Abstract: Cellulose degradation is usually characterized in terms of either the chain scission number or the scission fraction of cellulose unit as a function of degree of polymerisation (DP) and cellulose degradation evolution equation is most commonly described by the well known Ekenstam equations. In this paper we show that cellulose degradation can be best characterized either in terms of the percentage DP loss or in terms of the percentage tensile strength (TS) loss. We present a new cellulose degradation evolution equation expressed in terms of the percentage DP loss and apply it for having a quantitative comparison with six sets of experimental data. We develop a new kinetic equation for the percentage TS loss of cellulose and test it with four sets of experimental data. It turns out that the proposed cellulose degradation evolution equations are able to explain the real experimental data of different cellulose materials carried out under a variety of experimental conditions, particularly the prolonged autocatalytic degradation in sealed vessels. We also develop a new method for predicting the degree of degradation of cellulose at ambient conditions by combining the master equation representing the kinetics of either percentage DP loss or percentage TS loss at the lowest experimental temperature with Arrhenius shift factor function.
Abstract: In this paper we present a time-temperature superposition method for predicting the permanence of paper by extrapolating accelerated paper ageing data to ambient conditions. The presented method includes a test for the presence of shift factors to superpose all of the raw accelerated ageing data over the temperature range studied to obtain a master curve, a numerical fit of the master curve for producing a master equation representing the kinetics of paper degradation, a critical examination of applying Arrhenius equation for explaining the relationship between the empirically determined shift factors and the accelerated ageing temperature, and a verification of the Arrhenius activation energy extrapolation assumption. Unlike the usual approach that extrapolates the Arrhenius relationship between lifetime and temperature, without corroborating evidence, to ambient temperatures, we test the Arrhenius activation energy extrapolation assumption by determining the influence of acidity on cellulose hydrolysis reactions, and have found that detection and identification of the acid-sensitive linkages in cellulose substances is an ultra-sensitive and reliable method to measure degradation of cellulose and paper in what is normally the extrapolation region (ambient temperatures). Taking the examples of natural ageing data in literature from 18 bleached kraft dry-lap pulps for 22 years under ambient conditions and three handsheet samples for 22 years under controlled conditions, comparison of the predictions with natural ageing results has been addressed.
Abstract: We develop a kinetic model for the growth of electrical tree structures in solid polymeric insulation that allows for combined electrical and mechanical stresses. We present an energy balance analysis during the tree growth process and show the total tree extension driving force is not a material constant due to the existence of a damage process zone around the tree tip and the dissipated heat and the trapped energy remaining in the damage process zone. We derive both the tree growth rate equation and life formula of tree propagation to breakdown, based on the specific tree growth mechanism and the fractal nature of tree structures. We perform electrical tree growth tests in epoxy resin samples with and without mechanical residual stresses, and present results which show that the proposed model can give predicted tree propagation times to failure in good agreement with the experimental data of the tree growth subjected to a combined electrical and mechanical stress.
Abstract: Fiber reinforced polymeric insulation materials are composite insulating materials that are made from high-performance polymeric insulation material and high-performance fibers. This makes reinforcement of fiber-reinforced polymeric insulation material of interest in many structural insulation applications subject to high voltage or ultra high voltage environment. In this paper, the effect of residual stresses on the electrical insulation integrity is first summarized followed by the description of the laser Raman spectroscopy technique for the assessment of the residual stress in high-performance fiber reinforced polymeric insulation. The present investigation is concerned with the use of the laser Raman spectroscopy to analyze the residual stresses in a Twaron/spl reg/-type aromatic polyamide (aramid) fibers/Araldite/spl reg/ epoxy resin system.
Abstract: A model has been developed to predict the low cycle fatigue crack growth and life of short-fibre reinforced aluminium-matrix composites. The proposed model is based on the assumption that there is a fatigue-damaged zone ahead of the crack tip in which the local cyclic stress level approaches the ultimate tensile strength of the composite and the actual degradation process of the composite takes place. The cyclic plastic deformation in the fatigue-damaged zone is suggested as the main mechanical driving force for the crack growth. The cyclic J integral is calculated quantitatively from the cyclic plastic deformation characterisation in the fatigue-damaged zone. The range of crack tip opening displacement is correlated to the cyclic J integral in terms of the result of fracture mechanics. The low cycle fatigue crack growth per cycle is taken by an amount equal to one half of the range of crack tip opening displacement. The low cycle fatigue life is determined by integrating the crack growth rate equation from the initial crack length to a critical crack length. The empirical Coffin-Manson and Basquin laws and total strain amplitude vs. life relationship have been derived theoretically and compared with total-strain controlled low cycle fatigue data of pure aluminium Al99.85 reinforced with Al2O3 Saffil fibres of a volume fraction of 20 vol.% over a wide test temperature range from -100 °C to 150 °C. The results indicate that the model gives an excellent description of the crack growth process of short-fibre reinforced aluminium-matrix composites under low cycle fatigue conditions.
Abstract: The cyclic deformation behaviour of the die-cast magnesium alloy AZ91HP was investigated under total strain control at constant total strain amplitudes between 1.4 à 10-3 to 2 à 10-2 at room temperature and at 130 °C. Microstructural investigations in combination with the determination of crack-growth behaviour using the replica technique and measurements of changes of the stiffness (compliance) of the specimen during a fatigue experiment led to a detailed understanding of the evolution of damage and the main damage mechanisms. Based on these findings, a microstructurally based life-prediction concept was formulated.
Abstract: A low cycle fatigue model has been developed to predict the fatigue life of both the unreinforced aluminium alloy and the short-fibre reinforced aluminium alloy metal-matrix composites based solely on crack propagation from microstructural features. In this approach a crack is assumed to initiate and grow from a microstructural feature on the first cycle. The model assumes that there is a fatigue-damaged zone ahead of the crack tip within which the actual degradation of the material takes place. The low-cycle fatigue crack growth and the condition for failure are controlled by the amount of cyclic plasticity generated within the fatigue-damaged zone ahead of the crack tip and by the ability of the short fibres to constrain this cyclic plasticity. The fatigue crack growth rate is directly correlated to the range of crack-tip opening displacement. The empirical Coffin-Manson and Basquin laws have been derived theoretically and applied to compare with total-strain controlled low-cycle fatigue life data obtained on the unreinforced 6061 aluminium alloy at 25 °C and on the aluminium alloy AA6061 matrix reinforced with Al2O3 Saffil short-fibres of a volume fraction of 20 vol. % and test temperatures from - 100 to 150 °C. The proposed model can give predicted fatigue lives in good agreement with the experimental total-strain controlled fatigue data at both high strain low-cycle fatigue and low strain high-cycle fatigue regime. It is remarkable that the addition of high-strength Al2O3 fibres in the 6061 aluminium alloy matrix will not only strengthen the microstructure of the 6061 aluminium alloy, but also channel deformation at the tip of a crack into the matrix regions between the fibres and therefore constrain the plastic deformation in the matrix. The overall expected effect is therefore the reduction of the fatigue ductility.
Abstract: A (high strain) low-cycle fatigue (LCF) life prediction model of ultrafine-grained (UFG) metals has been proposed. The microstructure of a UFG metal is treated as a two-phase 'composite' consisting of the 'soft' matrix (all the grain interiors) and the 'hard' reinforcement (all the grain boundaries). The dislocation strengthening of the grain interiors is considered as the major strengthening mechanism in the case of UFG metals. The proposed model is based upon the assumption that there is a fatigue-damaged zone ahead of the crack tip within which the actual degradation of the UFG metal takes place. In high-strain LCF conditions, the fatigue-damaged zone is described as the region in which the local cyclic stress level approaches the ultimate tensile strength of the UFG metal, with the plastic strain localization caused by a dislocation sliding-off process within it. The fatigue crack growth rate is directly correlated to the range of the crack-tip opening displacement. The empirical Coffin-Manson and Basquin relationships are derived theoretically and compared with experimental fatigue data obtained on UFG copper (99.99%) at room temperature under both strain and stress control. Good agreement is found between the model and the experimental data. It is remarkable that, although the model is essentially formulated for high strains (LCF), it is also found to be applicable at low strains in the high-cycle fatigue (HCF) regime.
Abstract: A low-cycle fatigue life prediction model for particulate-reinforced metal-matrix composites (MMCs) is presented. The low-cycle fatigue behaviour of particulate-reinforced MMCs is treated as a localised damage development phenomenon activated by applied cyclic loading. The localised cyclic stress and strain concentration and fatigue damage evolution of microstructural elements within the fatigue damaged zone ahead of the crack tip are considered to dominate the whole low-cycle fatigue processes. In high-strain low-cycle fatigue conditions, the fatigue-damaged zone is described as the region in which the local cyclic stress level approaches the ultimate tensile strength of the composite and within which the actual degradation of the composite material takes place. The fatigue crack growth rate is directly correlated to the range of the crack tip opening displacement. The empirical Coffin-Manson relationship is derived and the model naturally predicts that in strain-controlled low-cycle fatigue tests the MMCs with the higher volume fraction of reinforcement particles exhibit shorter fatigue lives than composites with a smaller volume fraction. The theoretical predictions from this model are in good agreement with the low-cycle fatigue life data of AA6061-T6 aluminium alloy reinforced with 15 and 20 vol.% alumina particles, respectively, for a temperature range between - 100 and 150 °C in total-strain controlled mode.
Abstract: A statistical model of fatigue damage evolution has been developed for particulate-reinforced metal-matrix-composites (MMCs) by taking into considerations both the initial damage distribution and the effect of particulate reinforcement on fatigue damage development. The growth of microscopically fatigue-damaged regions in particulate-reinforced MMCs is considered as a stochastic process, and both the non-equilibrium statistical method and minimum strength principle are used to establish the evolution equation of fatigue damage. The fatigue damage evolution equation developed in the present study characterizes not only the kinetic process of fatigue damage evolution but also sets up the relationship between the mechanism of fatigue damage growth of the microscopically damaged regions and the result of fatigue damage, i.e. degradation of mechanical properties of particulate-reinforced MMCs. A new expression for calculating the cumulative fatigue damage and a new formula for predicting the average fatigue strength of the particulate-reinforced MMCs are derived. Experimental data of 2080Al/SiCp composites are analyzed and compared with results obtained with the present model. It is shown that the experimental results can be described well by the calculations.
Abstract: The dielectric breakdown phenomenon in the materials with quenched disorder is studied. By using renormalization group method the percolation limit for breakdown pc, the breakdown field Ec and the fractal dimension of the structure of dielectric breakdown df were obtained. The results indicate that the breakdown properties of the materials with quenched disorder are characterized by universal power laws, where the exponents are universal.
Abstract: A plastic flow-induced fracture theory for fatigue crack growth is presented. A new formulae for the fatigue stress intensity threshold and the fatigue crack growth rate law are derived by applying the principle of energy conservation in considering the fatigue crack growth process in the presence of local plastic flow ahead of the crack-tip. The present theory predicts not only the fatigue crack growth rate being just proportional to the rate of creation of dislocation at the crack-tip, but also the fatigue stress intensity threshold, which can be determined according to the applied fatigue stress amplitude and the characteristic size of microstructural fracture process ahead of the crack-tip, and can account for the fatigue crack growth characteristics at both low and high levels of applied fatigue stress intensity amplitude. All the results are universal and agree with the existing empirical results and experimental observations.
Abstract: In this paper we try to show how a reasonable equation for crack propagation in brittle materials naturally leads to self-affine or statistically self-similar failure structures. We argue that the fractal behaviours of crack propagation in brittle materials are in essence determined by the scale-invariance of the Fokker-Planck equation. This approach provides a good illustration for understanding the microscopic origin of fractal behaviour in real materials.
Abstract: A kinetic model of dielectric aging was presented, showing that there is a power-law relationship between the local electric field concentration and the rate of defect-tip initiated conducting crack growth. By applying such a power-law conducting crack growth rate expression to the evaluation of the life of solid dielectrics, the empirical classical aging law of insulation materials can be derived theoretically as a logical result. All results are universal and agree with the experimental data of oxide films.
Abstract: A non-equilibrium statistical theory of water treeing in polymeric cable insulators, which treats water tree growth as a stochastic process, is presented. In this treatment the deterministic equation for the rate of water tree growth is made stochastic by the addition of a fluctuation term. The fluctuations are used to model the effect of the complex topologically connected microstructure of the polymeric insulator on water tree growth. Such considerations lead to a generalized Langevin equation for the water tree's growth rate as well as an equivalent Fokker - Planck equation for the probability density distribution of the water tree length. Many of the major attributes of water tree growth are shown to be a natural consequence of this equation. The self-affine fractal object for water tree morphology is first constructed, based both on the self-affinity of the time-correlating fluctuations and on the scale-invariance of the fundamental dynamic equation dominating water tree growth. The empirical two-parameter Weibull distribution of water tree length in the literature is also derived. Good quantitative agreement between theory and previously reported experimental results is shown.
Abstract: An electro-chemical reaction model is proposed in this paper to describe electrical ageing of solid dielectrics under ac voltage. The present model has the ability to predict not only the scale-invariance property of the electrical ageing function, but also the classical electrical ageing law, i.e. the electric field strength - lifetime inverse power law relationship. The micromechanisms inherent in the model enable a direct physical interpretation of the macroscopic electrical ageing behaviour in terms of the microscopic processes. We argue that the fractal behaviour of electrical ageing and breakdown processes in solid dielectrics are by essence determined by the scale-invariance of the electrical ageing evolution equation.
Abstract: The dynamic aspects of dielectric breakdown was studied. A quantitative model based on impact ionization was presented. The formulas of micro-defect growth rate and lifetime prediction were derived, and the calculating values were in good agreement with the experimental data for silicon dioxide. All variables have definite physical meaning.
Abstract: A new development in the understanding of the fractal behavior in fatigue of materials is presented. The results from the investigation include the scaled Fokker-Planck equation and corresponding scaling solution for the crack size distribution (CSD) function. The problems of microcrack system evolution are discussed and the physical origin of the statistically self-similarity property in fatigue of materials is explored.
Abstract: A kinetic model of time-dependent dielectric breakdown for polymers is presented. The micromechanism of thermally activated bond-breakdown is developed for polymers to account for the dynamical process of dielectric breakdown. We first prove that the thermally activated polymer bond-breakdown processes can explain not only the burst of a conducting pathway nucleated in any defect and the following successive propagation, but also the time-dependent dielectric breakdown strength of polymers. The formulae for conducting microcrack growth rate and time to failure are derived and applied to the experimental data for polyethylene terephthalate films.
Abstract: The dynamic aspects of linear elastic fracture mechanics applied to dielectric breakdown are studied. In the same direction as the fracture mechanics analogue for dielectric breakdown described previously by some researchers, this paper develops the intrinsic, rate-dependent bond breakdown micromechanism to account for the dynamic process of dielectric breakdown. The formulae of conducting microcrack growth rate and lifetime prediction are derived and applied to the experimental data of SiO2 films.
Abstract: A model for hydrogen-induced cracking is proposed and developed on the assumption that
hydrogen enters the embrittling process zone (EPZ) ahead of the crack,tip and promotes localized
plastic flow which is proportional to its concentration. The formulae of the criterion for crack
propagation, KrH, and the crack growth rate dl/dt, are derived and applied to the experimental data
of AISI4340 superhigh-strength steels.
Abstract: 15. Ding, H. -Z., Wang, Z. D. and Jarman, P. âAccelerated ageing of oil-paper for the assessment of dielectric capabilities of aged transformers: Literature reviewâ in Proceedings of 14th International Symposium on High Voltage Engineering
Abstract: 12. Wang, Z. D. and Ding, H. -Z. âAssessment of dielectric capability of aged transformer insulationâ in Proceedings of First UHVNet Colloquium on Condition Monitoring and Ageing of High Voltage Plant/Equipment, 26th January 2005, Cardiff University, Cardiff, UK
Abstract: Ageing degradation of cellulose insulation in power transformers increases with operating cycles in a cumulative manner and may lead to the failure of power transformers. In this study a cumulative degradation model for the ageing of cellulose insulation is proposed. The concept of degradation accumulation is applied to predict the life of cellulose insulation; a degradation evaluation function, called the degree of degradation, is introduced to represent the ageing of cellulose paper insulation. The model has been applied to explain the experimental data of Kraft paper over a wide ageing temperature range from 60°C to 160°C. The results indicate that the model gives a good description of the evolution process of ageing degradation of cellulose insulation.
Abstract: Studies have been made of the development of micro-composite dielectrics with electrical breakdown resistance superior to the neat epoxy resin. Micron-sized aluminium trihydroxide particles were used as reinforcements to enhance the time to breakdown of an Araldite epoxy resin. Electrical tree growth tests have been performed in point-plane specimens in unfilled resin and resin-filled with aluminium trihydroxide in the 0-15 wt % range. Results are presented which show that the addition of filler particles in the epoxy resin can make considerable improvements in breakdown resistance by increasing the times to breakdown. In particular, the times to breakdown of microcomposite dielectrics increase with the increasing filler volume fraction. A model is proposed for electrical treeing breakdown-resistant composite dielectrics which accounts well for these experimental observations.
Abstract: In the present study, various amounts of nano- and micro-scale zinc oxide particles (62 nm and 1 into average diameter) were systematically introduced into an Araldite epoxy resin matrix for reinforcement purposes. The influence of these particles on the electrical treeing breakdown resistance was investigated experimentally. Samples of point-plane geometry were made and tested under 10 kV and 15 kV AC voltage. Comparative results are presented which show that the addition of a small amount (0.5 to 1 wt.%) of zinc oxide particles in the epoxy resin can make significant improvements in breakdown resistance by increasing the treeing time to breakdown, and nano-ZnO/epoxy composites are far more resistive to treeing breakdown than that of micro-ZnO/epoxy composites and neat epoxy resin. The possible reasons why and how smaller particles should improve the electrical treeing resistance are discussed.
Abstract: 9. Ding, H.-Z. and Varlow, B.R. âElectrical treeing studies on the Araldite LY/HY 5052 epoxy resin over a wide range of stressing voltageâ in
Electrical Insulation and Dielectric Phenomena, 2004. CEIDP '04. 2004 Annual Report Conference on 17-20 Oct. 2004, Page(s): 306- 309
