国内外PVB材性研究

主要介绍了国内外研究PVB(聚乙烯醇缩丁醛)材性的现状。国内外研究表明,PVB是应变率及温度敏感材料。应变率增加,弹性模量变大;温度升高,弹性模量和剪切模量均下降。同时,国内外进行了少量的实验,研究PVB的本构模型。总结发现,PVB的本构模型可描述为线弹性、弹塑性、线性粘弹性和非线性粘弹性四种,但本质上PVB是非线性粘弹性材料,不同的环境条件与计算要求可选择不同的本构模型。目前,国内外学者比较认可的是用超弹性考虑其非线性,用Maxwell模型考虑其粘弹性。     

Creep assessment in hyperelastic material by a 3D Neural Network Reconstructor using bulge testing

A new methodology based on a Neural Network approach is developed for three-dimensional reconstruction of pseudo-hemispherical geometry of a hyperelastic specimen during a creep bulge test. On the basis of this procedure, the inflated membrane stress and strain states are established to determine the material mechanical properties. A new and economic experimental apparatus, used to conduct the bulge test, is provided with a sliding crossbar for the dome image acquisition in order to detect its strain state. A pressure regulator is used to ensure constant pressure, essential for creep. The artificial neural network was trained by using the position (x, y, z) of the membrane points at different values of the pressure measured in the bulge test chamber. The main characteristics of the system, which consist of the experimental apparatus and the neural post-processing, are accurate characterization of the materials and the ability to analyze materials with moderate anisotropy. This method has been verified on carbon black-reinforced SBR. The hyperelastic parameters that were obtained are in agreement with the values reported in scientific literature.     

Modeling hyperelastic behavior of rubber: A novel invariant‐based and a review of constitutive models

A constitutive phenomenological model completing the Gent‐Thomas concept is carried out to formulate laws governing the hyperelastic behavior of incompressible rubber materials. It is shown that the phenomenological Gent‐Thomas model (1958) and the constrained chain model (1992) give similar precise results at small to moderate deformation. On the other hand, comparisons of the outcome of the proposed model with that of the molecular model from the combined concepts of Flory‐Erman and Boyce‐Arruda (2000), and with those of the phenomenological models of Ogden (1982), Yeoh‐Fleming (1997), Pucci‐Saccomandi (2002) and Beda (2005) are made. Residual inconveniences raised by attractive continuum models in rubber elasticity literature have been successfully overcome. Results from both the statistical and phenomenological mechanics concepts are compared with the data of some useful classical materials (rubbers of Treloar, Rivlin‐Saunders, Pak‐Flory and Yeoh‐Fleming). The results permit one to see salient equivalence of the two theories for a more reliable prediction of stress‐stretch response for all states of any mode of deformation. A complete and exhaustive analysis of the Mooney plot that combines small and very large extension‐compression has been quite essential in assessing the validity of models. A method of identification of material parameters is presented and data of the simple tension suffice for the determination of the parameter values. It is shown that the ordinary identification procedures, such as the usual least squares, a very much used numerical method in materials investigation, can be unsuitable in some cases of hyperelastic modeling. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1713–1732, 2007     

Comparison of simple and pure shear for an incompressible isotropic hyperelastic material under large deformation

The concepts of simple and pure shear are well known in continuum mechanics. For small deformations, these states differ only by a rotation. However, correlations between them are not well defined in the case of large deformations. The main goal of this study is to compare these two states of deformation by means of experimental and theoretical approaches. An incompressible isotropic hyperelastic material was used. The experimental procedures were performed using digital image correlation (DIC). The simple shear deformation was obtained by single lap joint testing, while the pure shear was achieved by means of planar tension testing. Classical hyperelastic constitutive equations available in the literature were used. As a consequence, the results indicate that simple shear cannot be considered as pure shear combined with a rotation when large deformation is assumed, as widely considered in literature.     

Novel strategy for the hyperelastic parameter fitting procedure of polymer foam materials

The most widely used approach to model the large strain elastic response of polymer foams in a finite element (FE) simulation is the use of the Ogden–Hill compressible hyperelastic material model. This model is implemented and termed as “hyperfoam” material model in the commercial FE software ABAQUS. The hyperfoam model is able to characterize the large compressibility (in volumetric sense) of the foam material. In order to find the material parameters of the model for a particular foam specimen, we need to fit the simulated responses to the available experimental data. This task is easier for incompressible hyperelastic materials because we can use the incompressibility constraint to eliminate the transverse stretch from the stress solutions. However, this simplification cannot be used for the hyperfoam model, therefore, in the stress-strain relations, the transverse stretch is included, which makes the parameter fitting procedure more complicated. In this paper, a novel strategy is proposed for the parameter fitting task. The performance of the new algorithm is demonstrated by presenting fitted material responses for a particular polymer foam material. The major advantage of the new strategy is that it can be used with any third-party optimization solver and there is no need to write our own code.     

Constitutive model and failure locus of a polypropylene grade used in offshore intake pipes

BorECO^®™ BA212E is a polypropylene block co-polymer which has become a common material in the manufacturing of large diameter non-pressurized gravity offshore intake pipelines. These lines are used for transportation of sea water for cooling of petrochemical process plants. The pipe sections are joined by butt heat fusion welding to create the pipeline. Recently a few premature failures of such pipelines have been reported in the field. Hence, there is a need to characterize the constitutive behavior of the pipe and weld material in order to properly design these pipes. The aim of this work is to determine the material constitutive behaviors of the pipe material and the welded joint material. Uniaxial tensile tests of both the pipe and weld joint material are conducted at various strain rates. Both the pipe and weld material show a rather high strain rate dependency, with the weld material having about half the yield strength than that of the pipe material. An analytical constitutive material model is developed for both the pipe and weld material, incorporating the effect of strain rate. The failure locus, expressed in terms of the equivalent plastic strain at failure vs. the stress triaxiality, for both materials is also determined as part of the constitutive model using notched dumbbell specimens. The constitutive model and failure loci for the pipe and weld material are implemented in a finite element model (FEM) and are validated by conducting a series of independent four-point bend experiments on both material types. The validation is carried out by comparing the FEM results of the four-point bend model with the experimental results, which show a rather good agreement.     

Morphology and strain rate effects on heat generation during the plastic deformation of polyethylene/carbon black nanocomposites

The phenomenon of internal heat generation during the plastic deformation of polyethylene/carbon black nanocomposites at high strain rates was investigated using a high resolution thermal camera. Material morphology, strain rate and carbon black (CB) content were found to be critical factors that affected heat generation during tensile testing, and consequently changed the mechanical behaviour. Two processing methods (M1 and M2) were used to prepare the materials, with CB contents of 0.5, 1 and 3 wt.%. The results showed a significant increase in internal heat generation after yielding, with temperatures exceeding 70 °C for materials processed using M1 and 55 °C for materials processed using M2. The temperature increase was dependent on the processing method, the CB content and the strain rate. The increase in temperature due to plastic heat generation affected the properties of the material, reducing the plastic hardening and reducing the tensile strength at high strain rates. This is of significance when considering the use of these materials in applications involving high strain rates, such as impact protection.     

A new high frequency dynamic mechanical analysis system: An approach to direct high frequency testing of tire tread rubber

Dynamic Mechanical Analysis (DMA) systems are measurement devices for obtaining master curves and complex modules of viscoelastic materials, such as rubbers. The conventional DMAs measurement systems in market have several limitations, which restrict their ability for operating at high frequencies. Thus, Williams, Landel and Ferry (WLF) relation is used to produce master curves and predict the material properties at high frequencies. In conventional DMAs, experiments are done in a range of temperatures, and then a master curve is made for a chosen reference temperature by shifting the measurements data to high frequencies. Therefore, the obtained results, which are not based on direct measurements, can be inaccurate. In order to overcome this problem a new simple shear high-frequency DMA (HFDMA) system is designed and built to directly measure the dynamic mechanical properties of viscoelastic material at high frequencies and the strain levels sufficient for tire manufacturers. The new HFDMA can be used to test any viscoelastic materials which have glass transmission temperature (Tg) lower than room temperature (about 23 °C) such as the Styrene-butadiene rubber (SBR). The SBR is the base material for tire tread. The designing process of this new HFDMA is presented in this paper. The rubber specimen shape is chosen by taking into account the shear elastic wave effect, bending, buckling effect and heat generation in the specimen. The repeatability test is accomplished to ensure that the results obtained from the new HFDMA are repeatable and the repeatability uncertainty is about 0.04%. The new HFDMA is validated by comparing to the direct test results of conventional DMA at 100 Hz. The direct high frequency (5 kHz) complex shear modulus and damping factor are compared with the master curve of the conventional DMA developed by the use of WLF relation for SBR. This comparison revealed that the complex shear modulus and damping factor of the SBR obtained from the HFDMA at 5 kHz and 0.05% strain amplitude are about 7% and 6.5% higher than those obtained from the conventional DMA, respectively.     

一种基于分子链统计理论的橡胶超弹性混合本构模型

橡胶材料因其独特的超弹性在实际中广泛应用,通过解析应力-应变关系可以为橡胶力学性能的工程应用提供理论指导。为了更准确地描述橡胶材料力学性能,提出一种适用于橡胶材料的超弹性混合本构模型。新模型基于Gaussian模型与八链模型,引入有关拉伸比的权重函数将二者耦合,在拉伸比较小的情况下,新模型退化成Gaussian形式,在拉伸比较大的情况下,新模型可以依靠权重函数改善八链模型在小变形区域的不足。针对取向硬化的应力-应变曲线、无取向硬化的应力-应变曲线、不同拉伸比应力-应变曲线三种力学特征曲线,从单轴拉伸、等双轴拉伸和纯剪切三类实验数据进行预测验证。结果表明,新模型同时保留了Gaussian模型在小变形范围和八链模型在大变形范围内的预测优势,且对拉伸比和应力应变曲线形式没有依赖性,均能给出高精度的预测结果,突破了Gaussian模型和八链模型的使用限制,为橡胶超弹性力学性能的预测提供了新的思路。     

Low density polyethylene,expanded polystyrene and expanded polypropylene: Strain rate and size effects on mechanical properties

Polymeric foam materials may be used as energy absorbing materials for protection in impact scenarios, and design with these materials requires the mechanical properties of foams across a range of deformation rates, where high deformation rate testing often requires small samples for testing. Owing to their cellular macrostructure, and the large deformations that occur during loading of foams, the measured stress-strain response of a foam material may be influenced by the sample size. In this study, the mechanical properties of three closed-cell polymeric foams (Low Density Polyethylene, Expanded Polystyrene and Expanded Polypropylene) at two different densities were investigated over a range of deformation rates from 0.01 s⁻¹ to 100 s⁻¹. For each foam material, three different nominal sample sizes (10 mm, 17 mm and 35 mm) were tested. On average, the polymeric foam materials exhibited increasing stress with increasing deformation rate, for a given amount of strain.Density variation was identified at the sample level, with smaller samples often exhibiting lower density. Expanded Polystyrene demonstrated the highest variability in sample density and corresponding variability in mechanical response, qualitatively supported by observed variations in the macrostructure of the foam. Expanded Polypropylene exhibited variability in density with sample size, and observable variability in the material macrostructure; however, the dependence of the measured mechanical properties on sample size was modest. Low Density Polyethylene was found to have a relatively consistent cell size at the macrostructure level, and the material density did not vary significantly with sample size. In a similar manner, the dependence of measured mechanical properties on sample size was modest. The effect of sample size was identified to be material specific, and it is recommended that this be assessed using sample-specific density measurements and considering different sized samples when testing foam materials.     

单向应力条件下松弛时间率相关的非线性粘弹性本构模型

基于单向拉伸实验研究和内变量理论 ,提出了一种新的简单的一维非线性粘弹性本构关系 .对两种粘弹性材料 ,即高密度聚乙烯和聚丙烯进行了不同加载速率作用下的拉伸实验研究 ,实验结果表明 ,两种材料的应力应变关系与加载速率相关 ;对材料的应力应变实验数据进行拟合发现 ,材料的松弛时间具有很强的应变率相关性 ,当应变率发生数量级变化时 ,材料的松弛时间也发生数量级的变化 .采用内变量理论 ,导出了在单轴应力条件下松弛时间率相关的非线性粘弹性本构关系的迭代形式 ,并给出其收敛条件 .当采取一次迭代形式时 ,本构关系退化为松弛时间率相关的Maxwell模型 .数值拟合的结果表明 ,一次迭代形式的本构关系就可以很好地拟合和预测实验结果 .     

Dynamic uniaxial extension of elastomers at constant true strain rate

Elastomers are widely used for damping components in various industrial contexts because of their remarkable dissipative properties: they can bear severe mechanical loading conditions, i.e., high strain rates and large strains. Depending on the strain rate, the mechanical response of these materials can vary from purely rubber-like to glassy. In the intermediate strain rate range (1-100/s), uniaxial extension experiments are classically conducted at constant nominal strain rate. We present here a new experimental methodology to investigate the mechanical response of soft materials at constant true strain rate in the intermediate strain rate range. For this purpose, the displacement imposed on the specimen by the tensile machine is an exponential function of time. A high speed servo-hydraulic machine is used to perform experiments at strain rates ranging from 0.01 to 100/s. A specific specimen is designed in order to achieve a uniform strain field (and thus a uniform stress field). Furthermore, an instrumented aluminium bar is used to measure the applied force; which overcomes the difficulties due to dynamic effects. Simultaneously, a high speed camera enables the measurement of strain in the sample using a point tracking technique. Finally, the method is applied to determine the stress-strain curve of an elastomer for both loading and unloading responses up to a stretch ratio λ = 2.5; the influence of the true strain rate on both stiffness and dissipation of the material is then discussed.     

Tension behaviour of HNBR and FKM elastomers for a wide range of temperatures

This article presents uniaxial tension tests of three different elastomer compounds commonly applied as seal materials in the oil and gas industry. The tests were performed at five different temperatures, ranging from −20 to 150 °C. Optical measurements were used to ensure high quality stress–strain data. The material samples were exposed to a cyclic deformation history, enabling the viscoelastic behaviour to be explored. A considerable effect of temperature changes was found, with a pronounced increase of stiffness and viscosity for the lowest temperatures. A dip in the stress–strain curve was seen for one of the hydrogenated nitrile butadiene rubbers tested at low temperatures. Matrix-particle debonding simulations qualitatively described this stress dip. For the tests performed at the highest temperatures, a considerable number encountered material failure.     

Hyperelastic characterization of the interlamellar domain and interphase layer in semicrystalline polyethylene

The interphase layer in semicrystalline polyethylene (PE) serves as the transition between the crystalline lamellae and the amorphous domains and is recognized as the third constituent of PE. When PE undergoes large deformations, this interphase layer together with the amorphous phase behaves hyperelastically. Because of the metastable nature and nanometric size of the interphase and its intimate mechanical coupling to the neighboring crystal and amorphous domains, detailed characterization of its hyperelastic properties have eluded detailed experimental evaluation. To extract these properties, a combined algorithm is proposed based on applying the constitutive relations of an isotropic, compressible, hyperelastic continuum to the molecular dynamics simulation results of a PE stack from Lee and Rutledge (Macromolecules 2011, 3096–3108). The simulation element is incrementally deformed to a large strain, during which the stress–strain information is recorded. Assuming a neo‐Hookean model, the tensorial constitutive equation is derived. The hyperelastic parameters for the central amorphous phase, the interphase layer, and the interlamellar domain are identified with the help of the optimization notion and a set of nonnegative objective functions. The identified hyperelastic parameters for the interlamellar domain are in good agreement with the ones estimated experimentally and frequently used in the literature for the noncrystalline phase. The specifically developed sensitivity analysis indicates that the shear modulus is identified with a higher degree of certainty, in contrast to the bulk modulus. It is also revealed that the presented continuum mechanics analysis is able to capture the melting/recrystallization and rotation of crystalline chains that take place during the deformation. The evolutions of the boundaries of the hyperelastic elements are also identified concurrently with the hyperelastic parameters as the by‐product of the presented methodology. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013 , 51, 1692–1704     

Microcalorimetric qualitative analysis of biofilm development in porous media used in wastewater treatment by constructed wetland

In wastewater treatment by constructed wetland, the biodegradation capability of the biomass developed in the soil is one of the most important factors. For this kind of treatment unit, soil properties are studied to improve its filtration capacity and hydraulic residence time of the wastewater. The impact of soil properties like porosity and soil components on biomass development and biodegradation capacity seem to be less studied certainly due to the complexity of microbial identification techniques currently used. The study presented here is a preliminary work to validate that calorimetric technique could be a tool in the understanding of biodegradation capacity of wastewater treatment processes. Biofilm is preliminary developed in columns filled with different porous materials of well known porosity and constitutive components. These columns are fed with the same continuous flow of synthetic solution (C, N, and P) as a substrate amending during 3 weeks. Then each week, 2 mL samples of porous media from these columns are analyzed in isothermal calorimeter for 48 h. Net heat flow is recorded before and after substrate injection. This work results in the definition of the procedure for batch experiments in calorimeter for wastewater process efficiency. The results of these experiments show that the microbial reaction due to substrate amendment is highly depending on the porous material used for biofilm growth. Indeed calorimetric signals recorded lead to conclude that biofilm grown on plastic beads has a faster and more intensive reaction to glucose amendment than biofilm grown on glass beads. At least, two glass beads samples analyzed in the calorimeter after the same duration of feeding with synthetic solution have very different response to glucose or synthetic solution.     

Using developed creep constitutive model for optimum design of HDPE pipes

Unlike metal pipes, high density polyethylene (HDPE) pipes are not susceptible to erosion and corrosion. However, the most important mechanical feature of the HDPE pipes is that this material creeps even at room temperature. Therefore, it is essential to study the creep behavior of this material in order to develop a model. In this paper, creep behavior of HDPE at different temperature and stress levels has been experimentally studied to obtain the creep constitutive parameters of the material. These parameters are used to predict the creep behavior of different structures such as HDPE pipes. For this purpose, a number of specimens have been machined from industrial manufactured pipe walls. Uniaxial creep tests have been carried out and creep strain curves with time for each test were recorded. Then, a constitutive model is proposed for HDPE based on the experimental data and optimization methods. The results of this model have been compared with the test data and good agreement is observed. The developed constitutive model and reference stress method (RSM) were used to produce graphs which provide optimum creep lifetime and design conditions for HDPE pipes that are subjected to combined internal pressure and rotation. These graphs can facilitate the design process of HDPE pipes.     

Cyclic viscoplasticity of high-density polyethylene/montmorillonite clay nanocomposite

Observations are reported on high-density polyethylene (HDPE) and nanocomposite, where HDPE matrix is reinforced with montmorillonite (MMT) nanoclay, in uniaxial cyclic tensile tests with various cross-head speeds ranging from 1 to 50 mm/min. Each cycle of deformation involves tension up to the maximal strain ?_max = 0.1 and retraction down to the zero stress. The study focuses on low-cycle deformation programs with N = 5 cycles in each test.A constitutive model is derived for the viscoplastic response of polymers at three-dimensional cyclic deformations with small strains. Given a strain rate and a maximum strain, the stress-strain relations involve eight material constants that are found by fitting the experimental data. Good agreement is demonstrated between the observations and the results of numerical simulation. It is shown that the rate of cyclic deformation affects the adjustable parameters in a physically plausible way.     

Strain rate effects on the mechanical response of polypropylene-based composites deformed at small strains

The mechanical properties and response of two polypropylene (PP)-based composites have been determined for small strains and for a range of strain rates in the quasi-static domain. These two materials are talc-filled and unfilled high-impact PP. Uniaxial tensile tests were performed at different strain rates in order to characterize the mechanical response and the strain rate effect. The experimental results showed that both unfilled and talc-filled high-impact PP were sensitive to strain rate and exhibited nonlinear behavior even at relatively low strains. SEM analysis was conducted to obtain a better comprehension of deformation mechanisms involved during loading by observations of the microstructure evolution. For each of these two materials, two existing modeling approaches are proposed. The first one is a three-parameter nonlinear constitutive model based on the experimental results. The second is a micromechanically based approach for the elastic-viscoplastic behavior of the composite materials. The stress-strain curves predicted by these models are in fairly good agreement with our experimental results. Published in Russian in Vysokomolekulyarnye Soedineniya, Ser. A, 2008, Vol. 50, No. 6, pp. 1051–1059. This article was submitted by the authors in English.