Double Slip Research Articles

Zimbabwe's Presidential Elections 2002: Evidence, Lessons and Implications (review)

Reviewed by: Zimbabwe's Presidential Elections 2002: Evidence, Lessons and Implications David Moore (bio) Henning Melber (ed) (2002) Zimbabwe's Presidential Elections 2002: evidence, lessons and implications. Discussion Paper 14, Uppsala: Nordiska Afrikainstitutet At first glance it seems very impressive to have what is almost a book (an 88 page 'discussion paper') out within weeks of the event with which it is concerned. Zimbabwe's Presidential Elections 2002 is noteworthy on that score and more —even if many of the nasty things one could say about the crooked victory the book marks could have been said (and often were) about the June 2000 parliamentary elections, and even if the structural context of both elections was substantially the same (see Bond and Manyanya 2002, for a view taking structural adjustment and debt as the relevant fundamental constraints, or even causes). Indeed, the book could almost be entitled 'Zimbabwe's Long Election'. The Zimbabwean African National Union-Patriotic Front (not 'popular' front: Ian Taylor makes a double slip with that one) was shocked into more repressive action than usual by its defeat at the February 2000 constitutional referendum. It had been pushed into holding that contest in order to meet a challenge mounted by the National Constitutional Assembly, but that provocation also signalled a contest with the September 11, 1999-born Movement for Democratic Change. From then on ZANU-PF and its ageing but cagey leader used every trick in the book to maintain power in the midst of economic meltdown —and invented a few new ones, learning as the 'long election' moved along through the June 2000 parliamentary election to its final stage in March 2002. Perhaps the most intriguing new wile was the 'land invasions', apparently led by the war veterans association (with lots of help from the army). To get to the root of that phenomenon would entail examining yet another power struggle, going back at least to the southern spring of 1997 when a [End Page 145] group of 'war vets' confronted the president to make demands that would destroy Zimbabwe's fiscus and catalyse the latent powder-keg of land redistribution for many years. When one looks to that moment and its consequences,1 one is forced to ask another 'election' oriented question: will Zimbabwe's next era be ruled by any 'party' in the sense of which orthodox political scientists are familiar, or will it be ruled by 'warlords'? Melber's team does not quite grapple with that question, or the antecedent of just what is the nature of the relationship between the 'veterans' and the ruling party, although there is lots of evidence in the book to force one to ask it, especially if one has been reading Norma Kriger (2001 and forthcoming) or Joanne Macgregor (2001) in the context of writers such as William Reno (1998) and Mark Duffield (2001) on the nature of war-torn Africa (Moore 2002). There is enough evidence in Zimbabwe's Presidential Elections 2002 to contemplate seriously the probability of Zimbabwe slipping down into the vacuum of shadowy states and networked wars. Kenneth Good's 'Dealing with Despotism,' at 23 pages the longest essay in the book, admirably rounds up Zimbabwe's most recent events with a comprehensive historical scan. Reminding us of the Matabeleland massacres in the mid-1980s, and Zimbabwe's involvement in the Democratic Republic of the Congo's war, his words bring in both domestic and international aspects of President Mugabe's penchant for solving problems with force and leaves plenty of room for contemplating 'warlords' gaining accumulative power abroad and at home. Tandeka Nkiwane's observations on the motley crew of election observers and Ian Taylor's prognosis on Zimbabwe's effect on NEPAD's hopes for peer reviewed good democratic governance illustrate how well ZANU-PF's foreign policy makers can manipulate global 'democracy' promoters and their closer peers' alacrity to rally around perceived threats to their 'sovereignty': thus it looks like Zimbabwe's near and far neighbours will leave it to its fate. Amin Kamete persuades us to ask just when the urban 'rebellion' will become more violent: what will happen, for example, if MDC-run Harare demands that ZANU-PF (note...

Mechanical testing device for in situ experiments on reversibility of dislocation motion in silicon

We present a general-purpose mechanical testing device built and tested for in situ experiments using synchrotron radiation. A new setup was used in order to investigate the reversibility of dislocation motion under stress reversal. The observations were done by in situ transmission X-ray topography, at the ID 19 beamline of the ESRF. Specimens of FZ silicon single crystals, with a [ 1 1 ̄ 4 ] axis (double slip orientation), were submitted to load cycles in tension and compression ( τ=10–20 MPa) at high temperature (650–750 °C). Dislocations were created at surfaces by Vickers indents. They were observed using a monochromatic beam ( λ=0.035 nm) and a 220 diffraction vector. The images were recorded both on X-ray films and using the FRELON X-ray camera. The first observations show that dislocations may behave in different ways within the same specimen, and that their motion can be partially reversible.

Cyclic deformation behaviour and dislocation structure of Ti-5 at.% Al single crystals oriented for double prism slip

Abstract Cyclic deformation behaviour of Ti-5at.%Al single crystals oriented for [21 10] was investigated in the total (i.e. elastic + plastic) axial strain amplitude from 0.195% to 0.798% at room temperature in air. The crystals displayed an initial cyclic hardening followed by a striking softening period, and then a saturation stage was reached at different cyclic strain amplitudes, except at the lowest of 0.195% at which the initial hardening stage was absent. The cyclic stress-strain curve in Ti-5 at.% Al single crystal with double prism slip contains a clear plateau with a saturation shear stress of 86–89 MPa over the complete range of applied strain amplitudes. (1010) and (1100) double-prism-slip systems can be distinguished from the traces on the (0001) surface by trace analysis with an optical microscope. Slip marking is planar and becomes denser as the cyclic strain amplitude increases. Two groups of planar dislocation lines lying along the (1010) and (1100) prism plane traces were determined fro...

Cyclic deformation behaviour and dislocation structure of Ti-5 at.% Al single crystals oriented for double prism slip

Cyclic deformation behaviour and dislocation structure of Ti-5 at.% Al single crystals oriented for double prism slip

Cyclic crystal plasticity analyses of stationary, microstructurally small surface cracks in ductile single phase polycrystals

ABSTRACT This paper explores the effects of microstructural heterogeneity on the cyclic crack tip opening and sliding displacements for stationary, microstructurally small transgranular surface cracks in a single phase metallic polycrystal using planar double slip crystal plasticity computations. Crack tip displacements are examined under plane strain conditions for stationary cracks of different lengths relative to grain size as a function of the applied nominal strain amplitude for tension‐compression and cyclic shear. Nominal strain amplitudes range from well below to slightly above the nominal cyclic yield strength for each type of loading condition. Results indicate the complex nature of the crack tip sliding and opening displacements as functions of nominal strain amplitude and orientation of the nearest neighbour grains, the influence of the free surface in promoting the cyclic opening displacement even for cracks in the first surface grain, the rather restricted limits of applicability of linear elastic fracture mechanics, and very interesting crack tip plasticity effects which include crack tip displacement ratcheting or progressive accumulation, even for completely reversed, proportional applied loading. Results are compared for cases with and without crack face friction.

Torsion/Simple Shear of Single Crystal Copper

We analyze simple shear and torsion of single crystal copper by employing experiments, molecular dynamics simulations, and finite element simulations in order to focus on the kinematic responses and the apparent yield strengths at different length scales of the specimens. In order to compare torsion with simple shear, the specimens were designed to be of similar size. To accomplish this, the ratio of the cylinder circumference to the axial gage length in torsion equaled the ratio of the length to height of the simple shear specimens (0.43). With the [110] crystallographic direction parallel to the rotational axis of the specimen, we observed a deformation wave of material that oscillated around the specimen in torsion and through the length of the specimen in simple shear. In torsion, the ratio of the wave amplitude divided by cylinder circumference ranged from 0.02–0.07 for the three different methods of analysis: experiments, molecular dynamics simulations, and finite element simulations. In simple shear, the ratio of the deformation wave amplitude divided by the specimen length and the corresponding values predicted by the molecular dynamics and finite element simulations (simple shear experiments were not performed) ranged from 0.23–0.26. Although each different analysis method gave similar results for each type boundary condition, the simple shear case gave approximately five times more amplitude than torsion. We attributed this observation to the plastic spin behaving differently as the simple shear case constrained the dislocation activity to planar double slip, but the torsion specimen experienced quadruple slip. The finite element simulations showed a clear relation with the plastic spin and the oscillation of the material wave. As for the yield stress in simple shear, a size scale dependence was found regarding two different size atomistic simulations for copper (332 atoms and 23628 atoms). We extrapolated the atomistic yield stresses to the order of a centimeter, and these comparisons were close to experimental data in the literature and the present study.

Crack tip displacements of microstructurally small surface cracks in single phase ductile polycrystals

A planar double slip crystal plasticity model is applied to the evaluation of crack tip opening (CTOD) and sliding (CTSD) displacements for microstructurally small stationary cracks under monotonic loading for a material with nominal stress–strain behavior that is representative of a relatively high strength helicopter rotor hub material. Two-dimensional plane strain finite element calculations are presented for CTSD and CTOD of microstructurally small transgranular surface cracks in a polycrystal subjected to monotonic loading. The effects of crack length relative to grain size, orientation distribution of nearest neighbor grains, stress state and stress level are considered for nominal stress levels below the macroscopic yield strength. The CTOD and CTSD are computed for stationary crystallographic surface cracks with various realizations of crystallographic orientations of surrounding grains. It is found that (i) the opening displacement is dominant for remote tension even for crystallographic cracks oriented along the maximum shear plane in the first surface grain, (ii) there is a strong dependence of the CTOD on the proximity to grain boundaries, but lesser dependence of the CTSD, and (iii) that the elastic solutions for CTOD and CTSD are valid below about 30% of the 0.2% offset-defined yield strength.

Localized deformation and hardening in irradiated metals: Three-dimensional discrete dislocation dynamics simulations

When irradiated, metals undergo significant internal damage accumulation and degradation of mechanical properties. Damage takes the form of a high number density of nanosize defect clusters (stacking-fault tetrahedrons (SFTs) or interstitial loops). The alteration of mechanical properties is manifested in a hardening behavior and localized plastic deformation in defect-free channels. This work uses discrete dislocation dynamics (DD) to capture these effects. It sets the framework for the elastic interaction between gliding dislocations and defect clusters and details a scheme for loop unfaulting and absorption into dislocations. Here, it is shown that SFTs represents weaker pinning points for dislocation motion than parent dislocation loops. It is also shown that appreciable yield drop can be attributed to high density of defects decorating the dislocations. Strong obstacles cause dislocations in Cu to continually double cross slip causing the formation of defect-free channels. Finally, the correlation between yield stress increase and defect number density is in excellent agreement with the experiment.

Orientation dependence of cyclic deformation behavior and dislocation structure in Ti–5at.% Al single crystals

Cyclic deformation behavior and substructure in single crystal Ti–5at.% Al alloy with different orientations fatigued at Δ ε t/2=0.4% were examined. The selected loading orientation includes: single prism slip, A; double prism slips [ 2 1 ̄ 1 ̄ 0 ], B; pyramidal slip, C; and twinning [0001], D. The testing results show that the crystallographic orientation has a strong effect on the cyclic stress response and the plastic deformation mode of Ti–5at.% Al single crystals. The crystals displayed an initial cyclic hardening followed by a striking softening period, and then a saturation stage was reached in specimens A and B. In contrast, an obvious cyclic saturation stage was obtained after first cyclic hardening until to fracture in specimens C and D. Trace analysis on the surface of specimens with an optical microscope shows that the ( 1 1 ̄ 00 ) single prism slip was operated in specimen A during cycling. The ( 10 1 ̄ 0 ) and ( 1 1 ̄ 00 ) double prism slips can be distinguished from the traces on the (0001) surface in specimen B. The ( 1 1 ̄ 01 ) pyramidal slip and the ( 1 1 ̄ 00 ) prism slip were activated simultaneously in specimen C. Twinning is the primary plastic deformation mode in specimen D. The twinning type includes: { 11 2 ̄ 1 }, { 10 1 ̄ 1 }, { 11 2 ̄ 2 } and { 10 1 ̄ 2 }. The substructure in the fatigued specimens was examined using TEM. Typical dislocation configuration is well developed saturation bundle structure (SBS) in specimen A, while it is the planar edge dislocations which are tangled on the primary ( 10 1 ̄ 0 ) plane and arranged parallel to the [0001] direction in specimen B. Fully developed loop patches were formed in specimen C. Typical deformed structure was the twin bundles and dislocations among twins in specimen D. The effects of plastic deformation mode on the cyclic stress response and the corresponding dislocation configuration of Ti–5at.% Al single crystals are then discussed.

Subgrain formation during deformation: Physical origin and consequences

The formation of subgrains in the course of plastic deformation is explained as a result of a trend to make the deformation easier by locally reducing the number of active slip systems. Local preference of one slip system changes the crystal orientation with respect to stress (Schmid factor), thus leading to geometrical softening or hardening. The trend to subgrain formation is treated in the framework of continuum mechanics as an instability against internal bending for the simple case of a crystal originally oriented for symmetric double slip. Once formed, the boundaries of the subgrains lead to hardening as they induce long-range internal back stresses in the interior of the subgrains by forcing the mobile dislocations to take a bowed configuration. Simple dislocation-based and Cosserat models are recalled to explain the size-dependent subgrain hardening, where smaller subgrains are stronger.

A large deformation atomistic study examining crystal orientation effects on the stress–strain relationship

Abstract We performed large deformation molecular dynamics calculations using the embedded atom method in order to examine yield and plastic flow of single crystal nickel. A parametric study using an analysis of variance technique was used to efficiently determine the influence of the deformation path, temperature, size scale, strain rate, and crystal lattice orientation with respect to yield and the plastic energy. The analysis of variance study revealed that orientation had a primary influence on the volume averaged shear stress, demonstrating the continued importance of slip within the crystal lattice for small sized metals and high applied strain rates. Consequently, other orientations were examined with a focus on averaged shear stress, axial stresses, and stress distributions. Results are presented for crystals oriented for single, double, quadruple, and octal slip, as well as a pseudopolycrystal. One last interesting point is that orientation effects on the stress-strain curves could be only approximately normalized by application of the Schmid factor.

411 摩擦力顕微鏡による原子摩擦過程の分子動力学シミュレーション

In order to dynamically clarify the friction mechanism between specimen surface and probe tip in the surface observation by atomic force microscope (AFM) or friction force microscope (FFM), several molecular dynamics simulations have been performed. In the simulation, a 3-dimensional simulation model is proposed where the specimen and the probe are assumed to consist of mono-crystalline copper and rigid diamond or carbon atom, respectively. The effect of cantilever stiffness of AFM/FFM is also taken into considered. The surface observation process on a well-defined Cu {100} is simulated. From the simulation results, it is verified that the surface images and the 2-dimensional atomic-scale stick-slip phenomenon just as observed through the real AFM/FFM surface observation can be detected from spring force acting on the cantilever. From the evaluation, it is clarified that the double slip and the dispersion in the stick-slip period occurs when the stiffness in the scan direction of the cantilever is low.

Planar dislocation bands formed in Ti-5at.% Al single crystals deforming by double prism slips

Planar dislocation bands formed in Ti-5at.% Al single crystals deforming by double prism slips

108 拡散-反応方程式による二重すべり条件下での転位組織の形成過程の解析

108 拡散-反応方程式による二重すべり条件下での転位組織の形成過程の解析

Computational nanoscale plasticity simulations using embedded atom potentials

In determining structure–property relations for plasticity at different size scales, it is desired to bridge concepts from the continuum to the atom. This raises many questions related to volume averaging, appropriate length scales of focus for an analysis, and postulates in continuum mechanics. In a preliminary effort to evaluate bridging size scales and continuum concepts with descritized phenomena, simple shear molecular dynamics simulations using the Embedded Atom Method (EAM) potentials were performed on single crystals. In order to help evaluate the continuum quantities related to the kinematic and thermodynamic force variables, finite element simulations (with different material models) and macroscale experiments were also performed. In this scoping study, various parametric effects on the stress state and kinematics have been quantified. The parameters included the following: crystal orientation (single slip, double slip, quadruple slip, octal slip), temperature (300 and 500 K), applied strain rate (10 6– 10 12 s −1 ), specimen size (10 atoms to 2 μm), specimen aspect ratio size (1:8–8:1), deformation path (compression, tension, simple shear, and torsion), and material (nickel, aluminum, and copper). Although many conclusions can be drawn from this work, which has provided fodder for more studies, several major conclusions can be drawn. • The yield stress is a function of a size scale parameter (volume-per-surface area) that was determined from atomistic simulations coupled with experiments. As the size decreases, the yield stress increases. • Although the thermodynamic force (stress) varies at different size scales, the kinematics of deformation appears to be very similar based on atomistic simulations, finite element simulations, and physical experiments. Atomistic simulations, that inherently include extreme strain rates and size scales, give results that agree with the phenomenological attributes of plasticity observed in macroscale experiments. These include strain rate dependence of the flow stress into a rate independent regime; approximate Schmid type behavior; size scale dependence on the flow stress, and kinematic behavior of large deformation plasticity.

Boundary layers in constrained plastic flow: comparison of nonlocal and discrete dislocation plasticity

Simple shear of a constrained strip is analyzed using discrete dislocation plasticity and strain gradient crystal plasticity theory. Both single slip and symmetric double slip are considered. The loading is such that for a local continuum description of plastic flow the deformation state is one of homogeneous shear. In the discrete dislocation formulation the dislocations are all of edge character and are modeled as line singularities in an elastic material. Dislocation nucleation, the lattice resistance to dislocation motion and dislocation annihilation are incorporated into the formulation through a set of constitutive rules. A complementary solution that enforces the boundary conditions is obtained via the finite element method. The discrete dislocation solutions give rise to boundary layers in the deformation field and in the dislocation distributions. The back-extrapolated flow strength for symmetric double slip increases with decreasing strip thickness, so that a size effect is observed. The strain gradient plasticity theory used here is also found to predict a boundary layer and a size effect. Nonlocal material parameters can be chosen to fit some, but not all, of the features of the discrete dislocation results. Additional physical insight into the slip distribution across the strip is provided by simple models for an array of mode II cracks.

Mechanism of the formation of deformation steps of nanometric sizes at the surface of plastically deformed crystals

The mechanism of the formation of nanometric-size deformation steps at the surface of plastically deformed crystals is discussed theoretically. Such steps are detected by means of a scanning tunneling microscope or by high-resolution speed filming. The analysis shows that the exponential step distribution by height is due to the double cross slip (DCS) of screw dislocations and that the growth kinetics of separate steps is deter-mined by the kinetics of the Frank-Read dislocation sources appearing as a result of the DCS.

On Transient Motion of a Bowed String.

The transient behavior of a bowed string from a Helmholtz motion to a double slip motion was observed using a simple apparatus consisting of nylon lines which served as a bow, and a string. As the bow speed increases, the motion transits from a Helmholtz motion to a double slip motion. Two Helmholtzian waves with different amplitudes and phases occur during the transition from a Helmholtz motion to a perfect double slip motion. This behavior is examined using numerical simulations, which provide good agreement with the experiments.

Non-Basal Slips in Magnesium and Magnesium-Lithium Alloy Single Crystals

Magnesium and magnesium - lithium single crystals were stretched in [1120] direction to investigate the deformation behavior by non-basal slips. In the case of magnesium, { 1122} second order pyramidal slip is active in a range from 77K to 293K and the yield stress decreases with increasing temperature in a range from 77K to 133K, while the stress increases with increasing temperature above 133K. Second order pyramidal slip is also activated in magnesium -3.5 at.%, -7.5 at.% and - 10.6 at.% lithium alloy single crystals, and its yield stress shows anomalous temperature dependence similar to the case of pure magnesium. The deformation mode due to the pyramidal slip is investigated by observation of slip band. Narrow slip bands are formed at lower temperature just after yielding. The width of slip bands at yielding increases continuously with increasing temperature and covers the whole specimen surface at 293K. The results of the observation by TEM showed that double cross slip of (c+a) screw dislocations occur more frequently during deformation at 293K than at 77K. At 293K, there are few (c+a) edge dislocations, while many a and c edge dislocations are observed in the crystals. The yield stress due to the pyramidal slip decreases with addition of lithium.

Dislocation patterning: from micro- to mesoscale description

During the plastic deformation of crystalline material the dislocations, being the carriers of the plastic flow, tend to form different patterns. Because of the long range nature of dislocation-dislocation interaction, the origin of this self-ordering phenomenon is still an open question. The paper presents a stochastic two-dimensional model derived directly from the properties of individual dislocations making it possible to investigate the problem on a mesoscale. Numerical results obtained in double slip configuration indicate the development of cell structure with fractal character.