Showing papers on "Slip (materials science) published in 1975"

Generation of Pseudotachylyte by Ancient Seismic Faulting

Abstract: Summary Pseudotachylyte occurs as vein material infilling highly brittle shear and extensional fractures developed along the western margin of the late Caledonian, Outer Hebrides Thrust zone in NW Scotland. Vein geometries and textures show clearly that the pseudotachylyte has been through a melt phase. From the composition of the pseudotachylyte matrix which is close to that of a basaltic andesite, probable melt temperatures of around 1100°C are inferred. Field and theoretical studies demonstrate that the pseudotachylyte was generated by relatively high stress seismic faulting in crystalline sialic crust devoid of an intergranular fluid, most probably at the time of thrust inception and at a depth of around 4–5 km. A study of pseudotachylyte-bearing ‘single-jerk’ microfaults shows that the slip (d) is related to the thickness of the pseudotachylyte layer (a) by the equation, where d and a are measured in centimetres. Work-energy calculations based on this empirical relationship suggest that the pre-failure shear stress on the microfaults must have been as high as 1.6 kbar to overcome the initial frictional resistance (τf), which decreases with increasing slip during a single movement according to the relationship, which may arise solely from the viscous shear resistance of the melt layer. Delineation of palaeoseismic zones by the recognition of those cataclastic rocks which are necessarily the products of earthquake faulting, may assist in the determination of ancient plate boundaries.

Metallurgical factors affecting fracture toughness of aluminum alloys

Abstract: Crack extension in commercial aluminum alloys proceeds by the “ductile” or fibrous mode. The process involves the large, ~1 μm to ~10μm, Fe-, Si-, and Cu-bearing inclusions which break easily, and the growth of voids at the cracked particles. The linking-up of the voids is accomplished by the rupture of the intervening ligaments, and this is affected by the fine, ~0.01μm precipitate particles that strengthen the matrix. The ~0.1μm Cr-, Mn-, and Zr-rich intermediate particles are more resistant to cracking and may enter the process in the linking-up stage. The fracture toughness of aluminum alloys therefore depends on a) the extent of the heavily strained region ahead of the crack tip, which is a function of the yield strength arad modulus, b) the size of the ligaments which is related tof c, the volume fraction of cracked particles, and c) the work of rupturing the ligaments. An approximate analysis predicts KIc varies asf c-1/6, and this is in agreement with measurements on alloys with comparable yield strength levels. Studies in which the aging conditions are altered for the samef cshow that the toughness decreases with increasing yield strength level. This degradation in toughness is related to the localization of plastic deformation. The tendency for localization is illustrated with the help of “plane strain” tension and bend specimens whose behavior is related to the toughness. Measurements of the strain distribution on the microscale show that slip is relatively uniformly distributed in a 7000-type alloy with low inclusion and particle content when the material is in the as-quenched and overaged conditions. In contrast the distribution is highly nonuniform in the peak aged condition where slip is concentrated in widely spaced superbands involving coarse slip bands with large offsets that crack prematurely. The connection between the tendency for slip localization and the fine precipitate particles which strengthen the matrix remains to be established. In overaged alloys grain boundary ruptures occur within the superbands. The amount of intergranular failure increases with grain size and is accompanied by a loss of fracture toughness.

Strain localization in cyclic deformation of copper single crystals

Abstract: The localization of plastic strain into macroscopic groups of persistent slip bands has been determined on copper single crystals strain-cycled into saturation. The macroscopic bands traverse the whole cross section and undergo complete reversibility of plastic strain with load reversibility. Although overall, surface slip steps form in proportion to the applied plastic strain, individual steps are not always completely reversed, leading to the rapid formation of a notch-peak topography within the bands. The dislocation structure of the persistent slip bands was determined, permitting a critical assessment of the various mechanisms proposed for saturation in low-strain cyclic deformation. Using existing ideas, an adequate model of saturation is based upon the cooperative movement of primary links between the persistent slip-band walls, but impeded in their motion by point-defect clusters and small dislocation debris.

Rotational friction coefficients for ellipsoids and chemical molecules with the slip boundary condition

Abstract: Friction coefficients are calculated numerically for ellipsoids rotating about their principal axes and for a benzene molecule rotating normal to its axis of symmetry under conditions of creeping flow and slip boundary conditions. It has been shown previously, that if a benzene molecule is approximated by an oblate spheroid, the predicted and experimental friction coefficients differ by more than 40%. The present study employs a more realistic shape for benzene and finds a difference of 10%, which is within the limits of the numerical and experimental uncertainty.

The hertz contact problem with finite friction

Abstract: The indentation of an elastic half-space by an axisymmetric punch under a monotonically applied normal force is formulated as a mixed boundary value problem under the assumption of Coulomb friction with coefficient μ in the region of contact. Within an inner circle the contact is adhesive, while in the surrounding annulus the surface moves inwards with increasing load. The slip boundary between the two regions depends on μ and the Poisson ratio v, and is found uniquely as an eigenvalue of a certain integral equation.

Strain accumulation and release mechanism of the 1906 San Francisco Earthquake

Abstract: Reexamination of geodetic data has shown that significant crustal deformation both preceded and followed the great 1906 earthquake (M = 8.3). In the ∼50 years prior to 1906, tensor shear strain rates averaged over an ∼100-km-wide region spanning the San Andreas system ranged from 0.4 to 0.9×10−6 yr−1, rates being higher east of the San Andreas fault than west of it. Part of this accumulated strain was released abruptly by seismic slip on the fault in 1906, but aseismic strain release continued at a rate of ∼1.2×10−6 yr−1 near the fault for about 30 years following the earthquake. Since that time the rate has been near 0.3×10−6 yr−1 both close to the fault and in a broad region surrounding it. The mechanism suggested to account for these observations is the progressive failure of most of a 450-km-long segment of the San Andreas plate boundary during an ∼80-year interval. The seismic slip, averaging near 4 m, is relatively shallow, being constrained by the data to depths no greater than about 10 km. The seismic moment determined from long-period surface wave amplitudes is 4.0×1027 dyn cm and agrees well with the geodetic estimates. Continued aseismic slip of 3–4 m below the seismic zone to depths of ∼30 km explains all of the postseismic data well. The small number of preearthquake observations are consistent with rapid slip on the fault at greater depths. Finally, the observation of a high rate of strain accumulation prior to 1906 casts considerable doubt on the often quoted 100-year periodicity of great earthquakes on the San Andreas fault, which appears to have originated with H. F. Reid's inferences based on the pre-1906 deformation.

Basal slip and twinning in α-titanium single crystals

Abstract: Single crystals of α-titanium, with small Schmid factors for prismatic slip, have been deformed in tension between 78 and 1120 K. At low temperatures, {1012} twinning has been observed in specimens having the angle between the basal plane and the tensile axis,x B , close to 90 deg, whereas at intermediate orientations withx B = 60 deg and 47 deg twinning occurs on the {1121} planes. A critical resolved shear stress law is not obeyed for either twinning mode. First order prismatic slip in the microstrain region appears to be responsible for the nucleation of {1121 twins. Slip is unlikely a pre-requisite for {1012} twinning. Basal slip without interference from twinning is observed in a variety of orientations at temperatures above 500 K. Plastic flow above 900 K may be described by an equation of the form:γ=Aτ n e-Q/kT The relative ease of basal and prismatic slip in Ti and Zr is discussed in terms of the hcp ⇆ bcc allotropic transformation.

Production of gaseous pore pressure during rock slides

Abstract: When a rock mass slips, the local heating of the slip surface transforms pore water into vapour if the surface of failure is deep enough. It is possible to calculate, as a first approximation, a critical displacement of the mass necessary to create vapour in the slide zone. A second approximation gives the relation between critical displacement and rate of shear displacement as the slide mass creeps towards catastrophe.

Non-parallel dissociation of dislocations in thin foils

Abstract: The equilibrium shape of dissociated dislocations in thin foils of Cu-10 at.% Al is investigated by weak-beam electron microscopy. It is found that dislocations assume a truncated triangular shape caused principally by the reduction in line energy when Shockley partials rotate towards screw character. The effects of interaction image stresses and surface slip steps are generally small, but the former may be important in the case of spontaneous cross-slip of a dislocation at a surface.

Prismatic slip in α-titanium single crystals

Abstract: Single crystals of Ti, oriented favorably for prismatic slip and containing two levels of interstitial impurities, have been deformed in tension at temperatures between 78 to 1120 K. The stress-strain curves exhibit three stages of hardening similar to those of Zr and of fcc crystals. The work hardening rate in Stage II, θII/G is lower in Ti than in Zr. From the strain rate dependence of the stress at the onset of Stage III, the stacking fault energy on the prism planes of Ti has been estimated as 0.145 Joules/m2. The relative values of stacking fault energy for Ti and Zr are consistent with a dissociation model which is based on the hcp ai bcc transformation. The thermally activated prismatic slip below 250 K is controlled by the interaction of dislocations with interstitial solute atoms. Above 250 K the dislocation mechanism for plastic flow is not clearly understood.

Some numerical results for the BGK model: Thermal creep and viscous slip problems with arbitrary accomodation at the surface

Abstract: Numerically ’’exact’’ results for the thermal creep and viscous slip problems for the BGK model and Maxwellian diffuse specular reflection at the wall are obtained by using equations derived earlier by Cercignani, Loyalka, and Cipolla. Results obtained by the use of both the half range and the ’’full‐range’’ expansions are found to be in complete agreement and provide standards of comparison for various approximate methods.

The influence of temperature and cyclic frequency on the fatigue fracture of cube oriented nickel-base superalloy single crystals

Abstract: Carbon-free single crystals of Mar-M200 were tested in pulsating tension, stress-controlled fatigue at temperatures and frequencies ranging from 1033 to 1255°K and 0.033 to 1058 Hz, respectively. The axis of loading was parallel to [001], the natural growth direction for directionally-solidified nickel-base alloys. Except for the lowest frequency at the higher temperatures where creep damage was extensive, crack initiation occurred at subsurface microporosity. Cracks initiated and propagated in the Stage I mode (crystallographic cracking on the {111} slip planes) at the lower temperatures and higher frequencies, whereas Stage (perpendicular to the principal stress axis) crack initiation and propagation was found at the higher temperatures and lower frequencies. Often a transition from Stage II to Stage I crack propagation was observed. It was established that Stage I cracking occurred under conditions of heterogeneous, planar slip and Stage II cracking under conditions of homogeneous, wavy slip. A thermally activated recovery process with an activation energy of 368 KJ/mole (88 Kcal/mole) determined the instantaneous slip character,i.e., wavy or planar, at the crack tip. In addition, it was found that an optimum frequency existed for maximizing fatigue life. At frequencies below the optimum, creep damage was detrimental, while at frequencies greater than the optimum, intense, planar slip was detrimental. The optimum frequency increased with increasing temperature.

Prismatic slip in zirconium single crystals at elevated temperatures

Abstract: Single crystals of Zr oriented favorably for prismatic slip have been deformed in tension over a range of strain rates at temperatures between 473 and 1113 K. A temperature independent plateau is observed between 600 and 800 K and dynamic strain aging occurs in the vicinity of 723 K. The flow stress is temperature dependent both above and below this temperature interval. Plastic flow above 850°K is represented by an equation of the form: {ie1217-05} where {ie1217-06} is the shear strain rate,A is a constant whose value is 680 ± 20 (MN/m2)−4.3. The stress exponentn = 4.3 ± 0.3 and the activation energyQ = 2.05 ± 0.15 eV. It is proposed that the high temperature prismatic slip in Zr is controlled by a glide-climb process where the rate of plastic flow is determined by the rate of climb of dislocations.

Preferred Orientation in Quartz Ribbon Mylonites

Abstract: Quartz mylonites composed of elongate ribbon quartz without appreciable recrystallization at grain boundaries were examined and contrasted One was from a lower greenschist facies environment at Mount Isa, Australia, in which the c -axis preferred orientation of the ribbons is either a pronounced orthorhombic distribution or a small-circle distribution (with a small opening angle) about the normal to the foliation and lineation The other was an upper greenschist or lower amphibolite facies mylonite from Risfjallet in the Swedish Caledonides, in which the c -axis preferred orientation of the ribbon is a maximum lying close to the foliation and normal to the lineation Variation in preferred orientation can be accounted for by temperature and (or) strain-rate differences, with basal-slip mechanisms predominant at lower temperatures and prismatic slip (and possibly other slip systems), together with diffusion-controlled processes, predominant at higher temperatures

From antimoment to moment: Plane-strain models of earthquakes that stop

Abstract: Some simple cases of the seismic gap model are examined in plane strain. Numerical solutions are found for near-field elastodynamic motion coupled to frictional sliding on a fault. Rupture velocity is specified, but the dynamic solution is otherwise dependent only on the initial state. The initial seismic gap is a non-uniformity of past slip on the fault plane. It is termed an antidislocation and is characterized by its antimoment and its energy. Sliding stops naturally in this model, even with uniform material properties. The moment of the calculated earthquake is dependent on the difference between the kinetic friction level and the background shear-stress level and is independent of rupture velocity. If initial states are scaled to have equal antimoments, and if the stress scale is normalized according to the initial energy, then the earthquake moment is nearly independent of the shape of the initial antidislocation. The ratio of radiation reaction stress (apparent stress) to weighted mean stress drop is 0.36±0.04 for all cases calculated. Non-dependence of earthquake moment on rupture velocity and the shape of the initial antidislocation is expected to carry over to three-dimensional cases. The final fault dislocation is smoother than the initial dislocation in every case calculated. Nonuniform friction or a nonplanar fault is required to get a dislocation rough enough to produce succeeding earthquakes.

Temperature and Orientation Dependence of Plastic Deformation in GaAs Single Crystals Doped with Si, Cr, or Zn

Abstract: Stress-strain curves are presented for Si-doped GaAs single crystals deformed in compression parallel to and at temperatures ranging from 250° to 550°C. In both orientations, slip occurs only on {111} . Repeated-yielding experiments on GaAs crystals, both undoped and doped with Si, Cr, or Zn, indicate that Si-doped crystals (n-type) have a greater yield stress than undoped crystals, that undoped crystals have a greater yield stress than Zn-doped crystals p-type), and that Cr doping does not significantly affect the yield stress. Extensive crack formation resulting from deformation was not observed. Compression specimens normally failed by breaking into several columnar fragments with their long dimension parallel to the stress axis.

Dislocation structures in deformed single-crystal Ni3(Al, Ti)

Abstract: A transmission electron microscopy investigation of deformed Ni3(Al, Ti) single crystals has shown that, over the temperature range − 107°C to 917°C, there exist five distinct regions, each having a characteristic dislocation structure. No primary cube slip was observed at temperatures below approximately 450°C, the temperature of the peak proof stress. This observation makes untenable any theory which attributes the anomalous, positive dependence of the proof stress to an interaction between primary cube and octahedral slip.

Texture strengthening and strength differential in titanium-6Al-4V

Abstract: The alloy titanium-6 Al-4 V has been processed in the laboratory to develop a wide range of textures, and determinations have been made of the yield loci for these conditions. Optical and electron metallography have been used to gain information about the operative deformation modes under various states of stress. Only limited texture strengthening is observed for conditions of tensile loading, although quite significant effects are found in compression. In strongly textured material a large strength differential occurs along thec axis direction, but this cannot be attributed to twinning. It is shown that slip can play a major role in plastic deformation, and that the observed mechanical properties result from the peculiar characteristics of the dislocations.