Ductile Adhesives Research Articles

Mode II interfacial fracture toughness of composite/adhesive interfaces obtained by in-mold surface modification

In order to conduct surface modification more effectively, we have investigated the in-mold surface modification technique for composite surface that uses microstructures fabricated by imprint lithography. In the present study, we performed end notched flexure (ENF) tests to evaluate the resistance to crack propagation under macroscopic mode II loading at the modified carbon fiber reinforced plastic (CFRP)/adhesive interfaces by comparing the behaviors of brittle adhesives and ductile adhesives. In addition, we also investigated the influence of the aspect ratios (A) of the microstructures on the fracture toughness. From the ENF tests and microscopic observation of crack propagation, the mode II interfacial fracture toughness (GIIC) of the modified surfaces were found to be improved than that of flat surfaces regardless of applied adhesives. In the case of applying Epoxy A (3M DP-100 clear), which has a Young׳s modulus that is approximately 6.56 times higher and a mode I fracture toughness that is 1.34 times higher than Epoxy B (3M DP-105 clear), the cohesive failures of the CFRP concavo-convex microstructures occurred in addition to the microscopic interfacial failures during mode II loading. In addition, GIIC showed an almost constant value regardless of aspect ratio A. Thus, the interfacial fracture toughness of CFRP/Epoxy A was comparatively low compared to the CFRP cohesive failure, and the substantial crack length was almost constant, regardless of A, because the crack penetrated the concavo-convex microstructures. In the case of applying Epoxy B (3M DP-105 clear), the crack propagated along the CFRP/Epoxy B interface, including the plastic deformation of Epoxy B. Therefore, GIIC increased with an increase in A. Thus, we concluded that the substantially increased crack length was mainly influenced by the interfacial fracture toughness of CFRP/Epoxy A, which was comparatively higher than the cohesive failure of Epoxy B.

Behavior of CFRP Laminates Bonded to a Steel Substrate Using a Ductile Adhesive

Strengthening of steel structures with adhesively bonded carbon fiber–reinforced polymer (CFRP) laminates (or plates) has received extensive research attention over the past few years. Existing studies have revealed that debonding of CFRP plates from a steel substrate is one of the main failure modes in such CFRP-strengthened steel structures. To better understand and model debonding failures, the behavior of CFRP-to-steel bonded joints needs to be well understood. Recent tests conducted by the authors’ have demonstrated that the bond strength of such bonded joints depends significantly on the properties of the adhesive used, and more specifically on the interfacial fracture energy rather than the tensile strength of the adhesive. The study has also showed that the bond-slip curves for nonlinear ductile adhesives have an approximately trapezoidal shape. This paper resents an analytical solution for the full-range behavior of CFRP-to-steel bonded joints with a ductile nonlinear adhesive. Predictions from the analytical solution are presented to explain the different stages of debonding failure and are compared with test data to demonstrate the validity of the analytical solution.

3D Modeling of the Elastic-Plastic Behavior of Thin Aeronautical Adhesive Films Suited for a Wide Range of Tensile/Compression-Shear Loads

Adhesive bonding is an interesting structural assembling technique for weight saving in modern commercial aircraft, in which the use of composites materials is increasing. In order to meet both optimization and respect of safety conception constraints, the development of accurate numerical strategies is required. Thus, improvement in the experimental characterization and in the design of reliable numerical tools for bonded assemblies is necessary. This paper presents the characterization of the elastic-plastic behaviour of four aeronautical adhesive films, consisting of two epoxy-based resins supported by two types of carrier. The characterization over a wide range of monotonic proportional tensile-shear loads is performed using a modified Arcan test device designed to strongly limit the influence of edge effects. Moreover, to obtain an accurate definition of the initial elastic limit of the adhesives, further experimental tests have been performed using a pressure vessel especially designed to study the influence of the hydrostatic stress. Inverse identification techniques using finite element analysis have been used to identify the material parameters of an elastic-plastic model based on the experimental results (the load-displacement curves). Results underline the potential of such a model to represent the non-linear behaviour of ductile adhesives under tensile/compression-shear proportional monotonic loads.

Experimental Analysis of the Bondline Stress Concentrations to Characterize the Influence of Adhesive Ductility on the Composite Single Lap Joint Strength

Adhesively bonded composite single lap joints were experimentally investigated to analyze the bondline stress concentrations and characterize the influence of adhesive ductility on the joint strength. Two epoxy paste adhesives—one with high tensile strength and low ductility, and the other with relatively low tensile strength and high ductility—were used to manufacture composite single lap joints. Quasi-static tensile tests were conducted on the single lap joints to failure at room temperature. High magnification two-dimensional digital image correlation was used to analyze strain distributions near the adhesive fillet regions. The failure mechanisms were examined using scanning electron microscopy to understand the effect of adhesive ductility on the joint strength. For a given surface treatment and laminate type, the results show that adhesive ductility significantly increases the joint strength by positively influencing stress distribution and failure mechanism near the overlap edges. Moreover, it is shown that high magnification two-dimensional digital image correlation can successfully be used to study the damage initiation phase in composite bonded joints.

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 (LO), 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.

Assessments for impact of adhesive properties: modeling strength of metallic single lap joints

Adhesively bonded joints are widely used in a variety of industrial and engineering activities. Their overall strength is dependent on the properties of the adhesives. In the present research, assessments of adhesive properties were performed systematically through defining both strength mixity and energy rate mixity and using them to characterize the overall strength of metallic single lap joints. By means of the cohesive zone model, the adhesive strength mixity was defined as the ratio of the shear and tensile separation strength, and the energy rate mixity was defined as the ratio of the area below the shear cohesive curve and the area below the tensile cohesive curve. For each specified group of mixity parameters, corresponding to the properties of a specified adhesive, the overall strengths and the critical displacements of bonded joints were characterized. A series of strength and energy rate mixities were taken into account in the present calculations. A comparison of the present calculations with some existing experiments was carried out for both brittle and ductile adhesives. Finally, in the calculations presented here, damage initiation and evolution of the adhesive layer were also undertaken. The results showed that the overall strength of the joints was significantly depended on the adhesive properties, which were characterized by the strength and energy rate mixities of the adhesive. Furthermore, the shear adhesive stress components played a dominate role in both the damage initiation and evolution in the adhesives, which were also affected by the overlap length of the joints.

Experimental Analysis of the Influence of Hydrostatic Stress on the Behaviour of an Adhesive Using a Pressure Vessel

The modelling of the non-linear behaviour of ductile adhesives requires a large experimental database in order to represent accurately the strains which are strongly dependent on the tensile-shear loading combination. Various pressure-dependent constitutive models can be found in the literature, but only a few experimental results are available, for instance, to represent accurately the initial yield surface taking into account the two stress invariants, hydrostatic stress, and von Mises equivalent stress. This paper presents the possibility of combining the use of tests on bulk specimens and tests on bonded assemblies, which strongly limit the influence of the edge effects, with a pressure vessel especially designed to study the influence of hydrostatic stress. The latter allows pressures up to 100 MPa to be applied during mechanical testing. For a given strain rate of the adhesive, experimental results using various stress paths are presented in order to analyse the influence of the hydrostatic stress on the mechanical behaviour of an adhesive. The analysis of the results focuses herein on the modelling of the initial yield surface, but such results are also useful for the development of the flow rules in the case of 3D pressure-dependent models.

Single Lap Joints with Rounded Adherend Corners: Experimental Results and Strength Prediction

The objective of the present study was to better understand the effect of the change in the geometry of the adherend corners on the stress distribution in single lap joints and, therefore, on the joint strength. Various degrees of rounding were studied and two different types of adhesives were used: one very brittle and another which had a large plastic deformation. Experimental results on the strength of joints with different degrees of rounding are presented. For joints bonded with brittle adhesives, the effect of the rounded adherend corners is larger than that with ductile adhesives. The strength of joints with brittle adhesives with a large radius adherend corner increases by about 40% compared to that with a sharp adherend corner. It is shown that for joints bonded with brittle adhesives, crack propagation occurs for a short period before it grows into catastrophic failure. However, for ductile adhesives, there is large adhesive yielding and small crack propagation before final failure. Another important feature of joints bonded with ductile adhesives is that there may be more than one crack in the adhesive layer before failure. This makes strength predictions more difficult. The second part of the paper presents an approximate method for predicting the strength of joints bonded with brittle and ductile adhesives, with and without adherend corner rounding. The predictions, based on an average value around the singularity, compare well with the experimental results, especially for joints bonded with ductile adhesives.

Influences of Loading Rates on Stress-Strain Relations of Cured Bulks of Brittle and Ductile Adhesives

Influences of loading rates on the stress-strain relations of cured adhesive were experimentally investigated. A brittle epoxy adhesive and a ductile epoxy adhesive modified with rubber particles were examined. Concave specimens of the cured adhesives were made, and strain gages for very large strain were bonded on the specimens. The specimens were tested in static conditions with a material testing machine and in dynamic condition with a drop-weight impact testing machine. The influences of loading rates on the stress-strain relations of the adhesives were identified. For the brittle adhesive, the modulus was constant in both the static and dynamic conditions, and the tensile strength and total strain increased with the increase of loading rates. For the ductile adhesive, large plastic deformation occurred in the static condition, and the tensile strength was 37 MPa and the total strain was 14%. In contrast, in the condition of high loading rate, the ductile adhesive became brittle, and the ultimate stress increased and the maximum strain decreased.

Mode II Fracture Toughness of a Brittle and a Ductile Adhesive as a Function of the Adhesive Thickness

The main goal of this study was to evaluate the effect of the thickness and type of adhesive on the Mode II toughness of an adhesive joint. Two different adhesives were used, Araldite ® AV138/HV998 which is brittle and Araldite 2015 which is ductile. The end notched flexure (ENF) test was used to determine the Mode II fracture toughness because it is commonly known to be the easiest and widely used to characterize Mode II fracture. The ENF test consists of a three-point bending test on a notched specimen which induces a shear crack propagation through the bondline. The main conclusion is that the energy release rate for AV138 does not vary with the adhesive thickness whereas for Araldite 2015, the fracture toughness in Mode II increases with the adhesive thickness. This can be explained by the adhesive plasticity at the end of the crack tip.

Numerical Prediction of the Response of Metal-to-Metal Adhesive Joints with Ductile Adhesives

The present work involves a numerical modelling of the Embedded Process Zone (EPZ) by utilizing the elastoplastic Mode I and Mode II fracture models for the simulation of plastically deforming adhesive joints. A traction-separation law was developed separately for Mode I and Mode II. For the analysis of the mixed-mode fracture processes, the cohesive zones in Mode I and Mode II fracture were assumed uncoupled. The experimental programme involved the fabrication and testing of Double Strap Joints (DSJs) and Single Lap Joints (SLJs). By fitting the numerical results to the experimental ones, the basic cohesive parameters of the problem were defined.

Influence of Specimen Configuration on The Stress-Strain Relation Measurement of High Ductile Adhesives

高靭性接着剤の応力・ひずみ関係を測定するのに適した試験片形状を本報告では提案する。近年，ゴム粒子などにより強靭化した接着剤の利用が増大し，その高い引張せん断強度とピール強度のため，高性能継手へ使用されつつある。このような接着剤の応力・ひずみ関係を求めることは，接合部の設計に重要であり，必要不可欠ともいえる。しかし引張試験ではゲージ部の不特定部位に局部的不安定変形が生じやすく，ひずみ計測が困難となる。そこでダンベル試験片の一種である新しい試験片形状を提案する。これにより最大変形領域が試験片中心部に限定でき，ひずみゲージのような従来のひずみ計測手法が適用可能となる。

Mixed-Mode Cohesive Damage Model Applied to the Simulation of the Mechanical Behaviour of Laminated Composite Adhesive Joints

A study of the mechanical behaviour of laminated composite adhesive joints is presented in this paper. The study consists of both numerical simulations and experimental tests. It concentrates on single lap-shear joints made of carbon–epoxy laminated composites and an epoxy adhesive. The main objective is to verify the adequacy of cohesive damage models for the strength prediction of bonded joints. These models are attractive in modelling fracture problems since they do not require the definition of an initial crack and they model damage propagation without any user intervention. A mixed-mode cohesive damage model is used to simulate damage onset and growth in the adhesive layer. The model is an extension of pure mode (I and II) trapezoidal cohesive damage models. The trapezoidal shape in the constitutive law is more valid when ductile adhesives are used. The mixed-mode model requires a complete characterization of the cohesive parameters in pure modes, determined with an inverse method. Tensile tests are performed for joints with different overlap lengths. It is shown that the numerical simulations represent with a reasonable accuracy the mechanical behaviour of the joints tested.

Joint strength optimization by the mixed-adhesive technique

An ideal adhesive lap joint is one in which the adhesive flexibility and strength properties vary along the overlap length. Because of greater adhesive shear strains at the edges of the overlap, a ductile and flexible adhesive should be used at the overlap ends, while in the middle a stiff and less-ductile adhesive should be used. This technique has been investigated in the past but only a few studies have reported any experimental evidence. In the present study, single-lap adhesive joints were manufactured and tested maintaining the same brittle adhesive in the middle of the overlap and using three different ductile adhesives of increasing ductility at the ends of the overlap. A simple joint strength prediction is proposed for mixed-adhesive joints. The mixed-adhesive technique gives joint strength improvements in relation to a brittle adhesive alone in all cases. For a mixed adhesive joint to be stronger than the brittle adhesive and the ductile adhesive used individually, the load carried by the brittle adhesive must be higher than that carried by the ductile adhesive.

Pure mode II fracture characterization of composite bonded joints

A new data reduction scheme is proposed for measuring the critical fracture energy of adhesive joints under pure mode II loading using the End Notched Flexure test. The method is based on the crack equivalent concept and does not require crack length monitoring during propagation, which is very difficult to perform accurately in these tests. The proposed methodology also accounts for the energy dissipated at the Fracture Process Zone which is not negligible when ductile adhesives are used. Experimental tests and numerical analyses using a trapezoidal cohesive mixed-mode damage model demonstrated the good performance of the new method, namely when compared to classical data reduction schemes. An inverse method was used to determine the cohesive properties, fitting the numerical and experimental load–displacement curves. Excellent agreement between the numerical and experimental R-curves was achieved demonstrating the effectiveness of the proposed method.

Modelling the tensile fracture behaviour of CFRP scarf repairs

An experimental and numerical study concerning the tensile behaviour of adhesively-bonded carbon–epoxy scarf repairs is presented, using scarf angles ranging from 2° to 45°. A mixed-mode cohesive damage model adequate for ductile adhesives was used to simulate the adhesive layer. The cohesive laws of the adhesive layer, composite interlaminar and composite intralaminar (in the transverse and fibre directions) in pure modes I and II, necessary to simulate numerically the experimental failure paths, were previously characterized using an inverse method. Validation of this methodology was accomplished in terms of repair initial stiffness, maximum load and the corresponding displacement, as well as the failure mode. A good agreement between the numerical predictions and the experiments showed that the proposed methodology can be successfully applied to joints or repairs bonded with ductile adhesives.

Non-Linear Analysis of a Ductile Adhesive in the Single Lap Joint Under Tensile Loading

In this article, potential use of ductile adhesives in structural applications is taken into consideration and their possible advantages over traditional structural adhesives are discussed. For this purpose, the performance of a very ductile adhesive with high strain to failure was investigated in the single lap joint (SLJ) configuration in tensile loading, and the joint was modeled using the finite element method (FEM) to analyze its non-linear solutions. In analyzing the joint, consideration is given to the stress distributions in the adhesive layer and the load to failure was predicted. The predicted values were well in agreement with the previous experimental results, and the stress distributions of the ductile adhesive differed in many respects from traditional structural adhesives. In order to see the differences, a structural modern epoxy has also been analyzed.

Application of Vibrating Beam Method to Determine Dynamic Properties of Flexible Adhesives

Nowadays, ductile adhesives are entering the arena of structural applications due to their high strains to failure that is able to tolerate possible stress—strain concentrations, and due to their high damping performance, which is useful for energy dissipation. In this article, dynamic properties of such an adhesive are investigated using a vibrating beam technique with free—free end conditions. For the measurements of flexural modulus and damping values of the adhesive, the free layer beam configuration with different layer thicknesses was used. It is shown that the technique is able to give the consistent results and the data can be used for design purposes. The results suggest that the dynamic properties of the adhesive are frequency dependent at room temperature, which means the adhesive has a viscoelastic behavior.

Crack equivalent concept applied to the fracture characterization of bonded joints under pure mode I loading

In this work, an accurate and suitable data reduction scheme is developed to measure the fracture energy of adhesive joints under pure mode I loading. The method is based on the crack equivalent concept and is applied to the double cantilever beam specimen. Using the proposed methodology it is not necessary to measure the crack length during propagation, which can introduce non-negligible errors on the fracture energy measurements. Moreover, it accounts for the fracture process zone effects which can be significant when ductile adhesives are used. The new method was compared to classical data reduction schemes and was validated numerically using a trapezoidal mixed-mode cohesive damage model. The fracture characterization in mode I using a developed trapezoidal cohesive damage model is performed by an inverse method. Excellent agreement between the numerical and experimental R-curves was achieved demonstrating the adequacy of the proposed method.

Cohesive and continuum mixed-mode damage models applied to the simulation of the mechanical behaviour of bonded joints

The objective of this work is to discuss the adequacy of cohesive and continuum damage models for the prediction of the mechanical behaviour of bonded joints. A cohesive mixed-mode damage model appropriate for ductile adhesives is presented. The double cantilever beam and the end-notched flexure tests are proposed in order to evaluate the cohesive properties of the adhesive as a thin layer under mode I and mode II, respectively. A new data reduction scheme based on the crack equivalent concept is also proposed to overcome crack-monitoring difficulties during propagation in these fracture characterization tests. An inverse method to determine the cohesive parameters of the trapezoidal softening law is discussed. A continuum mixed-mode damage model is developed in order to better simulate the cases where adhesive thickness plays an important role. The model is applied to evaluate the effect of adhesive thickness on fracture characterization of adhesive joints. Some important conclusions about the advantages and drawbacks of cohesive and continuum damage models are reported.