The objective of this study was to evaluate and develop an understanding of durability of an adhesive bonded system, for application in a future high speed civil transport (HSCT) aircraft structure. The system under study was comprised of Ti-6Al-4V metal adherends and a thermosetting polyimide adhesive, designated as FM-5, supplied by Cytec Engineered Materials, Inc. An approach based on fracture mechanics was employed to assess Ti-6Al-4V/FM-5 bond durability. Initially, wedge tests were utilized to find a durable surface pretreatment for the titanium adherends. Based on an extensive screening study, chromic acid anodization (CAA) was chosen as the standard pretreament for this research project. Double cantilever beam specimens (DCB) were then made and aged at 150° C, 177° C, and 204° C in three different environments; ambient atmospheric air (14.7 psia), and reduced air pressures of 2 psi air (13.8 KPa) and 0.2 psi air (1.38 KPa). Joints were aged for up to 18 months (including several intermediate aging times) in the above environments. The strain energy release rate (G) of the adhesive joints was monitored as a function of exposure time in the different environments. A 40% drop in fracture toughness was noted over the 18 month period, with the greatest degradation observed in samples aged at 204° C in ambient atmospheric air pressure. The loss in adhesive bond performance with time was attibutable to a combination of physical and chemical aging phenomena in the FM-5 resin, and possible degradation of the metal-adhesive interface(s). Several mechanical and material tests, performed on the bonded joints and neat FM-5 resin specimens, confirmed the above statement. It was also noted that physical aging could be "erased" by thermal rejuvenation, partially restoring the toughness of the FM-5 adhesive material. The FM-5 adhesive material displayed good chemical resistance towards organic solvents and other aircraft fluids such as jet fuel and hydraulic fluid. The results from the FM-5 adhesive and its bonded joints were compared and contrasted with VT Ultem and REGULUS polyimide adhesives. The FM-5 adhesive showed the best performance among the three adhesive systems. The effect of mode-mixity on the fracture toughness of the Ti-6Al-4V/FM-5 adhesive bonded system was also evaluated. DCB tests in conjunction with end-notched flexure (ENF) and mixed-mode flexure (MMF) tests, were used to fracture the bonded joints under pure mode I, pure mode II, and a combination of mode I and II loadings. The results showed that the mode I fracture toughness was twice as large as the mode II toughness. This was a rather surprising find, in sharp contrast to what several researchers have observed in the past. Our current understanding is that the crack path selection during the failure process plays a significant role in explaining this anomalous behavior. Finally, failure envelopes were generated for the titanium/FM-5 bonded system, both prior to and following thermal aging. These envelopes could serve as useful tools for engineers designing with Ti-6Al-4V/FM-5 bonds. / Ph. D.

We model the nonlinear dynamics of a cantilever beam with tip mass system subjected to different excitation and exploit the nonlinear behavior to perform sensitivity analysis and propose a parameter identification scheme for nonlinear piezoelectric coefficients. First, the distributed parameter governing equations taking into consideration the nonlinear boundary conditions of a cantilever beam with a tip mass subjected to principal parametric excitation are developed using generalized Hamilton's principle. Using a Galerkin's discretization scheme, the discretized equation for the first mode is developed for simpler representation assuming linear and nonlinear boundary conditions. We solve the distributed parameter and discretized equations separately using the method of multiple scales. We determine that the cantilever beam tip mass system subjected to parametric excitation is highly sensitive to the detuning. Finally, we show that assuming linearized boundary conditions yields the wrong type of bifurcation. Noting the highly sensitive nature of a cantilever beam with tip mass system subjected to parametric excitation to detuning, we perform sensitivity of the response to small variations in elasticity (stiffness), and the tip mass. The governing equation of the first mode is derived, and the method of multiple scales is used to determine the approximate solution based on the order of the expected variations. We demonstrate that the system can be designed so that small variations in either stiffness or tip mass can alter the type of bifurcation. Notably, we show that the response of a system designed for a supercritical bifurcation can change to yield a subcritical bifurcation with small variations in the parameters. Although such a trend is usually undesired, we argue that it can be used to detect small variations induced by fatigue or small mass depositions in sensing applications. Finally, we consider a cantilever beam with tip mass and piezoelectric layer and propose a parameter identification scheme that exploits the vibration response to estimate the nonlinear piezoelectric coefficients. We develop the governing equations of a cantilever beam with tip mass and piezoelectric layer by considering an enthalpy that accounts for quadratic and cubic material nonlinearities. We then use the method of multiple scales to determine the approximate solution of the response to direct excitation. We show that approximate solution and amplitude and phase modulation equations obtained from the method of multiple scales analysis can be matched with numerical simulation of the response to estimate the nonlinear piezoelectric coefficients. / Master of Science

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.

<p>Bonding techniques intended for assembling space microsystems are studied in this work. One of the largest problems in bonding pre-processed semiconductor wafers are the severe process restrictions imposed by material compatibility issues. Plasma processes have shown to be good for sensitive materials integration why the influence of different plasma parameters on the bondability of wafers is particularly studied. Conventional wet chemical and field-assisted methods are also examined. The resulting bond quality is assessed in terms of mechanical strength, homogeneity, and yield.</p><p>The effect of spaceflight environment on the reliability of wafer bonds is also investigated. Both high and low temperature annealed bonds are found to be very robust. Effects observed are that low temperature bonds are reinforced by thermal cycling in vacuum and that high temperature bonds degrade slightly by low dose γ irradiation.</p><p>Adhesion quantification is important for all bonding. Development of accurate quantification methods is considered necessary since most methods at hand are limited. This work includes the development of the blister test method. Former test structures are improved to be more practical to work with and to yield low experimental scatter. A physical stress model for the improved structure is suggested with which successful predictions of fracture for different test specimen configurations are made. The blister test method is used throughout this work to assess the strength of wafer bonds. The physics background and modelling of other common test methods are also thoroughly analysed. The methods’ practical capabilities and limitations are commented; origin and mitigation of measurement errors are discussed. It is shown that all methods can be significantly improved by small means.</p><p>Weibull statistics is introduced as a tool to characterise wafer bonds. This method is suitable to use in brittle materials design as the inherent variability in strength can be properly accounted for.</p>

Bonding techniques intended for assembling space microsystems are studied in this work. One of the largest problems in bonding pre-processed semiconductor wafers are the severe process restrictions imposed by material compatibility issues. Plasma processes have shown to be good for sensitive materials integration why the influence of different plasma parameters on the bondability of wafers is particularly studied. Conventional wet chemical and field-assisted methods are also examined. The resulting bond quality is assessed in terms of mechanical strength, homogeneity, and yield. The effect of spaceflight environment on the reliability of wafer bonds is also investigated. Both high and low temperature annealed bonds are found to be very robust. Effects observed are that low temperature bonds are reinforced by thermal cycling in vacuum and that high temperature bonds degrade slightly by low dose γ irradiation. Adhesion quantification is important for all bonding. Development of accurate quantification methods is considered necessary since most methods at hand are limited. This work includes the development of the blister test method. Former test structures are improved to be more practical to work with and to yield low experimental scatter. A physical stress model for the improved structure is suggested with which successful predictions of fracture for different test specimen configurations are made. The blister test method is used throughout this work to assess the strength of wafer bonds. The physics background and modelling of other common test methods are also thoroughly analysed. The methods’ practical capabilities and limitations are commented; origin and mitigation of measurement errors are discussed. It is shown that all methods can be significantly improved by small means. Weibull statistics is introduced as a tool to characterise wafer bonds. This method is suitable to use in brittle materials design as the inherent variability in strength can be properly accounted for.

In this thesis the Ramberg-Osgood nonlinear model for describing the behavior of many different materials is investigated. A brief overview of the model as it is currently used in the literature is undertaken and several misunderstandings and possible pitfalls in its application is pointed out, especially as it pertains to more recent approaches to finding solutions involving the model. There is an investigation of the displacement of a cantilever beam under a combined loading consisting of a distributed load across the entire length of the beam and a point load at its end and new solutions to this problem are provided with a mixture of numerical techniques, which suggest strong mathematical consistency within the model for all theoretical assumptions made. A physical experiment was undertaken and the results prove to be inaccurate when using parameters derived from tensile tests, but when back calculating parameters from the beam test the model has a 14.40% error at its extreme against the experimental data suggesting the necessity for further testing.

Structural adhesives are widely used with great success, and yet occasional failures can occur, often resulting from improper bonding procedures or joint design, overload or other detrimental service situations, or in response to a variety of environmental challenges. In these situations, cracks can start within the adhesive layer or debonds can initiate near an interface. The paths taken by propagating cracks can affect the resistance to failure and the subsequent service lives of the bonded structures. The behavior of propagating cracks in adhesive joints remains of interest, including when some critical environments, complicated loading modes, or uncertainties in material/interfacial properties are involved. From a mechanics perspective, areas of current interest include understanding the growth of damage and cracks, loading rate dependency of crack propagation, and the effect of mixed mode fracture loading scenarios on crack path selection. This dissertation involves analytical, numerical, and experimental evaluations of crack propagation in several adhesive joint configurations. The main objective is an investigation of crack path selection in adhesively bonded joints, focusing on in-plane fracture behavior (mode I, mode II, and their combination) of bonded joints with uniform bonding, and those with locally weakened interfaces. When removing cured components from molds, interfacial debonds can sometimes initiate and propagate along both mold surfaces, resulting in the molded product partially bridging between the two molds and potentially being damaged or torn. Debonds from both adherends can sometimes occur in weak adhesive bonds as well, potentially altering the apparent fracture behavior. To avoid or control these multiple interfacial debonding, more understanding of these processes is required. An analytical model of 2D parallel bridging was developed and the interactions of interfacial debonds were investigated using Euler-Bernoulli beam theory. The numerical solutions to the analytical results described the propagation processes with multiple debonds, and demonstrated some common phenomena in several different joints corresponding to double cantilever beam configurations. The analytical approach and results obtained could prove useful in extensions to understanding and controlling debonding in such situations and optimization of loading scenarios. Numerical capabilities for predicting crack propagation, confirmed by experimental results, were initially evaluated for crack behavior in monolithic materials, which is also of interest in engineering design. Several test cases were devised for modified forms of monolithic compact tension specimens (CT) were developed. An asymmetric variant of the CT configuration, in which the initial crack was shifted to two thirds of the total height, was tested experimentally and numerically simulated in ABAQUS®, with good agreement. Similar studies of elongated CT specimens with different specimen lengths also revealed good agreement, using the same material properties and cohesive zone model (CZM) parameters. The critical specimen length when the crack propagation pattern abruptly switches was experimentally measured and accurately predicted, building confidence in the subsequent studies where the numerical method was applied to bonded joints. In adhesively bonded joints, crack propagation and joint failure can potentially result from or involve interactions of a growing crack with a partially weakened interface, so numerical simulations were initiated to investigate such scenarios using ABAQUS®. Two different cohesive zone models (CZMs) are applied in these simulations: cohesive elements for strong and weak interfaces, and the extended finite element method (XFEM) for cracks propagating within the adhesive layer. When the main crack approaches a locally weakened interface, interfacial damage can occur, allowing for additional interfacial compliance and inducing shear stresses within the adhesive layer that direct the growing crack toward the weak interface. The maximum traction of the interfacial CZM appears to be the controlling parameter. Fracture energy of the weakened interface is shown to be of secondary importance, though can affect the results when particularly small (e.g. 1% that of the bulk adhesive). The length of the weakened interface also has some influence on the crack path. Under globally mixed mode loadings, the competition between the loading and the weakened interface affects the shear stress distribution and thus changes the crack path. Mixed mode loading in the opposite direction of the weakened interface is able to drive the crack away from the weakened interface, suggesting potential means to avoid failure within these regions or to design joints that fail in a particular manner. In addition to the analytical and numerical studies of crack path selection in adhesively bonded joints, experimental investigations are also performed. A dual actuator load frame (DALF) is used to test beam-like bonded joints in various mode mixity angles. Constant mode mixity angle tracking, as well as other versatile loading functions, are developed in LabVIEW® for use with a new controller system. The DALF is calibrated to minimize errors when calculating the compliance of beam-like bonded joints. After the corrections, the resulting fracture energies ( ) values are considered to be more accurate in representing the energy released in the crack propagation processes. Double cantilever beam (DCB) bonded joints consisting of 6061-T6 aluminum adherends bonded with commercial epoxy adhesives (J-B Weld, or LORD 320/322) are tested on the DALF. Profiles of the values for different constant mode mixity angles, as well as for continuously increasing mode mixity angle, are plotted to illustrate the behavior of the crack in these bonded joints. Finally, crack path selection in DCB specimens with one of the bonding surfaces weakened was studied experimentally, and rate-dependency of the crack path selection was found. Several contamination schemes are attempted, involving of graphite flakes, silicone tapes, or silane treatments on the aluminum oxide interfaces. In all these cases, tests involving more rapid crack propagation resulted in interfacial failures at the weakened areas, while slower tests showed cohesive failure throughout. One possible explanation of this phenomenon is presented using the rate-dependency of the yield stress (commonly considered to be corresponding to the maximum traction) of the epoxy adhesives. These experimental observations may have some potential applications tailoring adhesive joint configurations and interface variability to achieve or avoid particular failure modes. / Ph. D.

Carbon nanotubes, graphene and nano sized core shell rubber particles have all been extensively researched for their capability to improve mechanical properties of thermoset resins. However, there has been a lack of research on their evaluation for energy absorption in high velocity impact scenarios, and the fundamental mechanics of their failure mechanisms during highly dynamic stress transfer through the matrix. This fundamental research is essential for laying the foundation for improvement in ballistic performance in composite armor. In hard armor applications, energy absorption is largely accomplished through delamination between plies of the composite laminate. This energy absorption is accomplished through two mechanisms. The first being the elongation of the fiber reinforcement contained in the resin matrix, and the second is the propagation of the crack in between the discreet fabric plies. This research aims to fundamentally study the energy absorption characteristics of various nano-particles as reinforcements in thermoset resin for high velocity impact applications. Multiple morphologies will be evaluated through use of platelet, tubular and spherical shaped nano-particles. Evaluations of the effect on stress transfer through the matrix due to the combination of nano sized and micro scale particles of milled fiber is conducted. Three different nano-particles are utilized, specifically, multi-walled carbon nanotubes, graphene, and core shell rubber particles. The difference in surface area, aspect ratio and molecular structure between the tube, platelet and spherical nano-particles causes energy absorption through different failure mechanisms. This changes the impact performance of composite panels enhanced with the nanoparticle fillers. Composite panels made through the use of dispersing the various nano-particles iv in a non-contact planetary mixer, are evaluated through various dynamic and static testing, including unnotched cantilever beam impact, mixed mode fracture toughness, split-Hopkinson bar, and ballistic V50 testing. The unnotched cantilever beam testing showed that the addition of milled fiber degraded the impact resistance of the samples. Addition of graphene nano platelets unilaterally degraded impact resistance through the unnotched cantilever beam testing. 1.5% loading of MWCNT showed the greatest increase in impact resistance, with a 43% increase over baseline. Determining the critical load for mixed mode interlaminar shear testing can be difficult for composite panels that bend without breaking. An iterative technique of optimizing the coefficient of determination, R2 , in linear regression is developed for objectively determining the point of non-linearity for critical load. This allows for a mathematical method of determination; thereby eliminating any subjective decision of choosing where the data becomes non-linear. The core shell rubber nano particles showed the greatest strain energy release rate with an exponential improvement over the baseline results. Synergistic effects between nano and micro sized particles in the resin matrix during transfer of the stress wave were created and evaluated. Loadings of 1% milled carbon fiber enhanced the V50 ballistic performance of both carbon nanotube and core shell rubber particles in the resin matrix. However, the addition of milled carbon fiber degrades the impact resistance of all nano-particle enhanced resin matrices. Therefore, benefits gained from the addition of microsized particles in combination with nano-sized particles, are only seen in high energy impact scenarios with micro second durations. v Loadings of 1% core shell rubber particles and 1% milled carbon fiber have an improvement of 8% in V50 ballistic performance over the baseline epoxy sample for 44 mag single wad cutter gas check projectiles. Loadings of 1% multi-walled carbon nanotubes with 1% milled carbon fiber have an improvement of 7.3% in V50 ballistic performance over the baseline epoxy sample. The failure mechanism of the various nano-particle enhanced resin matrices during the ballistic event is discussed through the use of scanning electron microscope images and Raman spectroscopy of the panels after failure. The Raman spectroscopy data shows a Raman shift for the fibers that had an enhancement in the V50 performance through the use of nano-particles. The Raman band for Kevlar® centered at 1,649 cm-1 stemming from the stretching of the C==O bond of the fiber shows to be more sensitive to the residual axial strain, while the Raman band centered at 1,611 cm-1 stemming from the C-C phenyl ring is minimally affected for the CSR enhanced panels due to the failure mechanism of the CSR particles during crack propagation.

Global-local finite element analyses were used to study the damage tolerance of curvilinearly stiffened panels; fabricated using the modern additive manufacturing process, the so-called unitized structures, and that of adhesive joints. A damage tolerance study of the unitized structures requires cracks to be defined in the vicinity of the critical stress zone. With the damage tolerance study of unitized structures as the focus, responses of curvilinearly stiffened panels to the combined shear and compression loadings were studied for different stiffeners' height. It was observed that the magnitude of the minimum principal stress in the panel was larger than the magnitudes of the maximum principal and von Mises stresses. It was also observed that the critical buckling load factor increased significantly with the increase of stiffeners' height. To study the damage tolerance of curvilinearly stiffened panels, in the first step, buckling analysis of panels was performed to determine whether panels satisfied the buckling constraint. In the second step, stress distributions of the panel were analyzed to determine the location of the critical stress under the combined shear and compression loadings. Then, the fracture analysis of the curvilinearly stiffened panel with a crack of size 1.45 mm defined at the location of the critical stress, which was the common location with the maximum magnitude of the principal stresses and von Mises stress, was performed under combined shear and tensile loadings. This crack size was used because of the requirement of a sufficiently small crack, if the crack is in the vicinity of any stress raiser. A mesh sensitivity analysis was performed to validate the choice of the mesh density near the crack tip. All analyses were performed using global-local finite element method using MSC. Marc, and global finite element methods using MSC. Marc and ABAQUS. Negligible difference in results and 94% saving in the CPU time was achieved using the global-local finite element method over the global finite element method by using a mesh density of 8.4 element/mm ahead of the crack tip. To study the influence of different loads on basic modes of fracture, the shear and normal (tensile) loads were varied differently. It was observed that the case with the fixed shear load but variable normal loads and the case with the fixed normal load but variable shear loads were Mode-I. Under the maximum combined loading condition, the largest effective stress intensity factor was very smaller than the critical stress intensity factor. Therefore, considering the critical stress intensity factor of the panel with the crack of size 1.45 mm, the design of the stiffened panel was an optimum design satisfying damage tolerance constraints. To acquire the trends in stress intensity factors for different crack lengths under different loadings, fracture analyses of curvilinearly stiffened panels with different crack lengths were performed by using a global-local finite element method under three different load cases: a) a shear load, b) a normal load, and c) a combined shear and normal loads. It was observed that 85% data storage space and the same amount in CPU time requirement could be saved using global-local finite element method compared to the standard global finite element analysis. It was also observed that the fracture mode in panels with different crack lengths was essentially Mode-I under the normal load case; Mode-II under the shear load case; and again Mode-I under the combined load case. Under the combined loading condition, the largest effective stress intensity factor of the panel with a crack of recommended size, if the crack is not in the vicinity of any stress raiser, was very smaller than the critical stress intensity factor. This work also includes the performance evaluation of adhesive joints of two different materials. This research was motivated by our experience of an adhesive joint failure on a test-fixture that we used to experimentally validate the design of stiffened panels under a compression-shear load. In the test-fixture, steel tabs were adhesively bonded to an aluminum panel and this adhesive joint debonded before design loads on the test panel were fully applied. Therefore, the requirement of studying behavior of adhesive joints for assembling dissimilar materials was found to be necessary. To determine the failure load responsible for debonding of adhesive joints of two dissimilar materials, stress distributions in adhesive joints of the nonlinear finite element model of the test-fixture were studied under a gradually increasing compression-shear load. Since the design of the combined load test fixture was for transferring the in-plane shear and compression loads to the panel, in-plane loads might have been responsible for the debonding of the steel tabs, which was similar to the results obtained from the nonlinear finite element analysis of the combined load test fixture. Then, fundamental studies were performed on the three-dimensional finite element models of adhesive lap joints and the Asymmetric Double Cantilever Beam (ADCB) joints for shear and peel deformations subjected to a loading similar to the in-plane loading conditions in the test-fixtures. The analysis was performed using ABAQUS, and the cohesive zone modeling was used to study the debonding growth. It was observed that the stronger adhesive joints could be obtained using the tougher adhesive and thicker adherends. The effect of end constraints on the fracture resistance of the ADCB specimen under compression was also investigated. The numerical observations showed that the delamination for the fixed end ADCB joints was more gradual than for the free end ADCB joints. Finally, both the crack propagation and the characteristics of adhesive joints were studied using a global-local finite element method. Three cases were studied using the proposed global-local finite element method: a) adhesively bonded Double Cantilever Beam (DCB), b) an adhesive lap joint, and c) a three-point bending test specimen. Using global-local methods, in a crack propagation problem of an adhesively bonded DCB, more than 80% data storage space and more than 65% CPU time requirement could be saved. In the adhesive lap joints, around 70% data storage space and 70% CPU time requirement could be saved using the global-local method. For the three-point bending test specimen case, more than 90% for both data storage space and CPU time requirement could be saved using the global-local method. / Ph. D.