Critical Shear Research Articles

Correlation between plasticity and dislocation dissociation in ceramics

The influence of dislocation core extension on plastic properties is discussed. Dislocation dissociation has been studied in ceramics. Such investigation has been carried out for the past decades, but in spite of new results, there are still pending questions. Here, we review cases of glide and climb dissociation. The former has been found in 4H-SiC with implications not only on the plasticity but also on the crystal structure changes. The latter has been found in several oxides (sapphire, spinel, strontium titanate, etc.). It is, sometimes, associated with a maximum in the critical resolved shear stress (CRSS) dependence on temperature. We discuss the reasons for such effects, emphasising factors that can reduce the mobility of dislocations to justify why dissociation can influence plasticity and, sometimes, has negligible effects.

Modeling the strength and ductility of magnesium alloys containing nanotwins

ABSTRACTMagnesium alloys have been receiving much attention recently as potential lightweight alternatives to steel for automotive and other applications, but the poor formability of these alloys at low temperatures has limited their widespread adoption for automotive applications. Recent work with face centered cubic (FCC) materials has shown that introduction of twins at the nanometer scale in ultra-fine grained FCC polycrystals can provide significant increase in strength with a simultaneous improvement in ductility. This objective of this work is to explore the feasibility of extending this concept to hexagonal close packed (HCP) materials, with particular focus on using this approach to increase both strength and ductility of magnesium alloys. A crystal plasticity based finite element (CPFE) model is used to study the effect of varying the crystallographic texture and the spacing between the nanoscale twins on the strength and ductility of HCP polycrystals. Deformation of the material is assumed to occur by crystallographic slip, and in addition to the basal and prismatic slip systems, slip is also assumed to occur on the {1 0 $\bar 1$ 1} planes that are associated with compression twins in these materials. The slip system strength of the pyramidal systems containing the nanotwins is assumed to be much lower than the strength of the other systems, which is assumed to scale with the spacing between the nanotwins. The CPFE model is used to compute the stress-strain response for different microstrucrutral parameters, and a criterion based on a critical slip system shear strain and a critical hydrostatic stress is used to compute the limiting strength and ductility, with the ultimate goal of identifying the texture and nanotwin spacing that can lead to the optimum values for these parameters.

Theoretical Study on the Interaction between Shuffle 60° Dislocation and Hexavacancy in Silicon

The interaction of the shuffle 60° dislocation with a regular chain of hexavacancies was investigated via the molecular dynamics simulation with Stillinger-Weber potential. The results show that an attraction exists between the shuffle 60° dislocation and hexavacany. The attraction energy is dependent obviously upon the hexavacancy concentration. The dislocation can overcome the pinning of vacancies under a critical resolved shear stress, and a linear relationship is found between the critical stress and hexavacancy concentration.

STUDY ON THE PEIREL–NABARRO STRESS OF IRON ALUMINIDES

The modified Peirel–Nabarro model of dislocations is not valid for the superdislocation bounded by the antiphase domain boundaries in long-range ordered intermetallic. In this work, a new Peirel–Nabarro Stress model is developed to take into account the critical resolved shear stress of antiphase domain boundary (APDB). Based on it, the Peirel–Nabarro Stress of DO3 structure and B2 structure in iron aluminides are calculated. Comparing the Peirel–Nabarro Stress of dislocations and the crystal theoretical yield strength, the results demonstrate B2-type crystal has good plasticity. It coincides with the experimental results well.

Surface tension-induced global instability of planar jets and wakes

AbstractThe effect of surface tension on global stability of co-flow jets and wakes at a moderate Reynolds number is studied. The linear temporal two-dimensional global modes are computed without approximations. All but one of the flow cases under study are globally stable without surface tension. It is found that surface tension can cause the flow to be globally unstable if the inlet shear (or, equivalently, the inlet velocity ratio) is strong enough. For even stronger surface tension, the flow is restabilized. As long as there is no change of the most unstable mode, increasing surface tension decreases the oscillation frequency. Short waves appear in the high-shear region close to the nozzle, and their wavelength increases with increasing surface tension. The critical shear (the weakest inlet shear at which a global instability is found) gives rise to antisymmetric disturbances for the wakes and symmetric disturbances for the jets. However, at stronger shear, the opposite symmetry can be the most unstable one, in particular for wakes at high surface tension. The results show strong effects of surface tension that should be possible to reproduce experimentally as well as numerically.

Effect of Shrub Planting on Runoff and Sediment under Simulated Rainfall

Effect of shrub on runoff and sediment yield and its hydraulic mechanism of shrub were studied under rainfall intensities of 45, 87 and 127mm/h with 20°slope gradient using simulated rainfall experiment. the results showed that average runoff rates ranged from 39.7 to 126.0 L/min for bare plots and 0.77 to 4.83 L/min for shrub plots, and the runoff rates from shrub plots were much less than from bare plots. Average sediment yields varied from 3636.7 to 9436.3 g/min for bare plots and from 28.0 to 421.6 g/min for shrub plots. The critical flow shear stress of 1.65 N/m2 on shrub slope and 0.861 N/m2 on bare slope were got under experiment condition. The sediment yield increased with the increase of flow shear stress. The experiment results are meaningful for quantifying runoff and sediment reduction and deepening soil erosion mechanical process also.

Study of stress localisation in polycrystalline grains using self-consistent modelling and neutron diffraction

The time-of-flight neutron diffraction technique and the elastoplastic self-consistent model were used to study the behaviour of single and multi-phase materials. Critical resolved shear stresses and hardening parameters in austenitic and austenitic–ferritic steels were found by analysing the evolution of the lattice strains measured during tensile tests. Special attention was paid to the changes of the grain stresses occurring due to transition from elastic to plastic deformation. Using a new method of data analysis, the variation of the stress localisation tensor as a function of macrostress was measured. The experimental results were successfully compared with model predictions for both phases of the duplex steel and also for the austenitic sample.

Role of glide systems in channel die compression of commercially pure titanium alloy

Plastic deformation process of Ti alloys depends on the competition of active deformation modes: slip and twinning. The specific deformation mechanism depends on the availability of the specific deformation modes as well as on the critical resolved shear stress for slip and the twin activation stress. Effort has been made to provide information on the different slip systems other than prismatic slips, which get activated during the channel die compression process. It is found that, though at the initial stage of deformation, prismatic slip is prevalent, but other slip systems, especially pyramidal types, also appear at the later stage of deformation. Transmission electron microscopy results confirm the presence of pyramidal slips beside prismatic ones. It is also observed that twining does not take place in those grains having the most stable orientation as per the hexagonal close packed system. Information on the critical resolved shear stresses of different glide systems for T40 is also provided.

In-Situ Observation and Mechanical Criterion on Interface Cracking in Nano-Components

ABSTRACTWe have investigated the criterion of interfacial crack initiation in nanometer-scale components (nano-components) by means of a loading facility built in a transmission electron microscope (TEM). Three types of experiments are conducted in this project. (1) In order to clarify the applicability of conventional continuum mechanics to the nano-components, we prepare cantilever specimens with different size, which introduce different stress fields, containing an interface between a 20 nm-thick copper (Cu) thin film and a silicon (Si) substrate. These demonstrate the validity of the “stress” criterion even for the nano-scale fracture. (2) In order to examine the effect of microscopic structure on the mechanical property, we fabricate a bending specimen in the nano-scale with thin Cu bi-crystal (the thickness of about 100 nm) formed on Si substrate, of which understructure can be observed in situ by means of a TEM during the mechanical experiment. The initial plastic deformation takes place near the interface edge in a grain with a high critical resolved shear stress and expands preferentially in the grain. Then, the plasticity appears near the between Cu grain boundary and Cu/Si interface, and this development brings about the interfacial cracking from the junction. These indicate the governing influence of understructure on the mechanical property in the nano-components. (3) In order to investigate the fatigue behavior of metal in a nano-component, a cyclic bending experiment is carried out using nano-cantilever specimens with a 20 nm-thick Cu constrained by highly rigid materials (Si and SiN). The high strain region is in the size of 20-40 nm near the interface edge. The specimen breaks along the Cu/Si interface before the maximum load under the fatigue loading. The load-displacement curve shows nonlinear behavior and a distinct hysteresis loop, indicating plasticity in the Cu film. Reverse yielding appearing after the 2nd cycle suggests the development of a cyclic substructure in the Cu film. These indicate that the crack is caused by characteristic understructure owing to fatigue cycles.

Fabrication and characterization of integrally skinned‐oriented highly selective charged asymmetric low pressure poly(ether sulfone) membranes for nanofiltration

AbstractBACKGROUND: The effects of shear rate induced molecular orientation in polymeric based asymmetric membranes on performance, structural details, key properties, morphologies and pore size distribution were studied. Asymmetric membranes fabricated at five different potential shear rates ranging from 93.33–466.67 s−1 were evaluated based on nanofiltration test. The use of Spiegler–Kedem, steric‐hindrance pore and Teorell–Meyers–Sievers models enables an assessment to be made of the relationship between shear rates and membranes properties.RESULTS: Experimental and modeling results show that there is a significant correlation between shear and membrane characteristics, whereby as shear rate increases, the separation performances of nanofiltration membranes increase until an optimum (critical) level of shear is achieved. Beyond the optimum shear, the deterioration in membrane performance suggests that there exists an optimum shear rate which produces optimal structural details, key properties, morphologies and pore size distributions.CONCLUSIONS: Results showed that a skinned‐oriented highly selective charged asymmetric low pressure nanofiltration (ALP‐NF) membrane was successfully developed. The optimum shear rate (critical shear) was found to be 233.33 s−1. At the optimum shear rate, the fabricated ALP‐NF membranes produced the finest properties, morphology and narrowest pore distributions. The positive improvement in performance properties of ALP‐NF membrane provides the potential for producing a highly selective NF membrane for different applications in the future. Copyright © 2011 Society of Chemical Industry

Bayesian sediment transport model for unisize bed load

Fluvial sediment transport studies have long underscored the difficulty in reliably estimating transport model parameters, collecting accurate observations, and making predictions because of measurement error, natural variability, and conceptual model uncertainty. Thus, there is a need to identify modeling frameworks that accommodate these realities while incorporating functional relationships, providing probability‐based predictions, and accommodating for conceptual model discrimination. Bayesian statistical approaches have been widely used in a number of disciplines to accomplish just this, yet applications in sediment transport are few. In this paper we propose and demonstrate a Bayesian statistical approach to a simple sediment transport problem as a means to overcome some of these challenges. This approach provides a means to rigorously estimate model parameter distributions, such as critical shear, given observations of sediment transport; provides probabilistically based predictions that are robust and easily interpretable; facilitates conceptual model discrimination; and incorporates expert judgment into model inference and predictions. We demonstrate a simple unisize sediment transport model and test it against simulated observations for which the “true” model parameters are known. Experimental flume observations were also used to assess the proposed model's robustness. Results indicate that such a modeling approach is valid and presents an opportunity for more complex models to be built in the Bayesian framework.

Theoretical Prediction on the Shear Capacities of Shear Connectors in Steel Column Bases

Shear connectors are extensively used in steel column bases of steel structures to transfer loading from superstructure to concrete foundations, however the behavior and shear capacity of the steel shear connector subject to complex external loading are not well understood. This paper presents an approach for theoretically predicting the shear capacities as well as behavior of the shear connector in steel column bases subject to external loading such as both shear force and bending moment at the top. The load-displacement behavior at the top of the shear connector and the internal force distribution along the depth of the connector are also investigated at various states with different cases of boundary conditions. Two states of concrete compressive strain, i.e. the critical and ultimate states, are considered for evaluating the critical and ultimate shear capacities of the shear connector. The theoretical predictions for the behavior and load capacities of the shear connector are then compared with finite element results. The results from the verification study show that the proposed approach can provide satisfactory theoretical predictions for the behavior of the shear connector.

Validation of strain invariant failure analysis in an open hole off-axis specimen

We present a model that incorporates the effect of uncertainty in the microstructure of composite systems in failure analysis. The method involves a recently developed micromechanical enhancement model to integrate the effect of variability in the fiber volume fraction. The model determines the range of failure angles in an open hole off-axis specimen and the uncertainty in the critical effective shear strain in the matrix phase at failure due to uncertainties in the fiber volume fraction. A Bayesian methodology is used for model validation and the Bayes factor metric is used to quantify the confidence in the model prediction.

Experimental Study on Critical Flow Conditions Causing Damage of Grass-Covered Revetment

Flexible vegetations supply great effect on river bank protection. It is important to study the critical flow conditions, including bed shear stress, scour duration and frictional velocity etc, causing damage of grass-covered revetment. Laboratory experiments prove that, destruction of turf can be estimated by observing the variation of water head. The experimental shear stress above vegetation zone has nonlinear relation with velocity square, which agrees with theoretic derivation. Before the turf destruction occurring, shear stress above vegetation zone increases with the scour duration and then goes to stabilization. This mentioned Shear stress will change suddenly when local scour damage happens, or keep stabilization while affine damage is on-going. Under the same flow conditions, the critical bed shear stress of reinforced turf is larger than that of natural turf. As far as the same-form revetments are concerned, the bed shear stress at breaking point increases with the flow velocity.

Bed Load Transport under Complex Flow

In this paper, the backward-facing step flow and the sediment transport downstream step were studied experimentally. The critical incipient bed shear velocity is obtained by the results of bed shear velocity and sediment incipient probability. It was found that the critical incipient bed shear velocity depends on the flow structures under the complex flow. By using the new critical incipient bed shear obtained in this paper and calculating the Shields parameter based on instantaneous bed shear velocity, the bed load sediment transport rate downstream step was given. The time history of the bed profile downstream step was calculated by bed load sediment transport rate and compared that obtained by the digital images. Good agreement was observed.

Micro-cantilever testing of ⟨a⟩ prismatic slip in commercially pure Ti

Four sizes (1, 2, 5 and 10 µm wide) of single-crystal α-Ti micro-cantilevers were manufactured from polycrystalline commercially pure Ti samples using a focussed ion beam. A nanoindenter was used to conduct micro-bending tests. The crystal orientation and alignment of the micro-cantilever beams were selected so as to activate only ⟨a⟩ slip on prism planes. A crystal plasticity finite element model was used to simulate the deflection behaviour of the cantilevers. The critical resolved shear stresses (CRSS) were determined via a reverse process of fitting the simulation results to the experimental load–displacement curves. The CRSS increased from 144 MPa for the 10 µm wide cantilevers to 566 MPa for the 1 µm wide cantilevers.

Localization of Stresses in Polycrystalline Grains Measured by Neutron Diffraction and Predicted by Self-Consistent Model

Time of flight neutron diffraction method was applied to measure elastic lattice strains in austenitic steel during "in situ" tensile test. Comparing experimental data with self-consistent model, the critical resolved shear stress and hardening parameters were determined for polycrystalline grains. The result allowed us to determine the main component of the stress localization tensor, relating the rate of grain stress with the applied macrostress rate. The evolution of concentration tensor in function of the applied macrostress was analyzed. Finally, the load transfer between grains during yielding of the sample was studied.

Influence of Slip Localization on Surface Relief Formation and Grain Boundary Microcrack Nucleation

Slip localization is often observed in metallic polycrystals after cyclic deformation (persistent slip bands) or pre-irradiation followed by tensile deformation (channels). To evaluate its influence on surface relief formation and grain boundary microcrack nucleation, crystalline finite element (FE) computations are carried out using microstructure inputs (slip band aspect ratio/spacing). Slip bands (low critical resolved shear stress (CRSS)) are embedded in small elastic aggregates. Slip band aspect ratio and neighboring grain orientations influence strongly the surface slips. But only a weak effect of slip band CRSS, spacing and grain boundary orientation is observed. Analytical formulae are deduced which allow an easy prediction of the surface and bulk slips. The computed slips are in agreement with experimental measures (AFM/TEM measures on pre-irradiated austenitic stainless steels and nickel, copper and precipitate-strengthened alloy subjected to cyclic loading). Grain boundary normal stresses are computed for various materials and loading conditions. A square root dependence with respect to the distance to the slip band corner is found similarly to the pile-up stress field. But the equivalent stress intensity factor is considerably lower. Analytical formulae are proposed for predicting the grain boundary normal stress field depending on the microstructure lengths. Finally, an energy balance criterion is applied using the equivalent elastic energy release rate and the surface/grain boundary energies. The predicted macroscopic stresses for microcrack nucleation are compared to the experimental ones.

Comparative Studies of Mechanical and Interfacial Properties Between Jute Fiber/PVC and E-Glass Fiber/PVC Composites

Composites (50 wt% fiber) of jute fiber reinforced polyvinyl chloride (PVC) matrix and E-glass fiber reinforced PVC matrix were prepared by compression molding. Mechanical properties such as tensile strength (TS), tensile modulus (TM), bending strength (BS), bending modulus (BM) and impact strength (IS) of both types of composites was evaluated and compared. Values of TS, TM, BS, BM and IS of jute fiber/PVC composites were found to be 45 MPa, 802 MPa, 46 MPa, 850 MPa and 24 kJ/m2, respectively. It was observed that TS, TM, BS, BM and IS of E-glass fiber/PVC composites were found to increase by 44, 80, 47, 92 and 37.5%, respectively. Thermal properties of the composites were also carried out, which revealed that thermal stability of E-glass fiber/PVC system was higher. The interfacial adhesion between the fibers (jute and E-glass) and matrix was studied by means of critical fiber length and interfacial shear strength that were measured by single fiber fragmentation test. Fracture sides after flexural testing of both types of the composites were investigated by Scanning Electron Microscopy.

Experimental investigation of the relationship between the critical state shear strength of unsaturated soils and the soil-water characteristic curve

This paper presents the comparison of experimental results of the critical state shear strength of an unsaturated glacial till and the predicted shear strength using the Soil-Water Characteristic Curve (SWCC), determined following three different techniques, namely: (i) the conventional SWCC without application of any consolidation stress; (ii) the SWCC measured on a specimen subjected to a net stress equal to the net normal stress of experimental shear tests; (iii) the SWCC derived from the degree of saturation and matric suction of the sheared specimen under critical state conditions. The results of the study suggest that the best predictions of the shear strength are possible using the experimental SWCC information expressed as an apparent SWCC. The study also suggests that the relationship between the apparent degree of saturation and the matric suction of a specimen sheared to critical state is unique, and is not influenced by the shearing mode, or compaction water content (i.e. soil structure).