<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>

Over the last few decades, Shape Memory Alloys (SMAs) have been increasingly explored in order to take advantage of their unique properties (i.e., pseudoelasticity and shape memory effect), in various actuation, sensing and absorption applications. In order to achieve an effective design of SMA-based devices a thorough investigation of their behavior in the presence of cracks is needed. In particular, it is important to understand the effect of phase transformation on their fracture response. The aim of the present work is to study the effect of stress-induced as well as thermo-mechanically-induced phase transformation on several characteristics of the fracture response of SMAs. The SMA thermomechanical response is modeled through an existing constitutive phenomenological model, developed within the framework of continuum thermodynamics, which has been implemented in a finite element frame-work. The effect of stress-induced phase transformation on the mechanical fields in the vicinity of a stationary crack and on the toughness enhancement associated with crack advance in an SMA subjected to in-plane mode I loading conditions is examined. The small scale transformation assumption is employed in the analysis according to which the size of the region occupied by the transformed material forming close to the crack tip is small compared to any characteristic length of the problem (i.e. the size of the transformation zone is thirty times smaller than the size of the cracked ligament). Given this assumption, displacement boundary conditions, corresponding to the Irwin’s solution for linear elastic fracture mechanics, are applied on a circular region in the austenitic phase that encloses the stress-induced phase transformation zone. The quasi-static stable crack growth is studied by assuming that the crackpropagates at a certain critical level of the crack-tip energy release rate. The Virtual Crack Closure Technique (VCCT) is employed to calculate the energy release rate. Fracture toughness enhancement associated with transformation dissipation is observed and its sensitivity on the variation of key characteristic non-dimensional parameters related to the constitutive response is investigated. Moreover, the effect of the dissipation due plastic deformation on the fracture resistance is analyzed by using a Cohesive Zone Model (CZM). The effect of thermo-mechanically-induced transformation on the driving force for crack growth is analyzed in an infinite center-cracked SMA plate subjected to thermal actuation under isobaric mode I loading. The crack-tip energy release rate is identified as the driving force for crack growth and is measured over the entire thermal cycle by means of the VCCT. A substantial increase of the crack-tip energy release rate – an order of magnitude for some material systems – is observed during actuation as a result of phase transformation, i.e., martensitic transformation occurring during actuation causes anti-shielding that might cause the energy release rate to reach the critical value for crack growth. A strong dependence of the crack-tip energy release rate on the variation of the thermomechanical parameters characterizing the material response is examined. Therefore, it is implied that the actual shape of the strain- temperature curve is important for the quantitative determination of the change of the crack-tip energy release rate during actuation.

Functionally graded materials processed by the thermal spray techniques such as electron beam physical vapor deposition and plasma spray forming are known to have an orthotropic structure with reduced mechanical properties. Debonding related failures in these types of material systems occur due to embedded cracks that are perpendicular to the direction of the material property gradation. These cracks are inherently under mixed-mode loading and fracture analysis requires the extraction of the modes I and II stress intensity factors. The present study aims at developing semi-analytical techniques to study embedded crack problems in graded orthotropic media under various boundary conditions. The cracks are assumed to be aligned parallel to one of the principal axes of orthotropy. The problems are formulated using the averaged constants of plane orthotropic elasticity and reduced to two coupled integral equations with Cauchy type dominant singularities. The equations are solved numerically by adopting an expansion - collocation technique. The main results of the analyses are the mixed mode stress intensity factors and the energy release rate as functions of the material nonhomogeneity and orthotropy parameters. The effects of the boundary conditions on the mentioned fracture parameters are also duly discussed.

No description available.

Afin d'assurer le contrôle de son parc de production d'électricité, EDF doit maîtriser le vieillissement de ses installations pour en garantir le bon fonctionnement dans la durée. Dans ce but, il est nécessaire de disposer d’outils performants pour le modéliser et simuler la propagation des défauts dans les structures.Dans ces travaux de thèse, on s’intéresse à la propagation de fissures avec la méthode X-FEM et notamment à l’étape de localisation de la fissure par une technique de courbes de niveau. Nous avons proposé une approche fondée sur une méthode de propagation d’information de distance dite fast marching method pour rendre cette étape plus robuste. Elle est applicable à tous types de mailles,linéaires ou quadratiques.De plus, le calcul du taux de restitution d’énergie et des facteurs d’intensité de contrainte en pointe de fissure doit être suffisamment précis pour permettre de calculer la direction et l’avancée de la fissure. Dans ce but, nous avons proposé d’étudier une méthode d’intégrale de domaine pour laquelle on soulève plusieurs difficultés liées à la représentation de la fissure dans un espace tridimensionnel. Plusieurs améliorations sont proposées pour rendre les calculs plus précis et plus robustes.Dans le cas des fissures à front courbe, nous avons identifié les limites de l'utilisation des champs asymptotiques obtenus en pointe de fissure sous l'hypothèse des déformations planes comme champs auxiliaires d’une méthode d’intégrale d’interaction et nous avons proposé de nouveaux champs de déplacements auxiliaires qui prennent en compte la courbure du front de fissure. Toutes ces approches sont développées et validées dans le logiciel code_aster. / In order to ensure the control of its nuclear power plants, EDF must guarantee that they function effectively over the long term. For this purpose, it is necessary to have efficient tools tomodel and simulate crack propagation in structures. In this PhD work, we are interested in the propagation of cracks with the X-FEM method which allows using the same mesh as for a structure without default. We target especially the reconstruction of thelevel sets that characterize the position of the crack after propagation. We have proposed a fast marching method approach based on the propagation of distance information from the crack surface to the whole structure to make this step more robust in the X-FEM propagation process. It is applicable to all types of meshes, linear or quadratic. The calculation of information characteristic of thecrack status such as the energy release rate and the stress intensity factors must be accurate enough to obtain the direction and advance of the crack front ateach propagation step. For this purpose, we proposed to study a domain integral method, for which several difficulties related to the representation of the crackin a three-dimensional space are identified. Several improvements are proposed to make the calculations more accurate and more robust. In the case of curved cracks front, we have identified the limitations of using asymptotic fields obtained under the plane deformation hypothesis as auxiliary fields of an interaction integral method and we have proposed new auxiliary displacement fields that take into account the curvature of the crack front. All these methods are developed and validated with EDF software code_aster.

Original analytical theories are developed for partitioning mixed-mode fractures on rigid interfaces in laminated orthotropic double cantilever beams (DCBs) based on 2D elasticity by using some novel methods. Note that although the DCB represents a simplified case, it provides a deep understanding and predictive capability for real applications and does not restrict the analysis to a simple class of fracture problems. The developed theories are generally applicable to so-called 1D fracture consisting of opening (mode I) and shearing (mode II) action only with no tearing (mode III) action, for example, straight edge cracks, circular blisters in plates and shells, etc. A salient point of the methods is to first derive one loading condition that causes one pure fracture mode. It is conveniently called the first pure mode. Then, all other pure fracture modes can be determined by using this pure mode and the property of orthogonality between pure mode I modes and pure mode II modes. Finally, these 2D-elasticity-based pure modes are used to partition mixed-mode fractures into contributions from the mode I and mode II fracture modes by considering a mixed-mode fracture as the superposition of pure mode I and mode II fractures. The partition is made in terms of the energy release rate (ERR) or the stress intensity factor (SIF). An analytical partition theory is developed first for a DCB composed of two identical linear elastic layers. The first pure mode is obtained by introducing correction factors into the beam-theory-based mechanical conditions. The property of orthogonality is then used to determine all other pure modes in the absence of through-thickness-shear forces. To accommodate through-thickness shear forces, first two pure through-thickness-shear-force pure modes (one pure mode I and one pure mode II) are discovered by extending a Timoshenko beam partition theory. Partition of mixed-mode fractures under pure through-thickness shear forces is then achieved by using these two pure modes in conjunction with two thickness-ratio-dependent correction factors: (1) a shear correction factor, and (2) a pure-mode-II ERR correction factor. Both correction factors closely follow a normal distribution around a symmetric DCB geometry. The property of orthogonality between all pure mode I and all pure mode II fracture modes is then used to complete the mixed-mode fracture partition theory for a DCB with bending moments, axial forces and through-thickness shear forces. Fracture on bimaterial interfaces is an important consideration in the design and application of composite materials and structures. It has, however, proved an extremely challenging problem for many decades to obtain an analytical solution for the complex SIFs and the crack extension size-dependent ERRs, based on 2D elasticity. Such an analytical solution for a brittle interfacial crack between two dissimilar elastic layers is obtained in two stages. In the first stage the bimaterial DCB is under tip bending moments and axial forces and has a mismatch in Young s modulus; however, the Poisson s ratios of the top and bottom layers are the same. The solution is achieved by developing two types of pure fracture modes and two powerful mathematical techniques. The two types of pure fracture modes are a SIF-type and a load-type. The two mathematical techniques are a shifting technique and an orthogonal pure mode technique. In the second stage, the theory is extended to accommodate a Poisson s ratio mismatch. Equivalent material properties are derived for each layer, namely, an equivalent elastic modulus and an equivalent Poisson s ratio, such that both the total ERR and the bimaterial mismatch coefficient are maintained in an alternative equivalent case. Cases for which no analytical solution for the SIFs and ERRs currently exist can therefore be transformed into cases for which the analytical solution does exist. It is now possible to use a completely analytical 2D-elasticity-based theory to calculate the complex SIFs and crack extension size-dependent ERRs. The original partition theories presented have been validated by comparison with numerical simulations. Excellent agreement has been observed. Moreover, one partition theory is further extended to consider the blister test and the adhesion energy of mono- and multi-layered graphene membranes on a silicon oxide substrate. Use of the partition theory presented in this work allows the correct critical mode I and mode II adhesion energy to be obtained and all the experimentally observed behaviour is explained.

Cross-ply composite laminates can develop very high density of transverse cracks in the 90-layer under severe thermal and mechanical loading conditions. At such high crack densities, two adjacent cracks start to interact, and a stress gradient is created in the region between these cracks. Due to the presence of high stress gradients, thickness averaging of longitudinal stress becomes obsolete. Thus, a detailed analysis of stress state along the thickness direction becomes necessary to study growth conditions of fiber sized microcracks initiated at the interface between 0-layer and 90-layer. Stress analysis at various crack densities is carried out in this project using finite element analysis or FEM as the main tool. This analysis is coupled with strain energy release rate (ERR) studies for a microcrack which grows in transverse direction from one interface to the other. The growth of this microcrack is found to be strongly influenced by the stress gradients and a presence of compressive stresses along midplane under tensile loading conditions at high crack densities.

<p>Fracture of silicon solar cells in photovoltaic (PV) modules are widely reported and a wellknown issue in the PV industry, since it is exposed to adverse climatic conditions and varying temperature loads. A commercial silicon solar cell is mainly composed of four different layers. This thesis investigates delamination failure and thermal fatigue failure due to alternating temperature loads using finite element method (FEM) simulation.</p> <p><br></p> <p>The delamination of the encapsulant (EVA) layer and the cell interface was simulated using</p> <p>finite element (FE) simulations in the COMSOL Multiphysics software. The adhesion between the</p> <p>layers were modeled using the cohesive zone model (CZM). The CZM parameters such as normal</p> <p>strength and penalty stiffness were used for the bilinear traction-separation law for the cohesive</p> <p>model in a 90-degree configuration. The critical energy release rate (𝐺𝐺𝑐𝑐) was experimentally calculated as one of the CZM parameters. A uniaxial tensile test of the upper layer of the cell was conducted to determine the material properties of the solar cell layers, and that information was</p> <p>later used for FE simulations. To validate the simulation, we compared the peeling force graph</p> <p>from the experiment and FE simulation, and it was found both graphs showed a maximum peeling</p> <p>force of 120 N.</p> <p><br></p> <p>Finite element simulations were also conducted to predict the stress variations in the silicon</p> <p>solar cell layer due to alternating temperatures. An alternating temperature function was developed</p> <p>using triangular waveform equations in the COMSOL Multiphysics software. For this simulation,</p> <p>a 3D model of the cell with a 90-degree peel arm was used, like in the peeling simulation. A</p> <p>maximum stress of 7.31 x 10−3 𝑁/𝑚𝑚2 was observed on the encapsulant (EVA)/cell layer, but no</p> <p>delamination was observed for the given temperature range. In future work, we plan to explore the</p> <p>calculation of fatigue life using thermal simulation to predict the reliability of a solar cell.</p>

Road administrators have to make decisions regarding the maintenance and rehabilitation of many existing jointed Portland Cement Concrete (PCC) pavements in the road network. Since these pavements are in general expensive to rehabilitate, agencies often opt for overlaying the deteriorated PCC pavement with Hot Mix Asphalt (HMA), resulting in a composite pavement. Unfortunately, the tensile stresses and strains at the bottom of the overlay developed from the movement of the joints, which are caused by the traffic and the changes in temperature, will create cracks on the surface known as reflective cracking. Reflective cracking can reduce the life of a pavement by allowing water or other particles to get into the underlying layers, which causes the pavement structure to lose strength. To improve the performance of the composite pavement, road agencies have studied mitigations techniques to delay the initiation and propagation of those cracks reflected from the PCC joints and cracks. Traditionally, these studies have relied only on laboratory testing or nondestructive tests. This dissertation expands the traditional approach by adding full-scale Accelerate Pavement Testing (APT) to a laboratory effort to investigate enhanced asphalt overlays that delay the initiation and propagation of cracks reflected from the PCC joints. The study was organized into three complementary experiments. The first experiment included the first reflective cracking study of hot-mix asphalt (HMA) overlays over jointed Portland cement concrete pavements (PCCP) conducted at the Virginia APT facility. A Heavy Vehicle Simulator (HVS) was used to compare the reflective cracking performance of a Stone Matrix Asphalt (SMA) control mix with a modified mix with a synthetic fiber. The discussion includes the characterization of the asphalt mixes, the pavement structure, construction layout, the equipment used, the instrumentation installed, and lessons learned. Results showed that the fiber-modified mix had a higher resistance to fracture, which increases the pavement life by approximately 50%. The second experiment compared the cracking resistance of the same control and modified mixes in the laboratory. Four cracking resistance tests were performed on each mix. These four tests are: (1) Indirect Tensile Asphalt Cracking Test (IDEAL-CT), which measures the Cracking Test index (CTindex); (2) Semicircular Bend Test-Illinois (SCB-IL), which measures the critical strain energy release rate (Jc); (3) Semicircular Bend-Louisiana Transportation Research Center (SCB-LTRC), which measures the Flexibility Index (FI); and (4) Overlay Test (OT), which measures the Cracking Propagation Rate (CPR). The results from the four tests showed that the fiber-modified mix had a better resistance to cracking, confirming the APT test results. The laboratory assessment also suggested that the IDEAL-CT and SCB-IL test appear to be the most practical for implementation. The third phase evaluated the performance of mixes designed with a high content of Reclaimed Asphalt Pavement (RAP) and an enhanced asphalt-rubber extender, which comprises three primary components: plain soft bitumen, fine crumb rubber and an Activated Mineral Binder Stabilizer (AMBS). The experiment evaluated the fracture resistance of nine mixes designed with different rates of recycled asphalt pavement (RAP) and asphalt-rubber, compare them with a traditional mix, and propose an optimized mixture for use in overlays of concrete pavements. The mixes were designed with different rates of RAP (15, 30, 45%) and asphalt-rubber extender (0, 30, and 45%) following generally, the design requirements for an SMA mix in Virginia. The laboratory test recommended in the second experiment, IDEAL-CT and SCB-IL, were used to determine the fracture resistance of the mixes. The results showed that the addition of RAP decreases fracture resistance, but the asphalt-rubber extender improves it. A mix designed that replaced 30% of the binder with asphalt-rubber extender and 15% RAP had the highest resistance to fracture according to both. Also, as expected, all the mixed had a low susceptibility to rutting. / Doctor of Philosophy / Reflective cracking can reduce the life of a pavement by allowing water or other particles to get into the underlying layers, which causes the pavement structure to lose strength. To improve the performance of the composite pavement, road agencies have studied mitigations techniques that will delay the initiation and propagation of those cracks reflected from the PCC joints. Traditionally, these studies rely only on laboratory testing or nondestructive tests that will assist in the decision-making stage in a short time manner. This dissertation focusses on a reflective cracking study conducted through Accelerate Pavement Testing (APT) using a Heavy Vehicle Simulator (HVS) and laboratory testing. The first task used an HVS to evaluate reflective cracking of a Stone Matrix Asphalt (SMA) control mix and a modified mix with synthetic fiber. One lane was constructed with two layers of 1.5-inches of a control Stone Matrix Asphalt (SMA) mix and the second lane with an SMA mix modified with the synthetic fiber. Results from APT demonstrated that the modified SMA has a higher resistance to fracture which increases the pavement life by approximately 50%. The second task estimated the fracture resistance of the mixes studied in task one following the laboratory test: Indirect Tension Asphalt Cracking Test (IDEAL-CT), Texas Overlay Test (OT), Semi-Circular Bend-Louisiana Transportation Research Center (SCB-LTRC) and Semi-Circular Bend-Illinois (SCB-IL) to estimate the Cracking Test Index (CTindex), Cracking Propagation Rate (CPR), critical strain energy release rate (Jc) and Flexibility Index (FI), respectively. Results showed that the modified mix had a better resistance to cracking, confirming the APT test results. Specifically, CTindex results showed that the modified mix is more resistant than the control, with indices of 268.72 and 67.86. The estimated Jc indicated that less energy is required to initiate a crack for the control mix that achieved 0.48 kJ/m2 compared to the modified mix with synthetic fibers 0.54 kJ/m2. FI results for the control and fibers were 2.16 and 10.71, respectively. The calculated CPR showed that the control mix propagates a crack at a higher rate of 0.188 compared to the modified mix with a CPR of 0.152. The third phase evaluated the performance of mixes designed with a high content of Reclaimed Asphalt Pavement (RAP) and an enhanced asphalt-rubber extender, which comprises three primary components: plain soft bitumen, fine crumb rubber and an Activated Mineral Binder Stabilizer (AMBS). The experiment evaluated the fracture resistance of nine mixes designed with different rates of recycled asphalt pavement (RAP) and asphalt-rubber, compare them with a traditional mix, and propose an optimized mixture for use in overlays of concrete pavements. The mixes were designed with different rates of RAP (15, 30, 45%) and asphalt-rubber extender (0, 30, and 45%) following generally, the design requirements for an SMA mix in Virginia. The laboratory test recommended in the second experiment, IDEAL-CT and SCB-IL, were used to determine the fracture resistance of the mixes. The results showed that the addition of RAP decreases fracture resistance, but the asphalt-rubber extender improves it. A mix designed that replaced 30% of the binder with asphalt-rubber extender and 15% RAP had the highest resistance to fracture according to both. Also, as expected, all the mixed had a low susceptibility to rutting.

As hydrogel products are manufactured and used for applications ranging from biomedical to agricultural, it is useful to characterize their behavior and interaction with other materials. This thesis investigates the adhesion between two different solvated semi-interpenetrating polymer network (S-IPN) silicone hydrogels and a cyclo-olefin (COP) polymer through experimental, analytical, and numerical methods. Interfacial fracture data was collected through the application of the wedge test, a relatively simple test allowing for the measurement of fracture properties over time in environments of interest. In this case, the test was performed at discrete temperatures within range of 4Ë C to 80Ë C. Two COP adherends were bonded together by a layer of one of the S-IPN silicone hydrogels. Upon the insertion of a wedge between the two adherends, debonding at one of the two interfaces would initiate and propagate at a decreasing rate. Measurements were taken of the debond length over time and applied to develop crack propagation rate versus strain energy release rate (SERR) curves. The SERR values were determined through the application of an analytical model derived for the wedge test geometry and to take into account the effects of the hydrogel interlayer. The time-temperature superposition principle (TTSP) was applied to the crack propagation rate versus SERR curves by shifting the crack propagation rates with the Williams-Landel-Ferry (WLF) equation-based shift factors developed for the bulk behavior of each hydrogel. The application of TTSP broadened the SERR and crack propagation rate ranges and presented a large dependency of the adhesion of the system on the viscoelastic nature of the hydrogels. Power-law fits were applied to the master curves in order to determine parameters that could describe the adhesion of the system and be applied in the development of a finite element model representing the interfacial fracture that occurs for each system. The finite element models were used to validate the analytical model and represent the adhesion of the system such that it could be applied to future geometries of interest in which the S-IPN silicone hydrogels are adhered to the COP substrate. <i>[Files modified per J. Austin, July 9, 2013 Gmc]</i> / Master of Science