High strain rate behavior of graphene-epoxy nanocomposites

This work consists of the synthesis of high purity graphene nanoflakes (GNF), the manufacturing of GNF-epoxy nanocomposites and the mechanical characterization of the nanocomposite at high and quasi static strain rates, (2750/s - 1.E−5/s). GNF were synthesized by using the electric arc discharge technique. Thermogravimetry/Differential Thermal Analysis (TG/DTA) of synthesized graphene reveals high purity and high crystallinity. Raman spectra and the broad Brunauer-Emmet-Teller (BET) specific surface area indicate that the synthesized graphene has several layers. Following the solution mixing manufacturing process of GNF-epoxy nanocomposites, the influences of strain rate on the mechanical behaviors are investigated under quasi static and dynamic loadings. High strain rate uniaxial compression tests (1270–2750/s) using Split Hopkinson Pressure Bar (SHPB) and quasi static compression tests (1.E−3 and 1.E−5/s) of GNF-epoxy with two graphene contents (0.1 and 0.5 wt %) are performed at room temperature. The maximum elasticity modulus achieved by the GNF-epoxy with 0.5 wt% at the strain rate of 2350/s corresponds to a 68% increase compared to the neat epoxy. The yield strength of the material is doubled under dynamic loading conditions compared to the quasi static loading.     

Effect of strain rate on the dynamic tensile behaviour of UHMWPE fibre laminates

Ultra-high molecular weight polyethylene (UHMWPE) fibre has great potential for strengthening structures against impact or blast loads. A quantitative characterization of the mechanical properties of UHMWPE fibres at varying strain rates is necessary to achieve reliable structural design. Quasi-static and high-speed tensile tests were performed to investigate the unidirectional tensile properties of UHMWPE fibre laminates over a wide range of strain rates from 0.0013 to 163.78 s⁻¹. Quasi-static tensile tests of UHMWPE fibre laminates were conducted at thicknesses ranging from 1.76 mm to 5.19 mm. Weibull analysis was conducted to investigate the scatter of the test data. The failure mechanism and modes of the UHMWPE fibre laminates observed during the test are discussed. The test results indicate that the mechanical properties of the UHMWPE fibre laminate are not sensitive to thickness, whereas the strength and the modulus of elasticity increase with strain rate. It is concluded that the distinct failure modes at low and high strain rates partially contribute to the tensile strength of the UHMWPE fibre laminates. A series of empirical formulae for the dynamic increase factor (DIF) of the material strength and modulus of elasticity are also derived for better representation of the effect of strain rate on the mechanical properties of UHMWPE fibre laminates.     

Viscoelastic and relaxation properties of a polystyrene melt in axial extension

On axial extension of polymer melts at constant deformation rates, the development of high-elastic deformation is of predominant importance during the initial period. High-elastic deformation is accompanied by a rise in viscosity and in the modulus of high-elasticity and by retardation of the relaxation processes in the region of large relaxation times. At relatively low deformation rates, the rise in viscosity and high-elasticity modulus and the retardation of relaxation processes may give way to a decrease in viscosity and high-elasticity modulus and acceleration of relaxation processes, so that stationary flow regimes are attained. The transition from strain regimes with increasing viscosity and modulus of high elasticity to those with a decrease of these quantities corresponds to an increase in the rate of accumulation of irreversible deformation. Accordingly, a competing influence due to the orientation effect and to destruction of the network of intermolecular bonds becomes evident while stationary flow is being attained. The orientation effect must be responsible for the retardation of the relaxation processes, whereas rupture of the intermolecular network bonds results in structural relaxation accelerating relaxation processes. In contrast to shearing, during extension the orientation effect is of predominant importance. Hence in stationary flow regimes the viscosity may not only remain independent of the rate of strain, but even increase with it. In this case the contribution of the large relaxation times to the relaxation spectrum increases with increasing stress in stationary flow regimes. The fact that the longitudinal viscosity and the modulus of high elasticity are independent of the stress in stationary flow regimes does not guarantee linearity of the mechanical properties of the polymer in the prestationary stage of deformation when complex changes occur in its relaxation characteristics. At high deformation rates the viscosity and the modulus of high elasticity keep rising with increasing deformation until rupture occurs. Determination of the strength of polystyrene samples vitrified after extension showed that it is due not to the entire degree of extension, but only to the value of accumulated high-elastic deformation. The strength of the vitrified samples is to a first approximation independent of the rate at which the melt was extended.     

Effect of strain rate on the compression behaviour of a woven fabric S2-glass fiber reinforced vinyl ester composite

Quasi-static (˜10⁻³s⁻¹) and high strain rate (>500 s⁻¹) compression behavior of an S2-glass woven fabric/vinyl ester composite plate was determined in the in-plane and through-thickness directions. In both directions, modulus and failure strength increased with increasing strain rate. A higher strain rate sensitive modulus was found in the through-thickness direction while a higher strain rate sensitive failure strength was found in the in-plane direction. In the in-plane direction, the failure mode was observed to change from splitting followed by “kink banding” (localized fiber buckling) to predominantly splitting at increasing strain rates, while it remained the same in the through-thickness direction.     

Measurement of the stress-relaxation modulus in the primary transition region

Difficulties have been encountered in experimentally measuring the stress-relaxation modulus for systems with steep slopes in the primary transition region. The usually applied “factor-of-ten” rule is shown to apply in these cases as well as in cases where the relaxation is slower; with the steep slope, however, the rule offers little help, since the decay in the modulus is so fast that stresses are usually very small at times when direct modulus calculations may be made. A technique is suggested which allows calculation of the modulus in this difficult region. A slow constant strain-rate deformation is followed by a constant strain period. Modulus values for times during the constant strain rate period are calculated using well known relationships. Long-time values are calculated from the definition of the modulus (the factor-of-ten rule being employed) and a recursion relation is developed which is used for modulus calculations during the constant strain period at relatively short times where effects of strain rate are important. Starting values for the recursion relation are long-time moduli which can be calculated directly.     

Electrorheological fluid characterization via a vertical oscillation rheometer

A custom designed vertical oscillation rheometer (VOR) is used for the rheological measurements of electrorheological (ER) fluids consisting of 15 and 20 vol.% semiconducting polyaniline particles suspended in silicone oil. The viscoelastic material functions, including complex viscosity and complex shear modulus, are measured via geometric parameters, measured force, and applied strain of the VOR. Viscoelastic properties of the ER fluids are also measured as a function of applied electric field strength and particle concentration. The VOR, equipped with a high voltage generator, can easily be constructed and used to measure ER properties. It is further found that polyaniline suspensions behave as viscoelastic materials in an electric field. In linear viscoelastic conditions, elasticity was promoted with the increment of electric field due to particle chain structure in the presence of the applied electric field. It is also found that the applied electric field rather than particle concentration enhanced the elasticity of ER fluids.     

Variations in the mechanical properties of end products from mixed plastics

From well selected polymers, using an experimental plan methodology, we show the different influences (nature, processing conditions, composition) on the properties of end products from mixed plastics such as strain and stress at rupture, flexural modulus, impact strength and extrusion flow rate. From models, it is possible to adjust necessary formulations to obtain a good quality level and, eventually, an improvement of poor properties. Mixed plastics can be considered as a polyethylene matrix containing other polymers such as polypropylene (PP), polystyrene (PS) or poly(vinyl chloride) (PVC). Poly(ethylene terephthalate) (PET) is neglected in this study. Properties such as strain at rupture or impact strength are drasticly affected by the level of polypropylene, polystyrene or poly(vinyl chloride): 5 or 10% nullify these properties. Other properties such as stress at rupture, flexural modulus or extrusion flow rate highly depend on the relative ratio of polymers. To improve the poor properties or to obtain a good level in a property, it is necessary to add well adapted compatibilizers or to dope the mixed plastics material by one of the polymers.     

Combination of glass fibres and minerals in polyamides

Polyamides reinforced with glass fibres are characterised by high values for modulus of elasticity and notched impact strength but also have the negative properties of rough surfaces and anisotropy - particularly anisotropic shrinkage. Mineral-filled polyamides have good surface properties, are isotropic and have high impact strength, however, the elasticity modulus is only slightly better than for normal polyamides and notched impact strength values are relatively low. Through a combination of glass fibres and mineral filler, surface properties can be improved and anisotropy reduced. In order to achieve the optimum mechanical properties a range of 0 - 50 wt.-% glass fibre content and 0 - 40 wt.-% mineral content was systematically examined using statistically designed experiments. This investigation showed the relationship between the “Responses”, surface appearance, shrinkage, modulus of elasticity, impact strength and tensile properties and the “variables”, glass fibre and mineral content. Optimal products, based on the model functions, can be chosen in this way.     

Characterizing the constitutive response and energy absorption of rigid polymeric foams subjected to intermediate-velocity impact

As an optimum energy-absorbing material system, polymeric foams are needed to dissipate the kinetic energy of an impact, while maintaining the impact force transferred to the protected object at a low level. Therefore, it is crucial to accurately characterize the load bearing and energy dissipation performance of foams at high strain rate loading conditions. There are certain challenges faced in the accurate measurement of the deformation response of foams due to their low mechanical impedance. In the present work, a non-parametric method is successfully implemented to enable the accurate assessment of the compressive constitutive response of rigid polymeric foams subjected to impact loading conditions. The method is based on stereovision high speed photography in conjunction with 3D digital image correlation, and allows for accurate evaluation of inertia stresses developed within the specimen during deformation time. Full-field distributions of stress, strain and strain rate are used to extract the local constitutive response of the material at any given location along the specimen axis. In addition, the effective energy absorbed by the material is calculated. Finally, results obtained from the proposed non-parametric analysis are compared with data obtained from conventional test procedures.     

戊二醛蒸汽交联明胶材料的性能研究

利用戊二醛蒸汽对明胶材料进行交联改性。研究了交联反应时间对明胶材料力学性能、溶出性能和溶胀特性的影响。研究发现,随着交联时间的延长,交联反应从明胶瓣表面至内部逐步进行,由此可获得交联度呈梯度变化的明胶材料。研究结果表明,明胶材料的拉伸强度、模量和冲击强度随交联反应时间的延长而增加,而溶出速率和溶胀率随交联反应时间的延长而减小。蒸汽交联明胶材料的溶胀动力学不能用二次速率方程来描述。     

不同液体环境下聚丙烯酰胺的单分子力学

利用单分子力谱研究了非离子型聚丙烯酰胺(NPAM)及其水解产物阴离子型聚丙烯酰胺(APAM)在不同液体环境下的单链弹性. 利用改进的自由连接链(M-FJC)模型中的单链弹性模量参数(K₀)描述NPAM在不同pH值水溶液中的单链弹性. 实验结果表明, K₀随着溶液pH值增加而增大, 表明NPAM在碱性溶液中的水解度具有pH依赖性. 由于K₀和高分子链的净电荷正相关, K₀增大表明NPAM链净电荷增多, 结构单元之间的静电排斥作用增强使高分子链呈现高度伸展的构象. 在此基础上研究了APAM在不同pH值溶液中的构象, 单分子力谱数据表明, APAM在酸性水溶液中为柔性链, 在碱性水溶液中呈现较伸展的构象, 从而在分子水平上阐明APAM链构建的水凝胶网络的溶胀机理. 单分子层面的深入研究有望阐明这类高分子的减阻机理.     

Shear elasticity of a liquid in bulk and in boundary layers

Resonance measurements at 7.5 × 10⁵ Hz reveal a measurable bulk shear elasticity for various liquids (water, acetone, benzene, alcohols, acetic acid, CCl₄, oil). The shear modulus of a nonpolar liquid is independent of the layer thickness. A polar liquid shows a sharp increase in the shear elasticity at distances less than 10^-5 cm from the surface of the quartz.     

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。     

Oscillatory shear response of moisture barrier coatings containing clay of different shape factor

Oscillatory shear rheology of barrier coatings based on dispersed styrene-butadiene latex and clay of various shape factors or aspect ratio has been explored. Barrier performance of these coatings when applied to paperboard has been assessed in terms of water vapour transmission rates and the results related to shape factor, dewatering and critical strain. It has been shown that a system based on clay with high shape factor gives a lower critical strain, dewatering and water vapour transmission rate compared with clays of lower shape factor. The dissipated energy, as calculated from an amplitude sweep, indicated no attractive interaction between clay and latex implying a critical strain that appears to be solely dependent on the shape factor at a constant volume fraction. Particle size distribution was shown to have no effect on the critical strain while coatings of high elasticity exhibited high yield strains as expected. The loss modulus demonstrated strain hardening before the elastic to viscous transition. The loss modulus peak was identified by a maximum strain which was significantly lower for a coating based on clay with a high shape factor. The characteristic elastic time was found to vary between 0.6 and 1.3s. The zero shear viscosity of barrier dispersion coatings were estimated from the characteristic elastic time and the characteristic modulus to be of the order of 25-100 Pa s.     

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.     

Extreme hardening of PDMS thin films due to high compressive strain and confined thickness

Polymers confined to small dimensions and that undergo high strains can show remarkable nonlinear mechanics, which must be understood to accurately predict the functioning of nanoscale polymer devices. In this paper we describe the determination of the mechanical properties of ultrathin polydimethylsiloxane (PDMS) films undergoing large strains, using atomic force microscope (AFM) indentation with a spherical tip. The PDMS was molded into extremely thin films of variable thickness and adhered to a hard substrate. We found that for films below 1 μm in thickness the Young's modulus increased with decreasing sample thickness with a power law exponent of 1.35. Furthermore, as the soft PDMS film was indented, significant strain hardening was observed as the indentation depth approached 45% of the sample thickness. To properly quantify the nonlinear mechanical measurements, we utilized a pointwise Hertzian model which assumes only piecewise linearity on the part of the probed material. This analysis revealed three regions within the material. A linear region with a constant Young's modulus was seen for compression up to 45% strain. At strains higher than 45%, a marked increase in Young's modulus was measured. The onset of strain induced stiffening is well modeled by finite element modeling and occurs as stress contours expanding from the probe and the substrate overlap. A third region of mechanical variation occurred at small indentations of less than 10 nm. The pointwise Young's modulus at small indentations was several orders of magnitude higher than that in the linear elasticity region; we studied and ruled out causes responsible for this phenomenon. In total, these effects can cause thin elastomer films to become extremely stiff such that the measured Young's modulus is over a 100-fold higher than the bulk PDMS. Therefore, the mechanics of a polymer can be changed by adjusting the geometry of a material, in addition to changing the material itself. In addition to understanding the mechanics of thin polymer films, this work provides an excellent test of experimental techniques to measure the mechanics of other nonlinear and heterogeneous materials such as biological cells.     

Mechanical properties of polyamide-12 layered silicate nanocomposites and their relations with structure

The mechanical properties of polyamide-12/Cloisite 30B (PA12/C30B) nanocomposites prepared by melt compounding were studied as a function of clay volume fraction φ under various processing conditions. All measured mechanical characteristics, Young's modulus, yield stress, strain at break and stress at break, exhibit a transition at φ_p1%, identified with a percolation threshold. Also, the linear and non-linear mechanical properties appeared to depend on the degree of exfoliation of the structure, which can be tuned by the processing conditions. The three-phase Ji's theoretical model was used to predict Young's modulus as a function of clay concentration, focusing on the influence of the degree of exfoliation. Experimental yield stress data were fitted to Pukanszky's model and discussed in terms of PA12/C30B interfacial adhesion.     

基于金属配位键的自修复、可重塑聚丁二烯橡胶的合成及性能

橡胶材料通常因经过硫化及补强等工艺处理而呈现出热固性,因而难以被回收处理,容易造成严重的资源浪费和环境污染.本文通过在聚丁二烯上修饰羧酸基团,再加入锌离子(Zn2+)与羧酸配位,制备了基于金属配位键交联的自修复橡胶(PB-COOH/Zn2+).该橡胶具有良好的机械性能和优秀的自修复及重塑性能,在70℃下修复3 h,其韧性可以恢复到初始强度,修复效率可达100%. PB-COOH/Zn2+较高的聚合物链段运动能力及配位键交联网络良好的动态性不仅赋予其优异的修复性能,还使得其在较温和的条件下可以进行多次重塑,在70℃及5 MPa的条件下重塑3次仍能保持原有的机械性能.此外,通过在PB-COOH/Zn2+中掺杂适量的碳纳米管,不仅增强了其机械性能,还使其具备了电致修复及传感能力,扩宽了PB-COOH/Zn2+作为环境友好型材料的应用前景.     

The application of fracture mechanics to the impact behaviour of rubber-toughened polyamides

Rubber-modification of polyamide is a widely-applied method of improving material resistance under high strain rate loading. The processing conditions used for preparing such two-phase blends strongly influence their structure and thus their subsequent impact properties. In the present work the relationships between production parameters, phase structure and impact resistance have been studied, and the rǒle of the rubber phase in promoting energy absorption investigated. It has been found that improved impact resistance, defined as a combination of high resistance to crack initiation and to crack propagation, is achieved by decreasing the polyamide phase viscosity while increasing the extrusion and injection temperatures, the mixing shear rate and the rubber phase volume fraction; an EPR modifier offers superior performance to a polybutadiene modifier; the dominant mechanisms of energy-absorption are shearing and void formation, there being no clear evidence of crazing; the J-integral technique of plastic fracture mechanics can be applied to Charpy impact testing; TEM allied with image analysis techniques provides quantitative morphological information on polymer blends.     

Tensile properties of semi-crystalline thermoplastic polymers: Effects of temperature and strain rates

This work deals with the study of temperature and time dependency of tensile properties of a PA 12-based polymer. The range of variation of parameters in experiments was linked to in-service conditions of components manufactured with this material (temperature interval from ?25 °C to 50 °C and average strain-rate magnitudes from 0.00028 s^?1 to 9.4 s^?1). For tests with different temperatures and low speed, an electro-mechanical machine, Zwick Z250, equipped with an incremental extensometer was used. To study the effect of strain rate at medium speeds, a servo-hydraulic system, Schenk PC63M, equipped with a strain-gauge extensometer was used, while at high speeds a servo-hydraulic machine, Instron VHS 160/20, equipped with a high-speed camera for strain evaluation by digital image correlation was employed. The changes of the rate of deformation with strain as well as elastic modulus variation with strain were studied. An increase in the elastic modulus and yield strength was observed with a drop in temperature and an increase in the strain-rate, temperature having a stronger influence on the variation of mechanical properties. The collected data was assembled in an elasto-plastic material model for finite-element simulations capable of rendering temperature- and strain-rate-dependency. The model was implemented in the commercial software Abaqus, yielding accurate results for all tests.