| Aluminum alloys, magnesium alloys and fiber reinforced plastics are now being used widely in the development of lightweight designs. But there are difficulties in using traditional connection methods to join these materials, because of the concentration of stress and localised deformation around the connection points. However it may be possible to overcome these difficulties by incorporating the new macromolecular polymer adhesives into jointing technology. If this potential is to be realized, it is important to understand the mechanical properties of adhesively-bonded joints, particularly with respect to joint strength and service life.The strength of adhesive joints is influenced by many factors but in general both static and fatigue failures begin at the interface point. The research reported here addressed this weakness. Experiments and analyses were based on a digital image correlation method, singularity theory, Goland-Reissner (G-R) one-dimensional beam theory and finite element theory. Experiments were carried out using adhesively-bonded single-lapped joints formed in 5052 aluminum alloy sheets. The following investigations were conducted:Firstly, the deformations of interface points in adhesively bonded SLJs were measured using a digital image correlation method. Secondly, sample joints were tested under various tension-shear loading sand the data were analyzed using ANSYS software based on singularity theory. Joints were also tested under different levels of fatigue stress. The fracture surfaces of the samples were examined using scanning acoustic microscopy and scanning electron microscopy, to assist with characterization of the failures. Finally, sample joints were modified by reinforcements whose design was based on a G-R one-dimensional beam theory model. The intention of the modifications was to improve the strength and reduce the singularity property of the adhesively bonded SLJs.The research work was divided into several parts as follows:1. Analysis of deformation of interface point of adhesively-bonded SLJs based on digital image correlation method.Sample joints were subjected to tension-shear loading. Data about deformation in the elastic range, the plastic range and the critical state of rupture were acquired by using a digital image correlation method. The concentration of strain at the interface point was remarkable, presenting a "high-low-high" tendency. The strength failure began near the point of fixed constraint.2. Analysis of the strength of the interface point of adhesively-bonded SLJs based on singularity theory.Adhesively-bonded SLJs in 5052 aluminum alloy were prepared with various bondline thicknesses and tested under tension-shear loading. The results were analysed in ANSYS software based on singularity theory with a mesh refinement that gave good agreement with test results. Then, the influence of different overlap length, bondline thickness, constraint length and boundary load in a singularity point was studied based on the sensitivity coefficient of stress intensity factor Φ and a finite element analysis(FEA) model, which provided reference information for studying the relationship between the influencing factor and the adhesive joints. The stress intensity factor H(H1、H2) decreased with the increasing lap length and constraint length. There is a significant linear correlation between boundary load and stress intensity factor: H1=0.00425F-0.0001,H2=0.00152F.3. Analysis of the residual strength of adhesively bonded-SLJs based on cycle load.Adhesively bonded SLJs in 5052 aluminum alloy were tested under different levels of fatigue stress. Scanning acoustic microscopy and scanning electron microscopy were used to examine the fracture surfaces of the joints and characterize the failure mechanism. During the fatigue test, the fatigue failure began from the interface point and the formation of the main crack continued throughout the fatigue life of the adhesive joints. However, once the main crack formed, it propagated rapidly and fractured immediately. The rigidity of adhesive joints remained stable. The residual strength increased at first and then decreased with increasing cyclic loading.4. Analysis of the deformation of adhesively-bonded SLJs with reinforcements based on one-dimensional beam theory.Joint reinforcements were designed based on G-R one-dimensional beam theory. Adhesively-bonded SLJs whose adherends were 5052 aluminum alloy (AA joints), 5052 aluminum alloy with pasting aluminum(AAA joints),5052 aluminum alloy with pasting copper (AAC joints) and copper alloy with pasting copper(CCC joints)were separately tested under tension-shear loading.Fatigue load tests were performed on all joint types and the resulting data were examined using singularity analysis. The effectiveness of the experimental data was analyzed with reference to Normal Distribution and Weibull Distribution curves. The results showed that the mechanical properties of joints are influenced favourably by increasing the stiffness at the end of the overlap region. It appears that appropriate reinforcement can improve the strength and reduce the singularity property of adhesively-bonded SLJs.The research reported here increased the knowledge about the deformation mode, strength analysis, strength prediction, fatigue performance and failure analysis of adhesively-bonded single lap joints in aluminium alloys and to provide a reference for further research. |
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