Fracture mechanics and optimization — a useful tool for fibre-reinforced composite design

This paper will demonstrate the application of fracture mechanics and optimization techniques for the optimum design of fibre-reinforced composite laminates (FRC). First, a boundary-value problem of a cracked composite laminate is solved within the framework of linear elastic fracture mechanics (LEFM). The solution relates the stress intensity factor at a crack tip and the crack-induced interfacial stresses to the laminate configuration. These results are then used in two types of the optimum design of fibre-reinforced composite laminates. In the first type of optimum design, namely a crack-insensitive design of the laminate, the crack driving force and interfacial principal tensile stress are both minimized by using single- and multicriterion optimization techniques. The second type of optimum design involves in situ strength design of multidirectional angle-ply laminates. In this case, a set of in situ strength parameters are proposed based on theoretical analysis and experimental observations. This optimization problem is a min {max} one and non-differentiable. A proper treatment of the non-differentiability is introduced and the min {max} optimization problem is converted into a differentiable single-criterion one using the bound-formulation technique. All the optimization problems are solved by non-linear mathematical programming. The results show that optimization can greatly enhance the load carrying capacity of the laminates.     

Grid indentation analysis of composite microstructure and mechanics: Principles and validation

Several composites comprise material phases that cannot be recapitulated ex situ, including calcium silicate hydrates in cementitous materials, hydroxyapatite in bone, and clay agglomerates in geomaterials. This requirement for in situ synthesis and characterization of chemically complex phases obviates conventional mechanical testing of large specimens representative of these material components. Current advances in experimental micro and nanomechanics have afforded new opportunities to explore and understand the effect of thermochemical environments on the microstructural and mechanical characteristics of naturally occurring material composites. Here, we propose a straightforward application of instrumented indentation to extract the in situ elastic properties of individual components and to image the connectivity among these phases in composites. This approach relies on a large array of nano to microscale contact experiments and the statistical analysis of the resulting data. Provided that the maximum indentation depth is chosen carefully, this method has the potential of extracting elastic properties of the indented phase which are minimally affected by the surrounding medium. An estimate of the limiting indentation depth is provided by asssuming a layered, thin film geometry. The proposed methodology is tested on a “model” composite material, a titanium-titanium monoboride (Ti–TiB) of various volumetric proportions. The elastic properties, volume fractions, and morphological arrangement of the two phases are recovered. These results demonstrate the information required for any micromechanical model that would predict composition-based mechanical performance of a given composite material.     

Growth of III–V semiconductors by molecular beam epitaxy and their properties

Advances in solid state device technology in the sixties established III–V materials as a new class of semiconductors for high speed microwave and highly efficient optical devices. Molecular beam epitaxy (MBE) is an extremely versatile thin film technique which can produce single-crystal layers with atomic dimensional controls and thus permit the preparation of novel structures and devices tailored to meet specific needs. Important factors to achieve high quality MBE growth such as in situ analysis, substrate preparations, growth conditions and layer properties are discussed.     

Synthesis and mechanical properties of niobium aluminide-based composites

Niobium aluminide-based composites reinforced with in situ and externally added Al₂O₃ and TiC particulates were fabricated by hot-pressed sintering at 1400 °C. In particular, Nb₂Al–Al₂O₃–TiC in situ composites were successfully obtained from the raw powder mixtures of Nb60Al40 (in at.%)–TiO₂20C8 (in wt.%) by means of this process. The influences of ceramic particulates on the microstructures, flexural strength and fracture toughness were examined. The experimental results indicate that the presence of ceramic particulates yielded a remarkable improvement in both the strength and fracture toughness in comparison with previous results for monolithic niobium aluminide compounds.     

PECVD沉积和原位退火时间对h-BN薄膜组成及光学带隙的影响

采用射频等离子增强化学气相沉积设备, 以高纯N₂和B₂H₆为气源, 制备了系列h-BN薄膜, 得到适合生长h-BN薄膜的最佳工艺条件。在此条件下, 研究了不同沉积时间和退火时间对薄膜组成和光学带隙的影响。采用傅立叶变换红外光谱仪、紫外可见光分光光度计和场发射扫描电子显微镜对样品进行了表征。实验结果表明: 在衬底温度、射频功率和气源流量比率一定的条件下, 沉积时间对h-BN薄膜成膜质量和光学带隙都有较大影响, 且光学带隙与膜厚呈指数关系变化。700℃原位退火不同时间对h-BN薄膜的结晶质量有所影响, 而物相和光学带隙基本没有改变。     

Thermal and mechanical load induced damage behaviour of a low alloy steel: Mechanisms and modelling

A series of experiments, including macroscopic damage measurement and in situ microscopic observation at room temperature and tensile tests at eight different temperatures ranging from 20 to 900°C, is carried out. Mechanical load induced ductile damage evolution law and micromechanisms are presented, where damage evolution law is measured through a new a.c. potential system and the micromechanisms of damage and fracture are observed through an in situ technique in conjunction with a scanning electron microscope (SEM) equipped with a tensile platform. A continuum damage mechanics (CDM) model for ductile fracture proposed by Wang [Engng Fracture Mech. 42, 177–183 (1992)] is employed to model and to analyse the evolution law of damage in the steel. Comparison of experimental and modelling results is presented and good agreement is found. The effect of stress triaxiality on damage evolution is also discussed in the framework of CDM. The effect of temperature rise on tensile properties including Young's modulus, yield and ultimate tensile strength and ductility (elongation and reduction in area), is also reported.     

CuGaSe2 thin films for photovoltaic applications

Of the I-III-VI₂ group chalcopyrites, CuInSe₂ has already proved its suitability for thin film solar cells owing to its excellent optical and transport properties. CuGaSe₂ is expected to exhibit comparable properties from this point of view. With its band gap of 1.7 eV it is a candidate for use in photovoltaic tandem systems.

The preparation of CuGaSe₂ thin films by means of the vacuum evaporation of the constituent elements (four-temperature method) is described. The structural, electrical and optical properties of these films were investigated. Secondary electron microscopy, energy-dispersive X-ray analysis, X-ray diffraction examination and measurements of the optical transmission, resistivity and thermoelectric power were used to determine the film properties relative to the preparation parameters and stoichiometry. The growth conditions were optimized for solar cell applications. Heterojunctions were prepared by the in situ evaporation of Zn_xCd_1−xS onto the CuGaSe₂ films. The characteristic data of the cells are a short-circuit current of 6 mA and an open-circuit voltage of 620 mV at an illumination at air mass 1.5 on an area of 1 cm².     

In Situ Cross-Calibration of In-Fiber Bragg Grating and Electrical Resistance Strain Gauges for Structural Monitoring Using an Extensometer

This study addresses the direct calibration of optical fiber strain sensors used for structural monitoring and is carried out in situ. The behavior of fiber-Bragg-grating-based sensor systems when attached to metal bars, in a manner representative of their use as reinforcement bars in structures, was examined and their response calibrated. To ensure the validity of the measurements, this was done using an extensometer with a further calibration against the response of electrical resistance strain gauges, often conventionally used, for comparison. The results show a repeatable calibration generating a suitable geometric factor of extension to strain for these sensors, to enable accurate strain data to be obtained when the fiber-optic sensor system is in use in structural monitoring applications.     

In situ transmission electron microscopy observation of the appearance and disappearance of channels in Al-Mn thin films

In this paper the observation of the appearance and disappearance of channels in Al-Mn thin films during a heating treatment is reported. This observation was made in situ in a transmission electron microscope using a heating holder. This is the first time that a report on such a phenomenon has been made.     

Efficacy of Norethindrone Acetate and Norgestomet Implants in Suppressing Estrus in Female Beef Cattle

Beef females with corpora lutea on day 12 of the estrus cycle were implanted with 11.5 mg norethindrone acetate implants (7 implanted and 7 nonimplanted controls) at the same time they were administered a luteolytic dose of prostaglandin F_2aL. Although the implants released 340 to 740 μmlg of norethindrone acetate daily, the implants did not suppress estrus. Norgestomet implants were inserted into beef females 5 days after estrus and implants were left in situ for 16 days. The numbers of cows in the norgestomet study were 27 nonimplanted, 19 with 6-mg implants, and 21 with 8-mg implants. Control cows were not detected in estrus until day 17 of the estrous cycle (12 days after the time of implantation) and 10 of the 27 cows (37%) were detected in estrus over a 5-day period beginning on day 17 (12 to 16 days after the time of implantation). None of the cows implanted with 8-mg implants were detected in estrus while the implants were in situ. However, 3 of the cows with 6-mg norgestomet implants were detected in estrus 13 to 16 days post-implantation. Based on implant secretion data, cows with 6-mg implants began displaying estrus when the implants released less than 138 pg of norgestomet daily. Eight-milligram implants released 137 pg of norgestomet or greater per day during the entire 16-day implantation period.     

Coasting point optimisation for mass rail transit lines using artificial neural networks and genetic algorithms

Energy consumption of a rail transit system depends on many parameters. One of the most effective methods of reducing energy consumption in a rail transit system is optimising the speed profile of the trains along the route. A new efficient method will be presented for the optimisation of the coasting points for trains in a global manner. The proposed approach includes realistic system modelling using multi-train, multi-line simulation software and application of artificial neural networks (ANN) and genetic algorithms (GA). The simulation software used can model regenerative braking and train performance at low voltages. Using ANN and GA together, optimal coasting points for long line sections covering five stations and two lines are achieved. Simulation software is used for creating training and test data for the ANN. These data are used for training of the ANN. Trained ANNs are then used for estimating energy consumption and travel time for new sets of coasting points. Finally, the outputs of the ANN are optimised to find optimal train coasting points. For this purpose, a fitness function with target travel time, energy consumption and weighting factors is proposed. An interesting observation is that the use of ANN increases the speed of optimisation. The proposed method is used for optimising coasting points for minimum energy consumption for a given travel time on the first 5 km section of Istanbul Aksaray-Airport metro line, where trains operate every 150 s. The section covers five passenger stations, which means four coasting points for each line. It has been demonstrated that an eight input ANNs can be trained with acceptable error margins for such a system.     

In situ fabrication of high Tc Y---Ba---Cu---O thin film by resistive evaporation

We have recently used a simple conventional oil-pumped vacuum system equipped with a resistively heated boat for in situ fabrication of Y---Ba---Cu---O high T_c superconductivity phase thin films at a relatively low substrate temperature. A well-ground mixture of yttrium, BaF₂ and copper, weighed in the atomic proportion to yield a stoichiometric YBa₂Cu₃O_7−δ film, was evaporated from a resistively heated boat onto MgO substrates. The substrate temperature was then raised to about 700 °C and the oxygen partial pressure was raised to about 8 Pa for 30 min. The oxygen was injected through a nozzle placed close to the substrate surface. The resulting films, with no further heat treatment, were characterized by a four-probe d.c. method and zero resistance was detected at 85 K.     

Ellipsometry study of poly(o-methoxyaniline) thin films

Spin-coated poly(o-methoxyaniline) (POMA) thin films on various substrates were investigated using spectroscopic ellipsometry (SE) in the 1.5–4.5 eV photon energy range. Spin-coating process parameters are reported (spin speed and concentration). Substrates with higher surface energy were found to increase polymer film thickness and decrease roughness. An optical model was developed using SE data along with complementary data from atomic force microscopy and UV–vis spectroscopy to obtain optical properties—refractive index n and extinction coefficient k for POMA. The model includes Lorentz oscillators for the POMA film and a Bruggeman effective medium approximation for roughness. In-plane film optical anisotropy was not observed, but a small out of plane anisotropy was detected for the polymer.     

Physics and diagnostics of laser ablation plume propagation for high-Tc superconductor film growth

The formation, composition and propagation of laser-produced plasmas used for pulsed laser deposition (PLD) of Y₁Ba₂Cu₃O_7−x have been studied under film growth conditions. Four complementary spatially and temporally resolved in situ diagnostic techniques are applied to characterize the expansion of the laser plume into both vacuum and ambient gases: optical emission and absorption spectroscopy, fast ion probe measurements, and fast photography with a gated, image-intensified charge-coupled detector-array (ICCD) camera system. Transient optical absorption spectroscopy reveals large densities of ground state atoms, ions, and molecules in the plume as well as a slower component to the plume transport than is indicated by the plasma fluorescence and ion current.

Ablation into background gases results in scattering and attenuation of the laser plume. The exponential attenuation of the positive ion flux transmitted through 50–300 mTorr background oxygen is measured and used to define an overall ion-oxygen reaction cross-section σ_i−O2 = 2.3 × 10⁻¹⁶ cm² under the described film growth conditions.

The slowing of the laser plasma and formation of shock structures due to collisions with the ambient gas are described using ion probe measurements and ICCD photographic comparisons of expansion into vacuum and background oxygen. At the pressures used for PLD, distance-time R−t plots derived from the photographs and ion probe waveforms indicate that the higher pressure plume initially expands through the ambient gas in accordance with a drag model (where R = x_f[1 − exp( − βt)]), experiencing little slowing until a visible shock structure forms. Following a transition period, in which the plume appears to have two components, a single-component shock structure propagates in better agreement with a shock, or blast wave (R = ξ₀(E/₀)^1/5t^2/5) model.     

Enhanced photoluminescence from polycrystalline ZnO films resulting from oxygen processing

We report studies of photoluminescence from polycrystalline ZnO films deposited on sapphire as a function of the in situ oxygen pressure during growth and ex situ annealing. The ultraviolet photoluminescence was observed to increase by more than two orders of magnitude as a result of the annealing treatment. Enhanced cathodoluminescence was observed from the same films. The role of oxygen defects is discussed.     

Photoinduced Electron Transfer to Fulleropyrrolidinium Salts Studied by EPR

The photoinduced formation of fullerene anion radicals from mono- and bis-N-methylfulleropyrrolidinium salts in the presence of TiO₂ or triethylamine as electron donating substrates was investigated using in situ EPR spectroscopy in various solvent mixtures. The photoreduction yield and stability of anion radicals generated are affected by the solvent properties, such as relative permittivity and hydrogen bond donation ability.     

氧缺陷Bi2WO6-x可见光催化剂的制备和性能

采用乙二醇溶剂热法原位制备氧缺陷Bi₂WO_6-x催化剂, 利用XRD、SEM、N₂吸附-脱附、XPS、ESR、UV-Vis DRS、PL及电化学方法对样品的理化性能进行了表征, 考察了样品在可见光下(λ > 400 nm)对气相苯的光催化降解性能。结果表明: 乙二醇溶剂热法制备的催化剂具有较大比表面积, 形成了Bi-O_v和W-O_v氧缺陷中心; 缺陷的生成改变了催化剂的能带结构, 缩小其光响应带隙宽度, 并有效抑制了光生电子-空穴对的复合, 催化剂的活性增强。Bi₂WO_6-x降解苯的转化率和矿化率分别为52.5%和80.6%, 是Bi₂WO₆的1.72倍和1.84倍。     

Effect of soil organic matter (SOM) and soil texture on the fatality of indigenous microorganisms in intergrated ozonation and biodegradation

In situ ozonation has been proposed as a method to remediate soils contaminated with organic pollutants. Soil column experiments were performed on eight different soils in order to investigate the effects of soil properties, such as soil organic matter (SOM) and soil texture on the survival and regrowth of indigenous microorganisms after in situ ozonation. Indigenous microorganisms were found to be very sensitive to ozone in the soil column experiments. The microbial fatality revealed a linear relationship with the SOM content in the range of 1.72–2.42% of SOM content, whereas water content was poorly correlated. Four weeks of incubation of ozone-treated soil samples allowed for the regrowth of indigenous microorganisms with inverse relation to ozonation time. The regrowth was also significantly influenced by the SOM content in the same soil texture. Oxidation and removal rate of hexadecane was affected by particle size distribution. Especially, sand exhibited the highest oxidation rate of hexadecane, which resulted from having the lowest SOM content, water content, and surface area with respect to the other samples. The soil samples ozonated for 90–180 min were determined to exhibit the lowest concentration of hexadecane, with the exception of sand, after 4 weeks of incubation. This study provided insight into the influence of SOM and soil texture on indigenous microbial potential to degrade hexadecane in integrated ozonation and biodegradation.     

Ellipsometric investigation of quadratic electroptic and electrostrictive effects in anodic tungsten oxide films

Anodic oxide films on tungsten were studied in situ during formation using a computer-controlled ellipsometer. They were found to be optically homogeneous, isotropic and non-absorbing with no field applied but to become anisotropic with a field applied. The ordinary and extraordinary indexes decreased, and the thickness increased quadratically with the applied field.