结构微裂纹混频非线性超声检测方法研究

给出了一种基于混频效应的非线性超声微裂纹检测方法。首先,对结构损伤混频检测机理及信号特征提取方法进行了理论分析,之后,根据试件中差频分量及和频分量幅值分布随激励信号频率变化关系,优化确定出混频检测参数。最后,进行了异侧混频激励下无损检测试验研究,并分析了激励信号频率变化对混频检测效果的影响。结果表明,异侧激励混频检测模式不仅可以实现结构中疲劳微裂纹检测,而且可以实现缺陷的定位。且检测信号频率选择对混频检测信噪比有一定的影响。当检测信号中的混频分量幅值最大时,混频检测效果最佳。因此,在优化检测参数基础上,异侧混频激励检测模式可以很好实现结构微裂纹的检测与定位。      

基于分布式激光超声的带筋构件R区缺陷检测方法

针对带筋构件R区多类型缺陷的快速检测和精确识别等难题，提出基于分布式激光超声的带筋构件R区缺陷检测和分类识别方法。通过建立有限元模型研究了激光超声在带筋构件中的传播规律及其与R区表面裂纹、近表面气孔等缺陷的相互作用机理，进而设计制作了含典型人工缺陷的6061铝合金带筋构件试样，开展激光超声检测实验。实验获得的缺陷时域信号与B-scan特征图像及仿真结果一致，基于反射与衍射原理实现了对带筋构件R区缺陷定位与定量检测，从而验证了所提分布式激光超声检测方法的可行性，为带筋构件制造缺陷的快速检测和分类识别提供了新思路。     

基于空耦超声Lamb波的金属板状结构内部缺陷检测方法

针对接触式超声检测方法在金属板结构内部缺陷实际工程检测中存在的环境要求高、效率低、操作难度高等问题,提出了空耦超声Lamb波检测方法,该方法能更好地适应现场应用环境,提高检测效率,减少传感器数量。通过有限元仿真和实验分析比较了空耦超声检测与接触式超声检测两种方法接收到的信号和成像效果。结果表明:有限元仿真和实验中,空耦超声检测方法对缺陷位置的定位误差分别为2 mm和3.6 mm,接触式检测方法对缺陷位置的定位误差分别为2 mm和11.3 mm,空耦检测具有较高的定位精度;单侧激励条件下,适合采用A0模态Lamb波对板内缺陷进行检测;空耦超声检测可以通过调整信号接收角度接收单一模态Lamb波,避免伪像产生。该方法为后续金属板状结构内部缺陷的空耦超声检测提供参考。     

不锈钢折弯板裂纹Lamb波检测技术*

金属弯板广泛应用于车辆、船舶、飞机等大型装备结构件中，折弯处易形成应力集中产生裂纹，直接影响构件的使用安全和寿命。本文开展了不锈钢弯板裂纹缺陷Lamb波检测技术研究，计算了钢板的频散曲线，优选三种不同频率的S0模态：0.25MHz、0.5MHz、1MHz。利用COMSOL软件建立频域仿真模型，模拟了Lamb波在3mm厚“L”形弯板内部的传播情况，开展折弯板裂纹缺陷检测实验，并使用小波包变换的方法对实验信号进行分析。结果表明，经小波变换后，0.25MHz、0.5MHz、1 MHz的S0模态Lamb波均可用于裂纹缺陷的识别检测.     

闭合裂纹非共线混频超声检测试验研究

针对结构中微裂纹检测难题,发展了一种闭合裂纹非共线混频超声检测方法。在对非共线混频超声检测机理分析基础上,进行了结构中疲劳裂纹混频非线性超声检测实验。对有无裂纹试件中检测信号进行了滤波和时频分析,结果表明,可根据信号滤波后时域波形中是否存在明显的混频波包或时频分析中是否存在明显的和频分量,实现有无闭合裂纹的判识;通过移动激励探头的位置,控制两列入射声波在试件中的交汇位置,实现试件中不同深度位置的混频非线性检测。并根据测得的混频非线性系数沿试件深度方向上分布,实现了闭合裂纹沿深度方向上长度的测量。研究工作为结构中微裂纹定量评价做了有益探索。      

基于太赫兹波探测的墙体并排钢筋检测系统

为解决现有的太赫兹墙体钢筋系统在多根并排钢筋检测时,由于相邻钢筋反射信号的串扰而导致无法测量的问题,提出了一种基于太赫兹波的墙体内多根并排钢筋的检测方法。通过在太赫兹源前加装可调透镜组,达到在钢筋埋深处聚焦波束的效果,解决太赫兹波束同时照射到相邻两根钢筋产生混叠影响测量的问题;同时利用改进的直径计算算法,实现存在埋深差的并排钢筋的检测。实验结果表明:在水泥埋深40mm,并排钢筋间隔25mm时,测量误差小于0.5mm,并且相邻钢筋在埋深差40mm(最浅埋深40mm)的情况下,测量误差小于1mm。     

基于斜入射光反射差技术检测微倾角

利用斜入射光反射差技术对表面光滑、上下面存在倾角为0.23°的20号钢样品表面进行扫描成像.成像结果显示:位置对斜入射光反射差信号产生明显影响,在扫描的0.3mm×0.3mm范围内,2条对角线上的信号变化率分别为81.5%与33.3%.对于上下面存在微倾角的柱状样本,不同位置存在微小厚度差,使得光程发生变化,反射率及反射光相位发生改变,从而改变倍频信号的强度.因此,斜入射光反射差技术可检测材料表面的微倾角.     

涂层厚度与粘接质量的电磁声谐振无损检测

为了解决电磁超声(EMAT)涂层质量检测中接收信号信噪比较低的问题,将电磁声谐振技术(EMAR)应用于涂层质量检测,有效提高了超声接收信号的信噪比。在EMAR仿真中,通过变步长离散仿真得出了谐振与非谐振状态下的时域波形和频谱图,验证了利用电磁声谐振进行涂层质量检测的可行性。以电磁超声谐振驻波作为检测信号,实验中谐振信号的幅值为未谐振反射回波的2.52倍,信噪比由26 dB(反射回波)提高到34 dB(谐振信号)。利用EMAR信号的谐振频率差进行涂层厚度检测,对0.2~0.8 mm的试件的检测误差在5%以内且涂层厚度越大,谐振频率越多,谐振频率差越小,检测精度越高;通过功率谱密度函数(PSD)表征不同粘接程度的试件,不同试件的PSD比值为61.17:4.15:1,区分度相对于峰峰值比值提升明显。      

基于激光超声临界频率的表面缺陷检测与评价

激光超声能够激发宽频带的表面波,可实现金属材料表面微缺陷的定位分析与深度检测。根据表面波在缺陷处的透射/反射阈值现象,提出了测量表面缺陷深度的临界频率法;基于光弹激发原理搭建了激光超声检测平台,得到了铝合金样品的B扫成像,实现了表面缺陷的定位;结合小波变换分析了表面缺陷的透射与反射波形能量分布,测得透射/反射阈值θ_0=1/4,进而采用临界频率法实现了表面微缺陷的深度估计。此外,还分析了激发点与探测点距离、待测样品材料对透射/反射阈值的影响。结果表明,基于激光超声临界频率法可以实现表面微缺陷的定位及深度检测,且透射/反射阈值大小与表面波传播距离、待测样品材质无关。     

基于衍射波的孔类缺陷超声相控阵定量方法研究

针对常规超声检测方法对小孔型缺陷难以准确定量的问题,利用超声相控阵的声束偏转聚焦和成像技术,基于声波在体积缺陷处的衍射特性,提出采用一次反射回波和衍射回波来确定体积缺陷的直径尺寸。以横孔模拟孔型缺陷,利用动态光弹技术对声波入射横孔的传播规律进行研究,并采用超声相控阵对孔径为Φ1-Φ4mm不同埋深以及相同埋深下Φ0.8-Φ4.8mm的等差横孔进行了试验。研究结果表明利用本文提出的定量方法能快速有效的确定孔型缺陷直径尺寸,实际计算值与理论值误差较小,对相控阵的定量问题研究具有一定的参考价值。     

Self-organization of five species in a cyclic competition game

Cyclic competition game models, particularly the “rock–paper–scissors” model, play important roles in exploring the problem of multi-species coexistence in spatially ecological systems. We propose an extended “rock–paper–scissors” game to model cyclic interactions among five species, and find that two of the five can coexistent when biodiversity disappears, which is different from the “rock–paper–scissors” game. As the number of fingers is five, we named the new model the “fingers” game, where the thumb, forefinger, middle finger, ring finger, and little finger cyclically dominate their subsequent species and are dominated by their former species. We investigate the “fingers” model in two ways: direct simulations and nonlinear partial differential equations. An important finding is that the number of species in a cyclic competition game has an influence on the emergence of biodiversity. To be specific, the “rock–paper–scissors” model is in favor of maintaining biodiversity in comparison with the “fingers” model when the variables (population size, reproduction rate, selection rate, and migration rate) are the same. It is also shown that the mobility and reproduction rate can promote or jeopardize biodiversity.     

Contrastive analysis on tone between Putonghua and Taiwan Mandarin

The differences of the pitch and duration of Chinese syllables between Putonghua （PTH） and Taiwan Mandarin （TM） were studied. The speech materials to be used are not only isolated syllables, but also sentences. The results reveal that： For the isolated syllables, T1 and T2 in TM are influenced by Minnan dialect, therefore their pitch are lower than those in PTH. T3 is fall-rise in PTH, while it is fall in TM. Moreover, the syllable duration sequence for different tone is T3〉T2〉T1〉T4 in PTH, while it is T1〉T2〉T3〉T4 in TM. For the syllables in sentences, T2 is mid-rise in PTH, while it is mid-level in TM. And the T3 is longer than T4 but shorter than T1 or T2 in PTH, while it is the shortest in TM. Furthermore the effects of prosodic phrase boundary on duration for different tones are almost the same in PTH, but the lengthening part of T1 or T2 is longer than that of T3 or T4 in TM.     

Experimental study on downstep in Chinese intonation

Downstep in pitch contour of Chinese Putonghua is examined using subtly designed sentences by controlling tone combinations. The results show both automatic and nonautomatic downstep phenomena exist in Chinese. In non-automatic downstep, low tones compress downwards the pitch range of the following syllables. and the main influence of downstep is on topline. Low tone not only lower the topline behind it, but also raise the high tones before it, the effects are compatible with each other. In automatic downstep, the topline of pitch contour in intonational phrase is presented as a linear downtrend, but it differs among speakers due to the effect of personal stress practice. In comparison with downstep phenomenon in other tone or non-tone languages, the downstep ratio in Chinese is not constant, and the domain of downstep is not limited within the adjacent tones.     

Abelian-Higgs phase of SU(2) QCD and glueball energy

It is shown that SU(2) QCD admits an dual Abelian-Higgs phase, with a Higgs vacuum of a type-Ⅱ superconductor. This is done by using a connection decomposition for the gluon field and the random-direction approximation. Using a bag picture with soft wall, we presented a calculational procedure for the glueball energy based on the recent proof for wall-vortices [Nucl. Phys. B 741(2006)1].     

Theoretical analysis and simulation of conjugate heights for dual-conjugate AO system in lidar

A multi-conjugate adaptive optics （MCAO） can offer a possibility of widening field of view （FOV） characterized by the isoplanatic angle, and the choose of conjugate height becomes a basic problem for MCAO, which influences the size of iosplanatic angle. Considering the application of lidar, the isoplanatic angle＇s expressions of two deformable mirrors （DMs） MCAO for uplink and downlink are deduced. The effects of conjugate heights for dual-conjugate AO are thoughtfully discussed, and the isoplanatic angles are further analyzed. The results show that the isopanatic angle varies with the conjugate height and reaches the maximum as the conjugate height is at the optimal altitude. Moreover, the optimal conjugate height changes with the propagation distance.     

Quick single-photon detector with many avalanche photo diodes working on the time division

Due to the limit of response speed of the present single-photon detector, the code rate is still too low to come into practical use for the present quantum key distribution (QKD) system.A new idea is put up to design a quick single-photon detector.This quick single-photon detector is composed of a multi-port optic-fiber splitter and many avalanche photo diodes (APDs).Au of the ports with APDs work on the time division and cooperate with a logic discriminating and deciding unit driven by the clock signal.The operation frequency lies on the number N of ports, and can reach N times of the conventional single-photon detector.The single-photon prompt detection can come true for high repetition-rate pulses.The applying of this detector will largely raise the code rate of the QKD, and boost the commercial use.     

Signal generation in an isotropic medium in scanning electron acoustic microscope

Based on the research in Ref. [5][Materials Science and Engineering, 1989; A122： 57 63], an improved model of heat source is set up, the different modes of Lamb wave in an isotropic sample generated by a chopped electron beam at frequency f are obtained with integral transform and normal function expansion method, and the output signal of PZT coupled at the back surface of the sample is found out. The generation mechanism of SEAM （Scanning Electron Acoustic Microscopy） signal is discussed. It shows that the SEAM is a near field imaging technique with high spatial resolution and its best lateral spatial resolution is about 2√2α （α is the radius of the focused electron beam）. Some of experimental results of SEAM images are presented in the paper and it shows that the spatial resolution of SEAM is better than 0.5 μm and smaller than the thermal diffusion length of the sample. Therefore the character of near field imaging in SEAM is also proved experimentally.     

High efficiency filtering using two-dimensional photonic crystal coupled cavity structures

Transmission spectra of coupled cavity structures (CCSs) in two-dimensional (2D) photonic crystals (PCs) are investigated using a coupled mode theory, and an optical filter based on CCS is proposed. The performance of the filter is investigated using finite-difference time-domain (FDTD) method, and the results show that within a very short coupling distance of about 3λ, where λ is the wavelength of signal in vacuum, the incident signals with different frequencies are separated into different channels with a contrast ratio of 20 dB. The advantages of this kind of filter are small size and easily tunable operation frequencies.