Polymers

Research

16 pages, 882 KiB

Open AccessArticle

Time-Dependent Mechanical Properties in Polyetherimide 3D-Printed Parts Are Dictated by Isotropic Performance Being Accurately Predicted by the Generalized Time Hardening Model

by A. G. Salazar-Martín, A. A. García-Granada, G. Reyes, G. Gomez-Gras and J. M. Puigoriol-Forcada

Polymers 2020, 12(3), 678; https://doi.org/10.3390/polym12030678 - 19 Mar 2020

Cited by 8 | Viewed by 3857

Abstract

The Fused-Deposition Modelling (FDM) technique has transformed the manufacturing discipline by simplifying operational processes and costs associated with conventional technologies, with polymeric materials being indispensable for the development of this technology. A lack of quantification of viscoelastic/plastic behavior has been noted when addressing [...] Read more.

The Fused-Deposition Modelling (FDM) technique has transformed the manufacturing discipline by simplifying operational processes and costs associated with conventional technologies, with polymeric materials being indispensable for the development of this technology. A lack of quantification of viscoelastic/plastic behavior has been noted when addressing FDM parts with Polyetherimide (PEI), which is currently being investigated as a potential material to produce functional end-products for the aerospace and health industry. Primary and secondary creep along with stress relaxation tests have been conducted on FDM PEI specimens by applying stresses from 10 to 40 MPa for 100 to 1000 min. Specimens were 3D printed by varying the part build orientation, namely XY, YZ, and XZ. Creep results were fitted to the Generalized Time Hardening equation (GTH), and then this model was used to predict stress relaxation behavior. FDM PEI parts presented an isotropic creep and stress relaxation performance. The GTH model was proven to have a significant capacity to fit viscoelastic/plastic performances for each single build orientation (r > 0.907, p < 0.001), as well as a tight prediction of the stress relaxation behavior (r > 0.998, p < 0.001). Averaged-orientation coefficients for GTH were also closely correlated with experimental creep data (r > 0.958, p < 0.001) and relaxation results data (r > 0.999, p < 0.001). FDM PEI parts showed an isotropic time-dependent behavior, which contrasts with previous publications arguing the significant effect of part build orientation on the mechanical properties of FDM parts. These findings are strengthened by the high correlation obtained between the experimental data and the averaged-coefficient GTH model, which has been proven to be a reliable tool to predict time-dependent performance in FDM parts. Full article

(This article belongs to the Special Issue Mechanical Behavior of Polymers)

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14 pages, 895 KiB

Open AccessArticle

The Importance of the Mixing Energy in Ionized Superabsorbent Polymer Swelling Models

by Eanna Fennell, Juliane Kamphus and Jacques M. Huyghe

Polymers 2020, 12(3), 609; https://doi.org/10.3390/polym12030609 - 7 Mar 2020

Cited by 12 | Viewed by 4998

Abstract

The Flory–Rehner theoretical description of the free energy in a hydrogel swelling model can be broken into two swelling components: the mixing energy and the ionic energy. Conventionally for ionized gels, the ionic energy is characterized as the main contributor to swelling and, [...] Read more.

The Flory–Rehner theoretical description of the free energy in a hydrogel swelling model can be broken into two swelling components: the mixing energy and the ionic energy. Conventionally for ionized gels, the ionic energy is characterized as the main contributor to swelling and, therefore, the mixing energy is assumed negligible. However, this assumption is made at the equilibrium state and ignores the dynamics of gel swelling. Here, the influence of the mixing energy on swelling ionized gels is quantified through numerical simulations on sodium polyacrylate using a Mixed Hybrid Finite Element Method. For univalent and divalent solutions, at initial porosities greater than 0.90, the contribution of the mixing energy is negligible. However, at initial porosities less than 0.90, the total swelling pressure is significantly influenced by the mixing energy. Therefore, both ionic and mixing energies are required for the modeling of sodium polyacrylate ionized gel swelling. The numerical model results are in good agreement with the analytical solution as well as experimental swelling tests. Full article

(This article belongs to the Special Issue Mechanical Behavior of Polymers)

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20 pages, 7107 KiB

Open AccessArticle

Influence of Thermally-Accelerated Aging on the Dynamic Mechanical Properties of HTPB Coating and Crosslinking Density-Modified Model for the Payne Effect

by Yongqiang Du, Jian Zheng and Guibo Yu

Polymers 2020, 12(2), 403; https://doi.org/10.3390/polym12020403 - 11 Feb 2020

Cited by 17 | Viewed by 3775

Abstract

Hydroxyl terminated polybutadiene (HTPB) coating is widely used in a solid rocket motor, but an aging phenomenon exists during long-term storage, which causes irreversible damage to the performance of this HTPB coating. In order to study the effect of aging on the dynamic [...] Read more.

Hydroxyl terminated polybutadiene (HTPB) coating is widely used in a solid rocket motor, but an aging phenomenon exists during long-term storage, which causes irreversible damage to the performance of this HTPB coating. In order to study the effect of aging on the dynamic mechanical properties of the HTPB coating, the thermally-accelerated aging test was carried out. The variation of maximum elongation and crosslinking density with aging time was obtained, and a good linear relationship between maximum elongation and crosslinking density was found by correlation analysis. The changing regularity of dynamic mechanical properties with aging time was analyzed. It was found that with the increase of aging time, T_g of HTPB coating increased, T_α, tan β and tan α decreased, and the functional relationships between the loss factor parameters and crosslinking density were constructed. The storage modulus and loss modulus of HTPB coating increased with the increase of aging time, and decreased with the increase of pre-strain. The aging enhanced the Payne effect of HTPB coating, while the pre-strain had a weakening effect. In view of the Payne effect of HTPB coating, the crosslinking density was introduced into Kraus model as aging evaluation parameter, and the crosslinking density modified models with and without pre-strain were established. The proposed models can effectively solve the problem that the Kraus model has a poor fitting effect under the condition of small strain (generally less than 1%) and on the loss modulus, which have improved the correlations between the fitting results and the test results. Full article

(This article belongs to the Special Issue Mechanical Behavior of Polymers)

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24 pages, 3196 KiB

Open AccessArticle

Deformation Instabilities and Lamellae Fragmentation during Deformation of Cross-linked Polyethylene

by Zbigniew Bartczak and Alina Vozniak

Polymers 2019, 11(12), 1954; https://doi.org/10.3390/polym11121954 - 28 Nov 2019

Cited by 15 | Viewed by 3597

Abstract

The effect of the topology of the amorphous phase and phase interconnectivity on the stability of the deformation of semicrystalline polyethylene was investigated. The chain topology was modified by crosslinking the samples with electron beam irradiation. The samples were deformed by plane-strain compression, [...] Read more.

The effect of the topology of the amorphous phase and phase interconnectivity on the stability of the deformation of semicrystalline polyethylene was investigated. The chain topology was modified by crosslinking the samples with electron beam irradiation. The samples were deformed by plane-strain compression, while the accompanying structural changes were monitored with X-ray and differential scanning calorimetry (DSC). At the true strain around of e = 0.3, the lamellar stacks parallel to the loading direction experienced microbuckling instability, which shortly led to the cooperative kinking of lamellae. Macroscopically, this showed up as the ‘second yield.’ Buckling is driven by the different stiffness levels of the hard and soft layers and their strong connectivity—for given layer thickness, the critical strain for buckling appeared proportional to the stiffness of the amorphous phase. Above e = 1.0, lamellae fragmentation was observed. This resulted from the localization of crystallographic slip, which was triggered by stress concentrations generated at lamellae faces by taut ‘stress transmitter’ (ST) chains. Accordingly, the fragmentation was found to be dependent on the surface fraction of STs at the amorphous-crystal interface: a low concentration of STs resulted in fewer but stronger stress concentrations, which led to earlier slip localization, followed quickly by lamellae fragmentation. The observed instabilities, either lamellae kinking or fragmentation, profoundly influenced the deformation process as well as the resultant structure. Both phenomena relieved much of the structural constraints imposed on deforming lamellae and make further strain accommodation easier. Full article

(This article belongs to the Special Issue Mechanical Behavior of Polymers)

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18 pages, 11821 KiB

Open AccessArticle

Finite Element Study on the Impact Resistance of Laminated and Textile Composites

by Jun Xing, Chunlin Du, Xin He, Zhenqiang Zhao, Chao Zhang and Yulong Li

Polymers 2019, 11(11), 1798; https://doi.org/10.3390/polym11111798 - 1 Nov 2019

Cited by 11 | Viewed by 4173

Abstract

The impact resistance of fiber-reinforced polymer composites is a critical concern for structure design in aerospace applications. In this work, experiments were conducted to evaluate the impact performance of four types of composite panels, using a gas-gun test system. Computational efficient finite element [...] Read more.

The impact resistance of fiber-reinforced polymer composites is a critical concern for structure design in aerospace applications. In this work, experiments were conducted to evaluate the impact performance of four types of composite panels, using a gas-gun test system. Computational efficient finite element models were developed to model the high-speed ballistic impact behavior of laminate and textile composites. The models were first validated by comparing the critical impact threshold and the failure patterns against experimental results. The damage progression and energy evolution behavior were combined to analyze the impact failure process of the composite panels. Numerical parametric studies were designed to investigate the sensitivity of impact resistance against impact attitude, including impact deflection angles and projectile deflection angles, which provide a comprehensive understanding of the damage tolerance of the composite panels. The numerical results elaborate the different impact resistances for laminate and textile composites and their different sensitivities to deflection angles. Full article

(This article belongs to the Special Issue Mechanical Behavior of Polymers)

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18 pages, 4611 KiB

Open AccessArticle

Modelling the Mechanical and Strain Recovery Behaviour of Partially Crystalline PLA

by John Sweeney, Paul Spencer, Karthik Nair and Phil Coates

Polymers 2019, 11(8), 1342; https://doi.org/10.3390/polym11081342 - 13 Aug 2019

Cited by 10 | Viewed by 4881

Abstract

This is a study of the modelling and prediction of strain recovery in a polylactide. Strain recovery near the glass transition temperature is the underlying mechanism for the shape memory in an amorphous polymer. The investigation is aimed at modelling such shape memory [...] Read more.

This is a study of the modelling and prediction of strain recovery in a polylactide. Strain recovery near the glass transition temperature is the underlying mechanism for the shape memory in an amorphous polymer. The investigation is aimed at modelling such shape memory behaviour. A PLA-based copolymer is subjected to stress–strain, stress relaxation and strain recovery experiments at large strain at 60 °C just below its glass transition temperature. The material is 13% crystalline. Using published data on the mechanical properties of the crystals, finite element modelling was used to determine the effect of the crystal phase on the overall mechanical behaviour of the material, which was found to be significant. The finite element models were also used to relate the stress–strain results to the yield stress of the amorphous phase. This yield stress was found to possess strain rate dependence consistent with an Eyring process. Stress relaxation experiments were also interpreted in terms of the Eyring process, and a two-process Eyring-based model was defined that was capable of modelling strain recovery behaviour. This was essentially a model of the amorphous phase. It was shown to be capable of useful predictions of strain recovery. Full article

(This article belongs to the Special Issue Mechanical Behavior of Polymers)

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20 pages, 6722 KiB

Open AccessEditor’s ChoiceArticle

Effect of Porosity on Mechanical Properties of 3D Printed Polymers: Experiments and Micromechanical Modeling Based on X-ray Computed Tomography Analysis

by Xue Wang, Liping Zhao, Jerry Ying Hsi Fuh and Heow Pueh Lee

Polymers 2019, 11(7), 1154; https://doi.org/10.3390/polym11071154 - 5 Jul 2019

Cited by 212 | Viewed by 11942

Abstract

Additive manufacturing (commonly known as 3D printing) is defined as a family of technologies that deposit and consolidate materials to create a 3D object as opposed to subtractive manufacturing methodologies. Fused deposition modeling (FDM), one of the most popular additive manufacturing techniques, has [...] Read more.

Additive manufacturing (commonly known as 3D printing) is defined as a family of technologies that deposit and consolidate materials to create a 3D object as opposed to subtractive manufacturing methodologies. Fused deposition modeling (FDM), one of the most popular additive manufacturing techniques, has demonstrated extensive applications in various industries such as medical prosthetics, automotive, and aeronautics. As a thermal process, FDM may introduce internal voids and pores into the fabricated thermoplastics, giving rise to potential reduction on the mechanical properties. This paper aims to investigate the effects of the microscopic pores on the mechanical properties of material fabricated by the FDM process via experiments and micromechanical modeling. More specifically, the three-dimensional microscopic details of the internal pores, such as size, shape, density, and spatial location were quantitatively characterized by X-ray computed tomography (XCT) and, subsequently, experiments were conducted to characterize the mechanical properties of the material. Based on the microscopic details of the pores characterized by XCT, a micromechanical model was proposed to predict the mechanical properties of the material as a function of the porosity (ratio of total volume of the pores over total volume of the material). The prediction results of the mechanical properties were found to be in agreement with the experimental data as well as the existing works. The proposed micromechanical model allows the future designers to predict the elastic properties of the 3D printed material based on the porosity from XCT results. This provides a possibility of saving the experimental cost on destructive testing. Full article

(This article belongs to the Special Issue Mechanical Behavior of Polymers)

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15 pages, 6806 KiB

Open AccessArticle

Effect of Stone-Wales Defect on Mechanical Properties of Gr/epoxy Nanocomposites

by Maoyuan Li, Peng Chen, Bing Zheng, Tianzhengxiong Deng, Yun Zhang, Yonggui Liao and Huamin Zhou

Polymers 2019, 11(7), 1116; https://doi.org/10.3390/polym11071116 - 1 Jul 2019

Cited by 13 | Viewed by 3657

Abstract

Due to its superior mechanical properties, graphene (Gr) has the potential to achieve high performance polymer-based nanocomposites. Previous studies have proved that defects in the Gr sheets could greatly reduce the mechanical properties of Gr, while the Stone-Wales (SW) defect was found to [...] Read more.

Due to its superior mechanical properties, graphene (Gr) has the potential to achieve high performance polymer-based nanocomposites. Previous studies have proved that defects in the Gr sheets could greatly reduce the mechanical properties of Gr, while the Stone-Wales (SW) defect was found to enhance the interfacial mechanical strength between Gr and epoxy. However, the combined effects of defects on the overall mechanical properties of Gr/epoxy nanocomposites have not been well understood. In this paper, the effect of the SW defect on the mechanical properties of Gr/epoxy nanocomposites was systematically investigated by using molecular dynamic simulations. The simulation results showed that the SW defect would degrade the mechanical properties of nanocomposites, including the Young’s modulus and in-plane shear modulus. Surprisingly, the transverse shear modulus could be remarkably enhanced with the existence of SW. The reinforcing mechanisms were mainly due to two aspects: (1) the SW defect could increase the surface roughness of the Gr, preventing the slippage between Gr and epoxy during the transverse shea; and (2) the nanocomposite with defective Gr enables a higher interaction energy than that with perfect graphene. Additionally, the effects of temperature, the dispersion and volume fraction of Gr were also investigated. Full article

(This article belongs to the Special Issue Mechanical Behavior of Polymers)

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20 pages, 4275 KiB

Open AccessArticle

Radial Basis Function Neural Network-Based Modeling of the Dynamic Thermo-Mechanical Response and Damping Behavior of Thermoplastic Elastomer Systems

by Ivan Kopal, Marta Harničárová, Jan Valíček, Jan Krmela and Ondrej Lukáč

Polymers 2019, 11(6), 1074; https://doi.org/10.3390/polym11061074 - 21 Jun 2019

Cited by 34 | Viewed by 4445

Abstract

The presented work deals with the creation of a new radial basis function artificial neural network-based model of dynamic thermo-mechanical response and damping behavior of thermoplastic elastomers in the whole temperature interval of their entire lifetime and a wide frequency range of dynamic [...] Read more.

The presented work deals with the creation of a new radial basis function artificial neural network-based model of dynamic thermo-mechanical response and damping behavior of thermoplastic elastomers in the whole temperature interval of their entire lifetime and a wide frequency range of dynamic mechanical loading. The created model is based on experimental results of dynamic mechanical analysis of the widely used thermoplastic polyurethane, which is one of the typical representatives of thermoplastic elastomers. Verification and testing of the well-trained radial basis function neural network for temperature and frequency dependence of dynamic storage modulus, loss modulus, as well as loss tangent prediction showed excellent correspondence between experimental and modeled data, including all relaxation events observed in the polymeric material under study throughout the monitored temperature and frequency interval. The radial basis function artificial neural network has been confirmed to be an exceptionally high-performance artificial intelligence tool of soft computing for the effective predicting of short-term viscoelastic behavior of thermoplastic elastomer systems based on experimental results of dynamic mechanical analysis. Full article

(This article belongs to the Special Issue Mechanical Behavior of Polymers)

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29 pages, 7249 KiB

Open AccessArticle

Modelling the Temperature Dependent Biaxial Response of Poly(ether-ether-ketone) Above and Below the Glass Transition for Thermoforming Applications

by Josh A. Turner, Gary H. Menary, Peter J. Martin and Shiyong Yan

Polymers 2019, 11(6), 1042; https://doi.org/10.3390/polym11061042 - 12 Jun 2019

Cited by 5 | Viewed by 3902

Abstract

Desire to accurately predict the deformation behaviour throughout industrial forming processes, such as thermoforming and stretch blow moulding, has led to the development of mathematical models of material behaviour, with the ultimate aim of embedding into forming simulations enabling process and product optimization. [...] Read more.

Desire to accurately predict the deformation behaviour throughout industrial forming processes, such as thermoforming and stretch blow moulding, has led to the development of mathematical models of material behaviour, with the ultimate aim of embedding into forming simulations enabling process and product optimization. Through the use of modern material characterisation techniques, biaxial data obtained at conditions comparable to the thermoforming process was used to calibrate the Buckley material model to the observed non-linear viscoelastic stress/strain behaviour. The material model was modified to account for the inherent anisotropy observed between the principal directions through the inclusion of a Holazapfel–Gasser–Ogden hyperelastic element. Variations in the post-yield drop in stress values associated with deformation rate and specimen temperature below the glass transition were observable, and facilitated in the modified model through time-temperature superposition creating a linear relationship capable of accurately modelling this change in yield stress behaviour. The modelling of the region of observed flow stress noted when above the glass transition temperature was also facilitated through adoption of the same principal. Comparison of the material model prediction was in excellent agreement with experiments at strain rates and temperatures of 1–16 s⁻¹ and 130–155 °C respectively, for equal-biaxial mode of deformation. Temperature dependency of the material model was well replicated with across the broad temperature range in principal directions, at the reference strain rate of 1 s⁻¹. When concerning larger rates of deformation, minimum and maximum average error levels of 6.20% and 10.77% were noted. The formulation, and appropriate characterization, of the modified Buckley material model allows for a stable basis in which future implementation into representative forming simulations of poly-aryl-ether-ketones, poly(ether-ether-ketone) (PEEK) and many other post-yield anisotropic polymers. Full article

(This article belongs to the Special Issue Mechanical Behavior of Polymers)

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17 pages, 17080 KiB

Open AccessArticle

Formability and Failure Mechanisms of Woven CF/PEEK Composite Sheet in Solid-State Thermoforming

by Bing Zheng, Xiping Gao, Maoyuan Li, Tianzhengxiong Deng, Zhigao Huang, Huamin Zhou and Dequn Li

Polymers 2019, 11(6), 966; https://doi.org/10.3390/polym11060966 - 3 Jun 2019

Cited by 43 | Viewed by 6780

Abstract

In this study, the formability of woven carbon-fiber (CF)-reinforced polyether-ether-ketone (PEEK) composite sheets in the solid-state thermoforming process were investigated, and the failure mechanisms were discussed. The formability of the woven CF/PEEK sheets were analyzed using flexural tests, Erichsen test, and microscopic observation. [...] Read more.

In this study, the formability of woven carbon-fiber (CF)-reinforced polyether-ether-ketone (PEEK) composite sheets in the solid-state thermoforming process were investigated, and the failure mechanisms were discussed. The formability of the woven CF/PEEK sheets were analyzed using flexural tests, Erichsen test, and microscopic observation. The results show that the formability of CF/PEEK sheets significantly increases as the temperature rises from 165 to 325 °C, and slightly decreases as the deformation speed rises from 2 to 120 mm/min. The deformation of the sheets is caused by plastic deformation, shear deformation and squeeze deformation, without plastic thinning and fiber slippage, which is due to the restriction of the solid matrix and locked fibers. Moreover, the wrinkles will cause fiber fracture at lower temperatures and delamination at higher temperatures. At higher temperatures, the wrinkles mainly occur at the position with [0°/90°] fibers due to the squeezing of the matrix and fibers. Full article

(This article belongs to the Special Issue Mechanical Behavior of Polymers)

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13 pages, 2129 KiB

Open AccessArticle

Development of an Advanced Dynamic Microindentation System to Determine Local Viscoelastic Properties of Polymers

by Esther Ramakers-van Dorp, Thomas Haenel, Dominik Ciongwa, Bernhard Möginger and Berenika Hausnerova

Polymers 2019, 11(5), 833; https://doi.org/10.3390/polym11050833 - 8 May 2019

Cited by 5 | Viewed by 4031

Abstract

This study presents a microindentation system which allows spatially resolved local as well as bulk viscoelastic material information to be obtained within one instrument. The microindentation method was merged with dynamic mechanical analysis (DMA) for a tungsten cone indenter. Three tungsten cone indenters [...] Read more.

This study presents a microindentation system which allows spatially resolved local as well as bulk viscoelastic material information to be obtained within one instrument. The microindentation method was merged with dynamic mechanical analysis (DMA) for a tungsten cone indenter. Three tungsten cone indenters were investigated: tungsten electrode, tungsten electrode + 2% lanthanum, and tungsten electrode + rare earth elements. Only the tungsten electrode + 2% lanthanum indenter showed the sinusoidal response, and its geometry remained unaffected by the repeated indentations. Complex moduli obtained from dynamic microindentation for high-density polyethylene, polybutylene terephthalate, polycarbonate, and thermoplastic polyurethane are in agreement with the literature. Additionally, by implementing a specially developed x-y-stage, this study showed that dynamic microindentation with a tungsten cone indenter was an adequate method to determine spatially resolved local viscoelastic surface properties. Full article

(This article belongs to the Special Issue Mechanical Behavior of Polymers)

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11 pages, 5240 KiB

Open AccessArticle

Effect of Temperature on the Tear Fracture and Fatigue Life of Carbon-Black-Filled Rubber

by Wenbo Luo, Ming Li, Youjian Huang, Boyuan Yin and Xiaoling Hu

Polymers 2019, 11(5), 768; https://doi.org/10.3390/polym11050768 - 1 May 2019

Cited by 27 | Viewed by 4074

Abstract

The mechanical behaviour of carbon-black (CB)-filled rubber is temperature-dependent. It is assumed that temperature affects the fatigue life of rubber products by changing the tear energy of the material. The static tearing behaviour and fatigue crack propagation behavior of CB-filled rubber at different [...] Read more.

The mechanical behaviour of carbon-black (CB)-filled rubber is temperature-dependent. It is assumed that temperature affects the fatigue life of rubber products by changing the tear energy of the material. The static tearing behaviour and fatigue crack propagation behavior of CB-filled rubber at different temperatures were investigated in this study. The critical tear energy of the material was measured through static tear fracture tests at different temperatures; it is shown that the critical tear energy decreases exponentially with increasing temperature. A fatigue crack growth test of a constrained precracked planar tension specimen was conducted at room temperature; the measurements verify that the fatigue crack growth follows a Paris–Erdogan power law. Considering the temperature dependence of the critical tear energy, the temperature dependent fatigue crack growth kinetics of CB-filled rubber was established, and the fatigue life of the material at high temperatures was predicted based on the kinetics. The predictions are in good agreement with experimental measurements. Full article

(This article belongs to the Special Issue Mechanical Behavior of Polymers)

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17 pages, 5915 KiB

Open AccessEditor’s ChoiceArticle

Direct Numerical Simulation of Gas-Liquid Drag-Reducing Cavity Flow by the VOSET Method

by Yi Wang, Yan Wang and Zhe Cheng

Polymers 2019, 11(4), 596; https://doi.org/10.3390/polym11040596 - 2 Apr 2019

Cited by 23 | Viewed by 3629

Abstract

Drag reduction by polymer is an important energy-saving technology, which can reduce pumping pressure or promote the flow rate of the pipelines transporting fluid. It has been widely applied to single-phase pipelines, such as oil pipelining, district heating systems, and firefighting. However, the [...] Read more.

Drag reduction by polymer is an important energy-saving technology, which can reduce pumping pressure or promote the flow rate of the pipelines transporting fluid. It has been widely applied to single-phase pipelines, such as oil pipelining, district heating systems, and firefighting. However, the engineering application of the drag reduction technology in two-phase flow systems has not been reported. The reason is an unrevealed complex mechanism of two-phase drag reduction and lack of numerical tools for mechanism study. Therefore, we aim to propose governing equations and numerical methods of direct numerical simulation (DNS) for two-phase gas-liquid drag-reducing flow and try to explain the reason for the two-phase drag reduction. Efficient interface tracking method—coupled volume-of-fluid and level set (VOSET) and typical polymer constitutive model Giesekus are combined in the momentum equation of the two-phase turbulent flow. Interface smoothing for conformation tensor induced by polymer is used to ensure numerical stability of the DNS. Special features and corresponding explanations of the two-phase gas-liquid drag-reducing flow are found based on DNS results. High shear in a high Reynolds number flow depresses the efficiency of the gas-liquid drag reduction, while a high concentration of polymer promotes the efficiency. To guarantee efficient drag reduction, it is better to use a high concentration of polymer drag-reducing agents (DRAs) for high shear flow. Full article

(This article belongs to the Special Issue Mechanical Behavior of Polymers)

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20 pages, 1773 KiB

Open AccessArticle

Calibration of a Cohesive Model for Fracture in Low Cross-Linked Epoxy Resins

by Dery Torres, Shu Guo, Maria-Pilar Villar, Daniel Araujo and Rafael Estevez

Polymers 2018, 10(12), 1321; https://doi.org/10.3390/polym10121321 - 28 Nov 2018

Cited by 2 | Viewed by 3264

Abstract

Polymer-based composites are becoming widely used for structural applications, in particular in the aeronautic industry. The present investigation focuses on the mechanical integrity of an epoxy resin of which possible damage results in limitation or early stages of dramatic failure. Therefore, a coupled [...] Read more.

Polymer-based composites are becoming widely used for structural applications, in particular in the aeronautic industry. The present investigation focuses on the mechanical integrity of an epoxy resin of which possible damage results in limitation or early stages of dramatic failure. Therefore, a coupled experimental and numerical investigation of failure in an epoxy resin thermoset is carried out that opens the route to an overall micromechanical analysis of thermoset-based composites. In the present case, failure is preceded by noticeable plasticity in the form of shear bands similar to observations in ductile glassy polymers. Thus, an elastic-visco-plastic constitutive law initially devoted to glassy polymer is adopted that captures the rate- dependent yield stress followed by softening and progressive hardening at continued deformation. A general rate-dependent cohesive model is used to describe the failure process. The parameters involved in the description are carefully identified and used in a finite element calculation to predict the material’s toughness for different configurations. Furthermore, the present work allows investigation of nucleation and crack growth in such resins. In particular, a minimum toughness can be derived from the model which is difficult to evaluate experimentally and allows accounting for the notch effect on the onset of failure. This is thought to help in designing polymer-based composites. Full article

(This article belongs to the Special Issue Mechanical Behavior of Polymers)

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32 pages, 5896 KiB

Open AccessArticle

The Continuum Approach to the Description of Semi-Crystalline Polymers Deformation Regimes: The Role of Dynamic and Translational Defects

by Yurii V. Grinyaev, Nadezhda V. Chertova, Evgeny V. Shilko and Sergey G. Psakhie

Polymers 2018, 10(10), 1155; https://doi.org/10.3390/polym10101155 - 16 Oct 2018

Cited by 7 | Viewed by 6884

Abstract

This paper presents a new approach to describe the mechanical behavior of semi-crystalline polymers, the plastic deformation of which is determined by their two-phase structure. To describe the plastic behavior of semi-crystalline polymers, a two-phase model is used. In the framework of this [...] Read more.

This paper presents a new approach to describe the mechanical behavior of semi-crystalline polymers, the plastic deformation of which is determined by their two-phase structure. To describe the plastic behavior of semi-crystalline polymers, a two-phase model is used. In the framework of this model, one phase is in a hard (crystalline) state, and the other in a soft (amorphous) state. The two-phase material is modeled by a single-phase homogeneous continuum based on the approximation of the effective medium. It is assumed that two infinitely close material points of the continuum are connected in series by elastic and viscous bonds, which corresponds to the Maxwell model. It is shown that, in this case, the Maxwell continuum is a pseudo-Euclidean space. Generalizing the definition of defects from a three-dimensional space to a four-dimensional pseudo-Euclidean space, we obtained a dynamic system of nonlinear, interrelated equations to describe the behavior of translational-type defects in the solid phase and dynamic defects in the amorphous phase. As an example of an application for these equations, the phenomenon of creep under uniaxial loading is considered. It is shown that the formalism of the proposed two-phase model makes it possible to describe creep phenomenon regularities, which correspond to both the aging theory and the flow theory. Full article

(This article belongs to the Special Issue Mechanical Behavior of Polymers)

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11 pages, 4267 KiB

Open AccessArticle

Mechanisms of the Complex Thermo-Mechanical Behavior of Polymer Glass Across a Wide Range of Temperature Variations

by Weidong Liu and Liangchi Zhang

Polymers 2018, 10(10), 1153; https://doi.org/10.3390/polym10101153 - 16 Oct 2018

Cited by 9 | Viewed by 3938

Abstract

This paper aims to explore the mechanisms of the complex thermo-mechanical behavior of polymer glass across a wide range of temperature variations. To this end, the free vibration frequency spectrum of simply supported poly(methyl methacrylate) (PMMA) beams was thoroughly investigated with the aid [...] Read more.

This paper aims to explore the mechanisms of the complex thermo-mechanical behavior of polymer glass across a wide range of temperature variations. To this end, the free vibration frequency spectrum of simply supported poly(methyl methacrylate) (PMMA) beams was thoroughly investigated with the aid of the impulse excitation technique. It was found that the amplitude ratio of the multiple peaks in the frequency spectrum is a strongly dependent on temperature, and that the peaks correspond to the multiple vibrational modes of the molecular network of PMMA. At a low temperature, the vibration is dominated by the overall microstructure of PMMA. With increasing the temperature, however, the contribution of the sub-microstructures is retarded by β relaxation. Above 80 °C, the vibration is fully dominated by the microstructure after relaxation. The relaxation time at the transition temperature is of the same order of the vibration period, confirming the contribution of β relaxation. These findings provide a precise method for establishing reliable physical-based constitutive models of polymer glass. Full article

(This article belongs to the Special Issue Mechanical Behavior of Polymers)

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11 pages, 5359 KiB

Open AccessArticle

Mechanical Behavior—Microstructure Relationships in Injection-Molded Polyamide 66

by Noëlle Billon, Joan Giraudeau, Jean Luc Bouvard and Gilles Robert

Polymers 2018, 10(10), 1047; https://doi.org/10.3390/polym10101047 - 20 Sep 2018

Cited by 11 | Viewed by 4284

Abstract

Clear relationships between the semi-crystalline microstructure of injection molding polymers and their mechanical behavior are not yet totally established for all polymers. Part of this relative lack of understanding is because an unambiguous scientific approach is difficult to build up. The processing of [...] Read more.

Clear relationships between the semi-crystalline microstructure of injection molding polymers and their mechanical behavior are not yet totally established for all polymers. Part of this relative lack of understanding is because an unambiguous scientific approach is difficult to build up. The processing of samples promotes a microstructure which is not uniform and can be described in various ways on different scales. This introduces uncertainty in the correlations. Most completed studies were conducted on polyolefin, which exhibits an evolution of microstructure that is quite easy to observe and to correlate to mechanical properties. This paper intends to illustrate a more diffuse case. To achieve this, combined characterizations along the flow path and throughout the thickness of a plaque as well as characterizations of the local microstructure and tensile behavior of polyamide 66 are described. The microstructure was explored in terms of skin-core structure, spherulites sizes, crystallinity ratio and lamellae organization. Mechanical properties were addressed with non-monotonic tests with the use of DIC (Digital Image Correlation) to assess true behavior. The effect of humidity is also accounted for. It is demonstrated that small changes in lamellae or interlamellar amorphous phase are likely to be responsible for non-uniform mechanical properties, whereas more macroscopic levels (skin core structure, spherulites level of crystallinity ratio) appeared to be irrelevant levels of description. Consequently, the usual simplified analyses based on optical microscopy and differential scanning calorimetry (DSC) can be inefficient in improving knowledge in that field. Full article

(This article belongs to the Special Issue Mechanical Behavior of Polymers)

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11 pages, 4489 KiB

Open AccessArticle

Significant Reduction of the Friction and Wear of PMMA Based Composite by Filling with PTFE

by Dapeng Gu, Longxiao Zhang, Suwen Chen, Kefeng Song and Shouyao Liu

Polymers 2018, 10(9), 966; https://doi.org/10.3390/polym10090966 - 1 Sep 2018

Cited by 23 | Viewed by 4704

Abstract

Polytetrafluoroethylene/Poly(methyl methacrylate) (PTFE/PMMA) composite was prepared by mixing PTFE into PMMA matrix which synthesized by the PMMA powder mixture and methyl methacrylate (MMA) liquid mixture. The effects of the filling mass ratio of PTFE and powder/liquid (P/L) ratio on the friction and wear [...] Read more.

Polytetrafluoroethylene/Poly(methyl methacrylate) (PTFE/PMMA) composite was prepared by mixing PTFE into PMMA matrix which synthesized by the PMMA powder mixture and methyl methacrylate (MMA) liquid mixture. The effects of the filling mass ratio of PTFE and powder/liquid (P/L) ratio on the friction and wear properties of PTFE/PMMA composites against bearing steel were studied by a ball-on-disk tribometer. Fourier transform infrared (FTIR), field emission scanning electron microscopy (FESEM), and energy dispersive X-ray spectroscopy (EDS) were used to characterize the synthesis of PTFE/PMMA composite. The shore hardness and glass transition temperature (T_g) were obtained respectively by shore hardness tester and differential scanning calorimetry (DSC). The results show that the friction coefficient and wear rate of PMMA based composite, comparing with the unfilled PMMA, can be significantly reduced by filling with PTFE. With the increasing of PTFE filling mass ratio, the wear rate of PTFE/PMMA composite increases. The friction coefficient and wear rate of the unfilled PMMA and PTFE/PMMA composite generally decrease with the P/L ratio increasing. The main wear mechanism of the unfilled PMMA is adhesive wear. While the main wear mechanisms of PTFE/PMMA composites are fatigue wear and abrasive wear. Full article

(This article belongs to the Special Issue Mechanical Behavior of Polymers)

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18 pages, 1299 KiB

Open AccessArticle

Mathematical Modeling and Simulations for Large-Strain J-Shaped Diagrams of Soft Biological Materials

by Kazuhiko Mitsuhashi, Swapan Ghosh and Hiroshi Koibuchi

Polymers 2018, 10(7), 715; https://doi.org/10.3390/polym10070715 - 29 Jun 2018

Cited by 16 | Viewed by 4257

Abstract

Herein, we study stress–strain diagrams of soft biological materials such as animal skin, muscles, and arteries by Finsler geometry (FG) modeling. The stress–strain diagram of these biological materials is always J-shaped and is composed of toe, heel, linear, and failure regions. In the [...] Read more.

Herein, we study stress–strain diagrams of soft biological materials such as animal skin, muscles, and arteries by Finsler geometry (FG) modeling. The stress–strain diagram of these biological materials is always J-shaped and is composed of toe, heel, linear, and failure regions. In the toe region, the stress is almost zero, and the length of this zero-stress region becomes very large (≃150%) in, for example, certain arteries. In this paper, we study long-toe diagrams using two-dimensional (2D) and 3D FG modeling techniques and Monte Carlo (MC) simulations. We find that, except for the failure region, large-strain J-shaped diagrams are successfully reproduced by the FG models. This implies that the complex J-shaped curves originate from the interaction between the directional and positional degrees of freedom of polymeric molecules, as implemented in the FG model. Full article

(This article belongs to the Special Issue Mechanical Behavior of Polymers)

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