Different Modes Of Deformation Research Articles

Pencil‐Shaped Necking of Thick Al Whisker Grown by Stress‐Induced Migration and Enhancement of Tensile Strength

This study aims to characterize the mechanical response of thick Al whiskers under tension. Wire‐shaped whiskers are fabricated by stress‐induced migration using an original sample structure. Straight and single‐crystal whiskers are suspended between a W probe tip attached to a micromanipulator and an atomic force microscope tip attached to a nanostage using electron beam irradiation hardening glue. Tensile tests are conducted by pulling the probe tip. A tensile strength of 1 GPa is obtained for a whisker with a diameter of 500 nm. Observations using a high‐voltage electron microscope show that the whisker is deformed by pencil‐shaped sharp necking with signs of recrystallization at the tip, which displays a novel mechanical response of Al whiskers. The obtained tensile strength is more than twice that of whiskers with similar diameters reported in the literature, indicating that the tensile strength of not only thin, but also thick whiskers can be enhanced by activating a different mode of deformation.

Rigid-plastic behavior and bearing capacity of thin flat reinforced rotating disks

In the framework of the model of the rigid-plastic body, the limiting behavior of thin hybrid reinforced flat disks rotating in a steady-state regime is considered. The disks are fixed on the inner contour and have blades on the outer contour. Rigid-plastic materials of components of the composition are asymmetric in tension/compression and have piecewise linear yield conditions. Bind material has cylindrical anisotropy. The structural model with a two-dimensional stress state in components is used. The complete solution is constructed. Different modes of deformation are considered. Analytical expressions for the limit speed of rotation are obtained. Numerical examples of radial-circumferential and spiral reinforcement are given.

Modeling the mechanical behavior of torque-tightened screw lap joints

This paper presents an analytical approach to model the mechanical behavior of pre-tightened / torque-tightened screw lap joints under axial loading through a spring-based method. The presence of pre-torque in a screw joint significantly influences the load-deformation behavior since it induces frictional forces between the joint components. These frictional forces introduce different modes of deformation in the joint and are captured here by modeling the entire joint as a spring-mass system where the individual joint components and their deformation modes are approximated by linear springs. Subsequently, the whole model is presented by a set of equilibrium equations, which are solved to acquire the mechanical behavior of the screw lap joint. The proposed analytical model is validated against the performed experimental results and predictions from the 3D finite element model. Finally, the effect of different pre-loads and clearances on the behavior of the screw lap joints is analyzed through the proposed model.

Fabricating 3D Moisture- and NIR Light-Responsive Actuators by a One-Step Gradient Stress-Relaxation Process.

Polymeric materials that can actuate under the stimulation of environmental signals have attracted considerable attention in fields including artificial muscles, soft robotics, implantable devices, etc. To date, the improvement of shape-changing flexibility is mainly limited by their unchangeable shapes and structural and compositional distributions. In this work, we report a one-step treatment process to convert 2D poly(ethylene oxide)/sodium alginate/tannic acid thin films into 3D-shaped moisture- and NIR light-responsive actuators. Spatial surface wetting of the film leads to the release of residual stress generated in film formation in a gradient manner, which drives the wetted regions to bidirectionally bend. By controlling the position and bending amplitude of the wetted regions, designated 3D shapes can be obtained. Moreover, Fe3+ ions in the aqueous solution used for surface wetting can coordinate with carboxylate groups in sodium alginate chains to form a gradient cross-linking network. This gradient network can not only stabilize the resulting 3D shape but also render the film with moisture-responsive morphing behaviors. Fe3+ ions can also self-assemble with tannic acid molecules to form photothermal aggregates, making the film responsive to NIR light. We further show that films with versatile 3D shapes and different modes of deformation can be fabricated by a one-step treatment process. This strategy is convenient and extendable to develop 3D-shaped polymer actuators with flexible shape-changing behaviors.

Non-invariant elastic moduli of bi-level architected lattice materials through programmed domain discontinuity

Effective elastic moduli of lattice-based materials are one of the most crucial parameters for the adoption of such artificial microstructures in advanced mechanical and structural systems as per various application-specific demands. In conventional naturally occurring materials, these elastic moduli remain invariant under tensile and compressive normal modes or clock-wise and anti-clock-wise shear modes. Here we introduce programmed domain discontinuities in the cell walls of the unit-cells of lattice metamaterials involving a bi-level microstructural design to achieve non-invariant elastic moduli under tensile and compressive normal modes or clock-wise and anti-clock-wise shear modes. More interestingly, such non-invariance can be realized in the linear small deformation regime and the elastic moduli can be tailored to have higher or lower value in any mode compared to the other depending on the placement and intensity of the discontinuities in a programmable paradigm. We have derived an efficient analytical framework for the effective elastic moduli of lattice materials taking into account the influence of domain discontinuity. The axial and shear deformations at the beam level are considered along with bending deformation in the proposed analytical expressions. The numerical results ascertain that the domain discontinuities, in conjunction with unit cell level geometric parameters, can impact the effective elastic constants significantly under different modes of far-field stresses. It is further revealed that the degree of auxeticity of such lattices can be programmed to have target values (including non-invariance under different modes of deformation) as a function of the intensity and location of domain discontinuity when axial and shear deformations are included at the beam level. Realization of the unusual non-invariant elastic moduli of bi-level architected lattice materials would lead to a range of technologically demanding niche applications where one mode of deformation requires more or less force to deform compared to the opposite mode. Besides being able to perform as a load-bearing component, the proposed metamaterial can be used as an integrated sensor for measuring the level of stress or strain in structures.

A multiscale model for multiaxial inelastic behavior of elastomeric particulate composites

This article addresses the problem of the multiscale constitutive representation of the multiaxial inelastic behavior of elastomeric particulate composites. A fully three-dimensional model is proposed within a micromechanical treatment to describe the multiaxial inelastic response in relation to the reinforcement mechanisms. A network decomposition based on the tube confinement theory is used to consider the combined effects of multiaxiality and inelasticity. The near-field direct interactions between the particles and the rubber networks are physically described using the Eshelby inclusion theory. The capabilities of the microstructure-based model are evaluated for different modes of deformation over a wide range of filler concentrations. It is found being able to successfully reproduce the significant features of the multiaxial macro-response upon monotonic and cyclic loading sequences. Important insights about the effective role of the reinforcement mechanisms on the multiaxial dissipation are revealed.

Response of curved aluminum panels subjected to localized blast loading at extreme temperatures

An experimental investigation was conducted to understand the effect of curvature on blast response of aluminum panels under extreme temperatures. Three aluminum 2024-T3 panels: one flat and two curved panels with radii of curvatures of 304.8 mm and 111.8 mm, were used for the study. A shock tube apparatus was utilized to impart controlled blast loading. Propane flame torches were used to heat the specimens and experiments were conducted at 25 °C, 200 °C, and 350 °C. High-speed photography coupled with 3D Digital Image Correlation (DIC) technique was used to obtain full-field deflection, velocity, in-plane strain and curvature change on the back face of the panels. Different modes of deformation and mode transitions during the blast loading of the three panels have been identified. The effect of temperature on the deformation modes was investigated. The deflection of the panel with 304.8 mm radius of curvature was found to be higher than the other two panels. With an increase in temperature, deflections increased, and strain was seen to get localized. The deformation modes were strongly dependent on initial curvature and less sensitive to temperature.

Mechanical behavior and forming of commercially-pure niobium sheet

This work investigates the mechanical and forming behavior of pure niobium sheet, which has broad applications in low-temperature physics including the proposed International Linear Collider. An extensive suite of mechanical tests is performed on commercially-pure niobium (CP-Nb), including uniaxial tension, strain-rate-jump, biaxial tension and disc compression, which are used to characterize the plastic behavior of the CP-Nb sheet. The material is found to have significant plastic anisotropy, high R-values, significant ductility, and non-negligible rate-dependent behavior. Subsequently, two types of forming experiments, deep-drawing and stretch-forming, are conducted with CP-Nb blanks of different diameters. These two experiments subject the CP-Nb sheet to different modes of deformation and stress states, and are meant to highlight the strengths and deficiencies of constitutive models. To model the forming behavior of the CP-Nb sheet, the material hardening at large strains is extracted from the uniaxial tension test, including the strain-rate effect. The plastic anisotropy of the CP-Nb sheet is modeled with the Yld2000-2D and Yld2004-3D non-quadratic yield functions. The friction coefficient of 0.25 is inversely identified from the deep-drawing experiments, by matching the predicted and measured punch force–displacement responses. While the features of the deep-drawing experiment are captured well using the constitutive framework described above, those of the stretch-forming experiment are not. Agreement with the latter is accomplished only after a targeted recalibration of the anisotropic yield function. This underscores the general conclusion that successful modeling of plastic deformation processes requires an accurately calibrated yield function which captures the dominant, process-specific stress states.

Nonlinear rheological models and their application to describe the mechanical behavior of highly oriented polymer materials

The nonlinear viscoelasticity of uniaxial oriented polymer materials is considered. To explain the deformation mechanisms of oriented polymers and the possibility of predicting their mechanical behavior in various operating modes, new nonlinear rheological models have been proposed. The application of the simplest rheological model of a real viscoelastic solid to the description and explanation of the recovery process in polymer materials is studied. From the standpoint of rheology, a model of an ideal viscoelastic solid is introduced. Using the balance equation for the number of transitions through energy barriers, a method for calculating the new nonlinear rheological model has been proposed. To eliminate the shortcomings of the ideal viscoelastic solid model associated with the impossibility of predicting creep and stress relaxation modes for long times, a generalized rheological model of a real viscoelastic solid was obtained. In the corrected model, the simplest elements are connected in parallel, which means the presence of not one, but several energy barriers in the materials, the transmission through which have their own relaxation times. To describe the recovery processes in polymer materials, the model of an ideal viscoelastic solid is supplemented by a parallel connected elastic spring. An additional Hooke spring replaces the interfibrillar interaction between the individual elements of the structure and is responsible for possible obstacles during jump-like transitions through the energy barrier. Using the method of rheological models and describing interfibrillar bonds within the framework of the theory of elasticity, a constitutive equation was obtained for the case of recovery processes. Based on the constitutive equation of viscoelasticity for uniaxial oriented polymer materials, a new nonlinear highly elastic element is introduced, which replaces the Maxwell’s element in the theory of linear viscoelasticity. A new rheological model of parallel connection of elastic elements is shown. An explanation of the retardation of the recovery process in polymers is given. The simplest rheological model of a real viscoelastic solid is proposed, in which an elastic spring is responsible for interfibrillar bonds. A constitutive equation is obtained that describes the recovery process in polymers. This equation can be integrated by quadratures and gives a solution that is an analogue of the Newton-Leibniz formula for the proposed model. It is shown that the deformation recovery process in polymer does not depend on the level of initial deformation and the loading method. The obtained result is confirmed by experimental data for polyamide and polyethylene film yarns. When specifying a certain initial level of deformation, generalized recovery curves of these materials are obtained. The proposed simplest rheological model of a real viscoelastic solid makes it possible to predict the reducing properties of polymer materials. And also makes it possible to determine the height of the energy barrier and the magnitude of the elastic modulus in the model. Based on the new rheological models, it is planned in the future to consider the issues of modeling and forecasting different modes of deformation.

A hyperelastic model to capture the mechanical behaviour and histological aspects of the soft tissues

It is well established that the soft connective tissues show a nonlinear elastic response that comes from their microstructural arrangement. Tissues' microstructure alters with various physiological conditions and may affect their mechanical responses. Therefore, the accurate prediction of tissue's mechanical response is crucial for clinical diagnosis and treatments. Thus, a physically motivated and mathematically simplified model is required for the accurate prediction of tissues' mechanical and structural responses. This study explored the ‘Exp-Ln’ hyperelastic model (Khajehsaeid et al., 2013) to capture soft tissues' mechanical and histological behaviour. In this work, uniaxial tensile test data for the belly and back pig skin were extracted from the experiments performed in our laboratory, whereas uniaxial test data for other soft tissues (human skin, tendon, ligament, and aorta) were extracted from the literature. The ‘Exp-Ln; and other hyperelastic models (e.g. Money Rivlin, Ogden, Yeoh, and Gent models) were fitted with these experimental data, and obtained results were compared between the models. These results show that the ‘Exp-Ln’ model could capture the mechanical behaviour of soft tissues more accurately than other hyperelastic models. This model was also found numerically stable for all modes and ranges of deformation. This study also investigated the link between ‘Exp-Ln’ material parameters and tissue's histological parameters. The histological parameters such as collagen content, fibre free length, crosslink density, and collagen arrangement were measured using staining and ATR-FTIR techniques. The material parameters were found statistically correlated with the histological parameters. Further, ‘Exp-Ln’ model was implemented in ABAQUS through the VUMAT subroutine, where the mechanical behaviour of various soft tissues was simulated for different modes of deformation. The finite element analysis results obtained using the ‘Exp-Ln’ model agreed with the experiments and were more accurate than other hyperelastic models. Overall, these results demonstrate the capability of ‘Exp-Ln’ model to predict the mechanical and structural responses of the soft tissues.

Proposal of a generic constitutive model for deformation-dependent dielectric constant of dielectric elastomers

Dielectric elastomers (DEs) are electroactive polymers that could be used as soft actuators, stretchable generators and sensors. Their performance is dependent on both their mechanical and dielectric properties. Particularly, the dielectric constant has been shown to be dependent on the state of mechanical stretch, which has to be comprehensively characterized across different modes of deformation and different classes of DEs. We conduct experiments to characterize the stretch-dependence of dielectric constant for three classes of elastomers: acrylic (VHB), silicone and natural rubber. In our characterization, we selected three distinct modes of mechanical deformation: equal-biaxial, unequal-biaxial and uniaxial. We reviewed the five most common stretch-dependent dielectric constant models and evaluated their fit with our experimental data which were based upon stretch-dependent dielectric constant of elastomers. Comparing all the model fittings, we find that, although the polarization-based lumped parameter model presents the best fit, both localized dipole relaxation and long chain polymer polarization were absent in the model. In view of the statistical long-chain dynamics in elastomers, we therefore propose a generic, semi-empirical inverse power-law relationship to generalize and standardize the characterization of stretch-dependence of dielectric constant along with log–log curve fitting. We hope to establish a consistent and general framework for dielectric characterization of dielectric elastomers.

Influence of pore-size distributions and pore-wall mechanics on the mechanical behavior of cellular solids like aerogels.

The pore-size distributions play a critical role in the determination of the properties of nanoporous cellular materials like aerogels. In this paper, we propose a micromechanical model, and by further designing artificial normal pore-size distributions, we inspect their effect on the macroscopic stress-strain curves. We show that the location of the mean pore size as well as the broadness of the distribution strongly affects the overall macroscopic behavior. Moreover, we also show that by using different damage criteria within the proposed model, the elastic, inelastic, and brittle nature of the macroscopic material can be captured. The damage criteria are based on the different modes of deformation in the pore walls, namely, elastic buckling, irreversible bending and brittle collapse under compression, and combined bending and stretching under tension. The proposed model approach serves as a reverse engineering tool to develop cellular solids with desired mechanical properties.

Humidity‐Responsive Liquid Crystalline Network Actuator Showing Synergistic Fluorescence Color Change Enabled by Aggregation Induced Emission Luminogen

AbstractImitating the structures and behaviors of natural creatures is of great significance to scientists to explore novel materials for practical applications. However, the design and fabrication of biomimetic devices with complex and outstanding performances is still on the way. A bilayer film composed of liquid crystalline networks (LCN) film and hydrochromic aggregation‐induced‐emission molecule‐doped hydrophilic layer is prepared. Under different relative humidity, the composite film can deform and change fluorescence color simultaneously. The influence of the content of the hydrophilic matrix on the fluorescent property and humidity‐responsive behaviors of the bilayer film is investigated. Thanks to the mechanical anisotropy provided by uniform‐aligned LCN film, different modes of deformation of the bilayer film are achievable, like bending, curling, and twisting. More importantly, due to the independence of the LCN film and the non‐mesogenic molecules brought by this bimorph strategy, complex alignment of the LCN film and modification by immiscible molecules are realized in a single LCN actuator. Based on the functional composite film, artificial flowers showing synergistic blooming and shape‐changing is prepared. By regulating the molecular alignment of the LC mesogens of the LCN film, the artificial flowers can imitate various blooming behaviors of natural flowers like confederate jasmine and jade lotus.

Different Modes of Deformation of Soft Triangular Honeycombs at the Sub-5nm Scale.

Patterning on the sub-5nm length scale is a contemporary challenge for further miniaturization of microelectronic circuits. Here, the first soft self-assembled triangular patterns are reported showing transitions between regular and two different kinds of isosceles (acute and obtuse angled) triangles on this length scale, formed by liquid crystalline honeycombs of polyphilic block molecules involving a fluorinated oligo(para-phenylene ethynylene) core. The type of formed triangular pattern depends on the degree and position of fluorination and on temperature. They are the first soft honeycombs combining tilted and nontilted organizations in a uniform nanostructure, where the tilted molecules in only one or two sides of the triangular prismatic cells dominate the shape and the size of the morphology.

Adhesion and Interface Properties of Polydopamine and Polytetrafluoroethylene Thin Films

Abstract Polytetrafluoroethylene (PTFE) has been studied as a low friction surface coating since its discovery. The high wear-rate of PTFE reduces the usefulness of the polymer for mechanical purposes; however, combining PTFE with polydopamine (PDA) has been shown to greatly reduce the film wear-rate. During rubbing tests involving PDA/PTFE thin films, a tenacious layer of PTFE remains intact after substantial testing even though pure PTFE film layers are destroyed quickly. Understanding the interface mechanics that allow PTFE and PDA to adhere so well during experimental rubbing tests is necessary to improve the wear-rate of PDA/PTFE thin films. In this study, we use density functional theory (DFT) and molecular dynamics (MD) simulations to investigate the adhesive properties and interface deformation mechanisms between PDA and PTFE molecules. Steered molecular dynamics (SMD) is then performed on isolated pairs of PDA and PTFE molecules to investigate different modes of deformation from equilibrium. PDA trimer oligomers were identified as the most adhesive to PTFE and selected to use in a PDA/PTFE thin film, where nano-indentation and scratch tests are performed. Our results indicate that a combination of the unique deformation mechanisms of PDA molecules and the penetration of PTFE molecules into the PDA substrate provide the PTFE/PDA interface with its wear resistance.

Validation of homogeneous anisotropic hardening model using non-linear strain path experiments

We conducted non-linear strain path experiments on a dual-phase steel-sheet sample. The first strain path in each of these experiments, called pre-strain, was always uniaxial tension in the direction transverse to the sheet (90° from the rolling direction). For this purpose, we clamped large tensile specimens in a suitable gripping system and deformed them to produce a sufficiently large area with a uniform strain distribution. We machined new specimens from this area, and we subjected them to different modes of deformation, namely uniaxial tension in different directions from the transverse direction, simple shear, and biaxial tension with different load ratios. In addition, we performed uniaxial tension-compression tests consisting of either one or three full cycles. We obtained the coefficients of a distortional plasticity model—the homogeneous anisotropic hardening (HAH) model—using an inverse analytical identification tool. First, we calibrated two sets of reverse loading coefficients using the tension-compression stress–strain curves obtained with either one or three full cycles. Then we used the 90° tension–45° tension and the 90° tension–0° simple-shear sequences to determine two sets of cross-loading coefficients. We compared the other independent experimental flow curves with simulations using the HAH model with combinations of the different sets of coefficients. These comparisons showed the influence of the selected input data on the calculations of the coefficients calibrated and demonstrated the advantages and limitations of the present HAH model.

The effects of particle shape on the yielding behaviour of crushable sand

Discrete element method simulations are used to investigate the effects of particle shape on the plastic behaviour of sand. This is achieved by using four different shapes of crushable particles. The behaviours of the different particles when subjected to isotropic compression, triaxial shearing, and a range of stress path tests are compared, revealing the important role of particle shape on key aspects of behaviour. Yield surfaces are established and analysed with respect to different modes of deformation, and are also shown to depend on the particle shape. A simple state boundary surface is then obtained, which is confirmed by a performing an additional stress path test on a sample that had been sheared to a critical state.

Deformation Maps for Bolted T-Stubs

Deformation maps on the plastic and ultimate failure of T-stubs with a single bolt-row in tension are developed. The maps condense a large body of information within a two-dimensional parameters space onto which different modes of deformation, including the régime boundaries, are plotted for any practical combination of geometric and material properties encountered in a T-stub. Its fidelity is demonstrated with experimental data from literature and through a detailed parametric investigation by high-fidelity finite-element analysis. The predictive capability of two existing analytical models is also compared against predictions by the finite-element model. Their shortcomings and applicability are critically assessed and discussed.

Role of isothermal omega phase precipitation on the mechanical behavior of a Ti-Mo-Al-Nb alloy

The formation and the evolution of the athermal and isothermal omega phase precipitates in a commercial metastable β–titanium alloy, β-21S, under controlled heat treatments, and its consequent impact on tensile properties has been presented in this paper. The structural and compositional changes of the omega precipitates have been tracked using SEM, TEM, and atom probe tomography (APT). Upon quenching from above the β-transus, the microstructure consisted of a single phase β without any discrete athermal omega precipitates and this condition exhibited the highest tensile ductility. Following β -solutionizing, when the samples were aged below the omega solvus temperature, there was an increase in the size scale of the omega precipitates with increasing time period. The larger and more solute-depleted omega precipitates, resulted in increased tensile strength and reduced ductility. The details of the deformation behavior were further analyzed using electron backscattered diffraction (EBSD) and TEM. Two different modes of deformation were noted in the system, depending on the initial microstructure. While the single phase condition (β solutionized sample) exhibited deformation occurring mainly via (110)β slip, the presence of larger isothermal omega precipitates in the aged condition (β solutionized + 350 °C/100H) resulted in the activation of not only (110) β slip but also (112) β slip. The reasonably high tensile ductility noted in this aged condition has been attributed to the activation of both these slip systems.

Joining sheets to tubes by annular sheet squeezing

This paper proposes a new joining by forming process to secure a sheet to a tube away from the tube end. The process is an alternative to welding, adhesive bonding, mechanical fastening or riveting and consists of squeezing the sheet adjacent to the outer tube radius, instead of applying direct loading on the tube itself as is customary in existing joining by forming processes. This process is cost effective, can be easily used on site and prevents cracking of the plastically deformed beads in tubular materials with low fracture toughness.The presentation combines experimentation and finite element modelling to identity the major process parameters, understand their influence in plastic flow and characterize the different modes of deformation that lead to acceptable and unacceptable joints. Utilization of a deformation-zone geometry parameter Δ, defined as the ratio of the sheet thickness to the cross-section recess length of the punch, is successfully utilized to characterize plastic flow inside the sheet thickness and to establish fundamental design rules. Appropriate values of the deformation-zone parameter in the range Δ=2−3 and placed in-between the limiting values corresponding to inhomogeneous and homogeneous deformation, revealed adequate to obtain sound sheet-tube connections. The maximum pull-out forces are sensitive to the direction of applied loading due to the non-symmetric shape of the joints but no detachments were observed for forces up to 2 kN.