Pronounced Strain Hardening Research Articles

High‐melt‐elasticity poly(ethylene terephthalate) produced by reactive extrusion with a multi‐functional epoxide for foaming

ABSTRACTA multi‐functional epoxide oligomer, Joncryl ADR‐4368 (ADR), is used as a modifier to prepare foamable poly(ethylene terephthalate) (PET) by reactive extrusion and compared with common tetra‐functional modifier pyromellitic anhydride (PMDA) as a reference. Torque evolution reveals that ADR has a faster reaction with PET than PMDA. The reactions generate long‐chain branches and gel structures, which are confirmed by rheological methods. Shear rheological studies show that PET modified with both ADR and PMDA display higher complex viscosity and lower loss tangent than unmodified sample. In particular, at a given viscosity level, ADR leads to a lower loss tangent than PMDA. Moreover, compared to PMDA, the addition of ADR results in a higher die pressure during extrusion and a more pronounced strain hardening during uniaxial elongation. These results indicate that ADR‐modified PET is less viscous but more elastic than PMDA‐modified PET. Owing to the higher elastic properties, ADR‐modified PET presents better foaming performance in batch foaming process with CO2 as a blowing agent. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 45805.

Unexpected cyclic stress-strain response of dual-phase high-entropy alloys induced by partial reversibility of deformation

The recently developed dual-phase high-entropy alloys are characterized by pronounced strain hardening and high ductility under monotonic loading owing to the associated transformation induced plasticity effect. Fatigue properties of high-entropy alloys have not been studied in depth so far. The current study focuses on the low-cycle fatigue regime. Cyclic tests were conducted and the microstructure evolution was studied post-mortem. Despite deformation-induced martensitic transformation during cycling at given plastic strain amplitudes, intense strain hardening in the cyclic stress-strain response is not observed. This behavior is attributed to the planar nature of slip and partial reversibility of deformation.

Long chain branching as an innovative up-cycling process of polypropylene post-consumer waste – Possibilities and limitations

Long chain branching (LCB) was used the first time as an innovative tool for value adding to PP from household post-consumer waste. Due to the highly improved melt properties, the possible application profile is extended and not only a “re-cycling” process, even a real “up-cycling” is presented. The used PP was collected from commingled household polyolefin waste, which contained different types of PP and macromolecular impurities such as 10% of polyethylene with high density (PE-HD). In addition, a single PP waste fraction from cleaned beverage and yoghurt cups was manually sorted. The up-cycled PP from single polymer waste, as well as the post-consumer blend, showed pronounced strain hardening and increased melt strength, which was comparable to LCB-PP prepared from virgin PP. However, the up-cycled post-consumer blend showed weaker mechanical performance especially low elongation at break due to PE-HD.

Biosourced blends based on poly (lactic acid) and polyamide 11: Structure–properties relationships and enhancement of film blowing processability

AbstractThe purpose of the present work is to develop a new biomaterial with suitable melt strength, stiffness‐to‐toughness balance, and the required thermal performance for food packaging applications. The study is dedicated to investigating a new physical compatibilization approach of biosourced materials as well poly (lactic acid) (PLA) and polyamide 11 (PA11) in comparison with the chemical compatibilization through addition of a multifunctionalized epoxide. For this reason, this paper deals with gaining better understanding physical compatibilization in the biosourced blend with the incorporation of an acrylic melt strength enhancer into PLA/PA11 blend. The main focus of this paper is studying how physical compatibilization improves melt strength in comparison with chemical approach. Hitherto, the chain extension–branching balance was demonstrated to be difficult to be controlled and decoupled from chemical compatibilization at the polymer–polymer interface. Hence, the physical approach is presented as an original route in this work to develop a PLA‐based blend with good film blown processability and engineering properties. Morphological, rheological and thermomechanical properties of the new obtained biosourced blends were studied. The physical compatibilization effect was confirmed by the tuning interfacial properties. The extensional rheology highlighted that melt strength for the physical blends was also improved without any pronounced strain hardening in comparison with reactive blends. Besides, a great enhancement of the blowing processing windows of physically PLA‐PA11 compatibilized blend was highlighted while higher blow‐up ratio and take‐up ratio values were obtained. Overall, a correlation between the obtained thermal and crystalline properties was performed for the optimal biosourced blown film.

Grain boundary sliding, triple junction disclinations and strain hardening in ultrafine-grained and nanocrystalline metals

A theoretical model is suggested which describes grain boundary (GB) sliding and its accommodation through dislocation slip in ultrafine-grained and nanocrystalline metals. The initial stage of the accommodating dislocation slip represents emission of lattice dislocations from triple junctions into grain interiors. The lattice dislocations emitted from a triple junction slip across a grain and are absorbed by an opposite GB where they are dissociated into GB dislocations that climb along the GB. In the situation where these GB sliding and accommodating processes are dominant, stress-strain dependences are calculated in ultrafine-grained copper. With the calculated dependences, we found that pronounced strain hardening occurs which is related to the accommodation processes and associated formation of disclinations at triple junctions of GBs. It is theoretically revealed that the special (new) strain hardening mechanism under discussion can play a significant role in enhancing ductility of ultrafine-grained and nanocrystalline metals at comparatively low temperatures.

Ti-Fe-Sn-Nb hypoeutectic alloys with superb yield strength and significant strain-hardening

In this letter new Ti-Fe-Sn-Nb hypoeutectic alloy comprised of primary dendritic β-Ti and ultrafine (β-Ti+TiFe) eutectic was developed showing superior mechanical properties. The as-cast Ti67Fe27Sn3Nb3 (at.%) alloy exhibited exceptionally high yield stress (2.18GPa) comparable to that of bulk metallic glasses. Most importantly, the sample presented significant strain-hardening (320MPa) and enhanced plasticity. The slip deformation and dislocation accumulation in the β-Ti dendrite contribute to the plasticity and the pronounced strain-hardening, whereas the high strength stems from the ultrafine (β-Ti+TiFe) eutectic structure as well as the solution hardening in the multicomponent system.

Rheological behaviour of a high-melt-strength polypropylene at elevated pressure and gas loading for foaming purposes

The rheological properties of a long chain branched polypropylene (LCB-PP) were investigated at the processing conditions of foam extrusion, namely high pressure, gas loading and high shear rates, as well as in elongational deformation. For measuring the rheological properties of PP at moderate and high deformation rates, an in-line rheometer was used. Comparison of the results to standard rotational rheometry showed good agreement. The effect of the processing parameters was quantified using shift factors for temperature, pressure and gas concentration. The influence of pressure on the shear viscosity was found to be of minor importance. In contrast, the shear viscosity was distinctly affected by CO2 concentration, reducing it to one third of its gas-free value at a concentration of 6 wt% at a specific shear rate. The change of viscosity by a variation of temperature is similar in magnitude compared to the variation due to dissolved blowing agent. Furthermore, thermo-rheological complex behaviour was observed. In the foaming process, thermo-rheological complexity could contribute to a better morphology control of long chain branched polymers compared to linear ones. The elongational viscosity was measured using both a hyperbolic die and a film stretching tool (UXF) for comparison. It is almost three decades higher than the shear viscosity in the non-linear region, due to pronounced strain hardening of the melt.

Finite strain damage-elastoplasticity in double-network hydrogels

Abstract Finite tensile behavior and fracture toughness of double-network (DN) and triple-network (TN) hydrogels synthesized from 3-sulfopropyl acrylate potassium salt (SAPS) and acrylamide (AAm) are discussed. The mechanical behavior of pseudo-semi-interpenetrating polymer network (pseudo-SIPN), pseudo-interpenetrating polymer network (pseudo-IPN) and TN hydrogels prepared from the two types of DN networks are considered. In general, the mechanical behavior of tough hydrogels is dependent upon the crosslink densities of the first and second networks in DN and TN hydrogels and the covalent connectivity of the second or third network to the previous network architecture. Pseudo-SIPN hydrogels, where no crosslinking agent is used to make the second network, exhibit necking for tensile deformations. When a very low concentration of crosslinking agent is used in the second polymerization step (pseudo-IPN), necking is suppressed and strain hardening occurs. Increasing the crosslink density of the second network produces more pronounced strain hardening, but also embrittlement of the hydrogel. For tough hydrogels where the first network is very brittle, i.e., very high crosslink density, it is difficult to prevent necking. The formation of a third, loosely crosslinked network within pseudo-SIPN and pseudo-IPN hydrogels that exhibit necking prevents necking and produces strain hardening. Loading – unloading tensile experiments conducted on DN hydrogels indicated that regardless of the stretch ratio used (high strains above or low strains below the yield point), the sample exhibited plastic flow and a residual strain. The value of the residual strain was insensitive to the magnitude of the strain at which the load was removed.

Impact of surface plasma treatment on the performance of PET fiber reinforcement in cementitious composites

The purpose of this study is to demonstrate the influence of plasma treatment on the surface properties of PET fibers used as micro reinforcement in cementitious composites. The stress transfer across cracks that untreated fibers are able to accomplish can be enhanced via plasma treatment. The increase in surface roughness and the activation of polar groups result in the reduction of surface energy as observed during wetting angle measurements. The improvement in adhesion between primary (non-recycled) fibers and the surrounding matrix brought about a more pronounced strain hardening of the specimens tested in four-point bending. Plasma treatment of thinner fibers from recycled PET led to a lower capacity for transferring tensile stresses due to the reduced cross-sectional area of the fibers. The considerable bridging force provided by the plasma treated primary PET fibers resulted in the prevention of excessive and abrupt cracking, and limited crack openings.

A master curve for the size and strain rate dependent large deformation behavior of PS nanofibers at room temperature

Unlike the nominally brittle behavior of bulk polystyrene (PS), PS nanofibers with particular combinations of molecular weight (MW) and submicron scale diameters exhibit stable necking followed by pronounced strain-hardening, resulting in simultaneous increase in strength and ductility. The ratio, Dnorm, of the fiber diameter, D, to the intrinsic macromolecular length scale described by the root-mean-square end-to-end chain distance, Ree ∼ MW, has been shown to be an effective scaling parameter to determine the initiation and evolution of necking and strain hardening in submicron scale unoriented PS fibers. In this study, the room temperature large deformation response of individual PS nanofibers is quantified for the first time over a broad range of strain rates between 10−4–102 s−1. It is shown that for all combinations of MW in the range 123,000–2,000,0000 g/mol and D in the range 200–750 nm that satisfy Dnorm < 10, an increasing strain rate results in monotonic increase of the stress amplitude without any reduction in the nanofiber stretch ratio to failure. Furthermore, the experimental stress vs. stretch ratio curves for Dnorm < 10 obeyed a multiplicative decomposition of stress into a shape and a rate component. This decomposition permits the construction of a normalized stress vs. strain master curve that captures well both the size effects, originating in the relative molecular and specimen length scales, and the strain rate effects on the mechanical behavior of PS nanofibers at room temperature.

Deformation of disentangled polypropylene crystalline grains into nanofibers

ABSTRACTIt was shown that a solid‐state deformation of polypropylene (PP) being in the form of partially disentangled powder is possible by blending with another molten polymer. During mixing of disentangled polypropylene powder with polystyrene at the temperature below melting of polypropylene crystals the shear forces deform powder grains into nanofibers. All disentangled powder particles larger than 0.7 µm underwent deformation into nanofibers having the mean thickness between 100 and 200 nm. Polypropylene nanofibers got entangled during blending and form a network within polystyrene matrix, reinforcing it. Network of entangled nanofibers can be further deformed with pronounced strain hardening and strength reaching 70 MPa at 135 °C. Blending resulted in generation of PP nanofibers and formation of PP nanofibers entangled network, thus formation of “all‐polymer nanocomposites” in one step compounding. The crucial feature for ultra‐deformation of PP grains by shearing during mixing is disentanglement of macromolecules. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 1983–1994

Seismic performance of steel H-beam to SHS-column cast modular panel zone joints

This paper sheds considerable light on the potential of using cast steel technology for seismic-resistant H-beam to tubular column joints. The considered cast modular joint was an integral assembly of joint zone, beam linking zone, and column linking zone, and the joint geometry was carefully designed to overcome some fracture-related issues that conventional joints may have. The research commenced with a set of tensile and cyclic coupon tests for a comprehensive understanding of the material properties of the entire cast modular joint at different key locations, where the material generally showed sufficient ductility, pronounced strain hardening, consistent yield and ultimate strengths, and plump hysteretic response. The following sub-frame tests covered two types of cast modular joints (Type-A and Type-B) with two column load ratios for each joint type. Each specimen represented a typical sub-frame consisting of a cast modular joint and connecting beams and columns. All the specimens exhibited good ductility and energy dissipation ability. No facture was observed at the welding zone which was relocated away from the column face, and as a result, ductile fracture was delayed to occur within the cast modular joint at very large drifts. Moreover, the shear deformation of the panel zone was sufficiently mobilised, and the energy dissipation mechanism of the sub-frame was featured by a combined action of panel zone in shear and beam section in flexural yielding. The codified design expressions for the strength and stiffness of the specimens were also revisited, and design comments were finally made.

BEHAVIOR OF STAINLESS STEEL LIPPED CHANNEL SECTIONS SUBJECTED TO ECCENTRIC COMPRESSION

The design philosophy of stainless steel requires appropriate recognition of observed material nonlinearity and pronounced strain hardening. A rational method namely, the Continuous Strength Method (CSM) has recently been to incorporate these effects but, in its current form, CSM yields better results for stocky sections. Individual capacities (i.e., pure compression and pure bending) for all types of sections and cross-section resistance against combined loading (i.e. compression plus bending) for RHS and I-sections can be predicted using CSM. The current research numerically investigates the performance of stainless steel lipped channel (LC) sections subjected to compression and bending. Nonlinear finite element models are developed and validated using available experimental results, and are consequently used to generate additional results for a wide range of cross-sections through parametric studies. Current CSM guidelines are used to propose a new set of formulations for predicting the section resistance of lipped channel sections subjected to combined loading.

Novel graft copolymers with aliphatic polyether and polyester main chains

A series of new graft copolymers was synthesized by a “grafting to” approach, in the course of which the polymer backbone was build up first by a chain extension reaction of polyether or polycaprolactone diols with bis-N-acyl lactams. During the first reaction step, benzoxazinone groups were introduced into the backbone which served as grafting sites for monoamino-terminated polyamide 12 or polystyrene. Chain extension and grafting could be performed successively and with high selectivity in the melt at 180–220 °C.The morphology and the thermo-mechanical behavior of the graft copolymers were strongly influenced by the type of the grafted side chains. Polymers with polyamide grafts exhibited a two phase morphology with a spherulitic superstructure in the micrometer scale. The tensile behavior of these polymers was characterized by a certain elastic component.In contrast, the phase morphology of a polystyrene grafted polymer was much finer. A strong influence on the glass transitions revealed improved compatibility between the components. Pronounced strain hardening was observed during the tensile test of polystyrene grafted samples.

The continuous strength method for steel cross-section design at elevated temperatures

When subjected to elevated temperatures, steel displays a reduction in both strength and stiffness, its yield plateau vanishes and its response becomes increasingly nonlinear with pronounced strain hardening. For steel sections subjected to compressive stresses, the extent to which strain hardening can be exploited (i.e. the strain at which failure occurs) depends on the susceptibility to local buckling. This is reflected in the European guidance for structural fire design EN1993-1-2 [1], which specifies different effective yield strengths for different cross-section classes. Given the continuous rounded nature of the stress–strain curve of structural steel at elevated temperatures, this approach seems overly simplistic and improved accuracy can be obtained if strain-based approaches are employed [2]. Similar observations have been previously made for structural stainless steel design at ambient temperatures and the continuous strength method (CSM) was developed as a rational means to exploit strain hardening at room temperature. This paper extends the CSM to the structural fire design of steel cross-sections. The accuracy of the method is verified by comparing the ultimate capacity predictions with test results extracted from the literature. It is shown that the CSM offers more accurate ultimate capacity predictions than current design methods throughout the full temperature range that steel structures are likely to be exposed to during a fire. Moreover due to its strain-based nature, the proposed methodology can readily account for the effect of restrained thermal expansion on the structural response at cross-sectional level.

Understanding the foamability and mechanical properties of foamed polypropylene blends by using extensional rheology

ABSTRACTIn this article, the influence of the rheological behavior of miscible blends of a linear and a high melt strength, branched, polypropylene (HMS PP), on the cellular structure and mechanical properties of cellular materials, with a fixed relative density, has been investigated. The rheological properties of the PP melts were investigated in steady and oscillatory shear flow and in uniaxial elongation in order to calculate the strain hardening coefficient. While the linear PP does not exhibit strain hardening, the blends of the linear and the HMS PP show pronounced strain hardening, increasing with the concentration of HMS PP. Related to the cellular structure, in general, the amount of open cells, the cell size, and the width of the cell size distribution increase with the amount of linear PP in the blends. Also mechanical properties are conditioned by the extensional rheological behavior of PP blends. Cellular materials with the best mechanical properties are those that have been fabricated using large amounts of HMS PP. The results demonstrate the importance of the extensional rheological behavior of the base polymers for a better understanding and steering of the cellular structure and properties of the cellular materials. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42430.

Buckling and crush resistance of high-density TRIP-steel and TRIP-matrix composite honeycombs to out-of-plane compressive load

The mechanical and structural responses of high-density TRIP steel and TRIP-steel/zirconia composite honeycomb structures were studied under uniaxial compression in the out-of-plane loading direction over a wide range of strain rates. Their mechanical response, buckling, and failure mechanisms differ considerably from those of conventional thin-walled, low-density cellular structures. Following the linear-elastic regime and the yield limit of the bulk material, the high-density square honeycombs exhibited a uniform increase in compression stress over an extended range of (stable) plastic deformation. This plastic pre-buckling stage with axial crushing of cell walls correlates with the uniaxial compressive response of the bulk specimens tested. The dominating material effects were the pronounced strain hardening of the austenitic steel matrix accompanied by a strain-induced α’-martensite nucleation (TRIP effect) and the strengthening effect due to the zirconia particle reinforcement. The onset of critical plastic bifurcation was initiated at high compressive loads governed by local or global cell wall deflections. After exceeding the compressive peak stress (maximum loading limit), the honeycombs underwent either a continuous post-buckling mode with a folding collapse (lower relative density) or a symmetric extensional collapse mode of the entire frame (high relative density). The densification strain and the post-buckling or plateau stress were determined by the energy efficiency method. Apart from relative density, the crush resistance and deformability of the honeycombs were highly influenced by the microstructure and damage evolution in the cell walls as well as the bulk material’s strain-rate sensitivity. A significant increase in strain rate against quasi-static loading resulted in a measured enhancement of deformation temperature associated with material softening. As a consequence, the compressive peak stress and the plastic failure strain at the beginning of post-buckling showed an anomaly with respect to strain rate indicated by minimum values under medium loading-rate conditions. The development of the temperature gradient in the stable pre-buckling stage could be predicted well by a known constitutive model for quasi-adiabatic heating.

An investigation on microstructure and mechanical propertiesof a Nb-microalloyed nano/ultrafine grained 201 austenitic stainless steel

The present study was aimed to investigate the mechanical properties of a nano/ultrafine grained Nb-containing 201 austenitic stainless steel. For this purpose, 90% cold rolled sheets with fully martensitic microstructure were isothermally annealed at 900°C for different times of 1 to 1800s, leading to the reversion of strain- induced α′-martensite to austenite and significant grain refinement. Ferritescopy, X-ray diffractometery and optical/electron microscopy techniques along with hardness measurements and tensile tests were used to study the evolution in microstructure and mechanical properties in the course of annealing. It was found that heavy cold-rolling promoted formation of Nb-rich carbonitrides which effectively retarded the growth of fine reverted austenite grains. The obtained results showed that the complete transformation of martensite to austenite took about 60s with the corresponding austenite grain size of about 90nm. This sample had an ultrahigh yield strength of 1170MPa, which was almost four times higher than that of the raw material and outstanding elongation of 37%. Further, the true stress–strain curves of the reversion annealed samples revealed two distinct uniform elongation stages (stage I and stage II), whereas, the onset of stage II was concurrent with pronounced strain hardening. This was related to the sharp increase in the formation of α′-martensite upon tensile straining.

Effect of zirconia and aluminium titanate on the mechanical properties of transformation-induced plasticity-matrix composite materials

Metal-matrix composite materials composed of an austenitic stainless steel with different ceramic particle reinforcements were investigated in this study. The test specimens were prepared via a powder metallurgical processing route with extrusion at room temperature. As reinforcement phase, either magnesia partially stabilized zirconia or aluminium titanate with a volume content of 5% or 10% was used. The mechanical properties were determined by quasi-static compressive and tensile loading tests at ambient temperature. The microstructure characteristics and failure mechanisms during deformation contributing to significant changes in strength and ductility were characterized by scanning electron microscopy including energy dispersive X-ray spectroscopy and electron back-scatter diffraction, and by X-ray diffraction. The composite materials showed higher stress over a wide range of strain. Essentially, the deformation-induced formation of α′-martensite in the steel matrices is responsible for the pronounced strain hardening. At higher degrees of deformation, the material behavior of the composites was controlled by arising damage evolution initiated by particle/matrix interface debonding and particle fracture. The particle reinforcement effects of zirconia and aluminium titanate were mainly controlled by their influences on martensitic phase transformations and the metal/ceramic interfacial reactions, respectively. Thereby, the intergranular bonding strength and the toughness of the steel/ceramic interfaces were apparently higher in composite variants with aluminium titanate than in composites with magnesia partially stabilized zirconia particles.

Rheological and foaming behavior of linear and branched polylactides

In this work, a chain extender (CE) was added to polylactide (PLA) to improve its foamability. The steady and transient rheological properties of neat PLA and CE-treated PLA revealed that the introduction of the CE profoundly affected the melt viscosity and elasticity. The linear viscoelastic properties of CE-enriched PLA suggested that a long-chain branching (LCB) structure was formed from the reaction with the CE. LCB-PLA exhibited an increased viscosity, more shear sensitivity, and longer relaxation time in comparison with the linear PLA. The LCB structure was also found to affect the transient shear stress growth and elongational flow behavior. LCB-PLA exhibited a pronounced strain hardening, whereas no strain hardening was observed for the linear PLA. Batch foaming of the linear and LCB-PLAs was also examined at foaming temperatures of 130, 140, and 155 °C. The LCB structure significantly increased the integrity of the cells, cell density, and void fraction.