Failure of elastic‐plastic core–shell microcapsules under compression

Characterization of the failure behavior of microcapsules is extremely important to control the release of their core actives by mechanical forces. The strain and stress of elastic‐plastic uninflated core–shell microcapsules at failure (rupture or bursting) has been determined using finite element modeling (FEM) and micromanipulation compression experiments. The ductile failure of polymeric microcapsules at high deformations is considered to occur when the maximum strain in the shell exceeds a critical strain, resulting in their rupture. FEM has been used to determine the maximum strains present in the capsule wall at different deformations for three types of shell material: elastic, elastic—perfectly plastic and elastic—perfectly plastic with strain hardening at large strains. The results obtained were used to determine the failure strain and stress of melamine‐formaldehyde microcapsules, with average population values of ～0.48 and ～350 MPa, respectively. Thus, the elastic‐plastic stress–strain relationship has been determined for the core–shell microcapsules tested. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Modeling of visco‐hyperelastic behavior of transversely isotropic functionally graded rubbers

In this article, visco‐hyperelastic constitutive model is developed to describe the rate‐dependent behavior of transversely isotropic functionally graded rubber‐like materials at finite deformations. Zener model that consists of Maxwell element parallel to a hyperelastic equilibrium spring is used in this article. Steady state response is described by equilibrium hyperelastic spring and rate‐dependence behavior is modeled by Maxwell element that consists of a hyperelastic intermediate spring and a nonlinear viscous damper. Modified and reinforced neo‐Hookean strain energy function is proposed for the two hyperelastic springs. The mechanical properties and material constants of strain energy function are graded along the axial direction based on exponential function. A history‐integral method has been used to develop a constitutive equation for modeling the behavior of the model. The applied history integral method is based on the Kaye‐BKZ theory. The material constant parameters appeared in the formulation have been determined with the aid of available uniaxial tensile experimental tests for a specific material and the results are compared to experimental results. It is then concluded that, the proposed constitutive equation is quite proficient in forecasting the behavior of rubber‐like materials in different deformation and wide ranges of strain rate. POLYM. ENG. SCI., 56:342–347, 2016. © 2016 Society of Plastics Engineers     

Three‐dimensional imaging of large compressive deformations in elastomeric foams

Specimens of silica‐reinforced polysiloxane foam pads were three‐dimensionally imaged during axial compressive loading to densification. The foams' behavior was highly nonlinear and showed the three characteristic regions of linear elastic, elastic buckling, and densification. A finite‐element technique, based upon conversion of the image voxels to finite elements, was used to calculate the mechanical properties of the foams. The results were compared with conventional mechanical testing and theory. The finite‐element calculations were in excellent agreement with experimental stress–strain data over the entire range of compressive loading. Theoretical models, on the other hand, overestimated the stiffness of the foam above the elastic buckling stress by not correctly predicting the abruptness of the transition from elastic buckling to densification. Three‐dimensional analysis of the deformed microstructures indicated that there was a critical foam density beyond which the cell morphology suddenly changed from open‐celled to closed‐celled and that this “phase”‐like transition was responsible for the abrupt increase in stiffness near densification. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1746–1755, 2001     

Rubber modeling using uniaxial test data

Accurate modeling of large rubber deformations is now possible with finite‐element codes. Many of these codes have certain strain‐energy functions built‐in, but it can be difficult to get the relevant material parameters and the behavior of the different built‐in functions have not been seriously evaluated. In this article, we show the benefits of assuming a Valanis–Landel (VL) form for the strain‐energy function and demonstrate how this function can be used to enlarge the data set available to fit a polynomial expansion of the strain‐energy function. Specifically, we show that in the ABAQUS finite‐element code the Ogden strain‐energy density function, which is a special form of the VL function, can be used to provide a planar stress–strain data set even though the underlying data used to determine the constants in the strain‐energy function include only uniaxial data. Importantly, the polynomial strain‐energy density function, when fit to the uniaxial data set alone, does not give the same planar stress–strain behavior as that predicted from the VL or Ogden models. However, the polynomial form does give the same planar response when the VL‐generated planar data are added to the uniaxial data set and fit with the polynomial strain‐energy function. This shows how the VL function can provide a reasonable means of estimating the three‐dimensional strain‐energy density function when only uniaxial data are available. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 837–848, 2001     

Application of digital image correlation method to biogel

This study adopts the digital image correlation (DIC) method to measure the mechanical properties under tension in agarose gels. A second polynomial stress–strain equation based on a pore model is proposed in this work. It shows excellent agreement with experimental data and was verified by finite element simulation. Evaluation of the planer strain field by DIC allows measurement of strain localization and Poisson's ratio. At high stresses, Poisson's ratio is found to exceed the standard assumption of 0.5 which is shown to be a result of pore water leakage. Local failure strains are found to be approximately twice those determined by crosshead displacements. Viscous properties of agarose gels are investigated by performing the tensile tests at various loading rates. Increases in loading rate do not cause much difference in the shape of stress–strain curves, but result in increases in ultimate stress and strain. POLYM. ENG. SCI., 50:1585–1593, 2010. © 2010 Society of Plastics Engineers     

Theoretical and finite element studies of interfacial stresses in reinforced concrete beams strengthened by externally FRP laminates plate

The paper presents the results of an analytical and numerical solution for interfacial stresses in carbon fiber reinforced plastic (CFRP)–reinforced concrete (RC) hybrid beams studied by the finite element method. The analytical analysis is based on the deformation compatibility approach where both the shear and normal stresses are assumed to be invariant across the adhesive layer thickness. The adherend shear deformations are taken into account by assuming a parabolic shear stress through the thickness of both the concrete beam and the bonded plate. In numerical analysis, the mesh sensitivity test shows that the finite element results for interfacial stresses are not sensitive to the finite element mesh. The finite element analysis then is used to calculate the interfacial stress distribution and evaluate the effect of the structural parameters on the interfacial behavior. It is shown that both the normal and shear stresses at the interface are influenced by the material and geometry parameters of the composite beam. Numerical results from the present analysis are presented both to demonstrate the advantages of the present solution over existing ones and to illustrate the main characteristics of interfacial stress distributions. We can conclude that this research is helpful for the understanding the mechanical behavior of the interface and design of the FRP–RC hybrid structures.     

The dynamic response and failure of Polycarbonate plate by soft body impact

This article is mainly to experimentally and numerically investigate the dynamic response and failure of Polycarbonate (PC) plate against strike by soft body. The experimental results show that high speed soft body impact leads to a large global displacement for PC sheet, though, the obtained strain data shows deformation of PC material are still small. This evidence allows us to employ a thermo‐viscoelastic constitutive model we proposed in our previous work, where the model parameters are determined based on the uniaxial tension test data of PC materials, to describe the PC plate. Then, the simulation is made in finite element (FE) software LS‐DYNA and computational results get a fair agreement with experiments including displacement, strain, and the crack propagations at high velocity impact. The temperature effect on mechanical behavior of PC sheet under impact is numerically studied as well. It is found that the effect gets more significant with the increase of impact velocity, and the higher temperature of PC sheet would lead to its larger deflection but smaller maximum resistance force and principal stress. POLYM. ENG. SCI., 56:1160–1168, 2016. © 2016 Society of Plastics Engineers     

Characterization and Finite Element Analysis of the Tensile Behavior of Electrospun Polymer Single Fibers

In this paper, polymer single fibers are prepared by electro‐spinning technology with different solvent rations, and the micromechanics properties are investigated together with the finite element analysis. It is found that the tensile stress–strain curves of single fibers can be attributed to two kinds of trends, which are independent of the solvent ratios. A possible arrangement model of polymer chains within the electrospun fibers is proposed according to the tensile stress–strain curves and the mechanics theories. The effect of polymer chains arrangement on the mechanical properties of the fibers is explored by the finite element analysis. This research shows a practical reference to predict the relationship between orientation degree of polymer chains and mechanical properties within the fibers.     

Finite strain 3D thermoviscoelastic constitutive model for shape memory polymers

A 3D thermoviscoelastic model is proposed to represent the thermomechanical behavior of shape memory polymers. The model is based on a physical understanding of the material behavior and a mechanical interpretation of the stress–strain–temperature changes observed during thermomechanical loading. The model is thermodynamically motivated and is formulated in a finite strain framework in order to account for large strain deformations. Model predictions capture critical features of shape memory polymer deformation and, in some cases, provide very favorable comparisons with experimental results. POLYM. ENG. SCI. 46:486–492, 2006. © 2006 Society of Plastics Engineers.     

Micromechanical approach to modeling damage in crystalline polyethylene

The purpose of this article is to describe how the concepts of continuum damage mechanics can be applied to modeling of polyethylene materials under different loading conditions. The increasing use of polyethylene in diverse applications motivates the need for understanding how its molecular properties relate to the overall behavior of the material. Although microstructure and mechanical properties of polymers have been the subject of several studies, the irreversible microstructural rearrangements occurring at large deformations are not completely understood. In this work, a three‐dimensional damage constitutive model for polyethylene is proposed. The material is analyzed from a microscopic viewpoint and considered as an aggregate of crystals. The model regards the crystals as rigid‐viscoplastic and incorporates the effects of atomic debonding on the overall mechanical behavior. To illustrate the capability of the proposed model, two simulations are carried out to capture the macroscopic stress–strain behavior and texture evolution under uniaxial tension and simple shear loading conditions. The results are compared with experimental data and numerical simulations from other references. POLYM. ENG. SCI., 47:410–420, 2007. © 2007 Society of Plastics Engineers.     

Synthesis and characterization of novel poly(dimethylsiloxane)/polyindole composites

A series of composites of polyindole (PIN) and poly(dimethylsiloxane) (PDMS) were synthesized chemically using FeCl₃ as an oxidant agent in anhydrous media. The composites were characterized by FTIR and UV‐visible spectroscopies, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), X‐ray diffraction (XRD), elemental analysis, inductively coupled plasma‐optic emission spectroscopy (ICP‐OES), magnetic susceptibility, stress–strain experiments, and conductivity measurements. The conductivities of PIN at different temperatures were also measured and it was revealed that their conductivities were slightly increased with increasing temperature. Moreover, the freestanding films of PDMS/PIN composites were prepared by casting on glass Petri dishes to examine their stress–strain properties. From thermogravimetric analysis results it was found that PDMS/PIN composites were thermally more stable than PIN. Thermal stabilities of PDMS/PIN composites increased with increasing PIN content. It was found that the conductivities of PDMS/PIN composites depend on the indole content in the composites. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Poly(dimethylsiloxane)/poly(hexamethylene oxide) mixed macrodiol based polyurethane elastomers. I. Synthesis and properties

The compatibilizing effect of poly(hexamethylene oxide) (PHMO) on the synthesis of polyurethanes based on α,ω‐bis(6‐hydroxyethoxypropyl) poly(dimethylsiloxane) (PDMS) was investigated. The hard segments of the polyurethanes were based on 4,4′‐methylenediphenyl diisocyanate (MDI) and 1,4‐butanediol. The effects of the PDMS/PHMO composition, method of polyurethane synthesis, hard segment weight percentage, catalyst, and molecular weight of the PDMS on polyurethane synthesis, properties, and morphology were investigated using size exclusion chromatography, tensile testing, and differential scanning calorimetry (DSC). The large difference in the solubility parameters between PDMS and conventional reagents used in polyurethane synthesis was found to be the main problem associated with preparing PDMS‐based polyurethanes with good mechanical properties. Incorporation of a polyether macrodiol such as PHMO improved the compatibility and yielded polyurethanes with significantly improved mechanical properties and processability. The optimum PDMS/PHMO composition was 80 : 20 (w/w), which yielded a polyurethane with properties comparable to those of the commercial material Pellethane™ 2363‐80A. The one‐step polymerization was sensitive to the hard segment weight percentage of the polyurethane and was limited to materials with about a 40 wt % hard segment; higher concentrations yielded materials with poor mechanical properties. A catalyst was essential for the one‐step process and tetracoordinated tin catalysts (e.g., dibutyltin dilaurate) were the most effective. Two‐step bulk polymerization overcame most of the problems associated with reactant immiscibility by the end capping of the macrodiol and required no catalysts. The DSC results demonstrated that in cases where poor properties were observed, the corresponding polyurethanes were highly phase separated and the hard segments formed were generally longer than the average expected length based on the reactant stoichiometry. Based on these results, we postulated that at low levels (∼ 20 wt %) the soft segment component derived from PHMO macrodiol was concentrated mainly in the interfacial regions, strengthening the adhesion between hard and soft domains of PDMS‐based polyurethanes. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 2026–2040, 2000     

High‐temperature large strain viscoelastic behavior of polypropylene modeled using an inhomogeneously strained network

The effects of microstructural rearrangements during the stretching of semicrystalline polymers and the resultant inhomogeneous strains are modeled by rigid spheres embedded in a polymer network. This results in strain concentrations in the network, which is then caused to yield at realistic overall strains. To simulate the collapse of the original spherulitic morphology, the radii of the spheres decrease at a rate dependent on the shear stress imposed on them by the surrounding network. This results in time‐dependent behavior. The resultant large strain viscoelastic model is implemented in a commercial finite element code and used to predict shapes of necking polypropylene sheet specimens at 150°C. Rate dependence of stress and stress relaxation are also predicted, and the model is shown to be generally effective in its predictions of shapes and forces up to large deformations. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 563–575, 1999     

Micromechanical discrete element modeling of fiber reinforced polymer composites

An analytical model of mechanical behavior of carbon fiber reinforced polymer composites using an advanced discrete element model (DEM) coupled with imaging techniques is presented in this article. The analysis focuses on composite materials molded by vacuum assisted resin transfer molding. The molded composite structure consists of eight‐harness carbon fiber fabrics and a high‐temperature polymer. The actual structure of the molded material was captured in digital images using optical microscopy. DEM was developed using the image‐based‐shape structural model to predict the composite elastic modulus, stress–strain response, and compressive strength. An experimental case study is presented to evaluate the accuracy of the developed analytical model. The results indicate that the image‐based DEM micromechanical model showed fairly accurate predictions for the elastic modulus and compressive strength. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Relationship between microstructure and mechanical properties of polyether block amide foams

The mechanical behaviors of five polyether block amide foams, obtained by mold-opening foam injection process, were investigated with regard to their microstructures. The materials vary in mass ratios of hard versus soft segments, and/or in process packing time. The resulting microstructures have been characterized in terms of cavity size and shape ratios, by analyzing scanning electron microscope images after careful sample preparation. The foam mechanical responses have been characterized in compression at small and large strain. At small strain, the initial linear part of the stress–strain curve is enhanced firstly by the hard segment mass ratio and secondly by the fineness of the microstructure. Similar results have been obtained at large strain. The foam viscoelasticity at large strain has been characterized by stress relaxation and strain recovery tests, relevant for foam applications. Reduced packing time and pressure have been shown to lead to the presence of undesired large cavities. The morphological defects appear to have a negligible impact on the macroscopic mechanical behavior of the foams at infinitesimal strain, but lead to critical inconsistency at large strain. Furthermore, the mechanical behavior of the tested polyether block amide foams is controlled first by hard versus soft segments ratio, and second by the microstructure fineness.     

Dynamic Behavior of Composites under Preloading

Polymers with characteristic damping properties are obtained by compounding them with additives. The mechanical behavior under vibration conditions of these compounds is characterized by dynamic moduli which depend on the static preload. The test results of a PU-compound are presented in this paper. The obtained results enable one to calculate the complex moduli of a special dynamic constitutive equation, which then enables one to calcualte stress and strain of dynamic deformations of structures under static preload. The paper presents a method to calculate such loading conditions on a system, too. A finite element analysis is used to determine the frequency response which characterizes the damping behavior of the structure.     

Effects of polymeric curing agent modified with silazanes on the mechanical properties of silicone rubber

Polymeric curing agent modified with hexamethyldisilazane (PCA‐D), or with hexamethylcyclotrisilazane (PCA‐T), was used to improve the mechanical properties of hydroxyl‐teminated polydimethylsiloxane (PDMS) rubber. The structure and the gel time of PCA were characterized by ²⁹Si NMR and shear viscosity measurement, respectively. The PCA modified with silazanes was more stable in storage than that without treatment (PCA‐0). Chemical bonds were formed during the reaction of silazanes and PCA according to ²⁹Si NMR results. The crosslink density (γ_e) and the mechanical properties of PCA/PDMS rubber were determined by swelling equilibrium and stress–strain tests. It was found that PCA treated with both silazanes could better enhance the mechanical properties of PCA/PDMS rubber compared with PCA‐0. PCA‐T/PDMS rubber, with additional crosslinks, was the best among the three types of PCA/PDMS rubber on the mechanical properties. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Creep and relaxation of cement paste caused by stress‐induced dissolution of hydrated solid components

The creep and relaxation of cement paste caused by dissolving solid hydration products is evaluated in this work. According to the second law of thermodynamics, dissolution or precipitation of solid constituents may be altered by the change in stress/strain fields inside cement paste via alteration of the stress power or strain energy. Thus, it is hypothesized that stress‐induced dissolution can affect the overall creep/relaxation behavior of cement composites. A novel, fully coupled thermodynamic, mechanical, and microstructural model (TM²) that uses the finite element method was developed to predict the time‐evolving properties of cement paste under prescribed strains and to test the hypothesis. In the model, the strain energy was incorporated to accurately predict the effect of stress and strain fields on cement microstructure change. From the simulation results, depending on the stress/strain levels and the choice of the domain (over which the thermodynamic equilibrium is enforced), stress‐induced dissolution of solid constituents can lead to significant creep/relaxation.