Simple Lap Joints Research Articles

An energy-based 2D model for predicting mechanical behavior of adhesively bonded CFRP laminates

This paper presents a two-dimensional energy-based model for the mechanical behavior of adhesively bonded carbon fiber-reinforced polymer (CFRP) joints subjected to tensile loading. The proposed energy model is better suited than available discrete models to capture failure modes in adhesively bonded CFRP joints of different configurations having complex geometries. The governing equations are first developed for a three-stepped lap joint configuration by employing the minimum potential energy theorem and then solved using the finite element method (FEM). The disbond behavior of the joint configuration is modeled using the bilinear cohesive law. Subsequently, the developed model is extended to capture the mechanical response of adhesive joints for simple lap and scarf joint configurations. The proposed energy model is verified against 3D-FEA and experimental studies. Thereafter, a comparative analysis is undertaken for the three configurations of adhesive joints studied here. This is followed by a parametric study for the three-stepped lap joint to understand the influence of geometric and material properties of the adhesive and adherends on the stiffness and load-carrying capacity of the adhesively bonded CFRP joint. Finally, a design criterion is proposed for the three-stepped lap joint configuration, aimed at improving its load-carrying capacity/strength by reducing the peak stress levels at joint corners/overlap edges.

Evaluation of Laser Lap Weldability between the Titanium Alloy Ti-6Al-4V and Aluminum Alloy 6060-T6

This work investigates laser weldability between non-ferrous dissimilar metallic materials, specifically the aluminum 6060-T6 alloy and titanium Ti-6Al-4V. These materials are used in several engineering applications, including aerospace. In a simple lap joint configuration, these were welded with a pulsed Nd:YAG laser, with direct incidence on the titanium piece. Preheating and post-weld heating were introduced to mitigate cracking issues. Based on the primary experiments, the main variables were the peak laser power, which varied between 60 and 70%, and the number of beads (a single bead and double beads). The quality of the welds was assessed via uniaxial tensile tests, subjecting the joint to shear loading. Additionally, SEM micrographs were obtained to analyze the quality of the fusion between the dissimilar alloys. The higher strength of the welded samples achieved was 90 MPa, which is close to the reported value for the aluminum base material. A fracture occurred near the weld bead in the heat-affected zone (HAZ). The observed microporosities and cracks explain the lower value as compared to the base material. Although these were mitigated through the thermal cycle strategy employed and the shielding gas, they were not entirely avoided.

Effect of Adhesive Type and Overlap Length on the Mechanical Resistance of a Simple Overlap Adhesive Joint

The present study focuses on the experimental determination of the mechanical behaviour of simple lap joints subjected to quasi-static tensile loads. Four different specimens were tested, resulting from the combination of two different adhesives (ductile and brittle) and two distinct overlap lengths. Cohesive fractures were observed on all specimens, which allowed for a direct comparison of experimental results and failure load prediction using analytical criteria adequate for adhesive joints. Three different criteria were used to predict the failure load, the Volkersen criterion, generalized yielding of adhesive and the adherend yielding criterion. The predictions based on the Volkersen criterion were found to agree well with experimental values obtained for the more brittle material. Moreover, increasing overlap length for the same material only led to a slight increase of the failure load of the joint. However, for the ductile material it was the generalized yielding criterion that better agreed with experimental results, where a significant and linear increase of the failure load was observed with increased overlap length. These results provide valuable insights for material selection and joint design procedures for adhesive joints loaded under large mechanical demands.

The Effect of Adhesive Layer Thickness on Joint Static Strength.

One of the most relevant geometrical factors defining an adhesive joint is the thickness of the adhesive layer. The influence of the adhesive layer thickness on the joint strength has not been precisely understood so far. This article presents simplified analytical formulas for adhesive joint strength and adhesive joint coefficient for different joint loading, assuming, inter alia: linear-elastic strain of adhesive layer, elastic strain of adherends and only one kind of stress in adhesive. On the basis of the presented adhesive joint coefficient, the butt joint was selected for the tests of the influence of adhesive thickness on the adhesive failure stress. The tests showed clearly that with an increase in the thickness of the tested adhesive layers (up to about 0.17 mm), the value of their failure stress decreased quasi linearly. Furthermore, some adhesive joints (inter alia subjected to shearing) may display the optimum value of the thickness of the adhesive layer in terms of the strength of the joint. Thus, the aim of this work was to explain the phenomenon of optimal adhesive layer thickness in some types of adhesive joints. The verifying test was conducted with use of single simple lap joints. Finally, with the use of the FE method, the authors were able to obtain stresses in the adhesive layers of lap joints for loads that destroyed that joints in the experiment, and the FEM-calculated failure stresses for lap joints were compared with the adhesive failure stresses determined experimentally using the butt specimens. Numerical calculations were conducted with the use of the continuum mechanics approach (stress-based), and the non-linear behavior of the adhesive and plastic strain of the adherends was taken into account.

Experimental and numerical investigations on built-up cold-formed steel beams using resistance spot welding

The lightweight steel structural systems such as trusses or built-up beams, consisting of thin gage steel elements, are highly efficient and have the advantages in ease of handling and construction. The major concern relates to the joining technique between each part of the element. Commonly, the connection between thin-walled elements is performed by self-drilling screws but the quantity of time and manpower necessary for a large number of connections, demands an improved solution. Conventional welding technology is not appropriate for connecting thin sheets, such those of 0.4 mm–1.0 mm, to thicker ones of 1.0 mm–3.5 mm. The electric resistance spot or seam welding are widely used in the automotive industry to connect thin-to-thin metal materials. These technologies can be applied in case of thin-walled cold-formed construction, too. The paper presents a study on a new technical solution for lightweight cold-formed steel beams made of corrugated sheet web and built-up steel profiles for the flanges, connected by spot welding. Starting from the investigations on simple lap joint specimens tested under tensile-shear loading for various thicknesses combinations, the research concerns also metallographic aspects and investigates the response of the full-scale beams subjected to bending. Based on the obtained results, the study intends to demonstrate the feasibility and advantages of this solution compared to self-drilling screw solution. Particularly, besides the structural performance, the spot-welding solution increase significantly the work productivity and enables for automated fabrication in order to develop the mass production of these types of beams.

Parametric study of size, curvature and free edge effects on the predicted strength of bonded composite joints

This paper presents the effects of size, curvature and free edges of laboratory lap joints on the debond fracture behaviour of joints that more realistically represent fuselage skin structures than conventional flat, narrow specimens. Finite Element Analysis is used in conjunction with Cohesive Zone Modelling (CZM) to predict the strength of selected joint features. The modelling approach was verified by simple single lap joint geometry. Four realistic joint features were then modelled by this validated modelling approach. The results show that moderate curvature has negligible effect on the peak load. There is a significant difference in the load vs displacement response of flat lab coupon joints with free edges and realistic curved joints with constrained edges. Further detail design features were investigated in this study, including (i) the joint runout and (ii) the presence of initial damage (thumbnail delamination). The modelling results show that the joggle configuration has an effect on the distribution of interlaminar stresses that affect the damage initiation and propagation. Fracture behaviour from different initial crack geometries associated with wider specimens has been simulated. From a design standpoint, an expansion of modelling capability is suggested to reduce the number of component tests in the traditional test pyramid.

Detection of fastener loosening in simple lap joint based on ultrasonic wavefield imaging

Joints in aero-mechanical structures are critical elements that ensure the structural integrity but they are prone to damages. Inspection of such joints that have no prior baseline data is really challenging but it can be possibly done using the Ultrasonic Propagation Imager (UPI). The feasibility of applying UPI for detection of loosened fastener is investigated in this study. A simple lap joint specimen made by connecting two pieces of 2.5mm thick SAE304 stainless steel plates using five M6 screws and nuts has been used in this study. All fasteners are tightened to 10Nm but one of them is completely loosened to simulate the damage. The wavefield data is processed into ultrasonic wavefield propagation video and a series of spectral amplitude images. The spectral images showed noticeable amplitude difference at the loosened fastener, hence confirmed the feasibility of using UPI for structural joints inspection. A simple contrast maximization method is also introduced to improve the result.

Effect of plasma treatment in Aluminum and composites bonding joints: Shear load tests results

This paper presents the results of simple adhesive lap joint shear strength tests of materials under atmospheric plasma treatment conditions. The effects of surface activation of the shear joint by cold atmospheric plasma in the perspective of the adhesion promotion are discussed. Specifically, the changes in the maximum strength introduced by the plasma of coupon samples of aluminum and carbon fiber reinforced materials. Finally, the results of tests and some conclusions are reported.

A new analytical approach for optimization design of adhesively bonded single-lap joint

In this study the three-dimensional nonlinear finite element method was used to analyze the stresses distribution in the adhesive layer used to joint two Aluminum 2024-T3 adherends. We consider in this study the effect of different parameters witch directly affect the values of different stresses. The experimental design method is used to investigate the effects of geometrical parameters of the single lap joint in order to achieve an optimization of the assembly with simple lap joint. As a result, it can be said that both the geometrical modifications of the adhesive and adherends edge have presented a significant effect at the overlap edge thereby causing a decrease in peel and shear stresses. In addition, an analytical model is also given to predict in a simple but effective way the joint strength and its dependence on the geometrical parameters. This approach can help the designers to improve the quality and the durability of the structural adhesive joints.

Effects of current densities on creep behaviors of Sn–3.0Ag–0.5Cu solder joint

Abstract

Fatigue life estimation of double lap simple bolted and hybrid (bolted/bonded) joints using several multiaxial fatigue criteria

In this investigation, the effects of torque tightening on the fatigue life of double lap simple bolted and hybrid (bolted/bonded) joints have been studied via experimental and multi-axial fatigue analysis. To do so, two kinds of joints, i.e. simple double lap and hybrid joints were studied. For each kind of the joints, three sets of specimens were prepared and subjected to tightening torques of 1, 2.5 and 5Nm and then fatigue tests were carried out at different cyclic longitudinal load levels. A nonlinear finite element ANSYS code was used to obtain stress and strain distribution in the joint plates due to torque tightening of the bolt and longitudinal applied loads. Fatigue lives of the specimens were predicted with six different multi-axial fatigue criteria by means of local stress and strain distribution obtained from the finite element analysis. Experimental tests revealed that the hybrid joints have higher fatigue life in comparison with the simple bolted joints. In addition, multi-axial fatigue analysis and experimental results revealed that the fatigue life of specimens was improved by increasing the clamping force resulting from the compressive stresses created around the hole. Furthermore, the investigation revealed the positive role of clamping force resulting from torque tightening on the fatigue life of both simple and hybrid joints.

A Comparative Study of Friction Stir Brazing and Furnace Brazing of Dissimilar Metal Al and Cu Plates

Friction stir brazing of 5-mm-thick Al and Cu plates in simple lap and stepped lap joint configurations was carried out in air as a modified process for friction stir lap welding of dissimilar metals, in which a pin-free tool and a 0.1-mm-thick Zn braze foil were used to eliminate both pin wear and keyhole and to achieve interface mixing by a metallurgical reaction route instead of vertical plastic mixing, respectively. The microstructure distribution features of friction stir brazing were characterized and compared with traditional furnace brazing to confirm the metallurgical advantages of the new process. For traditional furnace brazing, poor wettability at the Al side for lower brazing temperature or solidification cracking within too thick solidified microstructure (~150 µm) at higher brazing temperature was found. For friction stir brazing, oxide film removal could be achieved well in air without flux by rapid dissolution of the base metals into the molten braze; both solidified microstructure and solidification cracking could be eliminated by extruding excessive liquid phase out of the interface.

Transverse cracking of a simple lap joint under axial load

The failure mode of axially loaded simple, single lap joints formed between thin adherends which are flexible in bending is conventionally described as one of axial peeling. We have observed – using high-speed photography – that it is also possible for failure to be preceded by the separation front, or crack, moving in a transverse direction, i.e. perpendicular to the direction of the axial load. A simple energy balance analysis suggests that the critical load for transverse failure is the same as that for axial separation for both flexible lap joints, where the bulk of the stored elastic energy lies in the adhesive, and structural lap joints in which the energy stored in the adherends dominates. The initiation of the failure is dependent on a local increases in either stress or strain energy to some critical values. In the case of a flexible joint, this will occur within the adhesive layer and the critical site will be close to one of the corners of the joint overlap from which the separation front can proceed either axially or transversely. These conclusions are supported by a finite element analysis of a joint formed between adherends of finite width by a low modulus adhesive.

A Reduction Procedure for One-Dimensional Joint Models and Application to a Lap Joint

A reduction procedure for joint models that was developed in earlier work is extended to allow for relative motion between surfaces, and the effect of this procedure on timestep issues is considered. A general one-dimensional structure containing a frictional interface is considered. Coulomb friction is approximated with nonlinear springs of large but finite stiffness. The system of equations describing this structure is reduced in a procedure similar to Guyan reduction by assuming that the system deforms only in the shapes that it takes when the interface is massless. The result of this procedure is that the dynamics associated with the interface region are removed from the analysis. Following the development of the reduction procedure, the reduced formulation is specialized to the case of a simple lap joint. A numerical example problem is considered in which both the full and reduced equations of motion are integrated over time. It is seen that, for the example problem considered, the reduction procedure results in tremendous computational savings with little loss of accuracy. Based on the results of the simple example problem, it appears that the proposed reduction procedure has potential to be an accurate and effective method of alleviating the timestep difficulties associated with direct finite element analysis of joints in structural dynamics applications.

The interaction between corrosion management and structural integrity of aging aircraft

ABSTRACTAircraft joints feature prominently in aircraft structural degradation; fatigue cracking and corrosion damage are major issues, which can reduce joint strength and degrade service life. Protecting the structure against corrosion usually involves use of highly developed protective coatings – paints and sealants – and, increasingly, the application of corrosion inhibiting compounds (CICs) which retard corrosion principally by penetrating into crevices and cracks, and displacing water. A combination of coatings and CIC use can provide effective corrosion protection, but both interact – in different ways – with joint structural performance and overall system durability. This paper discusses the interaction between these two corrosion protection measures and fatigue performance of joints. The first issue relates to the extent to which application of CICs (or other lubricants) can cause a reduction in the fatigue life of mechanically fastened joints. The CICs are lubricants which will reduce the friction at the faying surface of the joint, and change the load transfer characteristics of the joint. This paper discusses results from a test program assessing the fatigue life and failure mode of simple riveted lap joints; the results show a distinct reduction in fatigue life for joints containing CICs, and the paper discusses the changes thought to be responsible for the reduction. The second issue discussed is the degradation of protective coatings in service. Joints are key locations for coating cracking and failure, because areas such as sheet ends and fastener heads, where displacements are concentrated, may produce concentrated strain in coatings. So far, however, the potential influence of aircraft loading on coating degradation prognostics has received little attention. This paper discusses the role of joint displacement in service as a factor contributing to degradation in aircraft coatings at joints, and argues that this local strain effect, and indeed structural loading history, needs to be considered in predicting and assessing rates of coating degradation. It describes analyses of displacements in aircraft joints, to identify the levels of strain and to identify the roles and relative contributions of the various deflections in the joints. The results indicate the potential for very large strains in coatings.

Numerical analysis and optimisation of cylindrical adhesive joints under tensile loads

This study is concerned with increasing the performance of adhesively bonded structures. Adhesively bonded joints offer many advantages, but stress singularities can contribute to the initiation and the propagation of crack in adhesive joints. Therefore, designing adhesively bonded joints which strongly limit the influence of edge effects can significantly increases the transmitted load by the assembly. Cylindrical joints are associated with high substrate strength in the radial direction, meaning that peel and cleavage forces have different effects compared to simple lap joints. But, for such assemblies, edge effects also exist. The aim of this paper is first to analyse stress concentrations in cylindrical joints in the case of axial loadings, starting from refined finite element computations under a linear elastic assumption. Thus as the stress state in the adhesive thickness can be complex associated to edge effects, simplified methods can lead to an overestimation of the transmitted load by the assembly as they generally are not able to represent effects of stress concentrations in the adhesive thickness. Secondly, geometries which strongly limit the influence of edge effects are proposed. An optimisation of the maximum transmitted load of cylindrical joints is proposed using a pressure-dependent elastic limit of the adhesive.

Effect of production variables on the fatigue behaviour of riveted lap joints

Results of an experimental research on the influence of several production related factors on the fatigue behaviour of simple riveted lap joint specimens under constant amplitude loading conditions are presented. The specimen geometry is representative for the longitudinal joints of the aircraft fuselage. The variables considered are the rivet type and material, the sheet material and the squeeze force. Some comparisons are made between results for Russian and Western aluminium alloys used in the aircraft industry for sheets and rivets. The measurements of the driven head dimensions for a range of squeeze force levels and fatigue test results for rivets installed with various squeeze forces indicate that the squeeze stress rather than the rivet driven head dimensions is a safe standard for the quality of the rivet installation. The superior fatigue performance of a rivet with the so-called compensator compared to the round head and universal rivet is noted. The underlying reason is the better hole filling achieved due to the presence of the compensator. Observations on crack growth indicate that crack initiation covers the major part of the joint fatigue life.

116 単純重ね合わせ接着継手の界面における接着部と端部の応力状態の比較(材料力学IV)

116 単純重ね合わせ接着継手の界面における接着部と端部の応力状態の比較(材料力学IV)

622 接着層厚さが接着剤の機械的性質に及ぼす影響

Adhesive sheet specimens with different thicknesses were manufactured, by hardening only an acrylic adhesive. Mechanical properties of their sheet specimens were examined by the tension tests. Using these mechanical properties, the deformation behaviour of simple lap joints was analyzed by the finite element method. These analytical results were compared with experimental results. Both didn't agree. It considers from this result that the mechanical properties of the adhesive are changing widely inside the adhesive layer. The acrylic adhesive causes molecular orientation in the adhesive interface neighborhood with metal. In the part which caused molecular orientation, the Young's modulus is low and the deformable quantity is large. Also, a center in the adhesive layer is supposed to have the opposite properties.

Electrodeposition of Adherent Polyaniline-Polypyrrole Composite Coatings on Low Carbon Steel

Uniform and adherent polyaniline-polypyrrole composite coatings were electrochemically polymerized on low carbon steel under aqueous conditions. The structure of the coatings was analyzed as a function of electrochemical deposition parameters (molar feed ratio of monomers and reaction time). X-ray photoelectron spectroscopy (XPS) was used to study the chemical composition of the coatings and also the amount of dopants (oxalate ions) incorporated into the polymeric coatings. The morphology of the coatings was studied using scanning electron microscopy (SEM). The adhesion strength of the coatings was measured using simple lap joint tests. The composite coatings formed by an equimolar feed of the monomers were much more adherent than the homopolymeric coatings.