This thesis presents the nonlinear in-plane behaviour of circular fixed arches subjected to thermal loading only. Due to the nonlinear prebuckling behaviour of arches and its effects, classical buckling theory which is founded on geometric prebuckling linearity can not predict the in-plane buckling of arches accurately. Based on a nonlinear formulation of the strain and displacement relationship and considering constant thermal distributions only, virtual work formulations are used to establish the differential equations of in-plane equilibrium and the statical boundary conditions, from which the nonlinear equilibrium equations are derived in closed form and which are suitable to use in design. By considering the adjacent buckled configurations, the differential buckling equilibrium equations are formulated from the principle of virtual work as well, and the analytical solutions for the nonlinear buckling of fixed arches are obtained. It is shown that nonlinear elastic buckling of a fixed in the plane of it curvature can not occur when it is subjected to thermal loading only, except if the arch is as a straight column. By using the algebraic representation of nonlinear in-plane equilibrium derived in this thesis, the elastic response of fixed arches at elevated temperatures and the attainment of first yield are examined in detail. The arch deflects transversely without bound in the elastic range at elevated temperatures, whereas it will yield first at the top extreme fibre of the cross section at the supports when a critical temperature is reached. The influence of several parameters such as the included angle is also considered. Based on the models of stress distributions at cross sections, the spread of yield both through the cross section and along the length of the arch is studied. It is indicated that the progress of yielding causes the first two hinges to form at the supports of the fixed arches, and then moment redistribution leads to the generation of the third hinge at the crown with an increase of temperature. Thus nonlinear plastic hinge analysis can be applied to the arch analysis under thermal loading.

Metals in industrial settings such as power plants are often subjected to high temperature and pressure aqueous environments, where failure to control corrosion compromises worker and environment safety. For instance, zircaloy (1.2-1.7 wt.% Sn) fuel rods are exposed to aqueous 250-310 oC coolant in CANDU reactors. The Pourbaix (EH-pH) diagram is a plot of electrochemical potential versus pH, which shows the domains of various metal species and by inference, corrosion susceptibility. Elevated temperature data for tin +II and tin +IV species were obtained using solid-aqueous phase equilibria with the respective oxides, in a batch vessel with in-situ pH measurement. Solubilities, determined via spectroscopic techniques, were used to calculate equilibrium constants and the Gibbs energies of Sn complexes for E-pH diagram construction. The SnOH3+ and Sn(OH) species were incorporated, for the first time, into the 298.15 K and 358.15 K diagrams, with novel G ͦ values determined at 358.15 K. / UOIT

The fatigue response of mechanical properties and life due to constant stress amplitude tension-tension(T-T)cyclic loading at elevated temperature in nanocomposite APC-2 laminates was investigated. From the basic testing the total amount of 1% by weight of SiO2 spreaded in the interfaces was proved optimally. Related experiments on unidirectional nanocomposite APC-2 laminates were completed, including static tension tests in [0]16¡B[30]16¡B[45]16¡B[60]16 and [90]16 and T-T cyclic tests in [0]16¡B[45]16 and [90]16 specimens at room temperature. After obtaining experimental data, such as ultimate strength and elastic modulus, which were found improved significantly, and then comparing with the basic theory of mechanics of composites, rule of mixtures was adopted to estimate the properties of cross-ply and quasi-isotropic nanocomposite APC-2 laminates and found the largest errors were within 25%. In the consideration of heterogeneous and anisotropic properties of the matrix and the reinforced fibers in nature, the results are reasonably acceptable. On the other hand, the S-N curves according to the experimental data of the fatigue tests were plotted. The vertical axis shows the nondimensional stress level, i.e., the applied maximum stress normalized by ultimate strength at room temperature, and the horizontal axis represents the logarithm of applied cycles. The S-N curves at room and elevated temperatures were also expressed by curve fitting from top to bottom as temperature increasing from RT to 150¢J for both cross-ply and quasi-isotropic nanocomposite laminates. However, when applied maximum stress was normalized by the corresponding ultimate strength, the positions of S-N curves were reverse, i.e., the curves were shown from bottom to top as temperature increasing from RT to 150¢J. That strongly hints us the resistance to fatigue at elevated temperature in both lay-ups of nanocomposite laminates is indeed significantly improved.

Because magnesium alloy tube has good formability at elevated temperature, uniaxial tensile tests were employed to evaluate the warm properties of AZ31 magnesium alloy. After that, this paper has designed and manufactured a hydraulic bulge warm forming machine which can be used to experiment with fixed tube length. Using this testing machine, the experiments of hydraulic bulge forming of AZ31 magnesium alloy and 6061 aluminum alloy tube at elevated temperatures were carried out. According to experimental results, the hydraulic bulge forming properties of tubes at different temperatures are discussed.

This dissertation presents the results of experimental and computational investigations on the behavior of steel simple beam end framing connections subjected to fire. While significant progress has been made in understanding the overall structural response of steel buildings subject to fire, the behavior of connections under fire conditions is not well understood. Connections are critical elements for maintaining the integrity of a structure during a fire. Fire can cause large force and deformation demands on connections during both the heating and cooling stages, while reducing connection strength and stiffness. Of particular importance are simple beam end framing connections. These are the most common type of connection found in steel buildings and are used at beam-to-girder and girder-to-column connections in the gravity load resisting system of a building. This dissertation focuses on one particular type of beam end connection: the single plate connection, also known as a shear tab vii connection. This connection is very commonly used in U.S. building construction practice. In this study, material properties of ASTM A992 structural steel at elevated temperatures up to 900°C were investigated by steady state tension coupon tests. Experimental studies on the connection subassemblies at elevated temperatures were conducted to understand and characterize the connection strength and deformation capacities, and to validate predictions of connection capacity developed by computational and design models. In the computational studies, a three-dimensional finite element connection model was developed incorporating contact, geometric and material nonlinearity temperature dependent material properties. The accuracy and limitations of this model were evaluated by comparison with experimental data developed in this research as well as data available in the literature. The computational studies investigated the typical behavior of the connection during heating and cooling phases of fires as well as the connection force and deformation demands. The finite element model was further used to study and understand the effects of several key building design parameters and connection details. Based on the test and analysis results, some important finding and conclusions are drawn, and future work for simple shear connection performance in fire are discussed. / text

Cigas S. Determination of low cycle straining parameters for weld metals of stainless steel: Master thesis of mechanical engineer / research advisor associate professor dr. R. Šniuolis; Šiauliai University, Technological Faculty, Mechanical Engineering Department.–Šiauliai, 2005.-68p. Strain and stress change during the exploitation depend on the type of material (hardening, softening or cyclically stabile), that is chosen for the constructions in low cycle loading. If we know the type of the material, we can determine the possibility of its application in concrete exploitation conditions. Real working conditions of the most constructions are close to loading with limited strain (hard straining), because elastic and plastic deformation is met in the zones of crack and stress concentration, that are surrounded with elastically deformed material. Analytical dependences between stress and strain in any semicycle k are expressed by summarized low cyclic stress strain curve. The low cycle loading curves parameters A, and are used for the computation of this curve. These parameters were obtained from the low cycle straining results. The other possible ways for the determination of parameters A, , and statistical methods for the evaluation of these parameters for weld metals of stainless steel at room temperature are presented in this work. Cyclic characteristics A, and were determined by methods shown in this work. It was determined, that the values of cyclic strain and... [to full text]

Extreme events such as the Mont Blanc Tunnel fire in 1999 (Bettelini et al. 2001) or the Windsor Tower fire in 2005 (Calavera et al. 2005) have shown how concrete failure at elevated temperatures can be hazardous to the safety of members of the public. Generally, there is an absence of understanding of the mechanical behaviour of both plain and reinforced concrete at elevated temperatures, which is essential for computational modelling. Since fire is an extreme event, a certain amount of damage within the structure would be seen to be permissible within its performance objectives. This necessitates analysis in the post-peak regime. As a material, concrete has a very low value of thermal conductivity. This means that large thermal gradients often occur within concrete, causing differential expansion of the material. This, coupled with the change in mechanical properties at elevated temperatures, further complicates analytical analysis procedures. This study investigates issues associated with computational modelling of plain and reinforced concrete at elevated temperatures and its residual behaviour (behaviour when tested after the material has been heated, for example in a fire, and then cooled). In order to achieve this, first the constitutive material properties of both plain and reinforced concrete at ambient and elevated temperatures were investigated. The study showed that mesh sensitivity and localisation of strain softening occurs in plain concrete under both tensile and compressive loading. Path dependency of the stress-strain behaviour of plain concrete was also demonstrated, when it was subjected to loading and heating. Tension stiffening was included in the reinforced concrete material model, to represent the interaction between concrete and reinforcing steel. Complex behaviours were seen for simple reinforced concrete benchmark tests, due to changing material properties at elevated temperatures and differential thermal expansion of steel and concrete. Non-linear load-displacement relationships were seen as a result of complex load-sharing between concrete and reinforcement. A hypothesis was proposed – that variation of temperatures during heating and cooling of a specimen will cause damage, and hence material degradation, in plain and reinforced concrete. On investigation, it was seen that damage due to differential thermal expansion plays a small part in the reduction of elastic load-displacement slope and peak strength seen in experimental data on residual tests, indicating that other factors identified in previous research also affect the residual behaviour of plain and reinforced concrete. Indeed, in reinforced concrete, when tension stiffening was included, it was found that damage due to differential thermal expansion and contraction had a negligible effect on the residual response in the pre-peak regime. The study also found that for a simply supported beam pure thermal expansion caused a localised response, while pure thermal gradient gave distributed yield. When both were present, in this study, distributed yield with no mesh sensitivity was seen. Realistic heating of a restrained reinforced concrete plane strain model caused compressive stresses accompanied by tensile longitudinal total strains and tensile longitudinal plastic strains throughout the depth of the slab, with the largest values occurring near to the model supports. Damage and recovery variables were found to have no effect on the response of the model. When a portal frame was exposed to heating, plastic strains were distributed throughout the beam, with column rotation limiting downward thermal bowing due to a uniformly distributed load or thermal gradient present. Application of displacement loading causing plastic damage changed the behaviour of the structure under heating – instead of symmetrical compressive plastic strains being induced, areas of varying tensile and compressive strain were caused within the beam. Throughout, simple, easily reproducible simulations were used so that single parameters could be altered and considered. This was important, so that the important parameters to computational modelling could be identified. These can be used to guide experimental series to ensure that they are investigated, in order to improve computational material models. Not all variations of parameters were investigated in this study, but it is clear where further repetition would be beneficial (e.g. in varying thermal expansion and thermal gradient ratios in heating regimes). This study looks to address experimentalists and people working in structural analysis, who would be interested in the parameters investigated, as well as practitioners who may want to use these results.

Exposure of concrete structures to high temperatures leads to significant losses in mechanical and physical properties of concrete and steel reinforcement as well as the bond characteristics between them. Degradation of bond properties in fire may significantly influence the load capacity of concrete structures. Therefore the bond behaviours need to be considered for the structural fire engineering design of reinforced concrete structures. At present, the information about the material degradations of concrete and reinforcing steel bars at elevated temperatures are generally available. However, the research on the response of the bond characteristic between concrete and reinforcing steel bar at elevated temperatures is still limited. Due to the lack of robust models for considering the influence of the bond characteristics between the concrete and steel bar at elevated temperatures, the majority of the numerical models developed for predicting the behaviour of reinforced concrete structures in fire was based on the full bond interaction. Hence, the main purpose of this research is to develop robust numerical models for predicting the bond-slip between concrete and the reinforcement under fire conditions. Therefore, the bond-slip between the concrete and reinforcement for conventional and prestress concrete structures at both ambient and elevated temperatures has been investigated in this research. Two models have been developed in this study: the first model is to simulate the behaviour of bond-slip of deformed steel bars in normal concrete at room temperature and under fire conditions. The model is established based on a partly cracked thick-wall cylinder theory and the smeared cracking approach is adopted to consider the softening behaviour of concrete in tension. The model is able to consider a number of parameters: such as different concrete properties and covers, different steel bar diameters and geometries. The proposed model has been incorporated into the Vulcan program for 3D analysis of reinforced concrete structures in fire. The second robust model has been developed to predict the bond stress-slip relationship between the strand and concrete of prestressed concrete structural members. In this model, two bond-slip curves have been proposed to represent the bond-slip characteristics for the three-wire and seven-wire strands. This model considers the variation of concrete properties, strands’ geometries and the type of strand surface (smooth or indented). The degradation of materials and bond characteristic at elevated temperatures are also included in the model. The proposed models have been validated against previous experimental results at both ambient and elevated temperatures and good agreements have been achieved. A comprehensive parametric study has been carried out in this research to examine the influence of bond-slip model on the structural behaviours of normal reinforced concrete structures. The study investigated the most important factors that can affect the bond characteristics between concrete and steel reinforcement at elevated temperatures. These factors are: the concrete cover, spalling of concrete, concrete compressive and tensile strengths.

Behaviour under fire circumstances is becoming more and more crucial for designing a concrete structureand authorities require more often a fire-resistance time. In fact, engineers need a powerful, user-friendly,accurate and non time-consuming method that can be used to design reinforced concrete structures. Inthis study, the author has developed a method to design any fire-exposed reinforced concrete crosssections under flexure that takes into account second order effect. The first part focuses on the thermal analysis of the reinforced concrete cross section. Fourier'sequation is solved using finite differences method and the development tool of Excel: Virtual BasicAdvanced macro. Thus, it could easily be used on every personal computer (reasonably powerful) andneeds no extra investment. The accuracy of this thermal analysis is checked by comparison with resultsfrom commercial softwares (FAGUS edited by Cubus and SAFIR developed by the university of Liege). The second part deals with the mechanical analysis. Indeed, the concrete compressive strengthtogether with the yield strength of the steel reinforcement bars will decrease when the temperature willraise inside the concrete cross section. This loss of characteristic will be regarded as a loss of area andnew dimensions are set up. Finally a classic analysis (as it can be done at ambient temperature) isperformed. The mechanical analysis which takes into account second order effect is based on the Eulerbuckling load. The last chapter presents a comparison study between this new method and the two commercialsoftwares FAGUS and SAFIR, both of them are using finite element method. Several cross sections havebeen modelled, T-shaped ones with various dimensions and rectangular ones with various dimensionsand various steel areas. The two aspects (mechanical and thermal) have been studied and the resultsshowed good correspondance. / Master thesis

The purpose of thesis aims to investigate the mechanical behavior and properties of a centrally notched hybrid Al alloy/Carbon-Fiber/PEEK(APC-2) laminate at elevated temperature. The high performance hybrid composite laminates of 0.5mm Aluminum alloy sheets sandwiched by APC-2 cross-ply and guasi-isotropic laminates were fabricated. The prepregs of APC-2 were stacked into cross-ply [0/90]s and quasi-isotropic [0/45/90/-45] laminates spread uniformly with nanoparticles SiO2. The sheet surface was treated by chromic acid anodic method to achieve perfectly bonding with matrix PEEK. The modified diaphragm curing process was adopted to fabricate Al/APC-2 hybrid nanocomposite laminates. The panels were cut into the specimens and then drilled an diameter hole in the center with diameters of 1,2,4,6 mm. The MTS 810 material testing machine was used to conduct the tension and fatigue tests. In addition, the MTS 651 environmental chamber was installed to control and keep the specific testing temperatures, such as ,25¢XC(RT), 75¢XC, 100¢XC, 125¢XC and 150¢XC. At first, the nominal stress(£mnom) and stress-strain diagram were obtained due to static tension tests at elevated temperature. The constant stress amplitude tension-tension cyclic tests were carried out by using load-control mode at a sinusoidal loading with frequency of 5Hz and stress ratio R=0.1. The received fatigue data were plotted in normalized S-N curves at variously elevated temperature. For the tensile tests, at the same temperature the nominal stress of cross-ply specimens was higher than that of quasi-isotropic specimens. Comparing with the notched and unnotched of cross-ply specimens, the nominal stress of notched specimens was about 60% to 80% that of unnotched specimens. Besides, as for the notched and unnotched quasi-isotropic specimens, the nominal stress of notched specimens was about 75% to 85% that of unnotched specimens. Then, the fatigue life and stress-cycles (S-N) curves of notched specimens were obtained often tension-tension fatigue tests. In the case of the same loading, notched specimens possess worse fatigue behavior, but in the same normalized stress ratio, the S-N curves of the unnotched were below the notched ones. The fatigue resistance of notched samples decrease as the temperature rising.