Homogeneous Slip Research Articles

Wall slip and hydrodynamics of two-dimensional journal bearing

In the present paper, based on the limiting shear stress model, a multi-linearity finite element algorithm and quadratic programming technique are used to study the influence of wall slip on the hydrodynamic lubrication performance of a two-dimensional journal bearing (finite length journal bearing). It is found that if the lubricated surfaces are designed as homogeneous slip surfaces, the hydrodynamic force will be decreased. If the shaft surface (rotation) is a slippery surface with very low limiting shear stress, almost no fluid load support can be generated. If the sleeve surface is designed as the homogeneous slip surface, a low fluid load support together with a small friction drag can be obtained. However, if the sleeve surface is designed as an optimized slip surface with a slip zone in the inlet region, a high load support and low friction coefficient can be obtained. Optimization of the shape and the size of the slip zone can give the journal bearing many advanced properties.

Deformation banding and shear banding in single crystals

A theory of rigid-plastic single-crystal deformation based on the hypothesis that the crystal follows a mode of deformation that minimizes the plastic power is developed. In addition to the usual homogeneous slip modes, plastic power minimization in the present theory also extends over inhomogeneous deformation modes in the form of deformation bands or shear bands. Bands are assumed to originate from unstable perturbations of the lattice orientation. The evolution of the banded substructure with continued deformation depends on the mobility of the interband dislocation boundaries. The theory predicts the evolution of substructural morphology and lattice orientation with deformation. These predictions agree quantitatively with experimental observations in initially ( 1 1 2 ) / [ 1 1 1 ¯ ] - and (0 0 1)/[1 1 0]-oriented copper single crystals that undergo shear banding and deformation banding, respectively, when subjected to plane strain deformation.

Effect of Periodic Overloads on Fatigue Crack Propagation in Aluminium Alloys

The influence of variable amplitude loading on fatigue crack propagation was investigated for two high purity versions of the alloy Al 2024 in sheet form, one with fine equiaxed grains, and the other with coarse elongated grains. Fatigue tests on center cracked specimens were conducted in vacuum at constant amplitude (R-ratio of 0.1) and with periodically applied single tensile overloads with an overload ratio of 1.5. The number of intermittent baseline cycles between consecutive overloads was varied (n=100 and n=10.000). Detailed fractographic investigations were carried out for the identification of changes in the fracture surfaces due to the overloads. Crack closure measurements were performed in all cases. The results revealed a strong influence of the overloads on the crack propagation rate. Whether overloads are retarding or accelerating the fatigue crack propagation depends on the crack propagation mechanism at constant amplitude loading and the number of intermittent baseline cycles. For n=100 retardation occurred for the fine grained alloy exhibiting homogeneous slip at constant amplitude while acceleration was observed for the alloy with coarse elongated grains showing pronounced slip band fracture at constant amplitude. For n=10.000, the formation of steps parallel to the direction of crack propagation by overloads is assumed to be the reason for the observed increase in fatigue crack propagation resistance resulting in retardation for both alloys compared to constant amplitude and n=100. The influence of crack closure on the overload effects was minor. This was verified by additional tests at R=0.5.

Rheological Behavior and Slip Casting of Al2O3–Ni Aqueous Suspensions

A strong effort has been devoted recently toward processing of metal–ceramic composites with tailored microstructure by colloidal methods. The aim of this work is to optimize the rheological behavior of concentrated Al2O3–nickel (Ni) aqueous suspensions and further slip casting in order to obtain dense green composites. Compositions with Ni relative contents ranging from 5 to 75 vol% were prepared from suspensions with high solids loadings (50 vol%) by adjusting the colloidal stability of each component in terms of pH, mobility, dissolution conditions, and influence of polyelectrolytes. The rheological properties were measured under controlled rate and controlled stress conditions at different basic pH conditions and contents of polyelectrolyte. Better rheological conditions of the mixtures were found for pH 10 and 1.0 wt% polyelectrolyte. Minimum viscosity was obtained for suspensions containing 15 vol% of Ni. The analysis of flow curves demonstrates that the suspensions form a structure at very low shear, hindering sedimentation. Homogeneous slip cast bodies with green densities up to 70% of theoretical and up to 75 vol% Ni were sintered in Ar to achieve dense biphasic composites.

Influence of measurement errors on stress estimated from single-phase fault/slip data

The influence of measurement errors on estimated stress is investigated in reduced sigma space on the basis of numerous sets of homogeneous fault/slip data generated under different extensional stress and then modified by varying measurement errors for the striation pitch. For a given measurement error, error propagation to a datum vector depends upon the controlling stress and fault orientation with respect to the controlling stress. For a given stress ratio, both the dispersion of datum vectors (i.e. root mean square cosine) and the misfit between estimated and controlling stresses (i.e. similarity and stress difference) increase with the increase in measurement error. The two measures appear generally smaller at small measurement error than at large error and have a strong tendency to increase with stress ratio for a given measurement error. The measurement error has an apparent effect in estimated stress only if it is greater than 10°, implying a limited effect on homogeneous fault slip data that is carefully measured at outcrop, in which a smaller error can be guaranteed in a majority of cases.

Environmental effects on fatigue failure micromechanisms in titanium

Fatigue behavior of titanium in inert environments has been studied from the microstructural viewpoints on thin-plate specimens subjected to alternating plane bending in vacuum (1.3 × 10−3 Pa) and in argon gas (purity 99.999%). Microstructural examinations were carried out in the process zone wake of a fatigue crack. Fatigue properties of titanium were better in vacuum than in argon gas and fatigue damage was distinguished between the testing atmospheres: in vacuum, homogeneous slip bands were uniformly formed and cracks propagated in transgranular mode, but in argon gas, twin systems as well as slip ones were activated and cracks propagated partly in intergranular mode. It was demonstrated that slip-bands cracking occurred at the junction of the intersecting slip bands developed on the {101¯0} planes during cyclic stressing in vacuum, and grain boundaries were subject to selective damage in argon gas.

Environmental effects on fatigue failure micromechanisms in titanium

Abstract Fatigue behavior of titanium in inert environments has been studied from the microstructural viewpoints on thin-plate specimens subjected to alternating plane bending in vacuum (1.3 × 10 −3 Pa) and in argon gas (purity 99.999%). Microstructural examinations were carried out in the process zone wake of a fatigue crack. Fatigue properties of titanium were better in vacuum than in argon gas and fatigue damage was distinguished between the testing atmospheres: in vacuum, homogeneous slip bands were uniformly formed and cracks propagated in transgranular mode, but in argon gas, twin systems as well as slip ones were activated and cracks propagated partly in intergranular mode. It was demonstrated that slip-bands cracking occurred at the junction of the intersecting slip bands developed on the { 1 0 1 ¯ 0 } planes during cyclic stressing in vacuum, and grain boundaries were subject to selective damage in argon gas.

Earthquakes as three stage processes

Relying on basic physics, laboratory and field evidence, and accounting for the different timescales involved, we derive an approximate energy function for earthquakes. The fracture term is disregarded because virtually all earthquakes occur on pre‐existing faults and because fault gouge has a very low fracture energy. Disregarding also the gravitational term, the importance of which depends on the type of seismic focal mechanism, yields that the energy function has only thermal and radiative terms. The self‐similarity ranges in the bulk rocks and gouge suggest taking the basic element as two cubes of 10 m side, with in common one face, over which slip occurs. An earthquake is a cascade of such slip processes on a series of neighbouring elements and the volume involved can be self‐similarly defined as the two megacubes embedding the set of elements participating in the slip. Dimensional analysis suggests that the slip process is composed of three stages. The first stage consists of a stick‐slip episode with an average velocity v∼ 10−1 m s−1 over a time t∼ 10−2 s. In this first stage, stage I, virtually all energy is converted into heat, with a temperature increase of the order of 102 K on the sliding surface. The second stage, stage II, in which the occurrence of further slip episodes is hardly relevant, consists of the propagation of the thermal wave generated in stage I to the whole shear zone, with a characteristic time of the order of 102 s. In light of the comparatively low permeability of fault gouge with respect to heat diffusivity, this temperature increase induces a pore fluid pressure increase. When the transition from hydrostatic to lithostatic pressure is completed over the whole shear zone, a third stage, stage III, is entered, in which, provided that the pressurization is maintained, virtually frictionless high‐velocity slip occurs, converting all the available energy into elastic radiation. The duration of this purely radiative stage, the amount of slip and the size of the earthquake depend on the number of elements cooperatively participating in the cascade slip, which is ruled, just as in the usual single‐stage cellular automata models, by the correlation length over which the strain level is near the rupture threshold. At odds with the classical single‐stage cellular automata, the model does not require the introduction of strong jumps in stress to be ignited and appears thus also capable of explaining the Coulomb failure stress quandary of very small triggering stresses, with the ignition of large events requiring excess stresses of just 10−2 to 10−3 MPa. The global seismic efficiency, under the assumption that granular effects and viscous resistance are disregarded, is ∼ 1. Assuming then statistical self‐similarity on the fault plane for the patches that slip cooperatively and approximating their pattern as a Sierpinski carpet, yields partial and approximately constant stress drop values independent of event size. These emerge from the fractal nature of the slip surface interpreted according to the seismological assumption of constant homogeneous slip on the Euclidean rectangular plane fault, which embeds the slipping patches.

Damage Mechanisms in Nanolayered Metallic Composites

The strengths of metallic multilayers, composed of alternating layers of soft metals such as Cu and Nb, approach the theoretical limit of material strength when the bilayer periods are on the order of a few nanometers. We have investigated the damage mechanisms in these ultra-high strength nanolayered composite materials subjected to monotonic deformation. Large strain plastic deformation such as room temperature rolling does not lead to any dislocation cell structure formation within the layers indicating that the deformation and dislocation storage mechanisms in nanostructured materials are completely different from the bulk. In bulk metals, dislocation pile-ups lead to heterogeneous slip, but in nano-materials, deformation by single dislocations on closely spaced glide planes results in more homogeneously distributed slip. The implications of the high tensile strengths and homogeneous slip on the fatigue properties of nanolayered materials are also discussed.

On the factors affecting the propagation of long fatigue cracks in thin-sheet wrought aluminium alloys

The paper deals with the propagation of long fatigue cracks in commercial heat-treatable thin-sheet wrought aluminium alloys with focus on the mid-region of the cyclic stress intensity factor. The experimental observations support the distinction of two groups of alloy conditions according to the degree of coherency of the strength-controlling precipitates. A model explaining the observations for the first group, which is characterized by predominantly incoherent precipitates, is proposed. These precipitates cannot be cut by dislocations and both a homogeneous slip distribution and a continuum-mechanics dominated crack path are favoured. The reasons why the alloys with a high degree of coherency of the strength-controlling precipitates deviate from the fatigue crack growth behaviour common of the representatives of the first group are analysed.

Near threshold fatigue crack growth at positive load ratio in aluminium alloys at low and ultrasonic frequency: influences of strain rate, slip behaviour and air humidity

Fatigue crack growth near threshold has been studied in 2024-T3 and in over-aged 7075 (7075-OA) using servo-hydraulic equipment (cycling frequency 20 Hz) and ultrasonic equipment (20 kHz). Experiments have been performed at load ratio R=0.05 and R=0.5 in ambient air and in vacuum. Below 10 −9 m/cycle, fatigue crack growth in vacuum is not influenced by the cycling frequency and no strain rate effects are found. Air humidity accelerates near threshold fatigue crack growth, and threshold stress intensities at 10 −10 m/cycle in ambient air are approximately 50–60% of those measured in vacuum. Similar threshold stress intensities at 20 Hz and 20 kHz, however, higher crack growth rates at the lower frequency at stress intensities above threshold were found in ambient air, where the frequency influence was most pronounced for fully reversed loading. Near threshold 2024-T3 shows planar slip behaviour and crystallographic (stage I-like) fatigue crack growth in vacuum, whereas stage II fatigue crack growth is found in ambient air. 7075-OA shows homogeneous slip properties and stage II fatigue crack growth in both environments. No apparent influences of load ratio and cycling frequency on fracture surfaces produced at very low growth rates are visible.

Near threshold fatigue crack growth in aluminium alloys at low and ultrasonic frequency: Influences of specimen thickness, strain rate, slip behaviour and air humidity

Near threshold fatigue crack growth in ambient air and in vacuum has been studied with ultrasonic fatigue testing equipment (cycling frequency 20 kHz) and with servo-hydraulic equipment (20 Hz). Aluminium alloy 2024-T3 and 7075, heat treated to promote planar slip (7075-UA) and homogeneous slip (7075-OA), respectively, have been tested at load ratio R=-1. In vacuum, no strain rate influences were found and similar crack growth rates were observed at both frequencies. Specimen thickness does not influence the threshold stress intensity amplitude ( K max,th) determined at 3.5×10 –13 m/cycle in vacuum whereas crack growth resistance increases with decreasing specimen thickness at higher growth rates. K max,th determined at 10 -10 m/cycle in ambient air are similar at 20 Hz and 20 kHz and are 45–55% of those measured in vacuum. In ambient air and at higher stress intensities, fatigue cracks may propagate by a factor of 5–50 faster at the lower frequency. In vacuum, 7075-UA shows better crack growth properties than 7075-OA, more pronounced at higher growth rates. In ambient air, the planar slip materials show a plateau like regime in the (Δa/ΔN vs K max)-curves, where growth rates are hardly affected by the stress intensity.

The Weak-Motion Modeling of the Skyros Island, Aegean Sea, M w = 6.5 Earthquake of July 26, 2001

The earthquake was modeled using regional broad-band stations in Greece (epicentral distances up to 340 km). Inversion of the amplitude spectra of complete waveforms (0.05–0.08 Hz), later confirmed by the forward waveform modeling, provided strike = 150°, dip = 70°, rake = 10°, scalar moment Mo = 4.1e18 Nm, and depth of 8 km. As the aftershock distribution had the same strike, the earthquake was interpreted as a left-lateral strike slip. The fault length was estimated by combining observed mainshock spectra and synthetic spectra of a weak event, representing impulse response of the medium. This gave the fault length estimate of 16 to 24 km. Similar results were obtained by means of a true Mw = 5 aftershock. The waveform modeling (0.05–0.20 Hz) was performed for the 20 × 10 km finite-extent fault, with a homogenous slip of 0.63 m. It showed that the rupture propagation along the 150° strike was predominantly unilateral, from NW to SE.

A microstructural model for the monotonic and the cyclic mechanical behavior of single crystals of superalloys at high temperatures

Abstract A constitutive model for the mechanical behavior at high temperatures of superalloys with a high volume fraction of the γ ′ phase is derived from an analysis of the main deformation mechanisms. The model assumes periodically distributed cubic γ ′ particles and an homogeneous slip distribution perpendicular to the slip planes inside the matrix channels and the particles. It accounts for octahedral and cubic slip in the γ channels, recovery by climb of loops around the precipitates until complete annihilation, shear of both matrix and precipitates by complete matrix dislocations and partial slip reversal during unloading. The back-stresses of the constitutive law are identified with the long-range internal stresses in the microstructure which are explicitly calculated by Fourier series for a simplified distribution of the plastic strains in the microstructure. In particular the three types of channels are distinguished. The internal variables of the model are the plastic strains produced by each mechanism, the number of mobile loops in the γ channels and the scalar dislocation density in the γ/γ ′ interfaces for each slip system.

Coseismic displacements and creeping along the Pernicana fault (Etna, Italy) in the last 17 years: a detailed study of a tectonic structure on a volcano

The Pernicana fault is a tectonic fault located on Mt Etna (Sicily). Movements across this structure have been monitored, over the last 17 years, by a levelling network operated by the Osservatorio Vesuviano. During the monitoring period, the network has recorded coseismic displacements associated with earthquakes of magnitude 4–5, as well as aseismic displacements. We present a detailed analysis of the displacement data, aimed at modelling the geometry and mechanism of this fault, as well as the size and mechanisms of individual earthquakes. We use, for interpreting the displacement data, two types of inverse methods. The first is able to determine, from ground deformation data, the geometry, mechanism and location of a system rectangular faults with homogeneous dislocation; the second finds, for a fixed fault system geometry, the heterogeneous slip distribution giving the best fit to the data. The total recorded displacements during the last 17 years are well described by a single, homogeneous rectangular normal fault, with dislocation and size corresponding to a magnitude 5.7 earthquake, thus revealing intense dynamics along such a fault. Coseismic displacements for individual earthquakes of considerable magnitude account for about 30% of the total deformation. When interpreted in terms of an homogenous slip fault as inferred from total deformation, some aspects of the data are not well explained, thus calling for heterogeneous slip models. It is hence possible, with a certain approximation, to distinguish the slip distributions associated with individual earthquakes from the almost homogeneous slip resulting from the sum of coseismic and aseismic movements. The inferred seismic moments for the two larger earthquakes that occurred along this fault are considerable higher than reported in seismic bulletins (about 1 degree of magnitude). The results obtained here support the important role of levelling data for the detailed study of a peculiar kind of tectonic structure on a volcano, aimed at understanding the complex interaction between volcano dynamics and induced movements along well defined structures.

Atomistic Simulations of Steps in Bimetallic Interfaces as Barriers to Interface Slip Transmission

ABSTRACTAtomistic models of coherent interfaces in the CuNi system with and without (111)-steps were used to study slip transmission across interfaces in CuNi metallic bilayers. The lattice mismatch of the CuNi system results in large coherency stresses at the interface. The (111)-steps afford a larger barrier to slip than the flat, coherent interface. The coherent flat interface dislocation barrier is largely due to the large compressive stresses in the Cu layer that must be overcome by applied tensile stresses. Additional Koehler forces are present as the dislocation in the elastically softer Cu approaches the stiffer Ni layer. The step, however, possesses a small residual edge dislocation with a Burgers vector equal to the difference of bCu and bNi times the height of the (111)-step in (111)-layers. We find that these steps are potent slip barriers, which suggests that homogeneous slip is preferred in such systems.

Investigation of film boiling thermal hydraulics under FCI conditions: results of analyses and a numerical study

Film boiling on the surface of a high-temperature melt jet or of a melt particle is one of key phenomena governing the physics of fuel–coolant interactions (FCIs) which may occur during the course of a severe accident in a light water reactor (LWR). A number of experimental and analytical studies have been performed, in the past, to address film boiling heat transfer and the accompanying hydrodynamic aspects. Most of the experiments have, however, been performed for temperature and heat flux conditions, which are significantly lower than the prototypic conditions. For ex-vessel FCIs, liquid subcooling is big and can significantly affect the FCI thermal hydraulics. Presently, there are large uncertainties in predicting natural convection film boiling of subcooled liquids on high-temperature surfaces. In this paper, research conducted at the Division of Nuclear Power Safety, Royal Institute of Technology (RIT/NPS), Stockholm, concerning film boiling thermal hydraulics under FCI condition is presented. Notably, the focuses are placed on the effect of water subcooling, high-temperature steam properties, the radiation heat transfer and mixing zone boiling dynamics on the vapor film characteristics. Numerical investigation is performed using a novel CFD modelling concept named as the local homogeneous slip model (LHSM). Results of the analytical and numerical studies are discussed with regards to boiling dynamics under FCI conditions.

Influence of grain size and stacking-fault energy on deformation twinning in fcc metals

This article investigates the microstructural variables influencing the stress required to produce deformation twins in polycrystalline fcc metals. Classical studies on fcc single crystals have concluded that the deformation-twinning stress has a parabolic dependence on the stacking-fault energy (SFE) of the metal. In this article, new data are presented, indicating that the SFE has only an indirect effect on the twinning stress. The results show that the dislocation density and the homogeneous slip length are the most relevant microstructural variables that directly influence the twinning stress in the polycrystal. A new criterion for the initiation of deformation twinning in polycrystalline fcc metals at low homologous temperatures has been proposed as (σ tw −σ 0)/G=C(d/b)A, where σ tw is the deformation twinning stress, σ 0 is the initial yield strength, G is the shear modulus, d is the average homogeneous slip length, b is the magnitude of the Burger’s vector, and C and A are constants determined to have values of 0.0004 and −0.89, respectively. The role of the SFE was observed to be critical in building the necessary dislocation density while maintaining relatively large homogeneous slip lengths.

Mechanical behavior of Al−Li−SiC composites: Part I. Microstructure and tensile deformation

The microstructure and tensile properties of an 8090 Al−Li alloy reinforced with 15 vol pet SiC particles were investigated, together with those of the unreinforced alloy processed following the same route. Two different heat treatments (naturally aged at ambient temperature and artificially aged at elevated temperature to the peak strength) were chosen because they lead to very different behaviors. Special emphasis was given to the analysis of the differences and similarities in the microstructure and in the deformation and failure mechanisms between the composite and the unreinforced alloy. It was found that the dispersion of the SiC particles restrained the formation of elongated grains during extrusion and inhibited the precipitation of Al3Li at ambient temperature. The deformation processes in the peak-aged materials were controlled by the S′ precipitates, which acted as barriers for dislocation motion and homogenized the slip. Homogeneous slip was also observed in the naturally aged composite, but not in the unreinforced alloy, where plastic deformation was concentrated in slip bands. The most notorious differences between the alloy and the composite were found in the fracture mechanisms. The naturally aged unreinforced alloy failed by transgranular shear, while the failure of the peak-aged alloy was induced by grain-boundary fracture. The fracture of the composite in both tempers was, however, precipitated by the progressive fracture of the SiC reinforcements during deformation, which led to the early failure at the onset of plastic instability.

Plastic deformation of polypropylene in relation to crystalline structure

The tensile drawing behavior of quenched and annealed films of isotactic polypropylene is investigated as a function of draw temperature and strain rate. A strain-induced structural change from the smectic to the monoclinic form is observed for the quenched films. A kinetic interpretation is proposed for the phenomenon. Data of thermal activation volume at the yield point indicate two regimes of plastic flow for the quenched sample, between 25 and 60°C, but only one regime for the annealed sample. Homogeneous and heterogeneous crystal slip processes are proposed to account for these regimes in relation to the nucleation and propagation of screw dislocations. The basic mechanism of molecular motion in the polypropylene crystal is suggested to be a wormlike motion of conformational defects along the 3/1 helix chains that allows a 120° rotation and a c/3 translation. The occurrence of the smectic form as a transitory state in the deformation pathway is discussed in terms of plasticity defect generation. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1873–1885, 1999