Material characteristics of an ER fluid and its influence on damping forces of an ER damper Part II: damping forces

This paper presents damping forces of a cylindrical ER damper for a small-sized passenger car. The arabic gum-based ER fluid filled in the damper is subjected to temperature variation and the variation of the damping forces is evaluated. The measured damping forces are compared with those predicted from Bingham model obtained at various temperatures in Part I. The ER damper is continuously operated up to 500 000 cycles and the variations of the operating temperature and damping forces are tracked. Using the Bingham model obtained from mechanical endurance test of the ER fluid in Part I, the variation of the damping forces is predicted with respect to the number of the operating cycle. The predicted damping forces are then compared with the measured ones to demonstrate the effectiveness of the Bingham models distilled from various environmental conditions in Part I.     

Influence of hydrostatic pressure on the mechanical behavior and properties of unidirectional, laminated, graphite-fiber/epoxy-matrix thick composites

This paper reviews and gives new insight into earlier work by the author and his co-workers on the experimental investigation of the influence of superimposed hydrostatic pressure on the mechanical behavior and properties of the epoxy used for the matrix and unidirectionally laminated, graphite-fiber/ epoxy-matrix thick composites. The direction of the fibers was, respectively, 0°, 45° and 90° for the compressive test samples and 0°, 45° -45° and 90° for the shear samples.

Hydrostatic pressure induces very significant, often dramatic changes in the compressive and shear stress/ strain behavior of composites, and consequently in the elastic, yielding, deformation and fracture properties. The range of pressures covered for the compressive experiments was 1 bar to 4 kbar, and for the shear tests 1 bar to 6 kbar. The shear modulus (G) of the epoxy increased bilinearly with pressure, with the break, or the discontinuity point, occurring at 2 kbar. The compressive elastic modulus (E) and the shear modulus (G) of the composites increase in the same manner as for the epoxy. The break, which is located at 2 kbar, represents a pressure at which physical changes in the molecular motion of the matrix epoxy occur. That is, segmental motion of molecules between the cross-links is frozen in by 2 kbar pressure. This pressure is known as the secondary glass transition pressure of the epoxy at room temperature. Alternatively, the sub-zero secondary glass transition temperature of the epoxy is shifted to ambient temperature by 2 kbar pressure. The increase in the moduli may also be given a mechanical interpretation. The elastic or shear modulus of an isotropic, elastic material due to small compressive or shear deformations, respectively, superimposed on a finite volume deformation, which is caused by hydrostatic pressure, increases with pressure. Such an increase in E or G has been predicted using finite deformation theory of elasticity.

The normally brittle epoxy develops yielding when the superimposed hydrostatic pressure exceeds 2 kbar. The shear yield stress (1% off-set) of the epoxy increases linearly with pressure above 2 kbar. This kind of yielding behavior can be predicted by a pressure-dependent yield criterion. The compressive yield strength of the 45° and 90° composites increases bilinearly with pressure, and the shear yield strength of the 0°, 45° and 90° composites also increases bilinearly with pressure. This bilinear behavior is also due to the secondary glass transition pressure of the matrix epoxy, being located at 2 kbar. The fracture strength of the composites also increases with pressure linearly and the greatest increase occurs in the 45° composite in compression and in the −45° composite in shear. The fracture modes of the composites undergo changes with increasing hydrostatic pressure. For instance, the 0° composite undergoes a brittle-ductile transition under shear stress, while no such transition appears to set in under compressive stress. The fracture mode of the 45° composite changes from matrix failure at lower pressures to fiber failure at high pressures under shear stress.     

温度对电流变液体流变行为的影响

研究了温度对硅铝酸盐、沸石、高分子、钛酸钡悬浮相电流变液体（ERF）抗剪强度及表观粘度的影响规律,结果表明,温度的作用规律主要受外加电场强度和悬浮相数量的影响,与剪切速率的大小无明显关系。温度升高,在高的外加场强下,硅铝酸盐系ERF和沸石系ERF的抗剪强度有明显峰值出现;高分子材料悬浮相ERF也有峰值现象;BaTiO3悬浮相ERF的抗剪强度则单调下降。     

温度对电流变流体流变行为的影响

研究了温度对硅铝酸盐、沸石、高分子、钛酸钡悬浮相电流变流体(ERF)抗剪强度及表观粘度的影响规律,结果表明,温度的作用规律主要受外加电场强度和悬浮相数量的影响,与剪切速率的大小无明显关系。温度升高,在高的外加场强下,硅铝酸盐系ERF和沸石系ERF的抗剪强度有明显峰值出现;高分子材料悬浮相ERF也有峰值现象;BaTiO3悬浮相ERF的抗剪强度则单调下降。     

Investigation of the strength and cyclic fracture toughness of BT-16 titanium alloy subjected to thermal cycling simulating diffusion welding

The effect of the thermal cycle of diffusion welding on the service characteristics of BT-16 titanium alloy are considered. Using standard test equipment the mechanical properties of the alloy after thermal cycling to 860 or 960°C were studied at test temperatures of 960, 860, 300 and 20°C and the fatigue strength and fracture toughness were studied at room temperature.

It was shown that although specimens welded at 960°C displayed superior plastic properties, these were only partially effective in arresting propagating cracks. A greater fatigue crack resistance was seen with material welded at 860°C and it is this temperature which is recommended for diffusion bonding BT-16 titanium alloy components.     

Strain-hardening in the oscillatory shear deformation of a dedoped polyaniline electrorheological fluid

An electrorheological (ER) fluid, consisting of polydisperse dedoped polyaniline (PANI) particles, having irregular shapes, dispersed in silicone oil, was subjected to cyclic strain annealing treatments using oscillatory shear, under an external electric field. After each annealing period, the sample was subjected to a controlled-strain sweep, to determine the yield stress, and to erase the ER structure. During each annealing cycle, the storage modulus and the yield stress were observed to increase, and the loss modulus to decrease, each eventually approaching an asymptotic constant value. These observations in oscillatory shear complement our previous observations of a strain-hardening effect in a PANI/silicone oil ER fluid subjected to unidirectional creep tests.     

Sol-Gel Preparation of Graphite/TiO2 Composite Particles and Their Electrorheological Effect

Graphite/TiO2 composite particles were obtained by sol-gel technique in this paper. The structure and characteristic of the composite particles are analyzed by XRD, SEM and TG-DTA. The electrorheological properties of the ER fluid containing the particles were measured by a Couette-type rheometer under shear rates of 1～136 s-1 and AC electric fields of 0～3 kV/mm. The experimental results show that the leaking current density of the ER fluid is higher than that of pure titanium dioxide particles dispersed in damping oil. The shear yield stress of the ER fluid increases with increasing electric field and exhibits a typical Bingham flow behavior. The suspension demonstrates an excellent ER performance (τ/τ0=1200) compared with conventional ER fluids (τ/τ0 ≤500). The sedimentation of the ER fluid is improved obviously due to the coating effect of the particles.     

电场作用下电流变液的拉伸、压缩和剪切特性

对外加直流电场作用下电流变液的拉伸、压缩和剪切特性进行了实验研究。实验结果表明,在外电场作用下,电流变液的压缩强度比剪切强度约高一个数量级,压缩强度是拉伸强度的2~3倍。压缩弹性模量约与压缩过程中电场强度的三次方成正比。压缩应力与电流变液本身性能、外加电压大小和压缩应变都有密切关系。拉伸屈服应力为剪切屈服应力的拉伸屈服应力和剪切屈服应力的3~4倍,据此计算得到剪切屈服应变角度在15°~18.5°之间。     

Evaluation of the Heat Treatment Response of a Multiphase Hot-Rolled Steel Processed by Controlled Rolling and Accelerated Cooling

The tensile properties of hot-rolled multiphase steel after heat treatment were analyzed on a laboratory scale. Subcritical treatments applied to the hot-rolled strip revealed an increase in the yield stress and elongation with increasing temperature. Normalizing of the strip at 920°C notably improved the ductile response, while both the yield stress and the anisotropy of every property evaluated at 0°, 45°, and 90° to the rolling direction in the rolling plane were significantly reduced.     

Thermo‐mechanical methods for improving fatigue performance of wrought magnesium alloys

Wrought magnesium alloys AZ80 and ZK60 were extruded at 300 °C with extrusion ratios of ER = 12 and 44. Resulting microstructures, crystallographic textures and mechanical properties were investigated. Extruding led to profound reduction in grain size, which drastically improved yield stress, tensile elongation and HCF performance. Strength differentials in ZK60 after extruding at ER = 12 were more pronounced than after extruding at ER = 44, whereas no such effect of ER was observed in AZ80. Swaging after extruding further increased yield stress and endurance limit, while strength differential increased and ductility was lowered.     

Thermodynamic optimization of ejector actuated refrigerating cycles

A cold generation system featuring a Rankine cycle powered refrigeration cycle actuated by a supersonic ejector was theoretically investigated in view of the thermo-fluid-dynamic optimization of the working fluid characteristics.

The ejector model was validated against well established performance charts relating to water. A reference system was considered in which a Rankine cycle at moderate top temperature delivers its expansion power by means of an ideal turbine to an ideal compressor of a refrigeration cycle. Two main optimizing variables were ascertained: the fluid critical temperature and the complexity of the fluid molecule. The best performance of such reference cycle is around 80% of that of an ideal fully reversible, Carnot cycle based, system (COP of 2.0 for t_E,PC = 150 °C, t_E,RC = 5 °C, and t_C = 35 °C). As easily predictable the ejector compression introduces severe losses mainly due to the normal shock and the mixing of the motive and of the driven fluid. Overall COP for the above quoted temperatures decreases from 2.0 (reference cycle) to 0.4–0.7. The optimization of the working fluid showed that comparatively low critical temperatures are favoured and that a fluid complexity similar to that of CH₅N or CH₂Cl₂ gives the best performance. A detailed losses analysis explains this behaviour. In particular at low reduced temperatures the theoretical gain related to the better shape in the T–S plane of both the power and the refrigeration cycle is more than offset by the higher ejector losses due to the stronger normal shock needed to cope with an increased pressure ratio.

Notwithstanding an extensive fluid screening we did not succeed in finding a fluid that could be considered optimum from all points of view including ambient and safety issues. However, a number of traditional (non-zero ODP) chloro-fluoro-carbons and of new (zero ODP) refrigerants were found that yield, on the whole, a satisfactory performance.

Provided calculated COP will be confirmed by experimental testing, ejector powered refrigerators could compete with absorption systems in many applications.     

Calculation of Drag Force on an Object Settling in Gas-Solid Fluidized Beds

Gas-solid fluidized beds can be used for dry beneficiation of minerals. Estimation of the drag force on separated materials is important for the design and operation of separators. Previous research was based on the empirical correlation or on the hypothesis that fluidized beds behave as a Newtonian fluid. However, much experimental evidence showed that the hypothesis of Newtonian fluid was suspect. The drag forces on spheres passing through fluidized beds were calculated using the Bingham fluid model. The plastic viscosity and yield stress of a fluidized bed can be obtained by measurement of the terminal settling velocity of spheres. The relationship between drag coefficient (C D ), sphere size (d o ), settling velocity (u t ), bed bulk density ( ρ b ), plastic viscosity (µ), and yield stress ( τ 0 ) is expressed as C D = 24/ Re m (1 + 0.15 Re 0.687 m ), Re m = d o u t ρb /(µ + τ0 d o /3 u t The calculated results agree well with the experimental data.     

Life extension technology for steam pipe systems—I. Development of material properties

Material properties of A106B low-carbon steels were developed for life prediction analyses of steam pipes operated at elevated temperatures but in the sub-creep temperature range. Tensile, fracture toughness, fatigue crack growth rate and low-cycle fatigue properties were obtained on the piping steel at 24°C (75°F) and 288°C (550°F). The latter temperature corresponded to the highest operating temperature of nuclear plant steam piping. Increasing the test temperature from 24°C (75°F) to 288°C (550°F) decreased the yield strength and fracture toughness of the steel. Fatigue crack propagation rate properties at 24°C (75°F) and 288°C (550°F) were found to be comparable.

In the low-cycle fatigue tests, below a strain amplitude level of approximately 0.5%, cyclic softening was observed, while at higher strain levels, cyclic hardening was present. Based on the results of tensile and incremental-step fatigue testing, the strain-life curve was predicted. The predicted strain-life curve was found to be in agreement with the experimental result.

The fracture surfaces of fracture toughness specimens showed ductile fracture, while striations were observed on those of fatigue crack growth specimens. Fatigue striations were also observed on the fracture surfaces of low-cycle fatigue specimens. Fatigue initiation was associated with inclusions. It was shown that plastic straining in A106B steel could be detected by acoustic emission and by monitoring the eddy current response. These nondestructive evaluation techniques exhibit possibilities for in-situ monitoring of fatigue deformation.

While the development of material properties for the life prediction assessment of steam pipes is included in Part I of this paper, the establishment of a quantitative life prediction methodology and inspection criteria is contained in Part II. The developed life prediction methodology quantifies the effects of operating parameters on the remaining life of steam pipes using the material properties obtained in Part I.     

丙三醇/羧甲基淀粉/改性氧化钛复合颗粒的电流变性能

结合改进的水解法和溶胶-凝胶法,制备了丙三醇/羧甲基淀粉/改性纳米氧化钛复合颗粒,分析表明羧甲基淀粉和纳米氧化钛之间存在有相互作用;同时,介电测试表明此颗粒配制的电流变液的介电极化特性有很大改善.经流变学测试,此复合颗粒电流变液的动态剪切应力在颗粒质量分数为25%,剪切速率为100s-1,3kV/mm DC下为960Pa,剪切增幅比为35.同时,此电流变液的静态屈服应力在更高浓度,5kV/mm DC下可达到18kPa,比同等条件下改性氧化钛和CMS电流变液的静态屈服应力均有所提高.此外,这种电流变液还表现出较好的抗沉降性能.     

Rate sensitivity of mechanical properties and fracture characteristics of two-phase titanium alloys subjected to thermal cycles of diffusion welding

The effect of strain rate on mechanical properties and fracture characteristics of two dual-phase titanium alloys was studied using standard test equipment. The alloys had been subjected to various thermal cycles, simulating diffusion welding.

It is shown that diffusion welding is possible within the alloys β phase region without serious loss of service characteristics provided service temperature and rate of stress application are accounted for. A service temperature (which also depends on loading rate) of between 60 and 100°C is suggested for titanium articles — commensurable in size with that studied — to be diffusion welded in the β region.     

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.     

Effect of dynamic strain ageing on the deformation behavior of Incoloy alloy MA956

Incoloy alloy MA956 is an oxide dispersion hardened ferritic stainless steel produced by powder metallurgy. It is used as a candidate material for the high temperature components of gas turbines. This material underwent dynamic strain ageing at 300–400°C and strain rate of 1.2 × 10⁻³ s⁻¹. The following features of dynamic strain aging were observed: serrated flow at 300 and 400°C, a peak in the ultimate tensile strength normalized by the elastic modules versus temperature curve at 400°C, a plateau in the 0.2% offset yield stress-temperature curve at 300–400°C, a peak in the deformation rate-temperature curve at 300°C and the elongation-temperature plot showed a minimum at 400°C associated with shear fracture and with a minimum in the reduction in area-temperature plot. These features of dynamic strain ageing were discussed in the view of the recent models of dynamic strain ageing. The effect of dynamic strain ageing on the deformation and fracture behavior of this material was discussed.     

Adaptive composites incorporating shape memory alloy wires. Part 2: development of internal recovery stresses as a function of activation temperature

Shape memory alloy (SMA) wires have been integrated in composite laminates consisting of an epoxy resin reinforced by aramid fibres. In the first part of this series, a new methodology was proposed for the simultaneous measurement of stress and temperature in the reinforcing fibres during shape memory wire activation. Those tests were conducted at three activation temperatures (40, 60 and 100°C) and the internal compressive stress distribution was extracted for low wire volume fraction composites. It was observed that for higher wire volume fractions the high compressive recovery stresses generated during electrical resistive heating of the wires led to the geometric failure of the composite coupons. In this work, measurements are conducted on hybrid laminate coupons of dimensions that prevent geometric (global) specimen failure. The internal stress distribution is measured at relatively high activation temperatures of 80 and 100°C and a filtering technique based on fast Fourier transform (FFT) is employed to simulate the stress and temperature variability and to filter the associated experimental noise. The results show that the higher stresses appear in the middle of the mid-wire distance and that the values at 100°C activation are not significantly different than those at 80°C indicating the presence of an upper limit in the transmission of wire recovery stresses in these systems.     

Annealing effects on microstructure and mechanical properties of chromium oxide coatings

Reactive radio frequency magnetron sputter-deposited chromium oxide coatings were annealed at different temperatures and times. The influence of annealing temperature on the microstructure, surface morphology and mechanical properties was examined by X-ray diffraction, nanoindentation, pin-on-disc wear and scratch tests, respectively. X-ray results show that the chromium oxide sputtered at room temperature in low oxygen flux is primarily amorphous. Annealing below 400 °C did not cause much change, while annealing at higher temperature of 500 °C caused a significant change in microstructure and mechanical properties. Hardness increased from 12.3 GPa to 26 GPa, and the wearability improved with higher annealing temperature due to the formation of crystalline Cr₂O₃ phase, which occurs at 470 °C. Annealing time had little effect on mechanical properties and microstructure, although coating surface roughness increased with a longer annealing time. Coating adhesion was improved by annealing, due to residual stress relief and possible interfacial interdiffusion.     

Assessment of service exposed boiler tubes

Boiler tubes in power plants have finite life because of prolonged exposure to high temperature, stress and aggressive environment. Service-exposed platen superheater and reheater tubes (148,900 h) made of 2.25Cr-1 Mo steels in a 120 MW boiler of a thermal power plant were evaluated for remnant life. The investigation included hot tensile tests, hardness measurement, dimensional measurement, microscopy and creep tests. Experimentally determined yield and ultimate tensile strength, and estimated 10,000–100,000 h rupture strength in the temperature range 520–580 °C, exhibited a decreasing trend with increasing temperature. Microstructural study did not reveal any significant degradation in terms of creep cavities, cracks, graphitization, etc. Analysis of tensile and stress rupture data revealed that although there was degradation of the tubes due to prolonged service exposure in terms of the ultimate tensile strength values, stress rupture plots showed that the service exposed superheater and reheater tubes could remain in service for a length of more than 10 years at the operating hoop stress of 40 MPa/540 °C, provided no localised damage in the form of cracks or dents develop.