Determination of reliable fatigue life predictors for magnetorheological elastomers under dynamic equi-biaxial loading

Fatigue life prediction is of great significance in ensuring magnetorheological elastomer (MRE) based rubber components exhibit reliability and do not compromise safety under complex loading, and this necessitates the development of plausible fatigue life predictors for MREs. In this research, silicone rubber based MREs were fabricated by incorporating soft carbonyl iron magnetic particles. Equi-biaxial fatigue behaviour of the fabricated MREs was investigated by using the bubble inflation method. The relationship between fatigue life and maximum engineering stress, maximum strain and strain energy density were studied. The results showed that maximum engineering stress and stored energy density can be used as reliable fatigue life predictors for SR based MREs when they are subjected to dynamic equi-biaxial loading. General equations based on maximum engineering stress and strain energy density were developed for fatigue life prediction of MREs.     

Theoretical investigation of polymer molecular structure influence on dielectric properties and mechanical properties

Triboelectric nanogenerator (TENG) technologies have explosive development in the field of energy harvesting and self-powered sensing. As the key element of triboelectric devices, dielectric polymers have obtained much attention in recent years. The dielectric properties of polymer determine the output performance of TENG. In this paper, we take silicone rubber as an example of dielectric polymers, to study the properties of molecular structure influence on the dielectric properties and mechanical properties by the molecular dynamics simulation method. The free volume fraction, dielectric constant, and mechanical properties of silicone rubbers with different branch chains were calculated. The dielectric constant is highly related to the free volume distribution and the dipole moments of silicone rubbers with different amounts of branch chains. For fewer branch chains silicone rubber, the free volume distribution contributes most to the dielectric constant; for more branch chains silicone rubber, the dipole moment dominates the dielectric constant. Therefore, the silicone rubber ratio has a great influence on the dielectric constant of silicone rubber. With the increase of temperature, the dielectric constant of 2-chain silicone rubber increases at first and then decreases, and the maximum value is obtained near 300 K. Therefore, it is necessary to control the temperature when silicone rubber is used as a dielectric material. This work can be a guide for improving the dielectric properties of silicone rubber, and it provides a new approach to the optimal design of high-performance triboelectric nanogenerators.     

Improved resistance to crack growth of natural rubber by the inclusion of nanoclay

The effect of nanoclay on the fatigue crack growth behavior was investigated. Fatigue tests were carried out on edge notched specimens under cyclic tension loadings. A power–law dependency between crack growth rate and tearing energy was obtained. Natural rubber (NR) filled with 5 phr organically modified montmorillonite (OMMT) possessed the lowest value of the exponent, b, and the smallest crack growth rate at a given tearing energy, denoting the strongest resistance to crack growth. Strain‐induced crystallization was probed by synchrotron WAXD experiments, showing earliest occurrence and strongest ability of crystallization in NR with 5 phr OMMT due to the better exfoliation and orientation of clay layers. The study on the viscoelastic property by dynamic mechanical analysis indicated that NR filled with 10 phr OMMT had the largest contribution to tearing energy attributed to the viscoelastic dissipation in the viscoelastic region in front of the crack tip. This revealed that the strain‐induced crystallization played a more important role in the crack growth resistance than the viscoelastic dissipation for clay filled rubber. Copyright © 2010 John Wiley & Sons, Ltd.     

Predictive aging results in radiation environments

We have previously derived a time-temperature-dose rate superposition methodology, which, when applicable, can be used to predict polymer degradation versus dose rate, temperature and exposure time. This methodology results in predictive capabilities at the low dose rates and long time periods appropriate, for instance, to ambient nuclear power plant environments. The methodology was successfully applied to several polymeric cable materials and then verified for two of the materials by comparisons of the model predictions with 12 year, low-dose-rate aging data on these materials from a nuclear environment. In this paper, we provide a more detailed discussion of the methodology and apply it to data obtained on a number of additional nuclear power plant cable insulation (a hypalon, a silicone rubber and two ethylene-tetrafluoroethylenes) and jacket (a hypalon) materials. We then show that the predicted, low-dose-rate results for our materials are in excellent agreement with long-term (7–9 year) low-dose-rate results recently obtained for the same material types actually aged under bnuclear power plant conditions. Based on a combination of the modelling and long-term results, we find indications of reasonably similar degradation responses among several different commercial formulations for each of the following “generic” materials: hypalon, ethylene-tetrafluoroethylene, silicone rubber and PVC. If such “generic” behavior can be further substantiated through modelling and long-term results on additional formulations, predictions of cable life for other commercial materials of the same generic types would be greatly facilitated.     

Molecular Tailoring Approach for the Estimation of Intramolecular Hydrogen Bond Energy

Hydrogen bonds (HBs) play a crucial role in many physicochemical and biological processes. Theoretical methods can reliably estimate the intermolecular HB energies. However, the methods for the quantification of intramolecular HB (IHB) energy available in the literature are mostly empirical or indirect and limited only to evaluating the energy of a single HB. During the past decade, the authors have developed a direct procedure for the IHB energy estimation based on the molecular tailoring approach (MTA), a fragmentation method. This MTA-based method can yield a reliable estimate of individual IHB energy in a system containing multiple H-bonds. After explaining and illustrating the methodology of MTA, we present its use for the IHB energy estimation in molecules and clusters. We also discuss the use of this method by other researchers as a standard, state-of-the-art method for estimating IHB energy as well as those of other noncovalent interactions.     

自组织临界的OFC模型在聚丙烯介电弛豫中的应用

将自组织临界中OFC模型的能量传递概念及相关参量α引入极值动力学模型,得到了同时与KWW方程和幂律均相关的介电弛豫的函数关系.用时域介电谱方法测量了聚丙烯在-15～ 90℃慢极化电荷的释放过程,结果显示:在较低温度,弛豫为幂律;在较高温度,弛豫随时间的增加从幂律过渡到KWW关系.理论分析与实验结果基本一致,由此可知KWW方程的参量β反映了外界温度对材料弛豫单元的影响;参量a反映了能量传递的大小及温度的影响,当β较小或a较大时,弛豫的幂律关系较为明显.     

Fatigue testing of rubber materials and articles

The mechanism of rubber fatigue in tension, based on fracture mechanics, and the relationship between crack growth measurements and fatigue tests are revised. Deviations from linearity are considered. The importance of test conditions, such as constant stress or constant strain, in development work is stressed.

Poor precision of fatigue results is noted. As frequency distributions of test results often do not follow a Gaussian pattern, fitting a Weibull distribution function to experimental fatigue results is recommended as it allows estimation of confidence intervals of parameters such as median, mean, etc., and the statistical analysis of results.

The use of master curves for extrapolation of fatigue results to low tearing energies close to fatigue limit is revised. Extension of fatigue results to other deformation modes is briefly considered.     

Fatigue fracture of a highly stretchable acrylic elastomer

Dielectric elastomer has been extensively explored in various applications as soft active material. In most applications, dielectric elastomer is subjected to cyclic loading-unloading condition. As a result, a small initial defect in a dielectric elastomer may finally grow to a critical size to induce catastrophic rupture. In this article, we carried out an experimental study of the crack growth in an acrylic dielectric elastomer under cyclic loading-unloading. Pure-shear test specimens were used to measure the relationship between crack growth rate and energy release rate. Such relationship can be simply fit to a power-law. We further used the measured power-law to successfully predict the fatigue lifetime of the acrylic elastomer with an edge crack and subject to simple extension cyclic loading-unloading test.     

Morphological study of fatigue-induced damage in isotactic polypropylene

This article reports initial results of an investigation whose aim is to characterize fatigue damage induced in semicrystalline polymers subjected to uniaxial high cycle fatigue. Herein we report results obtained from fatiguing tensile bars of high molecular weight compression-molded alpha-phase iPP. Samples were fatigued for up to one million cycles at a frequency of 2 Hz. During fatigue, in situ measurements of dynamic mechanical response and energy densities were recorded. Postmortem morphological studies were also conducted using SEM of etched surfaces and TOM. The results show that damage formation occurs in a regularly spaced array of crazes. This damage, its evolution, and energetics are discussed as they relate to the overall fatigue life of the material. A methodology to isolate the energy consumption for the formation of a single craze is given. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2751–2760, 1998     

A 3D printable and self-healing polydimethylsiloxane elastomer

Silicone elastomers are broadly used in various fields because of their unique properties, such as flexibility, durable dielectric insulation, and excellent stability in hash environments. As a result, three-dimensional (3D) printing of silicone elastomers is frequently required to construct personalized structures. However, existing 3D-printing of silicone elastomers are less accurate, difficult to maintain shape, or require doping modification with thixotropic agents. Moreover, common 3D-printable silicone elastomers do not have self-healing capability, so they have to be discarded upon damaging. Herein, by introducing hydrogen bonds to improve the shape retention ability and induce network reversibility, we have developed a self-healing polydimethylsiloxane elastomer, which can be readily 3D-printed by fused deposition modeling (FDM) technology. We believe that this new silicone elastomer would be useful in the field of biomedical materials, flexible electronics, medical inserts, soft robots and so on.     

Adsorption energy distribution from the Aranovich-Donohue lattice density functional theory

We propose a new methodology projected for the estimation of the adsorption energy distribution from the monolayer part of a single nitrogen adsorption isotherm determined at 77 K based on the lattice density functional theory (DFT) via the Aranovich-Donohue formalism. At first sight, the presented approach is computationally more difficult than a classical one. However, it is more flexible and comprehensible. Next, we developed a numerical program and used it for the estimation of the adsorption energy distribution from the experimental data on carbon black samples. The main nitrogen molecule-carbon black surface interaction energy can be estimated as approximately 7-8 kJ/mol, but the heterogeneity of the investigated materials differs significantly. Furthermore, we compare the results obtained from the lattice DFT via the Aranovich-Donohue formalism with the solution of the integral equation with the kernel represented by the classical monolayer localized Fowler-Guggenheim isotherm equation. The similarity between these two independent approaches is observed. The proposed methodology can be used for the investigation of the energetic heterogeneity of not only the carbonaceous materials but also the other "flat-surfaced" solids.     

Influence of the Dielectric Environment on the Photoluminescence Intermittency of CdSe Quantum Dots

We show experimentally that the photoluminescence intermittency (blinking) of single CdSe quantum dots (QDs) is influenced by the dielectric properties of the embedding environment (matrix), the type of ligands and the capping shell. For the on‐times, we observe (and tentatively explain) a strong deviation from the commonly reported inverse power law behaviour, which can be taken into account by an exponential cut‐off at long times. We assign this component to the photoejection of the electron, while the power law behaviour is a combination of hole‐ and electron‐trapping processes. The cut‐off times and their distributions depend strongly on the polarity of the environment. Also, the off‐times show, though on a much longer timescale, deviations from the inverse power laws. We suggest a model including surface states and self‐trapped states, which quantitatively explains the experimental observations.     

纳米材料在超级电容器领域的有效设计与可控合成

伴随着电化学储能器件在便携式电子产品、混合动力电动汽车及大型工业规模的电力和能源管理中的应用,设计合成出结构新颖、性能优越的先进纳米电极材料显得至关重要.作为电化学储能器件中的重要一员,超级电容器以其功率密度高、循环寿命长等特点越来越受到人们的广泛关注,而电极材料的组成及结构是其性能高低的决定性因素.本文结合本科研团队近几年来的研究工作,综述了有关超级电容器纳米电极材料的设计与可控合成及其前沿研究进展.     

连续碳纤维增强的聚芳醚酮在Ⅰ型循环载荷下的层间破坏

用双悬臂梁(DCB)试件研究了连续碳纤维增强的聚芳醚酮复合材料(CF/PEK-C),在Ⅰ型循环载荷作用下的层间裂纹扩展行为.循环载荷采用载荷控制模式,最小载荷与最大载荷之比为0.5.在疲劳试验中,仍然发现有“阻力曲线”现象存在.层间裂纹扩展速率用指数定律与相应的应变能释放速率联系起来,并对结果进行了讨论.     

连续碳纤维增强的聚芳醚酮在Ⅰ型循环载荷下的层间破坏

 用双悬臂梁(DCB)试件研究了连续碳纤维增强的聚芳醚酮复合材料(CF/PEK-C),在Ⅰ型循环载荷作用下的层间裂纹扩展行为.循环载荷采用载荷控制模式,最小载荷与最大载荷之比为0.5.在疲劳试验中,仍然发现有“阻力曲线”现象存在.层间裂纹扩展速率用指数定律与相应的应变能释放速率联系起来,并对结果进行了讨论.     

A facile route for the fabrication of large-scale gate-all-around nanofluidic field-effect transistors with low leakage current

Active modulation of ions and molecules via field-effect gating in nanofluidic channels is a crucial technology for various promising applications such as DNA sequencing, drug delivery, desalination, and energy conversion. Developing a rapid and facile fabrication method for ionic field-effect transistors (FET) over a large area may offer exciting opportunities for both fundamental research and innovative applications. Here, we report a rapid, cost-effective route for the fabrication of large-scale nanofluidic field-effect transistors using a simple, lithography-free two-step fabrication process that consists of sputtering and barrier-type anodization. A robust alumina gate dielectric layer, which is formed by anodizing sputtered aluminium, can be rapidly fabricated in the order of minutes. When anodizing aluminium, we employ a hemispherical counter electrode in order to give a uniform electric field that encompasses the whole sputtered aluminium layer which has high surface roughness. In consequence, a well-defined thin layer of alumina with perfect step coverage is formed on a highly rough aluminium surface. A gate-all-around nanofluidic FET with a leak-free gate dielectric exhibits outstanding gating performance despite a large channel size. The thin and robust anodized alumina gate dielectric plays a crucial role in achieving such excellent capacitive coupling. The combination of a gate-all-around structure with a leak-free gate dielectric over a large area could yield breakthroughs in areas ranging from biotechnology to energy and environmental applications.     

Methods to ease the release of thin polydimethylsiloxane films from difficult substrates

Silicone elastomers are used as dielectric electroactive polymers for making actuators, generators, sensors, and as artificial muscles in medical applications. Current requirements in the actuator manufacturing put a strict limitation on the thickness of the elastomers, such that a maximum permissible thickness is around 25–50 µm. The relatively small Young's modulus for these elastomers is a requirement for actuation capabilities. However, peeling and release of such films during manufacture processes are very difficult. To ease the release of the films, techniques such as the use of release agents like surfactants and detergents, incorporating resins in the silicone matrix and grafting/adding low surface energy functionalities to the silicone elastomer have been tested. The methods used are required not to interfere with the Young's modulus and the dielectric permittivity in a negative way. Polysorbate‐20, a non‐ionic surfactant, fulfills all requirements and gives the lowest peel forces for the films. Copyright © 2013 John Wiley & Sons, Ltd.     

An Overview of Linear Dielectric Polymers and Their Nanocomposites for Energy Storage

As one of the most important energy storage devices, dielectric capacitors have attracted increasing attention because of their ultrahigh power density, which allows them to play a critical role in many high-power electrical systems. To date, four typical dielectric materials have been widely studied, including ferroelectrics, relaxor ferroelectrics, anti-ferroelectrics, and linear dielectrics. Among these materials, linear dielectric polymers are attractive due to their significant advantages in breakdown strength and efficiency. However, the practical application of linear dielectrics is usually severely hindered by their low energy density, which is caused by their relatively low dielectric constant. This review summarizes some typical studies on linear dielectric polymers and their nanocomposites, including linear dielectric polymer blends, ferroelectric/linear dielectric polymer blends, and linear polymer nanocomposites with various nanofillers. Moreover, through a detailed analysis of this research, we summarize several existing challenges and future perspectives in the research area of linear dielectric polymers, which may propel the development of linear dielectric polymers and realize their practical application.     

Fatigue life determinations of rubber springs

A method for the determination of fatigue lives of rubber springs is described which is based on a fracture mechanics approach. Fatigue experiments have been performed on model components consisting of solid rubber cylinders bonded between metal endpieces of the same diameter. Crack growth in uniaxial compression followed an approximately parabolic locus. Tearing energies were determined both experimentally and theoretically. The results could be plotted on a unique curve of tearing energy, T. versus crack growth rate, dc/dN, as could results from tests on thin strips of rubber fatigued in simple extension. Application of the approach is described to two case histories. The first is the design of a large spherical rubber joint to support an offshore oil-production platform, and the second is the fatigue analysis of a rubber antivibration mounting for a rail car after 2·5 years service.