Vibrations of bonded beams with a single lap adhesive joint

The natural frequencies and loss factors of the coupled longitudinal and flexural vibrations of a system consisting of a pair of parallel and identical elastic cantilevers which are lap-jointed by viscoelastic material over a length a_c from their free ends have been investigated. A complete set of equations of motion and boundary conditions governing the vibration of the system are derived. The solution of these equations, subject to satisfying the boundary conditions, yields the desired natural frequencies and associated composite loss factors. The numerical results have been compared with those from two other approximate methods.     

The transmission of Lamb waves across adhesively bonded lap joints

The transmission of Lamb waves across adhesively bonded lap joints is investigated using finite element analysis. The studies consider three modes for excitation and reception, s0, a0, and a1, applied to lap joints consisting of parallel aluminum sheets bonded with an epoxy adhesive. Transmission coefficient results for a two-dimensional range of bond thicknesses and bond overlap lengths are presented for all three modes. The transmission coefficients are calculated from the spectra of the received and transmitted signals using an approach which is insensitive to the presence of multimode signals and reverberated signals, and which approximates to a power transmission coefficient. Detailed analysis is then performed for one of the modes in order to investigate the nature of the mode conversion in the overlap region of the joint. It is found that the relative amplitudes of the different modes which propagate in the overlap region can be estimated reliably and simply from the properties of the incident wave mode. As well as demonstrating the physics of the mode conversion behavior, the study provides a basis for the selection of modes for nondestructive evaluation (NDE) of the bond region and for measuring the bond dimensions.     

循环载荷下粘接接头真空性能的实验研究

粘接接头在经历反复冷却、复温后能否保持良好的真空密封性,是决定无磁杜瓦耐用性的一个重要因素。该文实验研究了循环载荷对粘接接头气密性的影响。用MTS 810材料试验机对玻璃钢管状粘接试样施加循环载荷,并用动态真空法测量试样真空度的变化。结果一半试样的真空度随循环次数的增加而降低,而其余试样破坏前真空度变化不大。因此,无磁杜瓦在使用过程中反复冷却及复温所产生的循环应力载荷,是导致其真空性能下降的一个重要原因。     

A novel technique for measuring far-infrared absorption and dispersion

A recently developed femtosecond FIR light source was utilized to measure the absorption and dispersion spectra of LiTaO₃. The coherent time-domain detection method allows the dispersion spectrum to be obtained without using the Kramers-Kronig relation. Due to the compactness of the experimental setup involved, this new technique is ideally suited for low-temperature experiments.     

A novel 3D measuring method for discontinuous object by non-integral twin-frequency grating

A new 3D shape measurement method based on non-integral twin-frequency grating projection is proposed. In this paper, the projected composite grating is composed of two sinusoidal gratings, and the quotient of whose frequencies is not an integer. By using appropriate phase-shifting algorithms, two wrapped phases can be obtained from sixteen frames of the distorted grating patterns. In aid of appropriate phase unwrapping method, the unwrapped phase of high frequency fringe is obtained only from the relation of the two wrapped phases by a pixel-to-pixel phase unwrapping technique. Therefore the unwrapped phase of high frequency fringe pattern is independent of the low frequency fringe pattern, different from traditional integral twin-frequency grating projection methods in which the phase unwrapping error of low frequency fringe pattern may be propagated onto the unwrapped phase of high frequency fringe pattern. The new theory proves that the proposed method is applicable to measure discontinuous object, and has considerable measurement accuracy.     

A novel 3D deformation measurement method under optical microscope for micro-scale bulge-test

A micro-scale 3D deformation measurement method combined with optical microscope is proposed in this paper. The method is based on gratings and phase shifting algorithm. By recording the grating images before and after deformation from two symmetrical angles and calculating the phases of the grating patterns, the 3D deformation field of the specimen can be extracted from the phases of the grating patterns. The proposed method was applied to the micro-scale bulge test. A micro-scale thermal/mechanical coupling bulge-test apparatus matched with the super-depth microscope was exploited. With the gratings fabricated onto the film, the deformed morphology of the bulged film was measured reliably. The experimental results show that the proposed method and the exploited bulge-test apparatus can be used to characterize the thermal/mechanical properties of the films at micro-scale successfully.     

A novel technique for measuring absorption coefficients at 632.8 nm

A novel experimental technique is reported for the determination of the absorption coefficients of modern optical materials at the 632.8 nm with the help of a high power He-Ne laser. Sensitive dependence of the laser output on the intra-cavity optical losses of the laser has been utilized. Absorption in these materials are very small, and their measurements are beyond the capabilities of the existing instrumentation. Hence the technique described in this paper would excel. The absorption coefficients of boro-silicate crown glass and fused silica have been determined, and are found to be 0.008 and 0.001 cm^-1, respectively. This technique is capable of measuring absorptions ～ 10^-4 cm^-1, and can be improvised to measure absorptions ～ 10^-5 cm^-1.     

Low-cost speckle interferometry for measuring 3D deformation fields: Hardware and software

In the present paper the authors propose a portable and low-cost speckle interferometer for evaluating 3D deformation fields. The interferometer was designed and realized with the aim of carrying out measurements on small areas, hence it can be used to approach problems characterized by displacement field with highly localized gradients, such as the strain relief occurring in residual stress evaluation or the displacements which arise around notches or crack tips. The costs of the experimental equipment were reduced by employing laser diodes as light sources, a PZT actuator designed and calibrated by the authors and a control electronics realized on purpose. Moreover the configuration which was adopted allows further saving on optical components. The experimental results reported at the end of the paper, and obtained by a specimen subjected to 3D rigid body motions, show a high repeatability and accuracy. Furthermore the experimental results have shown that the geometry of the optical setup implies the variation of the sensitivity vectors on the inspected area, but this effect can be evaluated (analytically and/or experimentally) and taken into account in order to increase the accuracy of the measurements.     

A novel technique for measuring resistance fluctuations independently of background noise

A novel method is presented for measuring the spectral density of resistance fluctuations without the explicit determination of the voltage background noise (Johnson noise, pre-amplifier noise). The output of a standard ac bridge excited by a single-frequency alternating current is demodulated by two phase-sensitive detectors which operate in quadrature. When the phase difference between excitation and detection is properly set, the real part of the cross-spectral density of the two demodulators shows only the spectral density of the resistance fluctuations and not the disturbing background noise. The feasibility of our new method is demonstrated by measurements of 1/f noise of a thin-film A1 sample.     

能动抛光磨盘的变形实验研究

能动抛光磨盘实时产生不同的表面变形,在对大口径非球面光学元件进行精磨和抛光时实现与工件表面良好的大尺寸吻合,可以消除传统光学加工采用小尺寸磨头时带来的高频残余误差并提高加工效率。以加工直径1.3m左右,F/2的抛物面光学元件为例,对能动磨盘在不同离轴度时能够产生的变形进行了计算和实验。结果表明,能动磨盘能够以较高精度产生旋转对称或非对称的抛物面形状。对能动磨盘产生变形后的残余误差进行了分析。     

能动抛光磨盘的变形实验研究

 能动抛光磨盘实时产生不同的表面变形，在对大口径非球面光学元件进行精磨和抛光时实现与工件表面良好的大尺寸吻合，可以消除传统光学加工采用小尺寸磨头时带来的高频残余误差并提高加工效率。以加工直径1.3m左右，F/2的抛物面光学元件为例，对能动磨盘在不同离轴度时能够产生的变形进行了计算和实验。结果表明，能动磨盘能够以较高精度产生旋转对称或非对称的抛物面形状。对能动磨盘产生变形后的残余误差进行了分析。     

A novel technique for laser diode to single mode fiber coupling

An optical fiber coupler based on the semi-linear passive phase conjugate mirror is analyzed. The threshold coupling strength is determined from theoretical considerations. An estimate of 50% for the overall coupling efficiency is made, assuming there is good mode matching between the light incident on the fiber and the propagating mode. The coupler exhibits a high degree of alignment insensitivity allowing for simple manufacture.     

二维及三维流场的光学测量方法

对于复杂的非定常流动 ,流场的测量往往要求无干扰、非接触 ,并且能够瞬时记录流场的二维甚至三维信息。对近年来流场测量领域发展快速、应用广泛的几种光学测量方法 ,如 PIV技术及其由此发展而来的 DPIV和 HPV技术 ,做一些介绍和比较。     

A novel virtual four-ocular stereo vision system based on single camera for measuring insect motion parameters

A novel virtual four-ocular stereo measurement system based on single high speed camera is proposed for measuring double beating wings of a high speed flapping insect. The principle of virtual monocular system consisting of a few planar mirrors and a single high speed camera is introduced. The stereo vision measurement principle based on optic triangulation is explained. The wing kinematics parameters are measured. Results show that this virtual stereo system not only decreases system cost extremely but also is effective to insect motion measurement.     

A novel knowledge-based potential for RNA 3D structure evaluation

Ribonucleic acids(RNAs) play a vital role in biology, and knowledge of their three-dimensional(3D) structure is required to understand their biological functions. Recently structural prediction methods have been developed to address this issue, but a series of RNA 3D structures are generally predicted by most existing methods. Therefore, the evaluation of the predicted structures is generally indispensable. Although several methods have been proposed to assess RNA 3D structures, the existing methods are not precise enough. In this work, a new all-atom knowledge-based potential is developed for more accurately evaluating RNA 3D structures. The potential not only includes local and nonlocal interactions but also fully considers the specificity of each RNA by introducing a retraining mechanism. Based on extensive test sets generated from independent methods, the proposed potential correctly distinguished the native state and ranked near-native conformations to effectively select the best. Furthermore, the proposed potential precisely captured RNA structural features such as base-stacking and base-pairing. Comparisons with existing potential methods show that the proposed potential is very reliable and accurate in RNA 3D structure evaluation.     

A high sensitivity 3D experiment for measuring Calpha-Halpha residual dipolar coupling constants

A new sensitivity improved approach is presented to measure the Calpha-Halpha scalar and dipolar coupling constants in 13C/15N-labeled proteins using a HA(CA)CONH scheme. The proposed experiment has significantly higher sensitivity than the previously published (HA)CA(CO)NH sequence, and provides accurate and straightforward measurements of the scalar and residual dipolar coupling constants. The sequence is easy to implement, and has been demonstrated on the C-terminal domain of the human Ku-80 protein (152 amino acid residues). On average, sensitivity is improved by 40% for both isotropic and anisotropic samples. The sensitivity enhancement is more pronounced for structured regions than unstructured regions, with an average of 50-60% enhancement being observed in the well-structured regions of the protein.     

A polarization method for measuring absorption spectra of single molecules

A new method for measuring low-temperature absorption spectra of single impurity molecules using a laser confocal polarized-light transmission microscope is proposed. The sensitivity of this method is of the same order of magnitude as that of fluorescence excitation spectroscopy. Combined use of both methods makes it possible to study distributions of impurity molecules by the magnitude of fluorescence quantum yield and their correlations with the linewidth distributions, as well as to determine if spectral jumps lead to changes in the fluorescence quantum yield.     

A new technique for measuring spectral shape discrimination.

A new technique is described for studying the ability of listeners to discriminate between sounds on the basis of spectral shape, a process called "auditory profile analysis." The advantage of the technique is that it reduces the range of the random rove in level necessary to provide a specified limit on the performance which listeners could achieve by "level detection;" that is, by employing a detection strategy based solely on comparisons of stimulus level. Thresholds were measured for the just-discriminable "ripple" (a pattern of alternating intensity increments and decrements) in an equal-amplitude, multitone reference spectrum for a group of normal-hearing listeners. Broadband, high-pass and low-pass filtered conditions were tested. The results indicated that the thresholds obtained using the new technique were well below the lowest level achievable by level detection (referred to as the "level-detection limit") in all conditions using a 20-dB random within-trial rove in overall level. The lowest threshold occurred for the broadband stimulus while the highest threshold was observed for the most extreme high-pass filtered condition. The new technique appears to be well-suited for study of profile analysis in hearing-impaired listeners where stimulus bandwidth and rove range are limited.