Tensile Stress-strain Measurements Research Articles

Interactions of hydrophilic birch wood (Betula pendulaRoth) extractives with adhesives for load-bearing timber structures

This study investigated how water-soluble extractives of silver birch (Betula pendulaRoth) wood interact with the most common adhesives used for producing engineered wood products (EWPs), i.e., melamine-urea-formaldehyde (MUF) and one-component polyurethanes (PUR). Therefore, the extractives were characterized via various chromatographic techniques and mass spectrometry. The effects of extractives on the curing kinetics were investigated using rheometry. The impact of extractives on the chemical and mechanical properties of the cured adhesives was investigated with FTIR spectrophotometry and tensile stress-strain measurements, respectively. Moreover, a comparative study on the shear strength of birch wood with and without extractives was performed.The organic fraction of the water extractives of birch mainly consisted of phenolic glycosides, carboxylic (fatty) acids, and saccharides. The extractives decelerated the curing process of MUF, as observed in a rheological small-angle oscillatory shear experiment using wood substrates as the lower plate. This was indicated by a reduced gel time from 5 to 3.8 h on pristine vs. extracted wood. When measuring MUF on extracted wood, this deceleration was also reached by adding ∼1% (W/W) of the isolated extractives into the MUF resin. Similar experiments with PUR showed a slight acceleration when dispersing extract into the adhesive, indicated by a reduction of vitrification time of −0.9 ± 0.3 h per percent of extractives added. However, measurements with pure PUR on pristine and extracted wood showed no significant difference in curing kinetics due to the extractives’ limited mobility and solubility in PUR. Both PUR and MUF cured adhesive films showed a reduced stiffness, elastic stress limit and tensile strength in uniaxial tensile stress-strain measurements of adhesive films upon increasing the amount of hydrophilic birch extractives concentration. Comparative standard tensile shear strength measurements on pristine and extracted wood indicate a 7–19% strength increase due to extraction when bonding with MUF, while the bond strength of PUR bond lines was less affected by the extraction procedure.

A Rapid Method to Noninvasively Measure the Viscoelastic Properties of Synthetic Polymers Using Mechanical Vibrations and Photonics

Noninvasive measurement of the viscoelastic properties of both natural and synthetic polymers is important for the analysis of implant design and performance as well as in industrial material development. In this study, we used vibrational optical coherence tomography (VOCT) to compare the elastic and viscoelastic properties of silicone polymers with standard tensile stress–strain measurements. VOCT uses acoustic vibrations and infrared light to measure the resonant frequency of viscoelastic materials. The elastic modulus was calculated from the in-phase deformation of the material at fixed frequencies using an empirical calibration curve. Viscous loss was measured after pulsing the samples based on the ratio of mechanovibrational peak widths to heights. The results showed that the optimal cure time and modulus values obtained using VOCT were like those obtained using conventional tensile testing. VOCT could capture results that were comparable to conventional testing while not destroying the material, suggesting its usefulness for in vivo and in situ measurements as well as for early quality control environments during end-use application and fabrication experiments. We conclude that VOCT is a new technique that is comparable to conventional testing for noninvasively and nondestructively measuring the viscoelastic properties of polymers.

Mill Processability, Rheology and Structure Differences of Polyacrylate Elastomers

Abstract Two commercial polyacrylate elastomers, one having epoxide (EP) crosslinking sites and the other ethylidene norbornene (ENB) were selected. It was demonstrated that the former was more difficult to mill-process than the latter. The viscoelastic properties measured at very small strain in shear oscillation indicated that the ENB polymer was more branched than the EP polymer. The gel determination by filteration confirmed this. The tensile stress-strain measurements were conducted at four different deformation rates. These data were treated according to the strain-time correspondence principle to see if this principle applies to the data of these samples. The data of EP polymer did not form a master curve, thus, the above principle is not applicable; at the larger deformation the strain-hardening tendencies were observed. This indicates that the EP polymer has a rather extensive long branching, a type of which usually results from a chain transfer reaction. The data of the ENB polymer, on the other hand, form a master-curve; that is, the strain-time correspondence principle is applicable, in spite of the presence of the large amount of gel. This means that the gel in the ENB polymer is microgel. The micro-gel in this case is polymer-latex particle, crosslinked during emulsion polymerization with a difunctional comonomer. Thus, in this work polymer rheology was effectively used to find a major difference in the polymer chain structure. In this way it was shown that the results of polymerization may be related to mill processability.

Fabrication of polyurethane porous composite films using biomass-based Juncus effusus fibers for oil removal from water

Oil spills cause serious ecological threats to aquatic life. High-performance polymer composite films using biomass fibers for rapidly removing oil from wastewater are exceptionally attractive due to sustainable development and high efficiency. Herein, we report a novel polymer composite film composed of polyurethane (PU) and biomass-based Juncus effusus (JE), which was fabricated using the nonsolvent induced phase separation method. The Juncus effusus/polyurethane (JEPU) composite films exhibited a highly controllable porous structure with the addition of JE powders. Owing to the strong hydrogen-bond interactions between biomass JE and PU matrix, the JEPU composite films showed good interface compatibility and enhanced mechanical property. Tensile stress-strain measurements demonstrated that the 2%JEPU showed tensile strength and strain of 1.14 MPa and 280.61%, respectively. Furthermore, the JEPU composites possess desirable hydrophobicity/oleophilicity with a water contact angle of 127°, exhibiting a good oil absorption capacity. The developed JEPU composite films may broaden the application range of industrial crops and products in eliminating oil contaminants from water.

Non-linear elastic behavior of Ni-Fe-Ga(Co) shape memory alloy and Landau-energy landscape reconstruction

We study the elastic behavior of single crystals of the Ni-Fe-Ga(Co) ferromagnetic shape memory alloy in a broad temperature range. Using tensile stress-strain measurements and ultrasonic characterization, we show that austenite and martensite phases of the given alloy exhibit strongly non-linear elastic responses, and we utilize the non-linear stress-strain data for constructing an energy function describing the observed behavior within the framework of the 4th-order Landau theory. The resulting energy landscape is shown to capture realistically several key features of the examined alloy, such as the existence of the critical point indicated by the extrapolation of the stress-strain data or the value and temperature dependence of the transformation strain. We provide explicit formulas for the energy landscape, using linear and quadratic approximations to express the temperature evolution of the Landau-energy parameters.

PH-responsive cellulose\u2013chitosan nanocomposite films with slow release of chitosan

Cellulose–chitosan films were prepared using a physical method in which cellulose and chitosan were separately dissolved via freeze thawing in LiOH/urea and mixed in different proportions, the resulting films being cast and regenerated in water/ethanol. X-ray diffraction and Fourier transform infrared spectroscopy (FT-IR) spectroscopy verified the composition changes in the nanocomposites due to different mixing ratios between the polymers. Tensile stress–strain measurements indicated that the mechanical performance of the cellulose–chitosan nanocomposites slightly worsened with increasing chitosan content compared with that of films comprising cellulose alone. Field emission scanning electron microscopy revealed the spontaneous formation of nanofibers in the films; these nanofibers were subsequently ordered into lamellar structures. Water uptake and microscopy analysis of film thickness changes indicated that the swelling dramatically increased at lower pH and with increasing chitosan content, this being ascribed to the Gibbs–Donnan effect. Slow material loss appeared at acidic pH, as indicated by a loss of weight, and quantitative FT-IR analysis confirmed that chitosan was the main component released. A sample containing 75% chitosan reached a maximum swelling ratio and weight loss of 1500% and 55 wt%, respectively, after 12 h at pH 3. The study presents a novel way of preparing pH-responsive cellulose–chitosan nanocomposites with slow-release characteristics using an environmentally friendly procedure and without any chemical reactions.Graphical abstract

Improved experimental and diagnostic techniques for dynamic tensile stress–strain measurement with a Kolsky tension bar

Kolsky tension bar experiments were improved for dynamic tensile stress–strain measurements with higher fidelity and minimal uncertainties. The difficulties associated with specimen gripping, relatively short gage section, and geometric discontinuity at the bar ends all compromise the accuracy of the traditional strain measurement method in a Kolsky tension bar experiment. In this study, an improved three-channel splitting-beam laser extensometer technique was developed to directly and independently track the displacement of the incident and transmission bar interfaces. By adopting a dual-channel configuration on the incident bar side, the resolution and measurement range of this laser extensometer were coordinated between the two channels to provide highly precise measurement at both small and large strains under high strain-rate loading condition. On the transmission bar side an amplified channel, similar to that used on the incident bar side, was adopted to measure the transmission bar displacement with high resolution. With this novel design, a maximum resolution of approximately 500 nm can be obtained for the bar displacement measurement, which corresponds to a strain of 0.0079% for a specimen with 6.35 mm gage length. To further improve the accuracy, a pair of lock nuts were used to tighten the tensile specimen to the bars in an effort not only to prevent the specimen from potential pre-torsional deformation and damage during installation, but also to provide better thread engagement between the specimen and the bar ends. As a demonstration of this technique, dynamic tensile stress–strain response of a 304L stainless steel was characterized with high resolution in both elastic and plastic deformations.

Morphomechanics of dermis-A method for non-destructive testing of collagenous tissues.

Collagenous tissues store, transmit and dissipate elastic energy during mechanical deformation. In skin, mechanical energy is stored during loading and then is dissipated, which protects skin from mechanical failure. Thus, energy storage (elastic properties) and dissipation (viscous properties) are important characteristics of extracellular matrices (ECMs) that support the cyclic loading of ECMs without tissue failure. Uniaxial stress-strain measurements on decellularized human dermis have been made and compared to results of a non-destructive technique involving optical coherence tomography (OCT) combined with vibrational analysis. In addition, Poisson's ratio has been determined for tensile deformation of decellularized dermis. The modulus of decellularized dermis measured using standard tensile stress-strain tests and that determined from calculations derived from natural frequency measurements give similar results. It is also observed that Poisson's ratio for dermis is between 0.38 and 0.63 after correction for changes in volume that occur during tensile deformation. These results suggest that the assumption that dermis and other ECMs deform at constant volume is incorrect and will lead to differences in the calculated modulus by conventional tensile stress-strain measurements. It is proposed that OCT in conjunction with vibrational analysis is a convenient way to non-destructively measure the modulus of decellularized dermis, ECMs and other materials that have a positive curvature to their stress-strain curves. Tensile deformation of dermis and possibly other ECMs is associated with an increase in Poisson's ratio consistent with a model of fluid expulsion from collagen fibrils during stretching. The value of Poisson's ratio should be considered in analyzing the mechanical properties of ECMs since at least dermis appears to be compressible during tensile deformation. Fluid expression during tensile deformation may play a role in mechanotransduction in skin in a similar manner to cartilage and bone tissue.

A method for nondestructive mechanical testing of tissues and implants.

Numerous tests have been used to elucidate mechanical properties of tissues and implants including tensile, compressive, shear, hydrostatic compression, and three-point bending in one or more axial directions. The development of a nondestructive test that could be applied to tissues and materials in vivo would promote the analysis of tissue pathology as well as the design of implant materials. The purpose of this article is to present the results of preliminary studies demonstrating nondestructive in vitro testing of a tissue model, decellularized human dermis, and a model implant, silicone rubber, using a combination of optical coherence tomography (OCT), and vibrational analysis. The results presented suggest that nondestructive vibrational testing of tissues and materials can be used to determine the modulus of polymeric materials and the results are similar to those found using tensile stress-strain measurements. The advantage of this method is that the modulus can be obtained from vibrational methods without having to approximate the tangent to the stress-strain curve, which is difficult for nonlinear materials that have a rapidly changing slope. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 15-22, 2017.

Large tensile superelasticity from intermartensitic transformations in Ni49Mn28Ga23 single crystal

A multistep superelastic behavior, with up to a 12% strain, is reported in a 〈001〉P-oriented Ni49Mn28Ga23 single crystal. The observed behavior is produced by intermartensitic transformations during the tensile stress–strain measurements at temperatures between −140 °C and +60 °C. The tensile stress–temperature phase diagram and the stress dependence of the intermartensitic transformation entropies have been obtained. These results provide important input for theoretical modeling of the phase transformations in these alloys and show promising mechanical properties of the classical Ni-Mn-Ga ferromagnetic shape memory alloys.

Tuning the modulus of nanostructured ionomer films of core-shell nanoparticles based on poly(n-butyl acrylate).

In this study we investigate the structure-mechanical property relationships for nanostructured ionomer films containing ionically crosslinked core-shell polymer nanoparticles based on poly(n-butyl acrylate) (PBA). Whilst nanostructured ionomer films of core-shell nanoparticles have been previously shown to have good ductility [Soft Matter, 2014, 10, 4725], the modulus values were modest. Here, we used BA as the primary monomer to construct core-shell nanoparticles that provided films containing nanostructured polymers with much higher glass transition temperature (Tg) values. The core-shell nanoparticles were synthesised using BA, acrylonitrile (AN), methacrylic acid (MAA) and 1,4-butanediol diacrylate (BDDA). Nanostructured ionomer films were prepared by casting aqueous core-shell nanoparticle dispersions in which the shell -COOH groups were neutralised with KOH and ZnO. The film mechanical properties were studied using dynamic mechanical analysis and tensile stress-strain measurements. The use of BA-based nanoparticles increased the Tg values to close to room temperature which caused a strong dependence of the film mechanical properties on the AN content and extent of neutralisation of the -COOH groups. The Young's modulus values for the films ranged from 1.0 to 86.0 MPa. The latter is the highest modulus reported for cast films of nanostructured ionomer films prepared from core-shell nanoparticles. The films had good ductility with strain-at-break values of at least 200%. The mechanical properties of the films were successfully modelled using the isostrain model. From comparison with an earlier butadiene-based system this study demonstrates that the nature of the primary monomer used to construct the nanoparticles can profoundly change the film mechanical properties. The aqueous nanoparticle dispersion approach used here provides a simple and versatile method to prepare high modulus elastomer films with tuneable mechanical properties.

Preparation of Nafion 117™-SnO2 composite membranes using an ion-exchange method

Nafion 117™-SnO2 composite membranes were prepared by in-situ particle formation using an ion-exchange method. SnO2 was incorporated into Nafion 117™ membranes by ion-exchange in solutions of SnCl2. 2 H2O in methanol, followed by oxidation to SnO2 in air. By adjustment of the concentration of SnCl2 ∙ 2 H2O used in the ion-exchange step, compositions ranging from 2 to 8wt.% SnO2 with SnO2 homogeneously distributed as nanoparticles were obtained.The prepared nanocomposite membranes were characterized by powder XRD, 119Sn MAS NMR spectroscopy, electrochemical impedance spectroscopy, water uptake, tensile stress–strain measurements and thermogravimetric analysis. A SnO2 content around 4wt.% gave a significant increase in membrane stiffness without losses in conductivity.

Elastomeric electrospun scaffolds of poly(l-lactide-co-trimethylene carbonate) for myocardial tissue engineering

In myocardial tissue engineering the use of synthetically bioengineered flexible patches implanted in the infarcted area is considered one of the promising strategy for cardiac repair. In this work the potentialities of a biomimetic electrospun scaffold made of a commercial copolymer of (L)-lactic acid with trimethylene carbonate (P(L)LA-co-TMC) are investigated in comparison to electrospun poly(L)lactic acid. The P(L)LA-co-TMC scaffold used in this work is a glassy rigid material at room temperature while it is a rubbery soft material at 37 °C. Mechanical characterization results (tensile stress-strain and creep-recovery measurements) show that at 37 °C electrospun P(L)LA-co-TMC displays an elastic modulus of around 20 MPa and the ability to completely recover up to 10% of deformation. Cell culture experiments show that P(L)LA-co-TMC scaffold promotes cardiomyocyte proliferation and efficiently preserve cell morphology, without hampering expression of sarcomeric alpha actinin marker, thus demonstrating its potentialities as synthetic biomaterial for myocardial tissue engineering.

Synthesis and characterization of novel THTPBA/PEG-derived polyurethane scaffolds for tissue engineering

Novel polyurethane (PU) scaffold materials were designed and prepared on the basis of a coupling reaction between tetra-hydroxyl-terminated poly(butadiene-co-acrylonitrile) prepolymer (THTPBA) and poly(ethylene glycol) (PEG) via 1,6-hexamethylene diisocyanate as anchor molecule. The hydrophilicity, degradability, mechanical, and biomedical properties of the THTPBA/PEG PU materials were scrutinized by swelling and goniometry, FTIR and gravimetry methods, tensile stress–strain measurements and hemolysis, platelet activation, dynamic (erythrocyte aggregation) and static coagulation as well as MTT assays. The experimental results indicated that the hydrophilicity and mass loss were enhanced with increased concentrations and molecular weight (MW) of PEG. The degradation may be attributable to the cleavage of urethane or ester bonds in polymer chains. The in vitro blood compatibility and MTT cytotoxicity investigations elicited that the MW of PEG and mass ratios of THTPBA to PEG had important influence on the biomedical properties. The tensile stress–strain investigations showed that the highly crosslinked architecture offered high elastic modulus and mechanical strength. The PU scaffolds with proper component ratios and MW of PEG exhibited improved mechanical properties and biocompatibility as well as low toxicity, and can be employed as potential candidates for blood-contacting applications.

Microstructure and shape memory effect of Ti–20Zr–10Nb alloy

The microstructure, martensitic transformation behavior and shape memory effect of Ti–20Zr–10Nb shape memory alloy have been studied by X-ray diffraction (XRD), optical microscopy (OM) and transmission electron microscopy (TEM) observation, differential scanning calorimetry (DSC) and tensile stress–strain measurements. The results show that the recrystallization occurs in the cold rolled Ti–20Zr–10Nb alloy by annealing at 600 °C and the grain size goes up with the increasing annealing temperature up to 800 °C. The Ti–20Zr–10Nb alloy is primarily composed of α″ martensite and a small amount of ω phase which appears after annealing. A reverse martensite transformation temperature higher than 500 °C upon heating has been detected for the Ti–20Zr–10Nb alloy annealed at 600 °C, but no obvious exothermic behavior can be found upon cooling. The tensile strength and the failure strain of the alloy are measured to be 542 MPa and 13.1%, respectively, associated with a maximum shape recovery strain of about 2.5%.

A study on NiTiNbCo shape memory alloy

The influence of Co on the microstructure, phase transformation temperature, mechanical properties and shape memory effect of TiNiNb alloys has been investigated by X-ray diffraction, differential scanning calorimetry, tensile stress–strain measurements and bending tests, respectively. The results show that the addition of Co suppresses the brittle (Ti,Nb) 2Ni phase of NiTiNb alloys, and the phase transformation temperature and lattice parameter of Ni 46.5− x Ti 44.5Nb 9Co x alloys are decreased with the increase of Co content. The yield strength of NiTiNb alloys has been improved due to the addition of Co. In particular, Ni 45.5Ti 44.5Nb 9Co 1 alloy exhibits highest yield strength among the Ni 46.5− x Ti 44.5Nb 9Co x (Co = 0.5, 1, 1.5, 2) alloys, and a maximum completely recoverable strain of 7.8% has been achieved.

Nonfouling biomaterials based on polyethylene oxide‐containing amphiphilic triblock copolymers as surface modifying additives: Solid state structure of PEO‐copolymer/polyurethane blends

Three novel polyethylene oxide-containing amphiphilic triblock copolymers (PEO-PU-PEO) with PEO MWs 550, 2000, and 5000, were blended with a segmented polyurethane (PU). It was expected that the block copolymers would act as surface modifiers to produce surfaces rich in PEO. The solid state properties of the PEO-copolymer/PU blends were studied by infrared spectroscopy, differential scanning calorimetry, and tensile stress-strain measurements. Infrared analysis showed no significant hydrogen bonding between the PEO blocks of the copolymers and the PU matrix. Differential scanning calorimetry data indicated that for copolymer content up to 5 wt % the microphase structure of the blends was indistinguishable from that of the unmodified PU matrix; for copolymer content of 10% or greater, the blends showed phase separated structures. Similarly the tensile stress-strain properties of the blends were essentially the same as those of the matrix up to 5 wt % copolymer. At higher copolymer content, however, the tensile strength decreased with increasing content of the copolymers; for a given copolymer content the change in tensile properties increased with increasing PEO MW. The structures of the 20% blends were also investigated after extraction with toluene (copolymers soluble, matrix insoluble). Bulk compositional change upon extraction was determined by nuclear magnetic resonance spectroscopy. Surface compositional change was studied by X-ray photoelectron spectroscopy and water contact angles. Surface morphology was observed using scanning electron microscopy and atomic force microscopy. It was shown that the copolymers were removed from the blends by extraction and that the extent of removal increased with decreasing MW of the PEO block. After toluene extraction, the blend surfaces showed advancing water contact angle and surface elemental composition similar to those of the PU matrix. However in contrast to the relatively smooth matrix surface, the extracted blend surfaces were "decorated" with lacunae or pits. Consistent with the weight loss trends, the extent of pitting was greater for the copolymers having shorter PEO blocks, suggesting that surface enrichment of the copolymers increased with decreasing MW of the copolymers.

A study on TiNi(Fe,Mo) shape memory alloy

The influences of Mo on the phase transformation, microstructure and mechanical behavior of TiNi(Fe) shape memory alloy are investigated by differential scanning calorimetry, X-ray diffraction, and tensile stress–strain measurements. The results show that the lattice parameter of TiNi(Fe,Mo) increases with progressing replacement of Ni by Mo, while the martensitic transformation characteristic temperatures decrease simultaneously. The yield strength of TiNi(Fe) alloys has been remarkably improved by addition of Mo, e.g. Ti 50Ni 47Fe 2Mo 1 alloy (at.%) shows a yield strength of 400 MPa, which is 33% higher than that of Ti 50Ni 48Fe 2. Moreover, the martensite reorientation stress of Ti 50Ni 47Fe 2Mo 1 is 250 MPa at liquid nitrogen temperature and a shape memory effect of 6.4% has been achieved.

Axial tensile stress–strain characterization of ITER model coil typeNb3Sn strands in TARSIS

For a few years there has been an increasing effort to study the impact of (bending) strainon the transport properties of superconducting wires. As the stress distribution,originated by differences in the thermal expansion and electromagnetic load, isthe driving factor for the final strains, the axial and transverse stiffness of thestrand play a crucial role in the final performance. Since the strain state of theNb3Sn filaments in strands determines the transport properties, basic experimentalstress–strain data are required at the strand level for accurate modellingand analysis and eventually for optimizing cable and magnet design. Weperformed axial tensile stress–strain measurements on several types ofNb3Sn strands used for the manufacture of the International Experimental Thermonuclear Reactor(ITER) central solenoid and toroidal field model coils and a powder-in-tube processed wire.In total 48 wire samples were tested at boiling helium, boiling nitrogen and at roomtemperature. We present the computation of the stress–strain characteristic with astraightforward 1D model using an independent materials database, obtaining a goodagreement with the experimental results.The details from the take-off origin of the measured stress–strain curves are discussed andthe data are evaluated with respect to some commonly used functions for fittingstress–strain curves. The measurements are performed in the new setup TARSIS (testarrangement for strain influence on strands). A double extensometer connected to thesample enables us to determine the strain level whereas a load cell is used to monitor thestress level.For higher levels of applied stress (100 MPa), we found typically a higher strainfor bronze route wires compared to a powder-in-tube and internal tin typeof strand. Stress–strain results are essential to assess more accurately theimpact of thermal and electromagnetic induced stress on the strain state of theNb3Sn filaments for wires from various manufacturing processes.

Thickness Behavior of Biaxially Stretched PET Film in the Transverse Stretching Zone of a Tenter

An uneven thickness distribution can cause serious problems in practical applications. In this report the deformation and thickness behavior of poly (ethylene terephthalate) (PET) film during transverse stretching in a tenter is discussed. The experiments were performed in a pilot plant with extrusion, casting, machine direction (MD) stretching, transverse direction (TD) stretching and thermosetting and winding processes. The thickness distribution of the PET film was measured after uniaxial orientation (MD stretched) and again after biaxial orientation (TD stretched in the tenter). The thickness behavior during the transverse stretching in the tenter was calculated by a finite element method (FEM), assuming a rigid-plastic or elastic-plastic constitutive law with parameters determined from the tensile stress-strain measurements. The tensile stress-strain tests were performed on MD stretched film in the transverse stretching direction. The FEM analysis was carried out using the measured initial thickness distribution of the uniaxially oriented film (MD stretched). It was estimated from this analysis that the (TD) stretching of the film in the tenter initiated near the tenter clip (edge of film), spread to the center of the film and then finally moved from the center to the edge of the film under the experimental conditions of this study. Good agreement was obtained between the experimental and predicted FEM results for the final film thickness distribution after transverse stretching.