Step-lap Joint Research Articles

Effect of Adhesive Type on the Adhesively Bonded Stepped Joint: A Numerical Investigation

Single-lap joints are by far the most widely used adhesive joints and have been the subject of considerable research over the years. It is used in the automobile and aerospace industry where bolted or riveted joints are impossible. The joint strength in adhesively bonded joints depends on the adhesive and adherend properties and adherend geometry. In this paper, both the adhesive property and adherend geometry are considered. Two types of adhesives, i.e., SBT9244 (flexible) and DP460 (stiff), and three types of adherend geometry, Single Lap Joint (SLJ), One Step Lap Joint (OSLJ), and Three Step Lap Joint (TSLJ) are considered, and the effect of these are investigated by using a commercially available software Abaqus. The maximum peel stress occurs in a lap joint towards the edges of the joint and is minimum around the center region. The maximum peel stress is responsible for the failure of the joints, and the objective of this research was to reduce the peel stress, i.e., provide a more uniform stress distribution. Soft adhesive maximum peel stress and shear stress occur in [Type-I] 8.6 MPa and 6.4 MPa, respectively. Similarly, stiff adhesive maximum peel stress and shear stress occur in [Type-I] 37.14 MPa and 20.44 MPa, respectively. It is found from this investigation that if a relatively soft adhesive (SBT9244) is used in the joint, then the stress distribution reduces compared to a stiff adhesive (DP460). On the other hand, if steps are introduced in the bonded region, the stress distribution becomes more uniform and increases the bond strength.

Optimisation of bonded and hybrid step lap joints of thick carbon fibre/epoxy panels used in aircraft structures

The use of composite materials is widely spread across the Aerospace sector. Scarf and step lap joints are amongst the most commonly used methods for flush joint repairs; they restore original stiffness and nearly original strength. Prior investigations conducted by the authors have shown that bonded and hybrid step lap joints have superior static strength capabilities than bolted joints. Hybrid step lap joints have shown the highest fatigue resistance than its bolted and bonded counterparts. This investigation aims at optimising the previous step lap joint design containing five steps with a 90 mm long overlap length in order to improve joint efficiency. Four new step lap joint configurations are considered based on a parametric study conducted using Abaqus; two of which contain seven steps and a 130 mm long overlap (bonded and hybrid) whilst the other two contain six steps and a 105 mm long overlap (bonded and hybrid) with a tapered outer step region. Experimental tests right through to numerical modelling using three-dimensional (3D) finite element (FE) models have been investigated. Adhesive non-linear material properties, fastener contacts and friction forces were all included in the 3D FE models. The Multicontinuum Theory (MCT) is used to simulate the progressive failure process and determine the stress states in the four configurations. Overall the FE models are able to accurately predict the bonded and hybrid joint strengths and highlights the importance of not only optimising the number of steps and their lengths but also the individual step heights. This is a parameter which has often been overlooked by fellow researchers. Experimental results showed that all four step lap joint configurations have significant improvement under static and fatigue load cases; the greatest improvement is seen in the two bonded step lap joint configurations. Overall the hybrid joint configuration consistently outperformed the bonded joint configurations. The presence of fasteners supress crack propagation, minimise peak peeling stresses at the ends of the overlap and increase the durability of the joint. This enables early crack detection which can assist composite joint repair certification.

Damage Characteristics of a Step Lap Joint Exposed to Flexural Loading for Its Different Configurations.

Step lap joints are kinds of lap structures, where butted laminations of each layer are consecutively offset in succeeding layers in the same direction. They are mainly designed this way to reduce the peel stresses at the edges of the overlap area observed in single lap joints. In their service, lap joints are often subjected to bending loads. However, the performance of a step lap joint under flexural loading has not been studied in the literature yet. For this purpose, 3D advanced finite-element (FE) models of the step lap joints were developed via ABAQUS-Standard. DP 460 and A2024-T3 aluminum alloy were used for the adhesive layer and adherends, respectively. The polymeric adhesive layer was modelled using cohesive zone elements with quadratic nominal stress criteria and power law interaction of the energies to characterize the damage initiation and damage evolution, respectively. A surface-to-surface contact method with a penalty algorithm and a hard contact model was used to characterize the contact between the adherends and the punch. Experimental data were used to validate the numerical model. The effects of the configuration of the step lap joint on its performance in terms of the maximum bending load and the amount of energy absorbed were analyzed in detail. A step lap joint with three steps (three-stepped lap joint) was found to show the best flexural performance, and when the overlap length at the upper and lower steps was increased, the amount of energy absorbed by the joint increased markedly.

Fatigue Performance of a Step-Lap Joint under Tensile Load: A Numerical Study.

In many technical domains, adhesively bonded joints have been employed extensively. These joints perform poorly against peel stresses despite having good shear characteristics. A step-lap joint (SLJ) is one of the techniques used to reduce the peel stresses at the edges of the overlap area to avoid damages. In these joints, the butted laminations of each layer are successively offset in succeeding layers in the same direction. Bonded joints are subjected to cyclic loadings in addition to static loads. It is difficult to predict their fatigue life accurately; however, this information must be clarified to explain their failure characteristics. To this end, the fatigue response of an adhesively bonded step-lap joint subjected to tensile loading was investigated with the developed finite-element (FE) model. In the joint, toughened type DP 460 and A2024-T3 aluminium alloys were used for the adhesive layer and adherends, respectively. The cohesive zone model with static and fatigue damages were linked to each other and were used to represent the response of the adhesive layer. The model was implemented using an ABAQUS/Standard user-defined UMAT subroutine. Experiments found in the literature served as a basis for validating the numerical model. The fatigue performance of a step-lap joint for various configurations subjected to tensile loading was examined thoroughly.

The effect of geometry on joint strength in adhesively bonded joints with the same adhesive area

Studies to increase the joint strength of adhesively bonded joints used in the aerospace and automotive sectors are met with interest in the field of engineering. Many different methods are used to increase the strength of the bonded joint, and one of these methods is to change the joint geometry. This study aims to increase the joint strength by changing the geometry of the joint with the same adhesive area condition. In the study, the DP460 structural adhesive was used as the adhesive, the AA2024-T3 aluminum alloy was used as the adherend, and the One Step-Lap Joint (OSLJ), Double-Strap Joint (DSJ), and Stepped Double-Strap Joint (SDSJ) was used as the joint types. The joint strengths of the joints obtained by using different step lengths and different patch lengths for these three types of joints with the same adhesive area were investigated experimentally and numerically. Firstly, the strengths of the SLJ type obtained by using four different step lengths and the DSJ type obtained by using patches of four different lengths were investigated. In the light of these investigations, the mechanical properties of the joints obtained by changing the step length and patch length in the SDSJ type, a new joint type with the same adhesive area, were obtained. As a result, compared to the OSLJ type with the same adhesive area, the joint strength of the DSJ type increases by approximately 45% to 67%. In addition, while the strength of the new type of joint (SDSJ) obtained in the presented study increases between 7% and 35% according to the DSJ type, it increases between 56% and 126% according to the OSLJ type. These increases in the joint strength vary according to the bonding area. Additionally, in the presented study, experimental data were compared with numerical analysis, and it was observed that the data were quite consistent with each other.

Numerical investigation on the strength of different types of adhesively bonded lap joints

In recent years, adhesively bonded joints are extensively used in a multitude of technological applications, mainly in industries such as aerospace and automobile. Adhesive bonding is often chosen over other joining techniques like bolting, riveting as well as welding, due to the reduction of stress concentrations, easy manufacturing and reduced weight penalty, amongst other issues. For the investigation of the behavior of adhesive bonded joints, the stress–strain behavior of the structural adhesive is necessary. The present study aims to investigate the strength of various types of adhesively bonded lap joints by using the commercial finite element tool ANSYS. It is seen from the analysis that the double lap joint is the type of joint which is least affected by the forces acting on it followed by the single step lap joint.

Thoughts on the durability and damage tolerance assessment of adhesively-bonded joints

This paper first discusses the accuracy of the standard adhesively-bonded joint design code, the analytical A4EI code, that is used in the aircraft industry to determine the strain-energy density in a complex step-lap joint under shear loads. This joint geometry is then used to assess the ability of the A4EI code, and a standard finite-element analysis, to meet the static design requirement that there be no yield at 115% of the design limit load. Secondly, the analysis tools needed for the durability and damage tolerance (DADT) assessment of adhesively-bonded airframes are discussed. This study demonstrates the significant advantages of now adopting a Harman-Schijve approach, which considers a fatigue crack growth rate approach to DADT assessment. Indeed, this study reveals how very different approaches are required to assess the static versus the DADT performance of adhesively-bonded joints. A key finding is that, if a valid DADT assessment of fatigue crack growth in adhesively-bonded structures is to be performed, then the aspect of the fatigue crack growth data associated with the growth of small cracks in bonded joints needs to be resolved, together with the question of what is the value of the fatigue threshold associated with such small, naturally-occurring cracks in bonded structures.

Finite element studies of bolted, riveted, bonded and hybrid step-lap joints of thick plate

The application of the step lap joint has been increased due to its good characteristics such as lightweight, high stiffness, and good fatigue durability. Moreover, it has a wide application in the aerospace, automotive and marine sector because of its efficient strength and less stress concentration. The present study is concentrated on the effect of the kind of joint used for lap joint like the adhesive joint, riveted joints, and bolted joints. In addition, the length of the steps has been changed to study the behavior of adhesive lap joint in detail. Also, the hybrid joint such as a bolted-adhesive and riveted-adhesive joint is examined in the current study. The variation in the number of bolts and rivets are made for detailed analysis. The main objective of this project is to analyze the step lap joint with different length, different material for adherend and adhesive and also compare it with different mechanical fasteners such as rivets and bolts considering hybridization of it with and without the adhesive, Finite Element Analysis (FEA) is carried out in ANSYS Workbench for analysis and SolidWorks software is used for 3-D modelling.

Numerical analysis of elasto-plastic adhesively single step lap joints with cohesive zone models and its experimental verification

Adhesive bonding method is used as an alternative to mechanical bonding methods, and it is frequently used in automotive, space and aviation industries. In the present study; the displacement and failure load capacity characteristics of single step lap joint was measured experimentally and compared with results of finite element (FE) analysis. In addition, the change in the length and the thickness of the material and adhesive in this joint type, stresses on the adhesive was examined by numerical analysis that have affected. Also, the effects of overlap length and material thickness on the stresses in adhesive were examined with numerical analysis for the single step lap joint. AA2024-T3 aluminum alloy adhered material and double component DP460 structural adhesive was used as adhesives in the single step lap joint. In the numerical analysis of this joint type, two different material models including multilinear isotropic hardening (MISO) and cohesive zone model (CZM) were used in the adhesive layer. During the experimental tests, two cameras were used to measure the displacements and crack propagations. As a result, the data obtained from the numerical analysis were compared to the experimental data of the single step lap joint type, the use of CZM in the adhesive region was found to be very important in terms of compliance. Thus, the difference between CZM and MISO results in terms of damage load in adhesive is 7 %, while this difference is 32 % in terms of displacement. Also, it has a great importance to get the displacement data via a video extensometer for the compatibility of FE and experimental analyses. Another result obtained from the study is that changing the material thickness, the adhesive thickness and the overlap length in the joint, changes the stress concentrations and shear stresses at the ends of the overlap area.

Investigation of internal step and metal part reinforcement on joint strength in the adhesively bonded joint: Experimental and numerical analysis

Many studies have been performed to increase the failure load of adhesively bonded joints, which enable different types of materials to be joined together for use in the aerospace and automotive sectors. In studies from published literature, various methods have been used to increase the failure load of joints, such as altering the joint geometry, investigating additives to the adhesive or changing the overlap area parameters. This study experimentally and numerically investigated the mechanical properties of single lap joints obtained by applying internal steps of different lengths to the overlap region of the joints and adding metal pieces of different lengths. In the study, adhesive bonded single-lap joints were produced by using DP460 liquid structural epoxy as the adhesive, AA2024-T3 aluminum alloy as the adherend and AISI 1040 steel as the metal piece. As a result, the internal step lap joint (ISLJ) and metal piece reinforced lap joint (MRLJ) type was found to carry more load than single lap joint (SLJ) type. Also, the change in the step length in the ISLJ and the metal piece length in the MRLJ type had a significant effect on the failure load of the joint. The ISLJ and MRLJ geometry carried about 26–60% and 17–41% more load than the SLJ geometry, respectively. When the failure loads obtained in the experiments and the numerical analysis were examined, it was concluded that their results were quite compatible with each other when the cohesive zone model was used in the numerical analysis.

The experimental and numerical analysis of the adhesively bonded three-step-lap joints with different step lengths

Adhesive bonding is one of the most used joining methods for bonding composite or different types of materials. The joint type commonly used in adhesive bonding is the single lap joint (SLJ) type. However, in adhesively bonded joins, peel stress occurs due to the eccentric loading, which causes damage to the ends of the overlap area. Different types of joints are used to reduce these peel stresses. One of these joint types is the step-lap joint type obtained by creating steps in the overlap area. In this study, mechanical properties of the single lap joint (SLJ), one-step lap joint (OSLJ) and three-step lap joint (TSLJ) with five different step lengths subjected to tensile loading were examined experimentally and numerically by keeping the bonding area same for all samples examined. In this experimental study, AA2024-T3 aluminum alloy was used as the adherend and DP460, a structural adhesive with two components, was used as the adhesive. In addition, the progress of the crack in the overlap area was observed with a high-speed camera. As a result, the TLSJ type was found to carry more load than other joint types. Also, the change in the step length in the three-step lap joint type had a significant effect on the failure load of the joint. When the failure loads obtained in the experiments and the numerical analysis were examined, it was concluded that their results were quite compatible with each other when the cohesive zone model was used in the numerical analysis. Another result obtained from the study is the optimum length of the first step created at the ends of the overlap area in the three-step lap joint.

The tensile strength of mechanical joint prototype of lontar fiber composite

In the present study, an experimental activity has been programmed to investigate the effect of joint prototype configuration on tensile strength of lontar (Borassus Flabellifer) fiber composite. To do so, a series of tests were conducted to establish the tensile strength of different joint prototype configuration specimen of lontar fiber composite. In addition, post observation of macroscope was used to map damage behavior. The analysis of lontar fiber composite is a challenge since the material has limited information than others natural fiber composites materials. The results shown that, under static tensile loading, the tensile strength of 13 MPa produced by single lap joint of lontar fiber composite is highest compare to 11 MPa of tensile strength generated by step lap joint and double lap joint where produced the lowest tensile strength of 6 MPa. It is concluded that the differences of tensile strength depend on the geometric dimensions of the cross-sectional area and stress distribution of each joint prototype configuration.

Experimental and finite element studies of bolted, bonded and hybrid step lap joints of thick carbon fibre/epoxy panels used in aircraft structures

Abstract A prior study conducted by the authors had investigated the behaviour of thin composite double lap joint repairs. This investigation was an extension to the previous study and focused on analysing the behaviour of thick step lap joint repairs which are more complicated in nature. Finite element analysis (FEA) was performed to verify the static and fatigue strength of bolted, bonded and hybrid joint configurations. Thick carbon fibre/epoxy laminates and aerospace grade film adhesive and fasteners were selected. Several configurations were considered in this case. These include varying the number of fasteners in the joint region as well as inclusion of bondline defects. Adhesive non-linear material properties, fastener contacts and frictional forces were all included in the three dimensional (3D) finite element (FE) models. The progressive failure process was simulated and the Multicontinuum Theory (MCT) was used to determine the stress states in all specimens considered. The strain energy release rate (SERR) as a function of crack length was also utilised to gain further understanding into the fatigue behaviour. Overall the FE models are able to accurately predict the bonded, bolted and hybrid joint strengths. SERR results suggest it is vital to place fasteners closer to the ends of the overlap to suppress the peak peeling stresses and to delay the effects of early crack initiation. This overall improves the longevity of a conventional bonded joint design.

Fatigue Damage Monitoring of a Composite Step Lap Joint Using Distributed Optical Fibre Sensors.

Over the past few decades, there has been a considerable interest in the use of distributed optical fibre sensors (DOFS) for structural health monitoring of composite structures. In aerospace-related work, health monitoring of the adhesive joints of composites has become more significant, as they can suffer from cracking and delamination, which can have a significant impact on the integrity of the joint. In this paper, a swept-wavelength interferometry (SWI) based DOFS technique is used to monitor the fatigue in a flush step lap joint composite structure. The presented results will show the potential application of distributed optical fibre sensor for damage detection, as well as monitoring the fatigue crack growth along the bondline of a step lap joint composite structure. The results confirmed that a distributed optical fibre sensor is able to enhance the detection of localised damage in a structure.

Static and fatigue testing bolted, bonded and hybrid step lap joints of thick carbon fibre/epoxy laminates used on aircraft structures

Fibre reinforced polymer matrix composites are being increasingly used for aircraft structures because of their superior structural performance, such as high strength, high stiffness, long fatigue life, and low density. This study is targeted towards the repair of thicker composite step lap joint configurations. The aim is to compare the static strength and fatigue resistance of a hybrid joint consisting of both bonding and bolting, a purely fastened joint and a purely bonded joint. Parameters such as fastener array, bond strength and the influence of initial defects are investigated.Static tests results revealed there is minimal change in peak strength achieved by a hybrid joint compared to a bonded joint. Fatigue tests showed that hybrid joints have the greatest durability followed by bolted joints and finally bonded joints. The presence of fasteners in a hybrid joint reduces the peel stress and arrests crack propagation in the bondline. The fasteners also act as an adhesive bond failsafe mechanism preventing sudden catastrophic failure particularly for bondlines containing defects. Failure in the various specimen configurations ranged from bearing failure in the bolted joint cases whilst bonded and hybrid joint configurations showed first ply failure followed by net-tension failure as well as adhesive failure.

The strength of the adhesively bonded step-lap joints for different step numbers

Peel stresses developing at the edges of the overlap area of the adhesively bonded single lap joints subjected to static tensile loading have a profound effect on the damage of the joint. The reduction in the stress values formed at the edges of the overlap area or the transfer of these stresses to the middle part of the overlap area increase the strength of the joint. In this study, mechanical properties of the single lap joint (SLJ), one step lap joint (OSLJ) and three step lap joint (TSLJ) subjected to tensile loading were examined experimentally and numerically by keeping the bonding area same for all samples examined. In the samples produced for experimental study, AA2024-T3 aluminum alloy was used as adherent, while a flexible adhesive SBT9244 and a stiff adhesive DP460 were applied separately. After experimental studies on the three different joint types were conducted, stress analyses in the joints were performed with a three-dimensional finite element method by considering the geometrical non-linearity and the material non-linearities of the adhesive and adherend. As a result, it was observed that one-step lap geometry reduces the stress concentration developing at the edges of the overlap area while the highest decrease occurred in the three-step lap geometry. Additionally, the amount of reduction in the stress values supports the increase in the experimentally obtained load carrying capacity of the joint.

Stress Distribution in Single Lap Joints with a Cracked Composite Adherend—Part I: Single Lamina Adherends

The effect of a crack in the overlap region of a single step lap joint is studied on the shear distribution in the adhesive layer. Each adherend is considered to be a lamina with unidirectional fibers aligned in the direction of the applied load. Crack location is selected to be in the top adherend, in the form of cut fibers and matrix bays. The shear-lag model is used to derive the equilibrium equations which are then solved using eigenvector expansion. Additionally, a finite element model of the lap joint was prepared and solved using ANSYS. The results of the two methods perfectly match each other. The effects of crack location along the length of the overlap, crack size, edge cracks, adhesive thickness, and type of fibers were investigated on the shear distribution in the adhesive layer and its corresponding peak values. The effect of dissimilar adherends was also investigated on the adhesive shear stress distribution. According to the results, in the presence of a crack, the peak adhesive shear stress is very susceptible to adhesive thickness and type of fibers used in each adherend. Other factors also influence the peak shear stress to some degree.

Numerical computation for a new way to reduce vibration and noise due to magnetostriction and magnetic forces of transformer cores

Magnetostriction (MS) caused by the global magnetization of limbs and yokes and magnetic forces are the undisputed causes of the vibration and noise in power transformer cores. This paper presents a novel way to reduce the vibration and noise, in which nanocrystalline soft magnetic composite (NSMC) material with high permeability is used to fill the step-lap joint gaps of the power transformer magnetic cores. In order to numerically predict the effectiveness of the proposed method, a 3-D magneto-mechanical strong coupled model including MS and magnetic anisotropy of steel sheet was founded. Then, the numerical model was applied to analyze the step-lap joint region of the corner of magnetic cores. The analysis results illustrated that the deformation and noise of core with NSMC are lower than with the traditional epoxy damping material. Moreover, the validity of the proposed new way was verified by the simplified step-lap joint cores, which were achieved based on Epstein Frames.

Flux distribution in single phase, Si–Fe, wound transformer cores

This paper shows experimental results of longitudinal flux density and its harmonics at the limb, the yoke and the corner as well as normal flux in the step lap joint of a single phase, Si–Fe, wound transformer core. Results show that the flux density as well as the harmonics content is higher in the inner (window) side of the core and reduces gradually towards the outer side. Variations of flux density distribution between the limb and the corner or the yoke of the core were observed. A full record of normal flux around the step lap region of the model core was also obtained. Longitudinal and normal flux findings will enable the development of more accurate numerical models that describe the magnetic behavior of magnetic cores.

Evaluation of the Impact Damages of Scarf-Repaired Composites

A CFRP scarf-repaired specimen is manufactured using the autoclave method on the assumption that it is a part of the repaired composite airframes. Such specimens are composed of a scarf joint and a step-lap joint. Though the in-plane mechanical properties of the scarf-repaired composites have been studied, few researches on out-of-plane behavior have been reported. Thus, the mechanism of the impact damages in the scarf-repaired composites was clarified through two kinds of experiments, the low-velocity impact test and the static indentation test. Through the low-velocity impact test, the damage configuration was observed and the influences of the scarf angle and stacking sequence on impact damage were investigated. As a result, the debonding at the interface between the adhesive layer and the repair laminate was observed in all the specimens. Moreover, damage growth was observed through the static indentation test. The indentation results indicated that the debonding at the interface between the adhesive layer and the repair laminate occurs at the intersection of the adhesive layer and the delamination of the repair laminate.