【作者】 袁红梅；

【导师】 何存富；

【作者基本信息】 北京工业大学 ， 机械电子工程， 2010， 博士

【摘要】 粘结结构以其优越的性能而广泛应用于化工、航空航天及食品等各领域,但在其制造和使用过程中常常会出现界面缺陷,如粘结不良、气孔等现象,这些缺陷的存在将会影响材料使用的可靠性,在使用过程中由于界面缺陷的存在而造成的灾难性事故时有发生。为了保证产品质量和使用安全,对粘结结构界面质量的检测是十分有意义的。超声脉冲回波法以缺陷定位准确、适用范围广、操作方便等优点广泛应用于无损检测。采用脉冲回波法对粘结结构的界面特性进行检测分析时,由于超声波在各层介质之间发生多次反射和透射,不易分辨出各界面的回波。目前,尽管采用多种的信号处理方法用来解决该问题,但相对来说,处理方法比较复杂、繁琐,而且往往有时候不能得到满意的结果。尤其对于厚度较小(甚至于几百微米的厚度)的粘结结构界面质量的检测问题,缺陷特征信号的提取变得非常棘手。目前,国内有关这方面的研究相对缺乏。针对此问题,本文以两类厚度较小的粘结结构为研究对象,一类是软包装热封试件,另一类是粘接结构件,基于超声脉冲回波技术,提出一种基于超声背散射回波包络积分成像法的粘结结构界面缺陷检测方法,该方法将各界面回波作为一个总能量来考虑,通过有无缺陷时总能量的差异来判断缺陷的存在于否,该方法简单,快捷,不需要进行繁琐的信号处理,克服了由于试件厚度较小,常规的换能器难以分辨界面信号的局限性,为国内评价粘接材料界面质量性能提供了有效的实验手段和技术支持。主要工作如下:(1)粘结结构界面缺陷超声检测系统设计配合已有的硬件系统,开发设计了一套完整的粘结结构界面缺陷检测的软件系统,软件系统采用模块化思想进行设计,实现了检测过程的自动扫描、数据实时采集、实时显示、在线处理以及存储等功能;试验结果表明,所获得的背散射回波信号波形清晰,信噪比较大,满足检测要求,为进一步的粘结结构界面缺陷检测的试验研究提供了可靠保证。(2)粘结结构界面缺陷定性研究基于超声背散射回波包络积分成像方法(简称BEEI成像方法),采用中心频率为22.66MHz,-6dB横向分辨率为315μm的水浸式点聚焦传感器,对两类粘结结构界面缺陷,一类是软包装(聚乙烯膜)热封封口处50～150μm的通道型泄漏缺陷、夹杂缺陷;另一类是粘接试件中粘接界面出现的夹杂缺陷、气泡等进行检测;同时,以软包装热封封口处缺陷为研究对象,分析了成像矩阵中BEEI值的变异系数以及垂直于热封方向和平行于热封方向上BEEI值的变化趋势。试验结果表明,利用超声BEEI成像方法实现粘结结构界面缺陷检测是可行的,由于受传感器横向分辨率的影响,利用BEEI方法所测得缺陷尺寸比缺陷实际尺寸偏大;当成像矩阵中BEEI值的变异系数小于4%时,整个成像矩阵中BEEI值的离散程度较小;缺陷区的BEEI均值高于背景区的BEEI均值约30～40%。(3)粘结结构界面缺陷量化分析研究以软包装热封试样为研究对象,研究了扫描步长对BEEI图像质量的影响,结果表明,沿着热封长度方向,随着扫描步长的增大,图像质量明显下降,合理扫描步长是小于传感器-6dB的横向分辨率;并在合理扫描步长的基础上,通过详细分析不同尺寸范围内的缺陷尺寸与超声检测结果两者间的关系,从量值上评估了传感器横向分辨率对超声检测结果的影响大小。研究结果表明,缺陷的实际尺寸可近似为传感器-6dB的横向分辨率与超声检测结果二者的差值。(4)粘结结构界面缺陷检测机理研究超声背散射回波成像方法可以实现软包装热封后封口处微缺陷的检测,检出的最小缺陷达到52μm。鉴于考虑系统噪声和粘结结构的多层界面反射,如此微小的缺陷引起的背散射能量较小,加之试件本身厚度较小,微缺陷引起的回波会淹没在试件的上、下表面回波中,不能清晰地分辨出来,进而不能判断缺陷的存在与否。但在实际检测中,这些微缺陷能通过这种方法检测到。针对此问题,以两层聚乙烯膜热封封口处的微缺陷为研究对象,从声阻抗重建理论、ANSYS有限元热分析数值模拟以及缺陷截面扫描试验三个方面对其超声检测机理进行了深入的分析研究。结果表明,超声BEEI成像方法之所以能检测到微缺陷的存在,主要是因为缺陷周围区域由于高温作用,基体材质发生化学变化,使得缺陷周围产生与基体材质阻抗差异较大的热熔环形区,声波在传播过程中遇到环形区时,会产生较缺陷本身产生的较强的反射回波。更多还原

【Abstract】 More and more bonding structure have been widely applied in industry of chemical, aviation, aerospace and food for their superior performance. Disastrous accidents happen in process of manufacture and working at times for weak bonding or air hole of layered media. So it is urgently needed to make accurate detection for interface quality and insure the quality of products and working safety.Ultrasonic echo method is widely applied in non-destructive detection for its advantage of accurate defect locating, wide application, convenient operation and so on. When the interface properties of multi-layer materials is studied using pulse-echo method, the interface echo is not easy to be distinguished because of multiple reflections and transmission for ultrasound in layers of multi-layer materials. It is usually assessed by the echo of the overall energy, especially for a thin layered media, even thickness with a few hundred micrometers. In the current research, a novel ultrasonic imaging method, based on backscattered echo envelope integral (BEEI) imaging method is proposed and applied for the defect detection of bonding structure. Two kinds of interface defects are detected, one is the channel defects and inclusion embedded in bonded 2-sheet polyethylene film, the other is inclusion defect and air hole embedded in the adhesive structure. It has been demonstrated that the interface defects in bonding structure can be effectively detected using ultrasonic backscattered echo envelope integral imaging method. The main research works are given as follows.(1) System design for ultrasonic detection of defects in bonding structureUltrasonic system is developed for detecting interface defects embedded in bonding structure. The software system is designed based on the modularization, realizes experimental operation automatically controlled by computer, real time data acquiring, data on-line processing and storage, high efficiency compared with off-line processing. The results show that the software system has strong real time capability, high efficiency and reliability, and the echo with high signal-noise can be obtained to satisfy the need of signal analysis and processing.(2) Interface defects ultrasonic qualitative detection in bonding structureThe interface defects in bonding structure are detected using BEEI imaging method. The images are formed with an immersion spot focused 22.66-MHz ultrasound transducer which the -6dB pulse-echo beam diameter at the focus is 315μm. At the same time, the Coefficient of variation (CV) values of BEEI-matrix for each sample is studied for the channel defects and inclusion defects in bonded 2-sheet polyethylene film. The variation of the BEEI values in the direction vertical to the edge of the heat seal and in the direction parallel to the edge of the heat seal are compared for the channel defects. The results show that the interface defects in layered media can be effectively detected using ultrasonic BEEI imaging method, and the size of defects on the ultrasonic image is larger than its actual size which is correlated with the lateral resolution of the transducer. The CV values are lower than four percent for channel and inclusion defect, which indicate the variation of the BEEI values in matrix is small. The mean BEEI values on the defect region are about greater between thirty percent and forty percent than that on the background region.(3) Quantitative analysis for interface defects ultrasonic detection in bonding structureThe scanning size issue of the BEEI imaging method is investigated. The study of relationship between varying scanning step size and image quality for the BEEI imaging method is presented for the seal defects embedded in bonded 2-sheet polyethylene film. It show that the image quality degraded as the scanning step size increased in the direction parallel to the edge of the heat seal, and the reasonable scanning step size is small than -6dB focal beam diameter. Theoretical analysis for the influence of the transducer lateral resolution on ultrasonic result are made while scanning step size is less than -6dB lateral resolution of the transducer, and the relation between ultrasonic result and defects actual size is given in the different scope of size. The experimental results are in good agreement with the theoretical analysis.(4) Study of defects detection mechanism for bonding structureDefects detection mechanism for bonding structure is studied for the seal defects embedded in bonded 2-sheet polyethylene film from three aspects of reconstructed acoustic impedance, finite element heat conduction analysis and experiment. The BEEI imaging method is able to reliably inspect defects as small as 52-μm in diameter. The main reason is an annular impedance transition region surrounding the defect. The annular region is produced by material compounding during the heat sealing process. Because of the material compounding, this region with an impedance value higher than that of its surrounding medium. The impedance difference between the annular region and the defect region causes reflection. The reflection strength from annular region is strong compared with that of from defect.更多还原