厚胶层复合材料弱粘接结构的超声反射特性

开展了复合材料粘接结构粘接质量的非接触式超声反射特性的仿真与实验研究。首先,建立了水浸环境下纤维增强复合材料(FRP)单层板时域仿真模型,数值分析了不同入射角度下声反射系数频率谱的变化规律,并与已有理论方法进行了对比验证。基于此,建立了含厚胶层的复合材料粘接结构反射特性时域仿真模型,分析了粘接界面弱化程度对超声反射系数频谱特征点的影响规律。基于水浸超声反射特性测量系统,实验萃取了完好粘接状态的FRP粘接结构的超声反射系数频谱,与仿真结果吻合良好。随后,制备了具有不同粘接界面状态特征的FRP粘接结构试样,从实验角度揭示了单/双粘接界面弱化状态与超声反射特性间规律性的偏移特征。基于不同粘接状态与超声反射特性间的规律性分布特征,可为厚胶层复合材料粘接结构的粘接质量评估提供新的研究思路。     

复合结构界面粘接强度的声-超声评价研究

基于Ritec-SNAP测量系统建立了声-超声技术实验系统,分析了在声-超声技术评价过程中用声信号的幅频特性及应力波因子表征复合结构界面粘接强度的可行性。用粘接层的固化过程模拟复合粘接板粘接强度的变化过程,以粘接层的固化时间作为粘接强度的间接表征参量,借助于已建立的实验系统对复合结构粘接强度的评价问题进行了实验研究。实验结果表明,应力波因子与反映界面粘接强度的粘接层固化时间呈单调对应关系,且不同固化时期的应力波因子存在明显的不同特性,表明用应力波因子评价复合结构的界面粘接强度具有可行性。     

铝合金粘接件界面层超声检测的进展

本文简述了检测铝合金粘接件界面层质量的超声反射法中有关理论和实验技术.介绍了粘接件中多孔Al_2O_3薄层有效弹性常数的确定方法和Al_2O_3/有机涂层之间界面的粘弹体模型.对五层媒质组成的粘接件声反射系数的理论值和实验值进行了比较.指出了粘接件界面层超声检测中一些尚待解决的问题。     

脱粘缺陷对粘接结构频散特性的影响

采用全局矩阵理论,通过引入内聚强度弱化模型和弹簧模型,计算了脱粘缺陷对粘接结构频散特性的影响。分析了内聚强度缺陷、界面缺陷及混合缺陷等因素对粘接结构频散特性的影响,发现频散曲线的演化规律与材料声学性质密切相关。不同材料和不同组合,其规律有很大不同。针对各向同性粘接结构,可能通过考察特定频率段内频散模态的变化,实现对脱粘缺陷的检测。     

粘接界面特性的超声检测与评价

粘接界面特性的超声检测与评价既是物理学的前沿问题,又是经济建设和国防安全亟需解决的应用问题。高波阻介质下多层低波阻介质界面脱粘检测、滑移界面检测和粘接强度定量评价是粘接质量检测中遇到的三大难题。本文综述了课题组十余年来在攻坚三大难题方面的研究工作：建立了共振匹配理论,发明了六项关键技术并实现了技术集成,研制出全新概念的超声检测设备,解决了第一个难题;成功研制出横波直探头,用纵横波综合判断方法解决了第二个难题;对第三个难题,建立了粘接界面非线性弹簧模型,为利用非线性声学参量评价粘接界面奠定了理论基础,并探索性地利用统计性方法对粘接强度进行了预测,取得较好效果。     

层状粘接结构超声检测信号的同态解卷积脱粘界面定征

本文利用建立的多层粘接结构超声无损检测信号的卷积模型,推导出了检测信号的解析表达式,并用同态滤波解卷积方法有效地提取出粘接结构的系统响应,进而由其中界面多次反射响应的位置、幅度和包络等特征参数,实现了脱粘二界面（指空气脱粘）的定征和识别。仿真和实验数据的处理结果表明本文方法对脱粘检测的应用有积极的意义。     

粘接界面的非线性弹簧模型及实验验证

研究了纵波及横波垂直入射时在粘接界面处的非线性边界条件。在二阶微扰近似及粘接界面层厚度远小于入射声波长的条件下,给出了严格的界面非线性弹簧模型的表达式。数值计算结果表明,利用该模型得到的透射二次谐波与利用连续性边界条件得到的精确解吻合良好。针对钢(铝)的楔形粘接结构,在纵波垂直入射的条件下对该非线性弹簧模型进行了实验验证。实际测量结果显示该模型具有较高的精度;且被粘物与胶层的阻抗比越大,该模型的误差越小。      

钢-混凝土结构弱粘接界面缺陷的超声导波检测*

钢-混凝土结构是土木工程中的一种常用结构形式,钢与混凝土粘接处可能出现弱粘接甚至完全脱粘的缺陷,严重影响结构的安全性。该文提出利用空气耦合超声导波衰减的方法实现钢-混凝土结构粘接状态的非接触无损检测方法,分析不同厚度粘接界面对超声导波衰减的影响。基于全局矩阵技术对钢-混凝土结构求解理论频散方程和衰减曲线,得到界面层不同粘接条件下的理论参数及衰减特性。建立不同粘接条件的有限元模型,定量分析不同模态对粘接缺陷的检测敏感度。研究界面层厚度分别为1 mm和2 mm两种情况下S0能量的衰减情况。研究结果表明:S0模态可有效判断粘接结构的粘接状态,对于同一界面层厚度,随着界面粘接条件变弱,S0最大幅值与A0最大幅值比不断增大;不同厚度同一粘接条件下,2 mm相较于1 mm该值更大。该方法在钢-混凝土结构粘接界面缺陷的检测方面具有良好的应用价值和发展前景。     

金属基复合结构粘接强度的非线性超声混频检测

采用非线性超声混频方法对金属基复合结构的粘接强度进行表征研究。以不同固化剂含量的铝合金丙烯酸酯粘接结构作为检测对象,基于非线性超声混频的共振条件及其作用机制,并结合粘接固化机理,开展了非线性超声混频实验。非线性超声混频模式选取两列横波生成和频纵波的方式,实验测量并计算不同基频周期数下的超声非线性参量。通过拉伸实验标定粘接结构的粘接强度,并采用扫描电镜分析拉伸断面微观结构,进而分析超声非线性参量与粘接强度的关系。从实验结果可以看出在不同周期的基频信号激励下,超声非线性参量随着粘接强度的增大均呈现出减小的趋势。研究表明非线性超声混频信号对金属基复合结构粘接强度的变化比较敏感,可以适用于类似结构件粘接强度状态弱化的表征。     

三层介质超声谐振模式随材料和界面粘接性能变化的演变规律

在用超声波谐振对粘接材料的粘接强度进行无损评估时, 不同模式对粘接强度的敏感程度受到众多因素和参数的影响, 对检测结果的可靠性至关重要. 基于多层介质中声传播和界面弱粘接边界条件的理论模型, 将一个上下非对称的金属-粘接剂-金属三层结构的平面波反射系数函数中的谐振模式看作是上下铝金属层各自的Lamb波频散模式通过夹心粘接剂层相互耦合后叠加组成. 改变影响结构粘接强度的因素, 即粘接剂的性能参数(声阻抗、密度、厚度)和界面切向劲度系数k_t来分析三层结构谐振模式耦合方式的变化,得出结论: 粘接结构粘接性能的变化基本上不改变与被粘铝层相关的固有部分的Lamb波模式, 而它们的耦合模式则在谐振频率上产生平移并会与固有模式进行交换和替代; 不同参数的变化引起的模式演变有各自的规律, 大多可彼此区分.     

固-固界面退化特性的超声反射评价方法

提出了一种基于有限宽超声束反射的固-固界面退化特性评价方法,从理论和数值仿真角度进行了分析和计算。将两固体界面间的薄层简化为界面弹簧模型,以界面法向和切向劲度系数表征界面的退化程度。通过数值计算求得有限宽超声纵波束在不同入射角和界面不同退化程度下的反射横波、反射纵波的镜面反射系数。进一步地,通过建立二维有限元模型,仿真研究了有限宽超声纵波束在给定入射角及界面不同退化程度下镜面反射系数的变化规律。结果表明,反射纵波和反射横波的镜面反射系数随有限宽超声纵波束的入射角及界面劲度系数的改变而变化,且存在镜面反射系数随界面劲度系数单调且敏感变化的入射角,据此可准确评价界面的退化程度。     

双层粘接板界面的超声非线性谐振特性分析

采用声-力-电类比建立粘接界面非线性力学行为的等效非线性振荡电路,以求解双层粘接板的超声非线性谐振频率。理论上导出非线性谐振频率方程,确定双层粘接板非线性谐振频率与激励幅度、三阶弹性劲度系数的解析关系。双层粘接铝板的超声实验发现:在不良粘接情形下,超声谐振频率发生偏移,其值大于粘接完好区,且激发了较强的三次谐波,但二次谐波幅度变化不大。实验结果表明三次谐波幅度上升,超声谐振频率也显著增大,与理论导出非线性谐振频率变化规律相吻合,且三次谐波与基波、二次谐波的比值反映了非线性谐振频率变化趋势,证实粘接层三阶劲度系数是产生非线性共振频率偏移的主要因素。      

Effect of ultrasonic vibration on adhesive enhancement of plasma-modified nickel surface

Poor adhesion of nickel surface limits its further application in the aerospace field. In this study, plasma modification was conducted on the surface of the nickel plate pretreated by sandblasting, and then ultrasonic vibration was applied during the adhesively bonding process of the CFRP(Carbon fibre-reinforced polymer)/Ni joints. The bonding strength of the joints was increased by 65%. The adherend surface and the bonding interface were analyzed from microstructure, element distribution and chemical bonding to study the strengthening mechanism. By the sandblasting, irregular pits were formed on the nickel surface, effectively increasing the surface roughness. The plasma modification could introduce active functional groups including hydroxyl, amino and carbonyl on the nickel surface, which improved the surface wettability macroscopically. However, at a microscopic level, the adhesive with high viscosity and poor fluidity did not form a compact interface with the nickel. The ultrasonic application could promote the filling of the adhesive in irregular micro-scale pits on the surface, thereby strengthening the mechanical anchoring effect. Furthermore, the ultrasonic application produced dynamic impingement at the interface, enhancing the contact between the adhesive and the nickel plate. The adhesive molecules could fully collide and react with the active functional groups introduced on the nickel surface to form more chemical bonds, thus effectively improving the bonding strength of the CFRP/Ni joints.     

Nonlinear propagation effects on broadband attenuation measurements and its implications for ultrasonic tissue characterization.

A study is presented in which the influence of the pressure amplitude of the incident pulse on the estimated frequency dependency of the attenuation coefficient is shown. First, the effect is demonstrated with a simple theoretical model for both transmission and reflection measurements. Simulations show that for both measurement techniques a high-amplitude incident pulse results in a biased estimate of the attenuation coefficient due to nonlinear interaction of the different frequency components of the incident pulse. It is shown that in transmission and reflection measurements the biases have opposite signs. The effect of bandwidth, central frequency, and phase of the incident pulse on this bias is investigated. Second, the effect is demonstrated both in vitro, using a broadband through-transmission substitution technique on a tissue mimicking gelatine phantom, and in vivo, using reflection measurements with standard clinical equipment. The experimental results agree well with the theoretical model. The relevance of this study for ultrasonic tissue characterization is shown.     

Interfacial adhesion between semi-crystalline polymers (polypropylene and nylon-6): in situ compatibilized interface and fracture mechanism

The interfacial fracture toughness between semi-crystalline polymers (polyamide/polypropylene) were studied to understand the failure mechanisms at the interface, especially when the interface was reinforced by an in situ compatibilizer. Based on the observation of the interface using scanning electron microscopy and wide angle X-ray spectroscopy, it was revealed that crystalline structure of polypropylene was not affected by the in situ compatibilizer at the interface. The reinforcing mechanism could be qualitatively identified by investigating the evolution of fracture toughness as a function of annealing time and temperature. The adhesion strength increased with the annealing time. Depending on the annealing temperature, the fracture toughness passed a peak value and then reached a plateau after some bonding time. As long as the chain length of the compatibilizer is long enough to form entanglements with the molecules at both bulk sides, the fracture at the interface is decided by the balance between adhesion strength at the interface and cohesive strength in the weak modulus side; the failure locus follows the lower one. Thus, adhesive failure occurred first when the reaction at the interface did not occur long enough to provide high adhesive strength at the interface, but the cohesive failure occurred in the crack propagation side after the adhesive strength value became higher than the cohesive strength value.     

Inverse ultrasonic determination of imperfect interfaces and bulk properties of a layer between two solids

A method for determination of the complete set of physical, geometrical, and interfacial properties of an isotropic layer embedded between two known solids is discussed. These properties are: Lamé elastic moduli, density and thickness of the layer, and complex normal and transverse interfacial stiffnesses between the layer and the substrates. The properties are combined in the form of eight nondimensional parameters, which are determined from experimental reflection spectra at two incident angles: normal and oblique. The conditions for simultaneous determination of bulk layer properties and the interface normal and transverse springs with losses and the stability of the inversion method against data scatter are addressed. The inversion model is validated by experiment on normal and angular ultrasonic reflectivity from a layer between two semispaces in dry mechanical contact and from an environmentally degraded adhesive joint. The layer properties were measured independently, showing good agreement with the reconstructed results.