On the effect of pulsed laser ablation on shear strength and mode I fracture toughness of Al/epoxy adhesive joints

The present work describes an experimental study about the shear strength and the mode I fracture toughness of adhesive joints with substrates pre-treated by pulsed laser ablation. An ytterbium-doped pulsed fiber laser was employed to perform laser irradiation on AA6082-T4 alloy. Morphological and chemical modifications were evaluated by means of surface profilometry, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Thick adherend shear tests were carried out in order to assess the shear strength while the mode I fracture toughness was determined using the double cantilever beam. For comparison, control samples were prepared using classical surface degreasing. The results indicated that laser ablation has a favorable effect on the mechanical behavior of epoxy bonded joints; however, while a + 20% increase was recorded for shear strength, a remarkable threefold enhancement of fracture toughness was observed with respect to control samples. XPS analyses of treated substrates and SEM observations of the fracture surfaces indicated that laser pre-treatment promoted chemical and morphological modifications able to sustain energy dissipation through mechanical interlocking. As a result cohesive failure within the adhesive bond-line was enabled under predominant peel loading.     

Accuracy of cohesive laws with different shape for the shear behaviour prediction of bonded joints

The use of adhesive bonding as a joining technique is increasingly being used in many industries because of its convenience and high efficiency. Cohesive Zone Models (CZM) are a powerful tool for the strength prediction of bonded joints, but they require an accurate estimation of the tensile and shear cohesive laws of the adhesive layer. This work evaluated the shear fracture toughness (J_IIC) and CZM laws of bonded joints for three adhesives with distinct ductility. The End-Notched Flexure (ENF) test geometry was used. The experimental work consisted of the shear fracture characterization of the bond by the J-integral. Additionally, by this technique, the precise shape of the cohesive law was defined. For the J-integral, digital image correlation was used for the evaluation of the adhesive layer shear displacement at the crack tip during the test, coupled to a Matlab sub-routine for extraction of this parameter automatically. Finite Element Method (FEM) simulations were carried out in Abaqus® to assess the accuracy of triangular, trapezoidal and linear-exponential CZM laws in predicting the experimental behaviour of the ENF tests. As output of this work, fracture data is provided in shear for the selected adhesives, allowing the subsequent strength prediction of bonded joints.     

Mode I fracture toughness of adhesively bonded joints as a function of temperature: Experimental and numerical study

Adhesives used in structural high temperature aerospace applications must operate in extreme environments. They need to exhibit high-temperature capabilities in order to maintain their mechanical properties and their structural integrity at the intended service temperature. One of the main problems caused by high temperature conditions is the fact that the adhesives have different mechanical properties with temperature. As is known, adhesive strength generally shows temperature dependence. Similarly, the fracture toughness is expected to show temperature dependence.In this work, the Double Cantilever Beam (DCB) test is analysed in order to evaluate the effect of the temperature on the adhesive mode I fracture toughness of a high temperature epoxy adhesive. Cohesive zone models, in which the failure behaviour is expressed by a bilinear traction–separation law, have been used to define the adhesive behaviour and to predict the adhesive P–δ curves as a function of temperature. The simulation response for various temperatures matched the experimental results very well. The sensitivity of the various cohesive zone parameters in predicting the overall mechanical response as a function of temperature was examined as well for a deeper understanding of this predictive method. Also, issues of mesh sensitivity were explored to ensure that the results obtained were mesh independent.     

Application of adhesively bonded single lap joints for fracture assessment of adhesive materials

In this study, a method has been proposed to obtain the failure envelope of brittle adhesives using the experimental failure loads of precracked single lap joints (SLJs). The proposed technique is based on the principles of linear elastic fracture mechanics (LEFM), on J-integral relations, and on results of a numerical analysis. Compared to the previous approaches, the introduced experimental method has some advantages such as low manufacturing costs and simpler test procedure. The proposed method can also provide a wide range of mode mix ratios without the need of an additional apparatus. The fracture envelope obtained from the proposed method was then verified by performing some fracture tests including double cantilever beam (DCB), end-notched flexure (ENF), and single leg bending (SLB) specimens. Good correlation was seen between the fracture envelopes of the proposed method and the ones obtained from the fracture mechanics experiments.     

Comparison between the ENF and 4ENF fracture characterization tests to evaluate GIIC of bonded aluminium joints

ABSTRACT

Adhesively bonded joints have been increasingly used in structural applications over mechanical joints. Cohesive Zone Modelling (CZM) is the most widespread technique to predict the strength of these joints, and it uses the tensile fracture toughness (G_IC) and the shear fracture toughness (G_IIC). Different fracture characterization methods are available for shear loadings, among which the End-Notched Flexure (ENF) is undoubtedly the most popular. The 4-Point End-Notched Flexure (4ENF) is also available. This work consists of a detailed comparison between the ENF and 4ENF tests for the experimental estimation of G_IIC of bonded aluminium joints. Three adhesives were used: a strong and brittle (Araldite® AV138), a less strong but with intermediate ductility (Araldite® 2015) and a highly ductile (SikaForce®7752). Different data reduction methods were tested, and the comparison included the load-displacement (P-δ) curves, resistance curves (R-curves) and measured G_IIC. It was found that the ENF test presents a simpler setup and has a higher availability of reliable data reduction methods, one of these not requiring measuring the crack length (a) during its growth. For the 4ENF test, only one test method proved to be accurate, and the test geometry revealed to be highly affected by friction effects.     

Effect of high loading rate and low temperature on mode I fracture toughness of ductile polyurethane adhesive

The mode I fracture toughness of an adhesive at low temperatures under high loading rates are studied experimentally. Typical R-curves of the polyurethane adhesive under different loading rates (0.5?mm/min, 50?mm/min, 500?mm/min) at different temperatures (room temperature, ?20?°C, ?40?°C) respectively are obtained. From the experimental results, the mode I fracture toughness of this adhesive is extremely sensitive to the high loading rates and low temperatures. With the increase of the loading rate and decrease of temperature, the mode I fracture toughness of this adhesive decreases significantly. Under the loading rate of 500?mm/min at ?40?°C, the mode I fracture toughness of adhesive is 15% of the value at room temperature (RT) under quasi-static conditions. Through the experiment, the relationship between mode I fracture toughness of this adhesive, nominal strain rate and temperature is obtained.     

Lowering curing temperature in adhesive bonding of aluminum AA6061-T4

ABSTRACT

To minimize the part distortion and investment in the E-coat oven in adhesive bonding of metals for automotive applications, lowering the curing temperature of adhesive without apparent loss of the joint strength is desirable. The key to lower the curing temperature of adhesive bonding of lightweight materials is to accelerate the curing process of structural adhesives. In this study, curing agent (i.e., aliphatic polyamine) and curing accelerator (i.e., acetylacetone salt) were added into commercial Henkel 5089 adhesive and the effect of these curing additives on the curing temperature of Henkel 5089 in adhesive bonding of aluminum AA6061-T4 was studied. The test results showed that the addition of a curing agent and accelerator in Henkel 5089 lowered the curing temperature from 177°C to 130°C without sacrificing the strength of the adhesive-bonded aluminum AA6061-T4.     

Experimental and numerical investigations of crack face adhesive bonding effect on the mixed-mode fracture strength of PMMA

Experimental and numerical investigations have been conducted to evaluate the effect of adhesive bonding of crack surfaces on the mixed-mode (I and II) fracture strength and effective stress intensity geometry/loading factor of a plate with an edge crack. The experimental tests were carried out on five batches of simple edge crack and specimens in which adhesive bonding was used on crack faces at different distances from the crack tip. The cracked specimens made from poly methyl-methacrylate rectangular plates. The specimens’ fracture strength was obtained by employing a tensile testing machine at different loading angles using a modified Arcan fixture. In the numerical part, finite element simulations were used to model the test specimens and thereby establishing their stress intensity geometry/loading factors. The results show that the adhesive bonding of the crack surfaces has a significant effect on reducing the equivalent mixed-mode stress intensity factor for all loading angles. The bonded specimens show considerable fracture force enhancement compared to the simple edge crack specimens.     

The effect of a multicomponent silane primer on the interphase structure of aluminum/epoxy adhesive joints

The structure of films formed by a multicomponent silane primer applied to an aluminum adherend and the interactions of this primer with an amine-cured epoxy adhesive were studied using X-ray photoelectron spectroscopy, reflection-absorption infrared spectroscopy, and attenuated total reflectance infrared spectroscopy. The failure in joints prepared from primed adherends occurred extremely close to the adherend surface in a region that contained much interpenetrated primer and epoxy. IR spectra showed evidence of oxidation in the primer. Fracture occurred in a region of interpenetrated primer and adhesive with higher than normal crosslink density. The primer films have a stratified structure that is retained even after curing of the adhesive.     

Effect of hole drilling at the overlap on the strength of single-lap joints

Bonded unions are gaining importance in many fields of manufacturing owing to a significant number of advantages to the traditional fastening, riveting, bolting and welding techniques. Between the available bonding configurations, the single-lap joint is the most commonly used and studied by the scientific community due to its simplicity, although it endures significant bending due to the non-collinear load path, which negatively affects its load bearing capabilities. The use of material or geometric changes in single-lap joints is widely documented in the literature to reduce this handicap, acting by reduction of peel and shear peak stresses at the damage initiation sites in structures or alterations of the failure mechanism emerging from local modifications. In this work, the effect of hole drilling at the overlap on the strength of single-lap joints was analyzed experimentally with two main purposes: (1) to check whether or not the anchorage effect of the adhesive within the holes is more preponderant than the stress concentrations near the holes, arising from the sharp edges, and modification of the joints straining behaviour (strength improvement or reduction, respectively) and (2) picturing a real scenario on which the components to be bonded are modified by some external factor (e.g. retrofitting of decaying/old-fashioned fastened unions). Tests were made with two adhesives (a brittle and a ductile one) varying the adherend thickness and the number, layout and diameter of the holes. Experimental testing showed that the joints strength never increases from the un-modified condition, showing a varying degree of weakening, depending on the selected adhesive and hole drilling configuration.     

Experimental investigation of nanoparticles effects on cohesive model and bridging laws of mode I fracture in the adhesive joints

In this paper, an experimental study is conducted on bridging and cohesive mechanism of adhesive bonded joints including Nano-composite and Nano-adhesive for mode I fracture. Nano-composite adherents with glass fibers and alumina nanoparticle have been fabricated and underwent DCB test. The concentration of this study is on the comparison of three different types (ECM, CBT, and MCC methods) of traction-separation laws and the effect of adding nanoparticles on them. A modified trapezoidal model (P1) is presented. The obtained bridging laws and cohesive mechanism (traction-separation laws) for different nanoparticles wt% (in adhesive and adherent) can be used in the finite element method for numerical simulation. Values obtained for critical displacement by different methods have a good consistency with each other with a relatively similar value, except for the sample by 0.5% wt Nano-particle in adhesive. In addition, critical displacement increases with increasing nanoparticle content in the adhesive, and then decreases. In that, the greatest critical displacement was observed in adhesive samples with nanoparticle content of 0.5%. Although, critical stress obtained from CBT to MCC methods were consistent, they were different from ECM results. The critical stress decreased with further addition of nanoparticles to the adhesive, and then started increasing. In that, the lowest critical stress was observed in sample adhesives with nanoparticle content of 0.5%.     

预偏角对单搭接接头强度的影响

研究了在被粘物搭接区顸偏转角度对钢单搭接拉伸接头剪切强度的影响,并用弹性有限元法分析了预偏角变化时单搭接接头上胶层中的应力分布情况。数值分析的结果表明：当预偏角从0°增加到12°时,结构钢单搭接接头胶层中的所有应力峰值分量均显著下降。而在所采取实验条件下,接头的剪切强度最高值出现在预偏角为6°时。因而在进一步研究预偏角对单搭接接头承载能力的作用时,应将外载作用下接头的本征偏转情况考虑在内。     

Mode II fracture energy in the adhesive bonding of dissimilar substrates: carbon fibre composite to aluminium joints

The end-notched flexure (ENF) test calculates the value of mode II fracture energy in adhesive bonding between the substrates of same nature. Traditional methods of calculating fracture energy in the ENF test are not suitable in cases where the thickness of the adhesive is non-negligible compared with adherent thicknesses. To address this issue, a specific methodology for calculating mode II fracture energy has been proposed in this paper. To illustrate the applicability of the proposed method, the fracture energy was calculated by the ENF test for adhesive bonds between aluminium and a composite material, which considered two different types of adhesive (epoxy and polyurethane) and various surface treatments. The proposed calculation model provides higher values of fracture energy than those obtained from the simplified models that consider the adhesive thickness to be zero, supporting the conclusion that the calculation of mode II fracture energy for adhesives with non-negligible thickness relative to their adherents should be based on mathematical models, such as the method proposed in this paper, that incorporate the influence of this thickness.     

Effects of applied pressure and temperature during curing operation on the strength of tubular single-lap adhesive joints

Adhesive joints have been widely used for fastening thin adherends because they can distribute the load over a larger area than the mechanical joint, require no holes, add very little weight to the structure and have superior fatigue resistance. However, the load capabilities of adhesive joints are affected by both applied pressure and temperature during cure, as well as by service environments because the adhesion characteristics of adhesives are very sensitive to manufacturing and environmental conditions. In this study, the tensile load capabilities of tubular single-lap adhesive joints with an epoxy adhesive were experimentally investigated with respect to service temperature and the applied pressure and temperature during curing operation. The effects of the applied pressure on the tensile load capabilities of tubular single-lap adhesive joints were studied by measuring the actual cure finish temperature using thermocouples and dielectrometry. From the experiments, it was found that the actual cure finish temperature of tubular single-lap adhesive joints increased as applied pressure increased, which increased residual thermal stress in the adhesive layer to decrease the load capabilities of adhesive joints. From finite element analysis and experimental results of tubular singlelap adhesive joints, the optimal geometry condition for adhesive joints was also investigated.     

Experimental investigation of an adhesive fracture energy measurement by preventing plastic deformation of substrates in a double cantilever beam test

A method to prevent substrate damage in a double cantilever beam (DCB) test was experimentally investigated by changing bond-line width. Highly toughened adhesives are developed due to an increase in the demand for structural adhesives. Furthermore, structural materials that are lightweight and excellent in mechanical properties but have limitations in thickness are developed. In contrast, plastic deformation of the substrates is expected when the DCB test is performed with conventional DCB specimens using the aforementioned adhesives and materials. The reduction in the maximum stress on the substrate surface by narrowing a bond-line width is a practical method to prevent plastic deformation when a substrate possesses limited thickness. In this study, the influence of the bond-line width on an adhesive fracture energy in mode I was experimentally investigated by manufacturing DCB specimens in which the bond-line width is narrower than the substrate width. Additionally, the maximum bond-line width to prevent plastic deformation of the substrates was theoretically derived. The evaluated criteria for examining the existence of the plastic deformation by changing the bond-line width exhibited good agreement with the experiment results.     

Influence of graphene nanoplatelets (GNPs) on mode I fracture toughness of an epoxy adhesive under thermal fatigue

Abstract

The contribution of graphene nanoplatelets (GNPs) for enhancing the fracture toughness of a commonly used room-cured epoxy, used to bond E-glass/epoxy composite adherends, is evaluated. A comprehensive experimental investigation is conducted to examine the performance and degradation of adhesively bonded joints subject to cyclic thermal loading using the standard double cantilever beam (DCB) specimens. Several groups of DCB specimens were fabricated using the adhesive reinforced with four different GNPs weight-percentages (i.e. 0.0, 0.25, 0.5 and 1%). The specimens are subsequently subjected to various numbers of thermal cycles (to a maximum of 1000 heating/cooling cycles), and then tested, and the resulting mode I fracture toughness values are evaluated and compared. The extent and modes of damage captured through microscopy and scanning electron microscopy images are presented and discussed. In addition, a computational framework, using the cohesive zone modeling technique, is developed for predicting the response of the adhesives and their damage evolution.     

Effect of the specimen dimensions and the test speed on the fracture toughness of iPP by the essential work of fracture (EWF) method

The fracture parameters of an isotactic polypropylene are studied by the essential work of fracture method. The influence of the specimen height, width and thickness and the effect of the test speed are investigated. Results show that this method is very useful for studying the plane‐stress fracture of this kind of materials in form of films and sheets. Varying the width (30 to 60 mm) and the test speed (2 to 100 mm/min) has no relevant influence, whereas the results are only length independent in a range from 40 to 100 mm. The influence of the thickness is very high, obtaining an important decrease of the specific essential work as the thickness is increased in a range from 38 to 2500 μm. This result is justified with the fracture surfaces obtained, observed by SEM, in which an evolution of the fracture behavior is seen as a function of thickness (38, 100, 500, 1000, 2500 μm). © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 177–187, 1999     

An atomic force microscopy study of the effect of surface roughness on the fracture energy of adhesively bonded aluminium

The effect of roughening an initially polished aluminium surface using the Forest Products Laboratory chemical etch on the adhesive joint strength has been determined. It was found that while the lap shear strength increased rapidly with etching for short times, the fracture energy did not increase significantly until etching had occurred for at least 15 min. An atomic force microscope (AFM) was used to study the surface/interface morphology and to quantify the surface roughness. The AFM images showed that etching occurs heterogeneously across the aluminium surface and a correlation was found between the fracture energy and the fraction of etched surface. A model based on Griffith's fracture energy approach has been proposed to explain this observation. The lap shear strength was found to be more sensitive to a finer scale roughness which is generated at shorter etching times. Other observations regarding the mode of fracture and the variability in joint strength as a function of the surface roughness are explained on the basis of varying stress concentrations at the crack tip.     

Effect of an intumescent flame retardant on the fracture toughness (Mode I), thermal,and flame-retardant properties of continuous glass fibre-reinforced polypropylene composites

To solve the ‘wicking actions’ caused by fibre, intumescent flame retardant (IFR) was introduced into the resin of continuous glass fibre-reinforced polypropylene (CGF/PP) composites. The influence of IFR on the properties of composites were investigated by a double cantilever beam test, thermogravimetric analysis, limiting oxygen index, vertical burning test (UL-94), and cone calorimetric test. The results revealed that when the content of IFR was 15 wt-%, the initial Mode I interlaminar fracture toughness (G_{I init.}) and propagation fracture toughness (G_{I prop.})) were enhanced by 123.2 and 70.26%, respectively, compared to composites with no IFR. The maximum weight loss rate (T_max) was improved to various degrees. Samples could self-extinguish with an oxygen concentration of 32.4% and achieve a UL-94 V-1 rating. Furthermore, the peak of heat release rate, total heat release, fire performance index, and mass loss rate tests indicated that IFR could dramatically enhance the flame retardancy of the composites.     

Effects of mixing temperatures on the morphology and toughness of epoxy/polyamide blends

A new mixing process was explored to increase further the fracture toughness and to investigate the toughening mechanisms of epoxy/nylon blend. In this process, without mechanical mixing, the mixtures of epoxy and premade nylon 6 powder were heated without the curing agent to specific temperatures, referred to as the “mixing temperature.” For epoxy/nylon blends, at sufficiently high temperatures, a semi‐interpenetrating network‐like structure can be developed at the interphase via the reaction between the amine end group and the epoxide group. The depth of interphase and the extent of reaction depends on the mixing temperature. The strong dependency of the fracture energy on mixing temperature reveals the positive effect of the newly developed structure at the interphase. The increase of fracture toughness is possibly due to the enhanced crack fingering bifurcation/deflection mechanism resulting from the lamellae developed in the interphase and the enhanced plastic deformation of epoxy as a result of preyielding of the interphase. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 1055–1063, 1999