Embedded piezoelectric sensors to measure peel stresses in adhesive joints

Although peel stresses are believed to be responsible for failure in many adhesive joint geometries, the measurement of these peel stresses has been elusive. In this work, embedded poly(vinylidene fluoride) piezoelectric sensors were used to measure peel stresses in adhesively bonded joints. Piezoelectric KYNAR^® film was etched to produce multi-area stress sensors which were bonded into adhesive joints. Calibration results and results for single-lap and elastomeric butt joints are presented. The elastomeric butt joint was compared to an analytical solution for the bond-normal stresses, and the single-lap joint results were compared to finite-element analysis. Promising features and liminations of this technique are discussed.     

Development of an array of pressure sensors with PVDF film

 A new type of an array of pressure sensors, composed of PVDF (polyvinylidenefluoride), was devised and evaluated. In order to obtain the system transfer function of the PVDF system, a dynamic calibration was performed utilizing the signal from a 1/8 inch B&K microphone as input. The time history of the unsteady pressure was then reconstructed from the output of the sensor by using this transfer function. The reconstructed pressure signals showed good agreement with the reference signal in both temporal and spectral sense. This sensor was then used to measure the wall pressure fluctuations in a two-dimensional turbulent channel flow. Various statistical moments were obtained from the measurement data set. Comparison of these quantities with the existing studies demonstrated satisfactory agreement. These tests give credence to the relevance and reliability of this sensor for applications in more complicated turbulent flows. Received: 13 August 1996 / Accepted: 26 January 1998     

基于PVDF薄膜传感器的猪肝动态压缩力学性能测试

瞬态冲击载荷作用下肝脏的力学响应是损伤生物力学的重要研究内容.本文提出了一种可用于软组织动态压缩力学特性测试的改进SHPB(分离式霍普金森压杆,Split Hopkinson Pressure Bar)方法.该方法采用PVDF(聚偏氟乙烯,Polyvinylidene Fluor)压电薄膜传感器测量实验过程中试件两端面...     

Mechanics of adhesive joints

Summary Taking into consideration both the pulling and bending actions of the external force, the differential equations for bending of a partly attached tape in a peeling test have been derived. The equations relating the peeling load to the adhesive force were derived under the assumption that the peeling may proceed step by step from the attached end when the adhesive force is overcome by either the tensile stress along the interface (shearing peeling) or that which is perpendicular to the interface (tensile peeling). To verify the validity of the obtained equations, the dependence of the peeling load P on the angle between the direction of the action of the load and the adhering surface has been investigated using plasticized polymer films. In view of the elementary mechanics, the results were satisfactory, while a modification was attempted by introducing the stress concentration factor.     

Finite Fracture Mechanics model for mixed mode fracture in adhesive joints

Up to now the failure load assessment of bonded joints is still not fully understood. This work provides a new approach for assessing the crack initiation load of bonded joints. A failure model for single lap joints is proposed that is based on Finite Fracture Mechanics. Only two basic fracture parameters are required: the tensile strength and the fracture toughness of the adhesive. A coupled stress and energy criterion proposed in 2002 by Leguillon is used to model crack initiation in the adhesive layer. The theory of this criterion is outlined in detail, its relationship to other failure criteria is discussed and an overview of applications found in literature is given. An enhanced weak interface model that predicts a linear variation of the shear stresses in the adhesive layer is utilized to model the single lap joint. To compare joint designs and to reveal the limitations of the given approach a dimensionless brittleness number for mixed-mode loading is proposed. Along with a detailed discussion of the results for exemplary joint designs a comparison to experimental results from literature is performed. The two necessary fracture parameters are each taken from standard test results published in literature. A good agreement of the failure load predictions with the experimental results is observed. A remarkable outcome is that the presented failure model renders the adhesive thickness effect correctly. The paper concludes with a discussion of the limitations of the approach and the effect of material parameters.     

An elastic–plastic interface constitutive model: application to adhesive joints

An interface constitutive model which accounts for both reversible elastic, as well as irreversible inelastic separation-sliding deformations at the interface prior to failure has been developed and implemented in a finite-element computer program. A set of experiments has been conducted to determine the constitutive parameters in the interface model for adhesively-bonded components made from an aluminum alloy and a ductile polymeric adhesive. The constitutive model and the computational capability are shown to reasonably well predict the macroscopic deformed geometries and the load–displacement curves in: (a) T-peel experiments; (b) four-point bend experiments on bonded bi-layer edge-notch specimens; (c) lap-shear experiments.     

Behavior of adhesive joints in corrosive environment

An experimental study was undertaken to determine the effect of corrosive environments on the strength of adhesive joints between pairs of glass-fiber-reinforced-plastic (GRP) plates and plates of GRP bonded to aluminum (Al). Two corrosive agents, each in three concentrations, were used: sulphuric acid (30, 50 and 60 percent) and sodium chloride (15, 20 and 25 percent). The results indicate a loss of strength with immersion time that levels off after a certain time. This loss of strength was more severe for the sulphuric acid than for sodium chloride. Ultrasonic attenuation of the adhesive joints was also monitored and it was found to increase with the immersion time, suggesting that the corrosion created inhomogenieties in the bond. The loss of strength and the increase in ultrasonic attenuation were both greater for greater concentrations of each of the corrosive agents.     

A new technique for ultrasonic nondestructive evaluation of adhesive joints: Part II. Experiment

Transmission of a plane longitudinal ultrasonic wave through a lap joint immersed in water has been measured using piezoelectric transducers. The comparison between the measurements and theory developed in Part I was found to be extremely good for all five joints tested. Some preliminary work concerning the inverse problem was also carried out. The sensivity of the transmission transfer function to various joint parameters was studied in detail. These results indicate that using frequencies less than 10 MHz it should be possible to deduce the joint parameters from the measured transfer function to the following precision: ±10 m in adhesive and adherend thickness, ±0.1 percent in the adherend wave speed, and ±5 percent in the adhesive wave speed.Paper was presented at the 1991 SEM Spring Conference on Experimental Mechanics held in Wilwaukee, WI on June 9–13, 1991.     

Structural mode filtering of a discrete-parameter system using PVDF sensors

This study presents a modal filtering approach to separate the structural modes of a high-rise structure using shaped film sensors. A building-like model structure is constructed to realize the proposed model filtering approach. Since it is difficult to define a modal shape function for discrete-parameter structures, analytical, numerical, and experimental approaches are discussed to determine the shape of the sensor. In experiment, the first structural mode of a high-rise structure is successfully filtered and the robustness of the sensor output against parameter uncertainty is tested Published in Prikladnaya Mekhanika, Vol. 42, No. 2, pp. 136–142, February 2006.     

A new technique for ultrasonic nondestructive evaluation of adhesive joints: Part I. Theory

In this paper we proposed a new technique for ultrasonic nondestructive evaluation (UNDE) of adhesively bonded joints. We report an exact solution to the problem of reflection and tarnsmission of a plane, time-harmonic, longitudinal wave through anN-layered medium. The solution is valid for perfectly elastic as well as linear-viscoelastic materials, and for isotropic as well as anisotropic materials (for example, fiber-reinforced composite) so long as the wavepropagation vector coincids with one of the material coordinates. The transfer function,H ^*() is defined as the trans-mitted (or reflected) field normalized with respect to the incident field. A closed-form solution forH ^*() for the case of an adhesive joint (consisting of two adherends joined by an adhesive layer) immersed in an elastic fluid is derived. A detailed analysis of the sensitivity ofH ^*() to the wave speed and thickness of the adherends and the adhesive is carried out. An experimental verification of the analysis is the subject of Part II of this paper.Paper was presented at the 1991 SEM Spring Confernce on Experimental Mechanics held in Milwaukee, WI on June 9–13.     

Fully-coupled nonlinear analysis of single lap adhesive joints

This paper presents novel closed-form and accurate solutions for the edge moment factor and adhesive stresses for single lap adhesive bonded joints. In the present analysis of single lap joints, both large deflections of adherends and adhesive shear and peel strains are taken into account in the formulation of two sets of nonlinear governing equations for both longitudinal and transverse deflections of adherends. Closed-form solutions for the edge moment factor and the adhesive stresses are obtained by solving the two sets of fully-coupled nonlinear governing equations. Simplified and accurate formula for the edge moment factor is also derived via an approximation process. A comprehensive numerical validation was conducted by comparing the present solutions and those developed by Goland and Reissner, Hart-Smith and Oplinger with the results of nonlinear finite element analyses. Numerical results demonstrate that the present solutions for the edge moment factor (including the simplified formula) and the adhesive stresses appear to be the best as they agree extremely well with the finite element analysis results for all ranges of material and geometrical parameters.     

压电复合材料粘接界面断裂有限元模拟

根据数字化FRMM(Fix-Ratio Mix-Mode)断裂试验,得到了压电复合材料试件的断裂韧性和位移及应变场。本文在试验的基础上,通过非线性有限元软件ABAQUS及用户子程序UMAT进行了模拟分析,采用基于损伤力学的粘聚区模型(CZM)对压电复合材料界面的起裂和脱胶扩展进行了分析,并与VCCT方法进行了比较。计算得到的荷载位移曲线更接近于试验结果,但在裂纹扩展路径上的吻合需要对粘聚区法则进一步修正。通过进一步对CZM参数进行分析,表明界面粘结强度和界面刚度对计算结果的影响很大。研究结果表明,粘聚区模型可以很好地表征压电复合材料弱粘接界面脱胶断裂问题。