Single Lap Joint Configuration Research Articles

Experimental characterization of CFRP single lap joints under tension at various loading rates

The objective of this study is to experimentally characterize the performance of four different composite/composite single lap joint configurations at various loading rates. The four joint configurations are single bolted joint, simply bonded joint, single bolted hybrid joint (a single bolt is used along the lap length of a simply bonded joint) and multiple micro bolted hybrid joint (four micro-bolts are used along the lap length of a simply bonded joint). The lap joints are tested at various loading rates using tensile split-Hopkinson bar and fatigue testing machine. Experiments reveal that, for a given joint configuration, the peak load, initial stiffness, and energy absorbed generally increased with the loading rates, whereas the plateau load is relatively insensitive to the loading rates. Finite element analysis (FEA) using the commercial software Abaqus/Explicit solver for all the configurations is also performed to simulate experimental loading conditions. It has revealed that by accounting for rate dependency of adhesive one can qualitatively explain the experimental observations.

Using a non-linear mixed model behavior of CZM to predict the damage in single lap bonded joint with Bi-composite graded materials

The present work is a numerical prediction based initially on a reliable model validated by experiments under the same conditions used as ours with new concepts in Bi-composite plates. A system formulation approach of the adhesive in ductile and nonlinear damage behavior is used in this analysis This ductility is developed by the CZM technique implemented using the MATLAB program in ABAQUS computer code. For a numerical prediction of the ductile behavior of the adhesive, the CZM model is an extension of the pure modes (I and II) previously characterized in block by tests. This extension is trapezoidal in increasing stresses of the adhesive in its behavior law. The single lap joint configuration allows us to stress the glue joint to mixed-mode loading. The resistance of the assembled structures remains conditioned by the behavior of the adhesive until it is damaged and the rigidity of the attached plates. Therefore, our objective in this work is to cause the plasticization damage of the adhesive at different responses and levels. For this, the bi-composite graduation in concept and in graduation index largely plays the role of evaluation parameters. In order to test the adequacy for a good resistance, three different concepts of graduation in Bi-composite were proposed such as: concept1-concept1, concept2-concept2, and simple concept-simple concept. By these two cases of assemblies, these concepts have the advantage of having different assembly possibilities, The proposed concepts of graduated bi-composite plates showed an overcapacity of resistance on the assembly. The latter becomes important in the adequacy of concept of these assembled plates. Similarly, the results show that different responses and levels cause the damage by effect of these concepts.

Numerical method to assess the stress state and gradients induced by thermo-oxidation in adhesively bonded joints for aircraft engine applications

This paper presents a finite element numerical method to assess the stress state and gradients induced by thermo-oxidative ageing in Ti/epoxy/Ti adhesively bonded joints for aircraft engine applications. The method is dealing with couplings between thermal, chemistry and mechanics occurring during high-temperature ageing of the joint, taking into account post-curing stresses, and under mechanical loading. Calculations are carried out on a classical single lap joint configuration. An assessment of the stress state and the thermo-oxidation induced gradients at each step of the ageing process is carried out. It is shown that the stress state within the adhesive results from the superposition of several physical fields and is strongly affected by the thermo-oxidation gradients. Moreover, post-curing stresses cannot be neglected when compared to the adhesive yield stress. Numerical analyses are of paramount importance to assess the complex stress fields related to multiphysics couplings and for joint design.

An experimental and numerical investigation into the effects of bond line thickness variations on the strength of composite bonded joints

The structural integrity of adhesive joints used in the attachment of laminated composite panels has received considerable attention over the past few decades. Of particular concern and the topic of this paper is the effect that a nonuniform bond line thickness has on the load carrying capacity of a bonded joint. To this end, carbon fiber adherends were joined in a single lap joint configuration using a two-part epoxy adhesive. The joints were initially fabricated and tested with various uniform bond line thickness in a single lap joint configuration to establish a benchmark. Bonded single lap joints with linearly varying bond line thickness were then investigated. A finite element study was performed to predict bond strength of the uniform and linearly varying bond lines using the Critical Zone method. The failure strength of the uniform bond line thickness specimens was found to decrease with an increase in the bond line thickness. In the case of linearly varying bond line thickness specimens, the bond strength decreased as the bond thickness at one end of the bonded joint overlap deviated from its intended value. The finite element model using the Critical Zone method was able to predict the failure strength of the joints in both uniform and linearly varying bond thickness configurations. The reduction in failure strength was attributed to an increase in the stresses in the adherend ply adjacent to the bond line as determined through the finite element stress analysis.

Appraisal of surface preparation in adhesive bonding of additive manufactured substrates

This work emphasizes the critical need of surface preparation to improve the performance of adhesively bonded 3D printed Acrylonitrile Butadiene Styrene (ABS) adherends obtained using Fused Filament Fabrication (FFF). In particular, mechanical abrasion and two different plasma pre-treatments, i.e., Atmospheric Pressure Plasma (APP) and Low-Pressure Plasma (LPP), were compared through the qualitative analysis of surface wettability, using static and dynamic contact angle measurements, and the quantitative evaluation of surface roughness measured using optical profilometry. In addition, mechanical tests were carried out using the single lap joint configuration and the interaction between the treated ABS substrates and three different adhesive materials, epoxy, polyurethane and modified silane, were carefully ascertained. The results indicated that the plasma process at low pressure (LPP) enabled a substantial decrease of contact angle (θ < 10o), with little modifications of surface morphology and topography with respect to the reference solvent cleaned surface (θ > 80o). The improved wetting was accompanied by a relevant increase in the joint strength. The actual mechanism of fracture shifted from adhesive failure, typical of solvent cleaned and abraded surfaces, to full cohesive failure within either the substrates of the adhesive layer. Besides, a strong influence of the adhesive selected for manufacturing was reported. Indeed, using a tough bi-component epoxy adhesive in conjunction with the LPP pre-treatment led to substrate failure, representing the largest possible enhancement of joint strength. The shear strength was over 300% of that obtained with reference solvent cleaned, and 80% larger than that recorded on abraded surfaces.

On the influence of overlap topology on the tensile strength of composite bonded joints: Single overlap versus overlap stacking

The goal of this study is to explore new topologies for adhesively bonded composite overlap joints in order to improve their strength under tensile loading. Multiple stacked overlaps, also referred as finger joints, are compared with single overlap topologies. The quasi-static tensile behaviour of single lap joints with two overlap lengths 12.7 mm and 25.4 mm are compared to finger joints with 1 and 2 stacked overlaps through thickness with constant 12.7 mm overlap length. Two composite adherend stacking sequences are tested for each topology [0/90]4s and [90/0]4s.A non-linear FE-analysis is performed to analyse the shear and peel stresses along the adhesive bond line. A difference in peak shear and peel stress, at the tip of the bonded region could be observed: (i) for 1 finger, the peak peel stress is higher than in the single lap joint configurations because the beneficial effect of avoiding eccentricity in the finger joint is outperformed by the detrimental effect of reducing to half the adherend stiffness at the overlap; (ii) for 2 fingers, the stress field changes significantly leads to a 23% decrease in peak shear and 33% in peak peel stress, compared to the single lap joint topologies.In addition, experimental lap shear tests are performed and monitored using acoustic emission technique, to follow the damage events. Different trends at damage initation and at maximum load are believed to result from how the damage propagates inside the joint. A topology with 2 fingers and layup [90/0]4s, which fails entirely inside the adherend, provides the lowest peak shear and peel stress and the highest load at damage initiation. It is however outperformed in maximum load by a single lap joint topology with layup [0/90]4s, with mostly cohesive failure. It is further found that, unlike in single overlap topologies, the most dominant stress component for damage initiation inside the finger joints is the in-plane tensile stress, at the butt joint resin pockets, rather than peel stresses at the overlap region. Lastly, if weight efficiency is the main requirement, a finger joint design can effectively replace a single overlap joint design. However, for absolute maximum joint strength, the single overlap joint is a better choice than the finger joint.

On sequential ultrasonic spot welding as an alternative to mechanical fastening in thermoplastic composite assemblies: A study on single-column multi-row single-lap shear joints

In previous work, single-spot ultrasonically welded joints were found to feature similar load carrying capability in shear but significantly low capability in peel as joints with a representative single-mechanical fastener. This leads to questioning welding as an appropriate solution for the commonly-used single-lap joint configuration. The present paper investigates the mechanical performance of spot welded single-lap joints in thermoplastic composites in comparison to their mechanically fastened counterparts. Single-row joints, double-row joints with varying inter-row distance and multi-row joints with varying number of rows were investigated in this study. The results showed that, owing to higher joint stiffness and hence lower secondary bending and peel stresses, the load carrying capability of the spot welded joints was comparable to that of the mechanically fastened joints in all considered cases. Likewise, the effects of increasing the inter-row distance and of increasing the number of rows were similar for both types of the joints.

In situ spatial strain monitoring of a single-lap joint using inkjet-printed carbon nanotube embedded thin films

Adhesive bonds are particularly favored by lightweight structures due to their weight efficiency and insulating property regarding contact corrosion. However, it is challenging to interrogate directly over the bonding area due to its sandwiched structure. In this study, an inkjet-printed carbon nanotube (CNT) strain distribution sensing film is embedded at the interface between an adherend and the adhesive in a single-lap joint configuration to monitor the spatial strain distribution over the bonding area. The conductivity distribution reconstructed by an algorithm of electrical impedance tomography (EIT) results in similar trend to the correlated conductivity distribution numerically calculated from a three-dimensional (3D) finite element model. Moreover, the EIT result of a defect-embedded testing specimen shows a different conductivity distribution compared to the healthy specimen, indicating the existence of the debonding region.

Single Lap Joints Numerical Modelling and Comparison with Experimental Testing

Adhesive bonding is a joining technique which has been extensively employed in structural design as its ability to join different types of materials allows higher freedom for designers when choosing materials. One of the most common joint configuration used is the single lap joint configuration. In this work a set of experimental procedures were undertaken to characterize a structural adhesive and model the behaviour of a single lap joint manufactured using this adhesive. Two types of surface preparations were used to study its effect in single lap joint strength.

AddJoining: A novel additive manufacturing approach for layered metal-polymer hybrid structures

A novel assembling technique based on additive manufacturing and materials joining principles has been introduced for layered metal-polymer hybrid structures. The AddJoining technique produces layered hybrid structures, using polymer 3D printing methods. The feasibility of the technique was demonstrated using fused deposition modeling for single-lap joint configuration. Microstructure and mechanical strength of the joints were studied using two combinations of materials; aluminum 2024-T3 with acrylonitrile butadiene styrene and aluminum 2024-T3 with alternate layers of polyamide-6 and carbon-fiber-reinforced polyamide-6. The latter reached an average ultimate lap-shear strength of 21.9 ± 1.1 MPa, showing 19% superior performance to the adhesively bonded joints. This exploratory investigation showed the potential of AddJoining to produce metal-polymer layered structures.

Effect of the structural reinforcement of composite adherends on the mechanical strength of adhesive joints

AbstractThis study proposes a novel method to strengthen fiber reinforced plastic (FRP) adherends in a single lap joint (SLJ) by using Kevlar threads. The adherend reinforcement procedure consists of applying a zigzag thread in the fiber glass layers that comprise the adherend in order to subsequently manufacture the adherends using the vacuum infusion method. For this study, three single lap joint configurations were used: Adherends without Kevlar reinforcement (SLJ1), adherends with Kevlar reinforcement with a medium step zigzag (SLJ2) and adherends with Kevlar reinforcement with a small step zigzag (SLJ3). Mechanical tensile tests were carried out to determine the ultimate strength and displacement of the single lap joint configurations studied. Adherends reinforced by Kevlar threads increase the strength and displacement of the single lap joint. However, the influence of the step size of zigzag threads on the adherend does not significantly influence the strength, but significantly affects the maximum displacement of the single lap joint.

Analytical models for functionally graded adhesive single lap joints: A comparative study

Functionally graded adhesives yielding a varying adhesive Young's modulus along the overlap are increasingly considered in order to allow for a more uniform stress distribution and to reduce the occuring stress concentrations in the adhesive layer of adhesively bonded single lap joints. The present work provides a comparative study of the existing analytical models for planar functionally graded adhesive single lap joints. The respective modelling assumptions as well as the conditions of applicability of the different approaches are examined in detail. In addition, a novel approach and possible extensions of the presented approaches are outlined and discussed. A thorough comparative study comprising various single lap joint configurations and adhesive Young's modulus distributions along the overlap is performed in order to assist the engineer choosing the adequate model for a particular single lap joint design. For an assessment of the accuracy, the calculated adhesive stress fields are compared to numerical results obtained by Finite Element Analyses. In addition, the effects of the adhesive Young's modulus range on the peak adhesive stresses are studied and discussed.

The stresses in single lap joints with a functionally graded adhesive bondline: an efficient modelling approach

AbstractIn this work, an efficient analytical model for the stress analysis of single lap joints with a functionally graded adhesive bondline is proposed which considers peel as well as shear stresses in the adhesive. The model takes into account the nonlinear geometric characteristics of a single lap joint under tensile loading and allows for the analysis of various adhesive Young's modulus variations. The obtained stress distributions are compared to results of detailed Finite Element analyses and show a good agreement for several single lap joint configurations. In addition, different adhesive Young's modulus distributions and their effect on the peel and shear stresses are studied and discussed in detail. (© 2016 Wiley‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

An efficient analysis model for functionally graded adhesive single lap joints

Employing a functionally graded adhesive the efficiency of adhesively bonded lap joints can be improved significantly. However, up to now, analysis approaches for planar functionally graded adhesive joints are still not addressed well. With this work, an efficient model for the stress analysis of functionally graded adhesive single lap joints which considers peel as well as shear stresses in the adhesive is proposed. Two differential equations of the displacements are derived for the case of an axially loaded adhesive single lap joint. The differential equations are solved using a power series approach. The model incorporates the nonlinear geometric characteristics of a single lap joint under tensile loading and allows for the analysis of various adhesive Young׳s modulus variations. The obtained stress distributions are compared to results of detailed Finite Element analyses and show a good agreement for several single lap joint configurations. In addition, different adhesive Young׳s modulus distributions and their impact on the peel and shear stresses as well as the influence of the adhesive thickness are studied and discussed in detail.

Crack initiation in single lap joints: effects of geometrical and material properties

Brittle failure of adhesively bonded single lap joints is considered in this paper. A coupled stress and energy criterion in the framework of finite fracture mechanics is employed to study crack initiation by means of a numerical model presented by Hell et al. (Eng Fract Mech 117:112–126, 2014). Two different formulations of the coupled criterion are compared and the effect of geometrically nonlinear bending deformation of the adherends is analysed. A comparison to experimental results on the effect of the overlap length for single lap joints with composite adherends is given, showing a very good agreement of the failure load predictions. A detailed study of the effects of the geometrical and material parameters of a single lap joint configuration is given. As the energy release at crack formation is considered, the size effect of the adhesive layer thickness is covered correctly. The paper closes with an analysis of the effect of the unbonded adherend length. An approximate explicit expression for a minimum unbonded adherend length is given, which is required to optimize joint designs and to allow for the study of individual parameter effects in numerical and experimental studies.

The Effect of Moment and Flexural Rigidity of Adherend on the Strength of Adhesively Bonded Single Lap Joints

In the present study, mechanical properties of different single lap joint configurations derived from adherends with different thicknesses subjected to tensile loading were investigated experimentally and numerically. For this purpose, experimental studies were conducted on two different types of SLJ samples, the first type with identical upper and lower adherend thicknesses and the second with different upper and lower adherend thicknesses. For the first type, five different thickness values were tested. For the second type, the lower adherend thickness was constant while five different upper adherend thickness values were tested. The adhesive was prepared from a two-part paste. After the experimental studies, stress analyses on the SLJs were performed with three-dimensional finite element analysis by considering the geometrical non-linearity and the material non-linearities of the adhesive (DP460) and adherend (AA2024-T3). It was observed that, in single lap joint geometry, variation in the thickness of the adherend and the use of lower and upper adherends with different thickness values changed the stress concentrations at the edges of the overlap regions, affecting the experimental failure load of the joints.

Flexible adhesives for automotive application under impact loading

In automotive construction, adhesive bonding can be used for joining metals, plastics or combinations of the two, and it offers the further advantage that, because the joints are continuous, an overall increase in body stiffness is achieved, thus enabling thinner material to be used. Because of their ability to absorb energy adhesive joints can also contribute to the impacted system. The objective of this work is to investigate the behavior of a flexible adhesive under impact loading using the single lap joint configuration. This adhesive showed a high strain to failure with good strength. These features of the adhesive under impact loading improve structural crashworthiness. Results showed that the lap joint strength increases considerably under impact loading compared with those under quasi-static loading and that there is a relationship between the joint performance and the loading speeds.

Effect of Holding Time on the Diffusion Behavior at Interface of Dissimilar Metals Joint between Aluminum and Carbon Steel Joint Using Element Promoter

Holding time is used for optimizing the bond diffusion between aluminum Al and Carbon steel SS400. The objective of this research was to investigate the effects of holding time on the interface reactions of diffusion welding between aluminum and carbon steel. Holding time variations of 10, 15, 30 and 45 minutes were applied at 950°C using mixture of Cu and Fe powder as elements promoter. Single lap joint configuration was performed in vacum furnace to join the dissimilar materials which allowed bonding diffusion. Microstructure was examined on the same test piece. It was found that during diffusion process at 950°C, the interfacial zone between aluminum and carbon steel substrate features intermetallic layers. The intermetallic thickness increased with increasing the holding time. Crack or incomplete bonding appeared on the specimens with holding time up to 30 minutes and didn’t appear on the specimens with holding time of 45 minutes. Cu rich-element promoter made diffusion penetrated deeper than Fe rich-element promoter in the same holding time. Macrostructure, microstructure and SEM examinations revealed that Al-steel joint had the best result with element promoter content of 60/40 % at 45 minutes holding time. There was no interlayer gap at this specimen. Additionally, from mapping view it can be suggested that in terms of poor interface bonding, Cu molecules were located just around the interface area, on the other hand, in case of strong interface bonding, Cu molecules are scattered throughout the specimen. In fact, the position of Cu molecules can be used as a promising marker for the detection of quality of diffusion joint.

Experimental investigation of damage progression and strength of countersunk composite joints

An experimental investigation is conducted into the damage progression and strength of bolted joints with fibre-reinforced composite laminates and countersunk fasteners. The main goal of the experimental investigation is to characterise the effect of the countersink geometry on the load-carrying capacity of single lap joints in comparison to the straight-shank case. The effects of bolt torque, clearance and countersink height ratio on the damage progression and joint strength are also studied. Experimental tests and detailed microscopy studies are conducted on a bearing test specimen with a straight-edged hole, and several single-lap joint configurations with countersunk fasteners. It is found that introduction of the countersunk hole roughly halves the bearing stress, and causes delamination for some configurations. This delamination is primarily located at the start of the countersink region, though is found to be triggered by other damage mechanisms and has only minor influence on the results. Bolt torque increases the density of through-thickness damage though limits its extension from the hole edge, whilst bolt clearance causes localisation of the damage region. Increasing the ratio of the countersink depth to the laminate thickness reduces the extent of bearing and promotes bending, with a change to net section failure at large ratios.

Load Ratio Effect on the Fatigue Behaviour of Adhesively Bonded Joints: An Enhanced Damage Model

Structural adhesives are used widely in aerospace and automotive applications. However, fatigue damage in these adhesives is an important factor to be considered in the design of adhesively bonded structural members that are subjected to cyclic loading conditions during their service life. Fatigue life of adhesively bonded joints depends mainly on the fatigue load and the load ratio. A fatigue damage model is presented in this paper to include the effect of fatigue mean stresses on the failure behaviour of adhesively bonded joints. The fatigue damage model is developed using an effective strain-based approach. The model is implemented on a tapered single lap joint configuration and is validated by experimental test results. The adhesive layer in the tapered single lap joint is modelled by using a cohesive zone with a bi-linear traction-separation response. The adverse effect of increasing fatigue mean stresses on the failure behaviour of adhesively bonded joints is successfully predicted.