Search

Advanced structural adhesives are now an important joining technique in automobile and aerospace applications. The perceived uncertainty in the long-term structural performance of bonded members when subjected to static/fatigue loads in aggressive environments is probably restricting an even more widespread use of this joining technology. In this article, the effect of moisture on the static and fatigue resistances of adhesively bonded laminate joints was investigated. Experimental tests were performed on both aged and unaged adhesively bonded laminate joints for static and fatigue responses. Further, using a cohesive zone approach for the adhesive bondlines, a combined diffusion–stress analysis was developed to predict the progressive damage observed in the joints tested experimentally. The numerical predictions were found to be in good agreement with the experimental test results. [ABSTRACT FROM PUBLISHER]

Adhesively bonded metal laminates are used in aerospace applications to achieve low cost, light weight structures in the aerospace industry. Advanced structural adhesives are used to bond metal laminae to manufacture laminates, and to bond stringers to metal laminate skins. Understanding the failure behaviour of such bonded structures is important in order to provide optimal aircraft designs. In this paper, the static failure behaviour of adhesively bonded metal laminate joints is presented. A cohesive zone model was developed to predict their static failure behaviour. A traction-separation response was used for the adhesive material. Three joint configurations were considered: a doubler in bending, a doubler in tension and a laminated single lap. The backface strains and static failure loads obtained from experimental tests were used to validate the results from finite element modelling. The models were found to be in good agreement with experiments. [ABSTRACT FROM AUTHOR]

Adhesive bonding is an attractive joining method for fibre reinforced polymers (FRPs); however, if adhesive joints are to be used in structural applications, methods of accurately predicting mechanical performance are required. In many structural applications the joints will be subjected to cyclical loading, in which case fatigue performance must be considered. One of the most useful methods of analysing fatigue is through the application of fracture mechanics, in which the fatigue crack growth rate is related to an appropriate fracture mechanics parameter, such as strain energy release rate. This relationship is often plotted on a fatigue crack growth (FCG) plot and an empirical FCG law derived by fitting an appropriate form of equation to this plot. However, in bonded composite joints, anomalous FCG is seen when mixed mode samples are tested. In this paper, this anomalous behaviour is explained and a mechanistically based model for predicting this behaviour is proposed. The model is applied to experimental data and a good fit is seen. [ABSTRACT FROM AUTHOR]

Structural adhesives are used widely in aerospace and automotive applications. However, fatigue damage in these adhesives is an important factor to be considered in the design of adhesively bonded structural members that are subjected to cyclic loading conditions during their service life. Fatigue life of adhesively bonded joints depends mainly on the fatigue load and the load ratio. A fatigue damage model is presented in this paper to include the effect of fatigue mean stresses on the failure behaviour of adhesively bonded joints. The fatigue damage model is developed using an effective strain-based approach. The model is implemented on a tapered single lap joint configuration and is validated by experimental test results. The adhesive layer in the tapered single lap joint is modelled by using a cohesive zone with a bi-linear traction-separation response. The adverse effect of increasing fatigue mean stresses on the failure behaviour of adhesively bonded joints is successfully predicted. [ABSTRACT FROM AUTHOR]

The measurement of fatigue damage in adhesive bonding has been investigated. Bulk adhesive was used in this study for two reasons: the stress distribution of adhesives in bulk is simpler to investigate than adhesives in joints; and the specimen dimensions met fatigue test standards. Bulk adhesive was made from a film type of epoxy resin. In general, the characteristics and the behaviour of bulk adhesive may differ from adhesive in joint because of the presence of voids and the constraints imposed by the substrates. Low cycle fatigue tests with a load amplitude ratio of 0.1 at a frequency of 5 Hz were performed to determine the damage as a function of the number of cycles. Damage curves, i.e., the evolution of the damage variable as a function of number of cycles, were derived and plotted using an isotropic damage equation. Damage was evaluated using the decrease of stress range during the lifecycles of a constant displacement amplitude test. It was found that the damage curves were well fitted by a low cycle, fatigue damage evolution law equation. This equation was derived from a dissipation potential function. Curve fitting was performed using the robust least square technique rather than ordinary linear least square technique because damage curves have extreme points (usually near the failure point). It was found that the fitting process would not converge for adhesive fractures at high cycle values (Nf > 9000). Two damage constants A and β were found from the fitting process. Each fatigue set of data, at a certain level of von-Mises stress range for the undamaged state or at the stabilized hardened state, (Δσ*eq), had a different set of damage parameters A and β. Linear regression of these points was used to express A and β as a function of Δσ*eq. Using these expressions, damage curves for different levels of Δσ*eq could be predicted. [ABSTRACT FROM AUTHOR]

This paper presents a methodology for predicting moisture concentration in an epoxy adhesive under cyclic moisture absorption-desorption conditions. The diffusion characteristics of the adhesive were determined by gravimetric experiments under cyclic moisture conditions and the dependence of diffusion coefficient and saturated mass uptake on moisture history was determined. Non-Fickian moisture absorption was observed during absorption cycles while moisture desorption remained Fickian. The diffusion coefficient and saturated moisture content showed variation with absorption-desorption cycling. A finite element-based methodology incorporating moisture history was developed to predict the cyclic moisture concentration. A comparison is made between the new modelling methodology and a similar method that neglects the moisture history dependence. It was seen that the concentration predictions based on non-history dependent diffusion characteristics resulted in over-prediction of the moisture concentration in cyclic conditioning of adhesive joints. The proposed method serves as the first step in the formulation of a general methodology to predict the moisture dependent degradation and failure in adhesives. [ABSTRACT FROM AUTHOR]

A mesh-independent continuum damage model has been proposed to predict the residual strength of adhesively-bonded joints by introducing a displacement-based damage parameter into the constitutive equation of the damaged materials. This approach was originally developed for EA9321-bonded single-lap joints and this paper extends it to butt joints bonded with a different ductile adhesive (E32). This involves not only a different adhesive and joint configuration but the high hydrostatic stress requires a more realistic yielding model. A dry and a partially saturated steel joint were used to calibrate the moisture dependent damage parameters of the adhesive E32, and then these parameters were used without further modification to predict the failure of the other environmentally degraded steel butt joints and the aluminum butt joints. The FEA package ABAQUS was used to implement the coupled mechanical-diffusion analyses required. A von Mises yield model was used initially. Then, a linear Drucker–Prager plasticity model was used, as this could incorporate the hydrostatic stress dependency found in the adhesive. The predicted joint residual strengths agreed well with the corresponding experimental data, which exhibited a significant reduction in strength on exposure to moisture but still retained a cohesive failure mode in the adhesive. The mesh independence of the model was also demonstrated. [ABSTRACT FROM AUTHOR]

The use of a cohesive zone model (CZM) to predict the long-term durability of adhesively bonded structures exposed to humid environments has been investigated. The joints were exposed to high relative humidity (RH) environments and immersion in both tap and deionised water for up to a year before quasi-static testing to failure. Both stressed and unstressed conditions during aging were considered. The degradation was faster for the stressed joints and for those joints immersed in the more corrosive environments. Two mechanisms were suggested to explain this behaviour: cathodic delamination and stress-enhanced degradation. In the model, the cohesive zone parameters determine the residual strength of the joints. The degradation of these parameters was, in the first instance, related directly to the moisture concentration. The model was then extended to include degradation due to stress and more corrosive environments. Good correlation between the numerical modelling and the experimental results was obtained. [ABSTRACT FROM AUTHOR]

A progressive cohesive failure model has been proposed to predict the residual strength of adhesively bonded joints using a moisture-dependent critical equivalent plastic strain for the adhesive. Joints bonded with a ductile adhesive (EA9321) were studied for a range of environmental degradations. A single, moisture-dependent failure parameter, the critical strain, was calibrated using an aged, mixed-mode flexure (MMF) test. The mesh dependence of this parameter was also investigated. The parameter was then used without further modification to model failure in aluminum and composite single-lap joints (SLJ) bonded with the same adhesive. The FEA package ABAQUS was used to implement the coupled mechanical-diffusion analyses required. The elastic–plastic response of the adhesive and the substrates, both obtained from the bulk tensile tests, were incorporated. Both two-dimensional and three-dimensional modelling was undertaken and the results compared. The predicted joint residual strengths agreed well with the corresponding experimental data, and the damage propagation pattern in the adhesive was also predicted correctly. This cohesive failure model provides a simple but reliable method to model environmental degradation in ductile adhesive bonded joints, where failure is predominantly within the adhesive layer. [ABSTRACT FROM AUTHOR]

This work is concerned with developing numerical modelling techniques for predicting the environmental degradation of adhesively-bonded joints. Associated experimental data are also reported. The moisture-dependent mechanical properties of the adhesive were obtained by testing bulk specimens also exposed to various moisture contents. The diffusion parameters for moisture in the adhesive were determined by carrying out gravimetric experiments on bulk adhesive samples. The moisture-dependent interfacial bond strength of the adhesive system investigated has been determined by testing a mixed mode flexure (MMF) specimen, exposed to obtain various levels of moisture content at the interface. Progressive damage in the joints was modelled with a two-parameter cohesive zone model (CZM). The CZM parameters were determined by correlating the experimental data obtained from the MMF test with results from the numerical simulation. The parameters were then used to predict the response of another configuration, the notched coating adhesion (NCA) specimen. When the residual stresses were neglected in the modelling, the predicted NCA response was seen to be in good agreement with the experimental data. However, initial simulations that included the residual stresses resulted in poor predictions of the NCA response. Creep tests on the saturated adhesive, at the ageing temperature, showed large viscoplastic deformations at low loads. Coupled diffusion-stress modelling, including viscoplastic material properties for the adhesive continuum, showed that the residual stresses for the aged specimens decreased significantly and thus did not contribute strongly to the environmental weakening. Good predictions were then obtained for the NCA tests. [ABSTRACT FROM AUTHOR]

It is now well established that adhesives and fibre-reinforced polymers (FRPs) can fail at far lower loads under fatigue loading than under quasi-static loading. Most of the work on the fatigue of these materials has been under constant amplitude (CA); however, recent work has shown that fatigue crack growth acceleration and unstable crack growth can be seen under conditions of variableamplitude (VA) fatigue. In this paper, further evidence of this effect is given. This phenomenon is attributed to the influence of VA fatigue on the evolution of a damage zone ahead of the main crack front and results fromthe characterization of these damage zones are also presented. Finally, a simple fracture mechanics-based model for predicting crack growth under VA fatigue is proposed. This is shown to be entirely consistent with the experimental observations. [ABSTRACT FROM AUTHOR]

The main aim of this article is to investigate the effect of frequency on fatigue crack propagation in adhesively bonded joints. Adhesively bonded double-cantilever beam (DCB) samples were tested in fatigue at various frequencies between 0.1 and 10 Hz. The adhesive used was a toughened epoxy, and the substrates used were a carbon fibre-reinforced polymer (CFRP) and mild steel. Results showed that the crack growth per cycle increases and the fatigue threshold decreases as the test frequency decreases. The locus of failure with the CFRP adherends was predominantly in the adhesive layer, whereas the locus of failure with the steel adherends was in the interfacial region between the steel and the adhesive. The crack growth was faster, for a given strain energy release rate, and the fatigue thresholds lower for the samples with steel adherends. Tests with variable frequency loading were also carried out, and a generalised method of predicting crack growth in samples subjected to a variable frequency loading was introduced. The predicted crack growth using this method agreed well with experimental results. [ABSTRACT FROM AUTHOR]

Reliable predictive modelling of the environmental degradation of adhesively bonded structures is required for a more widespread use of this joining technique. Recent durability modelling has coupled moisture diffusion and stress analysis, where the joint response is controlled by continuum degradation of the adhesive. However, the joint response is more commonly controlled by degradation of the interface. Current research extends existing durability modelling to include interfacial degradation and failure. Experimental studies have been undertaken to provide the moisture uptake parameters and moisture-dependent properties, both for the constitutive behaviour of a bulk epoxy and for the fracture energy of an epoxy-steel interface that has been exposed to various uptake levels of moisture. The mixed mode flexure (MMF) test was used to determine the interfacial strength. It was found that the interface fracture energy reduces with increasing interfacial moisture concentration. Interfacial rupture elements were developed to model the complete progression of damage within a joint from a single FE analysis. These rupture elements were formulated for mixed mode conditions and followed a separation law that used the fracture energy and the tripping strain as the controlling parameters. The role of these parameters was investigated, and it was shown that as long as there is a continuous process zone these elements respond well. This can be achieved as long as the tripping strain remains below a (mesh-dependent) critical value. Moisture-dependent fracture energies and tripping strains were then determined by calibration using the initial crack length data from the MMF specimens. These parameters were subsequently used to predict the response with increasing crack length, and excellent predictions were obtained. [ABSTRACT FROM AUTHOR]

Predicting the service life of adhesive joints under fatigue loading remains a major challenge. A significant part of this task is to develop laws that govern the crack initiation phase. This paper contributes to this area through the development and application of the backface strain technique. A numerical study was carried out to investigate the effect of key parameters on the technique and to determine optimum gauge specification and location. Calibration curves were then produced relating the change in strain to the extent of damage. These numerical studies were then validated by undertaking a series of fatigue tests on both aluminium and GRP (glass-reinforced polymer)-bonded joints. Following various degrees of predicted damage the joints were carefully sectioned, polished, and studied using optical microscopy. The predicted and observed damage showed close correlation. The fatigue tests have also indicated that, for unmodified joints (intact fillets), even at high loads (50% static failure load) there was an initiation phase that accounted for about half the fatigue life of the joint. Removal of the adhesive fillet has been found to eliminate the initiation phase and consequently reduce fatigue life. [ABSTRACT FROM AUTHOR]