Non destructive evaluation of adhesively bonded composite structures using high frequency dielectric spectroscopy

Over the last ten years, the application of high frequency dielectric techniques for the assessment of adhesively bonded structures has been investigated. The technique has been used for the study of adhesively bonded aluminium structures and its application to carbon fibre reinforced plastic (CFRP) bonded structures forms the basis of this paper. The electrical conductivity of the carbon fibres in the CFRP composite materials is sufficiently high for adhesively bonded structures to exhibit the properties of a wave-guide. The non-conductive adhesive behaves as a dielectric. The time domain data allows the integrity of the structure to be explored and is sensitive to the orientation of the fibres at the adherent-adhesive interface. Furthermore, a good correlation is shown between time domain dielectric spectroscopy and gravimetric results. This study indicates that the success obtained in the application of high frequency dielectric measurements to adhesively bonded aluminium structures is also applicable to CFRP bonded structures. The dielectric studies not only indicate a new way to assess the state of such a structure but also are producing new insight into the application of dielectric time domain response (TDR) measurement to non-isotropic materials.     

Ultrasonic Study of Environmental Damage Initiation and Evolution in Adhesive Joints

This article reports on an experimental study of environmental degradation of adhesive joints by an ultrasonic angle—beam technique. The technique is based on measurements of the frequency response of the reflection of obliquely incident ultrasonic signals from a joint bondline. Ultrasonic measurements were performed using a special ultrasonic goniometer with only one ultrasonic transducer. By this method, the degradation of single lap adhesive joints was studied as a function of exposure in NaCl solutions at 68°C under static tensile load. It was found that joint degradation is accompanied by a shift of the ultrasonic reflection spectrum minimum to a lower frequency. Two stages of adhesive joint environmental degradation can be distinguished: a) a relatively slow adhesive joint degradation dominated by adhesive creep, and b) delamination along the adhesive/adherend interface, leading to failure. Several degradation mechanisms are found in the first stage to affect the position of the spectral minimum. The first is adhesive creep caused by normal-to-bond-plane stress concentration at the joint overlap edges. This mechanism is found to have the dominant effect on the ultrasonic signature. Second, changes of the effective density and elastic moduli of the adhesive layer also affect the spectrum of the reflected signal. The third mechanism is the degradation of the adhesive—adherend interface. In the second stage of the joint degradation process, delamination along the adhesive/adherend interface occurs and is followed by joint failure. While the time span of the first stage changes significantly from joint to joint, the time span of the second stage (failure by delamination) in our conditions is about 30—40 h. The delamination results in a significant additional spectral minimum shift to a lower frequency that can be used as an indicator of failure initiation.     

超声强化氨基化多壁碳纳米管改性环氧黏接接头性能研究

目的 探究超声处理对氨基化多壁碳纳米管(MWCNTs–NH₂)改性环氧黏接接头黏接性能和热稳定性的影响,为强化MWCNTs–NH₂改性环氧胶黏剂与铝合金的黏接提供参考。方法 通过机械搅拌与声波破碎的方法将质量分数为0.75%的MWCNTs–NH₂添加到环氧胶黏剂基体中,使用MWCNTs–NH₂改性环氧胶黏剂制备铝合金黏接接头,基于超声辅助黏接工艺在铝合金黏接过程中进行超声处理。通过傅里叶变换红外光谱仪(FTIR)分析MWCNTs–NH₂改性环氧胶黏剂基体官能团的变化情况。采用单搭接剪切强度试验测定黏接接头的拉伸剪切强度。通过扫描电子显微镜(Scanning Electron Microscope,SEM)观察黏接接头拉伸失效断面以及铝合金与胶黏剂间的黏接界面。通过热失重分析仪(TGA)测试并记录胶黏剂试样质量随温度变化的曲线。结果 经超声处理后,MWCNTs–NH₂与树脂基体间的化学反应增强。与纯环氧黏接接头相比,超声处理后的MWCNTs–NH_2...     

Polyphenylethersulfone adhesive for EPDM elastomer-to-stainless steel joints in a hydrothermal environment

Hot-melt polyphenylethersulfone (PES) adhesive was used to link ethylene-propylenediene terpolymer (EPDM) elastomer, which can protect geothermal drill pipes, to a stainless steel (SS) substrate.These joints then were exposed for up to 70 days in steam at 200 °C to evaluate the durability of the bond. Although the preparation and modification of EPDM and SS surfaces played an important role in ensuring a strong bond in the EPDM-to-SS adhesive joint system, the susceptibility of the PES adhesive to hydrothermal oxidation led to conformational transformation of its sulfone group into a fragmental sulfonic acid derivative during the exposure.Moreover, a prolonged exposure time caused the decomposition of polyphenylethersulfonic acid derivative, forming two additional derivatives, aryl radical and H2SO4. Hot H2SO4 favorably reacted with Fe in the SS to yield a water-soluble Fe2(SO4)3 reaction product. This reaction product generated at the interfaces between PES and SS caused a decrease in peel strength. In fact, the loss of adhesion occurred in the SS adjacent to the PES.     

Effect of filler content on the dielectric properties of anisotropic conductive adhesives materials for high-frequency flip–chip interconnection

The dielectric property of anisotropic conductive film (ACF) as an interconnect materials in the flip–chip joints is becoming important concern for device packaging solution at high-frequency due to low parasitic effect on the signal transfer. The effects of non-conductive, dielectric filler content on dielectric properties of ACA materials, like dielectric constant, loss factor and loss tangent, and conductivity at high-frequency were investigated. Frequency is dominating factor in determining dielectric constant, loss factor, and conductivity. However, the filler content is dominant only on dielectric constant, not on the loss factor, and conductivity at low-frequency range. The effect of low dielectric constant (low-k) filler addition on high-frequency behavior of ACF interconnection in flip–chip assembly was also investigated. Impedance parameters of low-k ACF with Ni filler and low-k SiO₂ filler extracted from measurement were compared with that of conventional ACF with only Ni filler. The resonant frequency of conventional ACF flip–chip interconnect was 13 GHz, while the resonant frequency of low-k ACF including low-k SiO₂ filler was found at 15 GHz. This difference is originated from capacitance decrease of polymer matrix between bump and substrate pad due to change in dielectric constant of polymer matrix, which was verified by measurement-based modeling. The high-frequency property of the conductive adhesive flip–chip joint, such as resonant frequency can be enhanced by low-k polymer matrix.     

Cohesive failure of adhesive joints

Abstract

Present interest in adhesives is intense, since in many ways they provide the ideal means of joining components, especially those assembled by robot techniques. In this paper, it is shown how the measured strength of an adhesive joint reflects the interaction between the properties of substrate, adhesive, and the interface between them. Work is discussed which shows how the peel strengths of model systems have been varied by both altering the interfacial bonding and changing the mechanical properties of the adhesive. Appropriate changes to surface topography as well as the introduction into the adhesive of second ‘phases’, which may be hard or soft, or even bubbles, can produce significant joint toughening in a range of adhesive joints. For the future, improved prediction of joint service properties from design data is to be expected. This will come from the further application to adhesive joints of fracture mechanics, time–temperature superposition techniques, slipline field theory, and finite element analysis.

MST/723     

Structural behavior of double-lap shear adhesive joints with metal substrates under humid conditions

Structural adhesive bonding is very often used joining method in aerospace and automotive industry, but in civil engineering, especially in façade applications, semi-flexible or semi-rigid adhesives are still rarely used. The article is focused on experimental analyses of structural adhesive joints intended for façade applications (e.g. bonding of façade cladding elements to the supporting substructure). The experimental study contains a comparison of the structural behavior of two different adhesives in joints with aluminum or zinc-electroplated steel substrates with various surface pre-treatments. The main goal of the study is a comparison of the mechanical properties of joints exposed and unexposed to laboratory ageing conditions; immersion on demineralized water according to ETAG 002 (Guideline for European Technical Approval for Structural Sealant Glazing Kits). Water content in adhesive layer can change significantly its mechanical properties and adhesion of glue to the substrate. Ageing resistance of joint can be improved by durability increasing of the substrate. For this reason, two different substrate materials with various surface treatments (mechanical roughening, smooth surface, anodizing) were tested. Different adhesive resistance against humid conditions was observed depending on the substrate material and pre-treatment. STP polymer joints showed strength reduction by 30% after immersion for almost all substrates, while acrylate adhesive proved 20% strength reduction for roughened aluminum substrate and 60% strength reduction for zinc-electroplated steel substrate with a roughened surface. The zinc-electroplated steel substrate showed problematic adhesion in case of the acrylate adhesive both reference set of specimens and specimens exposed to laboratory ageing. The positive effect of roughening on adhesion and ageing resistance was clearly observed in the specimens bonded by the acrylate adhesive.

     

Fatigue performance of metallic reverse-bent joints

Adhesively bonded lap shear joints have been investigated widely and several ideas have been proposed for improving joint strength by reducing bondline stress concentrations. These include application of adhesive fillets at the overlap ends and use of adhesive with graded properties in the overlap area. Another, less common, approach is to deform the substrates in the overlap area in order to obtain a more desirable bondline stress distribution.
Previous work carried out by the authors on a number of different substrate materials indicated that a reverse-bent joint geometry is useful for increasing joint strength. Results from static stress analysis and experimental testing demonstrated that significant improvements could be achieved. This paper presents results of further work carried out to assess the fatigue performance of reverse-bent joints. Substrates with different yield and plastic deformation characteristics were used and the effects of different overlap lengths on strength were examined. The results of this research show that the improvements obtained under static tests conditions translate to even higher benefits in fatigue. The paper also explains the failure mechanism of the joints under fatigue loading.     

Ultrasonic evaluation of environmental durability of adhesive joints

An angle-beam ultrasonic technique is used to investigate the interfacial properties of lap shear adhesive joints subjected to severe environmental degradation under load. The mode of failure during the process of degradation changes from cohesive (through the bulk adhesive) to predominantly interfacial (along the adhesive/adherent interface). It is found that the minimum shift in the frequency spectrum of the reflected ultrasonic signal is more sensitive to changes of properties in the interphasial region than in the bulk adhesive. The shift of the minimum to lower frequencies is related to the fraction of interfacial failure area and to the joint strength. This could lead to a methodology for predicting the residual lifetime of the joint in service. The joint degradation is described by a mathematical model of an adhesive joint with nonhomogeneous interphase structure. Reasonable agreement between the experimental results and computer simulations on the basis of the proposed model is obtained.     

Durability of metal joints bonded with aluminium powder filled epoxy adhesive

Abstract

Durability of the metal joints bonded with aluminium powder filled epoxy adhesive was investigated by measuring the joint strength by the single lap shear test before and after exposure to distilled water and the hot and humid Arabian Gulf atmosphere. Fractured specimens were examined by photography. The epoxy adhesive retained its strength with as much as 50 wt-% addition of aluminium filler. Moreover, varying the Al filler content in the adhesive did not have a significant effect on adhesive behaviour in either of the two environments studied. Exposure to atmosphere for as long as 6 months did not cause a deterioration of strength for metal joints bonded with aluminium powder filled epoxy. They failed almost completely within the adhesive, similar to the cohesive mode of unexposed specimens. However, a significant strength decrease was observed in adhesive joints after exposure to distilled water for 6 months. The joints failed in more than a single mode. The interior part of the adhesive lap area failed in cohesive mode while an adhesion failure mode was observed near the edges of the adhesive lap area, which is believed to be a result of moisture diffusion through the edges.     

胶粘剂在粘接接头内热分解动力学的计算

为了研究粘接接头内胶粘剂的耐热性能,采用X射线能谱分析确定了不同条件下粘接接头内胶粘剂的元素组成及其变化行为,利用X射线能谱计算了胶粘剂的热失重率,进而计算出聚酰亚胺薄膜粘接接头内胶粘剂的热分解动力学,并与空气环境下胶粘剂热分解活化能进行了比较.计算结果表明,粘接接头内胶粘剂的热分解速率低于空气环境下胶粘剂热分解速率,这种分析测试方法为原位表征粘接接头内胶粘剂耐热性能提供了一种新的分析方法.     

Impact of Frequency of Load Changes in Fatigue Tests on the Temperature of the Modified Polymer

In the article, the authors describe the analysis of the impact of the frequency of fatigue tests on the temperature of the modified polymer. The base material for producing the samples was the epoxy resin Epidian 57 cured with Z1 hardener. The presented study is part of a research program of adhesive joints made with the composition Epidian 57/Z1 aiming to determine the effect of physical modification of the adhesive composition on its properties and on the properties of adhesive joints made with such a modified composition. The tested adhesive compositions were modified by additions of micro- and nanoparticles in an amount of 1.85 % (micro- and nanoparticles) or 10 % (microparticles) in weight, depending on the type of the particles. In the studies, the authors used the particles of tungsten, microspheres and carbon nanotubes. The polymer samples produced by casting were loaded with compressive identical, one-sided loads at two different frequencies of load changes. During the tests, the authors recorded the temperature changes as a function of the number of cycles. The changes in the temperature field on the surface of the samples during the tests were observed by the infrared camera. As a result of the studies, it was possible to observe a significant impact of the composition of the polymer and the frequency of load changes during the test on the temperature of the sample.     

CONSIDERING ENVIRONMENTAL CONDITIONS IN THE DESIGN OF BONDED STRUCTURES: A FRACTURE MECHANICS APPROACH

The continued airworthiness of ageing aircraft and long-term durability of new airframes depends, in part, on the integrity of adhesive bonds used for repairs and joining structural components. Additionally, the advent of composite materials and advanced repair techniques incorporating composites has increased the number of adhesively bonded joints specified for use in aerospace structures. Traditionally, adhesive bonds have been analysed and designed using a dependable and rigorous stress-based approach. However, the need to address the effect of bondline flaws and to understand the fatigue characteristics of bonded joints has led to the adoption of a discipline already common in the design of metallic components—fracture mechanics. To understand the durability of bonded structures, however, it is further necessary to examine the effect of environmental exposure on the performance of the adhesive bondline. Familiarity with the stress-based and fracture mechanics analytical approaches, as well as an understanding of environmentally induced trends in bond performance is paramount to quality design. This paper will briefly discuss the attributes of the two main forms of bonded joint analysis, and will broadly outline a design approach that uses fracture mechanics and accounts for environmental effects. Experiments discussed in this paper were performed specifically to use fracture mechanics in assessing the environmental effects on a toughened epoxy adhesive. Results indicate that the Mode I fracture toughness and fatigue crack growth threshold of this adhesive are significantly reduced upon exposure to a high temperature, high humidity aircraft service environment. These results will be used to illustrate the philosophical arguments supporting the design approach.     

Mode II fracture energy characterization of brittle adhesives using compliance calibration method

The end‐notched flexure (ENF) test is widely used for measuring the Mode II critical strain energy release rate of adhesively bonded joints (ABJs). Unstable crack growth in ENF joints with brittle adhesives is a common phenomenon. Classic data reduction methods like the direct beam theory (DBT) and the compliance‐based beam method (CBBM) usually result in unacceptable scatter when crack grows unstable. In this study, the application of a compliance calibration method (CCM) for ENF adhesive joints with a brittle adhesive is experimentally investigated. For this purpose, ENF specimens were manufactured and tested. Different data reduction methods were considered for treating the results. Afterwards, the obtained fracture energies were used as an input parameter in a finite element (FE) analysis with a cohesive zone model to evaluate the validity of the experimental data. It is shown that the fracture loads obtained by the CCM have the best agreement with the experimental ones comparing with the other data reduction approaches. To study the effect of geometry on the CCM results, ENF specimens with different adhesive thicknesses, substrate thicknesses and span lengths were also considered in this study, and some general conclusions are made about the geometrical parameters effect on the Mode II fracture energy.     

Enhanced reliability of high-density wiring (HDW) substrates through new base substrate and dielectric materials

Flame retardant glass/epoxy composite (FR4) has been extensively used as a substrate material for microelectronic packaging due to its cost effectiveness and overall performance. However, to be able to fabricate high-density wiring with microvias, and embed capacitors, inductors, resistors, and RF and optoelectronic waveguides into a single substrate, we need materials other than FR4 as a base substrate to meet the stringent warpage requirements during fabrication. Typically, these base substrate materials should have a high modulus and good planarity in addition to having a coefficient of thermal expansion (CTE) that is close to that of silicon so that flip-chips can be attached directly to the substrate without the need for an underfill. Although low-CTE and high modulus base substrate materials can result in low warpage and can eliminate the need for an underfill, they can potentially cause delamination and cracking in the interlayer dielectric. This is due to the high CTE mismatch between the base substrate and a typical polymer dielectric. This paper aims to explore a combination of a base substrate material and an interlayer dielectric material such that the warpage is minimal, the dielectric will not crack or delaminate, and the flip-chip solder joints, assembled without an underfill, will not crack prematurely during qualification regimes or operating conditions. Non-linear finite element models with a design-of-simulations approach are used in arriving at optimized thermo-mechanical properties for the base substrate and the dielectric materials to enhance the overall reliability of the integrated substrate with flip-chip assembly. It is seen that an aluminum nitride base substrate with resin-coated foil C provides the best combination against dielectric cracking, high warpage, and solder joint fatigue. The results from the models have also been validated with experimental data.     

Statistical pattern recognition techniques: A sample problem of ultrasonic determination of interfacial weakness in adhesive joints

A pattern recognition approach was applied to the analysis of ultrasonic echo signals from two classes of aluminum-to-aluminum adhesive bonds. The two classes differed in the surface preparation of the adherends prior to bonding, resulting in different interfacial properties of the joints. These properties have a crucial effect on the long-term adhesive properties of the specimens. Application of advanced signal processing and pattern recognition techniques enabled the classification of the joints according to the surface preparation of the adherends, based on features extracted from the ultrasonic signals. The statistics yielded an upper bound for the probability of mis-classification of the specimens. The sensitivity of certain features, extracted from the ultrasonic signal, to the interfacial characteristics of the specimens is explained by means of the natural frequencies of a joint's components and surface condition of the adherends. This leads to a method for selecting the optimal probe frequency for carrying out the ultrasonic inspection.     

Application of interface finite elements to three-dimensional progressive failure analysis of adhesive joints

ABSTRACT The paper presents a new model for three‐dimensional progressive failure analysis of adhesive joints. The method uses interface elements and includes a damage model to simulate progressive debonding. The interface finite elements are placed between the adherents and the adhesive. The damage model is based on the indirect use of fracture mechanics and allows the simulation of the initiation and growth of damage at the interfaces without considering the presence of initial flaws. The application of the model to single lap joints is presented. Experimental tests were performed in aluminium/epoxy adhesive joints. Linear elastic and elastoplastic analyses were performed and the predicted failure load for the elastoplastic case agrees with experimental results.     

板状粘接结构中对称和反对称纵波与界面的相互作用

为了激发粘接结构中的导波或界面波,通常需要将声波从两半无限介质同相位或反相位地同时入射多层系统。针对此问题,基于矩阵方法,推导了界面处于理想连接的情况下,对称或反对称纵波从上下半无限空间入射时,三层板状粘接结构中纵、横波的反射与透射系数表达式。分析了入射角度、粘接层厚度以及基体材料等对声波反射(或透射)特性的影响。结果表明,对称或反对称纵波垂直入射时不发生波型转换。粘接结构中声波的反射(或透射)特性与入射角度、频率以及粘接层厚度等参数密切相关。在相同的粘接层厚度(或频率)范围内,随着声波频率(或粘接层厚度)的增加,谐振频率曲线向低频漂移。该方法可作为粘接结构中体波或导波传播特性研究的重要理论基础。     

Experimental Investigation of Oblique Impact Behavior of Adhesively Bonded Composite Single-Lap Joints

Applied Composite Materials - Determining the impact behavior of adhesive joints allows the designing of high-strength joints. Therefore, the dynamic behavior of adhesive joints has recently become...     

Dielectric spectroscopy — an analytical tool for monitoring the deterioration due to high energy radiation in ethylene propylene diene terpolymer (EPDM) vulcanizates

This paper demonstrates the use of a dielectric spectroscopy technique for monitoring the high energy radiation ageing and stabilization of ethylene propylene diene terpolymer vulcanizates. The measurement of dielectric constants and dielectric losses in the frequency domain help to quantify the physico-chemical changes in the bulk due to high energy irradiation. It is found that the relaxation time calculated using an empirical approach is exponentially related to the total exposed dose. A quantitative comparison of different antirads with a control vulcanizate has been demonstrated by using parameters defined as relative efficiency indices (REI) and relative stability indices (RSI). Three types of REI parameters are defined and computed from dielectric measurements after irradiation over a period of time at a fixed frequency. These REI values are a good estimate of antirad efficiency for energy scavenging and transient charge scavenging. Two types of RSI parameters are defined and computed from the frequency scans of dielectric properties for a material exposed to various doses of γ irradiation. These RSI values provide a good estimate of the rate and extent of changes in the physico-chemical structure due to irradiation. A third type of RSI, computed from equilibrium swelling measurements, closely matches that obtained from dielectric data. It is concluded that the deterioration is predominantly created by a cross-linking reaction even after the addition of antirads. It is observed that polymeric antirads provide the highest efficiency stability. This revised version was published online in November 2006 with corrections to the Cover Date.