A combination of bonding and bolting can potentially improve the joining efficiency of composite structures. Several researchers have shown that in specific cases, the joint yield and ultimate ...strength can be improved in this manner compared to the underlying joints separately. However, their results also demonstrate a strong sensitivity to various design parameters. In this study, the effect of adhesive layer compliance is experimentally investigated on the single-lap hybrid bonded-bolted joint strength. It is found that for a bonded joint with a low compliance adhesive, there is no benefit to adding a fastener on the initial (adhesive) failure. For a high compliance adhesive, it is found that the addition of a fastener significantly delays the initial failure. A mechanism is proposed – supported by a numerical model – that explains the observed behaviour.
Though aircraft joints conventionally employ either mechanical fastening or adhesive bonding, the hybrid bolted/bonded joint has recently appeared as an alternative with possible superior ...performance. A key aspect of the hybrid joint design is the load-sharing between the bolts and the bond, and performance can benefit from using a flexible adhesive. While the static properties of flexible epoxy adhesives are covered in the literature, the properties under cyclic loading are not fully understood. This study investigates the mechanical properties of flexible epoxy adhesives under cyclic loading and the corresponding effect on composite hybrid joints. This study is twofold. First, cyclic tensile tests on the bulk adhesive investigate the evolution of the adhesive stress/strain. The results show that the modulus and yield stress progressively decrease due to the accumulation of plastic deformation. As the load cycles continue, the stress/strain response converges limiting this accumulation. Subsequently, cyclic tension-tension tests are performed on hybrid joints. Attributed to the aforementioned behaviour of the adhesive properties, the bolt load-sharing is observed to progressively increase until a convergence is reached. This paper provides the understanding on the evolution of the mechanical properties of flexible adhesives under cyclic loading and further confirms their potential in hybrid joint applications.
Most studies usually use load-displacement curve or post-failure analysis to understand the mechanical behaviour of hybrid bonded-bolted (HBB) joints. While load sharing in HBB joints is the crucial ...design parameter and it is important to capture strain field changes or failures under real-time loading conditions. Therefore, two-dimensional digital image correlation (2D-DIC) technique is applied to experimentally investigate the behaviour of bonded and HBB composite joints under quasi-static tensile loading. A flexible epoxy paste adhesive is used. The bonded joint had a hole in the centre to determine the effect of the difference between the stress concentration due to the existence of the hole and the additional stress around a hole due to installation of the bolt. The strain fields around the bolt in a HBB joint and around the open hole in a bonded joint were compared accurately. The DIC technique was able to effectively and rapidly measure the strain field and identify the onset of failure around the fastener during the tests. Finally, the advantages and disadvantages of the DIC technique compared to the classical similar techniques for characterizing the composite joints are briefly explained.
Fiber-reinforced polymer (FRP) structures, due to their low mass, are valuable alternatives to traditional steel or concrete structures in seismic areas. However, to resist seismic actions, FRP ...structures must be able to dissipate a significant amount of inelastic energy. Since FRP materials are brittle, this dissipation must occur in the joints. Monotonic tension and cyclic tension–compression experiments were thus performed on adhesive and bolted double-lap joints composed of pultruded glass fiber-reinforced polymer (GFRP) profiles; a flexible adhesive was used in the adhesive joints. A significant amount of energy was dissipated in the adhesive joints at lower and medium displacement rates through viscoelastic friction and damage in the adhesive, while almost no energy dissipation occurred at the highest rate. The energy in the bolted joints was dissipated by progressive crushing and shear-out failures in the inner laminates. Although the dimensions and monotonic strength of the adhesive and the bolted joints were similar, the former dissipated significantly more energy at the two lower applied displacement rates. The obtained results can contribute to the seismic design of inelastic joints in FRP structures.
Various geometric and material parameters such as adhesive thickness, adhesive overlap, adherend thickness, and composite layup may affect the strength of the joint under quasi-static loading and ...failure mode of bonded composite-to-composite single-lap joints (SLJs) and are investigated by previous studies. The study herein broadens these findings by looking into the effect of lamina fiber angle adjacent to the adhesive layer on the damage initiation and evolution in detail. In this regard, a composite-to-composite adhesively bonded SLJ with adherends made of E-glass/epoxy composites and 04//θ/03 (where//shows the adhesive location) layups are manufactured and tested under quasi-static tensile loading. The adhesive type is semi-flexible Araldite 2015. Experimental results show that by increasing the fiber angle from 0° to 90°, the shear stress in the adhesive layer is decreased while the peel stress is increased. In examining typical fracture interfaces for each layup configuration, a full description of failure mode assessment is obtained. In particular, the SLJ is modeled in Abaqus using cohesive elements with bilinear traction-separation law. Numerical results indicate that the bilinear cohesive law cannot model the exact load-displacement curve due to semi-flexible behavior of the epoxy adhesive, but it can predict maximum strength precisely. The failure of composite joints is significantly influenced by shear stress.
This study investigates timber connections with flexible polyurethane adhesives, which prove to have the potential for timber-adhesive composite structures without mechanical connections for seismic ...regions. Results of conducted cyclic double lap-shear adhesive timber joints tests were compared with available experimental results on timber connections with standard mechanical dowel-type fasteners and with results of numerical finite element analysis. The study found that the shear strength, elastic stiffness and strength degradation capacity of the flexible adhesive connections were significantly higher compared to mechanical fasteners commonly used in seismic-resistant timber connections. The latter, however, manifested larger ultimate displacements but also yielded at lower displacements.
To study the influence of temperature on the fatigue performance of adhesive structures, polyurethane adhesive was used to make butt joints (BJs). Using an environmental-fatigue coupling loading ...devices, the temperature points of −40, −10, 20, 50 and 80°C were selected to carry out tension-tension fatigue tests on BJs. The S-N curves of the joints were obtained. The quasi-static and fatigue failure sections of the BJs were observed by macrography and scanning electron microscopy (SEM). The results show that temperature has a significant effect on the quasi-static and fatigue properties of the flexible adhesive. As the temperature increases, the failure strength, failure strain and Young's modulus of the adhesive decrease, and the fatigue performance of BJs decreases gradually. The fatigue failure sections of BJs vary greatly at different temperatures. When the temperature rises, the polymer chain begins to move and the viscoelastic property of the adhesive becomes obvious. Cavitation has obvious effect on the fatigue life of BJs. Fatigue failure is caused by multiple cracks and cavities, leading to localized stress concentration. The functional relationship for the temperature-nominal stress-fatigue life is established, which can provide a reference for fatigue performance prediction of vehicle bonding structures.
•A new simplified data reduction method based of the Wedge-Driven Test is proposed.•GIC values obtained from the DCB test and the WDT+ are in very close agreement.•The WDT+ is fully-objective and ...avoids crack length measurement or COF estimation.•The range of test velocities at which quasi-static conditions are met is increased.•The WDT+ is suitable for any type of adhesive and bondline thickness configuration.
The resistance to fracture of a bonded joint is usually evaluated through the experimental characterisation of its fracture toughness. Different test procedures and data reduction methods are available in the literature, such as the ISO-25217 test standard. The need for crack length measurement has led to inaccurate results when large adhesive thicknesses or flexible adhesives were studied. Advanced test protocols based on the insertion of a wedge, such as the Wedge Test (WT) in ASTM D3762 or the Wedge-Driven Test (WDT), simplify the test procedure, but most such procedures do not allow for the measurement of fracture toughness. Further, experimental results are scarce in the literature, especially when dealing with bonded joints and considerable thicknesses. In the present work, an advanced data reduction method based on the WDT is presented: The Wedge-Driven Test Plus (WDT+). The fracture toughness of specimens made by bonding two aluminium adherends with a rigid and a flexible adhesive was measured, and the effect of wedge thickness and test speed at low displacement rates on the test results was analysed. The results were compared with static tests performed in a double cantilever beam (DCB) following ISO-25217. It has been demonstrated that the proposed data reduction method is less sensitive to test speed and enables different adhesive types, and bondline thickness and wedge thickness configurations, thereby improving the WDT available in the literature.
Hybrid bolted/bonded joints are less effective when designed with strong structural adhesives as insignificant load is introduced to the bolt before the bond reaches failure. Advancement in hybrid ...joint design requires further knowledge on the behavior of flexible adhesives, which involve significant complexities such as large inelastic deformation. This study investigates the mechanical properties of EA9361 AERO, a representative flexible epoxy adhesive, within the context of the design of hybrid bolted/bonded joints. Tensile and shear tests were performed to obtain the tensile stress/strain relation, strength, strain-to-failure, Poisson's ratio, and adhesive's responses under pure shear state. A practical methodology for strain measurement of a thin bondline of adhesive was proposed using digital image correlation (DIC). It was concluded that the nonlinear tensile stress/strain relation of the flexible adhesive can be accurately represented with a bilinear elastic/plastic material model. The Poisson's ratio was found to significantly change throughout the strain development.
•Mechanical properties of EA9361 AERO, a representative flexible epoxy adhesive are characterized.•A practical methodology for strain measurement of a thin bondline of adhesive was proposed using DIC.•The nonlinear tensile stress/strain relation of the flexible adhesive can be represented with an elastic/plastic model.•The Poisson's ratio was found to significantly change throughout the strain development.
Recent experimental results of the FRP–concrete bonded joint using flexible adhesive showed that the most popular analytical models available in the literature underestimate the bond strength and the ...effective bond length of these experiments. Most of these existing models need to be modified to consider the type of adhesive layer. Consequently, the bond strength model proposed by Chen and Teng (2001) has been modified to consider the type of adhesive layer. An extensive database consisting of about 100 test results of FRP–concrete joint has been assembled to examine the validity of the proposed model taking the type of adhesive layer into consideration. The modified bond strength model is accurately capable of predicting the bond strength and the effective bond length.
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