Changes in load characteristics, deterioration with age, environmental influences and random actions may cause local or global damage in structures, especially in bridges, which are designed for long life spans. Continuous health monitoring of structures will enable the early identification of distress and allow appropriate retrofitting in order to avoid failure or collapse of the structures. In recent times, structural health monitoring (SHM) has attracted much attention in both research and development. Local and global methods of damage assessment using the monitored information are an integral part of SHM techniques. In the local case, the assessment of the state of a structure is done either by direct visual inspection or using experimental techniques such as acoustic emission, ultrasonic, magnetic particle inspection, radiography and eddy current. A characteristic of all these techniques is that their application requires a prior localization of the damaged zones. The limitations of the local methodologies can be overcome by using vibration-based methods, which give a global damage assessment. The vibration-based damage detection methods use measured changes in dynamic characteristics to evaluate changes in physical properties that may indicate structural damage or degradation. The basic idea is that modal parameters (notably frequencies, mode shapes, and modal damping) are functions of the physical properties of the structure (mass, damping, and stiffness). Changes in the physical properties will therefore cause changes in the modal properties. Any reduction in structural stiffness and increase in damping in the structure may indicate structural damage. This research uses the variations in vibration parameters to develop a multi-criteria method for damage assessment. It incorporates the changes in natural frequencies, modal flexibility and modal strain energy to locate damage in the main load bearing elements in bridge structures such as beams, slabs and trusses and simple bridges involving these elements. Dynamic computer simulation techniques are used to develop and apply the multi-criteria procedure under different damage scenarios. The effectiveness of the procedure is demonstrated through numerical examples. Results show that the proposed method incorporating modal flexibility and modal strain energy changes is competent in damage assessment in the structures treated herein.

Coal is likely to remain an important energy source for the next several hundred years and hence advances in coal combustion technologies have major practical impact. Detonation combustion of coal initiated by a plasma cartridge driven detonation wave holds promise for improving both system and combustion efficiencies. Both fragmentation and chemical kinetic pathways are qualitatively different in comparison to conventional coal combustion. The present work is a theoretical investigation of fragmentation due to detonation wave. The theoretical simulation starts with simple model and progressively incorporates more realistic analysis such as combined convective and radiative boundary condition. It studies the passing of detonation wave on coal particles suspended in air. Concepts of solid mechanics are used in analysing fragmentation of coal particles. A numerical model is developed which includes stress developed due to both thermal and volatilization effects. Weibull statistical analysis is used to predict the fracture time and fracture location resulting from principal stress induced. It is observed that coal particles fragment within microseconds. Radiation does not have much effect on developed stress. Volatilization does not have much effect on fragmentation for the particle size considered in this work and stress due to thermal effect dominated the fragmentation. Coal size distribution statistics is considered to obtain real regime. Coal is used as mixture of different sized particles in real combustors. Hence it is important to analyse the effect of detonation wave on mixture of coal particles. Results presented in this work from simulation run suggest that plasma assisted detonation initiated technology can fragment coal particles faster. Average fracture time of mixture of coal particles is far less than detonation travel time for the detonation tube considered here. Simulation results suggest that almost 90% of coal particles fragment early. Average fracture time reduces as Mach number increases. Same phenomena can be observed for volatile matter generated at fracture and ow of volatile matter at fracture. Hence it can be concluded that plasma assisted detonation combustion leads to different volatilization and fragmentation pathways.

Standard crystalline silicon photovoltaic (PV) modules are designed to continuously convert solar energy into electricity for 25 years. However, the continual generation of electricity by the PV modules throughout their designed service life has been a concern. The key challenge has been the untimely fatigue failure of solder interconnections of solar cells in the modules due to accelerated thermo-mechanical degradation. The goal of this research is to provide adequate information for proper design of solar cell solder joint against fatigue failure through the study of cyclic thermo-mechanical stresses and strains in the joint. This is carried-out through finite element analysis (FEA) using ANSYS software to develop the solar cell assembly geometric models followed by simulations. Appropriate material constitutive model for solder alloy is employed to predict number of cycles to failure of solder joint, hence predicting its fatigue life. The results obtained from this study indicate that intermetallic compound thickness (TIMC); solder joint thickness (TSJ) and width (WSJ) have significant impacts on fatigue life of solder joint. The impacts of TIMC and TSJ are such that as the thicknesses increases solder joint fatigue life decreases. Conversely, as solder joint width (WSJ) increases, fatigue life increases. Furthermore, optimization of the joint is carried-out towards thermo-mechanical reliability improvement. Analysis of results shows the design with optimal parameter setting to be: TIMC -2.5μm, TSJ -20μm and WSJ -1000μm. In addition, the optimized model has 16,264 cycles to failure which is 18.82% more than the expected 13,688 cycles to failure of a PV module designed to last for 25 years.

The effects of a composite overwrapped gun barrel with viscoelastic damping layers are investigated. Interlaminar stresses and constrained layer damping effects are described. The Modal Strain Energy method is developed for measuring the extent to which the barrel is damped. The equations of motion used in the finite element analysis are derived. The transient solution process is outlined. Decisions for selected parameters are discussed. The results of the finite element analyses are presented using the program written in FORTRAN. The static solution is solved with a constant internal pressure resulting in a calculated loss factor from the Modal Strain Energy Method. The transient solution is solved using the Newmark-Beta method and a variable internal pressure. The analyses conclude that strategically placed viscoelastic layers dissipate strain energy more effectively than a thick single viscoelastic layer. The optimal angle for maximizing the coefficient of mutual influence in a composite cylinder is not necessarily the optimal angle when viscoelastic layers are introduced between layers.

Tato práce je zaměřena na problematiku výpočtového a experimentálního modelování deformačně napjatostních stavů měkkých tkání se zaměřením na riziko ruptury u výdutě břišní aorty (AAA). V první části (kap. 1) je stručně nastíněn současný stav dané problematiky. Tato část shrnuje důležité poznatky publikované v dostupné literatuře. Pozornost je věnována zejména klíčovým faktorům pro stanovení rizika ruptury AAA. V další kapitole (kap. 2) je stručně popsána histologie cévní stěny a její výsledné mechanické chování, jakož i její patologie, především AAA. Druhá část práce (kap.3) je věnována experimentálnímu vyhodnocování deformačně napjatostního chování měkkých tkání, které je nutným předpokladem k věrohodnému výpočtovému modelování tohoto chování. V této kapitole je stručně popsáno experimentální zařízení speciálně vyvinuté pro testování měkkých tkání a typy zkoušek, které lze na tomto zařízení provádět. Dále jsou shrnuty klíčové faktory ovlivňující deformačně napjatostní chování měkkých tkání a experimentální ověření těchto faktorů na vzorcích z prasečích hrudních aort. V závěru této kapitoly jsou shrnuty nové poznatky vyplývající z experimentálního testování. Třetí část disertační práce (kap.4) je zaměřena na matematický popis deformačně napjatostního chování měkkých tkání, stručný popis používaných konstitutivních vztahu a postup při identifikaci parametrů pro tyto konstitutivní modely určované na základě provedených experimentálních zkoušek. Poslední část disertační práce (kap.5) je věnována výpočtovému modelování deformačně napjatostního chování AAA. V této kapitole jsou nejdříve shrnuty klíčové faktory a předpoklady pro vytváření modelů a pro vyhodnocování výsledku a dále jsou uvedeny materiálové parametry pro konstitutivní modely implementované do programu ANSYS. Byly provedeny testovací výpočty při použití hypotetické zjednodušené geometrie AAA, na kterých byly vyhodnoceny vlivy změny geometrie a vliv změny konsitutivního modelu na extrémní napětí ve stěně AAA. U reálné geometrie AAA byla navržena a otestována metoda výpočtu nezatížené geometrie z reálných CT snímků. Dále byl testován vliv zvýšení vnitřního tlaku jako rizika ruptury AAA. V závěru práce jsou shrnuty poznatky a možnosti výpočtového modelování a návrhy na další práce.

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.

Polymer electrolyte membrane (PEM) fuel cell (FC) stacks require elastomeric gaskets for each cell to keep the reactant gases within their respective regions[1]. If any gasket degrades or fails, the reactant gases can leak or mix with each other directly during operation or standby, affecting the overall operation and performance of the FC. The elastomeric gaskets used as FC seals are exposed to a range of environmental conditions, and concurrently, subjected to mechanical compression between the bipolar plates forming the cell. The combination of mechanical stress and environmental exposure may result in degradation of the seal material[2] over a period of time. In order to address the durability and make reliability predictions, the long-term stability of the gaskets in FC assemblies is critical. The aim of this study is to investigate the performance of elastomeric seals in a simulated FC environment in the presence of mechanical stresses. The overall scope of the study includes mechanical and viscoelastic properties characterization, and lifetime durability predictions based on an accelerated characterization approach. With the help of finite element analysis software, ABAQUS, a fixture was designed to perform strain-based accelerated characterization of seal material in air, deionized (DI) water, 50v/50v ethylene glycol/water solution, and 0.1M sulfuric acid solution. Dogbone samples were strained to different levels in the custom fixture and submerged in liquid solutions at 90°C and in air at 90°C and 120°C. It was observed that mechanical properties such as tensile strength, strain to break, 100% modulus, crosslink density, and tensile set degrade due to aging and the extent of change (increase or decrease) depends significantly on the strain level on the specimen. Trouser tear tests were conducted on reinforced specimens in air and deionized water (DI) to evaluate the tear resistance of an elastomeric seal material intended for proton exchange membrane fuel cells. Plots relating the crack growth rate with tearing energy were obtained at various temperatures and provided significant insight into the rate and temperature dependence of the tearing strength of the seal material. Stick-slip crack propagation was observed at all temperatures and loading rates, although the behavior was suppressed significantly at low loading rates and high temperatures. Crack growth rate versus tearing energy data at different temperatures was shifted to construct a master curve and an estimate on the threshold value of tear energy was obtained which may be helpful in designing components where material tear is of concern. Strain energy release rate (SERR) value, calculated using the J-integral approach for a pre-existing crack in ABAQUS, was used to estimate the crack growth rate in a given seal cross-section to predict lifetime. In order to assess the viscoelastic behavior and to investigate the long term stress relaxation behavior of the seal material, compression stress relaxation (CSR) tests were performed on molded seals, called as SMORS, over a range of environmental conditions using a custom-designed fixture. The effect of temperature and environment was evident on material property changes and presented in terms of momentary properties and stress relaxation behavior. Various mechanisms involved in material degradation, chain scission and crosslinking, were suggested and insights were gained into how cure state and level of antidegradants in a material dictate the material behavior during the first phase of environmental exposure leading to change in material properties. Ring samples made of silicone were also tested using the fixture to obtain insight additional into material degradation due to aging. Results presented from testing on SMORS showed a lot more variation in data as compared to neat silicone rings due to the complexity involved in making SMORS. For understanding the deformation behavior of an elastomeric seal and its sealing performance, finite element characterization of seal cross-section was carried out on O-ring and SMORS cross-section. The effect of a seal's layout on distribution and magnitude of contact stresses and contact width was investigated for the O-ring and the information obtained thereby helped to analyze a complex assembly such as SMORS, where several interfaces and boundary conditions are involved. Stress/strain profiles were generated to visualize their concentration and distribution in the seal cross-section. Frictionless and rough interfacial conditions between seal material and platens were assumed and it was found that its effect on contact width and peak contact pressure was insignificant. Results obtained from FEA on SMORS were validated through comparison with contact mechanics approach and experimental data and it was found that Lindley's equation correlates well with experimental data whereas ABAQUS overestimates the load values at a given compression. Lindley's approach may be used to develop contact pressure profiles that may help estimate peak contact pressure at a given time so leaking can be avoided. / Ph. D.

Six poly(ether ketone ketone)/carbon composite panels were manufactured from powder coated towpreg. All six panels were initially processed using a hot press equipped with controlled cooling. Four of the panels were used to investigate the effect of cooling rate on crystallinity. A fifth panel was used to investigate the effect of annealing the composite after completion of the standard fabrication process. The sixth panel was used to investigate changes in toughness due to manufacturing towpreg with polymer that had been reclaimed from the towpreg fabrication system’s air cleaner. Cooling rates of 2°C/min, 4°C/min, 6°C/min, and 8°C/min resulted in composites with crystallinities of 33%, 27%, 24%, and 23%, respectively. The principal investigation of the effect of cooling rate on crystallinity and mode I and mode II strain energy release rates, G_Ic and G_IIc, respectively, showed that G_Ic and G_IIc values increase with increasing cooling rate. Comparison of the toughness values as a function of crystallinity showed that the dependence of toughness on crystallinity is approximately equivalent to the dependence of toughness on cooling rate. Comparison of the data from the annealed panel to that from the analogous principal panel showed that annealing increased the crystallinity and decreased the mode I strain energy release rate. There was no effect, however, on the mode II strain energy release rate. Comparison of the data from the panel made with reclaimed polymer to that from its analogous principal panel showed that the reclaimed polymer panel had equivalent crystallinity and G_Ic values. On the other hand, the G_IIc values in this panel were lower than in the analogous principal panel. / Master of Science

Noise and vibration suppression is an important aspect in the design process of structures and machines. Undesirable vibrations can cause fatigue in a structure and are, therefore, a risk to the safety of a structure. One of the most effective and widely used methods of mitigating these unwanted vibrations from a system is passive damping, by using a viscoelastic material. This dissertation will primarily focus on constrained layer passive damping treatments in structures and the investigation of associated complex modes. The key idea behind constrained damping treatment is to increase damping as affected by the presence of a highly damped core layer vibrating mainly in shear. Our main goal was to incorporate viscoelastic material in a thick stiffened panel with plate-strip stiffeners, to enhance the damping characteristics of the structure. First, we investigated complex damped modes in beams in the presence of a viscoelastic layer sandwiched between two elastic layers. The problem was solved using two approaches, (1) Rayleigh beam theory and analyzed using the principle of virtual work, and (2) by using 2D plane stress elasticity based finite-element method. The damping in the viscoelastic material was modeled using the complex modulus approach. We used FEM without any kinematic assumptions for the transverse shear in both the core and elastic layers. Moreover, numerical examples were studied, by including complex modulus in the base and constraining layers. The loss factor was calculated by modal strain energy method, and by solving a complex eigenvalue problem. The efficiency of the modal strain energy method was tested for different loss factors in the core layer. Complex mode shapes of the beam were also examined in the study, and a comparison was made between viscoelastically damped and non-proportionally damped structures. Secondly, we studied the free vibration response of an integrally stiffened and/or stepped plate. The stiffeners used here were plate-strip stiffeners, unlike the rib stiffeners often investigated by researchers. Both plate and stiffeners were analyzed using the first-order shear deformation theory. The deflections and rotations were assumed as a product of Timoshenko beam functions, chosen appropriately according to the given boundary conditions. Unlike Navier and Levy solution techniques, the approach used here can also be applied to fully clamped, free and cantilever supported stiffened plates. The governing differential equations were solved using the Rayleigh-Ritz method. The development of the stiffness and the mass matrices in the Ritz analysis was found to consume a huge amount of CPU time due to the recursive integration of Timoshenko beam functions. An approach is suggested to greatly decrease this amount of CPU time, by replacing the recursive integration in a loop structure in the computer program, with the analytical integration of the integrand in the loop. The numerical results were compared with the exact solutions available in the literature and the commercially available finite-element software ABAQUS. Some parametric studies were carried out to show the influence of certain important parameters on the overall natural frequencies of the stiffened plate. Finally, we investigated the damped response of an adhesively bonded plate employing plate-strip stiffeners, using FSDT for both the plate and stiffeners. The problem was analyzed using the principle of virtual work. At first, we did not consider damping in the adhesive in order to validate our code, by comparing our results with those available in the literature as well as with the results obtained using ABAQUS 3D model. The results were found to be highly satisfactory. We also considered the effect of changing the stiffness of the adhesive layer on the vibration of the bonded system. As a second step, we included damping in the stiffened structure using complex modulus approach, a widely used technique to represent the rheology of the viscoelastic material. We observed an overall increase in the natural frequencies of the system, due to the damping provided by the viscoelastic material. Moreover, it was noticed that when the thickness of the adhesive layer is increased, the natural frequencies and loss factor of the stiffened structure decrease. A viscoelastic material with high loss factor and small thickness will be a perfect design variable to obtain overall high damping in the structure. / Ph. D.

The target of the ‘Green Overlays’ research was a cost effective, minimal disruption, sustainable and environmentally friendly alternative to the wholesale demolition, removal and complete reconstruction of the existing structural concrete pavement. The important problem of flexural resistance for strengthening concrete pavements with structural overlays has been scrutinised. A new mix design method for steel fibre reinforced, roller compacted, polymer modified, bonded concrete overlay has been proposed. The mixes developed were characterized of high flexural strength and high bond strength with the old concrete substrate. ‘Placeability’ and ‘compactability’ of the mix were two dominant issues during laboratory investigation. An innovative approach for establishing the relationship between Stress and Crack Face Opening Displacement for steel fibre reinforced concrete beams under flexure was developed. In addition, a new and simple method for calculating the interfacial Strain Energy Release Rate of both, a two-dimensional specimen and a three-dimensional model of the overlay pavement system were developed. This method can be readily and easily used by practicing engineers. Finally, a new test specimen and its loading configuration for measuring interfacial fracture toughness for concrete overlay pavements were established. The interfacial fracture toughness of a composite concrete beam, consisted of steel fibre-reinforced roller compacted polymer modified concrete bonded on conventional concrete and undergoing flexure, was assessed. In summary, this thesis presents four key findings: A new mix design method for steel fibre-reinforced roller compacted polymer modified concrete bonded on conventional concrete. A new method for establishing the fibre bridging law by an inverse analysis approach. A new, simplified method for calculating strain energy release rate at the interface of a composite beam. A new, innovative technique for calculating strain energy release rate at the interface of an overlaid pavement. The thesis contains a plethora of graphs, data-tables, examples and formulae, suitable for future researchers.