Show Strain Hardening Research Articles

Influence of processing on the flow properties of long‐chain branched polypropylene

Long‐chain branched polymers have high melt strength and show strain hardening in the elongational flow. These characteristics are important quantities for certain processing procedures, but are strongly affected by thermomechanical treatment. In this study, different rheological methods and material functions in shear and elongation are applied to determine the flow behavior that is affected by certain processing procedures. For this purpose, a series of blends of linear polypropylene (L‐PP) and long‐chain branched polypropylene (LCB‐PP) is prepared using different processing methods (mixing chamber with kneading elements, twin‐screw extruder, foaming) and varying processing parameters. The decrease of dynamic moduli and of flow activation energy depends on the number of processing stresses. Equilibrium states are created by solution treatment, and in this way process‐modification can be separated into reversible and irreversible parts. It is concluded that the irreversible process‐modification does not reduce the density of entanglements, but tears chains. POLYM. ENG. SCI., 58:1596–1603, 2018. © 2017 Society of Plastics Engineers

Fumed and Precipitated Hydrophilic Silica Suspension Gels in Mineral Oil: Stability and Rheological Properties.

Hydrophilic fumed silica (FS) and precipitated silica (PS) powders were suspended in mineral oil; increasing the silica volume fraction (φ in the suspension led to the formation of sol, pre-gel, and gel states. Gelation took place at lower φ values in the FS than the PS suspension because of the lower silanol density on the FS surface. The shear stresses and dynamic moduli of the FS and PS suspensions were measured as a function of φ. Plots of the apparent shear viscosity against shear rate depended on φ and the silica powder. The FS suspensions in the gel state exhibited shear thinning, followed by a weak shear thickening or by constant viscosity with an increasing shear rate. In contrast, the PS suspensions in the gel state showed shear thinning, irrespective of φ. The dynamic moduli of the pre-gel and gel states were dependent on the surface silanol density: at a fixed φ, the storage modulus G′ in the linear viscoelasticity region was larger for the FS than for the PS suspension. Beyond the linear region, the G′ of the PS suspensions showed strain hardening and the loss modulus G″ of the FS and PS suspensions exhibited weak strain overshoot.

A novel technique for conducting creep experiments in equibiaxial elongation

A recently developed rheological technique known as continuous lubricated squeezing flow (CLSF) is adapted to perform constant stress, or creep, experiments in equibiaxial elongation flows of polymer melts. By modifying the CLSF technique, which was developed for constant strain rate deformations, we demonstrate that the technique can also be used to generate constant stress flows. Measured steady state viscosities are compared to constant rate elongation results for polymer melts having different molecular characteristics. Linear polymers show strain softening and compare well in constant stress and constant strain rate deformations. The branched polymer shows strain hardening and a viscosity that is slightly higher in constant stress for low rates. Limitations of the current version of the CLSF technique for creep flows are also briefly discussed.

Production Technology for Precision Seamless Steel Tubes from the Perspective of Microhardness Changes

The production of precision seamless steel tubes in Železiarne Podbrezová is using hot rolled tubes with multiple cold drawing passes and intermediate annealing. It utilizes intensive plastic deformation during cold drawing, taking full advantage of the microstructural state from a physical point of view. In this paper, optimization of technological processes for cold drawn tubes made from ferritic-pearlitic steel has been elaborated. We use microstructural and substructural analysis, dislocation hardening theory and stress analysis. The subject of this article is the experiment with multiple drawing passes and intermediate annealing for production of precision steel tubes with dimensions of 31.8 x 2.6 mm. The drawing itself consists of 5 processes (also called „runs“) with 7 drawing passes in total. The results presented show strain hardening of the material after drawing along with relaxation mechanism during intermediate annealing. The possibility of utilizing the microhardness values on intensity assessment of these processes is investigated, too.

Tunable assembly of hydrophobic protein nanoparticle at fluid interfaces with tannic acid

Interparticle forces that govern the assembly of particles adsorbed at fluid interfaces determine the rheology and microstructure of particle laden interface. In this study, natural small molecule, tannic acid (TA), was used to modify particle-particle interactions and assembly behaviors of food grade hydrophobic protein particle (zein nanoparticle) at air-water interface. For bare zein particle (ZP), the rapid adsorption and high interparticle forces led to the low coverage and formation of large ZP aggregates at the interface. The hydrogen-bonding of TA weakened the particle hydrophobicity and thus interparticle forces at fluid interfaces. Therefore, the adsorption process and assembly rate of ZTP at the interface obviously reduced, which also provided enough time for ZTP to rearrange into orderly interfacial networks. Furthermore, the Lissajous plots of surface pressure versus deformation are used to understand the surface rheological response to large mechanical deformation in terms of interfacial microstructure. The rheological response of interface stabilized by ZTP showed strain hardening during extension and strain softening during compression. These findings indicate that the interfacial rheology and microstructure of zein particle-laden interface can be tuned by changing the TA content.

Experimental deformation of coarse‐grained rock salt to high strain

AbstractThe processes and deformation mechanisms (e.g., dislocation creep, pressure solution, grain boundary sliding, and recrystallization) of rock salt are still a matter of debate. In order to fill this gap, high strain constriction experiments at 345°C, atmospheric pressure and a strain rate of ~10−7 s−1 have been conducted on natural halite cuboids (60 × 60 × 45 mm) from the Morsleben mine of Northern Germany. Most samples were almost single crystals and contain a small amount of smaller grains (10–26%). The grain boundaries are decorated with fluid inclusions. The experiments were stopped at different final strains (εy = z of ~10, 20, 30, and 40%) corresponding to a maximum strain (εx) range of 20–170%. The halite is deformed by dislocation creep, and the size of developed subgrains corresponds to the applied stress. The combined Schmid factor and subgrain boundary analysis indicate that slip was largely accommodated by the {110} < 110 > slip systems, with possible minor contribution by slip on the {100} < 110 > slip systems. Some of the deformed samples show evidence of grain boundary migration. In addition, subgrains with small misorientations form that result in large cumulative misorientations within a single grain (>40°). However, no subgrain rotation recrystallization is observed (i.e., misorientation angles are <10°). All the experiments show strain hardening, suggesting that recrystallization by grain boundary migration was not extensive and did not reset the microstructure. The experiments show that high finite strain in coarse‐grained relatively dry rock salt can be accommodated by dislocation creep, without extensive dynamic recrystallization.

Rate dependence of ultra high toughness cementitious composite under direct tension

Ultra high toughness cementitious composite (UHTCC) usually shows strain hardening and multiple cracking under static tension loads. In practice, structures could be exposed to high strain rates during an earthquake. Whether UHTCC can maintain its unique properties and provide high structural performance under seismic loading rates largely determines whether it can successfully fulfil its intended function. To determine the rate dependence of UHTCC, uniaxial tensile tests with strain rates ranging from 4×10−6 s−1 to 1×10−1 s−1 were conducted with thin plates. The experimental results showed that UHTCC had significant strain hardening and excellent multiple cracking properties under all the rates tested. The ultimate tensile strain lay in the range of 3.7% to 4.1% and was almost immune to the change in strain rates. The rate of 1×10−3 s−1 seemed to be a threshold for dynamic increase effects of the first crack tensile strength, elastic modulus, ultimate tensile strength, and energy absorption capability. When the strain rate was higher than the threshold, the dynamic increase effects became more pronounced. The energy absorption capability was much higher than that of concrete, and the average ultimate crack widths were controlled below 0.1 mm under all rates. Several fitting formulas were obtained based on the experimental results.

Magnetic Field Assisted Electric Discharge Machining of Cryo-Treated Monel 400 Alloy

Monel alloys are pioneering materials which have exceptional engineering properties such as corrosion resistance, high toughness and show good response to cryogenic treatment. It finds uses in ship building, nuclear aerospace, missile and valve industries. These materials shows strain hardening effect which results in tool wear and in some cases tool breakage when machined by conventional methodshence, unconventional machining such as electrical discharge machining (EDM)discoverspurpose for machining of such materials. Researchers have recognized relation between electrical input process parameters of EDM process and output parameters of EDM process. But researchers have not investigated the influence of external magnetic field and cryo-treatment of work piece on EDM performance measures namely material removal rate (MRR) and tool wear rate (TWR). In vision of this the objective of present work was to study the effect of gap current, external magnetic field and cryogenic treatment of work part on MRR and TWR. Experiments were carried out by creating a 3 mm square hole on Monel400 alloys. Based on experimental results it was found that as gap current increases the MRR and TWR increases for untreated work part. For treated work part MRR increases and TWR decreases with increasein gap current. MRR and TWR increases with constant gap current for untreated work part, as magnetic field increases. For treated work part MRR increases and TWR decreases with increase in magnetic field at constant gap current.

Damage evolution in WC–Co after repeated dynamic compressive loading detected by eddy current testing

The failure behaviour of WC–Co hard metals with binder contents of 6 and 12wt.% was investigated under repeated dynamic loadings. The materials were consolidated by field-assisted sintering and exhibited WC grain sizes of about 200nm. Static and dynamic strength tests on a servo-hydraulic testing machine and a modified split Hopkinson pressure bar (MSHPB), respectively, resulted in compressive fracture strengths between 4370MPa and 6660MPa depending on the Co content and the applied strain rate. Moreover, the hard metal samples were repeatedly loaded in the MSHPB below their compressive strength at different stress levels, and subsequently investigated by eddy current testing after each loading step. The measured eddy current signal was dependent on the stress level and the number of impacts. Additionally, the damage of the samples was examined with the help of images from the scanning electron microscope (SEM).Static strength tests of the pre-damaged material revealed that the residual strength of WC–6Co decreases with an increasing number of impacts. In contrast, WC–12Co shows strain hardening caused by the multiple-impact loadings. The size of the fragments revealed intensified microcrack initiation.

Modeling the relaxation of polymer glasses under shear and elongational loads

Glassy polymers show "strain hardening": at constant extensional load, their flow first accelerates, then arrests. Recent experiments under such loading have found this to be accompanied by a striking dip in the segmental relaxation time. This can be explained by a minimal nonfactorable model combining flow-induced melting of a glass with the buildup of stress carried by strained polymers. Within this model, liquefaction of segmental motion permits strong flow that creates polymer-borne stress, slowing the deformation enough for the segmental (or solvent) modes then to re-vitrify. Here, we present new results for the corresponding behavior under step-stress shear loading, to which very similar physics applies. To explain the unloading behavior in the extensional case requires introduction of a "crinkle factor" describing a rapid loss of segmental ordering. We discuss in more detail here the physics of this, which we argue involves non-entropic contributions to the polymer stress, and which might lead to some important differences between shear and elongation. We also discuss some fundamental and possibly testable issues concerning the physical meaning of entropic elasticity in vitrified polymers. Finally, we present new results for the startup of steady shear flow, addressing the possible role of transient shear banding.

A new grafting monomer for synthesizing long chain branched polypropylene through melt radical reaction

A grafting monomer (p-(3-butenyl)styrene, BS)) containing two carbon–carbon double-bonds with different reactivity was used in melt radical reaction to prepare long chain branched polypropylene (LCB-PP). High-temperature size-exclusion (HT-SEC) coupled with triple detectors, rheometer and NMR were used to characterize the microstructure of the resultant LCB-PP. BS showed double functions in mediating radical reaction, i.e. stabilizing macroradicals and promoting branching reaction. Compared to styrene and divinylbenzene, using BS as a grafting monomer led to the formation of more uniformly distributed LCB structure on the PP backbone. Thus the resultant sample showed strain hardening in extensional flow and had high melt strength.

Study on Durability of Engineered Cementitious Composites

Engineered cementitious composites (ECC) has good ductility, with its unique strain hardening and multiple cracking characteristics. Through the research of uniaxial direct tension performance and durability tests of ECC blending with polyvinyl alcohol (PVA) fiber, the tensile stress-strain curves, the freeze-thaw resistances and the impermeability of ECC were analyzed. The tensile stress - strain curve results show strain hardening of ECC achieved under the uniaxial tensile load; PVA fiber has good crack resistance toughening effect, can significantly improve crack resistance and deformation capacity of cementitious composites. The maximum tensile strain of the ECC is between 3800με to 8657με (20-50 times that of polypropylene fiber concrete) displays high toughness and large deformation characteristics. The freezing level of the ECC is higher than F300, which is ideal for the maintenance and reinforcement of concrete structures in cold regions. Domestic and imported PVA fiber can significantly improve the impermeability and crack resistance of the ECC.

Effect of Particle Size to Sand’s Shear Strength and Deformation under High Pressure

This For mastering the effect of particle size to strength and deformation characteristics of sand under high pressure, DRS-1 high pressure direct shear test system is used in shear test for 3 groups of sand under pressure of 12 to 15 MPa. Through analysis of the test results, it is obtained that standard sand under high pressure shows strain hardening and volume contraction characteristics. The relationship curves of stress-strain and volume contraction-shear strain appear as hyperbola, they can be simulated by Duncan-Chang hyperbolic model. Sand with small particle size has better group, greater density and higher strength, but the shear contraction is not obvious. Sand with poor group has particle crushing under high pressure, which makes the soil particles rearrange and then the average porosity decreases at the same time large pores disappear, so finally it shows a more significant shear contraction characteristic.

Analysis and mathematical modelling of elevated temperature flow behaviour of austenitic stainless steels

High temperature flow behaviour of various grades of austenitic stainless steels viz. 304L, 304, 304 (as-cast), 316L and 15Cr–15Ni–Ti modified austenitic stainless steels (alloy D9) were analyzed by performing isothermal hot compression tests in a wide range of temperatures (1073 K to 1473 K for 304L, 304, 304 (as-cast), 316L and 1123 K to 1523 K for alloy D9) and strain rates (0.001–1 s −1). It has been observed that all these materials show strain hardening, strain rate hardening, thermal softening, coupled effect of temperature and strain, and temperature and strain rate on flow stress in the hot working domain. The modified Zerilli–Armstrong (MZA) model which considers the above significant effects on flow stress has been applied to predict the flow behaviour of these materials. The material constants of the MZA model for each material have been evaluated and subsequently applied to predict the flow stress. It has been demonstrated that the MZA model could adequately represent the elevated temperature flow behaviour of these materials over the entire ranges of strain, strain rate and temperature.

Elongational behavior of epoxy during curing

AbstractElongational behavior of epoxy (epoxy/curing agent = 100/0.5, w/w) cured at various conditions over the critical gelation time was investigated. Dynamic viscoelastic measurements of the epoxy system were performed and the critical gelation time of epoxy was determined according to the frequency dependence of G′ and G″ proposed by Winter and Chambon. Elongational behavior of epoxy cured for various times were measured. Epoxy, cured over the critical gelation time, showed strain hardening and elongational behavior similar to a crosslinked rubber. Increase of elongational viscosity of the sample occurred early, and the sample broke at small strain as curing time increased. The effect of strain rate on the elongational stress of epoxy cured near the critical gelation time was measured at various strain rates. For epoxy cured for critical gelation time only, high stress at a small strain rate was represented as strain rate increased. When increasing curing time further, the tensile stress converged on a single curve regardless of strain rate, and samples broke at nearly the same stress and strain. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Glass Transition Temperature Influence on Crosslinked and Entangled Polymer Interfaces

The influence of glass transition temperature (Tg) on crosslinked and entangled polymer interfaces was investigated using coarse grained molecular dynamics (MD). A crosslinked polymer interface and an entangled polymer interface were built and the Tg for each system were obtained by confining a thin film between two rigid walls. The physical properties of each system above and under Tg were compared. The mechanical properties were also explored by pulling the interfaces apart at different temperature. The results are qualitatively agreed with experimental observations. Furthermore, the present results show that, when under tensile loading at temperature higher than Tg, the entangled interface exhibits strain softening while the crosslinked thin film is still able to show strain hardening. The different performances may due to that, at high temperature, the high mobility of monomers tend to unravel the entangled chain in linear polymer system while in crosslinked system, monomers with high mobility tend to arrest the void and decrease the void propagation.

The rheology of Torlon ® solutions and its role in the formation of ultra-thin defect-free Torlon ® hollow fiber membranes for gas separation

Fundamental understanding of the material science and rheological engineering to fabricate Torlon ® 4000T-MV and 4000TF hollow fiber membranes with an ultra-thin and defect-free dense-selective layer for gas separation has been revealed. We have firstly investigated the rheology of Torlon ® 4000T-MV and 4000TF dope solutions, and then determined the effect of temperature-correlated shear and elongational viscosities on the formation of Torlon ® fibers for gas separation. Interestingly, Torlon ® 4000T-MV and 4000TF possess different rheological characteristics: the elongational viscosity of Torlon ® 4000T-MV/NMP solution shows strain thinning, while Torlon ® 4000TF/NMP solution shows strain hardening. The balanced viscoelastic properties of dope solutions, which are strongly dependent on the spinning temperature, have been found to be crucial for the formation of a defect-free dense layer. The optimum rheological properties to fabricate Torlon ® 4000T-MV/NMP hollow fibers appear at about 48–50 °C, and the resultant fibers have an O 2/N 2 selectivity of 8.37 and an apparent dense layer thickness of 781 Å. By comparison, the best Torlon ® 4000TF fibers were spun at 24 °C with an O 2/N 2 selectivity of 8.96 and a dense layer of 1116 Å. The CO 2/CH 4 selectivity of the above two Torlon ® variants is 47 and 53.5, respectively.

Model study for large deformation of physical polymeric gels

A model for large deformation of polymer gels with physical cross-linking is developed and shown to be in good agreement with experimental stress-strain curves which show strain hardening in intermediate strains followed by strain softening in large deformations near the yield strain. The model takes into account the coil-helix transition equilibrium and allows for the distribution of the end-to-end distance. The gel is considered to be formed by long flexible chains and crystalline zones acting as junctions of the chains. The number of segments contained in a flexible chain is variable due to the equilibrium between the two regions. As the end-to-end distance increases due to the deformation, more and more segments are reeled out from the junction zone. Finally, one end of the chain is librated from the junction and the chain becomes dangling. The appearance of dangling chains causes the strain softening because they cease to contribute to the elasticity. From the parameter dependence of the stress-strain relations, it was found that the yield behavior depends strongly on the distribution of end-to-end distance. The yield strain is approximately given by the ratio of the upper limit of the number of segments and the average end-to-end distance. The standard deviation of the end-to-end distance affects significantly the width of the peak in the stress-strain curve, thus determining the degree of strain softening.

Experimental investigation of mechanical properties of bedded salt rock

Because of salt cavern utilization for liquid, gas and solid waste storage, salt rock mechanical properties are needed for assessments of facility, stability and safety. Bedded salt deposits are widespread and used as much or more than diapiric salt bodies as storage facility hosts, but experimental data on the mechanical properties of bedded salt rock with impurities are far less common than data available on relatively pure diapiric salt rocks. Through laboratory uniaxial and triaxial compression experiments on rock salt (halite), interlayers (anhydrite) and bedded composite specimens (anhydrite–halite and mudstone–halite), differences in mechanical properties of the various lithologies are explored. In the composite specimens, the weakest or the most deformable component governs the behavior. Also, the properties of bedded composite lithology specimens tend to be in between the property ranges of the “pure” lithologies. The elastic modulus of the bedded salt rock increases from 5.3 to 24.1 GPa with an increase in the confining stress from 0 to 15 MPa, with some evidence of sample damage. The ductile transition for halite at the strain rates used is at about σ 3∼10 MPa. With increasing σ 3, the anhydrite–halite composite lithology deformation showed strain hardening and a strong trend to ductile behavior as the halite bands tended to dominate the behavior. Strain incompatibility effects exist along interfaces between creeping and non-creeping phases in anhydrite–halite composite lithologies. Mudstone–halite rocks tended to be extremely weak, compared with all other specimens.

Rheological properties and foaming behavior of polypropylenes with different molecular structures

This article investigates how rheological properties of polypropylenes with different molecular structures influence their foaming behavior. The molecular structure of the different polypropylenes is analyzed by size exclusion chromatography coupled with a light scattering detector, and by rheological means, such as the molar mass dependence of the zero shear-rate viscosity. The main focus of the rheological experiments is laid on the strain hardening and failure behavior of the melts in uniaxial elongational flow. For all linear polypropylenes investigated, a rupture of samples occurred before the maximum strain—accessible with the instrument used—was achieved. For the linear polypropylenes, a growth of the stress to rupture and the elongation at break were found with increasing molar mass, which go along with an increase of the expansion ratio in the foaming experiments. Besides the linear polypropylenes, which do not show strain hardening, several so-called high melt strength polypropylenes were investigated. It was found that the strain hardening of those polypropylenes causes not only a high melt strength, but also a more uniform sample deformation in the elongational experiments, and thus high elongations at break or even no rupture of the samples. Due to the superior homogeneity of deformation, the foams of polypropylenes, which show pronounced strain hardening at the strain rates relevant for foaming, possess higher expansion ratios than linear samples of the same melt strength. Furthermore, it was found that the foaming behavior of the polypropylenes is governed by the elongational behavior of the melts independently of the molecular constitution, which is responsible for it. This means that different molecular structures, which cause a similar elongational behavior of the melt, lead to a comparable foaming behavior.