Optimization of forming steps in the incremental forming of twisted shapes using a line array roll set (LARS)

The line array roll set (LARS) process is an effective metal forming process used to manufacture doubly curved metal plates. This study describes process optimization of forming steps that can be used to form a twisted shape. In the incremental forming of a twisted shape using the LARS process, a defect near the edge of the formed shape was observed. A plate was subjected bending and twisting simultaneously by the roll sets as part of this process; therefore, it was assumed that the edge defect resulted from the excessive longitudinal strain near the edge region of the plate. To obtain a desired shape with more precision and to minimize any possible defect, the process of forming the desired shape should be systematically and accurately designed. To obtain the optimized design and to ensure that the maximum longitudinal strain is under the limit of the critical value, an iteration scheme is used for the optimization. Edge profiles of deformed shapes for both a non-optimized design and an optimized design were compared. From the comparison, it was found that the optimized design can minimize the number of forming steps by decreasing the edge wave caused by excessive edge stretching.     

网格布断裂缺陷检测方法的研究

针对传统自动化设备在网格布断裂缺陷检测方面的不足,提出了一种基于机器视觉的网格布断裂缺陷检测方法。首先,获取网格布的高对比度图像,利用中值滤波去掉图像中的噪声,采用Otsu阈值分割算法实现目标和背景的分离。根据网格部分断裂和完全断裂的两种缺陷特点,分别设计针对性算法。网格部分断裂的特点是由背景生成的凸包区域内包含有目标物体,因此采用Blob分析和凸包检测相结合的方式,实现对此种缺陷的检测。当网格完全断裂时,缺陷区域与相近区域的位置信息会发生变化,通过分析网格骨架交点生成的圆形区域内连通区域的数目,可以完成网格完全断裂缺陷的识别。实验结果表明,该方法可以应用于网格布的断裂缺陷检测,缺陷识别率在97%以上,可以满足检测要求。     

Three-Dimensional Flow–Heat Coupling Model of a Wavy-Tilt-Dam Mechanical Seal

A three-dimensional flow–heat coupling model of a wavy-tilt-dam (WTD) mechanical seal is established to study the heat transfer in both the fluid and the solid domains. The upwind scheme is used to ensure the convergence of the numerical calculation. The inlet temperature boundary condition is discussed and two modified inlet boundary condition methods are proposed to eliminate the noncontinuity of the film inlet temperature. The method where the temperature gradient in the radial direction is assumed constant gives more reasonable results than the method where the temperature is assumed to vary linearly along the film thickness direction. The results show that the highest temperature is located near the inlet due to the waves, which will cause a backflow along the converging spaces formed between the rotor and seal faces (convex segments of the wave) in the circumferential direction. The thermal deformation in the radial direction is larger than the deformation that is caused by waves in the circumferential direction. In addition the deformation will increase the taper, which will result in an increase in the minimum film thickness. A parametric study shows that the maximum temperature of the film will decrease as the wave number increases. The rotor speed and pressure difference will obviously affect the performance of the WTD seal.     

Bloch wave treatment of symmetry and multiple beam cases in reflection high energy electron diffraction and reflection electron microscopy.

Bloch wave equations for the multiple beam cases in reflection high energy electron diffraction (RHEED) are derived from the integral equation by forward- and back-scattering Green function operators. A linearization is achieved through separation in a forward- and a back-scattering component for each beam. This leads to a set of fundamental equations similar to the transmission case, but with a non-Hermitian matrix, and the beams may be entered as either forward-, back-scattered, or both. The number of beams needed to be included in RHEED calculations is thus reduced, and so are the computing time and computer space required. The systematic row case, corresponding to reflections from planes parallel to the crystal surface, is treated in detail and illustrated by calculations of dispersion surfaces and rocking curves for Au(001). Symmetry relations for the systematic row and between reciprocal rows are discussed and illustrated.     

基于时间反转的管道导波缺陷参数辨识方法

针对导波检测中的缺陷辨识问题,通过对不同缺陷的时间反转导波检测缺陷反射率随时间反转信号截取窗宽变化关系的研究,确定缺陷散射波场中各导波模态的能量分布规律,并据此提出一种基于时间反转的管道导波检测缺陷辨识方法。在数值模拟的基础上,选择适当的特征参数用于表征反射率—窗宽关系曲线,建立与缺陷特征相对应的关系曲线样本库,并通过聚类分析方法在库中寻找到与待检测缺陷特征最为接近的样本。结果表明,该方法可达到评判缺陷特征的目的,为最终实现缺陷反演奠定基础。     

APPLICATION OF DOMAIN DECOMPOSITION IN ACOUSTIC AND STRUCTURAL ACOUSTIC ANALYSIS

Conventional element based methods for modeling acoustic problems are limited to low-frequency applications due to the huge computational efforts. For high-frequency applications, probabilistic techniques, such as statistical energy analysis (SEA), are used. For mid-frequency range, currently no adequate and mature simulation methods exist. Recently, wave based method has been developed which is based on the indirect TREFFTZ approach and has shown to be able to tackle problems in the mid-frequency range. In contrast with the element based methods, no discretization is required. A sufficient, but not necessary, condition for convergence of this method is that the acoustic problem domain is convex. Non-convex domains have to be partitioned into a number of (convex) subdomains. At the interfaces between subdomains, specific coupling conditions have to be imposed. The considered two-dimensional coupled vibro-acoustic problem illustrates the beneficial convergence rate of the proposed wave based prediction technique with high accuracy. The results show the new technique can be applied up to much higher frequencies.     

聚氨酯夹芯板发泡缺陷电磁超声导波快速检测

聚氨酯夹芯板在生产中因发泡不均容易在聚氨酯泡沫层形成空洞型缺陷,直接影响其保温效果,为此提出了一种从金属板外侧实施的电磁超声SH导波检测的快速检测方法.通过理论分析、解析推导、仿真计算和试验验证,建立了聚氨酯夹芯板中的SH导波透反射比系数与激励参数之间的关系,结果表明,当磁铁间距为4.5 mm时,控制SH0导波入射角度...     

固-流掺杂结构声子晶体中弹性波的缺陷模

为了研究弹性波在固-流掺杂结构声子晶体中缺陷模的特性,推导出了弹性波斜入射固-流掺杂结构声子晶体的转移矩阵。研究了固-流掺杂结构声子晶体中弹性波的透射系数,得到了由于掺杂在禁带的中央出现了一条狭长的缺陷模透射峰的结论。当杂质厚度固定时,缺陷模随入射角的增加向高频方向移动。当入射角固定时,缺陷模的中心频率随杂质厚度的增加近似呈线性减小。     

Wetting Analysis of Leadless Chips in Surface Mount Technology

It has been found that in wave soldering, when a chip entered the solder bath with its longitudinal axis parallel to the soldering direction, the resulting shape of the wetted surface of the incoming joint was concave, and the outgoing joint was convex. The difference in wetting of these two joints was analysed from the capillary viewpoints. Solder pads of various lengths were used to control the wetting conditions in infrared reflow soldering. The results show that the amount of tombstoning and joint opening are closely related to the pad length. The relationship between tombstoning, or open joint, and wetting was analysed based on theory, and it was found that the surface tension and the contact angle of the molten solder are the significant factors in the formation of the tombstoning or open joint defect.     

管道导波检测中的位移圆周分布调制方法

在研究管道导波检测中的位移圆周分布调制聚焦技术时,将存在的缺陷看作一个新的导波源,提出通过截取阵列中各传感器获得的检测信号中的缺陷反射回波及转换模态波包信息,分析该波源造成的波场在检测位置的位移圆周分布,从而得到将检测能量重新聚焦于缺陷所在圆周任一位置所需要的各激励信号幅值系数和时间延迟参数.有限元分析及试验表明,应用该方法,不仅可以减少运用位移圆周分布调制聚焦技术检测时的计算量,还可以灵活地应对检测过程中管道参数、传感器阵列及检测频率的变化,有利于该方法的推广和应用.     

SiCp颗粒增强铝基复合材料非共线非线性响应试验观察

采用粉末冶金方法制备的碳化硅颗粒增强铝基复合材料(SiCp/Al)受加工过程工艺影响较易出现SiCp颗粒分布不均等缺陷。传统的金相法及声速扫描法检测该缺陷存在破坏试样及定位困难且灵敏度不高等问题;基于谐波的穿透式共线非线性无损检测方法检测灵敏度高,但易受到其他非线性试验来源的干扰。为解决这一问题,利用基于波束混叠的非共线非线性超声检测方法,观察SiCp/Al的非共线非线性响应,探讨使用该方法检测粉末铝基复合材料非均匀性缺陷的可行性;建立非共线非线性超声检测试验系统,发现LY12铝合金及SiCp/Al试样内部的波型转换与波束混叠现象;利用非共线技术的空间可达、波型转换及频率方向可控等性质,成功剔除了非线性的试验来源,从试验上证明该方法能够观测试样内部的非线性,具备检测SiCp/Al试样均匀性的潜力。试验方法与结果为进一步研究非共线混叠波与粉末铝基复合材料均匀性的相关关系及扫查方法奠定基础。     

Directivity analysis of ultrasonic waves on surface defects using a visualization method

Ultrasonic testing uses the directivity of the ultrasonic wave, which propagates in one direction. The directivity is expressed as the relationship between the propagation direction and its sound pressure. The directivity of an ultrasonic wave is related to the choice of probe arrangement, testing sensitivity and scanning pitch, and correct measurement of defect size and location. This paper investigated the directivity of ultrasonic waves, which are scattered from a slit defect located in simulated butt weld joint using a visualization method. When the defect size was smaller than the wavelengths, clear directivity in the reflected wave was observed. When the ratio of defect size to wavelength is greater than 1.5, measured directivities almost agree with the theoretical directivity. The directivity of shear waves scattered from the slit defect varied according to probe direction (Far defect, Near defect). The angle of reflection wave became similar to angle of incidence as the height of excess metal in welded joint increased.     

Ultrasonic defect detection of a petroleum pipeline in a viscoelastic medium

During detection of the defects in the inner wall of a petroleum pipeline via an ultrasonic nondestructive evaluation method, the defect echo and the inner-wall echo often overlap. It is difficult to identify the arriving time of the defect echo. When an ultrasonic wave propagates in a viscoelastic medium, the stress relaxation and creep deformation result in a signal characterized by frequency. The wave speed and attenuation rate are dependent on the frequency; thus, the ultrasonic signal has different shapes along the wave propagation path. Sometimes the wave shape is wry. The empirical mode decomposition technology used for separating the overlapping echo signals has been presented in this paper. With such technology, the original ultrasonic signal can be decomposed and then some intrinsic mode functions (IMFs) and a residue can be obtained. Different IMFs contain different echo signals. Some useful IMFs are selected to reconstruct the ultrasonic signal. The peaks of the reconstructed ultrasonic signal envelope indicate the arriving time of the echo signals. The experimental results show that this method is effective for detection of the defects in the inner wall of a petroleum pipeline. The text was submitted by the authors in English.     

基于压电晶片阵列的管中导波时反检测方法研究

为提高超声导波技术对管中小缺陷的检测能力,提出一种采用窄带高压脉冲激发安装在管道外表面的压电晶片阵列实现管中导波时间反转检测的新方法。该方法采用窄带脉冲同时激励沿管道表面轴对称安装的压电晶片阵列,从各压电晶片接收到的反射回波中提取含L(n, 2)模态的缺陷信息进行时间反转,并用获得的时反波再次激励阵列中对应压电晶片,整个阵列将同时接收到较单一L(n, 2)模态信号。试验结果表明,该方法能较好地抑制导波的频散、多模态特性,提高缺陷回波信噪比,增加对小缺陷的检测能力。同时,时反前、后所检测到的管道端面和缺陷反射回波的导波模态几乎相同,可采用特定频率的L(0, 2)模态群速度作为时反后缺陷波包的传播速度;且时反后的幅值最大的缺陷波包能重构窄带初始激励信号幅值最大的波包,可有效增加缺陷波包的辨识能力。     

弹性波在一维掺杂声子晶体中的多缺陷模特征

推导出弹性波在一维掺杂声子晶体中的转移矩阵,研究了弹性波在一维掺杂声子晶体中的缺陷模特征。得出:弹性波在一维掺杂声子晶体中会出现多个缺陷模。缺陷模的数目随杂质厚度的增加而增加,缺陷模的半高宽随缺陷模数目的增加而减小。缺陷模的数目也随杂质声阻抗的增加而发生变化,各缺陷模的位置随杂质声阻抗的增加向禁带中心移动。这些特性可以作为设计多通道弹性波滤波器的理论依据。     

基于编码压缩的钢板电磁超声 Lamb 波检测方法研究

针对大型金属板材构件导波检测中EMAT分辨率低、SNR差,难以用于小裂纹检测等难题,建立了基于Barker码脉冲压缩技术的Lamb波EMAT检测过程多物理场有限元模型,以含预制裂纹的5.6 mm厚钢板为检测对象,仿真和实验相结合,研究了永磁体宽度和高度、曲折线圈匝数、Barker码序列长度和码元长度等参数对EMAT检测回波的影响,验证了脉冲压缩技术在Lamb波EMAT检测中的有益效果。结果表明,基于脉冲压缩技术的EMAT经优化后,SNR比传统tone-burst激励方式高出了9.69 dB,可检出10 mm长和0.5 mm深的小裂纹,缺陷波SNR为23.47 dB。当Barker码码元中心频率为1 MHz时,脉冲压缩后的Lamb波包发生模态分离,但A₀模态的缺陷波SNR仍可达35.23 dB,这对提升Lamb波EMAT检测能力具有重要的工程应用价值。     

光栅成像光谱仪同心光学系统研究

详细讨论了光栅同心光谱仪的波像差表达式。当入射光束与出射光束平行时,单色同心光学系统空间任一点成几何理想像。揭示了光栅同心光谱仪满足远心条件时,光瞳像差仅有三级球差和场曲及高级剩余像差,系统需要校正场曲和球差。光栅必须放在前后光学部分的焦平面上,满足远心条件。最佳光栅刻线是等间隔平行线。介绍了两种典型同心光学系统的结构和特性。     

Back-scattered electron imaging combined with EBSD technique for characterization of pearlitic steels

Microstructure of pearlitic steels subjected different heat treatments were characterized combining the usage of back-scattered electron imaging and electron backscatter diffraction in a scanning electron microscope. The results indicated that the method used in current study enabled the acquisition of pearlite nodule, colony and interlamellar spacing of pearlite structure only through sample preparation of one time. Both the morphology of pearlite lamellae and the crystallographic orientation of ferrite matrix can be released in back-scattered electron imaging image and electron backscatter diffraction micrograph acquired at the same region. The definitions of pearlite colony and the low-angle boundaries existed in ferrite matrix were also discussed based on this method.     

基于时间反转法的管道焊缝区缺陷检测研究

焊缝在整个管网系统中是比较薄弱的部分,在温度、压力、介质腐蚀和振动的影响下容易出现裂纹缺陷,对管道的安全运行产生危害,并且有的裂纹很小,通过超声导波不容易直接识别出来.对超声导波检测管道进行了研究,在分析了超声导波对焊缝小缺陷的检测后,提出了一种运用时间反转法基于L(0,2)模态超声导波进行数值模拟的方法,选择4％作为...     

A New Method for the Measurement of Real Area of Contact by the Adhesive Transfer of Thin Au film

In this study, a new experimental method is proposed to measure the real area of contact between a ceramic sphere and an Al surface based on the adhesive transfer of the Au film and the scanning electron microscope (SEM) in the back-scattered mode. A thin film of Au is sputtered on the ceramic sphere before the indentation with the Al surface. The success of this method relies on the fundamental assumption that the adhesive transfer of Au only occurs everywhere inside the contact area. A thin polymer (PMMA) film is deposited between gold film and the ceramic surface to further reduce adhesive strength. After indentation, the interfaces of the ceramic sphere and Al surface are observed by SEM. Experimental evidence that the adhesive transfer of the Au film occurs inside the contact area is given. The entire contact regions on the ceramic sphere and the Al surface are captured in the second electron and back-scattered images with a magnification of 220× (resolution: 432 nm, i.e., distance between neighboring pixels). The contact area can be identified based on both the distributions of the ceramic and Au on the ceramic sphere and Al surface, respectively. The back-scattered images with the magnifications of 5000× and 10,000× (resolution: 20 and 4 nm) are captured at four different locations along the radial direction (starting from the contact center), respectively. The real area of contact decreases from the center to the contact edge.