复合材料夹层壳振动分析的高阶剪切变形理论

本文提出了计及复合材料面板横向剪切变形的夹层壳高阶位移模式,该模式满足夹层壳上、下表面剪应力为零的条件,并以此推导出夹层壳自由振动的有限元方程,讨论了夹层壳的夹芯及面板的阻尼特性,给出阻尼矩阵的形成方式,将数值计算与实验结果进行了对比,最后给出了阻尼比Ψ随夹芯厚度h_c、剪切模量G和阻尼损耗因子β的变化曲线.     

Effect of presence of SiC and operating frequency on the damping behaviour of pure magnesium

Abstract

The objective of the present work was to investigate the effect of presence of SiC reinforcement and the vibration frequency on the overall damping characteristics of pure magnesium. The testing method uses a combination of the modified free – free beam method coupled with a circle fit approach. The effect of frequency on the damping property was studied by adding end masses to the specimen so as to alter the resonant frequency of the suspended beam. In the present study, the results are compared against a monolithic magnesium sample. The results revealed a higher damping capability of the composite specimen at all tested frequencies when compared to monolithic magnesium. Both monolithic and reinforced magnesium showed a progressive decrease in damping with an increase in frequency. An attempt is made to rationalise the damping behaviour of the composite in terms of the presence of a process induced plastic zone at the matrix/particulate interface and the operating frequency.     

Micromechanics and damping properties of composites integrating shear thickening fluids

Damping is an important parameter for vibration control, noise reduction, fatigue endurance or impact resistance of composite materials. In this study, a micromechanical model was used to predict the damping of a composite material containing shear thickening fluids (STFs) at the fibre–matrix interfaces. Predictions of the model and dynamical mechanical analysis results are in concert. The damping of the composites was improved significantly. The dynamic properties exhibited a strong dependence on both frequency and applied external load amplitude. Damping peaks appeared which coincided with the thickening of the STF at the fibre–matrix interface. The location of the peaks depends on the onset of thickening and post-thickening rheological behaviour of the STF. This work shows that a micromechanics approach can be useful for an appropriate choice of microstructural design and properties of STFs in order to control the stiffness and damping behaviour of composites. STFs can be integrated at the microscale of polymer composites to create new materials with load-controlled adaptive dynamic stiffness-damping properties.     

Residual stress of as-deposited and rolled wire+arc additive manufacturing Ti–6Al–4V components

Wire + arc additive manufacturing components contain significant residual stresses, which manifest in distortion. High-pressure rolling was applied to each layer of a linear Ti–6Al–4V wire + arc additive manufacturing component in between deposition passes. In rolled specimens, out-of-plane distortion was more than halved; a change in the deposits' geometry due to plastic deformation was observed and process repeatability was increased. The Contour method of residual stresses measurements showed that although the specimens still exhibited tensile stresses (up to 500?MPa), their magnitude was reduced by 60%, particularly at the interface between deposit and substrate. The results were validated with neutron diffraction measurements, which were in good agreement away from the baseplate.

This paper is part of a Themed Issue on Measurement, modelling and mitigation of residual stress.     

Zum Ermüdungsverhalten von Hartschaumstoffen

Fatigue Behaviour of Rigid Cellular Plastics The fatigue behaviour of crosslinked and non-crosslinked rigid cellular PVC had been investigated for tensile, compressive, and shear stresses, and displayed by means of Wöhler diagrams. In dynamic extension/compression loading, precompression shortens the life of the specimen, while pre-extension lengthens it - within certain limits. This behaviour can be explained by the different forms of the stress-strain curves in extension and compression. Detailed analysis of the hysteresis loops, including evaluation of the strains, energy uptake, and damping value, allows conclusions to be drawn about the behaviour of the material when subjected to dynamic loads and the degree of approach to failure. Although cellular plastics are poor thermal conductors there is very little rise in temperature when they are subjected to dynamic loads.     

Influence of Physicochemical Interactions on the Properties of Suppositories. III. Reeological Behaviour of Fatty Supfository Bases and its Effect on Spreading in Rats

Abstract

The viscosity and other rheological properties of the molten base at rectal temperature can markedly affect the rate of release and absorption of drugs' from fatty suppositories. The rheograms of pure mono-acid triglycerides, their mixtures and triglyceride suppository bases were determined at various temperatures using a rotational rheometer. Completely molten systems gave Newtonian behaviour, while incompletely molten mixtures, containing a suspension of higher melting triglyceride, exhibited plastic behaviour with thixotropy which reverted to Newtonian behaviour on removal, dissolution or melting of the higher melting component, The plastic yield values (67 Nm^?2) were less tnan the reported rectal pressure (300 Nm^?2), suggesting that they exert little effect on spreading in the rectum.     

Rheological Characterization of Tear Substitutes

Abstract

The steady state viscosity as a function of shear rate was determined for eight commerical tear substitutes and compared with data for high molecular weight sodium hyaluronate solutions. The Zero shear viscosity at steady state varied more than 100-fold from about 2 to 300 cP. Many samples were Newtonian, while some samples exhibited a varying degree of shear thinning (psedudoplastic) behaviour. The results are discussed in realation to the rheological behaviour of normal tears and mucin.     

复合材料层合板非线性热屈曲分析

本文基于高阶变形理论和修正型Hahn-Tsai非线性本构模型,提出一种复合材料层合板非线性热屈曲分析方法.针对四边简支反对称角铺设复合材料层合板,导出了非线性热屈曲临界温度封闭解.数值结果表明:材料非线性能显著降低层合板临界温度.      

Elevated temperature X-ray measurement of residual stresses in a fibre reinforced Al alloy

Thermal residual stresses have been measured using X-ray diffraction in an Al-2% Mg matrix with 10, 20 or 26 vol % Al₂O₃ short fibres. Stress measurements were made at room temperature as well asin situ at elevated temperatures up to 250?C. The thermal stresses arise due to the difference in coefficient of thermal expansion (CTE) between the matrix and the reinforcement. The largest CTE is found in the matrix, resulting in tensile residual stresses after a temperature drop, e.g. after processing or annealing. A high fraction of reinforcement implies higher matrix stresses than a low fibre content. The stresses decrease with increasing temperature for all fibre volume fractions. Measurements are compared with calculations using a modified Eshelby model for equivalent inclusions. Parameters taken into account in the model are coefficient of thermal expansion, Young's modulus, and volume fraction and geometric shape of the reinforcing phase. A good correlation between calculations and experimental results has been found, bearing in mind that no plasticity is taken into account in the Eshelby model. The plastic behaviour of the composites has been described using a model based on a rigid spherical cavity in an elastic-plastic matrix.     

Plasticity by martensite transformations in cobalt base metallic glasses?

The possibility of mechanical coupling to applied stresses of martensitic transformations in Co₈₀Nb₁₄B₆ and C0₈₄Nb₁₀B₆ metallic glasses reported by O'Handley and co-workers in the temperature range 295 to 473 and 513 K, respectively, was investigated. Since martensitic transformations strongly couple with externally applied shear stresses, internal friction scans and plastic resistance measurements in both tension and plane-strain compression were carried out in thermal cycles in the same temperature range, complemented with differential scanning calorimetry observations. No transformation plasticity was detected in these glasses as internal friction showed no peaks, DSC showed no heat evolution or absorption, and the temperature dependence of flow stress exhibited a monotonically decreasing behaviour.Visiting Associate Professor in the Department of Mechanical Engineering (1984–6), on leave from Middle East Technical University, Ankara, Turkey.     

Design and damping characteristics of magneto‐rheological fluid damper for special object

Aiming at the vibration characteristics of a small type steel frame, based on the theory of non‐Newton liquid, from the physical property of the magneto‐rheological fluids, the shear stress model of magneto‐rheological fluids damper is built up, the non‐linear characteristics of the magneto‐rheological damper is described, and the parameters of designed magneto‐rheological damper are obtained. The vibration characteristics of the frame structure is studied using magneto‐rheological damper, the results show that after the magneto‐rheological fluids damper fixed, the vibration response of the frame structure is decreased by 42 % for the first mode, which validates the damping effect of the developed magneto‐rheological damper, which provides the theoretical and experimental proof for the design of magneto‐rheological damper.     

折叠型双瓦楞纸板衬垫振动传递特性的试验研究

研究了3类折叠型双瓦楞纸板衬垫的防振性能,获得了不同静应力条件下的振动传递率曲线、峰值频率、传递率和阻尼比.试验结果表明,这3类衬垫的振动传递特性具有多模态性,有主次之分,而且静应力值对振动传递特性有明显影响.     

Effect of refrigerant oil additive on R134a and R123 boiling heat transfer performance

This paper investigates the effect that an additive had on the boiling performance of an R134a/polyolester lubricant (POE) mixture and an R123/naphthenic mineral oil mixture on a roughened, horizontal flat surface. Both pool boiling heat transfer data and lubricant excess surface density data are given for the R134a/POE (98% mass fraction/2% mass fraction) mixture before and after use of the additive. A spectrofluorometer was used to measure the lubricant excess density that was established by the boiling of the R134a/POE lubricant mixture before and after use of the additive. The measurements obtained from the spectrofluorometer suggest that the additive increases the total mass of lubricant on the boiling surface. The heat transfer data show that the additive caused an average and a maximum enhancement of the R134a/POE heat flux between 5 kW m⁻² and 22 kW m⁻² of approximately 73% and 95%, respectively. Conversely, for nearly the same heat flux range, the additive caused essentially no change in the pool boiling heat flux of an R123/mineral oil mixture. The lubricant excess surface density and interfacial surface tension measurements of this study were used to form the basis of a hypothesis for predicting when additives will enhance or degrade refrigerant/lubricant pool boiling.     

Residual stress due to parallel heat welding in small specimens of type 304 stainless steel

Abstract

The aim of this study was to verify the mechanism of parallel heat welding (PHW) and to investigate the thermal and residual stresses due to welding. In this study, autogenous gas tungsten arc welding was used on type 304 stainless steel. Wells' one-dimensional stress distribution model was assumed to explain the behaviour of stress introduced by conventional welding (CW) and PHW processes. A step by step method proposed by Tall was applied to calculate theoretical thermal and residual stresses. Comparison between experimental and analytical results showed that the predicted results calculated using a modified Wells' model are in agreement with the experimental results, and that the elevation of equilibrium temperature during the welding process is an important mechanism for the reduction of residual stress in the PHW process. The theoretical curves of residual stress calculated for thermal cycles corresponding to CW and PHW processes show fairly good agreement with experimental results in the vicinity of the fusion zone.

MST/1552     

Modelling of stretch forming behaviour in steel strip using finite element method

Abstract

Laboratory tests were carried out on CR1 grade steel strip involving punch stretching without lubrication and with PQ6 oil or polythene as lubricant. The finite element code was then used to simulate the forming behaviour during the test. The effects of normal anisotropy of the material and the contact friction between punch/die and the specimen on the forming behaviour were studied systematically. The predictability of the finite element modelling was confirmed by direct comparisons between the experimental measurements and the computer predictions. The aspects of further refinements required in the finite element models are discussed.

MST/1887     

Electroactive response of mesoporous silica and its nanocomposites with conducting polymers

Electroactive response of suspensions of mesoporous silica and its nanocomposites with conducting polyaniline and copolyaniline inside its channels were examined under an electric field, mainly focusing on their rheological characteristics. Initially these conducting polymer/mesoporous silica nanocomposites were synthesized and their physical properties were studied by scanning electron microscopy, transmission electron microscopy and N₂-adsorption isotherm. Then, mesoporous silica and its nanocomposites were dispersed in silicone oil as an electrorheological (ER) material. Typical ER behaviors of shear stress and shear viscosity curves as a function of electric field and shear rate were observed. Without an electric field, the suspensions behaved almost like a Newtonian fluid. However, under an electric field, their shear stresses increased with shear rate, demonstrating a yield stress. Compared with mesoporous silica and polyaniline, polyaniline/mesoporous silica-based ER fluid showed enhanced ER performance due to the anisotropic characteristics. In addition, it was found that a suggested shear stress model (Cho–Choi–Jhon model) well described the flow curves.     

A rational elasto‐plastic spatially curved thin‐walled beam element

Torsion is one of the primary actions in members curved in space, and so an accurate spatially curved‐beam element needs to be able to predict the elasto‐plastic torsional behaviour of such members correctly. However, there are two major difficulties in most existing finite thin‐walled beam elements, such as in ABAQUS and ANSYS, which may lead to incorrect predictions of the elasto‐plastic behaviour of members curved in space. Firstly, the integration sample point scheme cannot capture the shear strain and stress information resulting from uniform torsion. Secondly, the higher‐order twists are ignored which leads to loss of the significant effects of Wagner moments on the large twist torsional behaviour. In addition, the initial geometric imperfections and residual stresses are significant for the elasto‐plastic behaviour of members curved in space. Many existing finite thin‐walled beam element models do not provide facilities to deal with initial geometric imperfections. Although ABAQUS and ANSYS have facilities for the input of residual stresses as initial stresses, they cannot describe the complicated distribution patterns of residual stresses in thin‐walled members. Furthermore, external loads and elastic restraints may be applied remote from shear centres or centroids. The effects of the load (and restraint) positions are important, but are not considered in many beam elements. This paper presents an elasto‐plastic spatially curved element with arbitrary thin‐walled cross‐sections that can correctly capture the uniform shear strain and stress information for integration, and includes initial geometric imperfections, residual stresses and the effects of the load and restraint positions. The element also includes elastic restraints and supports, which have to be modelled separately as spring elements in some other finite thin‐walled beam elements. Comparisons with existing experimental and analytical results show that the elasto‐plastic spatially curved‐beam element is accurate and efficient. Copyright © 2006 John Wiley & Sons, Ltd.     

Thermal cycles effects on interlaminar shear strength (ILSS) and impact behaviour of carbon/PEI composites

In this paper, the effect of thermal cycle on the interlaminate shear strength (ILSS) and impact behaviour of unidirectional carbon fibre reinforced polyetherimide (PEI) matrix composites were studied. Samples were subjected to 100 thermal cycles (by immersing from boiling water (100°C) to ice water (0°C). The effects of thermal cycles were characterized by short beam shear and instrumented impact testers. Also Fractographic investigations were done using a scanning electron microscope (SEM). It is observed that the plastic deformations at the fibre/matrix and interlaminar interface as well as residual stresses lower the ILSS and flexural modulus of the material proportional with the number of thermal cycles. Up to the first 20 thermal cycles the material shows a brittle fracture with lower fracture energy, but after the 20th thermal cycles it is possible to observe that the material fractures with higher fracture energy at longer fracture time. A remarkable difference in the fracture morphology between the thermal cycled and un-treated materials has been observed. It is found that thermal cycles strictly affect the fracture morphology.     

Time dependent viscoelastic rheological response of pure,modified and synthetic bituminous binders

Bitumen is a viscoelastic material that exhibits both elastic and viscous components of response and displays both a temperature and time dependent relationship between applied stresses and resultant strains. In addition, as bitumen is responsible for the viscoelastic behaviour of all bituminous materials, it plays a dominant role in defining many of the aspects of asphalt road performance, such as strength and stiffness, permanent deformation and cracking. Although conventional bituminous materials perform satisfactorily in most highway pavement applications, there are situations that require the modification of the binder to enhance the properties of existing asphalt material. The best known form of modification is by means of polymer modification, traditionally used to improve the temperature and time susceptibility of bitumen. Tyre rubber modification is another form using recycled crumb tyre rubber to alter the properties of conventional bitumen. In addition, alternative binders (synthetic polymeric binders as well as renewable, environmental-friendly bio-binders) have entered the bitumen market over the last few years due to concerns over the continued availability of bitumen from current crudes and refinery processes. This paper provides a detailed rheological assessment, under both temperature and time regimes, of a range of conventional, modified and alternative binders in terms of the materials dynamic (oscillatory) viscoelastic response. The rheological results show the improved viscoelastic properties of polymer- and rubber-modified binders in terms of increased complex shear modulus and elastic response, particularly at high temperatures and low frequencies. The synthetic binders were found to demonstrate complex rheological behaviour relative to that seen for conventional bituminous binders.     

Thermal stability enhancement mechanism of engine oil using hybrid MoS2/h-BN nano-additives with ionic liquid modification

One of the primary causes of lubricating oil failure in automobile engines is thermal degradation caused by high temperatures, which prejudices the lube oil capacity between friction pairs. This research aims to comprehensively investigate the effect of the h-BN/MoS₂ hybrid nanoparticles and ionic liquid (IL) on the thermal performance of engine oil (5 W-40). The rheological results showed that nanolubricants (NL) behave like non-Newtonian fluids and IL promoted the viscosity value of lubricant due to the extension of alkyl chains. The thermal peculiarities of lube oil were analyzed by thermogravimetric (TGA/DTG/DSC) analysis to explore the thermal stability of h-BN/MoS₂ hybrid nano-lubricants modified by IL and further revealed how IL improved the thermal characteristics. The results strongly showed the synergistic effect between the IL and h-BN/MoS₂ hybrid nanoparticles produces a complicated interaction at high temperatures, forming a layer of film which can delay the erosion of high temperature on the surface of the nanoparticles. Ultimately, this study revealed a delay of burnout temperature in thermal degradation by approximately 8.1% under the use of hybrid nano-lube oils in a wide temperature range.