Vol 21, No 3 (2018)
- Year: 2018
- Articles: 12
- URL: https://journals.rcsi.science/1029-9599/issue/view/12072
Article
Strength and Fracture Toughness of Polyurethane Elastomers Modified with Carbon Nanotubes
Abstract
Very small additions of single-wall carbon nanotubes produce an anomalous change in the mechanical properties of a cross-linked polyurethane-amide-urea elastomer containing 10% of polyamide-6: its elastic modulus and ultimate stress reveal local maxima at a nanofiller content of hundredths and thousandths of a percent. Previously, the behavior of the elastic modulus was simulated reasoning from the formation of an intermediate phase layer in the elastomer at particle contact boundaries. Here, on the same basis, we simulate the behavior of its strength as a function of nanotube concentration and consider crack models accounting for the influence of nanotubes on the crack tip zone and fracture toughness.
Correlation between Geometrically Nonlinear Elastoviscoplastic Constitutive Relations Formulated in Terms of the Actual and Unloaded Configurations for Crystallites
Abstract
A correct description of severe plastic deformation requires the use of nonlinear kinematic and constitutive relations. The relations should meet certain criteria, such as the frame-independence, closed stress cycles and the absence of energy dissipation under elastic cyclic loading. A most complicated issue in formulating these relations is the decomposition of motion into quasi-rigid and strain-induced motions, which is an extremely difficult problem with no unambiguous solution. For the majority of structural metals and alloys, this decomposition can be done in a physically justified way on the level of crystallites. Earlier, using a multilevel approach we introduced a corotational coordinate system for crystallites responsible for quasi-rigid motion. This allowed us to formulate frame-independent mesoscopic constitutive relations in the actual configuration and to perform test calculations to show that the above criteria are satisfied with high accuracy. A new way of motion decomposition was proposed which is a multiplicative representation of the deformation gradient with an explicit extraction of the corotational frame motion. Elastoviscoplastic constitutive relations satisfying the above criteria were formulated in terms of an unloaded "lattice" configuration. However, it is more preferable to formulate nonlinear boundary value problems in terms of the actual configuration in rates. This paper is aimed to study correlation between the earlier derived constitutive relations formulated in terms of the actual configuration and in terms of the unloaded "lattice" configuration. Example problems are solved to demonstrate the closeness of results obtained with these constitutive relations.
Self-Oscillating Mode of a Nanoresonator
Abstract
The paper proposes a new graphene resonator circuit which operates on the principle of a self-oscillator and has no drawbacks typical of nanoresonators as mass detectors and associated with their law quality factor, eigenfrequency errors (measurements from resonance curves), and dependence of quench frequency on oscillation frequency (curves with quenching for nonlinear systems). The proposed circuit represents a self-oscillator comprising an amplifier, a graphene resonator, and a positive feedback loop with a graphene oscillation transducer, and its major advantage is in self-tuning to resonance frequency at slowly varying resonator parameters, compared to oscillation periods. The graphene layer with a conducting substrate beneath it forms a capacitor which is recharged by a dc voltage source as its capacitance varies due to graphene deformation, and the recharge current is an oscillation- dependent signal transmitted from the transducer to the amplifier input. The graphene layer is placed in a magnetic field and is deformed when a current from the amplifier output is passed through. By properly choosing the magnetic field direction and the amplifier gain, it is possible to provide swinging oscillation whose amplitude is limited by the amplifier nonlinearity. For the proposed system we present an electromechanical model, dimensionless equations of motion, and numerical data demonstrating the generation of steady-state oscillations with eigenfrequency. Also presented is an analysis showing that the system can have only one limit cycle and that this cycle is always stable. The proposed resonator circuit can be used as a mass detector which determines the added mass from a change in self-oscillation frequency.
Deformation Features on the Free Surface and Rigid Interface during Elastic Wave Reflection
Abstract
Analytical expressions defining the reflection coefficients have been derived in the framework of the known problem of elastic wave reflection from the free surface and rigid interface. The found reflection coefficients are used to analyze the dependences of different strain and rotation components during elastic wave reflection from the free surface and rigid interface in the case of ideal contact and possible slip on the wave incidence angle and elastic parameters of the medium. Deformation features of elastic media at the studied interfaces are considered for incident longitudinal and transverse waves.
Fracture Model of Anisotropic Rocks under Complex Loading
Abstract
The paper proposes a deformation and fracture model for anisotropic stratified rocks and presents theoretical and experimental data on how the rock strength and fracture geometry are influenced by principal stresses and their orientation to bedding planes. Two possible mechanisms are considered for rock fracture under true triaxial load: along bedding planes of weakness and along planes in which Mohr-Coulomb stresses reach a critical combination with cohesion coefficients and internal friction angles typical of the rock. The transition of rocks to inelastic deformation is described in the context of two criteria of which one accounts for the above fracture mechanisms and the other, being a semi-empirical analogue of the Hill yield criterion, accounts for the effect of normal stress. The experimental data presented are for the strain and strength properties of rocks sampled from the Fedorovskoye and Talakanskoye oil and gas fields and tested on an original loading system for true triaxial compression with lateral pressure (similar to the Karman scheme) and for generalized shear (three unequal and nonmonotonic principal stresses). The experimental and theoretical results, including total stress-strain curves, are in good qualitative agreement and demonstrate the possibility to evaluate the parameters entered in the model from tests of particular rocks.
Baikal Ice Cover as a Representative Block Medium for Research in Lithospheric Geodynamics
Abstract
The paper summarizes the results of long-term field research in the dynamics of the Baikal ice cover as a multiscale block medium similar to the lithosphere in structure, rheology, and seismotectonic features. The analysis covers data on deformation, seismicity, and contact interaction modes as well as on meteorological factors responsible for dynamic fracture of ice plates and strong ice shocks with earthquake-like vibrations. Similarity between seismic features in ice interface zones and zones of tectonic subduction, collision, and shear is discussed. Reasoning from dynamic analogies and similarities of destruction processes in the ice and lithosphere, the research data can help solving fundamental and applied problems, particularly those of earthquake prediction and assessment of contact interactions between lithospheric plates in fault zones.
Some Nonlinear Rock Behavior Effects
Abstract
The nonlinear rock behavior effects observed in loading diagrams are analyzed which are usually ignored in conventional models of elastoplastic media. The initial deformation stage and unloading of rock samples are considered. The nonlinear behavior on these loading stages is interpreted from the viewpoint of partial closure of cracks initiated during deformation beyond the elastic limit or in earlier loading history. Phenomenological relations are derived to account for the discussed nonlinear effects in numerical modeling. The postcritical deformation stage corresponding to the stage of strain localization and main crack formation is studied. Corrections are made to provide a more accurate determination of model parameters.
Effect of Nanostructure Size on Parameters of Rotational Fields Induced by External Compressive Stress
Abstract
This paper continues a series of studies on the formation and development of vortex structures in solids using the molecular dynamics method. This phenomenon is interpreted from the viewpoint of structural self-organization. The effect of the structure size on the formation of rotational fields has been studied to show that their appearance is not a consequence of the specimen nanosize. It is shown that the lateral nanostructure size influences the rotational field energy.
Strain-Induced Surface Roughening in Polycrystalline VT1-0 Titanium Specimens under Uniaxial Tension
Abstract
Surface roughening in uniaxially tensile specimens of commercially pure titanium VT1-0 has been investigated using electron backscatter diffraction, optical and atomic force microscopy, and numerical simulation. It is shown that intragranular slip leads to the rotation of surface grains, due to which the grain surface is inclined and a terrace is formed at the interface with neighboring grains. The effect of the crystallographic grain orientation on the grain shape change and the degree of grain rotation occurring under constrained plastic deformation is demonstrated.
Effect of Thermoelastic Characteristics of Components, Shape of Non-Isometric Inclusions, and Their Orientation on Average Stresses in Matrix Structures
Abstract
The paper presents model calculations on which to predict volume-average external stress under changes of local internal stress in matrix composites with non-isometric inclusions. It is assumed that the rise of local stress owes to different coefficients of linear thermal expansion of non-isometric inclusions and matrix. The inclusions are taken as ellipsoids of rotation (disks, short fibers) and their principal semiaxes as oriented either along three mutually perpendicular directions x, y, and z of a rectangular coordinate system, only along x and y, or only along x. The average stress in the heterogeneous material and its local stress within an individual inclusion are related through a stress concentration operator (fourth rank tensor) for which an explicit expression is derived in a generalized singular approximation of random field theory. The relations obtained for external stress take into account thermoelastic characteristics of the two components as well as inclusion concentrations and orientations in the matrix. The calculation is applied to estimate the average stress along three axes in a composite consisting of an ED-20 epoxy binder and non-isometric copper inclusions.
Effect of Hydrogen Charging on Mechanical Twinning, Strain Hardening, and Fracture of ‹111› and ‹144› Hadfield Steel Single Crystals
Abstract
This paper studies the effect of electrolytic hydrogen charging on the plastic deformation and fracture of Hadfield steel single crystals oriented for tension along the ‹111› and ‹144› directions, which the major deformation mechanism is mechanical twinning. Electrolytic hydrogen charging for five hours at a current density of 100 A/m2 slightly affects the stages of plastic flow, deformation mechanism, and the value of uniform elongation of ‹111› and ‹144› single clreystals. Hydrogen saturation causes shear microlocalization and a decrease of the strain hardening coefficient in twinning in one system, but slightly affects the strain hardening characteristics in multiple twinning. Hydrogen charging increases the fraction of the brittle component on fracture surfaces and leads to microand macrocracking near the fracture zone on the lateral surface of deformed specimens. It has been found experimentally that the stress relaxation rate in loaded ‹111› single clreystals after hydrogen saturation decreases. Mechanisms of describing this phenomenon have been proposed.
Erratum
Erratum to: “Numerical Study of Mechanical Properties of Nanoparticles of ß-Type Ti-Nb Alloy under Conditions Identical to Laser Sintering. Multilevel Approach”
Abstract
ACKNOWLEDGMENTS
The work was financially supported by Russian Science Foundation grant No. 15-19-00191. The ab initio calculations were performed by A.V. Ponomareva (with support by RFBR Grant 16-02-00797a) and A.Z. Zharmukhambetova (with support of the TSU Competitiveness Improvement Program).