Stress Elongation Research Articles

Influence of pre-straining path on high cycle fatigue performance of DP 600 steel

An evaluation of tensile and fatigue performances of dual phase steel subjected to change in the pre-strain path are presented in this work. Initially material is loaded up to a certain amount of pre-strain in the rolling or transverse direction, and after that specimen for tensile and fatigue tests are fabricated in the parallel or orthogonal to the pre-straining directions. Irrespective of pre-straining direction, all the pre-strained specimens show an increase in yield stress and simultaneous reduction in uniform elongation of the material, as well as improvement of high cycle fatigue life. The material exhibits better fatigue resistance for those specimens where fatigue testing is done orthogonal to the initial pre-straining direction.

Interrelationships of Pressure-Dependent Hole Fraction and Elongational Viscosity in Polymer Melts

The elongational flow behavior of polyethylene, polypropylene, polystyrene, poly(methyl methacrylate), and polycarbonate, temperatures from 70 to 290 °C and pressures up to 70 MPa, is examined with the Yahsi-Dinc-Tav (YDT) model and its particular case known as the Cross model. The viscosity data employed in the range of 3-405 s-1 elongational rates were acquired from the literature at ambient and elevated pressures. The predictions and the fitting results of the proposed YDT model with the same measurement data are compared with the Cross model. The average absolute deviations of the viscosities predicted by the YDT model range from 0.54% to 9.44% at ambient and 1.95% to 6.28% at high pressures. Additionally, the linear formulations derived from the YDT model are employed to relate the viscosity with temperature and hole fraction (“thermooccupancy” function) at zero level of elongational rate and constant elongational rate along with constant elongational stress. The effects of the four viscosity parameters (such as transmission and activation energy coefficients in these equations) on the elongational viscosity are analyzed in detail and some conclusions on the structural differences for the polymers are discussed.

The Effect of Viscosity-Modifying Admixture on the Extrudability of Limestone and Calcined Clay-Based Cementitious Material for Extrusion-Based 3D Concrete Printing.

To investigate the effects of viscosity-modifying admixture (VMA) on the extrudability of limestone and calcined clay-based cementitious materials, three mix designs with different dosages of VMA were proposed in this study. The ram extrusion was utilized as an extrusion model for exploring the fresh properties of printable materials. Two methods were used, based on the ram extruder setup—(a) extruding materials with the same extrusion speed at different rest times to determine how the pressure changes with time; (b) extruding materials with different extrusion speeds at the same rest time to investigate the material flow parameters using the Basterfield et al. model. The main findings of this study could be summarized as—(1) the extrusion pressure of all mix designs exhibited an increasing trend with time. At the same tested age, the extrusion pressure under 0.25 mm/s of piston speed was increased and the shape retention of the extruded filaments was enhanced by increasing the dosage of VMA; (2) the correlation between the experimental results and the Basterfield et al. model was excellent (R-squared value: 0.99). The mixture with a higher content of VMA showed an increased elongational yield stress, flow consistency, and shear yield stress.

Environmentally Stable Polymer Gels with Super Deformability and High Recoverability Enhanced by Sub‐5 nm Particles in the Nonvolatile Solvent

ABSTRACTIt remains challenging to satisfy the combined performances for hydrogels with excellent mechanical behavior, high deformability, and super recoverability under harsh environmental conditions. In this study, we first established a strong polymer network via the crosslinking of polymer chains on the surfaces of sub‐5‐nm calcium hydroxide nanospherulites in ethylene glycol solvent. The organic gel expressed excellent mechanical properties such as a recoverable compressive engineering stress of 249 MPa and an elongation stress of 402 KPa, which was attributed to the uniform nanosized crosslinking structure as characterized by SEM. Moreover, the nonvolatile solvent remained in the gel, meaning that the sample can resist a wide temperature range of −56 to 100 °C without losing the elastic properties. This novel organic gel could provide promising routes to develop the ideal elastic carriers for wearable devices, smart skin sensors, and damping materials. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 713–721

Effects of dislocation density, temperature and Cr concentration on helium behavior in α-Fe

Molecular dynamics (MD) simulation has been performed to study Helium (He) behavior and deformation feature of α-Fe with 0.1 at.% He injection. The effects of different pre-existing dislocation densities, temperatures and Cr concentrations were investigated in this study. Voronoi clusters (VCs) analysis and dislocation extraction algorithm (DXA) were performed to study the distribution of He clusters and the evolution of dislocations, respectively. The results show that He atoms are easier to segregate on pre-existing dislocations. Average size of He clusters is monotonically decreasing and total number is increasing with increasing dislocation density from 0 to 6.74 × 1011 cm−2. Furthermore, in tensile process, pre-existing dislocations accelerate the formation of larger He bubbles and these He bubbles enhance plasticity and promote dislocation multiplication. Higher dislocation density leads to higher yield stress, yield strain, and elongation stress. In addition, higher temperature (600 K and 800 K) can motivate He diffusion and improve dislocation mobility, whereas higher Cr concentration (9 at.% and 14 at.%) retards both of them. These increasing temperatures and decreasing Cr concentrations lead to decreasing yield stress, yield strain, but increasing dislocation density. All of these findings are helpful for understanding the atomic-level deformation process of He bubbles and dislocations evolutions in detail, which are elusive through experimental techniques. This will have profound significance for design and development of nuclear materials.

Selected Properties of Multilayer Films Applied for Vacuum and Modified Atmosphere Packaging Systems

AbstractThe aim of the research was to determine the selected properties of packaging materials applicable for vacuum and modified atmosphere packaging. Six samples of multilayer films with different composition were tested to evaluate the thickness, basic weight, density and the mechanical properties in elongation and puncture tests. Zwick/Roell apparatus equipped with elongation jaws and a puncture pin was used in the experiments. The tensile modulus, tensile strength, elongation at tensile strength, stress at break and work at break were investigated with the elongation test, whereas the breaking force, puncture work and extension at break were evaluated with the puncture test. The obtained results allow conclusion that the thickness and basic weight of the packaging films used in the vacuum system was higher comparing with the MAP system. Analyzing the results of the elongation modulus and stress at break it was reported that the materials used for food packaging in the MAP system were characterized with better properties, whereas elongation at the tensile strength was higher for films applied in the vacuum system, except for a film with a metalized layer. The highest resistance for puncture was evaluated for OPET/PE film used in MAP packaging. The extension at break under the puncture test was higher for films dedicated for vacuum packaging suggesting their better resistance for the mechanical damage by a thin pin. Several correlations between physical and mechanical properties of multilayer films were found.

Velocity Distribution of Extensional Flow Fields in an Eccentric Cylinder of an Extensional Extruder

The extensional extruder is a novel method of polymer processing which generates high elongation stress within a polymer. A mathematical model is proposed which describes the velocity distribution during polymer processing in such an extruder. The modeling results indicate that the dimensionless velocity profiles for power-law fluids decrease as the power-law index increases. Overall, the extensional extruder has an advantage for dispersion effects over traditional screw extruders based on the extensional rheology.

Modifying the rheological properties of polypropylene under elongational flow by adding polyethylene

Adding polyethylene greatly affects the rheological response of isotactic polypropylene (PP) under uniaxial elongational flow. Though strain hardening in the transient elongational viscosity barely appeared in pure PP, we induced strain hardening by adding low-density polyethylene (LDPE) to PP, even though the blends showed a phase-separated structure. During elongational flow, LDPE droplets dispersed in the PP were deformed in the flow direction. Because LDPE shows marked strain hardening in the elongational viscosity, the deformed LDPE droplets behaved as rigid fibers as the strain increased. Consequently, the PP between the fibrous LDPE droplets experienced excess localized deformation, which increased the apparent elongational viscosity. Furthermore, adding the high-density polyethylene (HDPE) increased the drawdown force—defined as the force required for uniaxial stretching of a molten polymer in the nonisothermal condition. This behavior comes from the rapid crystallization of HDPE, which causes the deformed HDPE particles to act as rigid fibers in the molten PP and enhances the PP crystallization, which increases the elongational stress.

High Impact Polystyrene/CNT nanocomposites: Application of volume segregation strategy and behavior under extensional deformation

The present work focuses on the analysis of two composites: a pure amorphous Polystyrene and a rubber modified Polystyrene (High Impact Polystyrene), both filled with Carbon Nanotubes (CNT) for generating Conductive Polymer Composites (CPC). Recently, CPC has been obtained with very low filler content following a volume segregation strategy which maintains the filler in one continuous phase. This phenomenon enhances the contacts between fillers and consequently gives birth to conductive composites with ultra-low filler concentration. In this work the two composites are analyzed using a specific set-up that gives the possibility to monitor both electrical conductivity evolution and the elongational stress in the melt state. This set-up allows determining the maximal deformation that nanocomposites can undergo before becoming insulating. In addition, the dynamic of the network evolution that is, its destruction and structuring has been analyzed. When the destruction is predominant, the two composites behave the same. However, we have observed a slight disparity when there is a possible competition between destruction and structuring during extensional deformation. This difference has been accounted to the presence of the nodules that might locally prevent the structuring mechanism during extensional deformation.

Elongational Stress and Velocity of Dilute Polymer Solutions Flowing into Small Apertures

Elongational Stress and Velocity of Dilute Polymer Solutions Flowing into Small Apertures

Preparation and characterization of poly(hydroxybutyrate) and hollow glass microspheres composite films: morphological, thermal, and mechanical properties

In this work, of poly(hydroxybutyrate) (PHB) and glass microspheres (HGM) in different compositions with polypropylene-grafted-maleic anhydride (PP-g-MA) composites were developed by melt intercalation, and then, films from the composites and the polymer matrix were obtained. Subsequently, the morphological, thermal, and mechanical properties were evaluated. A good distribution and intermediate dispersion of the filler throughout the matrix with the formation of some agglomerates was observed, while adhesion between the components was not achieved. The HGM did not affect significantly the thermal properties of the systems; there were discrete variations in the initial degradation temperature (Tonset), peak temperature (Tp), and melting temperature (Tm). However, the degree of crystallinity decreased with the addition of the HGM, and the film with 3% HGM had a variation above 11%. Additionally, it was noticed that the composites (regardless of the presence of PP-g-MA) presented small values of elongation and ultimate stress compared with those of pure PHB.

Effect of high molecular weight on pore formation and various properties of microporous membrane used for lithium-ion battery separator

In this article, iPP cast films with high molecular weight in middle layer were prepared to investigate the effect of high molecular weight component on pore formation and mechanical strength. The results of DSC, SEM and 2D-XRD indicated that higher molecular weight and die draw ratio (DDR) would not alter the thickness of lamellae but enhance the orientation and the content of extend-chain crystal. Then the tensile testing results show that higher DDR would improve the overall strength and the elastic recovery substantially but reduce the elongation at break dramatically. Additionally, the cast film containing higher molecular weight component has longer elongation and yield stress but shows a more gentle decrease of elastic recovery during the repeatedly stretching cycle. Moreover, further detailed characterization of the microporous structure after uniaxial stretching manifest that lower DDR is extremely unfavorable to pore formation, resulting in lowest porosity and poorer pore size distribution. While this inferior microporous structure would be improved by increasing DDR, accompanied by ascending porosity. Moreover, the membrane completely composed of high molecular weight also exhibits more coarse fibrils, which is stemmed from the undivided row-nucleated lamellae. Furthermore, the puncture and tensile strength were reinforced by boosting molecular weight and DDR significantly, which is more beneficial to battery assembly and safety.

Challenges in the formability of the next generation of automotive steel sheets

ABSTRACTImproving strength may lightweight car bodies by thinning the structure gauge while bearing the same crash loads. Development of the third generation advanced high-strength steels aiming to boost material ductility to over 30% total elongation has become the common sense of potential solutions for stamping. Not only total elongation but also several other parameters, involving presswork hardening, anisotropy ratio, strain rate sensitivity, uniform elongation, edge sensitivity and internal stress between phases, are critical in determining the formability under the complex stress and strain distributions during stamping. We clarify here the logic of formability and difficulties in this context of three types of steel: quenching and partitioning, δ-transformation-induced plasticity and medium Mn-TRIP.

Using microfluidic devices to study thrombosis in pathological blood flows.

Extreme flows can exist within pathological vessel geometries or mechanical assist devices which create complex forces and lead to thrombogenic problems associated with disease. Turbulence and boundary layer separation are difficult to obtain in microfluidics due to the low Reynolds number flow in small channels. However, elongational flows, extreme shear rates and stresses, and stagnation point flows are possible using microfluidics and small perfusion volumes. In this review, a series of microfluidic devices used to study pathological blood flows are described. In an extreme stenosis channel pre-coated with fibrillar collagen that rapidly narrows from 500 μm to 15 μm, the plasma von Willebrand Factor (VWF) will elongate and assemble into thick fiber bundles on the collagen. Using a micropost-impingement device, plasma flow impinging on the micropost generates strong elongational and wall shear stresses that trigger the growth of a VWF bundle around the post (no collagen required). Using a stagnation-point device to mimic the zone near flow reattachment, blood can be directly impinged upon a procoagulant surface of collagen and the tissue factor. Clots formed at the stagnation point of flow impingement have a classic core-shell architecture where the core is highly activated (P-selectin positive platelets and fibrin rich). Finally, within occlusive clots that fill a microchannel, the Darcy flow driven by ΔP/L > 70 mm-Hg/mm-clot is sufficient to drive NETosis of entrapped neutrophils, an event not requiring either thrombin or fibrin. Novel microfluidic devices are powerful tools to access physical environments that exist in human disease.

Understanding the break-up phenomena in an orifice-valve high pressure homogenizer using spherical bacterial cells (Lactococcus lactis) as a model disruption indicator

The break-up phenomena occurring in a high pressure homogenizer equipped with an interchangeable orifice valve were investigated by measuring the inactivation of Lactococcus lactis. Data were collected at varying the orifice size (80, 100, and 150 μm), the operating pressure (100–200 MPa), the number of passes (1–10), and the fluid viscosity (2.5–7.9 mPa s, changed by adding 0–50 % wt PEG 200 to buffered peptone water) to identify the correlations of the fragmentation occurring in the valve with the main fluid dynamic phenomena (turbulence, elongational and shear stresses, and cavitation). In addition, also the effects of a purely shearing or ultrasound treatment on cell break-up were considered.The results show that the most intense break-up phenomena occur for the smallest orifice size, highest pressure, and lowest viscosity. However, at low viscosity, turbulence, together with the elongational stresses appear to be the controlling factors of cell break-up, whereas, at higher viscosities, the shear stresses become increasingly important. The occurrence of cavitation is only slightly affected by viscosity, and mainly depends on the velocities reached in the homogenization valve.

Dispersion of carbon nanotubes in ultrahigh‐molecular‐weight polyethylene by melt mixing dominated by elongation stress

AbstractA novel melt‐mixing method and corresponding mixer for polymer materials are reported. The effects of carbon nanotube (CNT) loading, rotation rate and mixing time on the morphology and properties of CNTs/ultrahigh‐molecular‐weight polyethylene (UHMWPE) nanocomposites were experimentally investigated in detail using the mixer. Homogeneous dispersion of CNTs in intractable UHMWPE is successfully realized without the aid of any additives or solvents. Differential scanning calorimetry results showed that the crystallinity increases 13.8% when 1 wt% of CNTs is added into the composites. The maximum crystallinity increased 13.5% and then decreased slightly with increasing rotation rate. The mixing time had little effect on crystallinity. Rheological tests reveal that the effect of CNT loading on the storage modulus/complex viscosity is a result of competition between the viscosity decrease due to the selective adsorption of UHMWPE onto CNT surfaces and the viscosity increase caused by the formation of an interconnected polymer–nanotube network. The storage modulus/complex viscosity decreased with increasing rotation rate/mixing time. This is a synergic result of the selective adsorption of the long molecular chains onto the CNT surface and their thermomechanical degradation. The results showed that the mixing process dominated by elongation stress is a simple, efficient green way to prepare CNTs/UHMWPE nanocomposites via melt mixing. © 2018 Society of Chemical Industry

The effect of texture and grain size on improving the mechanical properties of Mg-Al-Zn alloys by friction stir processing

Friction stir processing (FSP) was used to achieve grain refinement on Mg-Al-Zn alloys, which also brought in significant texture modification. The different micro-texture characteristics were found to cause irregular micro-hardness distribution in FSPed region. The effects of texture and grain size were investigated by comparative analyses with strongly textured rolling sheet. Grain refinement improved both strength and elongation in condition of a basal texture while such led to an increment in yield stress and a drop in elongation and ultimate stress when the basal texture was modified by FSP.

Effect of Jet Stretch in the Fabrication of Polyethersulfone Hollow Fiber Spinning for Water Separation

The effect of jet stretch on the morphology, pure water permeation and sodium chloride rejection of hollow fiber membranes is analyzed by varying the spinning take up speed. Polyethersulfone hollow fibers were spun using dry–wet spinning technique. The membrane formulation of PES/NMP/Water/PVPk10 is spun at constant extrusion rate of 3.0 cm3/ min. The fiber take up speed during spinning varied from 19.7 to 29.5 cms–1, revealed that the flux of hollow fiber membranes is minimal when the fiber take up speed is equivalent to the velocity of dope extrusion. At low jet stretch, the permeability of membranes is high with elevated ionic solutes rejection produced. The influence of elongation stress towards hollow fiber membranes morphology and its performance for water separation is also highlighted.

Predicting Linear Elongation With Conductive Thread-Based Sensors

Conductive thread-based sensors have the potential to offer ambulatory measurement of body movement and posture. While such sensors have been previously described in terms of their overall correlation with existing measurement modalities, the ability of thread-based sensors to predict linear elongation has yet to be investigated. We constructed multiple stitched sensors, 20 cm in length, using two silver-coated nylon threads and seven different types of stitches, including cover and overlock stitches. Stitched sensors underwent elongation and cyclic stress tests while sensor resistance was recorded. Optimal predictive functions were derived for each sensor such that the range-normalized error was minimized. Responses were found to be governed by the combination of stitch geometry and physical properties of the threads. The average error in predicting elongation was 14% of the usable sensor range, suggesting that the stitched sensors are perhaps better suited to intermittent detection of stretched versus unstretched states rather than continuous prediction of absolute elongation. Nonetheless, coverstitches were generally superior to overlock stitches in terms of minimizing range-normalized error. The path of the conductive looper thread and the presence of stabilizing needle threads were important determinants of the sensor’s prediction capability and repeatability. Stress tests revealed a systematic upward drift of the resistance response curve for all sensors over repeated trials, suggesting that these stitched sensors require adequate recovery time between measurements. Given the possibility of applying stitched sensors non-destructively to existing garments, further study of these sensors—particularly those comprising multiple independent stitches in novel stitch geometries—remains worthwhile.

Characterization of 3D Printed Stainless Steel SS316L Powders Joined by TIG-, Plasma- and Laser Welding

Additive manufacturing or 3D printing is becoming more and more common in industry fabrication, especially for spare parts which cannot be found easily on the local market and for tailor made parts. For large parts and due to its size limitations, it is sometimes required to join 3D printed parts with other 3D printed parts or conventionally produced parts. In this work, 3D printed flat plates 4mm thick were prepared by direct laser sintering of SS316L powders. Mechanical properties and microstructure were investigated. Elongation and maximum tensile stress of 3D printed flat plates are with 1.3% and 704MPa smaller than the values 2% and 1241MPa of cold-rolled SS316 parts. The 3D printed flat plates were joined by welding using TIG welding, plasma welding and laser welding. The welded joints were tested for their mechanical properties and microstructure. For the used welding conditions, the fracture occurred outside the weld zone. The maximum stress of the welded parts is 65-80% of 3D printed flat plates. The elongation of the welded parts is above 3%.