Hygrothermal degradation of two rubber-toughened epoxy adhesives: Application of open-faced fracture tests

The absorption/desorption properties of two commercial, toughened epoxy adhesive systems were evaluated gravimetrically, and by X-ray photoelectron spectroscopy (XPS) and dynamic mechanical thermal analysis (DMTA). Fracture tests on degraded open-faced DCB specimens showed that these two adhesive systems have very different degradation behaviors. The steady-state critical strain energy release rate, G_cs, of an adhesive system 1 decreased rapidly with an exposure time in various hot-wet environments, reaching a relatively low value that was stable for over one year, while that of adhesive system 2 remained unchanged for more than one and a half years. A degradation mechanism which accounts for the different characteristics of the two adhesive systems was proposed. A model of fracture toughness degradation, analogous to Fick’s law, was then used to characterize the fracture toughness loss in an adhesive system 1, and the effects of temperature, RH and water concentration were evaluated. The results illustrate the wide variation in water absorption behaviors that can exist among toughened epoxy adhesives, and show how these differences relate to the degradation of fracture strength. The data were also used to assess the applicability of an exposure index (EI), defined as the integral of relative humidity over time, as a means of characterizing an aging history. The fracture strength degradation was measured after aging to achieve a range of EI values, and it was found that the strength loss was independent of the time-humidity path for sufficiently large EI.     

Predicting environmental degradation of adhesive joints using a cohesive zone finite element model based on accelerated fracture tests

A framework was developed to predict the fracture toughness of degraded adhesive joints by incorporating a cohesive zone finite element (FE) model with fracture data of accelerated aging tests. The developed framework addresses two major issues in the fracture toughness prediction of degraded joints by significant reduction of exposure time using open-faced technique and by the ability to incorporate the spatial variation of degradation with the aid of a 3D FE model. A cohesive zone model with triangular traction-separation law was adapted to model the adhesive layer. The degraded cohesive parameters were determined using the relationship between the fracture toughness, from open-faced DCB (ODCB) specimens, and an exposure index (EI), the time integration of the water concentration. Degraded fracture toughness predictions were done by calculating the EI values and thereby the degraded cohesive parameters across the width of the closed joints. The framework was validated by comparing the FE predictions against the fracture experiment results of degraded closed DCB (CDCB) joints. Good agreement was observed between the FE predictions and the experimental fracture toughness values, when both ODCB and CDBC were aged in the same temperature and humidity conditions. It was also shown that at a given temperature, predictions can be made with reasonable accuracy by extending the knowledge of degradation behavior from one humidity level to another.     

Mode-I adhesive fracture energy of carbon fibre composite joints with nanoreinforced epoxy adhesives

The effect of the addition of carbon nanoreinforcements to an epoxy adhesive on the strength and toughness of carbon fibre/epoxy composite joints was studied. The laminate surfaces, treated with peel ply, were characterised by profilometry, image analysis and wettability. The mechanical properties of the joints were determined by lap shear testing and double cantilever beam testing. The fracture mechanisms were studied by scanning electron microscopy.The addition of carbon nanofibres and carbon nanotubes caused an increase in the mode-I adhesive fracture energy, G_IC, of the joints while their lap shear strengths remained approximately constant. This improvement in the fracture behaviour was attributed to the occurrence of toughening mechanisms when carbon nanoreinforcements were added to the epoxy adhesive. Additionally, the use of carbon nanotubes improved the interfacial strength between the adhesive and the substrate, changing the crack growth behaviour and the macroscopic failure mode.     

Characterization and prediction of fracture properties in hygrothermally degraded adhesive joints: an open-faced approach

One of the challenges in the application of structural adhesive joints is the prediction of their long-term durability. During the service life, moisture diffuses into the adhesive layer and eventually degrades its fracture properties. Environmental degradation should thus be taken into consideration in the design and analysis of adhesive joints. This work first provides an overview, summarizing the recent efforts regarding the hygrothermal exposure of adhesive joints, accelerated aging methods, water diffusion modeling, and characterization of fracture properties in adhesively bonded joints. The second part presents a recent degradation methodology by which the fracture toughness evolution of adhesive joints can be predicted using fracture test data obtained using the accelerated open-faced degradation method.     

Effect of CNT dispersion methods on the strength and fracture mechanism of interface in epoxy adhesive/Al joints

The tensile and shear strengths of adhesively bonded aluminum (Al) joints were inspected in the presence of amino functionalized multi-wall carbon nanotubes (MWCNTs). Tensile and shear tests were carried out using butt and lap-shear joints. The main goal was to compare the effects of dispersion methods of functionalized-CNT into epoxy on the mechanical performances and failure mechanisms of Al joints. Two different types of dispersion procedures, distributing CNT in the hardener (HH method) and distributing CNT in the resin (RR method), were applied. To identify the failure mechanisms, the morphology of fracture surfaces were analyzed using scanning electron microscopy (SEM). Comparing two dispersion methods against one another ascertained that following the RR method for dispersing CNTs in the adhesive displayed larger shear strength, while applying HH method offered fairly greater tensile strength. Moreover, dispersing CNTs in the resin induced more uniform dispersion of CNTs as compared to distributing nanofillers in the hardener. Following RR method, CNTs good dispersion as well as the presence of effective crack growth dissipating mechanisms, increased the shear strength of CNT reinforced adhesive joint. Incorporating CNTs using HH approach encouraged the plastic void formation of epoxy around the agglomerated CNTs, and as a consequence, promoted the plastic deformation under tension.     

Effect of residual stresses to the crack path in alumina/zirconia laminates

Laminates with alternating layers are well known from nature. The strongly bonded alumina/zirconia (Al₂O₃/ZrO₂) layers can combine high fracture resistance with high strength and stiffness when properly tailored. The presence of compressive residual stresses formed in Al₂O₃ layers can suppress and deflect cracks propagating through the layers. The crack path is governed by both the elastic properties and the internal stress field of individual layers. The laminates with various layer-thickness ratios ranging from 0.1 to 3 were used to investigate the effect of residual stresses and influence of crack formation pattern on the crack path development. The indentation surface cracks observed in various alumina-zirconia laminates exhibit the same crack deflection independently on the level of internal stresses. The crack deflection observed on the fracture surfaces of bending specimens was related to the indentations cracks. The complicated crack path was explained experimentally by 3D reconstruction with the support of numerical simulations.     

Cohesive/adhesive failure interaction in ductile adhesive joints Part II: Quasi-static and fatigue analysis of double lap-joint specimens subjected to through-thickness compressive loading

This paper proposes a new methodology for the finite element (FE) modelling of failure in adhesively bonded joints. Cohesive and adhesive failure are treated separately which allows accurate failure predictions for adhesive joints of different thicknesses using a single set of material parameters. In a companion paper (part I), a new smeared-crack model for adhesive joint cohesive failure was proposed and validated. The present contribution gives an in depth investigation into the interaction among plasticity, cohesive failure and adhesive failure, with application to structural joints. Quasi-static FE analyses of double lap-joint specimens with different thicknesses and under different levels of hydrostatic pressure were performed and compared to experimental results. In all the cases studied, the numerical analysis correctly predicts the driving mechanisms and the specimens’ final failure. Accurate fatigue life predictions are made with the addition of a Paris based damage law to the interface elements used to model the adhesive failure.     

Strength and bonding characteristics of adhesive joints with surface-treated titanium-alloy substrates

This paper presents a study on the effect of surface treatments on the mechanical behavior of adhesively bonded titanium alloy joints. Several different treatments were selected for the preparation of Ti-6Al-4V alloy faying surfaces, and bonded joints were fabricated using surface-treated titanium alloy substrates and a film adhesive. Tensile tests were performed on single-lap specimens to evaluate the joint strength and to assess the failure mode, i.e. cohesive failure, adhesive (interfacial) failure or a mix of both. Contact angle measurements were also carried out, and the surface free energies of titanium alloys and the thermodynamic works of adhesion for the adhesive/titanium alloy interfaces were obtained. A three-dimensional finite element analysis was used to predict the strength of the specimens exhibiting cohesive failure. In addition, an expression of the relationship between the joint strength corresponding to interfacial failure and the thermodynamic work of adhesion was introduced based on the cohesive zone model (CZM) concept. It is shown that two surface treatments, Itro treatment and Laseridge, lead to cohesive failure and a significant increase in the joint strength, and the numerically predicted strength values are fairly close to the experimental values. These surface treatments are possible replacements for the traditional surface treatment processes. For degreasing, emery paper abrasion, atmospheric plasma treatment, sulfuric acid anodizing, nano adhesion technology and high-power lasershot, the specimens fail at the adhesive/substrate interface and the joint strength increases linearly with the thermodynamic work of adhesion as expected from our CZM-based expression.     

Effect of reinforcements at different scales on mechanical properties of epoxy adhesives and adhesive joints: a review

Improvement of the mechanical properties of adhesives and adhesive joints has been a subject of great interest in recent years. Up to now, several methods have been presented such as modifying substrate shapes, adding microparticles (MPs) and nanoparticles (NPs), and embedding micro and macrofibers in the adhesive layer. This review aims to investigate how these reinforcements of different scales in the adhesive layer influence the mechanical properties of adhesive joints and adhesives. Characteristics and applications of reinforcements are introduced in the first part. In the second part, the effects of several parameters commonly investigated by researchers on the strength, stiffness and fracture toughness improvement of polymeric materials are reviewed for reinforcements of different scales. Finally, damage mechanisms involved in increasing or decreasing the mechanical properties are reviewed and discussed.     

Critical current degradation and delamination crack observation of epoxy-coated REBCO superconducting tapes after thermal cycles in liquid nitrogen

The delamination and critical current (I_c) degradation caused by thermal stress after epoxy impregnation are threats for the application of REBa₂Cu₃O_7-x(REBCO, RE = Rare earth) superconducting magnets. In this work, two types of REBCO tapes were coated by Stycast 2850FT with controlled coating geometries. Critical currents of coated samples after thermal cycles in liquid nitrogen were measured. I_c degradation was found in coated samples with a free no-coating edge, when the surface coating layer was thicker than 1000 μm. It was also found that additional edge coating can help to suppress the I_c degradation. Samples with degraded I_c after thermal cycles showed an obvious delamination phenomenon. The morphology and location of delamination cracks were carefully observed by using focused Ion beam, scanning electron microscope, and transmission electron microscope. Delamination cracks propagated within the REBCO layer and stopped at reaching the Silver/REBCO interface. Simulations by finite element method suggest that delamination cracks are generated by the stress accumulation within the REBCO layer, which could be reduced by a full epoxy coating on both tape surface and edges.     

Fracture behaviour of a rubber nano-modified structural epoxy adhesive: Bond gap effects and fracture damage zone

This paper critically examined the fracture behaviour of a rubber-modified, structural epoxy adhesive with various bond gap thicknesses ranging from 0.05 mm to 6 mm. The main and very novel contribution is direct measurement of the fracture process zone, plastic deformation zone and intrinsic fracture energy dissipated in the fracture process zone. The shape and size of the fracture process zone and plastic deformation zone were identified using scanning electron microscopy, transmission electron microscope and transmission optical microscope. As the bond gap thickness increased, the fracture energy increased steadily from 2365 J/m² for 0.05 mm bond gap thickness to 6289 J/m² of 1.6 mm bond gap thickness, and then plateaued. The thickness and failure strain of the fracture process zone remained essentially constant, being approximately 0.052 mm and 0.55 respectively, for different bond gap thicknesses. The intrinsic fracture energy (dissipated in the fracture process zone) appeared to be a material property, which remained approximately 2738 J/m². The plastic deformation zone extended through the entire bond gap in thickness and occupied a significant length for all bond gap thicknesses. The effect of bond gap thickness on the fracture energy of the adhesive joints is hence directly attributed to the variation of the plastic deformation energy (dissipated in the plastic deformation zone) with bond gap thickness.     

Fatigue and fracture behavior of adhesive-bonded structures in the light of the surface morphology

In order to obtain light weight structures, the adhesive-bonding technique has been established since many years, especially in the aircraft industry. In principle, light weight structures consist of carbon fiber-reinforced plastics (CFRP) or Al-based alloys. Under external forces, the mechanical behavior of the adhesive-bonded structure was determined by cohesive and adhesive forces. Consequently both, the cohesive as well as the adhesive strength must be as high as possible to obtain structures that resist high external forces. The mechanical behavior of the bonded structure depends sensitively on the quality of the surface prior to the curing process. In case of adhesive - bonded Aluminum-based structures, the surface can be improved by a special laser beam treatment. It can be shown that the morphology of the internal surface between the substrate and the adhesive determines the adhesive strength and, consequently, the mechanical behavior of the bonded structure. The kind and critical force of fracture (cohesive or adhesive) can be explained by using a damage model that takes the kind and distribution of characteristic defects into account.     

Residual static strength and the fracture initiation path in adhesively bonded joints weakened with interfacial edge pre-crack

This paper deals with the application of fracture mechanics approaches for predicting the residual static strength and the crack kinking angle of adhesively bonded joints containing interfacial edge pre-cracks. The interfacial cracks are created due to different factors such as inappropriate surface preparation which cause a significant reduction of the joint strength. To investigate the residual strength of interfacial cracked adhesive joints and predict the crack kinking angle, three different approaches including the maximum tangential stress (MTS), the minimum strain energy density (SED) and the maximum tangential strain energy density (MTSED) were assessed. To this end, single lap joints (SLJs) containing a brittle adhesive material and with different pre-crack sizes and various substrate thicknesses were manufactured and tested. The results were also verified by applying fracture mechanics approaches on previously published experimental data. According to the results, it was concluded that in mode II dominant cases, the predictions of kinking angle using the MTS method was in good agreement with the experimental observations, while in mode I dominant cases the mentioned approach provided poor predictions. It was also found that the SED criterion could be a precise model for predicting the crack extension angle in mode I dominant conditions. The results also showed that the MTS criterion predicts the residual static strength of interfacial cracked adhesive joints very well.     

Vickers crack extension and residual fracture strength of annealed and thermally tempered glass in water

The effect of water contact on the propagation of microscale surface cracks is investigated in two types of glass: annealed and tempered glass. Initial flaw is artificially created on the glass surface using a Vickers indenter, and is covered with a water droplet for 20 min. Micrographs of the flaw taken before and after water contact confirms the increase in crack length from around 61–103 μm for thermally tempered glass. After water dipping, the maximum length to which the crack growth is approximately 57 % smaller in the thermally tempered glass than in the annealed glass. Despite the severe effect of water contact on crack propagation, it is found that the fracture strength is not substantially altered by water dipping; even though the crack length is enlarged, the fracture strength of glass is similar, and in the case of tempered glass, its fracture strength is slightly changed within 7% due to the blunting of the crack tip by water or others.     

Extensometric and Thermo-Elasticimetric Measurements to Characterize the Damage Evolution in Adhesively Bonded Joints: Application for the Adhesively Bonded Double Scarf Joint

In this article, two nondestructive experimental methods are compared in order to evaluate the influence of a singularity, created by geometry, on the damage evolution in the adhesively bonded joints. The first experimental method uses the “back face technique” to measure the disturbances of the strain fields induced by the cracks. The second experimental method uses an infrared focal plane array camera to measure the thermo-elastic field in the geometrical singularity. First of all, both methods are carried out independently: the analysis of the stress/strain and the thermo-graphic analysis. Then, these two methods enabled us to generate a database, the use of which improves the diagnosis of the damage state of the adhesively bonded joints.     

Application of a new constant G load-jig to creep crack growth in adhesive joints

A novel constant energy release rate load-jig, capable of applying loads in the full range of mode mixes from pure mode I to pure mode II, was developed for studying creep crack growth in structural adhesive joints. Since the load-jig applies only pure bending moments to uniform double cantilever beam (DCB) specimens, the expressions for the energy release rate and mode ratio are both simple and accurate. The new load-jig was used to study mixed-mode creep crack growth in DCB specimens which had either an intact fillet or a steady-state failure zone. Both a rubber-toughened and a mineral-filled epoxy adhesive were tested at room temperature, which is far below the glass transition temperature for either structural adhesive. In all cases, crack speeds were observed to decelerate, indicating that the adhesive were self-toughening over time. In addition, crack growth was observed to propagate by the initiation and coalescence of microcracks, rather than by the continuous advancement of a crack tip.     

Understanding and control of adhesive crack propagation in bonded joints between carbon fibre composite adherends II. Finite element analysis

Carbon fibre composites are being widely considered for many classes of heavily loaded components. A common feature of such components is the need to introduce local or global loads into the composite structure. The use of adhesive bonding rather than mechanical fasteners offers the potential for reduced weight and cost. However, such bonded joints must be shown to behave in a predictable and reliable way. A major aspect of this is to demonstrate that the progress of cracks through the bonds is well understood. The simulation work presented here complements the experimental work presented in Part I. The observed failure processes and their sequence are successfully described and modelled.     

An overall assessment of a new adhesive bonding process for composite materials,with regard to quality and cost

An assessment of innovative adhesive bonding process has been performed with regard to quality and cost. In this frame, the effect of two different atmospheric pressure plasma surface treatment conditions on the fracture toughness behaviour of adhesively bonded joints was experimentally investigated. Furthermore, the mechanical performance of a newly developed aerospace structural adhesive has been characterised experimentally in order to assess the quality of the bonded elements. To assess the feasibility of the new process, a complete cost-estimation analysis of the process has been carried out based on the activity-based costing modelling approach, thus serving to the estimation of the total cost/duration of the process. To this end, the newly developed process is assessed with regard to quality and cost. It could be shown that the new process offers tempting alternatives to the existing adhesive bonding and joining processes used in the aeronautic industry.     

Mixed-mode fracture behaviour of refractories with asymmetric wedge splitting test. Part I: Numerical implementation and validation

The suitability of asymmetric wedge splitting test (WST) for mode I/II mixed-loading was validated by FE simulation with a three-dimensional heterogeneous continuum FE model. The unsymmetrical strain and stress patterns were observed for mixed-mode loading. Compared with mode I loading of symmetric WST, the introduction of in-plane shear accelerates the crack extension and deviation from symmetry plane with a smaller fracture process zone. Additionally, the asymmetric WST with small, medium and large wedge angles were simulated for sensitivity analysis. With the increasing of asymmetric wedge angle, the recorded vertical load-displacement curves turn from “mild” to “steep”, and the ratio of mode I to mode II fracture energy G_I/G_II decreases accordingly while the symmetric one has the highest G_I proportion. The asymmetric WST with large wedge angle deforms the most at same applied vertical loading displacement, while all WSTs show similar damaged elements amount at the same crack mouth opening displacement.     

A contribution to the qualification process of surface pretreatment methods: Sensitivity of mechanical tests to adhesion and delamination

Focusing the adhesive bonding of thermoset carbon fiber reinforced plastics (CFRPs) due to release agent residues on a surface pretreatment prior to the bonding process is inevitable to guarantee sufficient adhesion. Based on different material and process-related phenomena, a huge variety of parameters (e.g. treatment time) influence the treatment effectiveness. This often leads to intense parameter studies in which pretreatment methods are qualified by different mechanical tests and which require significant effort in terms of time and resources. Those qualification procedures are sometimes standardized by different and mostly company-specific standards. To increase the efficiency of this qualification process by increasing the general validity, this paper presents a comparison of different mechanical test methods (namely single lap shear test, floating roller peel test, double cantilever beam (DCB) test, and cross-tension test) in terms of their sensitivity to the detection of adhesion or delamination defects, even if they normally test different aspects of a joint. However, it could be found that the most common test – the single lap shear test – can only reveal large defect levels, while different tests, e.g. DCB test, show a good sensitivity for all investigated failures and others show intermediate performance.