Progressive failure and experimental study of adhesively bonded composite single-lap joints subjected to axial tensile loads

Experimental tests and finite element method (FEM) simulation were implemented to investigate T700/TDE86 composite laminate single-lap joints with different adhesive overlap areas and adherend laminate thickness. Three-dimensional finite element models of the joints having various overlap experimental parameters have been established. The damage initiation and progressive evolution of the laminates were predicted based on Hashin criterion and continuum damage mechanics. The delamination of the laminates and the failure of the adhesive were simulated by cohesive zone model. The simulation results agree well with the experimental results, proving the applicability of FEM. Damage contours and stress distribution analysis of the joints show that the failure modes of single-lap joints are related to various adhesive areas and adherend thickness. The minimum strength of the lap with defective adhesive layer was obtained, but the influence of the adhesive with defect zone on lap strength was not decisive. Moreover, the adhesive with spew-fillets can enhance the lap strength of joint. The shear and normal stress concentrations are severe at the ends of single-lap joints, and are the initiation of the failure. Analysis of the stress distribution of SL-2-0.2-P/D/S joints indicates that the maximum normal and shear stresses of the adhesive layer emerge on the overlap ends along the adhesive length. However, for the SL-2-0.2-D joint, the maximum normal stress emerges at the adjacent middle position of the defect zone along the adhesive width; for the SL-2-0.2-S joint, the maximum normal stress and shear stress emerge on both edges along the adhesive width.     

The Effect of Adhesive Thickness on the Mechanical Behavior of a Structural Polyurethane Adhesive

One parameter that influences the adhesively bonded joints performance is the adhesive layer thickness. Hence, its effect has to be investigated experimentally and should be taken into consideration in the design of adhesive joints. Most of the results from literature are for typical structural epoxy adhesives which are generally formulated to perform in thin sections. However, polyurethane adhesives are designed to perform in thicker sections and might have a different behavior as a function of adhesive thickness. In this study, the effect of adhesive thickness on the mechanical behavior of a structural polyurethane adhesive was investigated. The mode I fracture toughness of the adhesive was measured using double-cantilever beam (DCB) tests with various thicknesses of the adhesive layer ranging from 0.2 to 2 mm. In addition, single lap joints (SLJs) were fabricated and tested to assess the influence of adhesive thickness on the lap-shear strength of the adhesive. An increasing fracture toughness with increasing adhesive thickness was found. The lap-shear strength decreases as the adhesive layer gets thicker, but in contrast to joints with brittle adhesives the decrease trend was less pronounced.     

Effect of hole drilling at the overlap on the strength of single-lap joints

Bonded unions are gaining importance in many fields of manufacturing owing to a significant number of advantages to the traditional fastening, riveting, bolting and welding techniques. Between the available bonding configurations, the single-lap joint is the most commonly used and studied by the scientific community due to its simplicity, although it endures significant bending due to the non-collinear load path, which negatively affects its load bearing capabilities. The use of material or geometric changes in single-lap joints is widely documented in the literature to reduce this handicap, acting by reduction of peel and shear peak stresses at the damage initiation sites in structures or alterations of the failure mechanism emerging from local modifications. In this work, the effect of hole drilling at the overlap on the strength of single-lap joints was analyzed experimentally with two main purposes: (1) to check whether or not the anchorage effect of the adhesive within the holes is more preponderant than the stress concentrations near the holes, arising from the sharp edges, and modification of the joints straining behaviour (strength improvement or reduction, respectively) and (2) picturing a real scenario on which the components to be bonded are modified by some external factor (e.g. retrofitting of decaying/old-fashioned fastened unions). Tests were made with two adhesives (a brittle and a ductile one) varying the adherend thickness and the number, layout and diameter of the holes. Experimental testing showed that the joints strength never increases from the un-modified condition, showing a varying degree of weakening, depending on the selected adhesive and hole drilling configuration.     

Finite element analysis of torsional free vibration of adhesively bonded single-lap joints

Adhesively bonding is a high-speed fastening technique which is suitable for joining advanced lightweight sheet materials that are dissimilar, coated and hard to weld. In this paper, the free torsional vibration characteristics of adhesively bonded single-lap joints are investigated in detail using finite element method. The effectiveness of finite element analysis technique used in the study is validated by experimental tests. The focus of the analysis is to reveal the influence on the torsional natural frequencies and mode shapes of these joints caused by variations in the material properties of adhesives. It is shown that the torsional natural frequencies and the torsional natural frequency ratios of the adhesively bonded single-lap joints increases significantly as the Young′s modulus of the adhesives increase, but only slight changes are encountered for variations of Poisson's ratio. The mode shapes analysis show that the adhesive stiffness has a significant effect on the torsional mode shapes. When the adhesive is relatively soft, the torsional mode shapes at the lap joint are slightly distorted. But when the adhesive is relatively very stiff, the torsional mode shapes at the lap joint are fairly smooth and there is a relatively higher local stiffening effect. The consequence of this is that higher stresses will be developed in the stiffer adhesive than in the softer adhesive.     

Three-dimensional finite element analysis of single-lap adhesive joints under impact loads

This paper deals with the stress wave propagation and stress distribution in single-lap adhesive joints subjected to impact tensile loads with small strain rate. The stress wave propagations and stress distributions in single-lap joints have been analyzed using an elastic three-dimensional finite-element method (DYNA3D). An impact load was applied to the single-lap adhesive joint by dropping a weight. One end of one of the adherends in the single-lap adhesive joint was fixed and the other adherend to which a bar was connected was impacted by the weight. The effects of Young's modulus of the adherends, the overlap length, the adhesive thickness and the adherend thickness on the stress wave propagations and stress distributions at the interfaces have been examined. It was found that the maximum stress occurred near the edge of the interface and that it increased with an increase of Young's modulus of the adherends. It was also seen that the maximum stress increased as the overlap length, the adhesive thickness and the adherend thickness decreased. In addition, strain response of single-lap adhesive joints subjected to impact tensile loads was measured using strain gauges. Fairly good agreements were observed between the numerical and experimental results.     

Optimization of adhesively-bonded single lap joints by adherend notching

The effect of adherend notching on the strength and deformation behavior of single lap joints was investigated. First, a parametric study was conducted using finite element analysis (FEA). This initial part of the research into the effect of notches on joint behavior involved determination of the optimum notch location and notch dimensions. This was done by using FEA in a series of models with different notch positions and geometries. The results of this parametric study were used to select the most promising lap geometries for further study. Next, more detailed FEA were conducted on the selected lap geometries. These data were compared with the experimental single-lap shear test results to assess the applicability of different failure criteria. Three different model adhesives were used: a rubber toughened film epoxy with nylon carrier, a styrene-butadiene-styrene block copolymer based deformable 'gel' adhesive, and a two-part, metal filled brittle epoxy adhesive. The FEA for single lap joints containing 'top notches' on the unbonded, top side of the adherends, at locations corresponding to the overlap ends, and bonded with the two-part metal filled epoxy provided the best agreement with the experimental results. The experimental results showed a 29% increase in joint strength with the introduction of the notches, which matched very well with the 27% decrease in the peak peel stress observed by the FEA results. For this brittle adhesive, the peel stress is almost certainly the governing failure stress. This was confirmed by matching of the FEA peak peel stress ratios with the experimental load ratios, for both the notched and unnotched specimens.     

On the von Mises elastic stress evaluations in the bi-adhesive single-lap joint: a numerical and analytical study

Bi-adhesive joints are an alternative stress-reduction technique for adhesively bonded joints. The joints have two types of adhesives in the overlap region. The stiff adhesive should be located in the middle and the flexible adhesive at the ends. This study is the extension of our previous paper to the von Mises stress evaluation and discusses the values and importance of the von Mises stresses in the bi-adhesive single-lap joint. Both analytical and numerical analyses were performed using three different bi-adhesive bondline configurations. The Zhao’s closed form (analytic) solution used includes the bending moment effect. In the finite element models, overlap surfaces of the adherends and the adhesives were modeled using surface-to-surface contact elements. The contribution levels of the peel and shear stresses for producing a peak von Mises stress are also studied. It is concluded that the contribution level of the shear stress at where von Mises stress becomes peak is more than that of the peel stress. Joint strength analyses were performed based on the peak elastic von Mises stresses. It is seen that joint strength can be increased using bi-adhesive bondline. The analytical and numerical results show that the appropriate bond-length ratio must be used to obtain high joint strength.     

Strength of epoxy adhesive-bonded stainless-steel joints

The strength of stainless-steel joints bonded with two epoxy adhesives was investigated. The experimental programme included tests on single-lap and butt joints, as well as thick-adherend and napkin ring shear tests. Results suggested that the tensile and shear strengths of the epoxy adhesives were quite similar. However, finite element (FE) analyses raised doubts on the true adhesive strengths, due to the complex stress state in joint tests and pressure-dependent adhesive behaviour. In spite of some uncertainties, FE analyses showed that failure could be fairly well predicted by a maximum shear strain criterion.     

Three-dimensional finite element analysis of single-lap adhesive joints subjected to impact bending moments

This paper deals with the stress wave propagations and stress distributions in single-lap adhesive joints subjected to impact bending moments with small strain rate. The elastic stress wave propagation and the stress distribution in single-lap adhesive joints of similar adherends subjected to impact bending moments are analyzed using three-dimensional finite-element method (FEM). A three-point impact bending moment is applied to the joint by dropping a weight. FEM code employed is DYNA3D. The effects of Young's modulus of the adherends, the lap length, the adherend thickness and the adhesive thickness on the stress wave propagation at the interfaces are examined. It is found that the maximum value of the maximum principal stress, σ₁, appears at the interface between the adhesive and the upper surface of upper adherend which is impacted. The maximum stress, σ₁, increases as Young's modulus of adherends, the lap length and the adhered thickness increase. It is also found that the maximum stress, σ₁ increases with decreasing adhesive thickness. In addition, experiments were carried out to measure the strain response of single-lap joints subjected to impact bending moments using strain gauges. A fairy good agreement was observed between the numerical and experimental results.     

Modeling of cohesive failure processes in structural adhesive bonded joints

This paper presents an approach to predicting the strength of joints bonded by structural adhesives using a finite element method. The material properties of a commercial structural adhesive and the strength of single-lap joints and scarf joints of aluminum bonded by this adhesive were experimentally measured to provide input for and comparison with the finite element model. Criteria based on maximum strain and stress were used to characterize the cohesive failure within the adhesive and adherend failure observed in this study. In addition to its simplicity, the approach described in this paper is capable of analyzing the entire deformation and failure process of adhesive joints in which different fracture modes may dominate and both adhesive and adherends may undergo inelastic deformation. It was shown that the finite element predictions of the joint strength generally agreed well with the experimental measurements.     

Effect of adhesive thickness and surface roughness on the shear strength of aluminium one-component polyurethane adhesive single-lap joints for automotive applications

Adhesively bonded technology is now widely accepted as a valuable tool in mechanical design, allowing the production of connections with a very good strength‐to‐weight ratio. The bonding may be made between metal–metal, metal–composite or composite–composite. In the automotive industry, elastomeric adhesives such as polyurethanes are used in structural applications such as windshield bonding because they present important advantages in terms of damping, impact, fatigue and safety, which are critical factors. For efficient designs of adhesively bonded structures, the knowledge of the relationship between substrates and the adhesive layer is essential. The aim of this work, via an experimental study, is to carry out and quantify the various variables affecting the strength of single-lap joints (SLJs), especially the effect of the surface preparation and adhesive thickness. Aluminium SLJs were fabricated and tested to assess the adhesive performance in a joint. The effect of the bondline thickness on the lap-shear strength of the adhesives was studied. A decrease in surface roughness was found to increase the shear strength of the SLJs. Experimental results showed that rougher surfaces have less wettability which is coherent with shear strength tests. However, increasing the adhesive thickness decreased the shear strength of SLJs. Indeed, a numerical model was developed to search the impact of increasing adhesive thickness on the interface of the adherend.     

Eco-economical polyurethane wood adhesives from cellulosic waste: Synthesis, characterization and adhesion study

This study reports the preparation of polyurethane adhesives using polyols obtained from cellulosic waste and detailed study on its adhesive strength in wood joints. Keeping in view the environmental hazards related to the huge paper-waste generation across the world, low-viscosity polyols have been prepared using magazine paper waste and vegetable oils with different physicochemical properties and were used to prepare two-component polyurethane adhesives for wood bonding. Polyurethane was analyzed by FTIR spectroscopy and TGA was used for the analysis of thermal properties. The adhesive strength was measured and compared with commercially available adhesives under different environmental conditions. The synthesized adhesive with NCO/OH ratio of 1.2 and curing time of 5 days was found to be superior to the commercial adhesives Fevicol™ and Araldite™ when compared simultaneously for the single-lap shear strength in different environmental conditions.     

Mechanical characterization and damage assessment of thick adhesives for wind turbine blades using acoustic emission and digital image correlation techniques

Adhesives play a key role in the structural integrity of the Wind Turbine Blades as they are one of the main load carrying materials. A deep knowledge of the adhesives' mechanical behaviour in terms of failure mechanisms and damage processes enhances the attempt to optimize the blade design. Therefore, a comprehensive experimental programme was performed in order to determine the static mechanical properties of the adhesives. Ultimate tensile strength, ultimate compression strength, ultimate shear strength and the elastic properties of the adhesive specimens were determined through tensile and compression tests on dogbone specimens and single-lap bonded joints. The Acoustic Emission (AE) technique was used to relate the acoustic activity in the specimens to their damage state. More specifically, a frequency-based methodology, analysing the AE data, was used for the identification of the different damage mechanisms into the material during the loading. In addition, Digital Image Correlation technique, as a full-field technique, was used to measure displacements and deformations.     

The use of the exponential Drucker-Prager material model for defining the failure loads of the mono and bi-adhesive joints

In this study, our previous experimental study was extended applying the exponential Drucker-Prager (EDP) yield criterion to define the numerical failure loads for mono and bi-adhesive single lap joints (SLJs) [Öz and Özer, 2016]. Bi-adhesive (or hybrid adhesive) joint is an alternative stress-reduction technique for adhesively bonded lap joints. The joints have two adhesives with different moduli in the overlap region. Non-linear finite element analyses were carried out for mono and bi-adhesive joints implementing the EDP material model. Distributions of EDP maximum principal stress, equivalent stress and shear stress were obtained along the middle of the adhesive thickness. Numerical failure loads were compared with our previous experimental failure loads. In addition, hydrostatic stress and equivalent plastic strain distributions for these joints under the failure loading were obtained. The general results show that experimental and numerical failure loads were in a good agreement. As a result, when bond-length ratios are selected properly and appropriate adhesives are used along the overlap length, the strength of bi-adhesive joints, compared to mono-adhesive joints, was found to increase considerably.     

The shear strength of composite secondary bonded single-lap joints with different fabrication methods

Abstract

The shear strength of composite secondary bonded single-lap joints was studied in this article. To optimize the adhesive thickness and ensure stable mechanical properties, an improved mold was applied. Based on this mold, a total of 15 specimens (180 samples) were examined and they were fabricated with various overlap lengths, curing pressures, adhesive thicknesses, ply angles, and surface treatment methods. The experimental results indicated that the improved mold not only significantly increased the uniformity of the adhesive thickness but also enhanced the shear strength of the joints and the shear strength was improved by approximately 13% compared to that of conventional methods. Moreover, the shear strength was decreased in specimens with increased overlap lengths and increased in samples with an increased curing pressure. Furthermore, the shear strength of the specimens was also affected by the adhesive thicknesses, ply angles, and surface treatment methods. The mechanisms can be ascribed to the effect of the fabrication method on the failure mode. A facile platform for optimizing these parameters is provided in this article. Based on this platform, the shear strength of the joints was enhanced to 33.5?MPa.     

Modelling adhesive joints with cohesive zone models: effect of the cohesive law shape of the adhesive layer

Adhesively-bonded joints are extensively used in several fields of engineering. Cohesive Zone Models (CZM) have been used for the strength prediction of adhesive joints, as an add-in to Finite Element (FE) analyses that allows simulation of damage growth, by consideration of energetic principles. A useful feature of CZM is that different shapes can be developed for the cohesive laws, depending on the nature of the material or interface to be simulated, allowing an accurate strength prediction. This work studies the influence of the CZM shape (triangular, exponential or trapezoidal) used to model a thin adhesive layer in single-lap adhesive joints, for an estimation of its influence on the strength prediction under different material conditions. By performing this study, guidelines are provided on the possibility to use a CZM shape that may not be the most suited for a particular adhesive, but that may be more straightforward to use/implement and have less convergence problems (e.g. triangular shaped CZM), thus attaining the solution faster. The overall results showed that joints bonded with ductile adhesives are highly influenced by the CZM shape, and that the trapezoidal shape fits best the experimental data. Moreover, the smaller is the overlap length (L_O), the greater is the influence of the CZM shape. On the other hand, the influence of the CZM shape can be neglected when using brittle adhesives, without compromising too much the accuracy of the strength predictions.     

Three-dimensional finite element stress analysis of single-lap adhesive joints of dissimilar adherends subjected to impact tensile loads

The stress-wave propagations and stress distributions in single-lap joints of dissimilar adherends were analyzed using an elastic three-dimensional finite-element method (DYNA3D). An impact tensile load was applied to the single-lap adhesive joint by dropping a weight. One end of the upper adherend in the single-lap adhesive joint was fixed and the other adherend (lower adherend) which was connected to a bar was impacted by the weight. The effects of Young's modulus and the thickness of each adherend on the stress wave propagations and stress distributions at the interfaces were examined. It was found that the maximum value of the maximum principal stress occurred near the edge of the interface of the fixed adherend. The maximum principal stress increased as Young's modulus of the fixed adherend increased. It was also observed that the maximum principal stress increased as the fixed adherend thickness decreased. In addition, strain responses in the single-lap adhesive joints of dissimilar adherends subjected to impact tensile loads were measured using strain gauges. Fairly good agreements were found between the FEM calculations and the experimental measurements.     

Strength prediction of adhesively bonded carbon/epoxy joints

A finite element approach has been used to obtain the stress distribution in some adhesive joints. In the past, a strength prediction method has not been established. Therefore in this study, a strength prediction method for adhesive joints has been examined. First, the critical stress distribution of single-lap adhesive joints, with six different adherend thicknesses, was examined to obtain the failure criteria. It was thought that the point stress criterion, which has been previously used for an FRP tensile specimen with a hole, was effective. The proposed method using the point stress criterion was applied to adhesive joints, such as single-lap joints with short non-lap lengths and bending specimens of single-lap joints. Good agreement was obtained between the predicted and experimental joint strengths.     

Three-Dimensional Finite Element Analysis of the Stress Distribution in Bi-Adhesive Bonded Joints

A 3-D elastic finite element model was developed to investigate the stresses distribution of bi-adhesive bonded joints (i.e., the bond line of joints filled with two adhesives of dissimilar toughness). The effects of the loading mode on the stress distribution of joints, including the single-lap joints under tensile loading (i.e., single-lap joints) and the butt joints under cleavage loading (i.e., cleavage joints), were also studied in detail. Results showed that higher stress, distributed at the contact position of the dissimilar adhesives placed along the bond line of bi-adhesive bonded joints. Also, the maximum stress of the adhesive layer decreased when the length ratios and bonding sequence along the bond line, filled with two dissimilar adhesives, was appropriately designed. At the same time, stress convergence in the adhesive layer of bi-adhesive joints was also obviously reduced in contrast to the mono-adhesive joints. The numerical investigation shows that it is necessary to take into account the change of loading modes when optimizing the bi-adhesive joint design, because of the uneven and complex loading modes of the adhesive bonding structure in the engineering applications.     

Progressive Failure Analysis of Reinforced-Adhesively Single-Lap Joint

The failure behavior of reinforced-adhesively single-lap joints was investigated experimentally and numerically. The reinforced adhesive was produced by mixing waste composite particles and an epoxy-based commercial adhesive. The single-lap joint was prepared with an adhesive and unidirectional fiber glass/epoxy composite plates with a (0°/90°)₃ stacking sequence. Three types of adhesive were used: an un-reinforced adhesive (ADH), an adhesive mixed with glass fiber-reinforced epoxy resin composite plate particles (GFRC), and an adhesive mixed with carbon fiber-reinforced epoxy resin composite plate particles (CFRC). The adhesive thickness (ta) and overlap length (lap) were 0.4, 0.8, 1.2, and 1.6 mm and 10, 20, 30, and 40 mm, respectively. Progressive failure analysis was performed with the ANSYS? 11.0 finite element program using ANSYS? parametric design language (APDL) code. In the numerical study, the failure loads of the composite and the adhesive were determined with the Hashin failure criteria and the Tresca failure criteria, respectively. The difference between the experimental and numerical studies ranged from 2% to 10%. The failure load of reinforced-adhesively single-lap joints was 1.3–22.8% higher than that of the un-reinforced adhesive.