The influence of mode mixity and adhesive system on the fatigue life of adhesive joints (original) (raw)
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Fatigue life estimation of adhesive joints at different mode mixities
The Journal of Adhesion, 2020
Fatigue life prediction of adhesive joints is a concern when using this bonding technique in structural components where the joints are subjected to multiaxial stress states. Thus, the development of a fatigue failure criterion that accounts for the effect of mode mixity is crucial. The main aim of the present study is to develop a fatigue model to be able to predict the fatigue life of an epoxybased adhesive loaded at different mode mixities. For this purpose, Arcan joints were manufactured and tested in pure modes (I and II), and mixed mode loading conditions. Using the obtained experimental data for pure mode loading conditions, a master S-N curve was constructed based on a proposed effective stress. According to this curve and by considering the numerical simulations, the fatigue lives of the joints under mixed mode conditions were estimated. Two different relations (Basquin and Stromeyer) were considered in this study. The influence of the number of load levels considered for the master curve was also analysed. It was found that more experimental results for the reference samples do not necessarily lead to a better estimation. Results show a good agreement between the experimental data and the predicted fatigue lives based on the proposed model.
Fatigue & Fracture of Engineering Materials & Structures, 2019
Adhesive joints usually experience mixed mode and mostly cyclic stresses conditions during their service life. The aim of the current research is to investigate the fatigue behaviour of a structural epoxy adhesive. Pure modes I and II and mixed mode tests were carried out to study the fracture and fatigue crack growth (FCG) behaviour of the adhesive. Compliance-based beam method was considered for experimental fracture energy measurement. The effects of load level and load ratio on the mode I FCG behaviour and Paris law parameters were also investigated. Result showed that the effect of load level on fatigue crack propagation is more pronounced for lower R ratios. It was found that when the crack faces are closer during the unloading process, the difference between the R 2 and G min /G max is higher. Some possibilities are the crack closure phenomenon, difficulty in measuring the G min , and the employed data reduction approach.
Effects of Mode Mixity on the Failure Mechanism of Aged Adhesive Joints
Journal on Mechanics of Solids
The strength and the performance of adhesive joints can be significantly influenced by the ageing procedure. However, the role of aging in the failure mechanism of adhesive joints as a function of loading conditions has not been studied yet. The current research aims to investigate the effect of mode mixity on the failure mechanism of aged adhesive joints using Arcan samples. Based on the results loading the tested joints in shear led to a higher percentage of interfacial failure than in tensile. However, it was also found that the shear loading mode is less sensitive (compared to tensile loading) to the ageing conditions.
Fatigue crack growth analysis of different adhesive systems: Effects of mode mixity and load level
Fatigue & Fracture of Engineering Materials & Structures, 2019
The aim of this paper is to investigate the fatigue fracture behavior of three different adhesive systems (epoxy-based, acrylic, and a rubber-like adhesive). To achieve this, double cantilever beam specimens were manufactured with different adhesives and tested under several mode mixities and different load levels. Fatigue crack growth rate was evaluated through a Paris law equation. For postprocessing, the compliance-based beam method was used. Results showed that the variation of the threshold energy with load level is more pronounced for the epoxy-based adhesive. The crack propagation life is higher for the acrylic adhesive. Although, for pure mode I conditions, the normalized threshold of the rubber-like adhesive is lower, for pure mode II, it was higher than the epoxy-based adhesive. Due to the normalization by the static fracture energy, the slope of the Paris law was approximately constant for all the adhesive systems.
Fatigue behaviour of an epoxy adhesive under mixed-mode conditions
Tecnologia em Metalurgia, Materiais e Mineração, 2021
The aim of this study is to characterize the fatigue behaviour of a structural epoxy adhesive loaded in mixedmode conditions. To this effect, several fracture tests were carried out: Double Cantilever Beam (DCB) for pure mode I, End Notched Flexure (ENF) for pure mode II, and mixed-mode testing, using an apparatus that enables the application of mixed-mode loadings to an adhesive and with high strength steel adherends. The fracture envelope was calculated using experimental data and it was found to be compatible with a quadratic criterion. A novel quadratic law criterion was developed to fit the envelope. The fatigue behaviour was studied recurring to fatigue crack growth curves for the same types of loads studied in the quasi-static tests. A relationship between mixed-mode phase angle and the Paris law slope was found, where the existence of mode II loads was found to delay crack propagation. By performing fatigue tests at 60% and 80% of the maximum load, it was noticed that increasing the load leads to the increase of the threshold energy release rate values.
A new testing methodology for the assessment of fatigue properties of structural adhesives
International Journal of Adhesion and Adhesives, 1996
The monotonic and fatigue shear behaviour of an epoxy adhesive joint has been studied using a short overlap-thick adherend configuration. A specific in situ bonding procedure has been developed in order to accurately control the initial stress state of the joint before testing. The strength of the joint has been found to be strongly dependent on the strain rate and the joint thickness. This dependency was associated with a change in damage mechanisms from cohesive failure (for small joint thickness or elevated speed) to adhesive failure (for large joint thickness and low speed). This was attributed to changes in peel stresses and/or joint morphology with thickness. Fatigue tests were carried out at imposed strain amplitude. Fatigue logs giving the changes in the shape of the transverse loaddisplacement loops were at first considered. The latter have been found to be useful to differentiate between the different failure modes (i.e. cohesive and adhesive). In a second step, all the results were summarised in a fatigue map giving the endurance properties and the failure mode of the joint as a function of the joint thickness and the strain level. This method should be generally useful whenever an accurate determination of the contribution of adhesive and interface behaviour is required for the assessment of joint durability.
Evaluation of Fatigue Damage in Adhesive Bonding: Part 1: Bulk Adhesive
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 (N f > 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, ( σ * 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 σ * eq . Using these expressions, damage curves for different levels of σ * eq could be predicted.
Almost all structural applications of adhesive joints will experience cyclic loading and in most cases this is irregular in nature, a form of loading commonly known as variable amplitude fatigue (VAF). This paper is concerned with the VAF of adhesively bonded joints and has two main parts. In the first part, results from the experimental testing of adhesively bonded single lap joints subjected to constant and variable amplitude fatigue are presented. It is seen that strength wearout of bonded joints under fatigue is non-linear and that the addition of a small number of overloads to a fatigue spectrum can greatly reduce the fatigue life. The second part of the paper looks at methods of predicting VAF. It was found that methods of predicting VAF in bonded joints based on linear damage accumulation, such as the Palmgren-Miner rule, are not appropriate and tend to over-predict fatigue life. Improved predictions of fatigue life can be made by the application of non-linear strength wearout methods with cycle mix parameters to account for load interaction effects.
Fracture Behavior of Typical Structural Adhesive Joints Under Quasi-Static and Cyclic Loadings
SAE International Journal of Materials and Manufacturing, 2010
Structural adhesive joints are expected to retain integrity in their entire service-life that normally involves cyclic loading concurrent with environmental exposure. Under such a severe working condition, effective determination of fatigue life at different temperatures is crucial for reliable joint design. The main goal of this work was thus defined as evaluation of fatigue performance of adhesive joints at their extreme working temperatures in order to be compared with their fracture properties under static loading. A series of standard double-cantilever-beam (DCB) specimens have been bonded by three structural 3M epoxy adhesives selected from different applications. The specimens were tested under monotonic and cyclic opening loads (mode-I) in order to evaluate the quasi-static and fatigue performances of selected adhesives at room temperature, 80°C and-40°C. The test results revealed that the fatigue damage occurred at relatively low load levels when compared to quasi-static fracture forces. At room temperature, the maximum cyclic fatigue forces varied between 25% and 40% of corresponding quasi-static fracture loads of selected adhesives. More significant reductions in adhesive mechanical performances were observed at 80°C. At cryogenic temperature, the adhesives had their own characterizations; mainly increasing the fatigue resistance but very sensitive to testing parameters such as loading rate or crosshead speed. In conclusion, the experimental observations showed a significant influence of fatigue loading on adhesive joints mechanical performances that should be considered in joint design, particularly at non-ambient temperatures.