Heterodyne polariscope for sequential measurements of the complete optical parameters of a multiple-order wave plate

A new heterodyne polariscope for sequential measurements of the complete optical parameters of linearly birefringent materials is proposed. A multiple-order crystalline quartz quarter-wave plate used as a sample was tested in two sequential setups. In the first setup we used an electro-optic modulator to modulate the circular heterodyne polariscope and then applied a phase-locking technique to measure the principal axis angle precisely. In the second setup, removing the first quarter-wave plate, resulted in a linear heterodyne polariscope, and again we used the phase-locking technique to extract the apparent retardance. Furthermore, by tilting the sample and placing a material of known thickness into the second setup, we determined the order, thickness, and refractive indices (ne and no) of a multiple-order wave plate by using the new algorithm. The proposed method has average absolute errors of 0.2167 degrees and 0.15% with respect to the principal axis angle and the apparent retardance, respectively. The order, thickness, and refractive indices are also in good agreement with the known sample data. In contrast to the conventional measurement schemes that could not measure more than two parameters, the proposed heterodyne polariscope uniquely measures six parameters.     

Surface and bulk stress/strain measurements in composite laminates with a fibre-optic Raman probe

Fibre stress/strain measurements in unidirectional, as well as, multidirectional aramid/epoxy composites have been conducted with the use of a laser Raman microprobe. The composite was incrementally loaded in tension while Raman measurements were taken. Fibre-optic probes sandwiched between adjacent laminae were employed for channelling the laser excitation light to a specified location within the bulk of the composite. The direction of the fibre-optic was either perpendicular or parallel to the reinforcing fibres. For comparison purposes, the same fibre-optic probe was used to scan the surface of the laminates. The perpendicular configuration was found to reduce the tensile strength of the as-received composite coupon by 10% whereas the parallel second configuration had no effect. In the unidirectional coupons the stress or strain in the principal fibre direction could be measured prior to loading and at every increment of applied tensile load up to fracture. The take-up of fibre strain for both bulk and surface set of measurements was identical with that obtained from the attached electrical resistance strain gauges. In the case of multidirectional coupons the stress or strain in the principal direction could be measured within successive plies situated at angles θ to the loading direction. The results for the 0° plies were in good agreement with those obtained by conventional laminate analysis whereas small deviations from linearity were observed in the angle plies. The proposed methodology paves the way for simultaneous in-service stress/strain measurements on the reinforcing fibres situated on the surface or within the bulk of a composite laminate.     

The effects of the biaxial stretching of leather on fibre orientation and tensile modulus

Leather has been subjected to different degrees of equal biaxial strain (up to 20%) during drying and its tensile modulus has been measured when dry. The collagen fibre orientation distribution in the dried leather has been assessed using wide angle X-ray diffraction. It was found that drying under biaxial strain caused the tensile modulus to increase markedly (by up to 400% at 20% biaxial strains) but with a dependence on the angle of test axis in relation to the principal axes of biaxial strain. The fibre orientation distribution in planes parallel to the surface was affected less by biaxial strain than in planes perpendicular to the surface and it is concluded that the latter type of fibre reorientation is the main factor responsible for the observed increases in tensile modulus.     

Fourier description of the propagation and focusing of an extraordinary beam in a planar uniaxial medium

The scalar-angular-spectrum approach is used to examine a focused extraordinary wave propagating along an arbitrary direction in a homogeneous uniaxial planar medium, and the Fraunhofer diffraction formula is generalized for this case. The size of the focal spot is found to be inversely proportional to an effective index, depending on the refractive indices and the propagation direction. The validity of the paraxial model is checked by nonparaxial (but scalar) numerical calculations. They show that the paraxial formulas predict the spot size correctly, but if the beam propagates neither parallel with nor perpendicular to the optic axis, they do not reproduce the symmetries of the amplitude distribution in the focal line.     

光学非均匀纤维复合材料对径受压圆盘应力分布－光学关系研究

本文报道利用显微激光分析仪研究微透明实际纤维复合材料对径受压圆盘径向应力分布的新尝试。结果表明,从I-θ曲线中提取出来的傅里叶变换四次基波的模A_‖4等于均质双折射材料在暗场下的等色线亮度的1/2.当用传统方法无法再获得实际纤维复合材料的条纹图象时,仍然可以利用显微激光分析仪得到A_‖4,且借助于A_‖4,可通过作图法求出对径受压圆盘的径向应力分布,所得结果与H.Okabo的理论计算相吻合。实验表明,采用上边方法可对透光度低达万分之六(0.06%)的实际纤维复合材料进行测试。      

Sinusoidal Microbending Loss of Orthogonal Polarized Modes in a Single-mode Step-index Fibre

Sinusoidal microbending losses of the HE₁₁ orthogonal polarized modes in a single-mode step-index fibre are studied and compared. The loss of the mode with polarization in a plane perpendicular to the plane of microbending is found to be higher than the loss of the orthogonal polarized mode with polarization in a plane parallel to the plane of microbending. The microbending loss difference of these two orthogonal polarized modes is more significant around the perpendicular directions with respect to the fibre axis than that in nearly parallel directions corresponding to different length periods of sinusoidal microbending.     

Off-axis fatigue cracking behaviour in notched fibre metal laminates

The off‐axis fatigue cracking behaviour of notched fibre metal laminates under constant amplitude loading conditions was investigated experimentally and numerically. It was found that the off‐axis fatigue crack initiation life decreased as the off‐axis angles increased. This indicated that the off‐axis laminates raised the applied stress level in the aluminium (Al) layer and subsequently resulted in earlier cracking in the Al layer. The off‐axis fatigue crack initiation lives of notched fibre metal laminates were predicted using lamination theory and an energy‐based critical plane fatigue damage analysis from the literature. After a crack initiated in the Al layer, it was observed that the crack path angles of the off‐axis specimens were neither perpendicular to the fibre nor to the loading direction. A finite‐element model was established for predicting the crack path angles.     

Structural characteristics and optical properties of vestan (polyester) fibres

Refractive indices and birefringence for skin and core changes with strain, produced by different stresses in undrawn vestan fibres, were measured interferometrically. Applications were carried out using multiple-beam Fizeau fringes in transmission to determine the Cauchy's constants and dispersive coefficient for the fibre layers. The resulting data were used to calculate the polarizability per unit volume for each layer. The scanning electron microscope (SEM), optical microscope and He-Ne laser beam were applied to estimate the geometrical parameters of the fibre cross-section. A comparative study between the three methods used for measuring the fibre diameter was made. The effect of temperature on the refractive index and birefringence for each fibre layer has been also investigated. The relationship between temperature and birefringence of the fibres was studied and the thermal coefficient of the refractive index was determined. An empirical formula is suggested to relate temperature and birefringence. Illustrations are given using microinterferograms.     

光学非均匀纤维复合材料对径受压圆盘应力分布－光学关系研究

本文报道利用显微激光分析仪研究微透明实际纤维复合材料对径受压圆盘径向应力分布的新尝试。结果表明,从I-θ曲线中提取出来的傅里叶变换四次基波的模A_‖4等于均质双折射材料在暗场下的等色线亮度的1/2.当用传统方法无法再获得实际纤维复合材料的条纹图象时,仍然可以利用显微激光分析仪得到A_‖4,且借助于A_‖4,可通过作图法求出对径受压圆盘的径向应力分布,所得结果与H.Okabo的理论计算相吻合。实验表明,采用上边方法可对透光度低达万分之六(0.06%)的实际纤维复合材料进行测试。     

Angular dependence of the reflectance from an isotropic polydomain medium: the influence of absorption

The angular dependence of the reflectance from an absorbing randomly oriented polydomain medium consisting of domains either small or large compared with the wavelength is investigated. Besides the two conventional cases, where the refractive index of the incidence medium is either smaller or larger than the averaged index of refraction (small-domain limit) or every principal index of refraction of the domains (large-domain limit), we also discuss a third principal case, which exists only in the large-domain limit. In this third case, only one of the principal indices of refraction is larger than that of the incidence medium, while the averaged index of refraction is smaller. Thus, in contrast to the small-domain limit, total reflection is completely suppressed even in the absence of absorption. A characteristic property of such a polydomain medium is its ability to considerably depolarize linear polarized light in spite of being optically isotropic. Additionally, the parallel polarized reflectance Rp can exceed the perpendicular polarized reflectance Rs over certain angle of incidence ranges. Absorption decreases these domain-size-dependent properties, even under the assumption of constant anisotropy. Nevertheless, for materials with low absorption indices, these effects can affect the optical properties significantly.     

X-ray measurements and the structure of polyacrylonitrile- and pitch-based carbon fibres

X-ray measurements were carried out on polyacrylonitrile- and pitch-based carbon fibres. The crystallites, disordered regions and microvoids in these carbon fibres were evaluated quantitatively by applying the methods previously proposed by the present authors. The structural parameters evaluated are the 1 1 plane spacing of the carbon layer, the average, standard deviation and asymmetry of the distribution of interlayer spacing, the stacking regularity parallel to the layer plane, the layer extents parallel and perpendicular to the fibre axis, the stacking height, the crystallite orientation, the volume fractions of crystallites, disordered regions and microvoids, the variation of the electron density in a microvoid, and the size parameters of the void cross-section perpendicular to the fibre axis, such as the area, radius of gyration, chord length and thickness. The mutual relationships of these structural parameters are presented, and parameters sensitive to the nature of the starting materials are pointed out.     

Study of the X-ray optical anisotropy of materials via X-ray interferometry

A new method for studying the X-ray optical anisotropy of materials which is based on the X-ray interferometric technique of measurement is described. Unlike the existing methods, in which the sample under study is placed in the path of one of the interfering beams, the test samples in the proposed method are placed directly in the path of two interfering beams; this eliminates the effect of other factors on the displacement of the interference moirefringes. In this method, moirefringes simultaneously occur during the same exposure both in the absence and in the presence of samples with different orientations of the optical axes. A relative displacement of moirefringes is observed in three different columns of the same beam, which makes it possible not only to simultaneously observe and immediately identify the presence of X-ray optical anisotropy but also to measure the values of refractive indexes n _o and n _e for a given sample (n _o is the refractive index for an emission with the polarization perpendicular to the principal cross section, and n _e is the refractive index for a beam with the polarization in the plane of the principal section and parallel to the optical axis). The new method is used to record the X-ray optical anisotropy of a cellophane film and to measure the values of refractive indexes n _o and n _e for cellophane. It is found that the cellophane film is an X-ray optically positive anisotropic medium.     

Serial bideposition of anisotropic thin films with enhanced linear birefringence

We describe a serial bideposition technique in which a tilted substrate is rotated stepwise by half a turn about a normal axis during the evaporation of a metal oxide from a single electron-beam source. Coatings formed by the new method develop a columnar nanostructure that is perpendicular to the substrate and has greatest width or bunching perpendicular to the common deposition plane. With appropriate choice of deposition parameters, the method produces biaxial films with large birefringence, principal axes aligned parallel and perpendicular to the substrate, and improved uniformity. Measured phase retardances for light incident normally on the films are double the corresponding values for tilted-columnar films.     

Detection and measurement of very small interfacial gaps in GRP

In situ detection and measurement of gaps of molecular dimensions between fibre and matrix materials in epoxy and polyester resin composites can be achieved by examination of the manner in which light is transmitted in a direction transverse to the fibre axis. Using the phenomenon of optical tunnelling following frustrated total internal reflection, it has been possible to detect and measure gap widths down to approximately 5 nm. It has been demonstrated that the presence of absorbed water gives rise to a small (≈0·3%) permanent increase in refractive index of the matrix resin.     

深埋隧道轴线合理布置与衬砌结构稳定性研究

深埋隧道常处于构造应力状态,且水平应力分布并不均匀。采用数值模拟和模型试验方法,研究自重应力场、水平大主应力与洞线平行和垂直、不同围岩级别隧道轴线选择对围岩和衬砌结构稳定性影响。水平大主应力与洞轴平行时,围岩最不利位置发生在边墙;水平大主应力与洞轴垂直时,围岩最不利出现在拱顶。水平地应力使得二次衬砌受力特征发生很大改变,自重应力条件下,衬砌首先发生裂缝部位为拱顶,发生压弯破坏,建议加强衬砌拱顶内侧纵向配筋,提高正截面抗拉能力。当大主水平应力与轴线垂直时,关键控制部位为墙脚,出现压剪破坏,应增设墙脚箍筋,提高抗剪承载力。规范提出隧道轴线宜与水平大主应力方向一致原则,是有适用条件的。研究成果可为深埋隧道设计、施工提供参考。     

The thermal conductivity of Kevlar fibre-reinforced composites

The thermal conductivities of a series of blocks consisting of Shell DX210/BF₃400 resin reinforced with Kevlar 49 fibre are reported in the approximate temperature range 180–270 K. The results are used to calculate the thermal conductivities of the fibres in directions parallel and perpendicular to their length. Varying the angle between the principal fibre directions of bidirectional laminates produced in-plane results that varied in a manner which was quantitatively consistent with expectation. The out-of-plane results proved to be independent of fibre orientation, as expected. In-plane and out-of-plane results for a Kevlar 49 fabric reinforced laminate proved to be essentially similar to results for a laminate reinforced with unwoven fibres of the same type, arranged in a 90° cross-plied disposition at the same fibre volume density.     

Propagation of stress corrosion cracks in aligned glass fibre composite materials

A modified fracture toughness test has been used to measure the growth of cracks in unidirectional glass fibre/polyester resin composite materials in the presence of a dilute hydrochloric acid. Crack growth rates perpendicular to the fibre axis have been measured over a range of stress intensities. Scanning electron microscope studies of the fracture surfaces have shown that the micromechanisms of nucleation and propagation are dependent on stress intensity. The use of crack growth data to predict component lifetimes and the existence of inherent flaws in the material are discussed.     

Magnetooptical study of the near-surface micromagnetic structure and local magnetic properties of NiFe/Cu nanocomposite microwires

The local near-surface magnetic properties and micromagnetic structure of NiFe/Cu nanocomposite microwires have been studied using the scanning Kerr microscopy technique. The local magnetization curves and the profiles of magnetization components (both parallel and perpendicular to the magnetic field H applied parallel to the microwire sample length) were measured using a 1-μm-diameter probing light beam that was scanned along the sample. The magnetization component parallel to H in all samples has the same sign, while the component perpendicular to the field changes sign. The observed magnetization distributions are indicative of the presence of circular near-surface domains with a ±70°–80° orientation of magnetization in the neighboring domains relative to the sample axis. It is shown that the dominating mechanism of magnetization reversal in the near-surface regions of nanocomposite microwires is via the rotation of magnetization in the circular domains.     

Multiple-scattering effects on infrared scattering measurements used to characterize droplet size and volume fraction distributions in diesel sprays

We quantify the maximum error due to multiple-scattering effects for an infrared scattering droplet izing technique. Errors in Sauter mean diameters (SMDs) and liquid volume fractions were estimated lased on experimentally determined polarization properties of the scattered light. Light that is multiply scattered from spherical particles becomes randomly polarized, whereas singly scattered light from a spherical particle contains no cross-polarization scattering component. Therefore measurement of the cross-polarization component (in this case parallel) of the scattering signal is a measure of the multiply scattered light. A ratio of parallel to perpendicular polarized scattered light was experimentally determined and used to calculate an error due to multiple scattering. The infrared scattering measurements and polarization measurements used to quantify the multiple-scattering errors were applied to a typical diesel spray that was injected into three different background conditions: a room ambient condition; a room-temperature, high-pressure condition; and a combusting condition. Droplet SMD, liquid volume fraction, and multiple-scattering errors were determined for a number of locations within the spray; results indicate that the combusting case is negligibly affected by multiple scattering. However, the room ambient case exhibited notable errors due to multiple scattering near the centerline of the spray, and the high-pressure case demonstrated susceptibility to multiple scattering throughout all regions investigated. It is important to note, however, that multiple-scattering errors in many cases translate into relatively small effects on the reported droplet sizes.     

Stress concentrations arising from a slot in a plate under biaxial stress

The stress distributions around a straight blunt slot under biaxial stress are found by adapting the classical solution to the stress concentration around an elliptical hole in an infinite plate under uniaxial stress. Firstly, the slot was converted into an equivalent thin ellipse of large major:minor axis ratio with its major equal to the length of the slot. The principle of superposition was then employed to find the stress state on an axis of symmetry at near and far locations from the notch tip. Stress states were found for any ratio of remotely applied stresses in directions perpendicular and parallel to the notch axis. The solutions were verified experimentally for specific tensile stress ratios by bonding strain gauges to an aluminium plate under hydraulic pressure.