Uniaxial Deformation Research Articles

A hygro-thermo-mechanical constitutive model for shape memory polymers filled with nano-carbon powder

ABSTRACT The nano-carbon powders are often used as fillers to endow the shape memory polymers (SMPs) with electroconductivity. It has been found that the shape memory effects (SMEs) of SMPs filled with nano-carbon powder can be triggered both by temperature and by water. To reveal the driving mechanism of SMEs, a constitutive model for describing the thermally activated and moisture activated SMEs of these shape memory polymer composite (SMPCs) is developed here. Because both of the SMEs share the same driving mechanism, the variable moisture is incorporated into the framework of a thermo-mechanical modeling approach to disclose the effect of moisture on the thermoviscoelastic properties. The SMPCs are regarded as isotropic materials and the effect of carbon powder on the mechanical properties of the matrix is also considered in the paper. Because the complete recovery may not be reached even they are exposed to the stimulus environment long enough, the blocking mechanism is also considered here. This is the mainly new contribution compared to the early work. Using the method of parameter determination presented here, the effectiveness of the proposed hygro-thermo-mechanical constitutive model is confirmed by comparing the model results with the test data of uniaxial deformation from the literature.

Plasticity in Amorphous Solids Is Mediated by Topological Defects in the Displacement Field

The microscopic mechanism by which amorphous solids yield plastically under an externally applied stress or deformation has remained elusive in spite of enormous research activity in recent years. Most approaches have attempted to identify atomic-scale structural "defects" or spatiotemporal correlations in the undeformed glass that may trigger plastic instability. In contrast, in this Letter we show that the topological defects that correlate with plastic instability can be identified, not in the static structure of the glass, but rather in the nonaffine displacement field under deformation. These dislocation-like topological defects (DTDs) can be quantitatively characterized in terms of Burgers circuits (and the resulting Burgers vectors) that are constructed on the microscopic nonaffine displacement field. We demonstrate that (i)DTDs are the manifestation of incompatibility of deformation in glasses as a result of violation of Cauchy-Born rules (nonaffinity); (ii)the resulting average Burgers vector displays peaks in correspondence of major plastic events, including a spectacular nonlocal peak at the yielding transition, which results from self-organization into shear bands due to the attractive interaction between antiparallel DTDs; and (iii)application of Schmid's law to the DTDs leads to prediction of shear bands at 45° for uniaxial deformations, as widely observed in experiments and simulations.

Uniaxial tensile deformation behavior of a sandwich-like structural TiNb-NiTi composite for biomedical applications

Uniaxial tensile deformation behavior of a sandwich-like structural TiNb-NiTi composite for biomedical applications

Comparison of the reactivity of isomeric 2H‐ and 3H‐naphthopyran mechanophores

AbstractNaphthopyrans are molecular switches that produce highly colored merocyanine dyes upon photochemical or mechanochemical activation in polymers. The mechanochromic behavior of these molecular force probes enables the straightforward visualization of stress and/or strain in materials. To date, research on the mechanochemistry of naphthopyran has largely focused on the 3H‐naphtho[2,1‐b]pyran (3H) scaffold, whereas isomeric 2H‐naphtho[1,2‐b]pyrans (2H) exhibit complementary properties as suggested from their photochemical reactivity. Here we directly compare the reactivity of two isomeric 2H‐ and 3H‐naphthopyran mechanophores in solution‐phase ultrasonication experiments and in crosslinked polydimethylsiloxane elastomers subjected to uniaxial tensile deformation. The prototypical 3H‐naphthopyran mechanophore produces a yellow merocyanine dye that reverts quickly, while the 2H‐naphthopyran mechanophore generates a red merocyanine dye that reverts significantly slower. The trends in absorption and reversion measured in solution are also reflected in solid polymeric materials activated in tension. Building on recent research into substituent effects, this study identifies naphthopyran isomerism as a simple lever for modulating the mechanochromic properties of the naphthopyran mechanophore used in the development of force‐responsive polymers.

Magnetic Alignment of Electrospun Fiber Segments Within a Hydrogel Composite Guides Cell Spreading and Migration Phenotype Switching.

Fibrous extracellular matrix (ECM) proteins provide mechanical structure and adhesive scaffolding to resident cells within stromal tissues. Aligned ECM fibers play an important role in directing morphogenetic processes, supporting mechanical loads, and facilitating cell migration. Various methods have been developed to align matrix fibers in purified biopolymer hydrogels, such as type I collagen, including flow-induced alignment, uniaxial tensile deformation, and magnetic particles. However, purified biopolymers have limited orthogonal tunability of biophysical cues including stiffness, fiber density, and fiber alignment. Here, we generate synthetic, cell-adhesive fiber segments of the same length-scale as stromal fibrous proteins through electrospinning. Superparamagnetic iron oxide nanoparticles (SPIONs) embedded in synthetic fiber segments enable magnetic field induced alignment of fibers within an amorphous bulk hydrogel. We find that SPION density and magnetic field strength jointly influence fiber alignment and identify conditions to control the degree of alignment. Tuning fiber length allowed the alignment of dense fibrous hydrogel composites without fiber entanglement or regional variation in the degree of alignment. Functionalization of fiber segments with cell adhesive peptides induced tendon fibroblasts to adopt a uniaxial morphology akin to within native tendon. Furthermore, we demonstrate the utility of this hydrogel composite to direct multicellular migration from MCF10A spheroids and find that fiber alignment prompts invading multicellular strands to separate into disconnected single cells and multicellular clusters. These magnetic fiber segments can be readily incorporated into other natural and synthetic hydrogels and aligned with inexpensive and easily accessible rare earth magnets, without the need for specialized equipment. 3D hydrogel composites where stiffness/crosslinking, fiber density, and fiber alignment can be orthogonally tuned may provide insights into morphogenetic and pathogenic processes that involve matrix fiber alignment and can enable systematic investigation of the individual contribution of each biophysical cue to cell behavior.

Mechanical Properties and Strain Transfer Behavior of Molybdenum Ditelluride (MoTe2) Thin Films

Abstract Transition metal dichalcogenides (TMDs) offer superior properties over conventional materials in many areas such as in electronic devices. In recent years, TMDs have been shown to display a phase switching mechanism under the application of external mechanical strain, making them exciting candidates for phase change transistors. Molybdenum ditelluride (MoTe2) is one such material that has been engineered as a strain-based phase change transistor. In this work, we explore various aspects of the mechanical properties of this material by a suite of computational and experimental approaches. First, we present parameterization of an interatomic potential for modeling monolayer as well as multilayered MoTe2 films. For generating the empirical potential parameter set, we fit results from density functional theory calculations using a random search algorithm known as particle swarm optimization. The potential closely predicts structural properties, elastic constants, and vibrational frequencies of MoTe2 indicating a reliable fit. Our simulated mechanical response matches earlier larger scale experimental nanoindentation results with excellent prediction of fracture points. Simulation of uniaxial tensile deformation by molecular dynamics shows the complete non-linear stress-strain response up to failure. Mechanical behavior, including failure properties, exhibits directional anisotropy due to the variation of bond alignments with crystal orientation. Furthermore, we show the deterioration of mechanical properties with increasing temperature. Finally, we present computational and experimental evidence of an extended c-axis strain transfer length in MoTe2 compared to TMDs with smaller chalcogen atoms.

Dynamic and static softening mechanisms of commercial-purity Zr during double-stage hot compressive deformation

The dynamic and static softening behavior of R60702 commercial-purity Zr was studied through single-stage and double-stage uniaxial hot deformation tests in the range of 600oC–800 °C. The microstructural evolution to uniaxial compression loading for the different conditions was investigated using a combination of optical microscopy and electron back-scattering diffraction (EBSD). The results show that the static softening fraction increases with an increase in temperature as well as holding time. At 600 °C, the peak stress of the second stage was greater than that of the single-stage, and the sample did not undergo static softening. At 800 °C, the peak stress of the second stage was less than that of the single-stage, and the sample underwent rapid static softening during holding time. As the holding time increased at 700 °C, the peak stress of the second stage of the sample decreased successively. At 700 °C and 1s holding time, the sample did not undergo static softening, and dynamic recovery and recrystallization occurred in the subsequent second stage. At 700 °C and 60s holding time, the sample underwent static recovery and recrystallization; larger grain growth and deformation appeared in the subsequent second stage. This change in the microstructural morphology with the flow stress curve is discussed in the subsequent sections.

On the issue of determination of the mechanism of ultrasonic velocity change in the process of uniaxial deformation of the rod

The molecular acoustics method is one of the types of non-destructive testing of materials during the operation of machines and mechanisms. Assessment of the performance of this or that material for further operation is carried out on the basis of changes in the ultrasonic velocity passing through the volume of the material. In this case, this assessment is carried out on the basis of a comparison of experimental data with data on the change in the ultrasonic velocity (UV) in a rod made of the same material subjected to uniaxial deformation. Thus, this method correlates the value of the ultrasonic velocity in the material, after a certain operating time as part of a machine or mechanism, with one or another area of deformation of the rod. They try to explain the changes in the ultrasonic velocity on the basis of the displacement of defects or the rotation of mesoscopic volumes in the material. However, the authors describing in their works the process, use expressions “most likely”, “probably”, which indicates that their explanations are an assumption, and not a strictly established mechanism for changing the ultrasonic velocity. This work shows that in the process of deformation, together with a change of the ultrasonic velocity, both the natural frequency of atomic vibration and the volume of the primitive cell change. The author proposes to estimate the change in the ultrasonic velocity during deformation on the basis of changes in the geometric parameters of the primitive cell in which ultrasound is transmitted along the edges of the primitive cells through the interaction of electron shells of atoms.

Effect of heating temperatures on AlSi coating microstructure and fracture during hot-tensile tests

In this article, the fracture behavior of different AlSi coating micro-structures is investigated. By changing the heating temperature, different AlSi coating micro-structures are obtained, due to varying diffusivity of iron. To study the fracture behavior of different coating micro-structures, uniaxial tensile tests are conducted at 700 °C. The hot tensile test involves heating the as-coated press hardening steel in a furnace to heating temperatures of 750, 920 and 1000 °C for 6 minutes of dwell time, after which the sample is cooled to the deformation stage at 700 °C. In this stage, the samples are uniaxially deformed for a fixed 20% macroscopic strain followed by cooling to room temperature. After the test, the coating micro-structure and fracture pattern are inspected under the microscope. For samples heated to 750 and 920 °C, coating fracture is observed; however, for samples heated to 1000 °C, no coating fracture is observed during the tensile deformation at 700 °C. The AlSi coating micro-structure, after heating at 1000 °C becomes sufficiently ductile to withstand 20% strain at 700 °C. The same micro-structure, however, fractures during uniaxial tensile deformation at 600 °C.

Maximum shear stress-controlled uniaxial tensile deformation and fracture mechanisms and constitutive relations of Sn–Pb eutectic alloy at cryogenic temperatures

Sn–Pb eutectic alloy is widely applied to the deep-space electronics that work under cryogenic environments, but little is known about its responses to the mechanical loading at cryogenic temperatures. In this work, a comprehensive investigation about the deformation and fracture behaviors as well as constitutive relations of the eutectic alloy at cryogenic temperatures was conducted through uniaxial tensile experiments over a full temperature range from 293 K to 77 K, in-situ cryogenic tensile experiments and fractography. With the declining temperature, the tensile strength and quasi-static toughness increase substantially while the elongation is maintained at 25%–30%; Sn–Pb eutectic alloy achieves the optimal combinations of strength, ductility and toughness at 123 K, which is due to a number of deformation twins activated in the Sn matrix together with the compatible deformation between the Pb-rich phases and the Sn matrix. Multiple 45° shear bands induced by the maximum shear stress contribute to reconcile the deformation difference between the two phases, which reaches its minimum at around 123 K. A ductile shear fracture on the 45° shear planes with respect to the tensile axis occurs under the maximum shear stress at temperatures ranging from 293 K to 123 K, while the maximum normal stress leads to a brittle fracture on the 90° planes at 77 K. Moreover, the Anand model fails in fitting the constitutive relations of Sn–Pb eutectic alloy when the temperature declines to 233 K or lower. Instead, the Hollomon equation has been successfully applied to fit the constitutive relations at these invalid temperatures.

Thermo-Mechanical Behavior of Poly(ether ether ketone): Experiments and Modeling.

Observations are reported on poly(ether ether ketone) (PEEK) in uniaxial tensile tests, relaxation tests and creep tests with various stresses in a wide interval of temperatures ranging from room temperature to 180 °C. Constitutive equations are developed for the thermo–mechanical behavior of PEEK under uniaxial deformation. Adjustable parameters in the governing equations are found by matching the experimental data. Good agreement is demonstrated between the observations and results of numerical simulation. It is shown that the activation energies for the elastoplastic, viscoelastic and viscoelastoplastic responses adopt similar values at temperatures above the glass transition point.

Uniaxial Compression Creep Characteristics and Acoustic Emission Characteristics of Two Different Kinds of Red Sandstone with Different Particle Sizes

In this study, we tested uniaxial compression deformation and uniaxial compression creep on red sandstone specimens with fine- and medium-sized particles; microstructure development and acoustic emission (AE) were recorded. The creep deformation characteristics and AE parameters of these two kinds of sandstone specimens were then compared and analysed. Results show that the microstructure of fine-grained sandstone is the granular sand-like structure, and the medium-grained sandstone has a grain-like microstructure. The particle size obviously influences uniaxial compressive mechanism; the medium-grained sandstone has a higher creep deformation than the fine-grained sandstone, while the AE amplitude, AE event rate and AE ring-down count rate of the medium- and fine-grained sandstone specimens have similar trend during the creep stage. The failure of both medium- and fine-grained sandstone mainly occurs in the transient and accelerated creep stages during the creep process. By combining the variation of AE parameters and AE-r value (the ratio of the accumulative hits and the cumulative energy) in the various creep stages, we found the creep damage to sandstone could be predicted to a certain extent.

Numerical Analysis of Strain Rate Dependency on Activities of Slip Systems in Polycrystalline α-Ti

Uniaxial tensile deformation of single-crystalline and polycrystalline α-Ti models was numerically simulated with strain rates ranging from 10-4 to 10-1 [/s] using a crystal plasticity finite element method, and we investigated the relationship between the activities of the basal and prismatic <a> slip systems and the critical resolved shear stresses (CRSSs) depending on the strain rates. The simulation of single-crystalline model showed that CRSS of prismatic <a> slip system was easier to increase than that of basal slip system although CRSS of both slip systems increased with strain rates. The nature of strain rate dependency affected the deformation mechanism of polycrystalline model: the simulation of polycrystalline model showed that the activity of basal slip system increased locally with strain rates, instead of reducing that of prismatic <a> slip system. However, the local activation of basal slip system was inhibited when 1st pyramidal <a> slip system was activated. These results indicate that the local activation of basal slip system can occur by strain redistribution between preferred regions for basal and prismatic <a> slips with strain rates while activation of another slip system can inhibit that of the basal slip system.

Dynamic Softening Mechanisms and Microstructure Evolution of TB18 Titanium Alloy during Uniaxial Hot Deformation

In this study, isothermal compression tests of TB18 titanium alloy were conducted using a Gleeble 3800 thermomechanical simulator for temperatures ranging from 650 to 880 °C and strain rates ranging from 0.001 to 10 s−1, with a constant height reduction of 60%, to investigate the dynamic softening mechanisms and hot workability of TB18 alloy. The results showed that the flow stress significantly decreased with an increasing deformation temperature and decreasing strain rate, which was affected by the competition between work hardening and dynamic softening. The hyperbolic sine Arrhenius-type constitutive equation was established, and the deformation activation energy was calculated to be 303.91 kJ·mol−1 in the (α + β) phase zone and 212.813 kJ·mol−1 in the β phase zone. The processing map constructed at a true strain of 0.9 exhibited stability and instability regions under the tested deformation conditions. The microstructure characteristics demonstrated that in the stability region, the dominant restoration and flow-softening mechanisms were the dynamic recovery of β phase and dynamic globularization of α grains below transus temperature, as well as the dynamic recovery and continuous dynamic recrystallization of β grains above transus temperature. In the instability region, the dynamic softening mechanism was flow localization in the form of a shear band and a deformation band caused by adiabatic heating.

Uncertainty Quantification Guided Parameter Selection in a Fully Coupled Molecular Dynamics-Finite Element Model of the Mechanical Behavior of Polymers.

The objective of investigating macroscopic polymer properties with a low computing cost and a high resolution has led to the development of efficient hybrid simulation tools. Systems generated from such simulation tools can fail in service if the effect of uncertainty of model inputs on its outputs is not accounted for. This work focuses on quantifying the effect of parametric uncertainty in our coarse-grained molecular dynamics-finite element coupling approach using uncertainty quantification. We consider uniaxial deformation simulations of a polystyrene sample at T = 100 K in our study. Parametric uncertainty is assumed to originate from parameters in the molecular dynamics model with a nonperiodic boundary (the force constant between polymer beads and anchor points, the number of anchor points, and the size of the surrounding dissipative particle dynamics domain) and a parameter to blend the energies of particles and continuum (weighting factor). Key issues that arise in uncertainty quantification are discussed on the basis of the quantities of interest including mass density, end-to-end distance, and radial distribution function. This work reveals the influence of key input parameters on the properties of polymer structure and facilitates the determination of those parameters in the application of this hybrid molecular dynamics-finite element approach.

Visualization of the Necking Initiation and Propagation Processes during Uniaxial Tensile Deformation of Crystalline Polymer Films via the Generation of Fluorescent Radicals.

To visualize and simultaneously quantify the necking behavior of crystalline polymer films during uniaxial stretching, tetraarylsuccinonitrile (TASN) moieties were introduced into polymers at the center of the main chain. TASN can produce a relatively stable radical that emits yellow fluorescence in response to mechanical stress. During the uniaxial elongation test of the TASN-centered crystalline polymers, the yellow fluorescence derived from the dissociated TASN radicals was used for microscale observations that showed the orientation of the polymer chains in the stretching direction. Furthermore, by comparing the radical generation in linear and star-shaped TASN-centered crystalline polymers during their tensile deformation, we found that the TASN dissociation ratio is higher in the star-shaped polymer, which has more chains connected to the lamellar crystal. Thus, the microforces generated in the amorphous region during uniaxial stretching were probed via the use of TASN, which allowed a direct visualization of the necking initiation and propagation processes as well as a quantification via electron paramagnetic resonance spectroscopy.

Structural-Mechanical Properties of Polyurethane Surface after Carbon Ion Subplantation

Ion-plasma modification of polymers has many potential applications, in particular, in the development of biomedical products. Treatment of soft polymers can easily damage the surface; low-energy plasma and subsequent investigation of the structural and mechanical properties of the surface are required. Polyurethane is a widely used block copolymer. Subplantation of carbon ions heterogeneously changes the structural and mechanical properties of the surface (relief, stiffness, thickness of the modified coating), forming a graphene-like nanolayer. Uniaxial deformation of the treated materials in some cases leads to the damage of the surface (local nanocracks, folds). Materials have increased hydrophobicity, good deformability (valid for certain treatment regimes) and can find application in design of products with improved biomedical properties.

Structural Basis for the Different Mechanical Behaviors of Two Chemically Analogous, Carbohydrate-Derived Thermosets.

Two renewable, structurally analogous monomers, isosorbide undecenoate (IU) and glucarodilactone undecenoate (GDLU) reacted with pentaerythritol tetrakis(3-mercaptopropionate) (PETT) via thiol-ene photopolymerization to form IU-PETT and GDLU-PETT thermosets. Despite their chemical similarity, uniaxial tensile testing showed that GDLU-PETT exhibited a strain-hardening behavior and is significantly tougher than IU-PETT. To understand this observation, in situ tensile testing and wide-angle X-ray scattering experiments (WAXS) were conducted. While the 2D WAXS patterns of IU-PETT displayed an isotropic halo during uniaxial deformation, they exhibited a change from an isotropic halo to a pair of scattering arcs for the GDLU-PETT samples. Density functional theory calculations further revealed that the GDLU alkyl chains are less angled than the IU alkyl chains. Based on these results, we postulate that the GDLU molecules can more easily order and align during uniaxial deformation, hence increasing intermolecular interactions between the GDLU molecules and contributing to the observed strain hardening behavior of their thermosets. This study exemplifies how molecules with subtle differences in their chemical structures can alter the structures and thermophysical properties of the resulting polymers in unpredictable ways.

A viscoelastic-viscoplastic constitutive model for glassy polymers informed by molecular dynamics simulations

This contribution presents a phenomenological viscoelastic-viscoplastic constitutive model informed by coarse-grained (CG) molecular dynamics (MD) simulations of pure glassy polystyrene (PS). In contrast to experiments, where viscoplasticity is caused by various effects simultaneously, these effects can be decomposed in MD simulations by adjusting the MD system. In the MD simulations considered here, neither bond breakage nor cross-links are introduced; instead, we focus on the intermolecular interaction of polymer chains. We employ a thermo-dynamically consistent generalized Maxwell framework parallelly comprising an elastic, a viscoelastic, and several elasto-viscoplastic modules with different yield stress to capture the viscoelastic and the viscoplastic mechanical behavior simultaneously. The yield stresses decrease with the maximum deformation the MD system has experienced. The constitutive model presented here is based on 10 material parameters, which can be identified by a few data sets and fits well the CG MD simulations of PS under uniaxial and biaxial deformation with time-proportional and cyclic loading conditions in a wide range of strain rates (0.1%/ns–20%/ns).

Density functional theory study on the influence of tension and compression deformation on the electrical and phonon properties of monolayer and bilayer graphene.

Based on first-principles calculations using density functional theory, this paper systematically studies the effects of uniaxial tension-compression deformation on the stability and electrical and thermal properties of monolayer graphene and AA stacked bilayer graphene. The study shows that the original symmetry of graphene is broken by the tensile and compression deformations, catalyzing the interlayer coupling of bilayer graphene. Its electronic energy band, phonon dispersion, and other physical properties have changed. The transition from metalloid to semiconductor has completed since the deformation weakens the stability of the graphene system to varying degrees and opens the band gap of monolayer graphene. The band gap becomes larger with the increase of tensile and compressive deformation, in which way it can be adjusted. Influenced by the tiny tensile deformation, metalloid properties are exhibited by a small band gap of intrinsic AA-stacked bilayer graphene, and then the band gap becomes larger as the deformation increases. A band gap appears in the system phonon dispersion curves when the compression deformation increases to -15%. The phonon mode softens and shows virtual frequency. The value of virtual frequency increases with the increase of compression deformation. At the very moment, the vibration mode is discontinuous, and the system is unstable.