M. Abdel Wahab

The effect of environment on the fatigue of bonded composite joints, part 1: testing and fractography

In this work, the effect that test environment and pre-conditioning had on the fatigue behaviour of CFRP/epoxy lap–strap joints was investigated. It was shown that the fatigue resistance of the lap–strap joints did not vary significantly until the glass transition temperature, Tg, was approached, at which point a considerable reduction in the fatigue threshold load was observed. It was also noted that absorbed moisture resulted in a significant reduction in the Tg of the adhesive. This must be taken into account when selecting an adhesive to operate at elevated temperatures. The locus of failure of the joints was seen to be highly temperature dependent, transferring from primarily in the composite adherend at low temperatures to primarily in the adhesive at elevated temperatures. It was also seen that as the crack propagated along the lap–strap joint, the resolution of the forces at the crack tip tended to drive it into the strap adherend, which could result in complex mixed mode fracture surfaces.

Prediction of fatigue thresholds in adhesively bonded joints using damage mechanics and fracture mechanics

The prediction of fatigue threshold in composite adhesively bonded joints using continuum damage mechanics (CDM) and fracture mechanics (FM) approaches has been investigated. Two joint types were considered in this study: double lap (DL) and lap strap (LS) joints. The substrates, which were made of uni-directional (UD) or multi-directional (MD) composite laminates, were bonded together using an epoxy film adhesive. The joints were tested under fatigue loading with a load amplitude ratio of 0.1 at various test temperatures. Damage evolution laws were derived using thermodynamics principles. The number of cycles to failure was then expressed in terms of the stresses in the adhesive layer and material constants. The stresses were calculated from non-linear finite element analyses, considering both geometrical and material non-linearities. The damage laws generated for the UD/DL joint data were then used to predict the fatigue crack initiation thresholds for the MD/DL, UD/LS, and MD/LS joints. The FM approach uses the crack closure integral method to compute the strain energy release rate at the threshold load (G th) from the results of geometrical non-linear finite element analysis. The G th value for an inherent crack size at the centre of the bondline in the UD/LS joint is used as the failure criterion in order to predict the fatigue threshold for the MD/LS, UD/DL, and MD/DL joints. It was found that the predictions using CDM were slightly more accurate than those obtained using the FM approach. In general, when predicting the fatigue thresholds of the LS joints using the DL joints data, or vice versa, good agreement was obtained between the measured and predicted thresholds at ambient and low temperatures, but poor agreement was seen at the high test temperature. This was attributed to the deleterious effect of creep, which was greater in the DL joints than in the LS joints.

Investigating fatigue damage evolution in adhesively bonded structures using backface strain measurement

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.

Environmental degradation of the interfacial fracture energy in an adhesively bonded joint

The mixed mode flexure and notched coating adhesion tests have been carried out in order to characterise interfacial fracture for a range of environmental exposure conditions and to find a meaningful interfacial strength parameter using a fracture mechanics approach. The moisture uptake of the adhesive was accelerated using an open-faced configuration. The critical loading to cause interfacial fracture was measured and was used in conjunction with finite element analysis (FEA) to determine the fracture energy under various exposure conditions. Moisture dependent material properties were incorporated in the FEA. Scanning electron microscopy was used to characterise the nature of the failure surface. Significant degradation of the fracture energy of the interface was found and this was matched by observed changes to the failure surface. The fracture energies were found to be largely independent of test method, exposure environment and time and was primarily related only to the moisture concentration.

Numerical prediction of fatigue crack propagation lifetime in adhesively bonded structures

Abstract In this short paper, a generalised numerical procedure using finite element (FE) analysis for prediction of the fatigue lifetime of adhesively bonded structures is proposed. The number of cycles to failure (Nf) is calculated by integrating a fatigue crack growth law between initial and final crack lengths. This crack growth law is formulated in terms of the strain energy release rate (SERR), which is determined, at any crack length, from an FE analysis. This complete process is implemented within the FE code, enabling automated calculation of the fatigue life for a given set of boundary conditions. This is a development of the approach outlined for single-lap joints [Int. J. Fract., 103 (2000) 41]. However, being fully implemented within an FE code it is not limited by the approximations of the simplified analytical expressions and furthermore can be applied to any structural configuration. The procedure was evaluated by application to a single-lap joint and good results were obtained in comparison with those using other methods. Furthermore, the use of the total SERR (GT) and mode I SERR (GI) as crack-propagation-controlling parameters are investigated and briefly discussed.

Coupled stress-diffusion analysis for durability study in adhesively bonded joints

Abstract Coupled stress-diffusion finite element analyses, which are required to study the durability of adhesively bonded joints aged in hot/wet environment, are presented. Two bonded joints have been considered in this study, namely, single lap joint and butt joint. The joints were immersed in water at 60°C for up to 60 weeks. For both joints, transient finite element diffusion analyses have been performed in order to determine the moisture distribution in the adhesive layer at different time intervals. The results of these simulations were coupled to non-linear stress finite element analyses in which the constitutive data of the adhesive was defined as a function of moisture concentration. Four different time intervals have been considered for each joint, namely, 0, 6, 12 weeks and fully saturated condition. The swelling strains have been taken into account in the stress analysis and have been introduced to the adhesive layer according to the moisture distribution at a particular time. These simulations can explain the experimental results of the effect of moisture on joint strength. Further, FE diffusion simulations have been carried out on various configurations of adhesive resin diffusion discs. Together with the experimental mass uptake data these have been used to study the effect of the interface on the moisture sorption.

Effect of Temperature on the Quasi-static Strength and Fatigue Resistance of Bonded Composite Double Lap Joints

Abstract Fibre reinforced polymer composites (FRPs) are often used to reduce the weight of a structure. Traditionally the composite parts are bolted together; however, increased weight savings can often be achieved by adhesive bonding or co-curing the parts. The reason that these methods are often not used for structural applications is due to the lack of trusted design methods and concerns about long-term performance. The authors have attempted to address these issues by studying the effects of fatigue loading, test environment and pre-conditioning on bonded composite joints. Previous work centered on the lap-strap joint which was representative of the long-overlap joints common in aerospace structures. However, it was recognised that in some applications short-overlap joints will be used and these joints might behave quite differently. In this work, double-lap joints were tested both quasi-statically and in fatigue across the temperature range experienced by a jet aircraft. Two variants on the double-lap joint sample were used for the testing, one with multidirectional (MD) CFRP adherends and the other with unidirectional (UD) CFRP adherends. Finite element analysis was used to analyse stresses in the joints. It was seen that as temperature increased both the quasi-static strength and fatigue resistance decreased. The MD joints were stronger at low temperatures and the UD joints stronger at high temperatures. It was proposed that this was because at low temperature the strength was determined by the peak stresses in the joints, whereas, at high temperatures, strength is controlled by creep of the joints which is determined by the minimum stresses in the joint. This argument was supported by the stress analysis.

Modelling interfacial degradation using interfacial rupture elements

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.

Diffusion of Moisture in Adhesively Bonded Joints

Abstract In this article, diffusion of moisture in adhesively bonded composite joints is discussed and analysed experimentally, analytically and numerically. The experimental studies concentrate on moisture diffusion in adhesive films and in unidirectional and multidirectional composite substrates exposed to two different conditioning environments, namely 45°C/85% RH and 90°C/97% RH for the absorption studies and 90°C/ambient for the desorption studies. The coefficients of diffusion are determined from the water uptake plots. The analytical solutions for diffusion in joints with impermeable adherends are based on the classical theory of diffusion and are used to derive equations in two-dimensions for different adhesive fillet shapes, namely radiused fillet, triangular fillet and rectangular fillet. In the finite element analysis, the diffusion of moisture from the composite substrates into lap-strap joints is also taken into account. Both unidirectional and multidirectional composites are considered, as well as two different fillet shapes, i.e., rectangular and triangular fillet. A comparison between the results obtained using FEA and those obtained using the analytical solution is made. Finally, fatigue test data for lap-strap joints aged and tested in different environments is presented and a tentative link between fatigue threshold and water concentration at the site of failure initiation is made, indicating a semi-empirical method of predicting the strength of joints subjected to moisture-induced degradation.

The effect of environment on the fatigue of bonded composite joints. Part 2: fatigue threshold prediction

In Part 2 of this paper, the fatigue strength of adhesively bonded joints is analysed using stress analysis and fracture mechanics. Composite lap strap joints are considered for this study. Substrates made of unidirectional and multidirectional composite laminates are bonded together using epoxy film adhesive. Non-linear stress and fracture analyses are performed in order to predict the strength of the joints in different hostile environmental conditions. The use of several threshold criteria is investigated. Criteria based on the principal stress provide good threshold prediction for small plastic deformation. The maximum principal strain, von Mises strain, shear stress and von Mises stress predict with good accuracy the fatigue thresholds of the joints that undergo large plasticity. The plastic zone size does not show correlation at the fatigue threshold load in the different joint types. Furthermore, elastic and elasto-plastic fracture parameters are calculated for two different inherent cracks. The first one is initiated at the adhesive/adherend interface, while the second is in the centre of the adhesive layer. The J-integral that accounts for plasticity and the elastic strain energy release rate have been shown to correlate with the threshold load for the different joints.