of approximation are examined. The results are in good agreement with the molecular dynamics c...

Mechanism of superplastic deformation in a magnesium alloy. I. Structural changes and operative deformation mechanisms

Abstract: Structural changes and deformation processes acting during superplastic flow of a magnesium alloy (Mg + 1.5% Mn + 0.3% Ce, grain size Do ≈ 10 μm) in different regions of the σ-f(epsi;) curve are studied by a combination of techniques. It is experimentally established that the superplastic deformation of the alloy occurs under the conditions of the activation of diffusion processes and is mainly caused by the action of two mechanisms — grain-boundary sliding and intragranular dislocation slip. Their interrelation and role in alloy deformation is analyzed on the basis of the data obtained on the peculiarities of the different processes. [Russian Text Ignored].