Uniaxial Extension Research Articles

First principles molecular dynamics studies of elastic constants, ideal tensile strength, chemistry of crack initiation, and surface and cohesive energies in amorphous silicon

First principles Kohn–Sham density functional theory (DFT)-based molecular dynamics (MD) is employed to investigate some physical and mechanical properties of amorphous Si (a-Si) samples, as-quenched and annealed containing dangling and floating bonds as well as distorted tetrahedral bonds. The total energy and true stress as functions of the engineering strain for a-Si samples subjected to uniaxial tensile stress as well as uniaxial extension are obtained. It is well-known that the electron density of the state of matters can be determined via ab initio DFT-based MD with high accuracy. Using this technique, such inherent properties as the elastic constants, ideal tensile strength, ultimate tensile strength, and surface and cohesive energies will be calculated. Since the employed ab initio MD, in contrast to the empirical potentials simulations, is capable of providing the evolution of the electronic charge distribution, we can afford to study the chemistry of crack initiation and reconstructed surfaces at final rupture. The calculated cohesive and surface energies are compared with the available theoretical and experimental results; Tyson’s empirical relation and universal binding energy relations (UBERs) are also examined. The calculated elastic constants using the symmetry-general scheme satisfy well the isotropic relation . To date, the ab initio MD samples of a-Si generated from the completely melted scheme were all free of three-fold-coordinated Si. In contrast, as we will show, by implementing special thermal treatments, generation of all inherent structural defects is possible. Based on the electronic charge distribution, dative bonds and trigonal prisms for, respectively, floating and dangling bonds have been observed.

Influence of Active Fillers in Elastomers on the Anisotropic Mullins Effect

The first uniaxial extension of an elastomer causes stress softening (the Mullins effect) not only in this loading direction but in any other direction of subsequent extension. However, the Mullins effect is less pronounced after a change of loading direction than in the direction of the initial loading. As a function of the material's deformation history, its mechanical behaviour is distinctly anisotropic. In the interests of providing a more comprehensive evaluation of the anisotropic Mullins effect, the role of the active fillers carbon black and silica was investigated. In particular, the influence of carbon black properties, such as specific surface area and structure, and of the amount of carbon black incorporated, was determined experimentally. Comparing carbon black- and silica-filled elastomers as well as different rubbers provided more insights into the anisotropic material behaviour of filled elastomers.

A theory for deformation dependent evolution of continuous fibre distribution applicable to collagen remodelling

This work explores the implications of a constitutive framework that introduces a particular remodelling process for materials having a continuous distribution of fibre directions. It is assumed that the matrix constituent is unaffected and that fibres remodel by a process of dissolution and reassembly. Dissolution results from a time dependent chemical kinetic process that is influenced by fibre stretch. Two possibilities for the reassembly of the fibres are considered: (1) fibres reassemble in a stress-free state or (2) fibres assemble in a pre-strained state matching that of the surrounding matrix. The composite is treated as a non-linear hyperelastic material. The constitutive framework accounts for the time dependent redistribution of fibre orientation and density. A directional fibre orientation density function is defined that combines fibre dissolution, reassembly and their dependence on deformation history. Expressions for the time dependence of this function are developed for the two fibre reassembly assumptions and different deformation histories. Numerical results for uniaxial extension are presented in the form of polar plots that illustrate fibre orientation density at different times and fibre density versus time plots that show evolution at different orientations.

Dispersed polypropylene fibrils improve the foaming ability of a polyethylene matrix

We compare the foaming ability of metallocene catalysed polyethylene (mPE) with that of mPE containing fibrillated polypropylene (PP) domains (mPE/fibrillated-PP) and mPE containing spherical PP domains (mPE/spherical-PP). We observe that mPE/fibrillated-PP shows the best foam morphology with the highest number of bubbles per unit volume. We explain this enhancement in foaming ability through rheological and crystallization studies. We identify that the fibrils form a percolated network at a fibril content of 4.5 wt% using the Winter-Chambon analysis. The network of entangled fibrils results in strain hardening in uniaxial extension. Shear thickening responses are observed in shear flow. Such responses are not observed for mPE or mPE/spherical-PP. Oscillatory shear flow investigation of the isothermal melt crystallization reveals a two decade decrease in the time for the onset of crystallization in the mPE/fibrillated-PP and a decade decrease in mPE/spherical-PP relative to neat mPE. We attribute the enhancement in foaming ability of mPE/fibrillated-PP to the concurrent increase in strain-induced hardening response and improved crystallization kinetics.

Deformability of a Flax Reinforcement for Composite Materials

In this work, the deformability of a flax fabric adopted as composite reinforcement is experimentally investigated. The fabric (commercialized as FLAXPLY UD 180 by LINEO) is a quasi-unidirectional woven fabric with thin weft yarns connecting thick slightly twisted warp using satin weave interlacing pattern. The study is dedicated to the understanding and measurement of the main deformation mechanisms occurring during forming processes. The deformation during extension is investigated under uniaxial and biaxial loading in the in-plane tow directions (i.e. warp and weft). Particular attention is dedicated to the behavior during shear loading because this is considered the primary deformation mechanism in the reinforcement forming. Uniaxial bias extension and picture frame tests are adopted to measure the shear deformation. The tests are assisted by digital image correlation (DIC) technique to have a continuous measurement of the local deformation in the fabric plane during loading.

A critical gel fluid with high extensibility: The rheology of chewing gum

Chewing gum provides an excellent everyday example of viscoelastic behavior, and understanding its rheological properties is important for application purposes. Here, we compare the rheological behavior of selected commercial chewing gums and bubble gums. Small amplitude oscillatory shear, shear creep, and steady shear demonstrated that both chewing and bubble gums behave like power-law critical gels in the linear regime. Nonlinear viscoelastic behavior was investigated using large amplitude oscillatory shear, shear creep, and start-up flows (in shear and uniaxial extension). Bubble gums showed more pronounced strain hardening and greater stresses to break in start-up of steady uniaxial extension than chewing gums. We argue that this combination of rheological signatures is sufficient to provide a new robust definition of chewing gum that is independent of specific molecular composition. There are potentially many different formulations and design routes that can achieve this distinctive rheological fingerprint.

Numerical prediction of nonlinear rheology of branched polymer melts

In a recent short communication [Read, D. J. et al., Science 333, 1871 (2011)], we showed that a computational scheme can describe the nonlinear flow properties for a series of industrial low-density polyethylene (LDPE) resins starting from the molecular architecture. The molecular architecture itself is determined by fitting parameters of a reaction kinetics model to average structural information obtained from gel-permeation chromatography and light scattering. Flow responses of these molecules in transient uniaxial extension and shear are calculated by mapping the stretch and orientation dynamics of the segments within the molecules to effective pom-pom modes. In this paper, we provide the details of the computational scheme and present additional results on a LDPE and a high-density polyethylene resin to illustrate the dependence of segmental maximum stretch variables on the flow rate.

Kinetics of the Polymorphic Transition in Isotactic Poly(1-butene) under Uniaxial Extension. New Insights From Designed Mechanical histories.

Isotactic poly(1-butene) (i-PBu) crystallizes upon cooling from the melt in a metastable tetragonal structure (form II), which slowly evolves toward the state of ultimate stability, i.e., the trigonal form I. It is well-known that this polymorphic transformation, which typically requires few weeks at room temperature, can be greatly accelerated by the application of mechanical stresses and/or deformation. However, the exact mechanism of this kinetics enhancement is not completely understood. In this work, the polymorphic transformation of i-PBu under tensile deformation is investigated in details. Thanks to properly designed mechanical histories–including experiments at different true strain and true stress rates–and to in situ wide-angle X-ray diffraction experiments, the role of the various deformation parameters is elucidated. The use of different time scales during the experiments enabled us to gain kinetics data on the transition, information which is disregarded in current literature. The set of exp...

Engineering a Segmented Dual-Reservoir Polyurethane Intravaginal Ring for Simultaneous Prevention of HIV Transmission and Unwanted Pregnancy

The HIV/AIDS pandemic and its impact on women prompt the investigation of prevention strategies to interrupt sexual transmission of HIV. Long-acting drug delivery systems that simultaneously protect womenfrom sexual transmission of HIV and unwanted pregnancy could be important tools in combating the pandemic. We describe the design, in silico, in vitro and in vivo evaluation of a dual-reservoir intravaginal ring that delivers the HIV-1 reverse transcriptase inhibitor tenofovir and the contraceptive levonorgestrel for 90 days. Two polyether urethanes with two different hard segment volume fractions were used to make coaxial extruded reservoir segments with a 100 µm thick rate controlling membrane and a diameter of 5.5 mm that contain 1.3 wt% levonorgestrel. A new mechanistic diffusion model accurately described the levonorgestrel burst release in early time points and pseudo-steady state behavior at later time points. As previously described, tenofovir was formulated as a glycerol paste and filled into a hydrophilic polyurethane, hollow tube reservoir that was melt-sealed by induction welding. These tenofovir-eluting segments and 2 cm long coaxially extruded levonorgestrel eluting segments were joined by induction welding to form rings that released an average of 7.5 mg tenofovir and 21 µg levonorgestrel per day in vitro for 90 days. Levonorgestrel segments placed intravaginally in rabbits resulted in sustained, dose-dependent levels of levonorgestrel in plasma and cervical tissue for 90 days. Polyurethane caps placed between segments successfully prevented diffusion of levonorgestrel into the tenofovir-releasing segment during storage.Hydrated rings endured between 152 N and 354 N tensile load before failure during uniaxial extension testing. In summary, this system represents a significant advance in vaginal drug delivery technology, and is the first in a new class of long-acting multipurpose prevention drug delivery systems.

MULTIAXIAL STRAIN ENERGY FUNCTIONS OF RUBBERLIKE MATERIALS: AN EXPLICIT APPROACH BASED ON POLYNOMIAL INTERPOLATION

ABSTRACTWe propose an explicit approach to obtaining multiaxial strain energy functions for incompressible, isotropic rubberlike materials undergoing large deformations. Via polynomial interpolation, we first obtain two one-dimensional strain energy functions separately from uniaxial data and shear data, and then, from these two, we obtain a multiaxial strain energy function by means of direct procedures based on well-designed logarithmic invariants. This multiaxial strain energy function exactly fits any given data from four benchmark tests, including uniaxial and equibiaxial extension, simple shear, plane–strain extension, and so forth. Furthermore, its predictions for biaxial stretch tests provide good accord with test data. The proposed approach is explicit in a sense without involving the usual procedures both in deriving forms of the multiaxial strain energy function and in estimating a number of unknown parameters.

Mobility evolution during tri-axial deformation of a glassy polymer

In this study, the evolution of mobility with deformation in a glassy polymer is compared in cases of dilatationally dominated longitudinal and deviatorically dominated uniaxial deformation. The mobility is evaluated via stress relaxation experiments performed at various points along the stress–strain curve, including pre-yield, yield, and post-yield regions. At Tg−5°C the mobility decreases with deformation in uniaxial extension, but increases with deformation in tri-axial extension. The τeff relaxation time calculated for longitudinal deformation exhibits a dramatically different dependence on the excess volume than the relaxation time obtained in structural relaxation experiments. Finally, the criterion for cavitational failure is proposed based on the thermodynamic stability considerations.

Regularities of lamellae ordering in the formation of polypropylene membrane porous structure

The lamellar structure of porous polypropylene membranes prepared by the extrusion of the polymer melt, annealing of extruded films, and their uniaxial extension has been investigated using data of computer analysis of scanning electron microscopy images. It has been shown that the formation of pores at the stage of uniaxial extension is accompanied by an ordering of lamellae or their self-organization controlled by the annealing temperature Tann. It has been found that the self-organization of lamellae can occur through two mechanisms depending on the spin draw ratio at the stage of extrusion of the polymer melt: in the first case, an increase in the temperature Tann leads to a gradual transition of the disorder-order type; in the second case, a bifurcation transition occurs with a nonmonotonic change in the order parameter. The conditions of the preparation of regular spatial lattices of lamellae have been discussed.

The biomechanical properties of canine skin measured in situ by uniaxial extension

IntroductionA uniaxial extension system was setup to analyze the mechanical properties of dog skin. Material and methodsPads were glued onto dog skin with constant reproducible geometrical parameters and the extension force was measured as a function of the extension values. Forty-one sites (82 cycling tests) were investigated in situ on 11 canine cadavers, half of them after surgically isolating the test area from the surrounding skin. Series of loading-unloading cycles of up to 5N or 10N or both loads were performed on each site. The elastic properties and the dissipative effects were characterized respectively by evaluating the secant Rigidity at maximum loads and the Fraction of dissipated energy. ResultsA hysteresis phenomenon, implying the need for preconditioning to attain equilibrium cycles, was apparent during mechanical characterization. Polynomial expressions were used to relate the measured Rigidities and the Fractions of dissipated energy with or without sample isolation. The latter were less affected by isolation. The ratios between the Rigidities at 5N to those at 10N displayed non-linearity in the investigated extension range in contrary to the Fractions of dissipated energy. Discussion/conclusionThe parameters confirming the dissipative non-linear elastic behavior of dog skin were identified and the correlation between Rigidity and Fraction of dissipated energy on isolated and non-isolated skin samples was quantitatively determined. This extension setup can now be used as a “true in vivo” mapping tool to determine the mechanical characteristics of the skin during healing processes or during the study of Human skin disease with the dog as an animal model.

Quantitative analysis of the debonding structure of soft adhesives

We experimentally investigate the growth dynamics of cavities nucleating during the first stages of debonding of three different model adhesives. The material properties of these adhesives range from a more liquid-like material to a soft viscoelastic solid and are carefully characterized by small strain oscillatory shear rheology as well as large strain uniaxial extension. The debonding experiments are performed on a probe tack set-up. Using high contrast images of the debonding process and precise image analysis tools, we quantify the total projected area of the cavities, the average cavity shape and growth rate and link these observations to the material properties. These measurements are then used to access corrected effective stress and strain curves that can be directly compared to the results from the uniaxial extension.

Effects of Wheat Bug (Eurygasterspp. andAeliaspp.) Infestation in Preharvest Period on Wheat Technological Quality and Gluten Composition

The effects of wheat bug infestation (Eurygaster spp. and Aelia spp.) on the composition of wheat gluten proteins and its influence on flour technological quality were investigated in the present study. Wheat samples of six wheat varieties, collected from two localities in northern Serbia, were characterized by significantly different level of wheat bug infestation. Composition of wheat gluten proteins was determined using sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS PAGE), while the selected parameters of technological quality were determined according to standard and modified empirical rheological methods (Farinograph, Extensograph, Alveograph, and Gluten Index). The surface morphology of the selected samples was viewed using scanning electron microscopy (SEM). Wheat from wheat bug-infested locality regardless of the variety had deteriorated technological quality expressed with higher Farinograph softening degree, lower or immeasurable Extensograph energy, and Alveograph deformation energy. The most important changes in the gluten proteins composition of bug-infested wheat were related to gliadin subunits with molecular weights below 75 kDa, which consequently caused deterioration of uniaxial and biaxial extensibility and dough softening during mixing.

Shape-responsive liquid crystal elastomer bilayers

Monodomain liquid crystal elastomers (LCEs) are shape-responsive materials, but shape changes are typically limited to simple uniaxial extensions or contractions. Here, we demonstrate that complex surface patterns and shape changes, including patterned wrinkles, helical twisting, and reversible folding, can be achieved in LCE-polystyrene (PS) bilayers. LCE-PS bilayer shape changes are achieved in response to simple temperature changes and can be controlled through various material parameters including overall aspect ratio and LCE and polystyrene film thicknesses. Deposition of a patterned PS film on top of an LCE enables the preparation of an elastomer that reversibly twists and a folding leaf-like elastomer, which opens and closes in response to temperature changes. The phenomena are captured through finite element simulations, in quantitative agreement with experiments.

A constitutive description of the anisotropic response of the fascia lata

In this paper we propose a constitutive model to analyze in-plane extension of goat fascia lata. We first perform a histological analysis of the fascia that shows a well-organized bi-layered arrangement of undulated collagen fascicles oriented along two well defined directions. To develop a model consistent with the tissue structure we identify the absolute and relative thickness of each layer and the orientation of the preferred directions. New data are presented showing the mechanical response in uniaxial and planar biaxial extension. The paper proposes a constitutive relation to describe the mechanical response. We provide a summary of the main ingredients of the nonlinear theory of elasticity and introduce a suitable strain-energy function to describe the anisotropic response of the fascia. We validate the model by showing good fit of the numerical results and the experimental data. Comments are included about differences and analogies between goat fascia lata and the human iliotibial band.

Anisotropic finite-strain models for porous viscoplastic materials with microstructure evolution

In this work, we propose a constitutive model for the finite-strain, macroscopic response of porous viscoplastic solids, accounting for deformation-induced changes in the size, shape and distribution of the voids. The model makes use of consistent homogenization estimates obtained by the “iterated variational linear comparison” procedure of Agoras and Ponte Castañeda (2013) to characterize both the instantaneous effective response of the porous material and the evolution of the underlying microstructure. The proposed model applies for general, three-dimensional loading conditions and can be implemented numerically for use in standard FEM codes. We also investigate the interplay between the evolution of the microstructure and the macroscopic stress–strain response, in the context of displacement-controlled, plane strain loading (bi-axial straining) of initially isotropic, porous, rigid-plastic materials with power-law hardening. We focus on the effect of strain triaxiality, and consider both extensional and contractile loading conditions leading to porosity growth and collapse, respectively. For both types of loadings, it is found that the macroscopic behavior of the material exhibits an initial hardening regime followed by a softening regime at sufficiently large strains. Consistent with earlier models and experimental results, the softening regime for extensional loadings is a consequence of porosity growth. On the other hand, the softening behavior predicted for contractile loadings is found to be a consequence of void collapse, i.e., of rapid changes in the average shape of the pores leading to crack-like shapes. For both types of loadings, the transition from hardening to softening in the macroscopic response can be identified with the onset of macroscopic strain localization. The associated critical conditions at the onset of localization are determined as a function of the strain triaxiality. The type of localization band ranges from dilatational to compaction bands as the bi-axial straining varies from uniaxial extension to uniaxial contraction.

Succinic acid in levels produced by yeast (Saccharomyces cerevisiae) during fermentation strongly impacts wheat bread dough properties

Succinic acid (SA) was recently shown to be the major pH determining metabolite produced by yeast during straight-dough fermentation (Jayaram et al., 2013), reaching levels as high as 1.6mmol/100g of flour. Here, the impact of such levels of SA (0.8, 1.6 and 2.4mmol/100g flour) on yeastless dough properties was investigated. SA decreased the development time and stability of dough significantly. Uniaxial extension tests showed a consistent decrease in dough extensibility upon increasing SA addition. Upon biaxial extension in the presence of 2.4mmol SA/100g flour, a dough extensibility decrease of 47–65% and a dough strength increase of 25–40% were seen. While the SA solvent retention capacity of flour increased with increasing SA concentration in the solvent, gluten agglomeration decreased with gluten yield reductions of over 50%. The results suggest that SA leads to swelling and unfolding of gluten proteins, thereby increasing their interaction potential and dough strength, but simultaneously increasing intermolecular electrostatic repulsive forces. These phenomena lead to the reported changes in dough properties. Together, our results establish SA as an important yeast metabolite for dough rheology.

Confined crystallization in end-linked PEO network under uniaxial extension

In this work extension induced crystallization of end-linked polyethylene oxide (PEO) network was investigated with in-situ small angle X-ray scattering (SAXS) and rheological measurement. The coupling between crystallinity and stress of crosslinked network makes the morphology time dependent. With a large strain the structure evolution in the stretched network is found to follow a two-stage process: i) crystals with high numerical density and small dimension form first, which laterally correlate with adjacent ones; ii) continuous melting and fusing of adjacent crystal happen after the first nucleation stage, leading to increase of crystal dimension and stability. Decreasing strain a gradual transformation to normal lamellae formation is found. Strain tuned confinement and crystallization induced confinement release is supposed to induce the structural change.