Bonds In Adhesive Joints Research Articles

Contact acoustic nonlinearity and local damage resonance for the detection of kissing bonds in structural adhesive joints

Adhesively bonded joints are susceptible to contamination of surfaces during manufacture and environmental deterioration in real operating conditions. These may cause the generation of so-called “kissing bonds” that can dramatically alter the strength of the joint leading to premature failure. Nonlinear acousto-ultrasonic (AU) techniques have shown great potential for monitoring kissing bonds with piezoelectric sensors permanently installed on the structural joint, thus enabling online and in-situ inspection. This paper investigated the combined contact acoustic nonlinearity (CAN) and local damage resonance (LDR) effects in the presence of kissing bonds in adhesive joints. Experimental nonlinear AU tests showed the formation of LDR frequency down-shifts and “jumps” of the fundamental damage resonance in adhesively bonded aluminium joints, with the kissing bond located internally to the overlapping region between the two adherends. These results were supported by a theoretical model based on the solution of the nonlinear Duffing's equation, under the assumption that the debonded region is a damped nonlinear harmonic oscillator subject to harmonic forcing. The Harmonic Balance Method was used to solve the nonlinear differential problem, showing the generation of frequency down-shifts via the dependence of the ratio between the excitation and LDR frequencies with the amplitude of the fundamental damage resonance. Additionally, two-dimensional finite element simulations using a reduced order model based on the Craig-Bampton technique were carried out to support experimental AU tests for the identification of the LDR frequency and the generation of nonlinear resonance effects. Good agreement between analytical, numerical, and experimental results revealed that a monitoring approach combining CAN and LDR is an extremely efficient and sensitive tool for ensuring integrity and safety of structural adhesive joints.

Investigation of Kissing Bonds in Adhesive Joints

Kissing bonds are critical defects in adhesively bonded parts. These defects result in a weak bonding in the interface region between the adherent and the adhesive. Unlike delaminations, kissing bonds induce a delicate change in contact properties. Thus, conventional non-destructive testing methods are not able to detect kissing bonds reliably. In this work various non-destructive testing techniques on kissing bonds were investigated. The manufactured kissing bond containing specimens were investigated with different non-destructive testing methods such as conventional ultrasound, aircoupled ultrasound, x-ray and shearography. Unfortunately, those non-destructive testing methods weren’t able to detect the kissing bonds dependably. On the contrary, the application of nonlinear ultrasound demonstrated the most reliable results in detecting kissing bonds. With this testing setup, the specimens were excited with a piezo shaker in a continuous wave mode and the vibration on the surface of the specimen was detected by laser Doppler vibrometry. After fast Fourier transform (FFT), a dramatic increase in the higher harmonic amplitudes were detected in an area with lower bonding force. The low adhesive forces lead to a higher nonlinearity of the kissing bond area, what results in substantial rise of the higher harmonic amplitudes. The measurements on the kissing bonds with their reduced adhesive force show a substantial rise of higher harmonic amplitudes, which makes it a suitable method for the detection of kissing bonds.

Linear vs nonlinear ultrasonic testing of kissing bonds in adhesive joints

Kissing bonds in adhesive joints are precursors to damage and failure in materials and components used in safety-critical industries. They are zero-volume, low-contrast contact defects widely regarded as “invisible“ in conventional ultrasonic testing. In this study, the recognition of the kissing bonds is examined in automotive industry-relevant aluminum lap-joints with standard bonding procedures using epoxy- and silicone-based adhesives. The protocol to simulate kissing bonds comprised customary surface contaminants PTFE oil and PTFE spray. Preliminary destructive tests revealed brittle fracture of the bonds with typical single-peak stress-strain curves indicating ultimate strength reduction due to adding contaminants. The curves are analyzed by using nonlinear stress-strain relation with the higher-order terms containing the higher-order nonlinearity parameters. It is shown that the lower-strength bonds manifest a high nonlinearity while the high-strength contacts are candidates for a low nonlinearity. Based on that, the nonlinear approach is set side by side with linear ultrasonic testing for experimental locating of the kissing bonds fabricated in the adhesive lap-joints. The sensitivity of the linear ultrasound is shown to be adequate to detect only a substantial bonding force reduction caused by the irregular interface defects in adhesives, while a minor contact softening due to kissing bonds remains undistinguishable. On the contrary, the probing of the kissing bonds vibration with nonlinear laser vibrometry reveals dramatic growth of the higher harmonic amplitudes and thus validates highly-sensitive detectabilty of these troublesome defects.

An efficient technique for extraction of nonlinear dynamic features in a model-based feature extraction scheme for machine learning–based structural health monitoring

Super-harmonic components in response to the harmonic excitation are sensitive indicators of damages such as breathing cracks in beams or kissing bonds in adhesive joints. In a model-based damage identification process using pattern recognition, these damage indicators can be extracted from the finite element model for all probable damage cases using stepped-sine simulation that necessitates nonlinear transient dynamic analysis with high computational costs. In this study, a procedure based on nonlinear autoregressive with exogenous input model is introduced as an alternative shortcut method for extraction of the damage indicators. As a case study, the finite element model of a beam connected to a rigid support via a flexible adhesive layer was used to investigate the efficiency of the proposed method. Kissing bond was introduced to the model as the source of nonlinearity via contact elements. The results prove that the super-harmonic components of orders up to 3, extracted from the nonlinear autoregressive with exogenous input model, agreed well with those extracted directly from the finite element model, whereas the computational time is reduced by a factor of 1/5. Consequently, the proposed method is very advantageous in the stage of damage pattern database creation in a real-world model-based damage identification process based on pattern recognition.

The analysis of kissing bonds in adhesive joints

Kissing bonds pose a serious threat and an obstacle to the application of adhesive bonding in engineering structures. This paper describes an investigation of the formation of kissing bonds using various surface contaminants. Double-lap joints have been prepared using several contaminants, and tested in quasi-static tension. The values of shear strength obtained from these tests are successfully compared with observations of the interfaces and fracture surfaces. Finite element analysis in combination with cohesive elements was used to simulate the adhesive joint tests. Fracture properties used in the analysis were obtained from independent material tests. The results indicate very good agreement between experimental and predicted shear strengths for both control and contaminated joints. The results of the validated finite element model are explored to find a strain ‘signature’ that could be used to detect kissing bonds. The results indicate that measurement of local lateral strain may form the basis of a technique for monitoring or detection of kissing bonds.

Evaluation of kissing bond in composite adhesive lap joints using digital image correlation: Preliminary studies

The assembly of aerospace and automotive structures in recent years is increasingly carried out using adhesives. Adhesive joints have advantages of uniform stress distribution and less stress concentration in the bonded region. Nevertheless, they may suffer due to the presence of defects in bond line and at the interface or due to improper curing process. While defects like voids, cracks and delaminations present in the adhesive bond line may be detected using different NDE methods, interfacial defects in the form of kissing bond may go undetected. Attempts using advanced ultrasonic methods like nonlinear ultrasound and guided wave inspection to detect kissing bond have met with limited success stressing the need for alternate methods. This paper concerns the preliminary studies carried out on detectability of dry contact kissing bonds in adhesive joints using the Digital Image Correlation (DIC) technique. In this attempt, adhesive joint samples containing varied area of kissing bond were prepared using the glass fiber reinforced composite (GFRP) as substrates and epoxy resin as the adhesive layer joining them. The samples were also subjected to conventional and high power ultrasonic inspection. Further, these samples were loaded till failure to determine the bond strength during which digital images were recorded and analyzed using the DIC method. This noncontact method could indicate the existence of kissing bonds at less than 50% failure load. Finite element studies carried out showed a similar trend. Results obtained from these preliminary studies are encouraging and further tests need to be done on a larger set of samples to study experimental uncertainties and scatter associated with the method.

Modelling and measurement of the nonlinear behaviour of kissing bonds in adhesive joints

The non-destructive evaluation of adhesively bonded joints is a subject of significant industrial and academic interest. It is thought that areas of interfacial weakness develop firstly into kissing bonds before finally disbonding. Critically, there is a requirement to detect such defects at the earliest possible stage. Whilst a number of techniques such as high frequency ultrasonic C-scanning are able to robustly detect disbonding, the detection of interfacial weakness has proved elusive. In this paper we focus our attention on the kissing bond state, which we consider to be a bond still in intimate contact, but with zero, or minimal, strength. Various researchers have shown that kissing bonds of this type behave nonlinearly when illuminated with ultrasound and have sought to make this the basis of a non-destructive evaluation procedure. In this paper we use a one-dimensional time domain model that includes an interfacial nonlinearity to predict the interaction of ultrasonic pulses with a kissing bond. With this model we show that the measured nonlinearity is highly dependent on the thickness of the adhesive layer to wavelength ratio. These effects are explained by the observation that the linear dynamics of the multi-layer bonded system act as a series of bandpass filters on the nonlinear signals generated at the kissing interface. It is suggested that, in order to make robust nonlinear measurements, these filtering characteristics should be known and compensated for.

Morphology and Surface Chemistry of Kissing Bonds in Adhesive Joints Produced by Using ElectRelease™ Adhesive

Kissing bonds are interfacial defects in which the substrate and adhesive are in intimate contact or couple through a weak bond or a thin layer of contaminant. In this study, the kissing bonds were produced by applying a low voltage to bonded specimens with an electrically disbonding adhesive, ElectRelease™. The results from field emission scanning electron microscopy (FESEM), combined with energy dispersive X-ray spectroscopy (EDS) analysis, indicated morphological changes that were observed at, or near, the weakened hardened steel/ElectRelease™ interface while chemical changes were determined at the interface and also within the adhesive after the application of a voltage. The adhesive chemistry and its curing mechanisms were better understood using Raman spectroscopy. A shift of a Raman peak to a higher wave number confirmed the results from EDS and could explain the weakening mechanisms at such interface and/or interphase. A considerable reduction in joint strength from mechanical testing for the joints subjected to the electric field was found.

Morphology and surface chemistry of kissing bonds in adhesive joints produced by surface contamination

Kissing bonds (KBs) or zero volume debonds are interfacial defects in which the substrate and adhesive are in intimate contact or couple through a weak bond or thin layer of contaminant. These KBs can dramatically alter the failure strength of the joint causing premature failure. Adhesive bonds containing a layer of Frekote 700NC, artificial sweat and cutting oil lubricant were produced on chromic acid etched aluminium substrates. All the joints with contaminants were examined using analytical field emission scanning electron microscope (FESEM) fitted with energy dispersive X-ray spectrometer (EDS) and also wavelength dispersive spectrometer (WDS). Photoacoustic Fourier transform infrared spectroscopy (FTIR) and mechanical testing were also performed. These methods were used to investigate whether such joints qualified as KBs. FESEM revealed changes in morphology and surface/interface chemistry related to the corresponding contaminants. Elemental distribution from EDS showed migration of artificial sweat and absorption of oil contaminant into the adhesives particularly when curing at elevated temperature while mould release agent (Freekote 700NC) remained at the metal/adhesive interface or within the interphase. FTIR revealed the presence of contaminants including comprehensive information of chemical interaction of the adhesives and contaminants within the interphase. Further understanding of the effect of various contaminants on wetting of contaminated substrates was obtained by contact angle measurement. A direct relationship was discovered amongst the reduction in surface energy of the substrate contaminated with Frekote, morphological changes and a reduction in joint strength, showing that Frekote is a potential contaminant in the production of kissing bonds.

Measurement of the ultrasonic nonlinearity of kissing bonds in adhesive joints

This paper presents a study of the detectability of kissing bonds in adhesive joints using an ultrasonic inspection technique to measure nonlinearity. Kissing bonds are defined as adhesive bonding defects in which the adhesive and the substrate are not fully bonded together but are in some way connected, for example, through contact between disbonded surfaces or through coupling via a very thin layer of contaminant. The result is a region that exhibits reduced normal or shear stiffness and is difficult to detect using conventional non-destructive testing techniques. It is hypothesised that when an ultrasonic wave of large magnitude propagates through the kissing bond area, the system will behave nonlinearly. This hypothesis is the starting point for this paper, and it is tested by measuring the degree of nonlinearity exhibited by perfectly and imperfectly bonded specimens. Perfectly disbonded adhesive joints are tested over a range of compressive loads from an initial zero-load ‘open’ condition to a final highly loaded ‘closed’ condition and the degree of nonlinear behaviour measured. It is shown that adhesive joints exhibit significant nonlinearity under light compressive loads and that the level of nonlinearity decreases rapidly with compressive load. Good agreement is also observed between experimental results and a nonlinear spring model of the adhesive joint, based on its quasi-static interfacial stiffness.

The detectability of kissing bonds in adhesive joints using ultrasonic techniques

This paper concerns a study of the detectability of dry contact kissing bonds in adhesive joints using three ultrasonic inspection techniques. Conventional normal incidence longitudinal and shear wave inspection were conducted on dry contact kissing bonds using a standard damped ultrasonic transducer and an electro-magnetic acoustic transducer (EMAT) respectively. The detectability of the dry contact kissing bonds was assessed by calculating the reflection coefficient of the imperfect interface at varying loads for a number of surface roughnesses. A high power ultrasonic method was also employed to determine the non-linear behavior of the adhesive interface. The non-linearity of the interface was determined by the ratio of the amplitudes of the first harmonic and fundamental frequencies of the transmitted waveform. It was found that the high power technique showed the greatest sensitivity to these kissing bonds at low contact pressures, however at high loads conventional longitudinal wave testing was more sensitive. It was also noted that a combination of two or more techniques could provide enhanced information about the kissing bond compared to a single technique alone.

The Effect of Compressive Loading on the Ultrasonic Detectability of Kissing Bonds in Adhesive Joints

The effect of compressive loading on the ultrasonic detectability of kissing bonds in adhesive joints was studied. Two distinct types of kissing bond were considered. The first type was a dry contacting interface that represented a disbonded adhesive interface subsequently subjected to compressive loading. The second type were manufactured by contaminating the joint with a thin layer of various liquids. The interaction of normal incidence 10-MHz ultrasound with both these kissing bond types has been quantified. In particular the effect of compressive loading on their detectability was investigated. A quasistatic spring model has been used to predict the interaction of ultrasound with the solid-solid interfaces in the dry-contacting kissing bonds. The reasons for the discrepancy between the experiments and the model are discussed.

Ultrasonic detection of kissing bonds in adhesive joints: NDE of adhesive bonds and bondlines, topical proceedings, valley forge, Pennsylvania (United States), 9–13 Oct. 1989. pp. 62–73. ASNT (1989)

Ultrasonic detection of kissing bonds in adhesive joints: NDE of adhesive bonds and bondlines, topical proceedings, valley forge, Pennsylvania (United States), 9–13 Oct. 1989. pp. 62–73. ASNT (1989)