Adhesive Fracture Energy Research Articles

The Role of the Surface Pretreatment in the Durability of Aluminium-Alloy Structural Adhesive Joints: Mechanisms of Failure

The durability of adhesively bonded aluminium-alloy joints have been investigated by undertaking cyclic-fatigue tests in liquid water and in 55% RH, to establish the threshold adhesive fracture energy, Gth, below which crack growth will not occur. Three surface pretreatments were employed: grit-blast and degreasing (GBD), phosphoric-acid anodising (PAA), and PAA followed by the application of an anti-corrosion primer (PAAP). The ranking of the durability for the six systems, as assessed by the value of Gth, is as follows: X-ray photoelectron spectroscopy, scanning electron microscopy, and energy-filtered transmission-electron microscopy have been used to assess the nature of the locus of failure and the mechanisms of failure. For the GBD pretreated joints, tested in both the liquid water and in 55% RH environments, failure occurs predominantly by the thermodynamic displacement of the adhesive from the metal substrate, according to the well-established principles of interfacial thermodynamics. In the case of the PAA and PAAP surface pretreatments, the anodising process provides additional adhesion forces and stability to the interface, which involves the formation of a microcomposite interphase region, and failure is shown to occur in a cohesive manner entirely within the adhesive for the tests undertaken in the 55% RH environment. For the specimens tested in liquid water, in the threshold region, the joints show loci of failure which are predominantly within the adhesive but with small areas of interfacial failure and oxide failure. The differences in durability for the PAAP and PAA when exposed to 55% RH or liquid water is ascribed to the kinetics of aggregation of water in the environs of the metal/adhesive interface, this phenomenon occurring much more rapidly for exposure to liquid water during the cyclic-fatigue process than for the exposure to water vapour at 55% RH. A method based on the XPS analysis of failure surfaces has been used to assess the extent of interfacial failure of the joint prepared from anodised stock. This is shown to be up to 95% depending on treatment and exposure conditions.

Experimental Study and Modelling of an Adhesive-Bonded Polycarbonate Assembly for Application in Consumer Electronics

In this work, adhesive bond strength and fracture energy were characterized using a standard Arcan fixture to provide data for product design simulation. Tests were performed on polycarbonate-adhesive-polycarbonate specimens to extract adhesive properties for numerical modelling. The bond strengths and fracture energy of the adhesive between polycarbonate substrates were determined at different loading rates and loading directions. Effects of adhesive aging were also investigated. Data from pure tensile (Mode I) and pure shear (Mode II) loadings were transformed into a cohesive law in a finite element model. Failure criteria were defined using damage theory. Simulations at different loading angles were compared with experimental results to evaluate the appropriateness of such a model. Simulation results were in good agreement with experiment in most cases. This study highlights the feasibility and challenges related to adhesive characterization using a standard Arcan test for modelling with a cohesive law.

Intermittent stick-slip dynamics during the peeling of an adhesive tape from a roller

We study experimentally the fracture dynamics during the peeling at a constant velocity of a roller adhesive tape mounted on a freely rotating pulley. Thanks to a high speed camera, we measure, in an intermediate range of peeling velocities, high frequency oscillations between phases of slow and rapid propagation of the peeling fracture. This so-called stick-slip regime is well known as the consequence of a decreasing fracture energy of the adhesive in a certain range of peeling velocity coupled to the elasticity of the peeled tape. Simultaneously with stick slip, we observe low frequency oscillations of the adhesive roller angular velocity which are the consequence of a pendular instability of the roller submitted to the peeling force. The stick-slip dynamics is shown to become intermittent due to these slow pendular oscillations which produce a quasistatic oscillation of the peeling angle while keeping constant the peeling fracture velocity (averaged over each stick-slip cycle). The observed correlation between the mean peeling angle and the stick-slip amplitude questions the validity of the usually admitted independence with the peeling angle of the fracture energy of adhesives.

The development of a novel test method to assess the durability of asphalt road–pavement materials

A better understanding of the adhesion mechanisms between the bitumen binder and the aggregate substrates, as employed in asphalt road pavements, is necessary to improve the durability of road–pavement materials. A new test protocol is outlined in the present paper to measure the characteristic fracture resistance of such bitumen–aggregate joints and to better understand the causes of failure of asphalt road surfaces, especially when associated with moisture ingress. The new test protocol is based upon a novel peel test which has been developed using a fracture mechanics approach to determine the adhesive fracture energy of bitumen–aggregate joints tested in both an unconditioned state (i.e., in the ‘dry’ state) and after being conditioned in an aqueous environment. Thus, the initial fracture resistance of the bitumen–aggregate can be assessed and the effects of any moisture-induced damage can be successfully quantified by the determination of the adhesive fracture energy. With these newly-developed techniques, the effects of using different aggregates and different bitumen grades may be quantitatively studied, since this test technique and recommended protocol may be readily adapted according to the requirements of different grades of bitumen, types of aggregate and required test conditions.

Effets de la rTMS haute et basse fréquence sur le cortex temporal : résultats préliminaires

Adhesive joints have a wide range of applications in the civil engineering, automotive and aircraft industries. In the present research, we use the finite element method to systematically study the overall strength and interface failure mechanism of single lap joints, which are subjected to tensile loading, focusing on the effects of various system parameters including fracture energy of the adhesive layer, overlap length and adhesive layer thickness on the load-bearing capability of the joints. The results show that the overlap length and the adhesive fracture energy have combined influences on the load-bearing capability. On the other hand, a preliminary damage analysis of the adhesive layer is carried out, considering the situations when the loads arrive to the peak values. Furthermore, the interface behavior is investigated, including the interface stress analysis and interface slip. The rotation of the joint during loading and its influence factors are studied as well. Obtained results suggest that the interface stress distributions are related to the slip and the rotation angle.

Taguchi analysis of bonded composite single-lap joints using a combined interface–adhesive damage model

The mechanical behaviour of bonded composite joints depends on several factors, such as the strength of the composite–adhesive interface, the strength of the adhesive and the strength of the composite itself. In this regard, a finite element model was developed using a combined interface–adhesive damage approach. A cohesive zone model is used to represent the composite–adhesive interface and a continuum damage model for the adhesive bondline. The influence of the composite–adhesive interfacial adhesion and the strength of the adhesive on the performance of a bonded composite single-lap joint was investigated numerically. A Taguchi analysis was conducted to rank the influence of material parameters on the static behaviour of the joint. It was found that the composite–adhesive interfacial fracture energy and the mechanical properties of the adhesive predominantly govern the static performance of the joints. A parametric study was performed by varying the most important material parameters, and a response surface equation is proposed to predict the joint strength. It is shown that the influence of experimental parameter variations, e.g. variation in adhesive curing and surface preparation conditions, can be numerically accommodated to investigate the static behaviour of bonded composite joints by combining finite element and statistical techniques. The methods presented could be used by practicing engineers to describe the failure envelope of adhesively bonded composite joints.

A multiscale parametric study of mode I fracture in metal-to-metal low-toughness adhesive joints

The failure of adhesively-bonded joints, consisting of metallic adherends and epoxy-based structural adhesive with a relatively low toughness ~200 J/m2, has been studied. The failure was via quasi-static mode I, steady-state crack propagation and has been modelled numerically. The model implements a ‘top-down approach’ to fracture using a dedicated steady-state, finite-element formulation. The damage mechanisms responsible for fracture are condensed onto a row of cohesive zone elements with zero thickness, and the responses of the bulk adhesive and of the adherends are represented by continuum elements spanning the full geometry of the joint. The material parameters employed in the model are first quantitatively identified for the particular epoxy adhesive of interest, and their validity is verified by comparison with experimental results. The model is then used to conduct a detailed study on the effects of (a) large variations in the geometrical configuration of the different types of specimens and (b) the adherend stiffness on the predicted value of the adhesive fracture energy, G a . These numerical modelling results reveal that the adhesive fracture energy is a strong nonlinear function of the thickness of the adhesive layer, the other variables being of secondary importance in influencing the value of G a providing the adhesive does not contribute significantly to the bending stiffness of the joint. These results which fully agree with experimental observations are explained in detail by identifying, and quantifying, the different sources of energy dissipation in the bulk adhesive contributing to the value of G a . These sources are the locked-in elastic energy, crack tip plasticity, reverse plastic loading and plastic shear deformation at the adhesive/adherend interface. Further, the magnitudes of these sources of energy dissipation are correlated to the degree of constraint at the crack tip, which is quantified by considering the opening angle of the cohesive zone at the crack tip.

Adhesive Strength and Toughness Improvement of Epoxy Resin Modified with Polystyrene-B-Polybutadiene-B-Poly (Methyl Methacrylate) Block Copolymer

Polystyrene-b-polybutadiene-b-poly (methyl methacrylate) block copolymer (SBM) was incorporated into epoxy resin to access the nanostructures in epoxy thermosets, knowing the different miscibility of polymeric blocks on epoxy matrix during the curing treatment. The morphology of modified SBM/epoxy resins was examined by Transmission and Field Emission Gun Scanning Electron Microscopy (TEM and FEG-SEM), reveling a nanostructured epoxy matrix with a dispersed micro-scale phase. The modified resins presented enhanced flexural properties, which were dependent on the composition of blend. The resin reinforced with low SBM contents (2.5 and 5 wt%) presented high values of flexural modulus and strength, while the blend reinforced with higher SBM load (10 wt%) showed higher elongation ability. This different behavior was explained by SEM analysis of fracture surfaces, which showed different toughening mechanisms. The adhesive strength and toughness were determined using carbon fiber/epoxy laminates with peel ply surface treatment as adherends. The results showed a dramatic enhancement of both properties for modified SBM/epoxy adhesives. The highest increases measured were 50 and 70% for lap shear strength and mode-I adhesive fracture energy, respectively.

Strength and interface failure mechanism of adhesive joints

Adhesive joints have a wide range of applications in the civil engineering, automotive and aircraft industries. In the present research, we use the finite element method to systematically study the overall strength and interface failure mechanism of single lap joints, which are subjected to tensile loading, focusing on the effects of various system parameters including fracture energy of the adhesive layer, overlap length and adhesive layer thickness on the load-bearing capability of the joints. The results show that the overlap length and the adhesive fracture energy have combined influences on the load-bearing capability. On the other hand, a preliminary damage analysis of the adhesive layer is carried out, considering the situations when the loads arrive to the peak values. Furthermore, the interface behavior is investigated, including the interface stress analysis and interface slip. The rotation of the joint during loading and its influence factors are studied as well. Obtained results suggest that the interface stress distributions are related to the slip and the rotation angle.

Interlayer adhesion in roll-to-roll processed flexible inverted polymer solar cells

The interlayer adhesion of roll-to-roll processed flexible inverted P3HT:PCBM bulk heterojunction (BHJ) polymer solar cells is reported. Poor adhesion between adjacent layers may result in loss of device performance from delamination driven by the thermomechanical stresses in the device. We demonstrate how a thin-film adhesion technique can be applied to flexible organic solar cells to obtain quantitative adhesion values. For the P3HT:PCBM-based BHJ polymer solar cells, the interface of the BHJ with the conductive polymer layer PEDOT:PSS was found to be the weakest. The adhesion fracture energy varied from 1.6 J/m 2 to 0.1 J/m 2 depending on the composition of the P3HT:PCBM layer. Post-deposition annealing time and temperature were shown to increase the adhesion at this interface. Additionally the PEDOT:PSS cells are compared with V 2O 5 cells whereby adhesive failure marked by high fracture energies was observed.

Characterisation of the Fracture Energy and Toughening Mechanisms of a Nano-Toughened Epoxy Adhesive

In this study the adhesive joint fracture behaviour of a nano-toughened epoxy adhesive was investigated. Two experimental test methods were used; (i) the standard tapered double cantilever beam (TDCB) test to measure the mode I adhesive joint fracture energy, GIC, as a function of bond gap thickness and (ii) a circumferentially deep notched tensile test to determine the cohesive strength of the adhesive for a range of constraint levels. It was found that the fracture energy of the adhesive followed the well-known bond gap thickness dependency [1]. SEM analysis of the TDCB fracture surfaces revealed significant plastic void growth. Finally, numerical modelling of the experimental tests suggested that most of the fracture energy was dissipated via highly localised plasticity in the fracture process zone ahead of the crack tip.

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.

Determining the Fracture Energy of Structural Adhesives from Wedge-Peel Tests

Analytical solutions for calculating the adhesive fracture energy from wedge-peel tests, where significant energy is absorbed by deformation of the substrates, are outlined. These are based on the analysis by Williams [12]. A range of structural adhesives, with fracture energies between 600 and 5300 J/m2, were used to bond aluminium-alloy substrates of thickness 1 or 2 mm, and tested using a range of pin diameters (i.e., wedge thicknesses). Three analysis methods were used, based on measuring either the radii of curvature of the substrates, the crack length or the force-per-unit-width during the test. All three methods gave good agreement with the independently measured values of the fracture energies (via linear elastic fracture mechanics). The methods were applied successfully to both cohesive and interfacial failure. The method using the force-per-unit-width was the easiest to apply, and gave the best agreement with the independently measured fracture energies.

Modelling the fracture behaviour of adhesively-bonded joints as a function of test rate

Tapered-double cantilever-beam joints were manufactured from aluminium-alloy substrates bonded together using a single-part, rubber-toughened, epoxy adhesive. The mode I fracture behaviour of the joints was investigated as a function of loading rate by conducting a series of tests at crosshead speeds ranging from 3.33 × 10 −6 m/s to 13.5 m/s. Unstable (i.e. stick–slip crack) growth behaviour was observed at test rates between 0.1 m/s and 6 m/s, whilst stable crack growth occurred at both lower and higher rates of loading. The adhesive fracture energy, G Ic , was estimated analytically, and the experiments were simulated numerically employing an implicit finite-volume method together with a cohesive-zone model. Good agreement was achieved between the numerical predictions, analytical results and the experimental observations over the entire range of loading rates investigated. The numerical simulations were able very readily to predict the stable crack growth which was observed, at both the slowest and highest rates of loading. However, the unstable crack propagation that was observed could only be predicted accurately when a particular rate-dependent cohesive-zone model was used. This crack-velocity dependency of G Ic was also supported by the predictions of an adiabatic thermal-heating model.

Comparison of fracture energy testing by means of double cantilever beam-(DCB)-specimens and lap joint testing method for the characterization of adhesively bonded wood

In this study, lap shear strength as well as specific fracture energy of bonded specimens from spruce wood were determined in order to verify whether the calculation of adhesive fracture energy is a suitable tool for the mechanical characterization of different adhesive systems, especially in comparison to the commonly used lap joint test method according to the European standard method EN 302-1. Two different test methods were applied for measuring the fracture energy by using a double cantilever beam (DCB). For both methods the DCB is separated along the bond in mode I fracture. While both adhesive systems applied showed similar shear strength values, differences were much more pronounced regarding the specific fracture energy of the bonded joints. Especially the application of the fracture energy concept, i.e., the separation of the DCB in a single load cycle, provided results with higher informative value than the results of standard lap shear tests.

Adhesion and degradation of hard coatings on poly (methyl methacrylate) substrates

The adhesion and time dependent crack growth behavior of polysiloxane based hard coatings on poly (methyl methacrylate) substrates were investigated. The adhesive fracture energies for different coatings were quantitatively characterized and varied between 1.4J/m2 and 22J/m2. Significant time dependent crack growth in various moist environments was observed and was consistent with a viscoelastic crack growth model. The effect of selected weathering treatments was also examined and resulted in a significant drop in coating adhesion. The coatings were analyzed using surface sensitive techniques; structural changes in the coatings resulting from various exposure doses were studied and mechanisms responsible for the observed degradation in adhesion were discussed.

Peeling Pressure-Sensitive Adhesive Tape from Thin Elastic Strip

In some applications of peeling of a pressure-sensitive adhesive tape, the substrate is thin and flexible, such as fabric, paper, leather, rubber, or skin. A thin strip, fixed at its ends, is considered as the substrate in this study. The strip and tape are assumed to be linearly elastic with negligible bending stiffness. The peel force depends on the adhesive fracture energy, the axial stiffnesses of the tape and strip, the initial slack of the strip, the initial location and length of the tape attached to the strip, and the peel angle of the force with respect to the line connecting the supports (or the local peel angle with respect to the side of the strip to which the tape is attached). Additional analyses involving an inextensible tape or an inextensible strip are included, along with an experimental result. The effects of various nondimensional parameters on the peel force are investigated.

Molecular structure and fracture properties of ZrOX/Epoxysilane hybrid films

The mechanical reliability of hybrid films depends critically on their fracture properties which are controlled largely by the film composition and molecular structure. We have investigated the adhesive and cohesive fracture properties of hybrid films processed from 3-glycidoxypropyltrimethoxysilane (GPTMS) and tetra n-propoxyzirconium (TPOZ), for which the roles of molecular structure and composition have not been well established. The influences of film Zr/GPTMS ratio, silane crosslinking, and substrate composition on fracture resistance were quantified in terms of the critical strain energy release rate, GC Film fracture energy was found to increase, then decrease with increasing Zr/GPTMS ratio. Removal of the epoxy rings of GPTMS from the film was found to drastically decrease the cohesive fracture energy of the film as well as the adhesive fracture energy of the film/epoxy interface. Finally, films deposited on silicon had much higher fracture energies compared to those deposited onto aluminum and titanium from identical sols. FTIR, XPS, and AFM were used to characterize the film structure and fracture surfaces. The molecular-scale mechanisms responsible for the observed trends are discussed. These results provide new insights into the interaction between the substrate chemistry, molecular structure, and mechanical reliability of hybrid sol-gel films.

Adhesive properties of ethylene and vinyl acetate copolymer. Deformation strength characteristics: Contribution of plastic deformation work

The deformation-stress curves of ethylene and vinyl acetate copolymer (EVA) are determined by traditional methods with respect to composition at various rates of deformation. The residual deformations after relief are estimated. The deformation work is calculated and the effect of copolymer composition on the value of plastic deformation is evaluated. According to the previously suggested model, the contribution of deformation characteristics of EVA adhesive to the adhesive joint fracture energy is estimated.

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 (GC). The test specimens were pressure-cycled in water-filled tanks for 1 to 4 weeks with an increment of 1 week. The GC of pressure-cycled specimens was compared with both control and water-soaked samples (without pressure-cycling). The results indicated that pressure-cycling decreased GC 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.