In this thesis an experimental method to calculate cohesive characteristics for an adhesive layer in a End Notched Flexure (ENF) specimen is presented and evaluated. The method is based on the path independent J-integral where the energy release rate (ERR) for the adhesive is derived as a function of the applied forces and the rotational displacements at the loading point and at the supports of the specimen. The major advantage with the method in comparison with existing theory known by the job initiator is that it is still applicable with ENF specimens that are subjected to yielding of the adherends. The structure of this thesis is disposed so that the theory behind the J-integral method is shortly described and then an evaluation of the method is performed by aid of finite element simulations using beam and cohesive elements. The finite element simulations indicates that the ERR can be determined with good accuracy for an ENF specimen where a small scale yielding of the adherends has occurred. However when a fully cross sectional yielding of the adherends is reached the ERR starts diverging from the exact value and generates a too high ERR according to input data in the simulations, i.e. the exact values. The importance in length of the adhesive process zone is also shown to be irrelevant to the ERR measured according to the J-integral method. Simulation performed with continuum elements indicates that a more reality based FE- simulation implies a higher value of the applied load in order to create crack propagation. This is an effect of that the specimen is allowed to roll on the supports which makes the effective length between the supports shorter than the initial value when the specimen is deformed. This results in a stiffer specimen and thus a higher applied force is needed to create crack propagation in the adhesive layer. An experimental set up of an ENF specimen is created and the sample data from the experiments are evaluated with the J-integral method. For measuring the rotational displacements of the specimen which are needed for the J-integral equation an image system is developed by the author and validated by use of linear elastic beam theory. The system calculates the three rotational displacements of the specimen by aid of images taken by a high resolution SLR camera and the system for measuring the rotations may also be used in other applications than for a specific ENF geometry. The validation of the image system shows that the rotations calculated by the image system diverge from beam theory with less than 2.2 % which is a quite good accuracy in comparison with the accuracies for the rest of the used surveying equipment. The results from the experiment indicates that the used, about 0.36 mm thick SikaPower 498, adhesive has an maximum shear strength of 37.3 MPa and a critical shear deformation of 482 µm. The fracture energy is for this thickness of the adhesive is determined as 12.9 kJ/m2. This report ends with a conclusion- and a suggested future work- chapter.

A research group at University of Skövde has developed an interphase element for implementation in the commercial FE-software Abaqus. The element is using the Tvergaard & Hutchinson cohesive law and is implemented in Abaqus Explicit version 6.7 using the VUEL subroutine. This bachelor degree project is referring to evaluate the interphase element and also highlight problems with the element. The behavior of the interphase element is evaluated in mode I using Double Cantilever Beam (DCB)-specimens and in mode II using End Notch Flexure (ENF)-specimens. The results from the simulations are compared and validated to an analytical solution. FE-simulations performed with the interphase element show very good agreement with theory when using DCB- or ENF-specimens. The only exception is when an ENF-specimen has distorted elements. When using explicit finite element software the critical time step is of great importance for the results of the analyses. If a too long time step is used, the simulation will fail to complete or complete with errors. A feasible equation for predicting the critical time step for the interphase element has been developed by the research group and the reliability of this equation is evaluated. The result from simulations shows an excellent agreement with the equation when the interphase element governs the critical time step. However when the adherends governs the critical time step the equation gives a time step that is too large. A modification of this equation is suggested.

<p>In this thesis an experimental method to calculate cohesive characteristics for an adhesive layer in a End Notched Flexure (ENF) specimen is presented and evaluated. The method is based on the path independent J-integral where the energy release rate (ERR) for the adhesive is derived as a function of the applied forces and the rotational displacements at the loading point and at the supports of the specimen. The major advantage with the method in comparison with existing theory known by the job initiator is that it is still applicable with ENF specimens that are subjected to yielding of the adherends.</p><p>The structure of this thesis is disposed so that the theory behind the J-integral method is shortly described and then an evaluation of the method is performed by aid of finite element simulations using beam and cohesive elements. The finite element simulations indicates that the ERR can be determined with good accuracy for an ENF specimen where a small scale yielding of the adherends has occurred. However when a fully cross sectional yielding of the adherends is reached the ERR starts diverging from the exact value and generates a too high ERR according to input data in the simulations, i.e. the exact values. The importance in length of the adhesive process zone is also shown to be irrelevant to the ERR measured according to the J-integral method.</p><p>Simulation performed with continuum elements indicates that a more reality based FE- simulation implies a higher value of the applied load in order to create crack propagation. This is an effect of that the specimen is allowed to roll on the supports which makes the effective length between the supports shorter than the initial value when the specimen is deformed. This results in a stiffer specimen and thus a higher applied force is needed to create crack propagation in the adhesive layer.</p><p>An experimental set up of an ENF specimen is created and the sample data from the experiments are evaluated with the J-integral method. For measuring the rotational displacements of the specimen which are needed for the J-integral equation an image system is developed by the author and validated by use of linear elastic beam theory. The system calculates the three rotational displacements of the specimen by aid of images taken by a high resolution SLR camera and the system for measuring the rotations may also be used in other applications than for a specific ENF geometry. The validation of the image system shows that the rotations calculated by the image system diverge from beam theory with less than 2.2 % which is a quite good accuracy in comparison with the accuracies for the rest of the used surveying equipment.</p><p>The results from the experiment indicates that the used, about 0.36 mm thick SikaPower 498, adhesive has an maximum shear strength of 37.3 MPa and a critical shear deformation of 482 µm. The fracture energy is for this thickness of the adhesive is determined as 12.9 kJ/m².</p><p>This report ends with a conclusion- and a suggested future work- chapter.</p>

<p>A research group at University of Skövde has developed an interphase element for implementation in the commercial FE-software Abaqus. The element is using the Tvergaard & Hutchinson cohesive law and is implemented in Abaqus Explicit version 6.7 using the VUEL subroutine. This bachelor degree project is referring to evaluate the interphase element and also highlight problems with the element.</p><p>The behavior of the interphase element is evaluated in mode I using Double Cantilever Beam (DCB)-specimens and in mode II using End Notch Flexure (ENF)-specimens. The results from the simulations are compared and validated to an analytical solution.</p><p>FE-simulations performed with the interphase element show very good agreement with theory when using DCB- or ENF-specimens. The only exception is when an ENF-specimen has distorted elements.</p><p>When using explicit finite element software the critical time step is of great importance for the results of the analyses. If a too long time step is used, the simulation will fail to complete or complete with errors. A feasible equation for predicting the critical time step for the interphase element has been developed by the research group and the reliability of this equation is evaluated.</p><p>The result from simulations shows an excellent agreement with the equation when the interphase element governs the critical time step. However when the adherends governs the critical time step the equation gives a time step that is too large. A modification of this equation is suggested.</p>

This thesis is intended to identify the mode I and mode II fracture toughness to characterize the thin laminate interface by using the Double Cantilever Beam test (DCB) and End Notched Flexure test (ENF). This study’s thin laminate was Polyethylene Terephthalate and Low-Density Polyethylene (PET-LDPE), which is mostly used by packaging industries in the manufacturing of packages to store liquid food. As PET-LDPE film is very flexible and difficult to handle, DCB and ENF tests cannot be performed directly so, sheet metal (Aluminium) was used as carrier material. PET-LDPE film is placed between two aluminum plates to reduce the flexibility and perform the tests. Therefore, the Aluminium plate was also studied to find the constitutive parameters (young’s modulus (E) and mixed hardening parameters (Plastic properties)) under the tensile test and three-point bending test. From the test response, energy release rate calculation has been done for different Pre-crack lengths to validate the DCB and ENF experimental setup, study the different Pre-crack lengths, and characterize the laminate interface. Finite Element simulation (FE simulation) for those tests were carried out in AbaqusTM2020. When needed, the force versus displacement response from FE simulation was optimized against experimental response to find the required constitutive parameters (Young’s modulus, Hardening parameters, and PET-LDPE material properties). Implementing of optimization algorithm and automated simulation has been done with the help of MATLAB code. In contrast, MATLAB works as a server, and Abaqus works as a client and connected two interfaces to run the optimization. The results obtained from experiments and FE simulations were compared to the results found in the literature.

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Le dimensionnement des joints collés est une préoccupation majeure du CNES pour lesapplications spatiales des futurs lanceurs. Pour dimensionner une structure collée, il est nécessaire depouvoir apprécier les caractéristiques mécaniques du joint collé.Dans cette étude, trois adhésifs structuraux ont été sélectionnés (Hysol®EA 9321, Hysol®EA9394 et Hysol EA® 9395). Après leur caractérisation massique, une étude statistique pour mettre enévidence les effets des différents paramètres (vitesse d’essai, géométrie éprouvette, le degré depolymérisation…) a été entreprise.La deuxième étape a pour objectif de fiabiliser l’analyse des essais de fissuration etd’améliorer la compréhension des mécanismes d’endommagement et de propagation de fissure dansles liaisons collées. Trois types d’essai ont été utilisés, à savoir, l’essai Double Cantilever Beam(DCB), pour l’étude du mode I, l’essai End Notched Flexure (ENF), pour le mode II, et l’essai MixedMode Bending (MMB), pour les chargements en mode mixte I/II. Nous avons développé de nouvellesinstrumentations et méthodologies d’analyse. Pour affiner le protocole de test standard, la techniquedite de « backface strain monitoring » a été utilisée. Elle consiste à positionnées des jauges dedéformation sur les surfaces de l’éprouvette de façon à enregistrer l’évolution du signalextensométrique durant la propagation de la fissure. Cette méthode permet une meilleure estimation dela position front de fissure ainsi que l'étude de la répartition des contraintes le long du joint de colle.La corrélation d'images numériques (DIC) a également été utilisée afin de proposer un nouveauprotocole de calibrage de la longueur de fissure et pour comparer un modèle analytique (poutre deTimoshenko sur fondation élastique) avec les résultats expérimentaux. / Adhesive bonding is being strongly considered in space applications CNES as anadvantageous assembly technique for future launchers. Correct design of adhesive joints is of majorconcern. Aerospace adhesives are tough viscoelastic matrices (special epoxy resins) reinforced withnano-, or microparticles. Extended use of adhesive joints in structural applications is limited due to thedifficulties in predicting in-service performance, frequently leading to over-conservative design.Three structural adhesives (Hysol®EA 9321, Hysol®EA 9394 and Hysol®EA 9395) wereselected. After their bulk characterization, statistical studies to highlight effects of different parameterse.g. speed, test piece geometry, degree of polymerization were undertaken.In the second stage, fracture mechanics tests were effected employing: the double cantileverbeam (DCB) configuration (mode I characterisation), the three point bending end-notched flexure(ENF) (mode II) and the mixed-mode bending (MMB) (combined mode I/II loading). Crack growth inbonded joints was investigated in a novel way. To refine standard test protocol, the backface strainmonitoring technique was used. Strain gauges were used to measure the strain on the exposed skin ofthe adherends during crack onset and propagation. This method allows better estimation of the crackfront position as well as fine investigation of the stress distribution along the bondline and in the crackfront vicinity. Digital image correlation (DIC) was also used to compare analytical models, e.g.Timoshenko beam on elastic foundation model with experimental results.