Uniaxial Elongation Research Articles

Transient extensional rheology of wormlike micelle solutions

A filament stretching rheometer is used to follow the evolution in the tensile force and the flow induced birefringence of a wormlike micelle solution experiencing a uniaxial elongation flow. The experiments are performed using a series of wormlike micelle solutions of cetyltrimethylammonium bromide and sodium salicylate in de-ionized water. The linear viscoelastic shear rheology of the wormlike micelle solutions is well described by an upper convected Maxwell model with a single relaxation time. In transient homogeneous uniaxial extension, the wormlike micelle solutions demonstrate significant strain hardening and a failure of the stress-optical, however, no stress-conformation hysteresis is observed. A quantitative fit to the extensional rheology of each of the wormlike micelle solution tested is achieved with a FENE–PM model having as few as two relaxation modes. At a critical stress, nearly independent of strain rate, the wormlike micelle solutions filaments are found to fail through a dramatic rupture near the axial midplane. This filament failure is not the result of elastocapillary thinning as is commonly observed in the filament stretching of weakly strain hardening polymer solutions. Instead, the filament failure might stem from the local scission of individual wormlike micelle chains. The energy of wormlike micelle chain scission is calculated to be roughly 4 kBT for all the solutions tested, nearly independent of both the imposed extension rate and the concentration of the surfactant and the salt.

Transformation of cubic symmetry for spherical microdomains from face-centred to body-centred cubic upon uniaxial elongation in an elastomeric triblock copolymer

Experimental observation of the transformation of cubic symmetry from face-centred cubic (f.c.c.) to body-centred cubic (b.c.c.) for spherical microdomains upon uniaxial stretching of poly(1,3-cyclohexadiene)-block-poly(ethylene-co-but-1-ene)-block-poly(1,3-cyclohexadiene) triblock copolymer, having φPCHD= 0.293,Mn= 5.73 × 104,Mw/Mn= 1.09, is reported. First, the sample was subjected to flow treatment at 493 K for 2 h; then it was annealed at 473 K for 2 h to prepare the f.c.c. structure. The sample was subsequently subjected to uniaxial stretching. After the load was completely removed, small-angle X-ray scattering measurements were conducted. It was found that the f.c.c. packing was transformed to the well ordered b.c.c. packing upon uniaxial stretching.

Mechanical properties of nasal fascia and periosteum

Objective. To determine under which layer the silicone implants should be inserted into, the biomechanical properties of fascia and periosteum were investigated. Design. Biomechanical testing of cadaveric tissues. Background. In silicone augmentation rhinoplasty, most complications are closely related to the depth of implant and the mechanical character of the tissue surrounding the implant. Methods. Biomechanical properties of human nasal periosteum and fascia were studied, including tensile strength, stress–strain relationship and stress relaxation under uniaxial elongation. Result. Although with less failure strain, the periosteum has more tensile strength than fascia. The slope of the linear part of stress–strain curve of the periosteum is bigger than that of fascia, which indicates the periosteum is stiffer than fascia. The stress-relaxation slope of periosteum is smaller than that of fascia. Conclusion. In the view of biomechanics, the periosteum is thicker, stronger and stiffer than fascia. Under periosteum the silicone implants are easier to be fixed at desired position, thus periosteum is more suitable for covering silicone implants. Relevance The periosteum is more suitable than fascia for covering silicone implants in augmentation rhinoplasty.

Multiaxial fatigue behavior of conventional and highly crosslinked UHMWPE during cyclic small punch testing.

Previous observations of reduced uniaxial elongation, fracture resistance, and crack propagation resistance of highly crosslinked ultrahigh molecular weight polyethylene (UHMWPE) have contributed to concern that the technology may not be appropriate for systems undergoing cyclic fatigue loading. Using a "total life" approach, we examined the influence of radiation crosslinking on the fatigue response of UHMWPE under cyclic loading via the small punch test. Our goal in this study was to evaluate the suitability of the small punch test for conducting miniature-specimen, cyclic loading, and fatigue experiments of conventional and highly crosslinked UHMWPE. We subjected four types of conventional and highly crosslinked UHMWPE to cyclic loading at 200 N/s and at body temperature in a small punch test apparatus. After failure, the fracture surfaces were characterized with the use of field emission scanning electron microscopy to evaluate the fatigue mechanisms. Cyclic small punch testing under load control was found to be an effective and repeatable method for relative assessment of the fatigue resistance of conventional and highly crosslinked UHMWPE specimens under multiaxial loading conditions. For each of the four conventional and highly crosslinked UHMWPE materials evaluated in this study, fatigue failures were consistently produced according to a power law relationship in the low cycle regimen, corresponding to failures below 10000 cycles. The fatigue failures were all found to be consistent with a single source of initiation and propagation to failure. Our long-term goal in this research is to develop miniature-specimen fatigue testing techniques for characterization of retrieved UHMWPE inserts.

Effect of uniaxial strain on the protective properties of coil-coatings

The effect of uniaxial deformation on coil-coated galvanised steel was investigated. Samples with several degrees of uniaxial elongation were exposed to a corrosive solution and the loss of protective properties was monitored by electrochemical impedance spectroscopy (EIS). Protection rapidly decreased with the degree of elongation. After 16% the coating degradation introduced by elongation was so high that EIS was not able to distinguish different strained samples. The loss of protection was interpreted in terms of the morphological changes experienced by the coated system as a consequence of strain, supported by scanning electron microscopy (SEM) observations.

Entry flow of LDPE melts in a planar contraction

Experiments using laser–Doppler velocimetry (LDV) and numerical simulations have been undertaken for the creeping entry flow of two rheologically well-characterised low-density polyethylene (LDPE) melts in a 14:1 planar contraction. LDV measurements of the flow reveal a distinct behaviour between the two melts regarding vortex formation. The flow of the melts has been modelled using an integral constitutive equation of the K-BKZ type with a spectrum of relaxation times, modified to properly account for strain hardening in the planar extensional flow relevant for the geometry used. Numerical values for the constants appearing in the equation have been obtained from fitting dynamic data ( G′ and G″) as well as data for the viscosities and normal stresses as measured in shear. Elongational data were derived from transient and steady uniaxial elongation experiments. The experiments show that for one LDPE melt the vortex keeps small with increasing flow rate, while for the other there is a vortex growth in the same range of flow rates. The numerical simulations also predict the same trend in agreement with the experiments. The differences in flow behaviour of the two branched melts are found to be attributed to big differences in the average relaxation times, giving rise to different De numbers or dimensionless stress ratios S R, for the same range of flow rates. Equally important is the more pronounced extensional strain-hardening behaviour of the vortex-producing polymer, giving rise to high Trouton ratios T R.

Elongation of Triblock Copolymer Melt: Elongation Flow Opto-Rheometry and Small-Angle X-ray Scattering Study

The elongational flow-induced morphological change of a triblock copolymer melt of polystyrene-b-poly(ethylene butylene)-b-polystyrene (SEBS) with polystyrene (PS) weight fraction of 30% has been investigated using elongational flow opto-rheometry (EFOR) and small-angle X-ray scattering (SAXS). The alignment of the PS cylindrical domains in a preferred direction is achieved by roll processing at 200 °C. In the EFOR measurement, the uniaxial elongation with a Hencky strain rate, ε0, of 0.01 s-1 is applied at 180 °C in the direction either parallel (case 1 elongation) or perpendicular (case 2 elongation) to the cylinder axis. Transient elongational viscosity, η(ε0;t), measured during the case 1 elongation exhibits an initial increase, followed by a gradual decrease to almost a steady state before rupture. One measured during the case 2 elongation shows three different regions with elongation time: an initial increase followed by a gradual decrease and a final, rapid increase. The SAXS measurement on the elongated films demonstrates that the alignment of the cylindrical domains persists during the case 1 elongation, whereas the cylindrical domains are rotated by 90° to be aligned parallel to the elongation direction during the case 2 elongation, indicating that the final orientation of the cylinders is not determined by the initial alignment of the domain, but by the Hencky strain rate and the tensile stress level applied on the specimen.

Rheological properties of hyperbranched polymer/poly(ethylene terephthalate) reactive blends

AbstractThe rheological properties in solution, in shear and in uniaxial elongation of poly(ethylene terephthalate) (PET) reacted together with hyperbranched polymers (HBPs) were investigated. Two different PET grades, of low and high molecular weights, were compounded with sub‐ to over‐stoichiometric concentrations of HBPs of second and fourth pseudo‐generation, and subsequently subjected to a solid‐state polycondensation (SSP). The formation of microgels, which occurs at high HBP concentration, gave rise to a large increase in melt elasticity and a related decrease in melt strength. At low HBP concentrations, the complex viscosity of the unreacted HBP/PET was considerably reduced, thus demonstrating a lubrication effect of the HBP molecules. During SSP, the intrinsic and shear viscosities exhibited a gradual increase, which was similar for both PET and HBP/PET blends, and was correlated to an increase in molecular weight, through linear‐chain extension and branching reactions. The elongational viscosity of the reactive blends was also increased as a function of reaction time, and this increase was much larger in the case of the HBP/PET blends. A 400% increase in melt strength of the PET was obtained by combining SSP and trace amounts of an HBP of second generation, without any decrease in drawability.

Study on Uniaxial Elongational Viscosity and Vacuum Molding Processability of the PP/PE Blends

The relationship between uniaxial elongational viscosity and vacuum molding processability of the PP/PE blends were investigated. Propylene homo-polymer (PP) and 3 kinds of high pressure method low density polyethylene with different molecular weight (PE-A, PE-B, PE-C) were used. Materials were melt blended to the ratio of PP/PE=90/10, 80/20 and 70/30wt% for the experiments. The PP/PE blends showed strain hardening of uniaxial elongational viscosity, while PP showed no strain hardening. The strain hardening of PP/PE increased with increasing of the blend ratio of PE to PP, and the molecular weight of PE. The morphologies of PP/PE under uniaxial elongational flow were observed by TEM. For the samples with the lower molecular weight PE, the larger deformation of PE domain was observed. The deviation of thickness of the vacuum molding products decreased according to the increase of the strain hardening of PP/PE.

血管再生のための足場材料としての異種血管の力学的性質

To develop small-diameter vascular grafts with high anti-thrombogenicity and the ability to grow, we prepared heparinized acellular matrices (HEPARINs) made from detergent-treated porcine carotid arteries. In this study, the mechanical properties of HEPARIN as well as the acellular matrix before heparinization (ACM) were investigated under uniaxial and biaxial deformation. The values of the initial Young's Modulus (E0) and tangent Modulus (Et) obtained by uniaxial elongation for the ACM were almost identical to those for a fresh porcine carotid artery (FRESH), but HEPARIN showed higher values than FRESH for the two mechanical parameters. The pressure at burst for ACM estimated by biaxial deformation was lower than the other samples, but was still much higher than the normal blood pressure. It was shown that HEPARIN is one of the promising materials as a scaffold for regeneration of blood vessels as long as mechanical properties are concerned.

A comparison of the stress and birefringence growth of dilute, semi-dilute and concentrated polymer solutions in uniaxial extensional flows

Filament stretching experiments are used to follow the evolution in the flow-induced birefringence (FIB) and the tensile force in polymeric fluid threads undergoing uniaxial elongation. The tests are performed with several monodisperse polystyrene solutions spanning concentrations from 0.025 to 12 wt.%, and the measurements are used to investigate the extent of hysteresis in the polymeric stress and molecular conformation during imposed stretching and subsequent stress relaxation. For Deborah numbers above De>0.5 (based on the longest or Zimm relaxation time), a pronounced stress-conformation hysteresis is observed in the dilute polystyrene solution at large Hencky strains, and the magnitude of the deviation from the quasi-static FENE connector force increases with imposed deformation rate. However, tests with a semi-dilute entangled polystyrene solution and a concentrated entangled polystyrene solution show that although the stress-optical rule is violated at large deformation rates and large strains, there is no evidence of any hysteresis during extension and relaxation. These observations are interpreted in terms of the relative magnitude of the stretching rate as compared to the reptation time, and also as compared to the Rouse time scales for stretching of the entire chain or for a single entangled segment. These distinct time scales depend on the number of entanglements in the fluid and govern the different relaxation mechanisms that exist in entangled polymer solutions for the polymeric stress and the molecular conformation.

Inhomogeneous transient uniaxial extensional rheometry

A filament stretching rheometer is used to probe the development of coil-stretch hysteresis in transient nonhomogeneous uniaxial elongation of a dilute polymer solution. Laser-Doppler velocimetry measurements are used to measure the steady centerline velocity profile in an elastic test fluid as it flows towards the throat of a 4:1:4 axisymmetric contraction/expansion over a range of Deborah numbers. The corresponding time-varying extension rate experienced by a fluid element as it flows into the contraction is then downloaded to a filament stretching rheometer in order to impose an identical inhomogeneous stretch history. Simultaneous optoelectronic measurements of the flow induced birefringence and mechanical measurements of the total tensile force in the filament allow independent determination of the evolution in the conformational anisotropy and stress. A pronounced stress-conformation hysteresis is observed with the relationship between the polymeric tensile stress and the average anisotropy in the polymer coil conformation evolving differently during the imposed extension and subsequent relaxation. An energy dissipation per unit volume can be calculated from the area enclosed by the stress-conformation hysteresis loop. The critical Deborah number for the onset of hysteresis and the scaling of the energy dissipation with an increase in deformation rate are found to correlate closely with the development of the additional drop in pressure that is measured macroscopically during viscoelastic flow through the contraction/expansion. These macroscopic and microscopic measurements both support the existence of an additional dissipative contribution to the polymer stress which must be resolved by constitutive models in order to accurately simulate complex flows of elastic polymer solutions.

Dynamic and Elongation Rheology of Yeasted Bread Doughs

ABSTRACTThe rheology of yeasted bread doughs is a little‐studied field despite yeast's importance in developing bread structure. A method of thermally inactivating the yeast within mixed bread doughs was developed to overcome the difficulty of yeast fermenting during rheological measurements. Sample stabilization by preshearing of dough samples at a stress amplitude of 1 Pa at 1 Hz for 10 sec improved the reliability of small amplitude oscillatory shear measurements, and resting 20 min within the rheometer was sufficient to produce reliable and consistent observations. Small amplitude oscillatory shear measurements were unable to detect any differences between yeasted and nonyeasted doughs nor any changes in linear viscoelastic properties due to fermentation. However, large strain uniaxial elongation measurements of yeasted doughs revealed a significant progressive decrease in elongational viscosities with fermentation. Size‐exclusion HPLC analysis of yeasted doughs showed an increase in unextractable polymeric dough proteins, which were interpreted as evidence of cross‐linking and therefore a potential improvement in dough properties. The apparent contradictions between uniaxial elongation and SE‐HPLC studies of fermenting yeasted doughs can be attributed to gas bubbles within the dough interrupting the increasingly cross‐linked protein network, resulting in the rheological weakness observed for fermenting yeasted doughs.

Tuning the Optical Properties of Inverse Opal Photonic Crystals by Deformation

Simple uniaxial elongation of the air spheres in polymer inverse opal is used to tune the material’s stop band. Stretching causes the distance between the (111) planes parallel to the film surface to decrease, resulting in a shift of the stop band to shorter wavelength. This is coupled with a shift to longer wavelength of the stop band associated with periodicity in the stretching direction. The Figure shows the resulting elongated air voids.

A new model for dilute polymer solutions in flows with strong extensional components

Ghosh et al. (2001) demonstrated that the Kramers chain captures the optical and rheological properties of dilute polymer solutions in rapidly varying elongational flows better than the finitely extensible nonlinear elastic dumbbell model. A new model, based on introducing an adaptive length scale (ALS) as an internal variable, is developed to reproduce the fine scale physics of the Kramers chain. The resulting ALS-model describes the polymer molecule as a set of identical segments in which each segment represents a fragment of the polymer that is short enough so that it can sample its entire configuration space on the time scale of an imposed deformation and, therefore, stretch reversibly. As the molecule unravels, the number of segments decreases, but the maximum length of each segment increases, so that the constant maximum contour length of the molecule is preserved. The ALS model gives very good predictions of stress growth in startup of uniaxial elongation and stress-birefringence hysteresis in a uniaxial elongational flow followed by relaxation. A closed form of the constitutive equation, the ALS-C model, is proposed. The rheological predictions of the ALS-C model resemble those of the ALS equation. This coupled with its small number of internal degrees of freedom suggests that this constitutive equation may be useful in modeling complex flows.

Induced crystallization and orientation of poly(ethylene terephthalate) during uniaxial and biaxial elongation

Stretching PET at a high strain rate above the glass transition temperature has a positive effect on the strength of the material. In a recent paper[1], we presented the influence of stretch and blow molding parameters on the properties of the final product, especially on the crystallinity induced by stretching. In this paper, we focus on the effects of loading, temperature, elongation and strain rate on macromolecular orientation and crystallization kinetics. We present experimental results from uniaxial and biaxial elongation tests carried out on injected PET specimens. To minimize the effect of quiescent crystallization, specimens are quickly heated with infrared lamps before the test and temperature is regulated during the test. Both uniaxial and biaxial tests are analyzed using a cross correlation technique[2] that compares a picture used as reference and the picture of the deformed specimen. This technique allows us to determine all strain components at each point of the specimen, even when the strain field is not homogeneous. In a second part, we present measurements of macromolecular orientation and crystallinity ratio performed after each test. The infrared dichroïsm technique is used to determine the orientation of the microscopic morphology of PET before and after the testing. DSC measurements and density measurements are carried out to calculate the crystallinity ratio. Influences of strain rate, temperature and strain path sequence are evaluated in order to build a database for recent models of induced crystallization[3],[4],[5].

Butadiene rubbers: topological constraints and microscopic deformation by mechanical and small angle neutron scattering investigation

Results of stress-strain measurements are reported for butadiene rubbers of varying crosslink density. The fluctuation of the effective tube diameter of the polymer networks was investigated under uniaxial elongation by mechanical measurements as well as by small angle neutron scattering (SANS) which probe the local orientation on a segmental scale. The effect of topological constraints on the microscopic deformation of the butadiene network chains is well described within a tube approach. For the first time, experiments at large deformations and for polydisperse sample are presented. Excellent agreement between the statistical mechanical model and the experimental results is obtained.

Flow through a convergence. Part 1: Critical conditions for unstable flow

AbstractThe critical conditions leading to fracture in elongation and different types of flow instabilities were examined in uniaxial elongation and in a capillary rheometer equipped with dies having different entry profiles. Either ductile or brittle fracture may be observed, ductile being related to necking of material. The critical stress approach was used to predict fracture in elongation. All linear polymers studied in this work exhibited ductile fracture in uniaxial elongation, but the transition to brittle fracture is discussed in relation to existing experiments with other materials. In a ductile fracture regime, critical stress and work both increase with an increasing rate of deformation, whereas in a brittle regime the critical values remain constant. The converging flow studies indicated that two types of flow instability that have been previously related to each other, namely, pressure oscillations and voltions distortions, are of different origins. The critical flow rate for pressure oscillations is independent of entry profile, and the origin for this type of instability lies along the wall of the capillary. On the other hand, the critical flow rate for volume distortions increased with a decreasing entry angle, indicating that volume distortions are not a consequence of pressure oscillation, nor are their origin at the capillary wall. Numerical simulations were used to determine the stress profiles within the flow, and it was shown that the onset of volume distortions is directly related to the magnitude of elongational stress and work, and may therefore be considered to be caused by fracture in elongation. In dies with 90° entry profile, volume distortions were observed simultaneously with pressure oscillations, making it difficult to distinguish between the two phenomena.

주조된 AI 합금의 기계적 성질에 미치는 HIP의 영향

The present paper investigates the effect of hot isostatic pressing (HIPing) on mechanical properties, e.g., tensile strength, ductility and impact absorption energy of sand and die casted aluminum alloys. After HIPing at various temperatures and pressure conditions, uniaxial tensile test and Izod impact test of the samples were carried out. The experimental results showed improvements in uniaxial tensile strength, elongation and Izod impact toughness of sand casted aluminum alloy, while deterioration of a tensile strength fur die casted aluminum alloy. The effect of HIPing for microstructure of the cast aluminum alloy was also investigated.

Microstresses Caused by Uniaxial Deformation and by Mechanical Surface Treatments

Abstract Plastic deformations of materials generally originate macrostresses as well as microstresses between the constituents of the material. Both parts may contribute to the results of stress determinations. Their influence on the material behaviour, however, is different. Therefore, a detailed assessment requires the separation of the stress kinds. The present paper deals with the portions and the depth profiles of macro- and micro-residual stresses after uniaxial elongation and after mechanical surface treatments. The possibilities to separate stresses are discussed. It will be distinguished between one-phase, quasi-one-phase, two-phase and multi-phase materials. The measurement techniques include X-ray and neutron diffraction and mechanical methods. The paper reviews the published results and reports on recent studies.