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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

The use of the J-integral vector to analyse adhesive bonds with and without a crack

The singularity problem in adhesive joints is mentioned in finite element analyses but it is often ignored. Adhesive joints may have cracks and certainly many sources of singularities due to the adherend corners. Cracks are easily dealt with using fracture mechanics concepts. However, angular wedged notches or bi-material wedges are more difficult to treat because the strength of the singularity must be known. Another point that must be born in mind is the fact that crack propagation in adhesive joints occurs in a mixed mode, especially for ductile materials. The present study shows that the use of the J-integral can treat most of the above problems. A very useful fracture mechanics method using the J-integral vector approach is discussed in connection with adhesive bonds but can be generalized to other singular problems. It is shown that the J vector can be used in adhesive bonds where there is always discontinuity in material properties regardless of whether there is a crack or not. One of the advantages of using the J vector in numerical analyses for homogeneous materials is that the accurate stresses (better known) and displacements in the far field can be used to calculate J instead of the not-so-accurate values near the singular point. A method for dealing with the singularity in adhesive bonds, which can be used in engineering design and analysis, has been established in this investigation.     

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.     

Modeling complex crack paths in ceramic laminates: A novel variational framework combining the phase field method of fracture and the cohesive zone model

The competition between crack penetration in the layers and cohesive delamination along interfaces is herein investigated in reference to laminate ceramics, with special attention to the occurrence of crack deflection and crack branching. These phenomena are simulated according to a recent variational approach coupling the phase field model for brittle fracture in the laminae and the cohesive zone model for quasi-brittle interfaces. It is shown that the proposed variational approach is particularly suitable for the prediction of complex crack paths involving crack branching, crack deflection and cohesive delamination. The effect of different interface properties on the predicted crack path tortuosity is investigated and the ability of the method to simulate fracture in layered ceramics is proven in relation to experimental data taken from the literature.     

Use of controlled oxidation to increase the surface area of coal: Application to a bituminous and a semi-anthracite coal

This Paper supplements previous work on the use of controlled oxidation to increase the surface area of coal by examining the effects of different temperatures of treatment and different ranks of coal. The work on an hvA bituminous coal showed no increase in the area by nitrogen when the oxidation was carried out at 375 °C, but at least a 72-fold increase at 400 °C. The increase in surface area measured by nitrogen adsorption for two other coals (a semi-anthracite and an hvC bituminous coal) similarly oxidized, varied from 3 to 20-fold, depending on whether comparison is made with the values for the raw coal or with values obtained after heating to 400 °C in nitrogen. Measurements made on the stability of the increased surface as a function of time showed that the newly created surface disappeared at rates as high as 30% per month.     

Application of surface modifying macromolecules in polyethersulfone membranes: Influence on PES surface chemistry and physical properties

Novel surface modifying macromolecules (SMMs) were developed for incorporation into polyethersulfone (PES) membranes, intended for pervaporation applications. These materials were synthesized with a diisocyanate, polypropylene oxide (PPO), and a fluoro-alcohol, and characterized for elemental analysis, molecular weight, and glass transition temperatures. PES/SMM blends with eight types of SMMs were characterized for surface and physical properties and compared with PES. Water droplet contact angle measurements and X-ray photoelectron spectroscopy data showed that the SMMs migrated to the surface and rendered the PES material more hydrophobic. While advancing contact angle data were equivalent to those of pure Teflon™, the highest average values of receding angles of these systems were less than those of commercial Teflon™. The opaqueness of PES/SMMs films and data from differential scanning calorimetry experiments showed that the SMMs were either immiscible or only partially miscible with PES. It was also observed, for a fixed PES concentration of 25 wt %, that increases in the molecular weight of the SMMs and the weight fraction of PPO in the SMMs led to phase separation in the ternary PES/SMMs/dimethylacetamide (i.e., membrane casting solution) system. On the other hand, in the binary PES/SMMs system (i.e., cast membrane film), an increasing weight fraction of fluorine in the SMMs contributed to an increase in the phase separation. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1363–1378, 1999     

The potential of zero total charge of Pt nanoparticles and polycrystalline electrodes with different surface structure: The role of anion adsorption in fundamental electrocatalysis

In the present paper anion effects are discussed on different shape-controlled Pt nanoparticles as well as on polycrystalline Pt electrodes of different surface structures. In particular, the potentials of zero total charge (pztc) were determined by using the CO displacement approach, both in sulphuric and perchloric media. The CO monolayer oxidative stripping was studied on the different samples. CO coverages were independent of the electrolyte provided that the true CO stripping charges, corrected for double layer contributions are compared to the charge involved only in the hydrogen adsorption region. The classical charge density normalization process is discussed and small changes are proposed. Then, criteria to evaluate the real surface area of platinum electrodes from charge measurements are proposed in both electrolytes.     

Application of chronopotentiometry and derivative chronopotentiometry with an alternating current to the study of a slow charge transfer in a surface confined redox system

General analytical expressions for the potential-time (E-t) and derivative-potential ((dt/dE)-E) curves of a electroactive monolayer exhibiting a quasi-reversible behaviour, corresponding to the application of an alternating current time function of the form I(t) = I₀ cos(ωt), are presented. The use of this programmed current gives rise to singular and characteristic electrochemical responses. The alternating current also allows to obtain cathodic or both anodic and cathodic responses depending on whether the depletion of the adsorbed species was complete or not, and without using more than one applied current. Moreover, in this last case, it is possible to distinguish a reversible or an irreversible process by means of a simple visualisation of the E-t or (dt/dE)-E curves. Easy methods for estimating thermodynamic and kinetic parameters of the electroactive film are proposed and experimentally tested and compared with those previously obtained by using Cyclic Voltammetry.