Dislocation model of localized plastic deformation initiated with a flat punch

We consider a classical contact mechanics problem, namely, the indentation of a ductile half-plane by a rigid flat punch (in plane strain), and revisit it using the dislocation mechanics approach. The dislocation nucleation and dislocation interaction beneath the indenter are examined. The threshold load for dislocation nucleation and the dislocation emission angle are obtained in analytical form. Moreover, based on the consideration of dislocation interaction, we explore the mechanism of contact load evolution (hardening). A triangular “dead zone” beneath the indenter, which could not be thus far accurately explained by traditional continuum models, is predicted in good agreement with the results of careful experiments that are reported in the literature. The proposed model is likely to be useful for the analysis of contacts at both the micro- and macro-scales.     

An indentation technique for estimating the energy density as fracture toughness with Berkovich indenter for ductile bulk materials

A technique is proposed to estimate the energy density as fracture toughness for ductile bulk materials with an indentation system equipped with a Berkovich indenter based on the theory of plastic deformation energy transforming into the indentation energy of fracture. With progressive increase of penetration loads, the material damage is exhibited on the effective elastic modulus. A quadratic polynomial relationship between the plastic penetration depth and penetration load, and an approximate linear relationship between logarithmic plastic penetration depth and logarithmic effective elastic modulus are exhibited by indentation investigation with Berkovich indenter. The parameter of damage variable is proposed to determine the critical effective elastic modulus at the fracture point. And the strain energy density factor is calculated according to the equations of penetration load, plastic penetration depth and effective elastic modulus. The fracture toughness of aluminum alloy and stainless steel are evaluated by both indentation tests and K_IC fracture toughness tests. The predicted S_cr values of indentation tests are in good agreement with experimental results of CT tests.     

压头不同晶向下纳米压痕的多尺度研究

为了研究压头晶体各向异性对纳米压痕的影响,采用多尺度准连续介质(QC)法模拟了不同晶向Ni压头与Ag薄膜的纳米压痕过程。通过对比不同晶向下压头在薄膜上触发的原子滑移,发现压头的晶向引起的界面失配位错在很大程度上决定薄膜开启初始原子滑移系的难易。然后对比了压头在不同晶向下测得的薄膜纳米硬度,发现其计算值是一样的。最后研究压头表面和压痕表面的正应力和切应力的分布,分析了应力分布与原子滑移系的关系。     

Flow of drag-reducing surfactant solutions in rough pipes

The paper reports the results of experimental study of the flow of hexadecyltrimethylammonium chloride (CTAC) solutions with addition of sodium salicylate (NaSal) in the rough pipes. Measurements were performed in the range of the surfactant concentration from 200 to 400 ppm at a constant molar ratio CTAC/NaSal of 1:2. Five pipes of the relative roughness k/D varying from 1.2 × 10^?2 to 5.6 × 10^?2, obtained by the covering of inner surface of the pipes with glued silicon carbide particles of different size, were studied. The roughness was observed to increase the drag of flow of CTAC/NaSal solutions already at Reynolds numbers higher than 800. With increasing relative roughness k/D, the critical value of Reynolds number, at which the drag reduction disappears, was found to decrease. However, no influence of the roughness on the critical shear stress was noted. The ratio of the critical Reynolds number for rough pipes to that of hydraulically smooth pipes was independent of the surfactant concentration. The degree of drag reduction by the flow of surfactants was greater in rough pipes than in smooth pipes.     

QUASICONTINUUM SIMULATION OF NANOINDENTATION OF NICKEL FILM

A large-scale atom simulation of nanoindentation into a thin nickel film using thequasicontinuum method was performed. The initial stages of the plasticity deformation of nickelwere studied. Several useful results were obtained as follows: (1)The response of the load versusindentation depth—on the load versus indentation depth curve, besides the straight parts cor-responding to the elastic property of nickel, the sudden drop of the load occurred several times;(2) The phenomena of dislocation nucleation—the dislocation nucleation took place when theload descended, which makes it clear that dislocation nucleation causes the drop of the load;(3)The mechanism of the dislocation emission—the Peierls-Nabarro dislocation model and a pow-erful criterion were used to analyze the dislocation emission. And the computational value was ingood agreement with the predict value; (4) The density of geometrically necessary dislocations.A simple model was used to obtain the density of geometrically necessary dislocations beneaththe indenter. Furthermore, the influence of the boundary conditions on the simulation results wasdiscussed.     

Hard-material Adhesion: Which Scales of Roughness Matter?

Background

Surface topography strongly modifies adhesion of hard-material contacts, yet roughness of real surfaces typically exists over many length scales, and it is not clear which of these scales has the strongest effect. Objective: This investigation aims to determine which scales of topography have the strongest effect on macroscopic adhesion.

Methods

Adhesion measurements were performed on technology-relevant diamond coatings of varying roughness using spherical ruby probes that are large enough (0.5-mm-diameter) to sample all length scales of topography. For each material, more than 2000 measurements of pull-off force were performed in order to investigate the magnitude and statistical distribution of adhesion. Using sphere-contact models, the roughness-dependent effective values of work of adhesion were measured, ranging from 0.08 to 7.15 mJ/m² across the four surfaces. The data was more accurately fit using numerical analysis, where an interaction potential was integrated over the AFM-measured topography of all contacting surfaces.

Results

These calculations revealed that consideration of nanometer-scale plasticity in the materials was crucial for a good quantitative fit of the measurements, and the presence of such plasticity was confirmed with AFM measurements of the probe after testing. This analysis enabled the extraction of geometry-independent material parameters; the intrinsic work of adhesion between ruby and diamond was determined to be 46.3 mJ/m². The range of adhesion was 5.6 nm, which is longer than is typically assumed for atomic interactions, but is in agreement with other recent investigations. Finally, the numerical analysis was repeated for the same surfaces but this time with different length-scales of roughness included or filtered out.

Conclusions

The results demonstrate a critical band of length-scales—between 43 nm and 1.8 µm in lateral size—that has the strongest effect on the total adhesive force for these hard, rough contacts.

     

Friction factor in micropipe gas flow under laminar,transition and turbulent flow regime

The present work deals with the compressible flow of nitrogen gas inside microtubes ranging from 30 to 500 μm and with different values of the surface roughness (<1%), for different flow regimes. The first part of the work is devoted to a benchmark of friction factor data obtained at DIENCA (University of Bologna) and the ENEA laboratories, using fused silica pipes of 50 and 100 μm. Data overlapping is excellent thus evidencing how the agreement of the experimental data with the classic theory is independent of the measurement system. The second part of the work demonstrates that classic correlations can predict friction factor in laminar flow without revealing any evident influence of the surface roughness. The laminar-to-turbulent transition starts for Reynolds number not lower than 2000 for smooth pipes, while tending to larger values (3200–4500) for rough pipes. Anyway, contrarily to other available results, no dependence of the critical Reynolds number on the L/D has been observed. Changes in the flow regime have been found of the sharp and smooth type, like for larger pipes; smooth transition looks typical of smooth pipes while the sharp transition in the flow pattern is associated with rough pipes. In the fully developed turbulent regime, obtained for both smooth and rough pipes, an agreement between experimental data and the Blasius correlation has been verified for smooth pipes, while for rough pipes the agreement with predictions given by the Colebrook equation is rather modest.     

镍单晶薄膜纳米压痕的准连续介质模拟

用准连续介质方法模拟了大规模原子的镍薄膜在纳米压痕下发生初始塑性变形的行为.主要得到了:(1)载荷-位移响应.在载荷位移曲线上除了反应晶体弹性性质的直线外还有数次的载荷突然下降过程.(2)位错形核现象.与载荷-位移曲线上的载荷突然下降相对应的在受压的晶体上发现了位错形核现象,说明载荷的下降是因为位错形核引起的.(3)位错的发射机制.用Peierls-Nabarro位错模型以及能量法分析了位错的发射机制,理论值与计算值吻合较好.(4)几何必需位错密度.用一个简单的模型计算了几何必须位错密度.此外还考虑了边界条件对模拟结果的影响.     

Diagnostics of dielectronic processes in laser produced samarium plasma

Spatially-resolved time-integrated X-ray spectra of laser produced samarium plasma were recorded, in the spectral range from 7 to 10 Å. The spectrum of samarium is characterized by the prominent pattern of transitions 3d – nf (n = 4–7) belonging to Co-like (Sm³⁵⁺), Ni-like (Sm³⁴⁺) and Cu-like (Sm³³⁺) ions. Spectral lines of Mn-like (Sm³⁷⁺) to Zn-like (Sm³²⁺) were identified. The appearance of these ionization stages as a function of distance from the target was measured. Transfer of the dominant ion stages to lower stages with increasing distance from the original target surface was demonstrated, probably indicating dielectronic recombination. The Hebrew University Lawrence Livermore Atomic Code was used to generate emission spectra for comparison with the experimental ones.A radiation-hydrodynamics code coupled to three non-Local Thermal Equilibrium ionization and equation of state models with different approaches for dielectronic processes was used to model the plasma. The simulated plasma ionization and electron densities and temperatures were found to be consistent with the experimental results.     

Transient dislocation emission from a crack tip under dynamic mode III loading

Summary  Transient dislocation emission from a crack tip under dynamic mode III loading is analyzed. By taking into account the dynamic interaction between the crack and dislocation, the governing equation for the dislocation motion is derived under the quasi-steady assumption. The behavior of dislocation emission is explored in detail by solving this equation numerically. A critical initial speed can be determined, which must be exceeded by dislocations to escape from the crack tip. The dislocation emission process is found to be completed in such a short time period that the applied load may be approximately treated as constant during dislocation emission. Based on this fact, an asymptotic criterion for transient dislocation emission is developed, from which the critical initial speed can be evaluated. In the case that the dislocation is emitted from rest, we recover the quasi-static criterion of dislocation emission. Received 22 November 2000; accepted for publication 20 March 2001     

Size effects in indentation response of thin films at the nanoscale: A molecular dynamics study

The indentation response of Ni thin films of thicknesses in the nanoscale was studied using molecular dynamics simulations with embedded atom method (EAM) interatomic potentials. A series of simulations were performed in films in the [1 1 1] orientation with thicknesses varying from 4 to 12.8 nm. The study included both single crystal films and films containing low angle grain boundaries perpendicular to the film surface. The simulation results for single crystal films show that as film thickness decreases larger forces are required for similar indentation depths but the contact stress necessary to emit the first dislocation under the indenter is nearly independent of film thickness. The low angle grain boundaries can act as dislocation sources under indentation. The mechanism of preferred dislocation emission from these boundaries operates at stresses that are lower as the film thickness increases and is not active for the thinnest films tested. These results are interpreted in terms of a simple model.     

纳米压痕试验中压头尺寸效应的准连续介质法分析

采用准连续介质法模拟了单晶铝纳米压痕试验过程,分析了不同宽度的刚性矩形压头所引起的初始塑性变形特点,获得了载荷-压深、应变能-位移和硬度-压深曲线.从位错理论的角度分析了压头尺寸对纳米压痕测试结果的影响.研究发现:随着压头宽度的不断增大,压头下方位错形核所需要的载荷和压深程度增大,需要的应变能增加,应变能的变化速率递增,纳米硬度值减小,呈现出明显的尺寸效应.同时表明在一定的压入深度下,硬度与压头尺寸之间存在着一定的比例关系,不同尺寸压头获得的硬度值可以相互换算,但当矩形刚性压头宽度大于或等于120时这种尺寸效应消失.研究结果为纳米压痕实验过程中压头尺寸的选择提供了参考依据.     

New Methodologies for Fracture Detection of Automotive Steels in Tight Radius Bending: Application to the VDA 238–100 V-Bend Test

Background

The VDA 238–100 tight radius V-bend test can be used to efficiently characterize the bendability and fracture limits of sheet metals in severe plane strain bending. Material performance in plane strain bending is critical for the selection of advanced high strength steels for energy absorbing structural components.

Objective

The detection of failure based upon a reduction in the punch force can lead to erroneous predictions of failure for ductile or thin gage alloys in the VDA 238–100 test. New failure criteria were proposed and evaluated across a range of automotive steels.

Methods

Four detection methods in the V-bend test were evaluated based upon the load drop, bending moment, novel stress metric and the strain rate for seven steels with strength levels from 270 to 1500 MPa. The appropriate failure threshold was identified from visual inspection of the surface during bending.

Results

The vertical punch force will decrease as a consequence of the mechanics in the V-bend test at intermediate bend angles even without fracture. The novel stress-based metric accounts for sheet thinning and could successfully identify “false positives” and punch lift-off when considering the strain-rate evolution.

Conclusions

Failure detection using the VDA load threshold method may significantly under-report the bend performance of alloys with intermediate-to-high bendability or thin gauges. The proposed stress-based metric can reliably detect fracture for bend angles in excess of 160° and be readily calculated using the existing data. The VDA load threshold for failure can work well for materials that exhibit significant cracking.

     

Effect of temperature on dislocation emission from crack tip for BCC metal Mo

The effect of thermally activated energy on the dislocation emission from a crack tip in BCC metal Mo is simulated in this paper. Based on the correlative reference model on which the flexible displacement boundary scheme is introduced naturally, the simulation shows that as temperature increases the critical stress intensity factor for the first dislocation emission will decrease and the total number of emitted dislocations increase for the same external load. The dislocation velocity and extensive distance among partial dislocations are not sensitive to temperature. After a dislocation emission, two different deformation states are observed, the stable and unstable deformation states. In the stable deformation state, the nucleated dislocation will emit from the crack tip and piles up at a distance far away from the crack tip, after that the new dislocation can not be nucleated unless the external loading increases. In the unstable deformation state, a number of dislocations can be emitted from the crack tip continuously under the same external load. The project is supported by the National Natural Science Foundation of China.     

On the impacts of coarse-scale models of realistic roughness on a forward-facing step turbulent flow

The present work explores the impacts of the coarse-scale models of realistic roughness on the turbulent boundary layers over forward-facing steps. The surface topographies of different scale resolutions were obtained from a novel multi-resolution analysis using discrete wavelet transform. PIV measurements are performed in the streamwise–wall-normal (x–y) planes at two different spanwise positions in turbulent boundary layers at Re_h = 3450 and δ/h = 8, where h is the mean step height and δ is the incoming boundary layer thickness. It was observed that large-scale but low-amplitude roughness scales had small effects on the forward-facing step turbulent flow. For the higher-resolution model of the roughness, the turbulence characteristics within 2h downstream of the steps are observed to be distinct from those over the original realistic rough step at a measurement position where the roughness profile possesses a positive slope immediately after the step’s front. On the other hand, much smaller differences exist in the flow characteristics at the other measurement position whose roughness profile possesses a negative slope following the step’s front.     

Structure of the turbulent boundary layer over a rod-roughened wall

Turbulent coherent structures near a rod-roughened wall are scrutinized by analyzing instantaneous flow fields obtained from direct numerical simulations (DNSs) of a turbulent boundary layer (TBL). The roughness elements used are periodically arranged two-dimensional spanwise rods, and the roughness height is k/δ = 0.05 where δ is the boundary layer thickness. The Reynolds number based on the momentum thickness is varied in the range Re_θ = 300–1400. The effect of surface roughness is examined by comparing the characteristics of the TBLs over smooth and rough walls. Although introduction of roughness elements onto the smooth wall affects the Reynolds stresses throughout the entire boundary layer when scaled by the friction velocity, the roughness has little effect on the vorticity fluctuations in the outer layer. Pressure-strain tensors of the transport equation for the Reynolds stresses and quadrant analysis disclose that the redistribution of turbulent kinetic energy of the rough wall is similar to that of the smooth wall, and that the roughness has little effect on the relative contributions of ejection and sweep motions in the outer layer. To elucidate the modifications of the near-wall vortical structure induced by surface roughness, we used two-point correlations, joint weighted probability density function, and linear stochastic estimation. Finally, we demonstrate the existence of coherent structures in the instantaneous flow field over the rod-roughened surface.     

On the elastic–viscoplastic behavior of nanocrystalline materials

A new constitutive law is introduced to quantify the macroscopic effect of grain boundary dislocation emission on the behavior of pure face center cubic nanocrystalline materials. It is postulated that an emitted dislocation ends its trajectory in the grain boundary opposite to the source causing mass transfer. Dislocation emission by grain boundary ledges, considered here as the primary grain-boundary sources, is modeled as a thermally activated mechanism and the penetration of an emitted dislocation is assimilated as a soft collision. The macroscopic behavior of the material is retrieved via the use of a secant self-consistent scheme. The material is seen as a two-phase composite where the inclusion phase represents grain cores, their behavior is driven by dislocation glide, and where the matrix phase, governed by the newly introduced dislocation emission and penetration mechanism, represents both grain boundaries and triple junctions. The long range stress field arising from the presence of grain boundaries is taken into account in the critical glide resistance stress at 0 K in the inclusion phase. The model is applied to polycrystal copper and results in pure tension and creep are compared to experiments. Good agreements between the experimental measurements and the model predictions are observed.     

Mechanics of adhesive contact at the nanoscale: The effect of surface stress

At small length scales, the adhesion and surface effect are of great significance, both of which play important roles in the contact between two elastic solids. In this study, the classical Johnson–Kendall–Roberts (JKR) adhesive contact theory is generalized to the nanoscale at which the surface effect is considered. The influence of the surface stress on the JKR adhesive contact is investigated by employing the non-classical Boussinesq fundamental solutions. It is found that, compared with the classical theory, the pull-off force increases while the critical contact radius decreases as a result of the surface effect. Numerical results show that a relative error of 10% can be introduced in the pull-off force when the indenter radius is less than 20 nm. A detailed theoretical analysis of this interesting phenomenon is presented based on dimensional analysis, and two scaling laws for the adhesive contact at the nanoscale are constructed. These two new scaling laws reveal that the pull-off force is relevant to the elastic properties of the bulk materials, which is different from the classical adhesive contact theory. The present work is promising for the engineering applications in micro-electro-mechanical systems (MEMS) and nano-intelligent devices.     

Temporal and spectral behavior of sub-picosecond laser-created X-ray sources from low- to moderate-Z elements

We report on a set of experiments in which solid targets of different atomic numbers (Z) were irradiated with laser pulses of time durations ranging from 300 fs to 33 ps, and energies up to 26 J. The time-resolved X-ray emission in the 7.6–8.1 Å spectral range was measured using an ultra-fast X-ray streak camera coupled with a conical Bragg crystal. In this way we were able to follow the dramatic modification of the spectral features as a function of the laser duration. The features evolve from a “ns-type” emission, characterized by narrow and well-defined spectral lines, to very broad spectral features, due not only to the Stark broadening but also to the proliferation of satellites lines. The measured spectra also show strong time dependence, which allows us to follow the time evolution of the hydrodynamic parameters. We then compare the derived parameter with the CHIVAS hydro-radiative simulations. The experimental results are also compared with the AVERROES/TRANSPEC collisional-radiative code, and with precise spectral line shape calculations (PPP and PrismSPECT). The results seem to indicate regimes of interaction where hot electrons play an important role on spectral line formation.