Within the European space programme, a new upper stage engine (Vinci) for the Ariane 5 launcher is being developed, and the Volvo Aero Corporation (VAC) is contributing with tur-bines for the fuel turbopumps. This MSc thesis investigates the possibility of designing the Turbine Exhaust Duct (TED) of the Vinci-engine in a carbon fibre composite material with adhesively attached titanium flanges. The focus of the project has been on stress analyses of the adhesive joints using Finite Element Methods (FEM), more specifically by using a cohe-sive zone material (CZM) to model the adhesive layer. Analysing adhesive joints is complex and an important part of the work has been to develop and concretise analysis methods for future use within VAC. To obtain the specialised material parameters needed for a CZM analysis, FE-models of ten-sile test specimens were analysed and the results compared to those of equivalent experimen-tal tensile tests. These parameters were then used when analysing the TED geometry with load cases specified to simulate the actual operation conditions of the Vinci engine. Both two-dimensional axisymmetric and fully three-dimensional models were analysed and, addition-ally, a study was performed to evaluate the effect of cryogenic temperatures on the strength of the joint. The results show that the applied thermal and structural loading causes local stress concentra-tions on the adhesive surface, but the stresses are not high enough to cause damage to the joint if a suitable joint design is used. Cryogenic temperatures (-150 °C) caused a significant strength reduction in the tensile specimens, partially through altered adhesive properties, but no such severe effects were seen in the temperature-dependent FE-analyses of the TED. It should be pointed out however, that some uncertainties about the material parameters exist, since these were obtained in a rather unconventional way. There are also several other impor-tant questions, beside the strength of the adhesive joint, that need to be answered before a metal-composite TED can be realised. / Volvo Aero deltar i utvecklingen av Vinci, en ny motor till det övre steget i den europeiska Ariane 5-raketen. Detta examensarbete behandlar möjligheten att tillverka ett turbinutlopp (TED) till vätgasturbinen i Vinci-motorn i kompositmaterial med flänsar i titan för att på så sätt uppnå en viktbesparing gentemot den tidigare konstruktionen i gjuten Inconel 718. Fokus har legat på att analysera hållfastheten i de limfogar som är tänkta att sammanfoga huvudröret med flänsarna, genom analyser med finita elementmetoden (FEM). Ett viktigt syfte har även varit att, för Volvo Aeros räkning, samla praktiska erfarenheter angående numerisk analys av limfogar, särskilt med användning av kohesiva zon-element för att modellera limfogen. FEM-analyser har gjorts av provstavsmodeller, där resultaten sedan jämförts med experimen-tella dragprovsresultat för att ta fram lämpliga material- och modelleringsparametrar för ana-lys med kohesiva zonelement. Därefter tillämpades dessa parametrar i analyser av den verkli-ga TED-geometrin med relevanta lastfall framtagna för att simulera driftsförhållandena i Vin-ci-motorn. Lastfallsanalyser med både tvådimensionellt axisymmetriska och tredimensionella geometrimodeller genomfördes, liksom uppskattningar av limfogens styrka vid kryogena driftstemperaturer. Resultaten pekar entydigt mot att en limfog med en ändamålsenlig tvärsnittsgeometri skulle hålla för de angivna lasterna utan att ta skada. De spänningskoncentrationer som uppstår ger lokalt höga spänningar i limmet, men inte på nivåer som skulle kunna orsaka brott. Det finns dock en viss osäkerhet angående riktigheten i materialparametrarna då en något okonventio-nell metod användes för att ta fram dessa. Flera stora frågor finns fortfarande kvar att besvara innan en metall-komposit konstruktion kan realiseras, inte minst hur flödeskammarens kom-plicerade geometri skall kunna tillverkas i kompositmaterial.

<p>An advanced bulk constitutive model is used with a new cohesive zone model that is stress state and rate-dependent in order to simulate the ductile to brittle failure transition in polycarbonate. The cohesive zone model is motivated by experimental evidence that two different critical energies per unit area of crack growth exist in glassy polymers. A higher energy state is associated with ductile failure (slow crack growth), while a lower energy state is associated with brittle failure (fast crack growth). The model is formulated so that as rate or stress state changes within a simulation, the fracture energy and thus fracture mode may also change appropriately. The ductile to brittle transition occurs when the cohesive opening rate is over a threshold opening rate and when the stress state is close to plane strain in a fracture specimen. These effects are coupled. The principal contribution of this work is that this is the first time a single set of material input parameters can predict the transition from slow to fast crack growth as test loading rate and sample thickness are varied. This result enlisted the use of an advanced constitutive model and the new cohesive zone model with rate and stress-state dependencies in three-dimensional finite element analysis.</p> / Dissertation

In wireless packet-forwarding networks, the nodes or users are always selfish to maximize their utilities in nature. Selfish users would not like to help others for forward each others¡¦ packets, which will cause the network performance degrades severely. To solve the packet-forwarding problem, we propose a novel coalitional game approach based orthogonal decode-and-forward (ODF) relaying scheme to encourage the selfish users for cooperation. In the game-theoretic analysis, we study the properties and stability of the coalitions thoroughly. Furthermore, we prove that the cohesive behavior can be obtained by the aspect of outage probability indeed in this game. Simulation results show that the proposed ODF coalitional game can enforce cooperation exactly and it is always beneficial to form the cooperative groups for all users.

This study aimed at discovering the role of lexical cohesive links in L2 reading comprehension. For this purpose, the researcher carried out lexical cohesion analysis of two TOEFL reading tests consisting of six texts. First, prior to the reading comprehension tests, the students were administered &ldquo / a vocabulary familiarity task&rdquo / . Second, the TOEFL reading tests were administered to fifty upper-intermediate and advanced level EFL students at Middle East Technical University, Ankara, Turkey. Third, after each test, a post-reading &ldquo / Lexical links recognition task&rdquo / was given. Finally, the data were analyzed comparing reader performance on each item, both within and across the groups. The researcher analyzed the reading test results besides the results of the accompanying lexical cohesive links tasks to see if there is a &ldquo / significant relationship&rdquo / between the three factors: vocabulary knowledge, reading comprehension level and recognition of lexical cohesive links. The results have indicated that awareness of lexical cohesive links noticeably contributes to reading test scores in L2 and that recognition-level vocabulary knowledge alone may not guarantee better reading comprehension scores. Finally, it is suggested that reading and writing teachers can develop some pedagogic exercises to teach lexical cohesive devices and in this way improve students&rsquo / knowledge of lexical cohesive sub-types, thus enhancing their reading performance.

Enhancing the effective peel resistance of plastically deforming adhesive joints through laser-based surface micro-machining Edwin Hernandez Diaz Inspired by adhesion examples commonly found in nature, we reached out to examine the effect of different kinds of heterogeneous surface properties that may replicate this behavior and the mechanisms at work. In order to do this, we used pulsed laser ablation on copper substrates (CuZn40) aiming to increase adhesion for bonding. A Yb-fiber laser was used for surface preparation of the substrates, which were probed with a Scanning Electron Microscope (SEM) and X-ray Photoelectron Spectroscopy (XPS). Heterogeneous surface properties were devised through the use of simplified laser micromachined patterns which may induce sequential events of crack arrest propagation, thereby having a leveraging effect on dissipation. The me- chanical performance of copper/epoxy joints with homogeneous and heterogeneous laser micromachined interfaces was then analyzed using the T-peel test. Fractured surfaces were analyzed using SEM to resolve the mechanism of failure and adhesive penetration within induced surface asperities from the treatment. Results confirm positive modifications of the surface morphology and chemistry from laser ablation that enable mechanical interlocking and cohesive failure within the adhesive layer. Remarkable improvements of apparent peel energy, bond toughness, and effective peel force were appreciated with respect to sanded substrates as control samples.

abstract: Cohesive zone model is one of the most widely used model for fracture analysis, but still remains open ended field for research. The earlier works using the cohesive zone model and Extended finite element analysis (XFEM) have been briefly introduced followed by an elaborate elucidation of the same concepts. Cohesive zone model in conjugation with XFEM is used for analysis in static condition in order to check its applicability in failure analysis. A real time setup of pipeline failure due to impingement is analyzed along with a detailed parametric study to understand the influence of the prominent design variable. After verifying its good applicability, a creep model is built for analysis where the cohesive zone model with XFEM is used for a time dependent creep loading. The challenge in this simulation was to achieve coupled behavior of cracks initiation and propagation along with creep loading. By using Design of Experiment, the results from numerical simulation were used to build an equation for life prediction for creep loading condition. The work was further extended to account for fatigue damage accumulation for high cycle fatigue loading in cohesive elements. A model was conceived to account for damage due to fatigue loading along within cohesive zone model for cohesive elements in ABAQUS simulation software. The model was verified by comparing numerical modelling of Double cantilever beam under high cycle fatigue loading and experiment results from literature. The model was also applied to a major industrial problem of blistering in Cured-In-Plane liner pipelines and a demonstration of its failure is shown. In conclusion, various models built on cohesive zone to address static and time dependent loading with real time scenarios and future scope of work in this field is discussed. / Dissertation/Thesis / Masters Thesis Mechanical Engineering 2016

As the use of fiber-reinforced composites has increased over the decades, so has the need to understand the complexity of their failure mechanisms as engineers seek to improve the damage tolerance of composite laminated structures. One of the most prevalent and limiting mode of failure within composite laminates is delamination, since it not only reduces a structures stiffness and strength, but can be very difficult to detect without the use of special non-destructive equipment. Industry testing organizations have utilized several fracture tests in order to characterize the fracture toughness of composite materials under different loading conditions. For this research, ASTM D5528, ASTM D7905 & 4ENF tests were performed to evaluate the fracture resistance of uni-directional pre-preg laminates; the 4ENF was used to compare its effectiveness as to ASTM D7905. Finite element methods such as the use of cohesive elements have been developed to simulate delamination within composite laminates. While there has been much work in evaluating the effectiveness of cohesive elements, very little exists within literature as to studying the success of cohesive surface contact for accurately modeling coupon level fracture testing. Cohesive contact interaction in Abaqus/Standard was used to simulate the mode I double cantilever beam (DCB) experiment of ASTM D5528. Cohesive contact was found to accurately and efficiently model DCB testing as the critical load- displacement values and steady state fracture agreed with experimental data. A parametric study was performed and found that cohesive contact was less sensitive in varying key model parameters than that commonly expected of cohesive elements.

Listeria monocytogenes is a major concern for the food industry. It is one of the major agents causing listeriosis. The objective of the first part of this study is to evaluate the effect of hydration level on attached listeria monocytogenes on stainless steel/High density polyethylene transferred to food products. Attached cells were prepared on stainless steel/High density polyethylene. Transfer experiments were conducted from inoculated surface material slides to cold smoked salmon fillets. This experiment was repeated 6 times. The results were analyzed with an analysis of variance by SAS. The differences between the different RH% and surface conditions were not statistically significant. There was variability in between packages, brands and over the course of storage after opening, and likely contributed to the variability of transfer observed in this set of experiments. The objective of the second part of the research is to study the effect of hydration level on the detachment of Listeria monocytogenes biofilm growing on stainless steel by using Atomic force microscope. Biofilms were grown on stainless steel in drip flow bioreactor at 32 °C for 72 h. Then biofilms were equilibrated over saturated salt solution at 20 °C for 48 h before the Atomic Force Microscope experiment. The results showed that cohesive energy value of the biofilm increased with biofilm depth. Only square shaped displaced 2.5X2.5 μm region were visualized after serious of raster scanning under high load which means that moisture condition of Listeria monocytogenes biofilm can significantly affect the cohesiveness between of Listeria monocytogenes biofilm.

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