弱粘接结构中谐振频率与刚度比的数值研究

利用超声波垂直入射的方法,研究了粘接层界面具有弹簧模型的边界条件下,波的反射系数与谐振频率的关系。首先,利用粘接层界面的一次反射导出切向和法向刚度比表达式;其次,推导了粘接层界面存在多次反射和透射时刚度比、谐振频率及反射系数之间的关系,得到了弱粘接结构的反射系数随着不同刚度比的变化规律;最后,分析了弱粘接层厚度的变化对谐振频率的影响。理论分析和数值计算的结果表明:随着刚度比的增加,谐振频率向高频方向漂移;并且,随着粘接层厚度的增加,粘接结构将出现多阶谐振频率。     

混合润滑状态下粗糙界面法向接触刚度计算模型与特性研究

机械装备系统的静态特性和动力学特性取决于系统接触界面法向接触刚度。基于粗糙表面形貌的Greenwood-Williamson统计模型描述与液体润滑界面的油膜共振模型和弹簧模型,推导了机械结构混合润滑粗糙界面固体接触刚度和液体润滑介质接触刚度,并实现粗糙微凸体固体接触刚度与液体润滑介质接触刚度的耦合,提出了一种混合润滑状态下粗糙界面法向接触刚度的计算模型,分析了接触界面形貌参数、润滑介质和接触基体材料属性对界面法向接触刚度的影响规律。结果表明:润滑介质的声阻抗是影响液体接触刚度的主要因素,声阻抗增大时,液体接触刚度减小;接触基体材料的表面形貌和弹性模量是影响固体接触刚度的主要因素,界面粗糙度和弹性模量增大时,固体接触刚度增大。混合润滑粗糙界面接触刚度计算模型的提出,为机械结构润滑接触界面的刚度计算、性能预测与优化提供理论和实验参考。     

增强相周期分布的多涂层纤维复合材料反平面问题的解析方法

采用增强纤维周期分布模型,通过引入非均匀的本征应变并结合双准周期Riemann边值问题理论,将非均匀介质问题转化为均匀介质的周期本征应变问题,获得了反平面载荷作用下多涂层纤维复合材料纤维、各涂层和基体中弹性场的解析解答。与有限元、广义自洽和渐进均匀化等方法的比较显示了该文方法的精度。讨论了涂层厚度和刚度等材料微结构参数对界面应力集中系数和有效反平面剪切模量的影响。该文为研究多涂层复合材料界面效应提供了一个有效的分析工具。     

涂层基体条件对梯度涂层残余应力影响研究

采用有限元法，对结构一定的Al/Ni-ZrO2梯度涂层在基体条件改变时涂层的残余应力进行了分析，结果表明，基体材料的热膨胀系数对涂层的残余应力有显著的影响，对于基体为圆柱形的涂层，其基体与涂层界面的残余应力梯度，最大轴向拉应力均随热膨胀系数的增大而线性增大，表面纯陶瓷层与次层界面残余应力梯度则随之减小，增大基体的直径和厚度，可缓和涂层残余应力，并在基体直径为36mm,厚度为20mm时各残余应力基本稳定。     

粘接结构中垂直入射纵波的准静态模型解

研究了板状粘接结构中垂直入射纵波的准静态模型解。在将粘接界面简化为准静态模型（QSM）的情况下,推导了界面为完好连接且上下基体为同种材料的板状粘接结构中垂直入射纵波的声反射和透射系数表达式。首先将纵波的准静态模型解和精确解进行了比较以阐释准静态模型的适用条件;接着分析了法向刚度系数和界面相对质量的变化对纵波反射和透射特性的影响。结果表明,在特定频率下,随着刚度系数或界面相对质量的增加,纵波的反射和透射系数的幅值分别减小和增大至某一稳定值。研究成果可为实验时采用纵波垂直入射检测粘接结构提供一定的理论指导。     

界面摩擦对热障涂层压痕响应的影响

以四层结构的热胀涂层薄膜／基体材料为基础，利用有限元建立模型分析了界面摩擦对压痕响应的影响。分析表明，考虑界面摩擦情况下，随着摩擦系数的增大，压痕响应越趋近于完全结合时的压痕响应；水平方向的最大位移值随着摩擦系数的增大也逐渐增大。界面光滑接触时接触界面力学响应可以忽略。     

圆柱型各向异性材料中螺型位错与非理想界面圆形夹杂的干涉效应

研究了圆柱型各向异性材料中螺型位错与含非理想界面圆形夹杂的弹性干涉问题。利用复变函数方法,获得了圆形夹杂和无限大基体区域复势函数的精确级数形式解答。利用Peach-Koehler公式导出作用于螺型位错上的位错力公式。主要讨论了材料各向异性性质与界面非理想性对位错力的影响规律。分析结果表明,界面非理想性吸引位错,而各向异性夹杂排斥各向同性基体材料中的位错。当夹杂的各向异性程度大于基体时,位错在非理想界面附近存在一个平衡位置。当界面非理想程度系数不变时,夹杂各向异性程度系数存在一个临界值改变作用在位错上位错力的方向。     

热喷涂涂层与基体结合界面研究进展EI北大核心CSCD

热喷涂再制造过程中,由于材料原因,涂层与基体之间往往存在一个异质界面问题。异质界面的形成与存在对再制造涂层服役性能有非常重要的影响。本文综述了热喷涂涂层与基体结合界面的研究发展现状,主要是结合界面形成机理和结合界面对涂层性能影响的研究发展现状。分析了热喷涂涂层与基体结合界面研究目前还存在的问题,并针对这些问题提出采用新技术与新手段深入研究涂层与基体结合界面的生长形成过程,揭示结合界面形成机理,并利用新表征方法实现涂层与基体结合界面形貌结构定量化表征,构建结合界面与涂层各项性能之间量化关系等的发展建议,进而为实现涂层性能的设计控制及寿命预测奠定基础。     

铷原子频标中微波谐振腔加载特性的微扰法分析

本文用微扰法分析了铷原子频标中圆柱形谐振腔内紧贴内壁加载一薄玻璃腔泡后对谐振腔谐振频率产生的影响.通过理论推导得到了由玻璃介质引起的谐振频率偏移的解析式,并对TE011模的谐振频率与玻璃厚度、径向长度的关系进行了数值计算与分析,得到了该种谐振腔的谐振频率随介质厚度和径向长度的不同而发生变化的曲线.结果表明,谐振腔加载玻璃腔泡后的谐振频率较空腔时变小,但随着腔壁厚度增大逐渐减小,而随着径向长度增大谐振频率先减小而后增大.以上结论对于微波谐振腔在原子频标中的设计应用有着一定的指导价值.     

丙烯酸改性多孔二氧化硅吸声涂层的制备与研究

采用丙烯酸改性多孔二氧化硅微粉为填料,以E-44环氧树脂为基体,乙二胺为固化剂制备出具有吸声性能的涂层,并对其耐磨性和吸声性能进行了测试。结果表明,涂层的耐磨性随着填料加入量的增加先增大后减小,填料含量为60%时磨损率最低;对于频率为1000Hz的声音,吸声系数随着填料的增加而增大,平均吸声系数可达0.22,同时,粒径小的填料吸声系数高于粒径大的填料吸声系数。     

Characterisation of interfacial imperfections of TiAl and 40Cr diffusion bonding by ultrasonic amplitude and phase

Abstract

A characteristics extraction algorithm is proposed to characterise the interfacial imperfections in TiAl and 40Cr diffusion bonding. The algorithm is based on analysing the variation of the ultrasonic amplitude and phase after interacting with the bonding interface. Ultrasonic measurements were performed by an ultrasonic imaging testing system, and broadband transducers with central frequency of the 10 and 20 MHz were employed. Metallographic analyses and shear tests were also performed on the joints. It was found that the amplitude of the reflection coefficient is almost a constant, and the phase of the reflection coefficient is the same for the perfectly bonded interface; for the kissing bond interface, the amplitude increases with the ultrasonic frequency, and the phase is the same at the low frequencies and opposite at the high frequencies; the amplitude does not vary with the frequency, and the phase is opposite for the unbonded interface.     

The determination of the viscoelastic properties of liquidmaterials at ultrasonic frequencies by CW mode impedance measurements

A method is described for measuring shear mechanical properties of liquids at ultrasonic frequencies. The method is based on the process of the electrical impedance of a sensor in contact with the liquid. The sensor is composed of a piezoelectric transducer bonded to a delay line. The process determines the complex reflection coefficient of shear waves reflecting from the delay line-liquid interface, and the mechanical impedance is deduced. Experimental determination of dynamic shear properties of a series of saccharose solutions is presented as an example of the method     

The chirp-Z transform applied to adhesively bonded structures

A signal-processing technique based on the chirp-Z transform is presented to evaluate the echo signals of longitudinal ultrasonic transducers in contact with bonded materials. Both a simulated glass-glass interface of variable thickness and a realistic double lap bonded aluminum sample were tested. The observed frequency dips and peaks in the transducer spectrum from pulse-echo and through-transmission modes were recorded and related to the condition of zero reflection coefficient at the interfacial layer. Resulting thickness predictions for different transducer center frequencies ranging from 5-20 MHz are in excellent agreement with experimental measurements     

Input impedance matching of acoustic transducers operating at off-resonant frequencies

The input impedance matching technique of acoustic transducers at off-resonant frequencies is reported. It uses an inherent impedance property of transducers and thus does not need an external electric matching circuit or extra acoustic matching section. The input electrical equivalent circuit includes a radiation component and a dielectric capacitor. The radiation component consists of a radiation resistance and a radiation reactance. The total reactance is the sum of the radiation reactance and the dielectric capacitive reactance. This reactance becomes zero at two frequencies where the impedance is real. The transducer size can be properly chosen so that the impedance at one of the zero-crossing frequencies is close to 50 Ω, the output impedance of signal generators. At this off-resonant operating frequency, the reflection coefficient of the transducer is minimized without using any matching circuit. Other than the size, the impedance can also be fine tuned by adjusting the thickness of material that bonds the transducer plate to the substrates. The acoustic impedance of the substrate and that of the bonding material can also be used as design elements in the transducer structure to achieve better transducer matching. Lead titanate piezoelectric plates were bonded on Lucite, liquid crystal polymer (LCP), and bismuth (Bi) substrates to produce various transducer structures. Their input impedance was simulated using a transducer model and compared with measured values to illustrate the matching principle.     

Ultrasonic evaluation of solid-state welds

Ultrasonic techniques for evaluating the quality of solid-state weld interfaces have been investigated over the past several years. Promising results have been obtained on a variety of solid-state welds by extracting features from the ultrasonic wave forms and applying pattern recognition algorithms to separate acceptable from unacceptable welds. The general conclusion is that the ultrasonic features most sensitive to interfacial bonding are those dependent on high frequencies. However, no single feature has been discovered that is adequate to yield separation of good vs. poor welds, since the microstructural response is also frequency dependent.Given the increase in sensitivity and resolution with high-frequency ultrasonic evaluation, selected specimens have been examined with acoustic microscopy. These specially prepared samples were inspected with focused transducers at frequencies in the 35–75 MHz range. The reflections observed indicated bond quality to vary in discrete regions with good and poor regions distributed across the diameter. Corresponding variations in the degree of bonding have been observed on the fracture surfaces of mechanically-tested specimens. The development of both low- and high-frequency acoustic microscopy has led to the possibility of sensing and imaging subtle changes in the reflection coefficient of the bond line. These acoustic images will improve our understanding of the mechanisms involved in evaluating solid-state bonds.     

粘结混凝土结构超声波时域和频域特性研究

装配式建筑在后浇混凝土过程中存在不同混凝土相互结合的问题,混凝土结合面粘结质量对粘结混凝土结构的受力性能具有重要影响。为研究粘结混凝土结构在不同粘结质量情况下的声波特性,运用有限元方法建立六类粘结混凝土结构在不同粘结质量情况下的物理模型,模拟了超声波在粘结混凝土结构模型中的传播过程。通过提取时域信号峰值,对频域数据进行积分处理,对比分析了各类粘结混凝土结构在不同粘结质量情况下的时域特征和频域特征。结果表明：不同混凝土强度等级相结合的粘结混凝土结构,界面反射较强,且强度等级差别越大,界面反射越强;时域信号峰值与频域积分随着粘结界面质量的提高而增大。根据这一特征,通过试验制作了三种不同粗糙度的粘结混凝土结构样本,采用斜测法对样本进行超声测试,结果表明界面粗糙度越高,超声波透射粘结界面区域内的时域信号峰值和频域积分越大。研究结果可为粘结混凝土结构质量检测提供参考和依据。     

机械结构流体层厚度超声测量方法理论研究

针对机械设备运行状态监控需要,对机械结构流体层厚度超声测量方法进行了理论研究。根据流体层厚度超声测量的实际情况,建立了超声波在三层介质中传播的理论模型,并由边界连续条件,得到流体层超声波反射系数的连续模型。通过对不同厚度流体层反射系数曲线的理论分析,得到流体层厚度超声反射系数测量的谐振模型和弹簧模型,即通过流体层的谐振频率和刚度系数来表征其厚度。同时,研究了三层介质的声学特性对流体层超声反射系数及厚度测量的影响。     

Sizing of disbonds at a thin-layer substrate interface from spectral peaks

An ultrasonic through transmission technique is used to inspect the interface of a thin layer and a substrate. An experiment is performed using a specimen constructed of a 0.040 inch (1.02 mm) polycarbonate layer bonded by an adhesive film to an aluminum plate. Circular flaws are induced with paper placed between the adhesive film and the polycarbonate layer to create disbonds. The flaws range in size from 0.5 inches (12.7 mm) to 0.067 inches (1.70 mm) in diameter. A through transmission ultrasonic inspection technique, which uses a flat transducer as a transmitter and a focused transducer as a receiver, is employed. Time signals from the receiving transducer are recorded for each flaw, transformed to the frequency domain by use of a fast Fourier transform (FFT), and are linearly deconvolved with the FFT of the time signals of the aluminum plate alone. The peaks of the resulting frequency response functions for each flaw predict the resonant frequencies of the flaws. The area of the thin layer above a disbond is modeled as a thin plate which is excited by the incident ultrasonic wave motion at its edges. The resonance peaks at the center of the plate are related to the size of the flaw. The principal result of the paper is that, based on the theory presented here, the size of the disbond can be obtained from measurements of the resonance peaks.