An adhesive bond strength prediction algorithm based on ultrasonic signal analysis

The problem of predicting adhesive bond defects for both surface preparation and undercure defects has been studied using an ultrasonic, experimental test bed system. This experimental test bed incorporates the ultrasonic and computer equipment necessary to acquire and process data from various types of adhesively bonded test specimens. The computer hardware and software have been developed to allow the design of reliable pattern recognition algorithms for the evaluation of surface preparation and bond cure. The specific problem studied is the inspection of the adhesive bond in an aluminium/aluminium step-lap joint whose strength could be affected by improper surface preparation or undercure. A set of 164 bond specimens was used to design an algorithm that is 91% reliable for separating the specimens into a good class, those bonds with no defects, or a week class, bonds with poor surface preparation on an undercured adhesive layer. A Fisher Linear Discriminant function was selected by the test bed as the best pattern recognition routine for this classification problem.     

Fatigue lifetime assessment of adhesive joints by ultrasonic and thermal wave imaging

Various nondestructive evaluation (NDE) methods are frequently employed to inspect the adhesive bonds of aircraft structures in service. The literature on the capability of various NDE techniques reveals a deficiency in linking NDE test parameter characteristics of the frequency or size of defects to critical failure properties such as the lifetime and the strength of adhesive bonds. In this study an attempt has been made to develop such correlations. A specimen geometry was employed so as to permit cleavage-type debonding under fatigue loading. This geometry and loading configuration provide for a simple fatigue testing program and simple analytical methods. Damage by flexural fatigue aging of these adhesively bonded specimens was induced at different intervals of their fatigue lifetime. The specimens were composed of materials that were commonly used in actual aircraft production during the 1970s. Pulse-echo ultrasonic C-scanning and thermal wave imaging were performed to inspect the adhesive joints at various percentages of the fatigue lifetime. A novel low-frequency ultrasonic method was used for making the C-scans; this technique was immune to signal amplitude changes due to interference phenomena caused by bond thickness variation. A direct correlation of the ultrasonic parameter (size of the debonded area) with the percentage lifetime of the adhesive joints was tentatively established. It was also found that this correlation was consistent when the scanning was conducted from either the top surface or the bottom surface of the adhesive joints. A similar correlation between the size of the debonded area and the percentage of fatigue lifetime of the adhesive joint was found using thennal wave imaging. Thus, it appears that the measurements obtained from both techniques are consistent.     

Ultrasonic evaluation of adhesive bond degradation by detection of the onset of nonlinear behavior

Quantitative nondestructive evaluation (QNDE) of the degradation of adhesive bonds remains one of the most challenging problems in QNDE. The objective of this research was to approach this problem by the detection of nonlinearity due to bond deterioration. The paper starts with experimental observations of the reflection of ultrasonic signals by adhesive bonds. The specimens for these tests had been subjected to cyclic loading which was expected to cause bond deterioration. Differences in the reflections could, however, be observed only if the adhesive bonds were subjected to static loads simultaneously with the ultrasonic testing. The higher the number of fatigue cycles, the lower the required load to display the differences in signals with unloaded cases. The second part of the paper presents a theoretical explanation of these ultrasonic measurements based on the postulate of nonlinear stress-strain behavior of the deteriorated bond. The ultrasonic tests provide the slope of the stress-strain curve and the results can therefore be used to determine the deviation of the stress-strain curve from linear behavior. For the higher numbers of fatigue cycles, this deviation, which is indicative of bond deterioration, starts at smaller load values.     

Modeling Concepts for Studying Ultrasonic Wave Interaction with Adhesive Bonds

Recent research has shown that such adhesive bondline defects as chemical segregation, variation in cure, gas entrapment or inadequate surface preparation are responsible for many adhesively bonded structural failures. Analytical models have been developed in this work that can be used to relate these “flaws” to the manner in which they affect the reflection of an ultrasonic pulse from such a bondline. The results of this study provide a substantial resource base for extended research through which ultrasonic inspection can become a reliable NDT technique for bond strength determination.     

Fracture mechanics testing of the bond between composite overlays and a concrete substrate

This paper presents a methodology for assessing the bond strength of composite overlays to concrete utilizing a fracture toughness test. The principles and practices of existing ASTM standards for determining the fracture toughness of adhesive bonds between double cantilever beam (DCB) metallic and composite specimens (D 3433-93 and D 5528-94a) have been extended to cover the case of an elastic composite layer bonded to a rigid concrete/masonry substrate. In the theoretical section, the dominant loading conditions, relevant ASTM standards, and the development of energy release rate concepts for analyzing a disbonding composite layer modeled as an elastic cantilever beam are presented. The experimental section covers specimen fabrication and preparation, experimental setup, test procedures, post-test evaluation of the specimens, and data processing. The discussion of test results focuses on explaining the variability in measured strain energy release rate, and identifies trends between the measured strain energy release rate and the fraction of the fracture surface retaining cement paste after disbonding. It was found that good-quality composite-to-concrete bond is associated with high fracture toughness of the adhesive and location of the crack path in the concrete substrate. Strict enforcement of surface preparation and adhesive handling procedures was found to play an important role in promoting good bond strength and high fracture toughness. The fracture toughness test developed in this paper can be used for screening various composite-repair systems, to assess the effect of different environmental attacks, and as a quality control tool.     

Effect of Pressure Cycling on Fracture Energy of Polyurethane/Aluminum Adhesive Bonds

An experimental study was conducted to investigate the effect of pressure-cycling on adhesive bond fracture energy of polyurethane/aluminum adhesive bond joints. Initially, two types of peel tests were conducted to characterize adhesive bond strength and challenges associated with pre-mature polyurethane cracking and failure during these tests are discussed. A modified double cantilever beam (MDCB) specimen configuration was specially designed and opening-mode loading conditions were employed to determine the interfacial adhesive bond energy (G_C). The test specimens were pressure-cycled in water-filled tanks for 1 to 4 weeks with an increment of 1 week. The G_C of pressure-cycled specimens was compared with both control and water-soaked samples (without pressure-cycling). The results indicated that pressure-cycling decreased G_C values to those of the control and water-soaked samples: hence, prolonged pressure-cycling could be problematic to polymer/metal adhesive bonds of hardware installed outboard of submarine pressure hulls.     

Surface preparation effect on performance of adhesively-bonded graphite/epoxy composites subjected to impact

The influence of pre-bond surface preparation on performance of adhesively-bonded composites subjected to impact was investigated in this study. Impact test was carried out on adhesively bonded graphite/epoxy composite specimens at different low impact energies ranging from 5 to 20 J using the drop-weight impact test. Post-impact ultrasonic evaluation was performed in order to determine the resulting internal damage due to impact on the adhesive bondline. The ultrasonic C-scan of the gated ultrasonic wave signal was acquired and the ensuing debond area in the adhesive bond was determined quantitatively for specimens made from substrates with different surface preparations such as paper peel ply, sandblasting, and sandpaper abrasion. In order to determine the flexural load bearing capacity and stiffness reduction after impact, a three-point bending test was conducted on unimpacted and impacted specimens. A comparative study was performed to evaluate the performance of adhesively-bonded composites with different surface preparations. The results revealed that paper-peel ply performed the best in terms of resistance to debond area formation in the adhesive layer, as well as in terms of retention of flexural load bearing capacity and stiffness after impact.     

Experimental and statistical study of three adherence tests for an epoxy-amine/aluminum alloy system: Pull-Off,Single Lap Joint and Three-Point Bending tests

The mechanical resistance of a bonded joint depends on the adhesive interaction onto the substrate and the mechanical properties of the adhesive itself. Many existing tests can be useful for measuring the adherence or evaluating mechanical adhesive response. All these tests do not provide the same information: in particular, adherence measurements can be split into initiation tests and propagation ones. In this paper, three adherence tests have been considered for the evaluation of the fracture initiation between a poly-epoxide adhesive (a mixture of pure epoxy and amine) and an aluminum surface (AA 2024-T3), namely the Pull-Off, Single Lap Joint (SLJ) and Three-Point Bending tests. Various surface preparation protocols before bonding have been tested and optimized for aluminum substrates, including mechanical and chemical surface treatments, followed by the application of an appropriate primer before bonding. This study paves the way for the future development of adhesive systems as it provides reliable surface preparation protocols for aluminum surfaces and gives an insight into the choice of an adequate adherence test dedicated to high-performance adhesives. The load at break (F_Max), the experimental error, the failure mode and statistical studies according to the Weibull model and Principal Component Analysis (PCA) were studied on each surface preparation configuration. It has been shown that the application of a primer, especially a sol-gel product increases the load at break and provides more reliable results. Then, this paper shows that the two tests can quantify the failure initiation and distinguish the different surface preparation efficiency, are the Single Lap Joint test (mode II or mode I + II) and the Three-Point Bending test (mode I), with an increase of the results reliability with the latter one. The Pull-Off test (mode I) is useful as a routine checking, and particularly interesting because its response does not depend on the substrate thickness, even though it cannot highlight the difference between all surface preparations.     

Improved Durability of Aluminum Adhesive Bonds with Phosphonic Acid Inhibitors

In this paper, we review our work on the use of hydration inhibitors to improve the durability of adhesively-bonded aluminum structures. Treatment of adherends given the Forest Products Laboratory (FPL) surface preparation in a solution of nitrilotris methylene phosphonic acid (NTMP) increases bond durability so that hydration of the adherend is no longer the limiting factor on crack propagation. In bonds with phosphoric-acid-anodized (PAA) adherends treated with NTMP, the transition to failure in the adhesive occurs at a shorter crack length. The initial strength of epoxy adhesive bonds is not changed by such pretreatments. Adsorption of NTMP onto FPL surfaces displaces the water initially present and forms P—O—Al bonds. Saturation coverage is approximately one monolayer. NTMP adsorbed onto PAA surfaces, which have little initial water, occupies residual active sites. Hydration of a PAA surface or an inhibited FPL surface proceeds in three steps: 1) reversible physisorption of water, 2) slow dissolution of the inhibitor-aluminum complex followed by rapid transformation of the freshly-exposed amorphous aluminum oxide to boehmite, and 3) formation of bayerite on the surface. We have identified several important criteria for inhibitors designed to improve bond durability; i.e. they should 1) displace water and occupy all active sites on the Al₂O₃ surface, 2) form strong inhibitor-surface bonds, 3) form a water-insoluble complex with aluminum, 4) be chemically compatible with the adhesive or primer, and 5) couple chemically or mechanically to the adhesive.     

Improved Durability of Aluminum Adhesive Bonds with Phosphonic Acid Inhibitors

In this paper, we review our work on the use of hydration inhibitors to improve the durability of adhesively-bonded aluminum structures. Treatment of adherends given the Forest Products Laboratory (FPL) surface preparation in a solution of nitrilotris methylene phosphonic acid (NTMP) increases bond durability so that hydration of the adherend is no longer the limiting factor on crack propagation. In bonds with phosphoric-acid-anodized (PAA) adherends treated with NTMP, the transition to failure in the adhesive occurs at a shorter crack length. The initial strength of epoxy adhesive bonds is not changed by such pretreatments. Adsorption of NTMP onto FPL surfaces displaces the water initially present and forms P—O—Al bonds. Saturation coverage is approximately one monolayer. NTMP adsorbed onto PAA surfaces, which have little initial water, occupies residual active sites. Hydration of a PAA surface or an inhibited FPL surface proceeds in three steps: 1) reversible physisorption of water, 2) slow dissolution of the inhibitor-aluminum complex followed by rapid transformation of the freshly-exposed amorphous aluminum oxide to boehmite, and 3) formation of bayerite on the surface. We have identified several important criteria for inhibitors designed to improve bond durability; i.e. they should 1) displace water and occupy all active sites on the Al₂O₃ surface, 2) form strong inhibitor-surface bonds, 3) form a water-insoluble complex with aluminum, 4) be chemically compatible with the adhesive or primer, and 5) couple chemically or mechanically to the adhesive.     

Effect of surface exposure time on bonds to aluminum

Lap joints with AF-126 adhesive were prepared from surfaces of 2024-T3, 2024-T3 alclad, and 6061-T4 aluminum alloys treated by either FPL etch, sandblasting, or vapor degreasing. The strength data were described by a two-parameter Weibull distribution. Allowing between 1 hr and 30 days to elapse between surface preparation and actual bonding had no appreciable effect on bond strength. This was true for all three alloys surface treated in each of the three ways as well as for bonds either tested at ambient conditions or aged for 30 days at 120°F and 95% R.H. 2024-T3 aluminum, both bare and alclad, formed bonds that showed better strength than 6061-T4 aluminum.     

Novel surface and interfacial analysis techniques as aids to the development of new, high fracture toughness film adhesives

An experimental film adhesive of high fracture toughness had given a promising range of adhesive properties on pickled 2024-T3 clad aluminum alloy, but a wide variation in peel performance resulted when this adhesive was used to bond anodized aluminum adherends. High resolution scanning electron microscopy/ energy dispersive X-ray analysis of the fracture interfaces coupled with transmission electron microscopy of ultramicrotomed sections through the same specimens has shown that the problem was non-wetting of the substrate on a microscopic scale. The rheological properties of the adhesive system have been altered to overcome the problem, yielding a high fracture toughness film adhesive of consistent performance.     

Ultrasonic Determination of Bond Strength Due to Surface Preparation Variations in an Aluminum-To-Aluminum Adhesive Bond System

This study demonstrates the application and use of analytical models in the experimental ultrasonic evaluation of interface conditions in an Aluminum-FM-47-Aluminum adhesive bond system. The results of the study show that a variation in bond strength due to surface preparation can be detected ultrasonically through careful inspection and signal processing analysis.     

Durability of bimetallic-adhesive laminate with ethylene–acid copolymer adhesives

A broadband multicoaxial cable has been developed, based upon the use of ethylene copolymer thermoplastic adhesives to bond tin-plated steel to copper. The characteristics of the adhesive and the integrity of the bond formed between the dissimilar metals were of prime interest. The copolymers of ethylene examined as adhesives were ethylene–acrylic acid (EAA) and ethylene–methacrylic acid (EMAA). The influence of high humidity on the durability of bimetallic laminates of copper or aluminum with tin-plated steel and ethylene–acid copolymer adhesives has been studied. Infrared spectrometric and differential thermal analysis data had shown that a contributing cause to failure of bonds between EAA copolymers and copper is the solubilization of an underlying weak copper oxide layer under condition of the test. Electrolytic chromate-treated copper and immersion chromate-treated aluminum bonded to degreased tin-plated steel resulted in joints having comparable durability under stress and humid environments. These results reflect the durability of tin–tin bonded specimens. These materials engineering studies have shown that ethylene–acid copolymer adhesive, properly used and stabilized, can be employed with confidence in adhesive joints.     

The Electrochemistry of the FPL (Forest Products Laboratory) Process and its Relationship to the Durability of Structural Adhesive Bonds

The FPL (Forest Products Laboratories) Process for preparing aluminum for structural adhesive bonding has been used in the aerospace industry since the early 1950's. Problems were encountered with the use of the process when the industry changed from phenolic to epoxy adhesives. In-service disbonds followed by corrosion were observed. This review article describes an investigation of the electrochemistry of the FPL etch process. Through a combination of electrochemical polarization measurements, surface chemical and surface morphological investigations and a thorough application of the Boeing wedge test, we are able to provide a mechanism of action of the FPL etch process. An oscillating electrochemical reaction was observed for low alloy aluminum which was ascribed to the dissolution and redeposition of copper on the aluminum during the etching process. A spatial variation in wedge test performance was found in that edge specimens demonstrated lower crack extensions than center specimens when using a low alloy aluminum that was etched in an FPL etch bath that was low in copper. These results are positioned in a historical perspective providing some insight into possible reasons for the irreproducibility of durability of structural adhesive bonds made with low alloy aluminum adherends that were prepared in a low copper content FPL etch bath.     

Metal Alkoxide Primers in Titanium/Epoxy Bonding

Factors influencing the durability of Ti-6Al-4V/metal alkoxide/epoxy interphases were determined by studying the chemical composition of three metal alkoxides and evaluating the bond durability of Ti-6Al-4V/epoxy bonds primed with these materials. The three alkoxides were sec-butyl aluminum alkoxide, tetra-isopropyl titanate and tetra-n-butyl titanate.

Because adhesive bonds made using phosphate fluoride (P/F) pretreated Ti—6Al—4V substrates were not durable, P/F treated Ti—6Al—4V was chosen as the substrate for testing the possible durability enhancement by the titanium and aluminum alkoxide coatings. Sec-butyl aluminum alkoxide significantly enhanced the bond durability of the P/F pretreated bonds, while the titanium alkoxide primers showed no improvement in durability. The locus of failure and infrared studies indicated the enhancement in durability by the aluminum alkoxide was due to the high concentration of hydroxyl groups on the alkoxide surface available to interact with the epoxy adhesive.     

Electrochemical Sensors for Nondestructive Evaluation of Adhesive Bonds

An in-situ corrosion sensor based on electrochemical impedance spectroscopy (EIS) has been used to detect moisture ingress into aluminum-aluminum and aluminum-composite adhesive bonds. Both wedge tests and tensile button tests (aluminum-aluminum bonds only) were performed. Upon moisture absorption, the impedance spectra change shape with the low-frequency region becoming resistive. The low-frequency impedance decreases by several orders of magnitude, depending on the adhesive and the experimental conditions. For bonds with stable interfaces, such as phosphoric acid anodized (PAA) aluminum, the absorbed moisture causes an initial weakening of the adhesive resulting in reduced strength or small crack propagation. A substantial incubation time prior to substrate hydration and bond degradation allows warning of potential joint deterioration and enables condition-based maintenance. For bonds with smooth interfaces with little or no physical bonding (mechanical interlocking), crack propagation can proceed interfacially with minimal moisture absorption. A comparison of the incubation times for Forest Products Laboratory (FPL, or sulfuric acid-sodium dichromate) etched surfaces, both bonded to epoxy adhesives and freely exposed to water or humidity at different temperatures, shows that hydration occurs with the same activation energy and, hence, the same mechanism, independent of whether or not the surface is covered with adhesive. However, the pre-exponential factor in the rate constant is dependent on the concentration of free moisture at the interface so that the hydration rate varies by several orders of magnitude.     

Quality assurance concepts for adhesive bonding of composite aircraft structures – characterisation of adherent surfaces by extended NDT

In order to ensure the performance of adhesively joined load-critical composite structures, suitable technologies are needed to steadily monitor adherent surfaces prior to bonding and to detect adhesion properties of bonded components. A novel class of non-destructive testing (NDT) techniques, classified as extended non-destructive testing (ENDT), is required to ascertain selected physicochemical properties which are important for the performance of adhesive bonds in place of detecting material defects like conventional NDT methods do. The European FP7 project, ‘ENCOMB – Extended non-destructive testing of composite bonds’ aims in the identification, development, adaptation and validation of ENDT methods for characterisation of adherent surfaces and adhesive bond quality. Here, recent NDT techniques such as optically stimulated electron emission (OSEE) and aerosol wetting test (AWT) as well as laser-induced breakdown spectroscopy (LIBS) were advanced and applied in field, and without contacting carbon fibre-reinforced polymer (CFRP) surfaces for detecting different contamination layers such as release agent, moisture or hydraulic oil as well as thermal degradation of CFRP adherent surfaces before performing an adhesive bonding process. Sensitivity and accuracy of these techniques allow distinguishing surface states which are suitable for bonding of CFRP adherents from surface states which are unfavourable for bonding. ENDT using OSEE, AWT and LIBS facilitated the detection of layers of release agent as thin as one nanometre and thin layers resulting from hydraulic oil. OSEE investigations of adherent surfaces before adhesive bonding allowed the indication of all surface states of potential CFRP adherents, which according to previous studies, were related to application scenarios reducing the joint strength of resulting adhesive joints by 20–70%.     

Electrochemical Sensors for Nondestructive Evaluation of Adhesive Bonds

An in-situ corrosion sensor based on electrochemical impedance spectroscopy (EIS) has been used to detect moisture ingress into aluminum-aluminum and aluminum-composite adhesive bonds. Both wedge tests and tensile button tests (aluminum-aluminum bonds only) were performed. Upon moisture absorption, the impedance spectra change shape with the low-frequency region becoming resistive. The low-frequency impedance decreases by several orders of magnitude, depending on the adhesive and the experimental conditions. For bonds with stable interfaces, such as phosphoric acid anodized (PAA) aluminum, the absorbed moisture causes an initial weakening of the adhesive resulting in reduced strength or small crack propagation. A substantial incubation time prior to substrate hydration and bond degradation allows warning of potential joint deterioration and enables condition-based maintenance. For bonds with smooth interfaces with little or no physical bonding (mechanical interlocking), crack propagation can proceed interfacially with minimal moisture absorption. A comparison of the incubation times for Forest Products Laboratory (FPL, or sulfuric acid-sodium dichromate) etched surfaces, both bonded to epoxy adhesives and freely exposed to water or humidity at different temperatures, shows that hydration occurs with the same activation energy and, hence, the same mechanism, independent of whether or not the surface is covered with adhesive. However, the pre-exponential factor in the rate constant is dependent on the concentration of free moisture at the interface so that the hydration rate varies by several orders of magnitude.     

Study on promotion of interface adhesion by ultrasonic vibration for CFRP/Al alloy joints

Abstract

Adhesively bonded CFRP/Al joints have been widely used in various engineering fields. However, the poor interface adhesion between the adhesive and the Al adherend limits its further use. In this study, ultrasonic vibration was applied to promote the interface adhesion, and the promotion mechanism was studied in detail. The vibration was exerted on the surface close to the bonding area after the adhesive was applied. According to the bonding strength test, this process improved the bonding strength and repeatability by approximately 32% and 48%, respectively. By comparing the failure behavior without and with ultrasound, it can be seen that ultrasound promotes interface adhesion of the adhesive/Al adherend significantly. Under the application of ultrasonic vibration, a tight microscopic bond was formed at the bonding interface, and a chemical reaction occurred to form chemical bonds. The opening of the epoxy group was promoted to allow Al to react with –O–C to form Al–O–C, because attack of electrophilic Al?+?on O– of the epoxy group was strengthened by high-frequency impact between the adhesive and the Al adherend at the interface caused by the ultrasonic vibration. It can be seen that the application of ultrasonic vibration during the adhesive bonding process can promote interface adhesion between Al and the adhesive in terms of physics and chemistry, thus significantly improving the performance of the adhesive bond.