The trend in aircraft design is to produce greener airplanes through lighter structures and/or structures with extended life and reduced maintenance. Bonded crack retarders (BCR) are one of the solutions towards that objective. BCR are reinforcing straps bonded to the structure in order to improve the fatigue and damage tolerance properties of the assembly. The aim of this study was to demonstrate that the BCR hybrid technology – beneficial for upper wing cover – could also be applied to lower wing covers. The project also focused on evaluating BCR most important parameters. The fatigue life improvement obtained from BCR was evaluated through a series of coupons and skin-stringer assemblies tested under constant and variable amplitude loading. While the coupon tests demonstrated a life improvement of only 17% under constant amplitude loading, the variable amplitude load tests performed on the skin-stringer assembly demonstrated increased fatigue lives with a factor of 5 and reduced crack growth rates with a factor of 5 to 6. A finite element calculation tool was developed in order to conduct a parametric analysis of BCR geometry through the evaluation of the substrate stress intensity factor in the case of fatigue loading. The main difficulty was to include the interacting mechanism of the substrate lead crack and the disbond of the adhesive layer. The novelty of the approach was to incorporate the fatigue delamination calculation in order to evaluate the fatigue disbond propagation with crack growth. This was embedded in a 3D finite element design tool ReSLIC (Reinforced Structures Life Improvement Calculation). A necessary step to the development of ReSLIC was the analysis of fatigue properties of the adhesive system in order to provide input data for fatigue delamination calculations. To that end, a series of fatigue tests were performed in pure Mode I, pure Mode II and mixed mode with ratios of 25%, 50% and 75% of mode II ... [cont.].

Chalcogenide based Phase Change Materials are currently of great technological interest in the growing field of optoelectronics. Ge₂Sb₂Te₅ (GST) is the most widely studied phase change material, and it has been commercially used in both optical and electronic data storage applications, due to its ability to switch between two different atomic configurations, at high speed and with low power consumption, as well as its high optical and electrical contrast between amorphous and crystalline states. Despite its well-known optical and electrical properties, the operation in combination of optical and electrical domains has not yet been fully investigated. This work studies the operation of GST nano-devices exposed to a combination of optical and electrical stimuli or mixed mode by asking, is it possible to electrically measure an optically induced phase change, or vice versa? If so, how do the optical and electrical responses relate to each other, and is it possible to operate GST with a combination of optical and electrical signals? What are the technical constraints that need to be considered in order to fabricate GST devices that could be operated either optically or electrically? In order to answer these questions, experiments that characterized the optical and electrical responses of GST based nano-devices were performed. It was found that different crystallization mechanisms may have influence in the response, and that the thermal and optical design characteristics of the device play a key role in its operation. Finally a proof of principle, of an opto-electonic memory device that can be read electrically, reset optically and write electrically, is presented. This opens up possibilities for the development of new opto-eloectronic applications such as non-volatile interfaces between future photonics and electronics, high speed optical communication detectors, high speed cameras, artificial retinas and many more.

La taille de la zone plastique en pointe de fissure dans un matériau est directement liée au facteur d’intensité de contrainte pour une configuration donnée. Cette étude utilise la technique d’indentation avec une très faible charge afin d’étudier la taille de la zone plastique cyclique et monotone au voisinage de la fissure. La zone plastique est caractérisée par la relation entre la dureté, le module réduit, le travail élastique, le travail plastique, l’indice de plasticité et la distance par rapport à la pointe de fissure. Les essais sont menés sur une éprouvette CT pour un alliage d’aluminium 2024T351. Dans une étude complémentaire, le contour des zones plastiques en fond de fissure en mode mixte, le rayon minimum de zone plastique (MPZR) et la direction de la fissure initiale pour l’alliage d’aluminium 2024T351 sur un “Compact Tension spécimen” est déterminé en utilisant le logiciel Matlab. Nous avons calculé la forme, la taille de la zone plastique au fond de fissure, le rayon minimum de la zone plastique par rapport à l’angle de chargement et le facteur d’intensité de contrainte en utilisant le critère de Von Mises. L’étude est effectuée pour les divers angles de chargement. Nous avons constaté que le chargement en mode mixte (13 = 600) est le plus néfaste pour le matériau. / The plastic zone size associated with a propagating crack in a material is directly related to the stress intensity factor for a given configuration. This work utilizes the ultra-low-load indentation techniques to study the cyclic and monotonic plastic zone size at the fatigue crack tip based on the relationship between the hardness, elastic work, plastic work, plasticity index and the distance from the crack tip. The study is conducted essentially on the aluminium alloy 2024T351. In a second part of this work, we study the contour of mixed mode crack-tip plastic zones, the minimum plastic zone radius (MPZR) and the direction of initial crack for the aluminium alloy 2024T351 in a Compact tension specimen by using Matlab software. We have computed the shape, size of plastic zone at crack-tip and the minimum plastic zone radius with respect to the loading angle and stress intensity factor in linear elastic fracture mechanics for plane strain condition according to Von Mises yield criteria, the study is conducted for various loading angle. We found that the mixed mode loading (F36O°) can lead to earlier material fracture earlier than any other biaxial loading.

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.

<p>QC 20110523</p>

The adsorption isotherm of two weak bases, Promethazine hydrochloride and Propranolol hydrochloride, were determined with isocratic reversed-phase liquid chromatography, with a 60 w% methanol in 20 mM sodium acetate buffer pH 4 as the mobile phase, and calculated by the elution by characteristic points method. The data obtained from the method were then fitted into the Langmuir isotherm and the electrostatically modified Langmuir. Propranolol fitted reasonably good into the models while Promethazine was not as good. When Promethazine and Propranolol were together in the same sample, there was indication of competition of the adsorption sites. For comparing retention and peak shape between a C18 column and a mixed mode column, Waters XBridge C18 and Thermo Scientific Acclaim WCX-1, were tested in gradient elution with 11.32 mM sodium acetate buffer and 10–70 % methanol. The mixed-mode column gave significantly better peak shapes, while the retention time were longer compared to the C18 column. / Adsorptions-isotermerna för två svaga baser, Prometazin hydroklorid och Propranolol hydroklorid, bestämdes med isokratisk omvänd-fas vätskekromatografi, med w% 60 metanol i en 20 mM natriumacetatbuffert pH 4 som mobil fas, och beräknad med metoden elution by characteristic points.  Från metoden erhållna data passades till Langmuir isotherm och den elektrostatiskt modifierade Langmuir. Propanolen passade ganska bra till de olika isotermerna, medan Prometazin var något sämre passad. När Prometazin och Propranolol var tillsammans i samma prov, fanns det indikationer på konkurrens om adsorptionsställen. För jämförelse av topparnas form och retentionstid mellan en C18-kolonn och en mixed-mode-kolonn, användes Waters XBridge C18 och Thermo Scientific Acclaim WCX-1, som testades i gradient eluering med 11, 32 mM natriumacetatbuffert och 10–70 % metanol. Mixed-mode-kolonnen gav åtskilligt bättre toppar, medan retentionstiden var längre jämfört med C18-kolonnen.

No description available.

The primary focus of this thesis was to investigate the critical strain energy release rates (G) for mixed-mode (I/II) fracture of wood adhesive joints. The aims of the study were: (1) quantifying the fracture properties of two material systems, (2) analyzing the aspects that influence the fracture properties of bonded wood, (3) refining test procedures that particularly address layered orthotropic systems in which the layers are not parallel to the laminate faces, of which wood is often a particular case, and (4) developing testing methods that enhance the usefulness of performing mixed-mode tests with a dual-actuator load frame. The material systems evaluated experimentally involved yellow-poplar (Liriodendron tulipifera), a hardwood of the Magnoliaceae family, as adherends and two different adhesives: a moisture-cure polyurethane (PU) and a phenol/resorcinol/-formaldehyde (PRF) resin. The geometry tested in the study was the double cantilever beam that, in a dual-actuator load frame, can be used for testing different levels of mode-mixity. The mixed-mode loading condition is obtained by applying different displacement rates with the two independently controlled actuators of the testing machine. Characteristic aspects such as the large variability of the adhesive layer thickness and the intrinsic nature of many wood species, where latewood layers are alternated with earlywood layers, often combine to confound the measures of the critical values of strain energy release rate, Gc. Adhesive layer thickness variations were observed to be substantial also in specimens prepared with power-planed wood boards and affect the value of Gc of the specimens. The grain orientation of latewood and earlywood, materials that often have different densities and elastic moduli, limits the accuracy of traditional standard methods for the evaluation of Gc. The traditional methods, described in the standards ASTM D3433-99 and BS 7991:2001, were originally developed for uniform and isotropic materials but are widely used by researchers also for bonded wood, where they tend to confound stiffness variations with Gc variations. Experimental analysis and analytical computations were developed for quantifying the spread of Gc data that is expected to be caused by variability of the adhesive layer thickness and by the variability of the bending stiffness along wooden beams. / Ph. D.

An experimental technique for conducting low speed impact of adhesively bonded automotive composite joints is presented. Based on the use of a modified drop tower, mode I, II, and mixed mode values for critical energy release rate were determined for a composite/epoxy system and used to create a fracture failure envelope. Because load measurements become erratic and unreliable at higher test rates, displacement-based relationships were used to quantify these energy release rates. Displacement data was collected with an imaging system that utilized edge detection to determine displacement profiles, end displacements, and opening displacements where applicable. Because of the resolution of the image-based approach used, determining crack length experimentally was extremely difficult. As a result, numerical methods were developed to objectively determine the crack length based on the available experimental data in mode I, II, and mixed mode I/II configurations. This numerical method uses a nonlinear fit to determine mode I crack lengths and a theoretical model based on cubic equations for mode II and mixed-mode I/II, where the coefficients of the equations are determined by using both boundary and transition conditions that are a result of the test setup. A double cantilever beam (DCB) geometry was chosen to collect mode I data, an end-loaded split (ELS) geometry was used for mode II, and a single leg bend (SLB) geometry was used for mixed-mode I/II. These geometries were used to determine the fracture characteristics of adhesively bonded automotive composites to create fracture failure envelopes as well as provide mode I, II, and mixed-mode I/II data to be used in finite element models. The chosen adhesive exhibited unstable, stick-slip crack growth, which resulted in very few data points being collected from each static DCB specimen as well as drastic drops in energy release rate between initiation and arrest points. Unstable growth also created issues in dynamic testing, as data points surrounding these "stick-slip" events were lost due to the insufficient sampling rate of the available imaging system. Issues also arose with differences between thick and thin composite adherend specimens. These differences could result from additional curing in thick adherend composite specimens due to the adherends retaining heat. DSC testing was conducted on uncured adhesive using a 2, 5, and 10 minute hold at the cure temperature, and significant additional curing was observed between the two and five minute cures. Due to the difference in relative stiffness between the 12 and 36 ply composite, the local loading rate at the crack tip was lower in the 12 ply adherends, possibly allowing for a larger plastic zone and thus a higher energy release rate. As a result, tests were conducted on 36 ply composite specimens at rates of 1 mm/min and 0.1 mm/min to determine if there were loading rate effects. This testing showed that higher initiation energy relase rates were found at the lower test rate, thus reinforcing the local loading rate theory. Due to issues with plastic deformation in aluminum adherends, mode II and mixed-mode I/II data were collected using only composite adherends. Only one data point was collected per specimen as the crack propagated directly into the composite after initiating from the precrack, thus multiple tests were conducted to collect sufficient data for constructing a failure envelope. Once mode I, II and mixed-mode I/II fracture data was collected, a fracture failure envelope was created. This failure envelope, combined with a predetermined factor of safety, could provide some of the necessary tools for design with this adhesive/composite system. / Master of Science

Many practical cases of fracture can be considered as one-dimensional, that is, propagating in one dimension and characterised by opening (mode I) and shearing (mode II) action only with no tearing (mode III) action. A double cantilever beam (DCB) represents the most fundamental one-dimensional fracture problem. There has however been considerable confusion in calculating its mixed-mode energy release rate (ERR) partition. In this work, new and completely analytical mixed-mode partition theories are developed for one-dimensional fractures in isotropic homogeneous and laminated composite DCBs, based on linear elastic fracture mechanics (LEFM) and using the Euler and Timoshenko beam theories. They are extended to isotropic homogeneous and laminated composite straight beam structures and isotropic homogeneous plates based on the Kirchhoff-Love and Mindlin-Reissner plate theories. They are also extended to non-rigid elastic interfaces for isotropic homogeneous DCBs. A new approach is used, based on orthogonal pure fracture modes. Two sets of orthogonal pairs of pure modes are found. They are distinct from each other in the present Euler beam and Kirchhoff-Love plate partition theories and coincide on the first set in the present Timoshenko beam and Mindlin-Reissner plate partition theories. After the two sets of pure modes are shown to be unique and orthogonal, they are used to partition mixed modes. Interaction is found between the mode I and mode II modes of the first set in the present Euler beam and Kirchhoff-Love plate partition theories. This alters the ERR partition but does not affect the total ERR. There is no interaction in the present Timoshenko beam or Mindlin-Reissner plate partition theories. The theories distinguish between local and global ERR partitions. Local pureness is defined with respect to the crack tip. Global pureness is defined with respect to the entire region mechanically affected by the crack. It is shown that the global ERR partition using any of the present partition theories or two-dimensional elasticity is given by the present Euler beam or Kirchhoff-Love plate partition theories. The present partition theories are extensively validated using the finite element method (FEM). The present beam and plate partition theories are in excellent agreement with results from the corresponding FEM simulations. Approximate 'averaged partition rules' are also established, based on the average of the two present beam or plate partition theories. They give close approximations to the partitions from two-dimensional elasticity. The propagation of mixed-mode interlaminar fractures in laminated composite beams is investigated using experimental results from the literature and various partition theories. The present Euler beam partition theory offers the best and most simple explanation for all the experimental observations. It is in excellent agreement with the linear failure locus and is significantly closer than other partition theories. It is concluded that its excellent performance is either due to the failure of materials generally being based on global partitions or due to the through-thickness shear effect being negligibly small for the specimens tested. The present partition theories provide an excellent tool for studying interfacial fracture and delamination. They are readily applicable to a wide-range of engineering structures and will be a valuable analytical tool for many practical applications.