Double Shear Lap Joints Research Articles

Effect of hole lubrication on the fretting fatigue life of double shear lap joints: An experimental and numerical study

In this research the affect that lubrication at a hole and pin connection has on the fatigue life of a double shear lap joint is studied both experimentally and numerically. The study focuses on the joint middle plate item, which is connected via a central hole to the outer plates by means of a clearance fitting pin, thereby placing the hole in double shear. In the experimental work three identical batches of fatigue specimens, which are made from aluminum alloy 2024-T3, were fatigue tested. In the first batch the surface of the fastener hole was not lubricated whilst the hole in the other two batches was lubricated – each batch using a different lubricant. The three batches of double shear lap joint specimens were fatigue tested and their S– N curves established. The results show that the specimens in which the holes were lubricated have better fatigue lives than the non-lubricated hole specimens. In the numerical study, FE simulations were performed to include hole lubrication effect on the stress distribution by using different friction coefficient at the interface of the hole and its fastener (pin). The FE results have helped to gain an understanding of the reasons for fatigue life improvement and also have helped to quantify the level of improvement.

Experimental and numerical investigations into the effect of an interference fit on the fatigue life of double shear lap joints

In this research the effect of bolt interference fit on the fatigue life of lap joints in double shear was investigated by conducting experimental fatigue tests and also analytically by FE simulation. In the experimental part, fatigue tests were carried out on specimens made from aluminium alloy 2024-T3 plates joined together as double lap joints and secured using bolts having fits ranging from zero clearance to different levels of interference. The results demonstrate how the failure is affected using different levels of interference fit. In the numerical study, 3-D FE models were used to simulate the different pin in hole fits considered and the results have been used to help explain the trends which were observed in the experimentally obtained S– N curve behaviour.

Effect of Elevated Temperature on Bond Behaviour of High Modulus CFRP/Steel Double-Strap Joints

This paper investigates and provides experimental evidence of the bond characteristics between carbon fibre reinforced polymer (CFRP) and steel plates under elevated temperature exposures. A series of tensile tests was carried out on CFRP/steel plates specimens joined together in double-lap shear joints and subjected to a range of environmental temperatures between 20 °C up to 60 °C, which would be usually encountered in civil infrastructure applications. High modulus (640 GPa) unidirectional carbon fibre plies were used in wet lay-up fabrication method to strengthen the composite matrix. Three different epoxy resins were used for the fabrication of the CFRP/steel specimens. The objective of the current experimental work is to determine the ultimate strength, failure patterns, elongation development, and lap-shear stress and slip variation under those exposures. This will help in enhancing the competency of using CFRP in retrofitting steel structures at high-temperature environments, usually experienced in civil construction applications.

Numerical prediction of load–displacement behaviors of adhesively bonded joints at different extension rates and temperatures

The present work focuses on simulation of nonlinear mechanical behaviors of adhesively bonded DLS (double lap shear) joints for variable extension rates and temperatures using the implicit ABAQUS solver. Load-displacement curves of DLS joints at nine combinations of extension rates and environmental temperatures are initially obtained by conducting tensile tests in a UTM. The joint specimens are made from dual phase (DP) steel coupons bonded with a rubber-toughened adhesive. It is shown that the shell-solid model of a DLS joint, in which substrates are modeled with shell elements and adhesive with solid elements, can effectively predict the mechanical behavior of the joint. Exponent Drucker-Prager or Von Mises yield criterion together with nonlinear isotropic hardening is used for the simulation of DLS joint tests. It has been found that at a low temperature (-20 degrees C), both Von Mises and exponent Drucker-Prager criteria give close prediction of experimental load-extension curves. However. at a high temperature (82 degrees C), Von Mises condition tends to yield a perceptibly softer joint behavior, while the corresponding response obtained using exponent Drucker-Prager criterion is much closer to the experimental load-displacement curve.

Strength of resin-coated-adhesive-bonded double lap-shear pultrusion joints at ambient temperature

The inherited adhesion limitation of polyester and vinyl ester resin-based pultruded GFRP makes pultrusions difficult to bond, especially when a thixotropic adhesive is used. While such an adhesive is necessary for gap filling, it has a limited wettability. Therefore, coating the adherend with low-viscosity epoxy resin, prior to bonding, improves wetting and hence increases joint strength. The paper describes the experimental methodology to achieve this, using double lap-shear (DLS) joints with various materials combinations. A significant strength improvement was reached as a result of coating the inner adherend in conjunction with using a “high adhesion” outer adherend. To further understand the effect of coating, numerical stress analysis was undertaken, including preliminary micro-models representing the composite/adhesive interface as well as overall DLS models.

An experimental and analytical study of the mechanical behaviour of adhesively bonded joints for variable extension rates and temperatures

In the present paper, mechanical behaviours of adhesively bonded joints are studied with the help of double lap shear (DLS) coupon tests conducted at different extension rates and temperatures. The joint specimens are made from dual-phase steel coupons bonded with epoxy resin. Tests are also carried out to ascertain the behaviours of these component materials. It has been found that at a high temperature, the adhesive joint exhibits a greater degree of strain rate sensitivity with a perceptible fall in the joint strength. However, at a low temperature, the joint strength remains comparable to that at room temperature. A new semi-analytical solution procedure is developed considering material nonlinearity to predict mechanical behaviours of adhesively bonded DLS joints. The joint behaviours using the semi-analytical approach are predicted separately using the Von Mises (VM) and Raghava yield criteria. It has been found here that the application of the Raghava criterion yields good correlation with test load–extension behaviours for most temperatures and extension rates considered; on the other hand, the VM condition gives rise to perceptibly softer joint behaviour when compared to test data at a high temperature.

Numerical and experimental investigation of three-dimensional strains in adhesively bonded joints

Strain distributions within adhesively bonded double-lap shear joints under tensile load have been investigated experimentally using the complementary experimental techniques of neutron diffraction (ND) and moire interferometry (MI). ND provides three-dimensional strain values within the samples, whereas MI gives high-resolution maps of the in-plane strain components at the surface. The resulting comprehensive datasets obtained from both aluminium and steel joints were compared against finite element (FE) predictions. The importance of a range of factors (e.g. two-dimensional versus three-dimensional geometry, sample asymmetry, residual strains) was assessed through development of the FE model and detailed comparison with the experimental data. The main conclusions were that FEA can be used accurately to predict strains in the adherends of bonded lap joints and that many of the simplifications of the joints commonly used in the analysis of bonded joints are justified as including them results in relatively small differences compared with sample-to-sample variation and the safety factors used in designing bonded joints.

Dual boundary element method modelling of aircraft structural joints with multiple site damage

The computation of crack growth from a bolt or rivet hole in a structural joint practically requires that the geometry be approximated to some degree. In this paper a simplified quasi-2D stress analysis method, using the boundary element method is presented, where the load transfer rate and the contact stresses at the hole edge for the full 3D geometry are fairly well approximated. Coupled with a dual boundary element formulation for the crack propagation problem, this model is used to evaluate stress intensity factors for through cracks emanating from holes in several double shear lap joint configurations. As the calculated stress intensity factors compare well with experimental data, this procedure is considered to approximate satisfactorily the load transfer rate and the contact stresses at the hole edge of the full 3D geometry, when secondary bending is not a factor.

Analysis of energy release rate for fatigue cracked metal-to-metal double-lap shear joints

This paper presents an analytical model for understanding crack propagation behaviour of adhesively bonded cracked double-lap shear joints subjected to tension–tension fatigue loading. The analytical model can be used to calculate the bending moments at the cross sections of the crack tips and strain-energy release rates at the two crack tips. The accuracy of the analytical model is established by comparing the calculated bending moments and energy release rates with those predicted using the finite element method. Numerical and preliminary experimental testing results show that after cracking initiates in one of the two adhesive layers it will then propagate a set distance before crack initiation and growth in the opposite adhesive layer begins. Subsequent crack growth in either adhesive layer then alternates as a function of time.

Stiffness prediction of the double lap shear joint. Part 2: Finite element modeling

To develop vehicles with adhesively bonded structures, one needs to be able to perform structural finite element (FE) analysis of the adhesive joints. In current structural analysis practice, adhesive joints are modeled as rigid joints, semi-rigid springs, or other equivalent representations. This paper is aimed at determining modeling methods that give reasonably accurate predictions of the stiffness of bonded joints, and yet are practical enough to be used at the component or vehicle level of modeling. Starting with the double lap shear (DLS) joint, different finite element modeling methods were investigated. Simulation results were compared with DLS test results and with the analytical solutions presented in Part 1 of this paper (Stiffness prediction of the double lap shear joint. Suitable modeling methods are recommended for ABAQUS and LS-DYNA.

Stiffness prediction of the double lap shear joint. Part1: Analytical solution

This paper presents an analytical solution for the in-plane stiffness response of adhesively bonded double lap shear (DLS) joints. The solution was first derived for a model with the consideration of shear deformation of the adhesive (Model 1) and then was extended to two other simplified models, where the adhesive was modeled as matrix of rigid links (Model 2) or as a line of rigid links (Model 3). Models 2 and 3 represent two common types of simplifications in current finite element structural analysis of bonded joints. The solution for Model 1 was verified with test results. Errors resulting from the simplifications of Models 2 and 3 were estimated. The effect of joint geometries and material properties on the joint stiffness was investigated through a parametric study of the solution for Model 1.

Microstructural effects on the behavior of Sn–Ag solder joints under repeated reverse stressing

Double shear lap joints made with eutectic Sn–Ag solder were cooled from 270 °C at different rates ranging from air-cooling to iced-brine quenching. These joints were subjected to reversed stressing with constant shear strain amplitude at room temperature. Shear strain amplitudes between 0.75 and 1 were imposed during the course of this investigation. Role of microstructural features on the surface damage accumulation and stress–strain behavior was investigated as a function of shear strain amplitude and number of cycles of repeated stressing. Shear banding was found to be along Sn dendrites in air-cooled specimens, while it cut across the small Sn grains present in the quenched specimens.

The integrity of bonded joints in large composite pipes

The structural integrity of composite pipework systems depends largely on understanding the behaviour of pipe joints under loading. This paper addresses the topic of adhesively bonded taper/taper connections. Both perfect joints and joints with artificial axisymmetric defects (debonds) were investigated in this study. Such geometry and the required property parameters are beyond the scope of conventional analytical theories normally applied to small parallel-sided tubular joints. Therefore, an extensive experimental and numerical programme was necessary to achieve this understanding. The mechanical testing focused on relatively small pipes (100 mm diameter) but the finite element analyses extended to larger pipes (up to 400 mm). Accordingly, the programme resulted in design guidance for the specific range of bonded pipe joints. Additionally, in order to correlate, compare and support the pipe joint analyses, thick steel adherend double-lap shear joints were also studied experimentally and numerically.

Analysis of a double shear lap joint with interference fit pin

This paper presents the study of stress distribution in a double-shear lap joint with interference fit pin subjected to inplane plate loads using FELJNT, a finite element software developed for this special purpose. FELJNT accounts for the effects of clamping and flexure of the bolt without having to adopt a laborious three-dimensional analysis. It carries out stress analysis in an axisymmetric region around the pin under non-axisymmetric loading. It uses an iterative method of contact stress analysis and a dummy element concept with frontal solver for finite element solution. This paper discusses the load for initiation of separation, through-the-thickness stress distribution, and qualitatively the beneficial effects of interference on fatigue life of the joint. The paper concludes with comparison of FELJNT results with those of JACKAL, a three-dimensional finite element analysis software for joints.

Approximate evaluation of the interfacial shearing stress in cylindrical double lap shear joints with application to dual-coated optical fibers

A simplified analytical model is developed for the evaluation of the interfacial shearing stress in a cylindrical double lap shear joint, with application to dual-coated optical fiber specimens subjected to pull-out testing, in situ measurements of Young's (shear) modulus of the primary coating material, and stripping of the coating from the glass. The objective of the analysis is to assess the effect of the material properties and specimen's geometry on the magnitude and distribution of the shearing stress.It is shown that the longitudinal distribution of this stress is nonuniform and that, for the given specimen's length, its maximum value increases with a decrease in the thickness of the primary coating. As far as the pull-out testing and Young's modulus evaluations are concerned, it is concluded that, while 1 cm long specimens with approximately 30 μm thick primary coating (such specimens are currently used in pull-out tests) are acceptable, shorter specimens will result in a more uniform stress distribution and, as a consequence of that, in more stable experimental data. As to the coating strippability, it is desirable that the stripping area be short, although satisfactory strippability is often achieved even for long stripping areas. It is concluded that a multiblade stripping tool might be worthwhile to consider if long portions of coating have to be removed from the fiber.The obtained results can be useful for comparing the adhesive strength of the primary coating in fibers of different lengths and with different coating designs, for the in situ evaluation of Young's modulus of the primary coating material from the measured axial displacement of the glass fiber, and for the assessment of the effect of material properties and fiber geometry on the strippability of the fiber coating.

Influence of temperature on structural joints with designed-in damping

An innovative means to enhance the inherent damping in structures is provided by the designed-in incorporation of viscoelastic materials in joints. The joints, as envisioned, are double-lap shear joints that dissipate energy when worked in an axial direction. The damping and stiffness characteristics of such joints have been evaluated experimentally at various temperatures and frequencies that are expected to be representative for large space structures. A new, nonresonant experimental technique has been utilized for this investigation. It provides the complex stiffness of the test specimen at extremely low strain levels, and accounts for elastic deformations in the test setup by means of carefully measured calibration factors. The temperature and frequency variations of the overall joint properties follow, in general, the same trends as the corresponding properties of the particular viscoelastic material used in the joint. The test data show that properly selected viscoelastic materials and design configurations can reduce the dependence of the joint properties on temperature and frequency variations. Significant damping benefits are possible without unacceptable stiffness penalties.

Analysis of design tradeoffs for passively damped structural joints

An innovative means to enhance the inherent damping in structures is provided by the designed-in incorporation of viscoelastic materials in joints. The joints, as envisioned, are double-lap shear joints that dissipate energy when worked in an axial direction. To better understand the relationship between structural stiffness and structural damping as a function of important physical parameters, a one-dimensional analysis of a typical joint was developed. A quasistatic analytical model based on the complex modulus description of viscoelastic behavior is utilized. Potential applications to parametric studies, preliminary design, and experimental data analysis are illustrated. The predictions correlate well with expected physical behavior, simplified approximate models, and available test data.

Stress Distribution of Single Spot Welded Lap Joint under Tension-Shear Load

The state of stress under tension-shear load was investigated experimentally and analytically on the single spot welded lap joint of single shear splice.Specimens were made from mild steel sheet of 1.0mm thickness. The stress distributions on the surfaces of the specimen were examined experimentally by the brittle coating and strain gage methods.According to the experimental results, the state of stress in the joint under tension-shear load appeared to be determined by the following factors.(i) Plane stress distribution near the weld nugget caused by the local transmission of load.(ii) Bending in the longitudinal direction resulted from the offset of lapped plates.(iii) Bending in the transverse direction, occured secondarily by the superposed effect of factors (i) and (ii).To clarify the state of stress owing to factor (i), the plane stress distribution was examined by the experiments on the middle plate of single spot welded double shear lap joint in tension, which was not affected by bending. Two-dimensional analysis by the finite element method was also made of the assumption of the idealized“Plane Model”.In connection with the state of stress due to longitudinal bending (factor (ii)), two-dimensional finite element analysis was employed on the“Section Model”of single spot welded single shear lap joint.Finally, the experimental stresses on the single spot welded single shear lap joint were compared with the analytical values, which were obtained by superposition of the stresses computed by Plane and Section Models.The results obtained are as follows:(1) In the stress measurement on the single spot welded single shear lap joint, the peak stress at 3mm distant above the nugget edge attained the value 3.9 or -1.3 times of the nominal tensile stress on the inner or outer surface of lapped plate, respectively.(2) The plane stress distribution computed by Plane Model agreed fairly well with the experimental result observed on the middle plate of double shear lap joint. In the finite element analysis by Plane Model, the lapped plates of the joint were doubly defined in the same plane.(3) The stress concentration factor deduced from the plane stress distribution was 2.2∼2.3 at the nugget edge of the joint, where the width of joint was 25mm and the nugget diameter was 5φ.(4) At the representative positions on the inner and outer surfaces of single shear lap joint, the superposed values of the stresses computed on the factors (i) and (ii) corresponded well with the axial stresses obtained from the experiments.