Experimental K-calibration for corner cracks under bending loading

The geometric correction function F in the expression for stress intensity factors has been determined experimentally by a compliance method for bar-shaped specimens of Al-7475-T761 (with a rectangular cross-section) cyclically loaded in bending at R = −1 and room temperature. The experimental method makes it possible to resolve the behaviour of the crack during the tensile and compressive portions of the loading cycle. This leads to the introduction of the initial notch size as an additional correction parameter. The development of the equilibrium shape of the crack during crack growth up to long surface distances has been investigated. These results have been used for the determination of F. A comparison with literature data is presented.     

The effect of the stress ratio on fatigue crack growth in a 3% NaCl solution

Corrosion fatigue crack growth tests have been carried out at various stress ratios for a low alloy steel SNCM 2 and type 304 stainless steel.

Measurements of the effective stress intensity factor range ratio U were performed to explain the effect of stress ratio R.

The corrosive environment decreased da/dN at R = 0.1, 0.4 and little affected da/dN at R = 0.9 for SNCM 2 and increased da/dN at all R ratios for SUS 304.

It was confirmed that there exists a threshold stress intensity factor ΔK_thCF in 3% NaCl solution for both materials tested.

The corrosive environment decreased ΔK_thCF for all conditions tested except at R = 0.1 and 0.4 for SNCM 2, where ΔK_thCF-values were nearly equal to ΔK_th-values in air. ΔK_thCF/ΔK_th was 0.6 at R = 0.9 for SNCM 2 and 0.8, 0.5 and 0.7 at R = 0.1, 0.7 and 0.9 for SUS 304, respectively.

It was shown that the complicated effect of stress ratios on crack growth for SNCM 2 can be explained using effective stress intensity factor ΔK_eff.     

Application of equations for incoherent internal reflections to HgCdTe/CdTe structures

The reflectance R and transmittance T of an epitaxial film of HgCdTe on CdTe were measured for a range of energies near the band edge. Equations for the incoherent addition of multiple internal reflections in multilayered media are derived and used to calculate the absorption coefficient and index of refraction from R and T. The exact calculation is compared with an approximation which simplifies the analysis of the data.     

Evaluation of optical properties of decorative coatings by spectroscopic ellipsometry

Some of the hard coatings (e.g. TiN, TiCN, TiCN, TiAlCN) occur in different colours. Using goniospectroscopy in the visible wavelength range colours are deduced by measuring the spectral reflectance factor R_λ. This factor depends on both the samples and the white reference as well as on the geometry of illumination and measurement. It is very difficult or even impossible to distinguish colours according to their origin and to relate them to bulk material composition, thin film coatings and interference layers by means of goniospectroscopy as unpolarized light is used. By using polarized light in spectroscopic ellipsometry these difficulties could be overcome as there is no reference to white and the geometric influences of illumination and measurement are negligible, but on the contrary the visual impression is not correctly described. From the measurement of the relative amplitude ratio tan Ψ and the relative phase shift cos Δ it is possible to calculate either the parallel and normal components R_p and R_s of the reflectivity R or the complex refractive index N = n + ik as well as the dielectric function = ₁ + i₂ which are strongly related to the electronic structure of the material used. It is shown that in this way it is possible to describe optical properties more physically, to separate interference effects and to detect changes in stoichiometry.     

Acoustic emission mechanisms during high-cycle fatigue

Acoustic emission was obtained during fatigue crack propagation in a D6 tool steel and a 1015 mild steel using two different load ratios, R. It was found that the slope of the acoustic emission count rate vs stress intensity factor was higher when R = 0.2 than when R = 0.4. These results are described in terms of several models of acoustic emission mechanisms available in the literature as well as by a proposed new model. It is concluded that the observed R dependence can be explained by the combination of the contribution of two different models. These two models are the new plastic yielding as well as the crack tip fracture processes.     

牦牛胫骨多级微孔参数化描述

经过若干世纪的选择进化,生物功能陶瓷牦牛胫骨的细观结构高度优化。通过SEM方法观测牦牛胫骨断面的微观形貌,发现多孔形态特征非常明显。对SEM图像进行数字化处理,实现具有较高灰度级分辨率的计算机视觉,借助MATLAB编程统计出孔的个数524,通过计算孔像素面积与骨断面的像素面积之比获得孔隙率0.063。分析获得牦牛胫骨的孔径随着水平级别的变化规律曲线,曲线趋向并收敛于0。获得孔径随个数的变化规律曲线,对其曲线进行多项式拟合获得了9阶函数表达式,描述了牦牛胫骨多级微孔分布的内在规律。     

Influence of R on effective stress range ratio and crack growth

The influence of stress ratio R and stress intensity range ΔK on crack closure and fatigue crack growth were studied. Crack closure and crack growth experiments were performed on 6063-T6 Al alloy. Crack closure stresses were measured using a surface-measurement technique with a COD gauge. The gauge was placed at different locations behind the crack tip, and it was found that the location of the gauge does not influence the closure load. The closure load was however found to be a function of R and ΔK. Fatigue crack growth rate is found to depend upon R, U and ΔK. A model for both U and da/dN has been developed.     

Mid-infrared (2.5 ≤ λ ≤ 12.5 μm) optical absorption enhancement of textured silicon surfaces coated with an antireflective thin film

High-performance solar cells and optical detection devices frequently incorporate microscopic surface texturing and antireflective (AR) thin films to reduce the reflection of incident radiation and, thus, enhance optical absorption. Using conventional electrochemical and single-crystal silicon micromachining techniques, porous silicon (PS) and textured surfaces composed of randomly spaced and sized pyramids (RSSPs) were fabricated and optically characterized over the mid-infrared (2.5 ≤ λ ≤ 12.5 μm) portion of the optical spectrum. The utility of a 1.53 ± 0.03 μm thick yttrium oxide (Y₂O₃) AR thin film was also investigated in an attempt to enhance optical absorption. The optical measurements were accomplished using a 21 ° incident illumination angle (measured with respect to the sample's normal) and a Bomem® total integrating sphere to quantify the total (specular and diffuse) reflectance (R). A highly-polished, uncoated, single-crystal silicon wafer was used as a reference surface (R_ave = 0.436 with R_σ = 0.033). The performance of the uncoated PS samples revealed R_ave = 0.205 with R_σ = 0.078, and the RSSP samples manifested R_ave = 0.090 with R_σ = 0.003. The AR coating significantly improved the performance of the reference and the RSSP textured surfaces: reference sample, (R_ave = 0.251 with R_σ = 0.040; RSSP samples, (R_ave = 0.024 with R_σ = 0.017). The AR coating did not improve the mid-infrared optical performance of the PS samples; however, the R characteristics for the 0.5 ≤ λ ≤ 2.5 μm portion of the optical spectrum were reduced by more than 50%.     

Stress evolution during isochronal annealing of Ni/Si system

Nickel films on Si(001) substrates were annealed in vacuum at a ramp rate of 5°C/min. The total force per unit width (F/W) in the film during isochronal annealing was determined using a laser scanning method for substrate curvature measurements. During heat treatment, several abrupt changes of F/W in the film were observed. A clear correlation between the evolution of F/W and the phase formation sequence was found. X-ray diffraction and sheet resistance measurements revealed that these changes of F/W coincide with the formation sequence of Ni₂Si, NiSi, and NiSi₂.     

电解质表面微结构对固体氧化物燃料电池的阴极极化电阻的影响

在固体氧化物燃料电池(SOFC)中, 电解质对阴极界面极化电阻(R_c)有着显著影响。通过测量以Sm_0.2Ce_0.8O_2-δ (SDC)为电解质、(La_0.85Sr_0.15)_0.9MnO_3-δ(LSM)和La_0.6Sr_0.4Fe_0.8Co_0.2O_3-δ(LSCF)为阴极的对称电池的交流阻抗谱, 研究SDC电解质表面微结构(晶粒大小和晶界长度)对R_c的影响。通过改变烧结温度和时间, 制备出具有不同晶粒尺寸和晶界密度的电解质。通过SEM得到微结构参数, 建立R_c与这些参数的联系。结果发现, 随着晶粒尺寸的减小、晶界密度的增加, R_c明显降低。此外, 对于LSM电极, 晶界密度的增加, 促进了阴极反应的氧离子传导。     

Fatigue damage in a unidirectional SiC/Al composite due to push-pull and zero-pull loading

Axial fatigue behaviour due to fully reversible and zero-tension cyclic loads on specimens cut from a 5 mm thick panel of a unidirectional SiC/A1 composite has been investigated at room temperature. The panel contained 40 vol.% SiC fibres (SCS-2), sandwiched between 32 layers of A1 6061 foils, which were bonded together by hot-pressing. The loading was always parallel to the fibres. Steady hysteresis loops were observed in the stress-strain plot after about 3 cycles of loading. A plot of S/N curves showed that at load ratio R = 0 the fatigue strength of the composite was about 3 times higher than that of the monolithic matrix metal. At R = −1, however, the fatigue strength of the composite was even lower than that of the matrix metal. At both R = 0 and R = −1, the composite suffered large modulus losses (about 15%) at cycles well before the final failure. At R = 0 the modulus loss involved fibre breakage and matrix cracks, which were transverse as well as parallel to the loading direction, while at R = −1 it involved delamination cracks and barrelling of outer layers. Fractography after the final failure at R = 0 showed secondary cracks and fibre pull-out.     

Porous CdS:CdO composite structure formed by screen printing and sintering of CdS in air

A porous CdS:CdO composite structure was formed by screen printing of CdS powder and sintering in air at high temperature. The X-ray diffraction analysis of CdS sintered at different temperatures revealed that a CdO formation in the CdS matrix is by the phase transformation of CdS, which may be stated as CdS–CdSO₄–CdO. The structure, composition and photosensitivity of this composite structure depend on the sintering temperature, sintering atmosphere and the flux to semiconductor (F/S) ratio. The results indicate that the screen printed CdS:CdO structure may be used as a photoconductor in solid state devices and as a photoelectrode in photo-electro-chemical energy conversion systems.     

A Modified S Chart for the Process Variance

Both the R and S charts are widely used in many manufacturing industries to monitor the process dispersion. The R chart is more popular among quality control practitioners especially when dealing with small sample sizes because of the simplicity of computing the range, R, from each sample. For larger sample sizes, the preferred choice is the S chart because it is slightly more effective than the R chart. The computation of the standard deviation, S, from each sample can now be made easily due to the availability of computers and scientific calculators. This article addresses the shortcomings of the conventional S chart and suggests a modified S chart to overcome these problems.     

New class of electrostatic energy analyzers with a cylindrical face-field

A new class of electrostatic energy analyzers based on a cylindrical face-field is presented. The focusing field used is a solution of a Laplace equation ²U(R,Z)=0 with boundary conditions U(R₁,Z)=U(R,0)=U(R,L)=0 and U(R₂,Z)=V and restricted by concentric cylindrical surfaces and two flat surfaces perpendicular to the axis of symmetry. Regimes of a second-order focusing were found for different types of sources, including a point-source, an extended surface source of large angular size and a flow parallel to the symmetry axis, while charged particles entered into the analyzer through a face-window arranged in a boundary electrode. Some results are described showing the capability of utilizing the prototype cylindrical face-field-analyzer for distant surface chemistry monitoring.     

Structural relaxation in poly(ethylene oxide)-salt solutions

We present the results of molecular dynamics simulations of PEO_xNaI polymer electrolytes, performed to study the influence of NaI on the dynamics of PEO. By calculating the mean square displacement of the CH₂ groups we observe that the Rouse-like motion, typical for short polymer chains, is slowed down under the influence of NaI. This slowing down is also observed in the self part of the intermediate scattering function F^s(k,t), as an increase in the relaxation time. By using the Gaussian approximation it is shown that the stretched exponential relaxation in the small k-limit of F^s(k,t) is a result of the Rouse-like motion of the polymers. We find that our results are in good agreement with neutron spin-echo experiments.     

Numerical investigation of conductance of porous media in high vacuum systems

In high vacuum systems or materials that have fine capillaries, the molecular transport can be characterized as being free-molecular flow regime. In this flow regime intermolecular interactions can be ignored and flow is determined entirely by molecule-surface collisions. The transport of gases and volatile compounds through porous media and filters with variety of geometries is of great interest in various industrial applications. Although the effect of porosity on gas flow in the most of the flow regimes has been explored, but there are a few investigations on gas transport in porous media and filers at free-molecular regime. In this investigation gas transport in porous media with various porosities and geometries is explored. Test Particle Monte-Carlo (TPMC) method is employed. The walls are assumed to be diffusive. The skeletal portion of the porous media (frame) is modelled by solid spheres. The developed numerical scheme is validated with non porous cases. The effect of porosity, sphere sizes of frame, porous geometry, gas type and temperature on the conductance is examined. The simulations are performed for a porous pipe and porous nozzle. Results demonstrate that porosity and filtration highly affects the conductance of pipe and nozzle and causes great pressure drop in high vacuum systems. The increase of sphere sizes at constant porosity causes conductance to grow. The gas type and temperature of gas affects the conductance of pipe and nozzle too.     

Numerical investigation of conductance of porous media in high vacuum systems

In high vacuum systems or materials that have fine capillaries, the molecular transport can be characterized as being free-molecular flow regime. In this flow regime intermolecular interactions can be ignored and flow is determined entirely by molecule–surface collisions. The transport of gases and volatile compounds through porous media and filters with variety of geometries is of great interest in various industrial applications. Although the effect of porosity on gas flow in the most of the flow regimes has been explored, but there are a few investigations on gas transport in porous media and filers at free-molecular regime. In this investigation gas transport in porous media with various porosities and geometries is explored. Test Particle Monte-Carlo (TPMC) method is employed. The walls are assumed to be diffusive. The skeletal portion of the porous media (frame) is modelled by solid spheres. The developed numerical scheme is validated with non porous cases. The effect of porosity, sphere sizes of frame, porous geometry, gas type and temperature on the conductance is examined. The simulations are performed for a porous pipe and porous nozzle. Results demonstrate that porosity and filtration highly affects the conductance of pipe and nozzle and causes great pressure drop in high vacuum systems. The increase of sphere sizes at constant porosity causes conductance to grow. The gas type and temperature of gas affects the conductance of pipe and nozzle too.     

The effect of load ratio on the threshold stresses for fatigue crack growth in medium carbon steels

A study of the effect of load ratio, R, on the low crack growth rates and the threshold stress conditions exhibited by five medium carbon steels has been conducted. It was found that decreased values of R retarded crack growth to an increasing degree as a defined threshold for crack growth was approached, such that this threshold showed a marked dependence on K max rather than ΔK. Comparisons of the five materials showed that the threshold increased as yield strength increased for a given R, but this effect could be normalised in terms of a constant value of the maximum plastic zone size at the crack tip.     

Evolution of microstructure and mechanical behavior of concretes utilized in marine environments

Concrete in marine environments is exposed to chemical mechanisms of deterioration, most of them involving chloride and sulfate ions. The principal motivation of this study is to try to minimize the expansive reactions between the aggressive ions and the cement matrix. However, the most effective protection will undoubtedly be the one that prevents the penetration of aggressive substances. The ingress of any aggressive substance in concrete is determined by concrete’s porous structure, especially the accessible connected porosity. This porosity is defined by the composition of the concrete and the chemical characteristics of the cement. The results clearly show that concrete that includes silica fume is significantly less porous and less permeable. In the rest of the mixtures studied, the porosity is higher, and the pore radius is the most decisive factor in defining the permeability.