Ductile Adhesives Research Articles

Numerical assessment of the Double-Cantilever Beam and Tapered Double-Cantilever Beam tests for the GIC determination of adhesive layers

ABSTRACTFracture mechanics-based techniques have become very popular in the failure prediction of adhesive joints. The most commonly used is cohesive zone modeling (CZM). For both conventional fracture mechanics and CZM, the most important parameters are the tensile and shear critical strain energy release rates (GIC and GIIC, respectively). The most common tests to estimate GIC are the Double-Cantilever Beam (DCB) and the Tapered Double-Cantilever Beam (TDCB) tests. The main objective of this work is to compare the DCB and TDCB tests to obtain the GIC of adhesive joints. Three adhesives with varying ductilities were used to verify their influence on the precision of the typical methods of data reduction. For both tests, methods that do not need the measurement of crack length (a) were tested. A CZM analysis was considered to reproduce the experimental load–displacement (P-δ) curves and obtain the tensile CZM laws of each tested adhesive, to test the suitability of the data reduction methods, and to study the effect of the CZM parameters on the outcome of the simulations. The CZM models accurately reproduced the experimental tests and confirmed that the data reduction methods for the TDCB test tend to underestimate GIC for ductile adhesives.

Comparison of different adhesively-bonded joint configurations for mechanical structures

Adhesively-bonded joints are widely used to join structural components. The most common joint configurations are single-lap joints (SLJ), double-lap joints (DLJ), stepped joints and scarf joints. One of the most important parameters that affects the joint strength is the overlap length (LO). The main objectives of this work are to carry out a comparative study involving several joint geometries using various adhesives with different characteristics and to check which type of adhesive is most suitable for a particular joint geometry. For this purpose, SLJ, DLJ, stepped joints and scarf joints were chosen for testing with three adhesives. The experimental results are compared with numerical results obtained using Abaqus® software using an integrated cohesive zone modelling (CZM) module. With this work, it was concluded that the optimal joint configuration depends significantly on the type of adhesive being used, such that less resistant and ductile adhesives are more suitable for joint geometries that exhibit large stress variations, while stronger but more brittle adhesives are recommended for joint geometries with more uniform stresses.

A closed-form solution of the interfacial stresses and strains in steel beams strengthened with externally bonded plates using ductile adhesives

Ductile adhesives are known to be beneficial in enhancing the capacity and ductility of bonded joints. However, there is a lack of closed-form analytical solutions for plated metallic beams that account for adhesive plasticity. This paper presents a first order, elastic-plastic bond analysis for beams strengthened with externally bonded fiber reinforced polymer (FRP) plates using ductile adhesives, based on a shear-lag formulation. This model is able to analyze arbitrary mechanical and thermal loading conditions and closed-form solutions under shearing and peeling are given. Following a review of the existing stress-based analytical solutions, a comparison between the existing and proposed analyses is also presented. The shear solution was validated by comparing with the experimental results of carbon fiber reinforced polymer (CFRP) strengthened steel beams under moderately elevated temperatures. A comprehensive parametric study has been conducted to illustrate the effects of different design parameters on the bond behavior. It is found that the adhesive shear toughness is the most critical parameter in determining the debonding failure load while the adhesive Young’s modulus does not significantly affect the bond stresses in the elastic-plastic regime. Further, the magnitude of the peak peeling stresses is self-limiting after shear yielding, and the use of a thinner plate with higher Young’s modulus is beneficial in further reducing the peeling stress.

Ductility of adhesively bonded timber joints

In the field of timber engineering, adhesive bonding remains a promising, though poorly developed, joining technique that may increase the structural stiffness and capacity of timber joints and structures. Selecting ductile adhesives may further allow to conceive ductile joints, which can compensate for the missing material ductility of timber. To demonstrate the potential of this approach, adhesively bonded double-lap timber joints were manufactured using a ductile acrylic adhesive and then subjected to axial tension and compression. The load–displacement responses were captured and compared to those of the same joints composed of a brittle epoxy adhesive. The effect of the different adhesives on the joint ductility has been studied and quantified.

Detection of Cracks in Adhesively Bonded Double-Strap Joints Using Artificial Neural Network Method

This study aims to survey the effects of crack length on the natural frequencies and mode shapes of adhesively bonded double-strap joints (DSJs) in the models with different adherend thicknesses and different adhesive ductility. Hence, the results of these analyses are used as a method of crack detection inside the adhesive layer. For this purpose, a rich database of results for DSJ models with cracks in length of 0 ≤ l C ≤ 10 mm were used for the aim of training the artificial neural network (ANN) algorithms. Subsequently, the results obtained from ANN models can be used to estimate the existence of crack and its length. The results show that the natural frequencies of the models with different adherend thicknesses and different adhesive ductility follow a logical trend, by which detection of cracks is possible for a wide range of geometries and material properties, using ANN analysis. By contrast, the results show that mode shapes are not affected by cracks. Therefore, the mode shapes are not useful characteristics for judgments.

Numerical modelling of adhesively-bonded double-lap joints by the eXtended Finite Element Method

The use of adhesive joints in industrial applications has been increasing in recent years because of the significant advantages offered compared to traditional joining methods such as welding, fastening and riveting. Thus, the existence of design tools is necessary to predict the joints’ strength with high accuracy. The eXtended Finite Element Method (XFEM) is emerging as a method to predict the joints’ behaviour, although this has not yet been adequately studied for the application to adhesive joints. This work presents an experimental and numerical study by XFEM of double-lap joints, in which adhesives ranging from brittle and strong, as the case of the Araldite® AV138, to more ductile adhesives, as the Araldite® 2015 and the Sikaforce® 7888, are applied. Aluminium substrates were considered (AW6082-T651) in joints with different overlap lengths (LO), subjected to a tensile load, in order to evaluate their performance. In the numerical study, an analysis of the stress distributions in the adhesive layer, a strength prediction by XFEM considering damage initiation criteria based on stresses and strains, and also a study on the energy criterion for damage propagation, were carried out. The XFEM analysis revealed that this method is very accurate when using specific damage initiation criteria, and a parameter of 1 in the damage propagation criterion, are considered. The choice of criteria that are not consistent with the adhesives’ behaviour and observed failure modes results in large errors.

Fracture toughness in Mode I (GIC) for ductile adhesives

Works carried out in this publication belong to a project that seeks the replacement of welded joints by adhesive joints at stress concentration nodes in bus structures. Fracture toughness in Mode I (GIC) has been measured for two different ductile adhesives, SikaTack Drive and SikaForce 7720. SikaTack Drive is a single-component polyurethane adhesive with high viscoelasticity (more than 100%), whose main use is the car-glass joining and SikaForce 7720 is double-component structural polyurethane adhesive. Experimental works have been carried out from the test called Double Cantilever Beam (DCB), using two steel beams as adherents and an adhesive thickness according to the problem posed in the Project, of 2 and 3 mm for SikaForce 7720 and SikaTack Drive, respectively. Three different methods have been used for measuring the fracture toughness in mode I (GIC) from the values obtained in the experimental DCB procedure for each adhesive: Corrected Beam Theory (CBT), Compliance Calibration Method (CCM) and Compliance Based Beam Method (CBBM). Four DCB specimens have been tested for each adhesive. Dispersion of each GIC calculation method for each adhesive has been studied. Likewise variations between the three different methods have been also studied for each adhesive.

Investigating the influence of material non-linearity in the fracture properties of ductile adhesives submitted to mixed-mode loading

The influence of the mechanical behaviour of a crash optimized adhesive in its fracture properties at different mode ratios in the plane I/II is discussed. The adhesive under investigation has been previously studied by means of standardized TDCB (Tapered Double Cantilever Beam) and MMB (Mixed Mode Bending) tests. In the current study, these tests were modelled numerically considering first the linear elastic case for the adhesive and secondly its non-linear behaviour using a spectral viscoelastic law. The simulations for the non-linear case have been performed under plane strain and plane stress assumptions. Crack initiation and propagation were simulated using cohesive zone modelling. The non-linearity of the adhesive material had minor effect in its fracture response under mode I loading. However, considerable impact has been observed while moving towards the mode II load case, where the fracture energy was calculated almost constant and equal to the one at mode I.

Strength and failure modes of single-L adhesive joints between aluminium and composites

Adhesive bonding is a process of permanent union between the components of a structure, which is used to manufacture complex shape structures, which could not be manufactured in one piece, aiming to provide a structural joint that theoretically should be at least as resistant as the base material. Composite materials reinforced with fibres are becoming increasingly popular in many applications, as a result of a number of competitive advantages over conventional materials. Regarding the manufacture of composite structures, although the currently used techniques reduce to the maximum the connections, these are still necessary due to the typical size of the components and design, technological and logistical limitations. Moreover, it is known that in many high performance structures, it is necessary to join components in composite materials with other light metals such as aluminium, for the purpose of structural optimization. This work aims to experimentally and numerically study single-L adhesive joints between aluminium components and carbon-fibre reinforced composite structures under peeling loads, considering different geometric conditions and adhesives. It was found that the adhesive ductility and aluminium plate thickness are highly relevant parameters to improve the joints strength.

Overview of different strength prediction techniques for single-lap bonded joints

Adhesive joints have been used in several fields of engineering, and their applications are vast. Due to their easy and quick fabrication process, single-lap joints are a common configuration. The increase of strength, weight reduction and resistance to corrosion are some of the advantages of this kind of joint over traditional joining methods. However, stress concentrations at the overlap edges are one of the main disadvantages. There are very few accurate design techniques for the diversity of bonded joints that can be found in real applications, which constitutes an obstacle to the use of this bonding method in structural applications. This work aims at comparing different analytical and numerical methods in the strength prediction of single-lap joints with different overlap lengths ( LO). The main objective is to evaluate which predictive method is the best. Adhesive joints were produced between aluminium adherends using a brittle epoxy adhesive (Araldite® AV138), a moderately ductile epoxy adhesive (Araldite® 2015) and a ductile polyurethane adhesive (Sikaforce® 7888). Different analytical methods were considered, together with two numerical techniques: cohesive zone models (CZM) and the extended finite element method (XFEM), allowing the comparative analysis. The analytical methods showed that they only give relatively accurate results in very specific conditions. The CZM analysis with the triangular law revealed to be a very accurate method, with the exception of joints with very ductile adhesives. On the other hand, the XFEM analysis was not adequate, especially for crack growth in mixed mode.

Application of the direct method for cohesive law estimation applied to the strength prediction of double-lap joints

Currently, significant efforts are being made in the design of aircraft and aeronautical applications to reduce weight and improve reliability. Thus, adhesive bonding techniques have been largely employed, which also enables the combined use of steel with lighter materials such as aluminium or high strength composites. Cohesive Zone Models (CZM) are a powerful tool for the design of bonded structures, but they require careful estimation of the cohesive laws for reliable results. This work experimentally evaluates by the J-integral/direct method the tensile and shear CZM laws of three adhesives with distinct ductility. The Double-Cantilever Beam (DCB) and End-Notched Flexure (ENF) specimens were considered to obtain the tensile and shear CZM laws of the adhesives, respectively. After obtaining the tensile and shear CZM laws, triangular, exponential and trapezoidal CZM laws were built to reproduce their behaviour. Validation of these CZM laws was undertaken with a mixed-mode geometry (double-lap joint) considering the same three adhesives and varying overlap lengths (LO). The strength prediction by this technique revealed accurate predictions for a given CZM law shape, depending on the adhesive ductility, although all CZM law shapes were moderately accurate.

Numerical Investigation of the Failure Phenomena in Adhesively Bonded Joints by Means of a Multi-Linear Equivalent Plastic Stress/Strain Approach

In this work, a multi-linear material model for elastic-plastic response of ductile adhesives is proposed. Indeed, the proposed formulation allows to evaluate equivalent stress and strains to be used as material model input in FE commercial codes instead of the classical true stress and true strains. The presented model, which is capable to simulate the plasticity related phenomena and the failure event, has been implemented in the FEM code ABAQUS and used to numerically simulate the mechanical behaviour of adhesively bonded joints in traction. Several joints configurations have been considered with ductile, fragile and mix adhesives’ behaviour to test the effectiveness and the range of applicability of the proposed model. Encouraging comparisons with literature experimental data demonstrates the added value of the suggested material model in predicting the failure of adhesively bonded joints.

Comparative evaluation of the Double-Cantilever Beam and Tapered Double-Cantilever Beam tests for estimation of the tensile fracture toughness of adhesive joints

The continuous development observed in bonded joints, along with the improvements of the adhesives’ properties, are resulting in an increase of the bonded joint applications, as well as the variety of applications. Regarding the strength prediction of adhesive joints, two highly relevant methods are Fracture Mechanics and Cohesive Zone Models (CZM). By Fracture Mechanics, this is usually carried out by an energetic analysis. CZM enable the simulation of damage initiation and propagation. The tensile critical strain energy release rate (GIc) of adhesives is one of the most important parameters for predicting the joint strength. Two of the most commonly used tests are the Double-Cantilever Beam (DCB) and the Tapered Double-Cantilever Beam (TDCB). This work aims to assess the capability of the DCB and TDCB test to estimate the value of GIc of adhesive joints. Three types of adhesives with different levels of ductility are used, to study the accuracy of the typical data reduction methods under conditions that are not always consistent with Linear Elastic Fracture Mechanics (LEFM) principles. For both test protocols, methods that do not require measurement of the crack length (a) during the test are evaluated. In the DCB test, these are the Compliance Calibration Method (CCM), Corrected Beam Theory (CBT) and Compliance-Based Beam Method (CBBM). The methods used in the TDCB test are the Simple Beam Theory (SBT), CCM and CBT. With few exceptions, the results were consistent between the different methods considered for each test. The discrepancy of results is higher when comparing the two types of tests, except for the brittle adhesive. It was concluded that the data reduction methods for the TDCB test are too conservative to measure GIc of ductile adhesives.

Special Issue: Papers from the 3rd International Conference on Structural Adhesive Bonding (AB 2015), Porto, Portugal, 2–3 July 2015

Guest Editorial This special issue of Applied Adhesion Science (AAS) contains selected papers presented at the 3rd International Conference on Structural Adhesive Bonding (AB 2015). AB 2015 was chaired by the two of us and was held in Porto, Portugal during July 2–3, 2015. The goal of the conference was to provide a unique opportunity to exchange information, present the latest results as well as to discuss issues relevant to structural adhesive bonding research today. Approximately 160 papers were presented by researchers from more than 30 countries. In order to disseminate the work presented at AB 2015, selected papers were prepared which resulted in the present Special Issue. The study of Schneider and Wrobel deals with use of high-energy laser radiation for surface preparation of magnesium for adhesive applications. High strength adhesive bonding of magnesium-alloys could be achieved. This fast physical process fulfils all legal requirements concerning environmental protection and occupational safety and also has low operating costs. Kemiklioglu et al. investigated the failure strengths of adhesively bonded joints with brittle and ductile adhesives by repeated impact. Useful information was generated to help designers who need information on mechanical properties of ductile and brittle adhesives under single and repeated impacts. The study of Moller et al. aims to determine the behaviour of a two-component adhesive regularly used to join pieces in acid mist extraction systems in mining applications. The specimens were tested at 20, 50 and 80 °C. From the results obtained, it is possible to appreciate that the adhesive at 50 °C decreased its strength by 14 % with respect to those at the reference temperature of 20 °C. Assih et al. describe an experimental and theoretical analysis of strengthened damaged concrete corbel by gluing carbon fibre fabrics. The study showed that the bonded composite fibre fabrics can be a convenient and effective strengthening method of concrete structures. A theoretical analysis is presented describing the behaviour of strengthened damaged concrete corbel using the damage theory of reinforced concrete beams. Applications of adhesive bonding are described in 2 papers. Ayyildiz et al. evaluate the shear bond strength (SBS) of three different cements to zirconia and lithium Open Access

Parametric Study of the Reinforcement Geometry on Tensile Loaded Scarf Adhesive Repairs

The recent enhancements in structural adhesives technology enabled the use of adhesive joints and repairs in several industrial fields. The scarf repair is the most efficient because of suppression of peel (σ) and shear (τ) peak stresses, arising from the tapered adherend edges. The repair of structures that have the requirement of flush surfaces is one of the major applications of the scarf geometry. This work is a numerical study on the ultimate tensile strength of two-dimensional (2D) scarf repairs of aluminum structures with external doublers as reinforcements. Two adhesives were considered: the brittle epoxy Araldite® AV138 and the ductile epoxy Araldite® 2015 (Huntsmann, Basel, Switzerland). The numerical analysis by finite elements (FE) was performed in Abaqus® (Dassault Systèmes, Vélizy-Villacoublay, France) and used cohesive zone models (CZM), thus enabling the strength prediction of the repairs. Validation of this methodology with scarf repair experiments was performed previously. The influence of the main reinforcement geometric parameters is studied for the two adhesives, such as doubler thickness and edge geometry. This work enabled to conclude that the repairs’ strength can be optimized by a significant amount by a careful selection of the doubler geometry, and that the results are dependent on the adhesive ductility.

Tensile fracture characterization of adhesive joints by standard and optical techniques

The use of adhesive joints has increased in recent decades due to its competitive features compared with traditional methods. This work aims to estimate the tensile critical strain energy release rate (GIC) of adhesive joints by the Double-Cantilever Beam (DCB) test. The J-integral is used since it enables obtaining the tensile Cohesive Zone Model (CZM) law. An optical measuring method was developed for assessing the crack tip opening (δn) and adherends rotation (θo). The proposed CZM laws were best approximated by a triangular shape for the brittle adhesive and a trapezoidal shape for the two ductile adhesives.

Adhesive Selection for Single Lap Bonded Joints: Experimentation and Advanced Techniques for Strength Prediction

The integrity of multi-component structures is usually determined by their unions. Adhesive-bonding is often used over traditional methods because of the reduction of stress concentrations, reduced weight penalty, and easy manufacturing. Commercial adhesives range from strong and brittle (e.g., Araldite® AV138) to less strong and ductile (e.g., Araldite® 2015). A new family of polyurethane adhesives combines high strength and ductility (e.g., Sikaforce® 7888). In this work, the performance of the three above-mentioned adhesives was tested in single lap joints with varying values of overlap length (LO). The experimental work carried out is accompanied by a detailed numerical analysis by finite elements, either based on cohesive zone models (CZM) or the extended finite element method (XFEM). This procedure enabled detailing the performance of these predictive techniques applied to bonded joints. Moreover, it was possible to evaluate which family of adhesives is more suited for each joint geometry. CZM revealed to be highly accurate, except for largely ductile adhesives, although this could be circumvented with a different cohesive law. XFEM is not the most suited technique for mixed-mode damage growth, but a rough prediction was achieved.

Comparative Evaluation of Single-lap Joints Bonded with Different Adhesives by Cohesive Zone Modelling

Structures built from several components require some means of joining. In this context, bonding with adhesives has several advantagescompared to traditional joining methods, e.g. reduction of stress concentrations, reduced weight penalty and easy manufacturing. Adhesives can be strong and brittle (e.g., Araldite® AV138) or less strong and ductile (e.g., Araldite® 2015). A new family of polyurethane adhesives combines high strength and ductility (e.g., Sikaforce® 7888). In this work, the performance of the three above mentioned adhesives was tested in single-lap joints with varying values of overlap length (LO). The experimental work carried out is accompanied by a detailed numerical analysis by Finite Elements, based on Cohesive Zone Models (CZM). This procedure enabled detailing the performance of this predictive technique applied to bonded joints. Moreover, it was possible to evaluate which family of adhesives is more suited for each joint geometry. CZM revealed to be highly accurate, except for largely ductile adhesives, although this could be circumvented with a different cohesive law.

Damage modelling and lifetime prediction of adhesively bonded joints under sustained loading with constant and variable amplitudes

AbstractThe lifetime of adhesively bonded joints under service loading is predicted by a linear viscoelastic traction–separation model, which is enhanced by an isotropic damage approach. Therefore, a scalar damage variable is defined according to the concept of effective stresses based on the hypothesis of strain equivalence in the framework of continuum damage mechanics. The damage evolution is driven by a specific equivalent stress, adapted for ductile adhesives. Experimental data acknowledge the validity of the proposed model for the lifetime prediction of adhesive joints. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Performance of Cohesive Zone Models for Brittle and Ductile Adhesives

Damage modeling approach is increasingly used to simulate fracture and debonding processes in adhesively bonded joint. In order to understand the relation between the delamination behavior of different types of adhesives and the type of cohesive zone models (CZMs), the pure tension and pure shear experiments were conducted used two distinct adhesives, an epoxy-based adhesive in a brittle manner and VHBTM tape adhesive in a ductile manner. The traction-separation relations of the two adhesives were extracted from the tension and shear experimental results. Three types of cohesive zone models (CZMs) are adopted, including the exponential, bilinear, and trapezoidal models. VUMAT subroutine of CZMs as the adhesive layer is used to simulate the specimen tension and shear debonding procedures. The results demonstrate that (i) the bilinear CZM more suitably describes the brittle adhesive and the exponential CZM suitably describes the ductile adhesive to simulate the tension and shear failure. (ii) cohesive strength and work of separation are the significant affections on the simulation results. and (iii) the shape of CZM is a significant affections on the simulation the pure tension and shear debonding procedure.