The thesis aims to investigate the mechanical behavior and properties of a centrally notched hybrid Magnesium/Carbon-Fiber/PEEK laminate at elevated temperature. The high performance hybrid composite laminates of 0.5mm Magnesium sheets sandwiched by Carbon-Fiber/PEEK (APC-2) guasi-isotropic and cross-ply laminates were fabricated. The Magnesium sheets were polished and cleaned by acetone, then underwent the surface treatment by CrO3-base solvent etchants, cured by the improved diaphragm curing process. The finished laminates were cut into the specimen than drilled a 4mm diameter hole in the center of specimen. At first, the ultimate strength, stiffness and stress-strain diagram were obtained due to static tension tests at elevated temperature, such as 25¢XC(RT), 75¢XC, 100¢XC, 125¢XC, and 150¢XC. Compare of them, the notched quasi-isotropic ones drop almost 50% in strength, and the notched cross-ply ones are half of unnotched ones. The two lay-up notched specimens are slightly below the unnotched ones in stiffness. The strength of the specimens are decrease as temperature rise. As the temperature rise the stiffness of quasi-isotropic ones drop, but it just change little in cross-ply ones. Then the notched specimen fatigue life and load-cycle (P-N) curves were obtained by tension-tension fatigue test. The P-N curves were adopt to prevent the stress concretion of the notched specimen. Consider the same loading, notched specimens has worse fatigue behavior, but in the same load ratio, the normalized P-N curves of the unnotched ones were below the notched ones means notched ones has better fatigue behavior. Recording the specimen image by video camera during the testing process, the cracks at the edge of hole were found. However delamination was not found. Necking was observed in quasi-isotropic specimens, but not in cross-ply. Observed by optical microscopy, the improved surface treatment will decrease the probability of delamination from 20% to less than 10% after hot press.

ABSTRACT EFFECTS OF POZZOLAN INCORPORATION AND CURING CONDITIONS ON STRENGTH AND WATER RESISTANCE OF NATURAL GYPSUM PASTES Cengiz, Okan Ph.D., Department of Civil Engineering Supervisor : Prof. Dr. Turhan Y. Erdogan September 2009, 189 pages There are large reserves of gypsum rock (CaSO4&middot / 2H2O) in Turkey and in various regions of the world. Heating gypsum rock to 140 - 190 0C causes it to lose three-fourths of its water molecules and turn into gypsum, called plaster of Paris / heat application over 190 0C results in loss of all the water molecules and this form of the material is called gypsum anhydrite. When gypsum is mixed with water, it gains cementitious property and hardens in a short time. Therefore, natural gypsum anhydrite and especially plaster of Paris are widely used in the construction industry. On the other hand, its not being water resistant and having low strength restrict the use of gypsum products for outdoor applications. In this study, the effects of pozzolan incorporation to natural gypsum pastes and application of various curing regimes for improving their water resistance and strength were investigated. Compressive strength and absorption tests on one thousand one hundred twenty two 5-cm cube specimens produced from v 13 different mixture types were conducted. Also the microstructures of these products were investigated using the analytical technique X-Ray Diffraction. The test results showed that water resistance and strength properties of pozzolan-incorporated gypsum products were improved. Curing of the product at elevated temperature regimes was an additional factor that contributed to this improvement. It was concluded that the natural gypsum mixtures prepared and cured at the above-mentioned conditions could also be used for outdoor applications. Keywords: Pozzolan, Natural Gypsum, Elevated Temperature Curing

ABSTRACT This thesis is aimed to investigate the fatigue life, strength, damage and fracture process in AS-4/PEEK APC-2 composite laminates subjected to both elevated temperature and loading sequences. Our main work is experiment. All specimens are 16-ply thick with lay-up of quasi-isotropic laminates. We accomplish static tensile test, fatigue test of constant stress amplitude, and two-step loading at elevated temperatures, i. e., 75¢Jand 125¢J. The residual strength and stiffiness are obtained. We also use Miner¡¦s rule to analyze the data acquired from experiment and to discuss the fatigue properties and fracture mechanism subjected to both elevated temperatures and loading sequences of combination. Finally, we perform ultrasonic C-Scan non-destructive test to examine laminates. In comparsion with experimental data, we can further understand the damage process and fracture mechanism in laminates. The experimental results can be concluded as follows. The damage of specimens at 125¢J is more serious than that at 75¢J.Furthermore, if the loading sequence is low-high, the cumulative damage value will be smaller than 1,whilst it will be larger than 1 due to reverse loading sequence. The sequence of decreasing residual strength and residual stiffiness of specimen associated with the temperature and stress level in combination is high temperature¡Ðlow stress, low temperature¡Ðlow stress, high temperature¡Ðhigh stress and low temperature¡Ðhigh stress.

The aim of this thesis is to manufacture Ti/APC-2 hybrid composite laminates and obtain its mechanical properties and fatigue characteristics at elevated temperatures. Ti/APC-2 laminates were composed of two layers of APC-2 and three layers of titanium sheets. For superior bonding ability between titanium and APC-2, chromic anodic method was adopted to treat titanium sheets in manufacturing process and APC-2 was stacked according to cross-ply [0/90]s and quasi-isotropic [0/45/90/-45] sequences. Then, the modified curing process was adopted to fabricate Ti/APC-2 hybrid composite laminates. Tension and fatigue tests carried out with MTS 810 and MTS 651 environmental control chamber to lift and maintain experimental temperatures, such as 25¢XC, 75¢XC, 100¢XC, 125¢XC and 150¢XC. From static tensile tests, the mechanical properties of cross-ply and quasi-isotropic composite laminates, such as ultimate strength, longitudinal stiffness were gained and the stress-strain diagrams of laminates were also plotted from testing data at elevated temperature. From fatigue tests we obtained laminate¡¦s fatigue resistance properties and the experimental data of applied stress vs. cycles were plotted as S-N diagrams at elevated temperature. From the tensile and fatigue tests, the important remarks were summarized as follows. First, no matter what the APC-2 stacking sequence was, the ultimate strength and longitudinal stiffness decreased while temperature rising, especially at 150¢XC; second, a turning point appeared at each stress-strain diagram that kink angle caused the decrease of stiffness while temperature rising; third, combining fatigue data and stress-strain diagrams we analogized a presumption that the region before turning point was in elastic behavior and after turning point in plastic deformation; fourth, quasi-isotropic laminates had better fatigue resistance than that of cross-ply laminates; sixth, the longitudinal stiffness before turning point was in good agreement with the prediction by using the modified ROM, however, after turning point the errors became large.

Aluminium alloys with Si as the major alloying element form a class of material providing the most significant part of all casting manufactured materials. These alloys have a wide range of applications in the automotive and aerospace industries due to an excellent combination of castability and mechanical properties, as well as good corrosion resistance and wear resistivity. Additions of minor alloying elements such as Cu and Mg improve the mechanical properties and make the alloy responsive to heat treatment. The aim of this work is studying the role of size and morphology of microstructural constituents (e.g SDAS, Si-particles and intermetalics) on mechanical properties of Al-Si based casting alloy at room temperatures up to 500 ºC. The cooling rate controls the secondary dendrite arm spacing (SDAS), size and distribution of secondary phases. As SDAS becomes smaller, porosity and second phase constituents are dispersed more finely and evenly. This refinement of the microstructure leads to substantial improvement in tensile properties (e.g. Rm and εF). Addition of about 280 ppm Sr to EN AC- 46000 alloy yields fully modified Si-particles (from coarse plates to fine fibres) regardless of the cooling conditions. Depression in eutectic growth temperature as a result of Sr addition was found to be strongly correlated to the level of modification irrespective of coarseness of microstructure. Modification treatment can improve elongation to failure to a great extent as long as the intermetallic compounds are refined in size. Above 300 ºC, tensile strength, Rp0.2 and Rm, of EN AC-46000 alloys are dramatically degraded while the ductility was increased. The fine microstructure (SDAS 10 μm) has superior Rm and ductility compared to the coarse microstructure (SDAS 25 μm) at all test temperature (from room to 500 ºC). Concentration of solutes (e.g. Cu and Mg) in the dendrites increases at 300 ºC and above where Rp0.2 monotonically decreased. The brittleness of the alloy below 300 ºC was related to accumulation of a high volume fraction damaged particles such as Cu- Fe-bearing phases and Si-particles. The initiation rate of damage in the coarse particles was significantly higher, which enhances the probability of failure and decreasing both Rm and εF compared to the fine microstructure. A physically-based model was adapted, improved and validated in order to predict the flow stress behaviour of EN AC- 46000 cast alloys at room temperature up to 400 ºC for various microstructures. The temperature dependant variables of the model were quite well correlated to the underlying physics of the material

Recently in the United States, there has been increasing interest in developing engineered approaches to structural fire safety of buildings as an alternative to conventional code-based prescriptive approaches. With an engineered approach, the response of a structure to fire is computed and appropriate design measures are taken to assure acceptable response. In the case of steel buildings, one of the key elements of this engineered approach is the ability to predict the elevated-temperature properties of structural steel. Although several past research studies have examined elevated-temperature properties of structural steel, there are still major gaps in the experimental database and in the available constitutive models, particularly for ASTM A992 structural steel, a commonly used grade. Accordingly, the overall objective of this dissertation is to significantly enlarge the experimental database of the elevated-temperature properties for ASTM A992 structural steel and developing improved constitutive models for application in structural-fire engineering analysis. Specific issues examined in this dissertation include the following: tensile properties at elevated temperatures; room-temperature mechanical properties after heating and cooling; and creep and relaxation properties at elevated temperatures. For the elevated-temperature studies of tension, creep and relaxation, constitutive models were developed to describe the measured experimental data. These models were compared to existing theoretical and empirical models from the literature. / text

The structural adhesives widely used in structural strengthening applications are thermoset ambient cure adhesive polymers. At ambient temperatures, these polymers are in a relatively hard and inflexible state. At higher temperatures, the material becomes soft and flexible. The region where the molecular mobility changes dramatically is known as the glass transition temperature Tg and often is presented as a single value. Epoxy polymers exhibit a significant reduction in mechanical properties near glass transition temperature Tg when they are exposed to elevated temperatures. Glass transition temperature Tg is used to characterise the change in epoxy adhesive properties with changing temperature. The mechanical properties of epoxies tend to improve with curing temperature. This is because the crosslink density between the adhesive molecular structures increases during the curing process consequently the Tg improves. The aims of this work are first to demonstrate the importance of curing temperature. Second, to investigate the influence of glass transition temperature !! improvement on the performance of EB-FRP strengthened steel structures in flexure at ambient and elevated temperatures. Third, to compare analytical results with experimental results from the flexure tests results. Finally, to compare the current design guideline recommendations with the flexure tests results. The most commonly used methods to evaluate Tg Dynamic Mechanical Analysis (DMA) and Differential Scanning Calorimetry (DSC) were used to study Tg. Two off-shelf structural adhesives were investigated to understand their property variation with temperature. Epoxy coupons were cured at different elevated temperature and humidity environments up to 28 days. A combination of two extreme relative humidity of 0 and 100% and variable curing temperatures between 15 to 80°C were considered. From a test matrix of 300 DMA and over 250 DSC coupons these conclusions were drawn. First, ambient cured thermosets have a linear relationship between Tg and curing temperature, but Tg is reduced if a certain temperature is reached. Second, a fully cured adhesive requires heating treatment. Without a curing regime, designed Tg may never be achieved. Finally, curing time is crucial at the low curing temperatures while it is less significant at the higher curing temperature. The results of Tg investigation were used to select appropriate curing temperature that the adhesives resistance to temperature can be maximised without damaging the mechanical properties. The study helps designs to understand and assess the behaviour of these two adhesives when they are exposed to extreme temperatures. The study increases the awareness that a fully cured adhesive may never be achieved at ambient or low temperatures. It is important to find the mechanical properties and Tg when the coupons are exposed to the same curing temperature. To investigate the influence of glass transition temperature Tg improvement on the performance of EB-FRP strengthened steel structures in flexure at ambient and elevated temperature, nine three metre length beams were designed to behave as a concrete-steel composite bridge deck. The beams were tested in four-point bending. Lap shear, DMA test, and pull-off adhesion samples were prepared and cured at the same conditions and tested at ambient temperature. Six beams were tested under only mechanically loading at ambient temperature, including the control specimen. Five beams were tested at ambient temperature to show the effects of adhesive curing on FRP strengthened sections. A significant increase of load capacity of the adhesive joints was achieved due to the curing of the joints at elevated temperature. The failure occurred was in the same manner. An increase in the load capacity was observed with increasing curing temperature. An increase of approximately 25% was noticed in the ultimate load capacity of the specimens cured at 50°C compared to the specimens cured at 30°C. The load capacity of lap-shear specimens cured at 50°C was 28% higher than the specimens cured at 30°C. Three specimens were tested under mechanical and thermal loading. A bespoke thermal chamber was designed and fabricated to apply a controlled thermal loading. The beams were loaded mechanically up to 350kN, first. The temperature of the specimens was then increased at a rate of 0.8°C/min. The sustained load 350kN remained constant during the heating phase. Digital Image Correlation (DIC) technique was used to detect the slippage of the tip of the FRP plates. The only specimen cured at 30°C showed relatively poor performance compared to the two specimens cured at 50°C. The plate ends started to slip when the adhesive storage modulus from the DMA runs reduced approximately by 15 and 18% for the beams cured at 30 and 50°C respectively. Pull-off adhesion tests confirmed that adequate surface preparation of over 25 MPa was achieved The flexural model for the composite steel section represented to predicate load-deflection behaviour of the specimens using semi-experimental constitutive material law. The model successfully predicts the load-deflection behaviour of specimens, considering the strain hardening contribution. A bond stress analysis is also presented, which counts for the effect of FRP plate moment effect. The experimental and theoretical FRP plate slippage assuming only adhesive degradation with temperature are compared. The analytical bond models cannot predict the experimental failure because the linear elastic material properties were assumed and the failure was adhesion.

Aluminium-silicon alloys have gained increasing market share in the automotive and aerospace industry because of increased environmental demands. These alloys have a high strength-to-weight ratio, good corrosion resistance, castability and recycling potential. However, variations in properties and limited performance at elevated temperature are restricting these alloys from use at elevated temperatures. During the last decades, researchers have investigated ways to improve the properties at elevated temperatures. However, the effect of some transition elements is not well understood. The aim of this work is to investigate the aluminium-silicon alloys with addition of cobalt and nickel for high temperature applications. Tensile testing and hardness testing were conducted on samples produced by directional solidification in a Bridgman furnace with condition generating a microstructure corresponding to that obtained in high pressure die casting, i.e. SDAS ~ 10 µm. The results show that cobalt and nickel improve the tensile properties up to 230 °C.

Steel beams with web openings are frequently used in construction to achieve attractive, flexible and optimised design solutions. These beams are used to provide passages for building services, to reduce the overall construction height and to achieve long spans. However, the presence of the openings may lead to a substantial reduction in the load carrying capacity of the beam at both ambient and elevated temperatures and introduce additional failure modes including shear-moment interaction at the location of the openings causing the Vierendeel mechanism. Steel beams in practical construction are axially restrained and the presence of this axial restraint can drastically change the behaviour of the beams in comparison to those without axial restraint. One particular issue is premature buckling of the compressive tee-sections around the openings. The aim of this research is to investigate the effects of openings on axially restrained steel beams at elevated temperatures so as to develop an analytical method for design consideration. The analytical derivation will be based on the results of extensive numerical simulations. The research starts with the behaviour of steel beams with web openings under combined axial compression, bending moment and shear force at ambient temperature. The results show that buckling of the compressive tee-sections at the openings can reduce the plastic moment capacity of the openings; and an analytical method has been proposed to incorporate the influences of axial compression and tee-section buckling into the existing shear-moment design equations. The elevated temperature simulations show that axially restrained steel beams with web openings may enter catenary action at much lower temperatures than the commonly accepted critical failure temperatures calculated assuming no axial restraint and no tee-section buckling. Therefore, at the commonly accepted critical failure temperatures, many perforated steel beams exert tensile forces on the adjacent connections. It is important that the connections have the strength and deformation (rotation) capacity to enable catenary action to develop. The parametric study examines, in detail, how changing the different design parameters may affect the elevated temperature behaviour of perforated beams. The examined parameters including load ratio, level of axial restraint, cross-section temperature distribution profile, opening shape, opening size and opening position. Based on the results of the numerical parametric study, an analytical method has been derived to obtain the complete axial force-temperature relationship for axially restrained perforated steel beams. The key points of the analytical method include initial stiffness, point of initial failure under combined axial compression, bending moment and shear force, transition temperature at which the axial force on the beam changes from compression to tension and the magnitude of the tensile force resulting from the beams going into catenary action. Using the analytical method, it is possible to assess the maximum tensile force in the beam and the corresponding temperature so that the safety of the connections can be checked.

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