Composite structures are commonly joined using adhesive or mechanical joints, withmechanical joints being preferred when components need to be removable for maintenancepurposes. However, the presence of mechanical joints introduces a discontinuity in theload path, which can serve as an initiation point for failure and needs to be taken intoaccount in the design of the joint. Additionally, delaminations may occur around thefastener hole during the manufacturing and assembly processes, further impacting thestrength of the laminate under compressive loading. While some studies have assessedthe residual strength of open-hole specimens, limited information exists regarding theresidual bearing strength in delaminated composite joints. This study aims to assessthe significance of delaminations of varying sizes on the bearing strength of single-bolt,single-lap shear joints under static loading using numerical analysis methods. The effectsof countersinking and bolt size are also examined. Stress and progressive failure analysisare utilized to evaluate different parameters and account for the nonlinear behavior of thematerials. The study reveals that the presence of delamination leads to degradation ofthe bearing strength of approximately five percent when bolt pretension is applied and15 percent in the absence of pretension. Countersinking increases maximum and averagestresses on the cylindrical section of the hole, while a larger bolt size enhances bearingstrength by reducing bolt bending in single-lap shear joints. / Kompositstrukturer sammanfogas vanligtvis med hjälp av lim eller mekaniska fogar,där mekaniska fogar är att föredra när komponenter måste kunna demonteras förunderhållsändamål. Mekaniska fogar introducerar dock en diskontinuitet i lastvägen, somkan fungera som en startpunkt för strukturella fel och måste beaktas vid utformningenav fogen. Dessutom kan delaminering uppstå runt fästelementhålet under tillverkningsochmonteringsprocesserna, vilket ytterligare påverkar laminatets tryckhållfasthet. Någrastudier har utvärderat resthållfastheten hos prov med öppna hål, men det finns begränsadinformation om den resterande tryckhållfastheten vid hålkanten hos delamineradekompositfogar. Denna studie syftar till att bedöma påverkan av delamineringar avvarierande storlek på tryckhållfastheten vid hålkanten hos enskärsförband med ettfästelement under statisk belastning med hjälp av numeriska analysmetoder. Ävenpåverkan av försänkning och bultstorlek undersöks. Analyser av spänning och progressivskada används för att utvärdera olika parametrar och ta materialens olinjära beteendetill hänsyn. Studien ställer fast att förekomsten av delaminering leder till en försämringav tryckhållfastheten vid hålkanten med cirka fem procent när fästelementet är underförspänning och 15 procent utan förspänning. Försänkning av fästelementets huvud ökarmaximala och genomsnittliga spänningar på den cylindriska delen av hålet, medan enstörre bultstorlek höjer tryckhållfastheten vid hålkanten genom att minska bultböjningen ienskärsförband.

Many applications require adhesives with high strength to withstand the exhaustive loads encountered in regular operation. In aerospace applications, advanced adhesives are needed to bond metals, ceramics, and composites under shear loading. The lap shear test is the experiment of choice for evaluating shear strength capabilities of adhesives. Specifically during single-lap shear testing, two overlapping rectangular tabs bonded by a thin adhesive layer are subject to tension. Shear is imposed as a result. Debonding occurs when the shear strength of the adhesive is surpassed by the load applied by the testing mechanism. This research develops a finite element model (FEM) and material model which allows mechanicians to accurately simulate bonded joints under mechanical loads. Data acquired from physical tests was utilized to correlate the finite element simulations. Lap shear testing has been conducted on various adhesives, specifically SA1-30-MOD, SA10-100, and SA10-05, single base methacrylate adhesives. The adhesives were tested on aluminum, stainless steel, and cold rolled steel adherends. The finite element model simulates what is observed during a physical single-lap shear test consisting of every combination of the mentioned materials. To accomplish this, a three-dimensional model was created and the cohesive zone approach was used to simulate debonding of the tabs from the adhesive. The thicknesses of the metallic tabs and the adhesive layer were recorded and incorporated into the model in order to achieve an accurate solution. From the data, force output and displacement of the tabs are utilized to create curves which were compared to the actual data. Stress and strain were then computed and plotted to verify the validity of the simulations. The modeling and constant determination approach developed here will continue to be used for newly-developed adhesives.

L’engouement pour le collage structural est important dans l’aéronautique. Actuellement, il n’existe pas de méthode de contrôle non destructive de l’adhésion dans un assemblage collé. Les méthodes de CND usuelles peuvent détecter au mieux des défauts majeurs de type décollement ou absence de colle. L’objectif de la thèse est donc de déterminer un indicateur ultrasonore en lien avec le niveau d’adhésion et la tenue structurale des assemblages collés.La première étape a consisté en l’élaboration d’éprouvettes étalons à adhésions variables de forme cisaillement simple. Trois traitements de surface différents ont été définis afin d’obtenir trois niveaux de force à rupture et donc trois niveaux d’adhésion distincts. Des cartographies détaillées du joint de colle sont obtenues par ultrasons. A l’issue des essais mécaniques les faciès de rupture sont analysés. Des contrôles supplémentaires par micro-tomographie X ont été réalisés. L’ensemble de ces essais ont permis de valider l’obtention d’éprouvettes homogènes et de niveaux d’adhésion maîtrisé. Un système expérimental spécifique a été réalisé pour développer des mesures d’acoustoélasticité qui permettent l’étude des variations locales de champ des contraintes. Pendant une sollicitation mécanique de type cisaillement simple, les variations de temps de vol dans l’aluminium en mode pulse-écho des éprouvettes sont analysées. Le dispositif est d’abord validé sur une éprouvette d’aluminium. Puis, il est démontré que sur une éprouvette de cisaillement simple, les bords d’un défaut, lieu de concentration de contraintes, sont visibles. Les simulations numériques réalisées donnent les mêmes tendances / The enthusiasm for structural bonding is important in aeronautic. Currently there is no method to test non-destructively the adhesion in a bonded assembly. The usual NDT methods can detect the most common defects like delamination or disbond. The aim of this thesis is to determine an ultrasonic indicator related to the level of adhesion and the structural strength of bonded assemblies.The first step was the development of calibrated samples. The specimens are single lap shear joints. Three different surface treatments have been developed to obtain three different levels of ultimate tensile strength and therefore three distinct levels of adhesion. Detailed cartographies of the adhesive joint are obtained by ultrasound. After mechanical testing the fracture surfaces are analyzed. Additional tests by microtomography were performed. They were used to validate the quality of samples. The objective of standards sample is achieved. A specific control system has been achieved to use acoustoelasticty to study the stress field in the bonded assembly. The variations of time of flight in the aluminum part in pulse-echo mode during mechanical test are analysed. First, the method is validated with an aluminum test piece. Then, it is shown, for a sample with defect, the edges of a defect are visible through the increase of stresses on its borders. Numerical simulations give the same trends.

Cartilage repair and/or replacement is necessary for many orthopaedic conditions including fissures from blunt trauma, autograft or allograft transplantation, and replacement of focal defects with biological or synthetic constructs. In cartilage repair, initial integration between the host tissue and repair site is desirable to allow for nutrient transport, molecular deposition to enhance fixation, and eventual stress transmission across the interface. It has been postulated that effective transport and crosslinking of newly synthesized collagen molecules across a repair site may be vital to the process of integrative repair, and recent experiments have correlated collagen deposition with the strength of such repair. Other investigations have shown that enzymatic degradation of the cartilage surface may enhance integrative repair and can increase bond strength of an adhesive to cartilage. This study explored a novel approach involving photochemical bonding of cartilage tissue samples through collagen crosslinking as a means to achieve rapid and effective initial fixation, with the goal of enhancing biological integration. Photosensitized collagen gels were first analyzed via FTIR to determine the crosslinking effects with respect to collagen type and photochemical mechanism. Using the photogellation FTIR results as a parametric guide, in vitro mechanical testing of photochemically bonded meniscal fibrocartilage and hyaline articular cartilage tissues was performed using a modified single-lap shear test. Finally, the cellular viability and bond stability of a photochemically bonded cartilage interface was evaluated over seven days of in vitro culture, where the bond strength was assessed by pushout of cores from annular defects. Results of this study have demonstrated the potential of combining enzymatic surface modification with photodynamic techniques to directly bond cartilage tissues for initial fixation.

Fabric Reinforced Cementitious Matrix (FRCM) composites utilize a mineral mortar matrix as a substitute for epoxy resin that is used for Fibre Reinforced Polymer (FRP). This eliminates issues associated with the low thermal compatibility of FRP with concrete, susceptibility to UV radiation, and sensitivity to high temperatures in which organic polymers undergo vitrification. This study discussed the effect of varying parameters like the number of Carbon-FRCM (C-FRCM) layers (1, 2 and 3 layers), different anchorage configurations (non-anchored, spike anchor, wrap anchor and mechanical anchor), bond length (300 or 200 mm), and the fabric type (unidirectional and bidirectional) on the direct shear behaviour of C-FRCM composites bonded to a concrete substrate, especially the fibre-matrix bond which is the most common debonding interface of FRCM composites. Calibrated models of the bond – slip behaviour are provided based on the fabric type and number of fabric layers. The results indicate that the anchor type and the overall composite thickness are the main factors that control the failure mode of the composite. All properly anchored specimens using spike and wrap anchors failed due to fabric rupture. Moreover, a considerable number of the non-anchored specimens failed due to composite-substrate debonding, although premature fabric rupture was frequently observed. Furthermore, specimens with bidirectional fabric demonstrated shallower penetration of the strain into the composite which may be due to the horizontal fabric strands providing some anchorage for the longitudinal strands. They also exhibited slip initiation at a higher stress compared to unidirectional specimens. In addition, slip initiation stress of unidirectional specimens decreased with more fabric layers which may indicate that the additional layers have a lower bond efficiency. For the same reason, specimens with three layers of fabric generally experienced deeper strain penetration into the composite than one-layered or two-layered specimens regardless of the anchor type. The results also indicate that the use of bidirectional fabric and anchorage systems decreases the strain penetration into the composite and correspondingly, the effective length is shortened. Surface strain measurements captured using digital image correlation generally did not match the internal fabric strain values obtained from strain gauges.

Plasma pretreatments are environmentally benign and energy efficient processes for modifying the surface chemistry of materials. In an effort to improve the strength of the titanium alloy/FM-5 polyimide adhesive joint for aerospace applications, oxygen plasma pretreatments and novel thin plasma polymerized (PP) films were investigated as adhesion promoters. Plasma treatments were carried out using custom-built, low pressure, radio frequency, inductively coupled plasma reactors. Ti-6Al-4V coupons were plasma treated and used to prepare miniature single lap shear (SLS) joints. The effects of plasma pretreatments on surface chemistry were studied using x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), Fourier transform infrared analysis (FTIR), and contact angle measurements. Relationships between composition, mechanical properties, and adhesion of PP films on Ti-6Al-4V and silicon wafers were investigated. The nanomechanical properties (modulus, hardness and adhesion) were studied using atomic force microscopy (AFM) nanoindentation and nanoscratch testing. A design of experiments (DOE) three factorial model was used to optimize the parameters for oxygen plasma treatments. Oxygen plasma pretreatments enhanced joint strength by cleaning the titanium surface and creating an extended oxide layer. Nanoindentation of oxygen plasma treated substrates showed no change in the surface mechanical properties due to the oxygen plasma treatment. This suggested that the improved SLS strength of the oxygen plasma pretreated substrates was due to the cleaning of the substrate and the removal of carbonaceous contaminants, rather than any changes in the morphology of the oxide layer. PP acetylene films were predominantly carbon, with oxygen as the other main constituent (incorporated mostly as C-O and C=O). For all SLS specimens tested, the adhesion between PP acetylene and FM-5 adhesive was adequate. However, the strength of SLS joints was limited by the adhesion of the PP acetylene to the Ti-6Al-4V substrate. The effects of a large number of plasma parameters, such as substrate pretreatment, carrier gas, input power, flow rate and film thickness were investigated. All samples failed at the PP film/Ti-6Al-4V interface or within the PP acetylene film, and thicker PP films yielded lower SLS strengths. PP films deposited at lower power exhibited higher hardness and reduced modulus than films deposited at higher power. Overall, thinner films exhibited higher hardness and reduced Young's modulus than thicker films. PP films of higher hardness yielded higher critical loads at debond (thickness normalized) during the nanoscratch test. Thin films were developed via the vapor plasma polymerization of titanium(IV) isobutoxide (TiiB). XPS results suggested that titanium was incorporated into the film as TiO2 clusters dispersed in an organic matrix. No evidence for Ti-C was obtained from the XPS spectra. PP films of TiiB were much more compliant than PP acetylene films. This behavior was attributed to decreased fragmentation and lower crosslinking that occurred during PP TiiB film deposition. These PP films did not exhibit sol-gel-like qualities, and because of the way titanium was incorporated into the films, a more appropriate name for these films might be "titanium dioxide-doped plasma polymerized films." / Ph. D.

L’assemblage des matériaux composites thermoplastiques tel que le PEEK est l’une des problématiques majeure de l’industrie aéronautique. Actuellement, différentes techniques sont développées pour assurer l’assemblage structural de ces matériaux, tels que : le soudage, le rivetage, le boulonnage et le collage. Les enjeux industriels majeurs sont principalement, à l’heure actuelle, la conception des structures simplifiées au maximum afin de réduire les coûts de production et la réduction des consommations énergétiques. A cet effet, l’industrie aéronautique fait fréquemment appel à l’assemblage par collage en raison de nombreux avantages qu’il offre (gain de poids, distribution régulière des contraintes, absence de trous) par rapport aux autres techniques existantes. Le PEEK (PolyEtherEtherKetone), est un matériau polymère semi-cristallin thermoplastique, à hautes performances. Ce matériau est souvent utilise dans l’industrie aéronautique principalement renforce par des fibres de carbone ou de verre. Cependant, du fait du niveau élevé de sa résistance chimique l’assemblage par collage du PEEK et de ses composites nécessitent des traitements de surfaces appropries et optimises. Or, afin d’obtenir un system collé à haute performance, la problématique scientifique et technique doit être concentrée sur la jonction entre les éléments à assembler. En effet, la qualité de cette jonction est de la plus haute importance car elle doit permettre un transfert optimal des contraintes thermomécaniques lorsque l’assemblage est soumis a ses conditions d'usage. Cette étude concerne donc, l’amélioration des propriétés mécaniques (monotones et cycliques) de l’assemblage par collage PEEK/PEEK. Dans cette optique, un traitement de surface simple de mise en œuvre est proposé. Ce traitement est le sablage, qui permet la modification topographique (morphologique) de surface. La compréhension des différents phénomènes d’interaction aux interfaces intervenant dans l’amélioration du comportement mécanique du joint de colle et qui s’inscrit dans la triptyque : « Rhéologie, Physico-chimie et topographie », est l’enjeu scientifique majeur dans cette thèse. Dans un premier temps, l’influence des paramètres du traitement tels que le temps de projection, la taille des particules, sur la morphologie de surface de différents matériaux à base de PEEK a été analysée, permettant ainsi d’établir la corrélation entre les paramètres morphologiques et les mécanismes de modification topographique de surface intervenant pendant le traitement de surface. L’un des facteurs clefs pour la compréhension des mécanismes d’interaction entre l’adhésif liquide et le substrat solide est la mouillabilité. L’analyse du comportement au mouillage en fonction des différents paramètres du traitement a été réalisée. La mouillabilité des surfaces traitées est fortement affectée par la rugosité de surface créée après ce traitement. La relation entre les paramètres morphologiques et la mouillabilité a été discutée. Enfin, l’influence des paramètres du traitement par sablage sur le comportement mécanique monotone et à long terme (essais de fatigue) sur la résistance du joint colle a été étudié à l’aide d’essais de cisaillement sur éprouvettes à simple recouvrement. Ceci a conduit, à la proposition de paramètres morphologiques surfaciques spécifiques pour l’optimisation du comportement mécanique du joint de colle des matériaux composites à matrice PEEK. / One of most problematic in the aeronautical industries is the structural joining of the high performance thermoplastic composites like PEEK composites. Actually, a lot of technologies are used for joining thermoplastic composites like welding, bolting, riveting, fastening and adhesive bonding. Due to the various advantages that characterize the adhesive bonding method, such an uniform stress distribution along the joint, weight‐light and cost reduction, makes this technique more desirable to join thermoplastic composites materials compared to the other joining techniques. PEEK (PolyEtherEtherKetone) is a semi‐crystalline thermoplastic material with high performance. This material is wildly used in aeronautical industries, principally, reinforced with carbon of glass fibres. However, its high chemical resistance makes the adhesive bonding of PEEK and its composites difficult and therefore an appropriate and optimised surface treatment is necessary. In the aim to obtain a bonded system with high performance, scientific and technical problematic should be focussed on the junction between adherents. Indeed, the quality of this junction is of utmost importance because it must allow optimum transfer of thermomechanical stresses when the assembly is subject to its terms of use. Though, at this time it is well known that thermoplastic composite materials are difficult to bond with‐out surface treatment. This study, therefore, relates to the improvement of mechanical properties (monotonic and cyclic) of the adhesive bonding system PEEK / PEEK. In this context, a surface treatment, easy to implement, is proposed. This surface treatment is sandblasting, which enables surface topographic (morphological) modifications. Understanding of various phenomena of interfaces interaction involved in the improvement of the mechanical behavior of the adhesive joint and is part of the triptych "Rheology, Physico‐chemistry and topography" is the major scientific challenge in this thesis. Initially, the influence of processing parameters such as the projection time, the particle size on surface morphology of various materials based on PEEK was analysed, thus allowing establishing the correlation between morphological parameters and modification mechanisms involved during surface treatment surface. One of the key factors for understanding the mechanisms of interaction between the liquid adhesive and the solid substrate is wettability. The analysis of the wetting behavior as a function of various parameters of the treatment was performed. The wettability of treated surfaces is strongly affected by surface roughness created after this treatment. The relationship between morphological parameters and wettability was discussed. Finally, the influence of sandblasting processing parameters on the mechanical behavior in monotoning and long‐term (fatigue tests) of the adhesive joint strength was studied, using single lap shear tests specimens. This has led to the proposal of specific surface morphological parameters for the optimization of the mechanical behavior of the adhesive joint of PEEK and its composites.

Adhesive bonding which has been in use for long as a traditional joining method has gained ground in the last couple of decades due to the introduction of advanced composite materials into the aerospace industry. Bonded structures have advantages such as high corrosion and fatigue resistance, ability to join dissimilar materials, reduced stress concentration, uniform stress distribution, good damping characteristics etc. They also have certain limitations like environmental degradation, existence of defects like pores, voids and disbonds, difficulty in maintenance and repair etc. A serious drawback in the use of adhesively bonded structures has been that there are no established comprehensive non-destructive testing (NDT) techniques for their evaluation. Further, a reliable evaluation of the effect of the existing defects on strength and durability of adhesive joints is yet to be achieved. This has been a challenge for the research and development community over several decades and hence, been the motivation behind this piece of research work. Under the scope of the work carried out in the thesis, some of the primary factors such as the existence of defects, degradation of the adhesive, stress and strain distribution in the bonded region etc., have been considered to study the bond integrity in composite to composite lap shear joints. The problem becomes complex if all the parameters affecting the adhesive joint are varied simultaneously. Taking this into consideration, one of the key parameters affecting the bond quality, viz., the adhesive layer degradation was chosen to study its effect on the bonded joint. The epoxy layer was added with different, definite amount of Poly vinyl alcohol (PVA) to arrive at sets of bonded joint specimens with varied adhesive layer properties. A thorough review of different non destructive testing methods applied to this particular problem showed that ultrasonic wave based techniques could be the right choice. To start with, preliminary experimental investigations were carried on unidirectional glass fiber reinforced plastic (GFRP-epoxy) lap joints. The adhesive joints were subjected to non destructive evaluation (NDE) using ultrasonic through transmission and pulse echo techniques as also low energy digital X-ray techniques. The results obtained showed a variation in reflected and transmitted ultrasonic pulse amplitude with bond quality. Digital X-Ray radiography technique showed a variation in the intensity of transmitted x-rays due to variation in the density of adhesive. Standard mechanical tests revealed that the addition of PVA decreased the bond strength. A plot of coefficient of reflection from the first interface and the bond strength showed a linear correlation between them. After obtaining a cursory feel and understanding of the parameters involved with the preliminary experiments on GFRP adhesive joints which yielded interesting and encouraging results, further work was carried on specimens made out of autoclave cured carbon fiber reinforced plastic (CFRP)-epoxy bonded joints. Normal incidence ultrasound showed a similar trend. Analyses of the Acoustic Emission (AE) signals generated indicate early AE activity for degraded joints compared to healthy joints. Literary evidences suggest that the ultrasonic shear waves are more sensitive to interfacial degradation. An attempt was made to use oblique incidence ultrasonic interrogation using shear waves. The amplitude of reflected shear waves from the interface increased with an increase in degradation. Further, a signal analysis approach in the frequency domain revealed a shift in the frequency minimum towards lower range in degraded samples. This phenomenon was verified using analytical models. An inversion algorithm was used to determine the interfacial transverse stiffness which decreased significantly due to increase in degradation. Conventional ultrasonic evaluation methods are rendered ineffective when a direct access to the test region is not possible; a different approach with guided wave techniques can be explored in this scenario. Investigations on CFRP-epoxy adhesive joints using Lamb waves showed a decrease in the amplitude of ‘So’ mode in degraded samples. Theoretical dispersion curves exhibited a similar trend. Frequency domain studies on the received modes using Gabor wavelet transform showed a negative shift in frequency with increased degradation. It was also observed that the maximum transmission loss for the most degraded sample with 40 percent PVA occurred in the range of 650 – 800 kHz. Non linear ultrasonic (NLU) evaluation revealed that the nonlinearity parameter (β) increased with increased degradation. Kissing bonds are most commonly occurring type of defects in adhesive joints and are very difficult to characterize. A recent non-contact imaging technique called digital image correlation (DIC) was tried to evaluate composite adhesive joints with varied percentage of inserted kissing bond defects. The results obtained indicate that DIC can detect the kissing bonds even at 50 percent of the failure load. In addition, to different experimental approaches to evaluate the bonded joint discussed above, the effect of degradation on the stresses in the bond line region was studied using analytical and numerical approach. A linear adhesive beam model based on Euler beam theory and a nonlinear adhesive beam model based on Timoshenko beam theory were used to determine the adhesive peel and shear stress in the joint. Digital image correlation technique was used to experimentally obtain the bond line strains and corresponding stresses were computed assuming a plane strain condition. It was found that the experimental stresses followed a similar trend to that predicted by the two analytical models. A maximum peel stress failure criterion was used to predict failure loads. A failure mechanism was proposed based on the observations made during the experimental work. It was further shown that the critical strain energy release rate for crack initiation in a healthy joint is much higher compared to a degraded joint. The analytical models become cumbersome if a larger number of factors have to be taken into account. Numerical methods like finite element analysis are found to be promising in overcoming these hurdles. Numerical investigation using 3D finite element analysis was carried out on CFRP-epoxy adhesive joints. The adherend – adhesive interface was modeled using connector elements whose stiffness properties as well as the bulk adhesive properties for joints with different amounts of PVA were determined using ultrasonic inspection method. The peel and shear stress variation along the adhesive bond line showed a similar trend as observed with the experimental stress distribution (DIC) but with a lesser magnitude. A parametric study using finite element based Monte-Carlo simulation was carried out to assess the effect of variation in various joint parameters like adhesive modulus, bondline thickness, adherend geometrical and material properties on peel and shear stress in the joint. It was found that the adhesive modulus and bond line thickness had a significant influence on the magnitude of stresses developed in the bond line. Thus, to summarize, an attempt has been made to study the bond line integrity of a composite epoxy adhesive lap joint using experimental, analytical and numerical approaches. Advanced NDE tools like oblique incidence ultrasound, non linear ultrasound, Lamb wave inspection and digital image correlation have been used to extract parameters which can be used to evaluate composite bonded joints. The results obtained and reported in the thesis have been encouraging and indicate that in specific cases where the bond line thickness and other relevant parameters if can be maintained or presumed reasonably non variant, it is possible to effectively evaluate the integrity of a composite bonded joint.