Composite Single-lap Joints Research Articles

Dynamic feature evaluation on streaming acoustic emission data for adhesively bonded joints for composite wind turbine blade

Damage mode identification and premature failure prevention for composite structures by acoustic emission have drawn a great deal of attention. Feature evaluation on streaming acoustic emission data is one of the significant issues in research of acoustic emission signal processing. This work conducts dynamic feature evaluation on 15 conventional acoustic emission features so as to seek a deeper insight into different features with damage accumulation. First, the procedure of dynamic feature evaluation is presented based on three basic algorithms. Second, the streaming acoustic emission data are collected from the adhesively bonded composite single-lap joint subjected to quasi-static tensile loads. Third, further efforts are made so as to explore the information contained as well as to interpret the effect of damage accumulation. It is found that different conventional acoustic emission features show distinctive functions, including damage mode identification, damage process indication, and both of them. Informative features for damage pattern recognition are independent on damage accumulation. Useful features for damage process description show sensitive dynamic characteristics with damage accumulation, especially before the complete failure of the specimen. Furthermore, dynamic feature evaluation can be used to detect singular signals.

Experimental investigations on fatigue life enhancement of composite (e-glass/epoxy) single lap joint with graphene oxide modified adhesive

With the increased use of composites in various sectors as a lightweight material exhibits high strength to weight ratio with tailor made properties. It becomes necessary to focus on various joining methods and different types of composite joints and their strength. The present study aims to improve the mechanical strength of single lap joint of composite material comprises of E-glass fibers and nano modified adhesive. Epoxy adhesive has been modified by dispersing Graphene Oxide (GO) to investigate the possibility of enhancement in the fatigue strength and fracture resistance of the single lap joint. Modified Hummer’s method has been used for synthesis of Graphene Oxide. Experimental investigations have been carried out for comparison of tensile and fatigue strength which shows significant improvement in the number of failure cycles for 0.25 wt.% and 0.75 wt.% GO concentrations respectively as compared to neat adhesive. Tension test results showed a significant increase in the fracture toughness of the joint due to addition of GO nanoparticles. There has been 33% and 19% increase in fracture toughness in 0.25 wt.% and 0.75 wt.% GO samples respectively as compared to neat adhesive. Improvement in fracture toughness, among all other nano-reinforcements has been obtained using GO, mainly because of its better capability of deviating the crack growth path to the longer path causing the final failure to retard and consequently improving mechanical properties of the adhesive for the tensile and fatigue strength parameters.

A strain-rate-dependent analytical model for composite bolted joints

In the present research, a novel analytical approach was developed to predict the bearing chord stiffness and the damage initiation bearing-load of single-lap composite bolted joints under medium strain rate loading. First, the elastic moduli, Poisson's ratio, and strength of a unidirectional composite ply at an arbitrary strain rate were predicted by available micromechanical equations. Then, the bearing chord stiffness of the joint at any arbitrary strain rate was predicted. For this purpose, the available spring-based model was modified. The strain-rate-dependent damage initiation bearing-load of the joint was predicted by using the moduli and the stress concentration factor of a pin-loaded unidirectional ply. Four types of single-lap joints with [-45/0/+45/90]s and [90/-452/+45]s layups with and without carbon nanofibers were tested at the strain rates of 0.0048 s-1, 0.36 s-1, and 0.89 s-1. The results of experiments showed that mechanical properties of single-lap composite bolted joints increased with increasing the strain rate. Also, employing carbon nanofibers has a significant effect on the mechanical properties of the joints. The predicted results in comparison with the conducted experimental data show good agreements.

Performance and failure assessment of adhesively bonded non-crimp fabric carbon fiber/epoxy composite single lap joints

Carbon fiber-reinforced plastic (CFRP) composite materials have experienced increased use for load-bearing structures in automobiles owing to their high specific mechanical properties. However, critical to widespread adoption of CFRPs are the development of robust joining methods with appropriate surface preparation to maximize joint strength, as well as an improved understanding of failure mechanisms to enable computational modeling for supporting design. In the present study, an adhesively bonded non-crimp fabric CFRP single lap joint (SLJ) was investigated to quantify the effect of the composite adherend surface treatment and stacking sequence on the lap shear strength, and to assess the corresponding failure mechanisms. Single lap shear test results revealed that the highest lap shear strength was achieved by abrading the surface with sandpaper, relative to an unprepared surface or one treated by grit blasting. Similarly, an adherend with a higher effective flexural longitudinal modulus, achieved through stacking sequence, increased the lap shear strength owing to reduced Mode I loading on the adhesive joint. Distinct failure processes were observed for CFRP adherends with different stacking sequences, where the locations of intra-ply and inter-ply cracks varied as a result of variations in the in-plane and out-of-plane stresses. Notwithstanding, the final failure event for all SLJ specimens was failure of the thin layer of matrix in the 0° ply of the adherends. Three-dimensional finite element analyses of the SLJ specimens were employed to determine the local stress components driving the observed failure mechanisms within the plies of the CFRP adherends. The results of this study provide a recommended surface treatment to maximize adhesive joint strength for CFRP adherends, along with a comprehensive assessment of failure mechanisms, to inform the design and application of CFRP materials in lightweight structures.

Strength prediction of composite single lap joints using the radial point interpolation method

This work applies a meshless method, the Natural Neighbours Radial Point Interpolation Method (NNRPIM), to a multi material structure composed of an anisotropic material, the composite substrates, and an isotropic material, the adhesive bonding the substrates. The strength predictions in this work were made using a recent continuum mechanics based criterion, the Critical Longitudinal Strain (CLS) criterion. The validity of the NNRPIM in this type of problem was proven by comparing it to the commonly used FEM, and the stress distributions for both numerical methods were similar. The CLS criterion proved to be accurate for the brittle Araldite®AV138, but when applied to the ductile adhesives used in this work, Araldite®2015 and Sikaforce®7888, the CLS criterion showed some limitations.

Experimental analysis and prediction of strength of adhesive-bonded single-lap composite joints: Taguchi and artificial neural network approaches

Adhesive-bonded joints made up of composite materials offer complex structures with the ease of joining similar or dissimilar materials. The failure behavior of adhesive-bonded joints is influenced largely by geometrical parameters (overlap length and adhesive thickness) and adherend surface preparations. The glass fiber-reinforced epoxy composites are prepared for single-lap adhesive joints to know their strength with different sets of geometrical factors. The adherend surfaces of composites are roughened to 2 ± 0.1 µm, prior to joint preparation. Taguchi L9 experimental matrix representing different combinations of overlap length and adhesive thickness is employed to know the behavior of failure load (FL) and shear strengths (SS) of the adhesive-bonded single-lap composite joints. The results showed that the impact of overlap length of adhesive-bonded joints is more compared to that of adhesive thickness. The interaction effects of geometrical parameters are found negligible toward both the outputs. The optimal factor levels received the highest load to fracture; the joints are found equal to 6096.1 N for FL and 80 MPa for SS, respectively. The empirical relationship based on multiple linear regression (MLR) equations is derived for both the failure load and shear strength. Levenberg–Marquardt algorithm-trained neural networks (NNs) are used for the prediction of both responses. Ten experimental cases are used to check the prediction capabilities of both MLR and NNs. The mean absolute percent error in prediction of both the responses is found equal to 2.27% for NNs and 3.12% for MLR. The NNs and MLR results in accurate prediction might be due to the model development process based on experimental input–output data, rather than assumption-based theoretical and numerical methods.

Study on bearing strength and failure mode of a carbon-epoxy composite laminate for designing bolted joint structures

A composite single-lap bolted joint was experimentally studied and its bearing behavior was predicted numerically. Prior to performing the experiment, an appropriate e/D ratio of the composite material was determined via double-lap bolted joint tests. Using the obtained e/D ratio, specimens were manufactured, and single-lap composite bolted joint tests were subsequently performed. Various joint tests were conducted with the number of bolt holes, pre-torque, and thickness as experimental variables. The preferred bearing failure mode was observed with the designed e/D ratio and bearing strength increased with increasing tightening torque. The bearing strength was significantly affected by the projected bearing area and exhibited a linearly proportional relation with the joint bearing area. The fracture mechanism was observed using optical microscopy, indicating fiber buckling, breakage, and delamination induced from the initial damage. An energy-based progressive failure analysis (PFA) was implemented to predict the composite bolted joints strengths. The adopted PFA accurately predicted the strength of the bolted joint in range of 12%.

A numerical sensitivity analysis on the cohesive parameters of a carbon-steel single lap joint

One of the key issues concerning the use of composite materials in the construction of steel ships, or vice versa, the use of steel in the construction of composite ships, is related to the ways of joining composite parts to the adjacent steel structure. Carbon-steel joints are of increasing interest for the maritime industry, but knowledge is scarce. Applications of adhesive joints as an alternative method to welding or riveting has numerous advantages. Still, gathering more information about the behaviour of such joints through numerical sensitivity analysis and simulation are necessary to speed up applications on ships and marine structures.The present investigation aims at assessing the sensitivity of the parameters that define the traction separation law on the numerical simulation of the de-bonding of a steel to composite single lap joint, following a previous experimental campaign. The test is intended to induce a mode II separation, although the slight bending moment caused by the misalignment of the two ends leads to mixed mode effects, as well.Two finite element software were compared, ADINA and LS-DYNA.The influence of peak traction, penalty stiffness and energy release rate were investigated, along with the effect of a mixed mode opening behaviour.

3-D interfacial stress analysis of adhesively bonded curved laminated FRP composite single lap joint

According to the fracture mechanics concept, the distribution of three-dimensional (3-D) three major stresses, i.e. peel stress for mode I, direct shear stress for mode II and transverse shear stress for mode III, are computed at different interfacial layers on curved laminated FRP composite single lap joint (SLJ), subjected to different ply orientation in adherend with application of tensile load. The three interfacial adhesive layers have been considered as; bottom layer (in between lower adherend and adhesive layer), middle layer (mid plane of adhesive layer) and top layer (in between upper adherend and adhesive) for determining the critical location of damage initiation by using failure index criteria. The results show that the variations of peel stress and shear stresses in the overlap region are very complicated in nature and hence a 3-D finite element analysis (FEA) is recommended to have a clear visualization of these stress variations. A comparative analysis due to ply orientation of adherends of SLJ has been carried out. It has been observed that the adhesion failures (in between adherend and adhesive) are more dominated than cohesion failure (within the adhesive layer).

Monitoring of Fiber-Reinforced Composite Single-Lap Joint with Electromechanical Impedance of Piezoelectric Transducer.

The single-lap joint of fiber-reinforced composites is a common structure in the field of structure repair, which has excellent mechanical properties. To study and monitor its quasi-static response behavior under external load, two methodologies called effective structural mechanical impedance (ESMI) and reduced-ESMI (R-ESMI) are presented in this article. A two-dimensional electromechanical impedance (EMI) model for a surface-bonded square piezoelectric transducer (PZT) is adopted to extract more sensitive signatures from the measured raw signatures. There are two major advantages of the monitoring scheme based on ESMI and R-ESMI signatures: (1) excellent monitoring results with less signatures to process, (2) the ability to monitor the quasi-static behavior of a single-lap joint with previously ignored susceptance signatures. Combining the extracted ESMI signatures with the index of root-mean-square deviation, the quasi-static behavior of single-lap joints can be effectively quantified. To test the effectiveness of ESMI methodology, verifying experiments were conducted. The experimental results convincingly demonstrated that the presented ESMI and R-ESMI methodologies have good feasibility in monitoring the quasi-static behavior of a fiber-reinforced composite single-lap joint. The proposed method has potential application in the field of structural health monitoring (SHM).

A study on mixed fracture characteristics of composite single lap joint (SLJ) combining mixed continuum damage model and critical zone size based approach

In this paper, authors present an improved method for predicting the mixed fracture characteristics of composite single lap joint using the mixed - mode continuum damage model and the critical zone size based approach. We apply a method based on the critical zone size to predict the failure of composite adherend and establish a procedure for more precisely calculating the critical zone size using the ABAQUS user subroutine. In addition, a bilinear/exponential mixed - mode continuum damage model is used to evaluate the failure of adhesive of single lap joint. The failure of adherend and the damage process of adhesives can be considered simultaneously by the method based on the critical zone size and the mixed - mode continuum damage model. For the validation of proposed model, the user subroutine of general purpose finite element software ABAQUS was used to simulate the failure process of single lap joint made of fiber reinforced composite materials and the obtained results were compared with the previous experimental results. The results obtained by the method presented in this paper are in good agreement with previous experimental results, and showed that the adhesive cohesive damage process before the global failure of the joint due to the failure of the adherend and the interlayer failure of the composite adherend can be predicted simultaneously.

A novel approach for the interfacial stress analysis of composite adhesively bonded joints

This paper presents a novel approach for the interfacial stress analysis of composite single-lap joints (SLJ). The analytical modeling is based on the interface elasticity where both the shear and peel stresses are assumed to be variant across the adhesive layer thickness. The unknown interfacial parameters are determined by using an experiment-based inverse method. The finite element simulation is then performed to calculate the interfacial stress distributions and evaluate the effect of the structural parameters on the interfacial behavior of the composite bonded joints. It is shown that both the peel and shear stresses at the interface are influenced by the interface energy and geometry parameters of the composite bonded joint. Numerical results are presented both to demonstrate the advantages of the present model over existing ones and to illustrate the main characteristics of interfacial stress distributions.

Effect of adhesive thickness and overlap on the behavior of composite single-lap joints

The effect of adhesive thickness and adherent overlap on behavior of composite single-lap joints (SLJ) under tensile load is studied by using a 3D finite-element model. A bilinear Cohesive Zone Model (CZM) law is used to represent the adhesive behavior and its parameters are determined as a function of adhesive thickness by carrying out Double Cantilever Beam and End-Notched Flexure tests. The performance of SLJ is defined by peak load, maximum shear, and peel stresses. For the range of adhesive thicknesses considered, both maximum shear and peel stresses decrease with increasing adhesive thickness, while they increase with increasing overlap length.

Effect of nanostructured reinforcement of adhesive on thermal cycling performance of a single-lap joint with composite adherends

Nanostructure of adhesive increased the failure load of bonded joints. Adhesively bonded joints are often exposed to alternating warm and cold temperatures in many applications. Hence investigation of their thermal cycle performance is very important. In the present study, the tensile strength of adhesively bonded single lap composite joints (SLCJs) was experimentally investigated under ambient temperature and thermal cycling conditions. In the study, carbon fiber fabric reinforced (CFFR) composites (0/90°) with Plain Weave (PW) were used as the adherend; DP460 tough and DP125 flexible adhesive types were used as the adhesive and 1 wt% Graphene-COOH, Carbon Nanotube-COOH and Fullerene C60 were used as the nanostructures. Six different thermal cycles were applied to SLCJs and their tensile failure loads and failure surfaces were investigated. As a result, when the joints obtained through nanostructure non-reinforced adhesives are exposed to thermal cycle, a significant decrease appears in the failure load of the joint. Yet, adding nanostructure to the adhesive enhances the thermal cycle performance of the joint. This enhancement in the thermal cycle resistance changes depending on the structural features of the adhesive, the type of nanostructure and thermal cycle condition.

An integrated constitutive model for tensile failure analysis and overlap design of adhesive-bonded composite joints

An integrated constitutive model is proposed for tensile failure analysis and overlap design of adhesive-bonded composite joints. In this model, two improved criteria are developed: for matrix failure and fiber failure of laminates, an improved 3D strain-based Tsai-Wu criterion is developed by introducing mode factors and an exponential progressive damage evolution; for delamination and debonding, an improved 3D coupled exponential cohesive zone model (CZM) is developed by introducing new tangential components. Both criteria are implemented in one explicit user subroutine to facilitate applications for engineering. The validity of the model is proved in comparison with the experimental results of adhesive-bonded composite single-lap joints and other numerical models. Based on the model, the influence of different overlap lengths on the tensile failure behavior of the adhesive-bonded composite single-lap joints is analyzed comprehensively. For the adhesive-bonded composite single-lap joints tested in this study, an equivalent load-weight ratio is proposed to reflect the relative load capacity, and the optimal value of the overlap length is discussed.

Hydrothermal ageing effect on the mechanical behaviour and fatigue response of aluminium alloy/glass/epoxy hybrid composite single lap joints

The effect of hydrothermal ageing on the mechanical behaviour and fatigue response of a hybrid (bolted/bonded) aluminium alloy (AA7075) and glass fibre reinforced epoxy (GRE) hybrid composite single lap joints (SLJs) were carried out in this work. This effect was investigated using hybrid joints as a joint configuration. An adhesive layer of Araldite epoxy and mechanical fasteners of Huck bolt were attached between the adherends as primary and secondary attachments, respectively. Three types of joint were exposed to a humid environment at 50 °C for long-term immersion (20, 40, 60, 80, 100, and 120 days) periods. Composite laminates (i.e. glass fibre epoxy GRE) and metal (i.e. aluminium alloy AA7075) were used as joint adherends. Quasi-static and fatigue tests were carried out to evaluate the evolution of the mechanical performance and the damage mechanisms of hybrid joints during the ageing exposition. The dissimilar-AA7075/GRE hybrid SLJ showed the highest joint strength and the longest failure strain. Moreover, the hybrid joint with dissimilar-AA7075/GRE achieved 83% and 30.2% higher fatigue strength than similar adherends of AA7075 and GRE composites, respectively. As for the damage mechanisms, shear specimens experienced a typical bearing mode.

An analytical approach to predict the mechanical behavior of single-lap single-bolt composite joints reinforced with carbon nanofibers

In the present research, the stiffness and the damage initiation load of single-lap composite bolted joints (SLJs) reinforced with carbon nanofibers (CNFs) were studied. To this end, an analytical approach was developed and in the first step, the spring-mass model was modified. By having the elastic properties of the fiber, matrix and nano reinforcement, layup and geometrical parameters of the joint and using the modified spring-mass model, the stiffness of the joint was predicted. Then, by calculating the modulus and the stress concentration factor of a unidirectional ply with a pin-loaded hole and using the maximum stress failure criteria, the damage initiation load of the joint was predicted. Two different SLJs with [−45/0/+45/90]s and [90/−452/+45]s layups with and without nanoparticles were studied. The present analytical approach was evaluated by the finite element model as well as conducting an experimental program.

Micro-Mechanical Based Load Distribution in Multi Bolt Single Lap Composite Joint

This paper presents an analytical method for evaluating a load distribution in multi-bolt single lap laminated composite joints using a micromechanical approach based on Classical Plate Theory. A MATLAB programme is developed for computing equivalent properties of laminated composite plate and a spring model is used for the computation of load distribution in multi bolt joint. The results are obtained for various lamination schemes, volume fibre fraction and loads. The comparison of present results with various literature shows good agreement.

In situ analysis of interfacial damage in adhesively bonded composite joints subjected to various surface pretreatments

Secondary bonding of carbon fiber-reinforced polymer (CFRP) using structural adhesives is a promising technology. However, the joint performance can be ultimately affected by surface modifications attained through different surface preparation strategies. Thus, there is still a need for experimental techniques capable of assessing the mechanical properties associated with different surface preparations, and their relation to the damage morphology at initiation. In this work, a refined experimental set-up was used to perform three-point bending tests on miniaturized single-lap composite joints. Such testing configuration features a peel-dominant stress field within the bondline and the possibility to track the evolution of adhesive deformation and damage in situ at the meso-scale using optical microscopy. Epoxy-bonded CFRP joints were evaluated as a function of various surface pretreatments. In situ local observation obtained during mechanical loading provided insights into the detailed mechanisms taking place depending on the surface preparation strategies.

Numerical study on the mechanical behavior of a polyurethane adhesive under high strain rate

The cohesive parameters of a polyurethane (PU) adhesive are experimentally tested under different strain rate conditions. As the strain rate of this adhesive is constantly changing under the actual impact conditions, this paper proposes a method, called weighted average strain rate method, to determine the effective strain rate of the adhesive under the high strain rate condition for numerical simulation. Based on this method, the relationship between cohesive parameters and the determined effective strain rate is established and a strain rate-dependent cohesive constitutive model of the adhesive is developed and implemented into commercial finite element analysis software via a subroutine. The model is calibrated and verified against experimental data for double cantilever beam (DCB), end notched flexure (ENF) tests and thick adherend shear tests (TAST). Finally, impact tensile shear test and numerical analysis on composite single-lap joints at high loading rates are carried out to validate the reliability of the subroutine. By comparing the FEA results with experimental results, a good agreement is achieved which proves the effectiveness of the strain rate determination method and the subroutine.