Dissertations / Theses on the topic 'CFRP'

La fabrication de structures hybrides associant pièces métalliques et composites vise à combiner résistance aux sollicitations multiples des pièces de structure et poids global limité, notamment dans l'industrie aéronautique. Les différences importantes de comportement mécanique qui existent entre les deux types de matériaux doivent être prises en considération lors de la mise en place d'une opération de perçage d'une structure hybride. L'objectif de cette thèse est de développer, en se basant sur l'expérimentation, des outils et une stratégie de perçage associée, pour les structures hybrides CFRP/Al2198 et CFRP/Ti6Al4V. Une étude de l'impact de la morphologie de l'outil et de la stratégie d'usinage a révélé un lien entre la morphologie du foret utilisée et les conditions de coupe d'une part, et la qualité du trou percé et les efforts de coupe d'autre part. Les outils proposés dans un premier temps ont combiné différentes morphologies existantes (hélicoïdal à deux lèvres, pointes extrêmes et étagé). Dans un deuxième temps, un outil répondant aux exigences a été développé et associé à une nouvelle stratégie d'usinage comprenant trois phases (ébauche, ½ finition et finition). Enfin, le délaminage en sortie de trou restant une préoccupation majeure lors du perçage des composites carbone à matrice polymère, un modèle analytique permettant le calcul de la force critique avant délaminage a été proposé. Ce travail de thèse a été réalisé dans le cadre d'un projet Industrilab financé par la Région Hauts de France et piloté par STELIA-AEROSPACE
The manufacturing of hybrid structures composite and metal parts aims to combine resistance to multiple stresses and to limit overall weight, particularly in the aviation industry. The significant differences in mechanical behavior that exist between the two types of materials must be taken into account in drilling operations of a hybrid structure. The objective of this thesis is to develop, based on the experimentation, tools and an associated drilling strategy for the CFRP/Al2198 and CFRP/Ti6Al4V hybrid structures. A study of the impact of the morphology of the tool and the machining strategy revealed a link between the morphology of the drill used and the cutting conditions, on the one hand, and the quality of the hole drilled and the thrust force, on the other hand. The tools proposed, at the first step, combine different existing morphologies (twist, square and step drill). In a second step, a tool meeting the requirements was developed and associated with a new machining strategy comprising three phases (roughing, ½ finishing and finishing). Finally, delamination at the exit of the hole remains a major concern when drilling carbon composites, an analytical model allowing the calculation of the critical force before delamination has been proposed. This work is part of Industrilab project financed by the Hauts de France Region and led by STELIA

Due to inadequate lateral stiffness, many reinforced concrete buildings are highly damaged or collapsed in Turkey after the major earthquake. To improve the behavior of such buildings and to prevent them from collapse, repair and/or strengthening of some reinforced concrete elements is required. One of the strengthening techniques is the use of CFRP sheets on the existing hollow brick masonry infill. While using the CFRP sheets their attachment to both structural and non-structural members are provided by CFRP anchor dowels. In this study, by means of the prepared test setup, the pull-out strength capacities of CFRP anchor dowels are measured. The effects of concrete compressive strength, anchorage depth, anchorage diameter, and number of fibers on the tensile strength capacity of CFRP anchor dowel are studied.

This thesis investigated the impact performance of CFRP products within the sports industry. The primary aim of this thesis was to evaluate different configurations, matrix system, and technologies to find the best performing solutions for impact. During this work, an extensive literature study was conducted and various solutions were reviewed. Further on, several tubes were manufactured, impacted and put through a 2 point bending test to find out the residual strength. It was found that TeXtreme R fabrics positively affected the impact performance when compared to conventional fabrics and UD depending on the placement location. Thin plies proved to be better than conventional plies. Newer technologies such as CNT stitching requires further investigation before it can be qualitatively assessed.

Most of the building structures needed in twenty years from now is already built. Building structures are ageing and they deteriorate and often the demands on the structure changes over time. It is often needed to reinforce the structure, especially when reconstructing, repairing or renovating a building. The traditional strengthening methods that are used today are strengthening the structure with beams and columns of steel. The beams and columns often require much space and problems might occur if there are a lot of installations. There is another more modern technique where a composite plate, sheet, grid or bar of relatively small thickness is bonded with an epoxy adhesive to the structure. These reinforcements don’t change the cross-section of the structure. The most common type of fibre reinforcement that is used for strengthening in the building industry is carbon fibre. The carbon fibre products are much more expensive when comparing to steel. Therefore the object of this report is to show when it is economically motivated to use carbon fibre instead of the more traditional material steel. To do this I have studied five real cases brought to me by WSP Construction in Linköping, Sweden. The study shows that in the cases where it’s possible to use composites strengthening the building structure it is also economically motivated to do so. Even if the material is more expensive than the more traditional materials the total cost is in the same price class as the traditional ones because of the short time of construction. But there is still a problem with only a few consultants and entrepreneurs that have the knowledge and the experience to work with composites when strengthening a structure. If the knowledge will be spread to the consultants I will say that carbon fibre composites are the materials of tomorrow even in the building industry.
En stor del av det byggnadsbestånd som kommer att behövas inom en tjugoårsperiod är redan uppfört idag. Byggnadskonstruktioner åldras och försämras, och ofta förändras förutsättningarna för byggnaderna med tiden. Ett ständigt återkommande moment är behovet av att förstärka den bärande konstruktionen, framför allt i samband med ombyggnation, reparation och renovering. De traditionella metoder man använder sig av idag innebär att stålbalkar och pelare monteras för att förstärka konstruktionen. Dessa tar utrymme i anspråk och problem kan uppstå med t ex installationer i tak. En nyare metod innebär att man fäster kolfiberkompositer på ytan av konstruktionen i tunna lager i form av laminat, väv, nät eller stavar som fräses ner i ytan. För att fästa kompositen på ytan använder man sig av epoxi. De tunna lagren gör att konstruktionens tvärsnitt och egenvikt inte förändras märkbart. Då kolfiber är ett dyrare material än stål är syftet med denna rapport att ta fram konkreta praktikfall där det är ekonomiskt motiverat att använda sig av kolfiberkompositer istället för de traditionella metoder som används idag. Fem praktikfall tillhandahållna av WSP Byggprojektering i Linköping har studerats. Det visade sig att i de fall det var tillämpligt att använda sig av kolfiber för förstärkning var det även ekonomiskt hållbart. Trots det dyrare materialet innebär den korta monteringstiden att totalkostnaden för förstärkningsjobbet ligger i ungefär samma prisklass som en förstärkning med stål. Problem kvarstår dock med alltför några kunniga konstruktörer och entreprenörer – men annars är kolfiberkompositen morgondagens material.

In environments of high moisture exposure, carbon fibre-reinforced polymer (CFRP) reinforcement is believed to be a good alternative to steel due to its non-corrosive properties. However the CFRP matrix experiences other types of changes, mechanical deterioration and transverse swelling, with exposure to moisture and sustained stresses. Specifically for structural applications, few studies have investigated the effects of moisture and stress-induced matrix-dominated changes to the CFRP-concrete bond when the CFRP rods are cast into concrete as prestressed reinforcement. Experiments were conducted to measure the transverse swelling and moisture uptake rates of stressed and unstressed CFRP rods immersed in water and concrete pore solution (CPS). CFRP rods were also embedded in concrete and immersed in water to observe whether the transverse swelling of the rods would cause the concrete to crack. Pull-out tests were conducted on stressed and unstressed CFRP rods cast in concrete cubes and immersed in water for up to 125 days. Six CFRP prestressed concrete prisms were fabricated, three were immersed in water for over 125 days and three were left in lab conditions before being tested in three-point bending. Moisture-induced transverse swelling did not appear to cause cracking of the concrete covering CFRP rods in water, contrary to the prediction of the thick-walled cylinder model, but possibly caused an increase in the CFRP-concrete radial contact pressure. The imposition of sustained bending stresses on wet CFRP rods was found to cause premature failure, believed to be a result of matrix softening and microcracking. These observations of swelling and matrix degradation were believed to affect the CFRP-concrete bond. The pull-out tests of unstressed CFRP rods indicated an increased likelihood for bond failure in the CFRP matrix with moisture exposure. This resulted in greater variations in the ultimate and residual bond stresses of the wet samples compared to the dry controls. For the prestressed pull-out samples, higher magnitudes of prestressing resulted in lower bond failure loads. Both outcomes are indicative of matrix weakening with moisture and stress exposure. However despite this, the three-point bending tests revealed no significant differences in behaviour between the wet and dry prestressed prisms. Although the wet prisms exhibited slightly greater variation in their post peak load behaviour compared to the dry prisms, in line with the observations from the pull-out tests. The work from this thesis reveals that despite observations of matrix deterioration and transverse swelling in CFRP rods exposed to moisture and stress, the effect of these changes to the overall behaviour of a CFRP prestressed concrete member are minimal. Any long-term effects are unlikely to cause significant changes to the behaviour of the member. However, the CFRP-concrete bond may be more variable in members exposed to moisture, which should be allowed for in the design process. Therefore CFRP is a promising alternative to steel reinforcement in high moisture environments, with good long-term durability.

Behovet av lätta konstruktioner ökar och nya lättare material används. Dessa behöver fogas samman. Att använda fogning med laser är ett sätt att göra detta och i detta examensarbete utvärderas fogning med laser av kolfiberkomposit (CFRP) och aluminium. Frågeställningarna i detta exjobb har rört huruvida aluminium och CFRP går att foga samman med hjälp av laser och hur hög skjuvhållfasthet som kan uppnås vid överlappsfogning med denna metod. Tre olika aluminiumlegeringar har fogats till PA6-CFRP och därför har även eventuella skillnader mellan fogarna undersökts. Fogning och testning visade att alla tre legeringar var fogbara till CFRP genom fogning med laser och att bindningsytan växte när fogningshastigheten minskade. AA5182 visade sig ge de starkaste fogarna och kunde fogas med CFRP vid en högre hastighet än AA6016 och AF200. AA5182 smälte dock lättare på ytan än de andra legeringarna. Genom LOM (ljusoptiskt mikroskop) och SEM (Svepelektronmikroskop) indikerades att bubblor i CFRP nära gränsskiktet gav starkare fogar än när bubblorna befann sig långt ifrån gränssnittet. Genom SEM-analysering syntes att materialen var tätt sammanfogade med varandra.
The need for light weight structures are growing stronger and new materials is being used. These materials need to be joined together. Laser joining is one possible way of doing that and in this thesis project laser joining of carbon fiber composite (CFRP) and aluminum is being analyzed. The scope of the thesis consists of questions whether aluminum and CFRP are joinable using this method and how high tensile strength that can be obtained in the lap joint. Three different aluminum alloys is being joint to the PA6-CFRP, and therefore it is also interesting to see if there is any difference between the joints. Joining and testing showed that all three alloys were joinable to CFRP through laser joining and that the bonding area increased for decreasing speed. AA5182 achieved the highest strength and was joined to CFRP at a significant higher speed than AA6016 and AF200. On the other hand AA6016 and AF200 did not melt as easily as AA5182.LOM (light optical microscope) and SEM (Scanning Electron Microscope) analysis showed that bubbles in the CFRP close to the interface seemed to give stronger joints than bubbles far away from the interface. Through SEM analyzes it was seen that a tight binding between the materials was achieved.

With the market introduction of both the Airbus A350XWB and the Boeing 787, Carbon Fibre Reinforced Plastics (CFRP) has been applied to primary structure of large commercial aircraft, as a means of enhancing overall performance. Both these aircraft are being developed and produced in a unique way where Airbus and Boeing are acting as System Integrators and using Risk Sharing Partners to develop the majority of the principal components. To support this new business and technological model it is necessary that the System Integrator has sufficient knowledge and tools to support the development of the components. Of particular interest are items such as the wing covers, as they are both heavy and expensive items, thus offering large opportunities for optimisation, in particular when the benefits of applying CFRP are considered. This creates the forum for this thesis, i.e. to thoroughly understand all factors that influence a CFRP wing cover, from which an optimisation methodology is developed, incorporating design constraints, while seeking the lightest weight solution, with a resultant Life Cycle Cost (LCC). Based on this, different solutions can be compared based on weight and LCC. In general stringer-stiffened panels are, from a weight perspective, the optimal configuration for wing covers, and thus are solely considered. Serendipitously, due to their prismatic shapes, buckling calculations of stringer-stiffened panels can be solved with reasonable accuracy and ease using the Finite Strip Method (FSM), as opposed to more time consuming methods such as the Finite Element Method. A suitable FSM program is available from ESDU, which when used in combination with a configured Excel spreadsheet can take into consideration constraints established from the extensive literature review. Once the lowest weight solution is obtained under buckling constraints, the solution is then checked for in-plane and if desired out-of-plane strength. Based on the structurally optimised wing cover, the manufacturing cost is calculated using a Process Based Cost Model (PBCM), which has been developed based on different CFRP materials for the skin and stringer fabrication, as well as suitable manufacturing and integration methods. In order to consider the LCC, i.e. all costs from cradle to grave, the PBCM factors in both the cost of recycling scrap material during manufacture and after retirement. Furthermore, when more than one solution is compared then the Economic Value of Weight Saving, which is based on the range equation, can be factored in to consider the financial benefit of weight saving. The optimisation methodology and PBCM has been evaluated on diverse wing cover examples, which has considered both uni-directional prepreg, non-crimp fabric and braids materials in combination with autoclave and liquid composite moulding techniques. The results demonstrated a trend which can be considered realistic, although the cost estimation is very much dependent on the assumptions made. In conclusion, the thesis and the optimisation methodology can be used to compare different configurations.

This study presents results from non-destructive testing to evaluate the degradation of the CFRP-masonry bond using thermal imaging. The goal of the research was to identify locations where there was evidence of bond deterioration that could subsequently be verified through destructive pull-off testing. Four full-scale masonry walls were built outdoors at the University of South Florida in 1995 to evaluate the effectiveness of CFRP for repairing settlement damage. Two of the settlement-damaged walls were repaired using single layer, commercially available unidirectional CFRP systems that used Tonen (wall 3) and Henkel (wall 2) epoxies. These two walls were the subject of this investigation. Before non-destructive tests were initiated, historical site data on temperature, humidity and rainfall variation was compiled. Over seventeen years, the walls experienced ambient temperatures as high as 98°F and as low as 25°F. The average rainfall in Tampa is about 34 inches and the annual average high humidity is around 87%#37;. Because of the high temperature and humidity, the CFRP-masonry bond was exposed to a particularly aggressive environment. Three types of thermal evaluation were carried out: thermocouple monitoring and both passive (solar) and active (localized heating) infrared thermal imaging. Twenty-four thermocouples were used to observe the spatial variations in temperature on the wall. Data showed that the surface temperatures of the wall are uneven with one end being hotter than the other. Measurements indicated that the wall temperatures went as high as 103°F during the week of data collection in late March and early April of 2012. In contrast, the highest ambient temperature over the same period was 92°F. The high temperature experienced by the wall is below the glass transition temperature for the epoxies, which ranges from 140°F to 180°F. A FLIR Tau 320 thermal imaging camera was used to identify localized de-bonding. Solar radiation heated the walls and the goal of thermal imaging was to detect hot spots which are indicative of de-bonding. Although this technique is ideal for exterior applications, initial attempts were unsuccessful. Once de-bonds were located by sounding, the camera was capable of confirming two hot spots on wall 2. A thermal scanner built by the university from a series of ten Omega OS137 thermal sensors was used to obtain more complete thermal images of the walls. This scanner had a heating element which supplied heat and allowed for active thermography. The scanner detected 16 hot spots not seen with the thermal camera. Ten of the twelve spots on wall 2 are concentrated on a region of the wall which experienced the highest daily changes in temperature, which indicates that higher thermal and environmental cycling has caused greater de-bond. Based on the number of hot spots found using both active and passive thermography the Tonen epoxy is performing better than the Henkel epoxy. In general, the bond has endured; however, there are a few localized areas that have de-bonded. Pull-off tests are recommended on walls 2 and 3. Five locations in regions suspected to have poor bond and five locations in regions suspected to have good bond are identified for each wall.

Composite sandwich structures are being used in the automotive and aerospace industries at an increasing rate due to their high strength and stiffness per unit weight.  Many teams in the world’s largest engineering competition for students, Formula Student, have embraced these types of structures and are using them in their chassis with the intent of increasing the torsional stiffness per unit weight.   The Formula Student team at Karlstad University, Clear River Racing, has since 2017 successfully built three carbon fiber based sandwich structure chassis. A big challenge when designing this type of chassis is the lack of strategy regarding torsional stiffness simulations. Thus, the goal of this thesis project was to provide the organization with a set of accurate yet relatively simple methods of modelling and simulating the torsional stiffness of the chassis.   The first step in achieving the goal of the thesis was the implementation of simplifications to the material model. These simplifications were mainly targeted towards the aluminum honeycomb core. In order to cut computational times and reduce complexity, a continuum model with orthotropic material properties was used instead of the intricate cellular structure of the core. To validate the accuracy of this simplification, the in-plane elastic modulus of the core was simulated in the finite element software Abaqus. The stiffness obtained through simulations was 0.44 % larger than the theoretical value. The conclusion was therefore made that the orthotropic continuum model was an accurate and effective representation of the core.   Furthermore, simplifications regarding the adhesive film in the core-carbon fiber interfaces were made by using constraints in Abaqus instead of modelling the adhesive films as individual parts. To validate this simplification and the overall material model for the sandwich structure, a three-point bend test was simulated in Abaqus and conducted physically. The stiffness for the sandwich panel obtained through physical testing was 2.4 % larger than the simulated stiffness. The conclusion was made that the simplifications in the material modelling did not affect the accuracy in a significant way.   Finally, the torsional stiffness of the 2020 CFRP chassis was found to be 12409.75 Nm/degree.   In addition to evaluating previously mentioned simplifications, this thesis also serves as a comprehensive guide on how the modelling of the chassis and how the three-point bend test can take place in regards to boundary conditions, coordinate system assignments and layup definitions.

The purpose of this publication is to analyse the hygroscopic ageing effects of a CFRP material subjected to fatigue. The tested material is provided by the company RiBa Composites. Because of the lack of data about fatigue in composites, there is not a specific test approved by any regulation for this analysis. Therefore, the tests are inspired by the ASTM D2344/D2344M, which is the test characterizing the short-beam strength and the moisture absorption of CFRP aged in deionized water. The difference, other than the type of applied load (the original test is performed by using quasi-static loading condition, meanwhile the fatigue test is performed by using CA loading), is in the number of specimens used. In fact, because of the long time spent by specimens to fail under low amplitude loadings, the use of five specimens per loading level for each ageing period is restricted to three specimens. Moreover, because of the definition of fatigue, for some ageing case more specimens are used and, sometimes, even different loading levels are considered. The final outcome is the analysis of the fatigue life. During the data analysis has been observed that even for CFRP materials can be made some fatigue life prediction by the use of the well know Wohloer diagram. Moreover, coefficients for the Basquin’s law have been obtained for all the ageing periods.

Behavior of lap splices formed by CFRP rolls has been studied. CFRP rolls have been prepared by using CFRP sheets of a certain width. Strengthening methods that use CFRP rolls as reinforcement may require an epoxy anchored lap splice due to the conditions at the strengthening regions. It may not always be possible to strengthen the region by using only one roll fan anchored at both ends, but using two rolls from opposite faces of the member and lap splicing them at the middle so that they act as a single roll. Lap splice behavior can be studied best by using flexural beam bond specimens if the reinforcing material is steel. Therefore, it has initially been suggested that flexural beam specimens reinforced for flexure with CFRP rolls as tension reinforcement can be used in studying the lap splice behavior. However, due to the difficulties encountered in the beam tests, another type of test specimen was introduced, which was a direct pull-out specimen. In this type of test specimen, lap spliced CFRP rolls have been tested under direct tension, in which the tension has been applied by making use of concrete end blocks that transfer the tension to the rolls. Eleven tests have been made in total. Full material capacity of the rolls could not be achieved due to premature failures. However, important conclusions and recommendations have been made for future studies.

During the service life of a building structure, the introduction of web openings into existing steel floor beams is often required to allow for new services such as air conditioning, sprinkler systems, telecommunications etc. However, the presence of large openings in the webs can significantly reduce the shear and bending strength capacity of the beams. Traditionally, the welding of additional steel plates around the opening areas is adopted as a means of strengthening and stiffening. This not only presents practical difficulties but can induce residual stresses which weaken fatigue performance of the section. The aim of this study is to explore the applicability of externally bonded carbon fibre reinforced polymer composites (CFRP) as an alternative means of strengthening for web openings in steel flexural elements. A numerical and experimental investigation was employed in the research reported in this thesis to achieve this aim. Using a non-linear finite element approach, the effects of strengthening arrangements and CFRP lengths were investigated with a view to determine the most structurally efficient layout of CFRP strengthening. The experimental tests were conducted later on four specimens, one control specimen (without opening and un-strengthened) and the rest with different web opening positions and CFRP strengthening. In order to further understand the limits of applicability of this approach, further numerical modeling was also performed to assess the proposed strengthening method when applied to full-scale steel beams with web openings at mid-span or in the high shear zone. The series of beams examined comprised the types of spans which are common in commercial frame buildings. The outcomes of this research show that the CFRP strengthening method is capable of recovering and in some cases exceeding the strength of the beam to that before the introduction of web openings. Similarly, the strengthening method increases the stiffness of the altered beam, thus bringing deflections back to a level similar to the beam before introduction of openings. In many of the strengthened cases, a reduction in ductility was observed; this can in part be due to over-specification of strengthening thickness and thus demonstrates the importance of choosing the optimum strengthening arrangement. In parallel with the reduction in ductility, it was observed that changes in failure mode and position can occur with certain strengthening arrangements in comparison to the unaltered beam. In the application of the method to full scale beams, the results of the simulations suggest that the technique of CFRP strengthening can be used effectively as a realistic and practical alternative solution to retrofitting of existing steelwork. When the CFRP strengthened beams were compared to those with traditional welded steel plate strengthening designed according to SCI P355 (2011), the results were found to be similar in terms of load capacity and overall failure mode.

Multilayer metallic/composite stacks are increasingly being used in wing and tail plane sections of modern commercial aircraft, with component assembly primarily through mechanical joining, hence the requirement for fixation holes. Currently, the individual material sections (titanium, aluminium and CFRP) are machined independently prior to assembly however; there is growing demand within the industry to produce holes through the stack in a single operation. The research detailed in the thesis involves evaluating the effect of operating parameters, drill geometry, tool materials/coatings and cutting strategy when single shot drilling three-layer Ti/CFRP/Al stacks. Performance was assessed against various process measures including thrust forces/torque, hole accuracy/quality, tool wear/life, burr formation and hole surface integrity (microhardness and microstructure). Statistical design of experiments and associated analysis techniques (main effects plots, ANOVA etc.) were employed to identify the significance of variable factors and preferred operating levels with respect to different responses. Based on the experimental results, a bespoke drill design was formulated, which was validated against current commercially available drills recommended for the drilling of multilayer stacks. Finally, the influence of cutting fluid pressure on temperature and hole quality was investigated.

The high cost of repairing reinforced or prestressed concrete structures due to steel corrosion has driven engineers to look for solutions. Much research has been conducted over the last two decades to evaluate the use of Fiber Reinforced Polymers (FRPs) in concrete structures. Structural engineering researchers have been testing FRP to determine their usability instead of steel for strengthening existing reinforced concrete structures, reinforcing new concrete members, and for prestressed concrete applications. The high strength-to-weight ratio of FRP materials, especially Carbon FRP (CFRP), and their non-corrosive nature are probably the most attractive features of FRPs. In this study, an experimental program was conducted to investigate the flexural behavior of prestressed concrete beams pre-tensioned with CFRP strands. The bond characteristics were examined by means of experimentally measuring transfer length, flexural bond length, and bond stress profiles. A total of four rectangular beams pre-tensioned with one 0.5-in. diameter CFRP strand were fabricated and tested under cyclic loading for five cycles, followed by a monotonically increasing load until failure. In investigating bond properties, the experimental results were compared to the equations available in the literature. The results from the four flexural tests showed that the main problem of CFRP strands, in addition to their liner-elastic tensile behavior, was lack of adequate bonding between FRP and concrete. Poor bonding resulted in early failure due to slippage between FRPs and concrete. As a result, a new technique was developed in order to solve the bonding issues and improve the flexural response of CFRP prestressed concrete beams. The new technique involved anchoring the CFRP strands at the ends of the concrete beams using a new "steel tube" anchorage system. It was concluded that the new technique solved the bond problem and improved the flexural capacity by about 46%. A computer model was created to predict the behavior of prestressed beams pre-tensioned with CFRP. The predicted behavior was compared to the experimental results. Finally, the experimental results were compared to the behavior of prestressed concrete beams pre-tensioned with steel strands as generated by the computer model. The CFRP beams showed higher strength but lower ductility.

The performance of reinforced concrete (RC) structures, such as bridges in the heavy haul industry, may be severely impacted by fatigue when subjected to repeated cyclic loading. Fatigue not only reduces load carrying capacity and serviceability limit states (SLS), but it can cause structural failure even when the components are subjected to low stress range cyclic loading. Corrosion damage exacerbates fatigue related problems as chloride induced pitting corrosion facilitates the formation and gradual propagation of cracks under cyclic loading. A common rehabilitation and retrofitting approach that involves patch repairing and fibre reinforced polymer (FRP) strengthening has proven effective to not only restore structural capacity, but also to enhance infrastructure service life. The structural repair process involves the replacement of deteriorated cover concrete with a less permeable patch repair mortar. The patch repair only restores durability of the structure; to restore or enhance structural capacity the repair process further involves bonding of FRP laminates. Particularly in the case of FRP's with a low elastic modulus, the design is often guided by serviceability limit states as opposed to ultimate limit states (ULS), resulting in an over-reinforced structural member. In addition, the reinforcement area of commercially available FRP strengthening may exceed the design requirements, especially at low levels of corrosion damage. In both the abovementioned considerations the design may result in an over-reinforced section. At the time when this researched was proposed, the effect of increasing damage extent on fatigue behaviour of over-reinforced RC beams was not clear and merited further investigation. A scientific experimental approach was developed to investigate the long-term performance of fifteen (15) full-scale 40MPa RC beams with dimensions 155x254x2000mm and ultimate capacity of 62.3kNm. Accelerated corrosion damage was induced in varied extents which included 450mm, 800mm, 1300mm and 1800mm length to a constant degree of 10% on all specimens. Specimens from each damage extent were patch repaired using SikaCrete214 and subsequently strengthened with externally bonded with SikaCarboDurS512 carbon fibre reinforced polymer (CFRP) laminates. Four-point bending monotonic loading tests were conducted on one (1) specimen from each damage extent. The results obtained from the quasi-static tests were used to determine two (2) cyclic loading stress ranges at which the remaining 2 specimens from each damage extent would be tested under. Under the 40% and 60% stress ranges four-point bending cyclic loading tests were carried out at a test frequency of 4Hz. Information was acquired on key performance indicators that included fatigue life, crack development, failure mode and stiffness degradation, where stiffness was assessed in terms of midspan deflection, composite material strains and neutral axis shift. Information on these parameters were collected using strain gauges, linear variable differential transducers (LVDT), DEMEC strain targets and digital image correlation (DIC). Ultimate failure loads under monotonic loading showed that despite having the highest degree of corrosion, the 450mm damage extent specimen had the highest failure load of 325kN. The failure load gradually reduced to 290kN as the damage extent was increased to 1800mm and the 0mm (control) specimen failed at the lowest load of 274kN. In contrast to the static behaviour, the specimen fatigue life enhanced by 106.3% as the damage extent was increased from 450mm to 1800mm. As expected, the 40% stress range tests yielded much longer fatigue lives than their 60% stress range counterparts. Furthermore, the experimentally obtained fatigue lives were compared to three fatigue life prediction models and the Helgason and Hanson model yielded the closest correlation with the experimental results. IV ABSTRACT Crack densities were found to increase with a longer fatigue life. An increase in damage extent was found to positively affect crack development and overall stiffness of the specimen during longterm fatigue testing. This finding was further substantiated by an assessment of midspan deflection, compression concrete strain and carbon fibre strain results, which all suggested a lower neutral axis and a lower stiffness reduction rate under fatigue loading as the damage extent was increased from 450mm to 1800mm. Furthermore, the tension concrete cracks propagated gradually during longer fatigue tests periods, while the tension steel and carbon fibre were comparably less affected by the resultant internal forces. Unfortunately, the neutral axis strain measurements using DEMEC targets were unable to assess the relative effect of an increase in damage extent as well as the compression concrete and carbon fibre strains were able to. During this experimental period, it was established that the laboratory layout was not conducive for carrying out the DIC process of long-term cyclic loading tests. The area in which testing took place did not adequately protect the camera against the environment and therefore required daily storage of the equipment. Regular movement of the camera for storage purposes introduced measurement inaccuracies which accumulated over longer test periods of up 20 days. However, for the short-term tests that did not require movement of the camera, the DIC process yielded favourable results. It was possible to capture the crack patterns early in the test period when the crack growth rate and development of new cracks was high using DIC. It was found that the high strain cracks coincided with the points of maximum vertical deflection (obtained through DIC) and eventual failure location of the specimen. The points of maximum deflection obtained from the DIC process were often not at midspan, which in the absence of the DIC process, would not have been possible to predict accurately. The results have shown that the specimens with the longer damage extents exhibit improved fatigue performance than their shorter counterparts. This revealed a stark contrast to their monotonic loading performance which favoured shorter damage extents. Furthermore, DIC holds potential to predict failure location more accurately than conventional approaches used for structural health monitoring (SHS).

Adhesive-bonded applications are widely used in industry, because of significant advantages such as uniform stress distribution, design flexibility and suitability to bond similar and dissimilar structural materials. This study focuses the adhesive-bonded long overlap of steel/carbon fibre reinforced polymer (CFRP) composite double lap shear (DLS) joints. The purpose of the work is to predict and assess the structural failure and behaviour of the DLS joint, including delamination of the composite, and to determine the effects of the design parameters of adherend thickness, overlap length and fabric orientation on the joint’s failure. There are different ways for such a joint to fail, which makes predicting failure very difficult. Another important difference is the failure mode of composites, where the relatively low interlaminar shear or tensile strength of the resin system causes failure of the composite before failure of the adhesive bondline occurs. Both experimental and numerical methods were used for the analysis. The experimental programme includes fabrication, mechanical testing and failure examinations of various joints. The numerical methods are based on 2D models, using strength of materials and cohesive zone modelling (CZM) approaches. In order to model adhesive joints accurately and efficiently, fracture tests were implemented to determine the fracture criteria. Mode-I and mode-II fracture energies were obtained by double cantilever beam (DCB) and end notched flexure (ENF) tests. An inverse method was used to define the cohesive parameters of the bilinear relation, fitting the numerical and experimental load-displacement curves. The DLS model has been created in Abaqus software, and results for each approach have been presented. Critical locations of stress concentrations in the DLS joint were identified, and the CZM successfully predicted the delamination initiation and propagation region observed in the experiment. As a result, it was concluded that the data obtained from the analysis showed good agreement with the experimental results, and in addition to the fibre orientation angles of the CFRP laminate markedly affecting the failure load of joints, the failure mode and stress distributions appeared in adhesive and composite. Furthermore, the study shows that the cohesive elements enable the numerical results to be obtained in shorter simulation times than the strength of materials approach, which should encourage use of CZM to analyse large structural applications.

The low areal weight requirements of telescopes in aerospace applications has driven the study on composite mirrors for several years. For example, the primary parabolic mirror in a balloon-borne, Cassegrain telescope required an optical quality better than 30 microns in figure RMS error. A parametric study on composite sandwich mirrors was conducted by using finite element analysis as well as optical analysis. The factors covered the cell sizes, core materials, core thicknesses, face layups, and support configurations. Based on theoretical calculations, many high quality spherical composite sandwich mirrors were generated by using a non-heat curing process. The CFRP faces and Nomex core were chosen as the baseline materials for mirror fabrication due to their high strength and low weight. The proposed replication method applied an interface layer between face and surface coat to eliminate print-through problems. Many important goals have been realized in those mirror samples with optical laser interferometer testing. These include the figure RMS error less than 2 microns and the surface RMS error less than 0.05 micron. The areal weights of the mirror samples are less than 7 kg/m². The thermal stability of these mirrors was observed from the optical results with thermal cycling tests. The proposed 2-meter parabolic composite sandwich mirror, with an areal weight of less than 10 kg/m², would consist of either [0/90/45/-45](s) layup faces with an optimal 3'' core or (QC) layup faces with a total core thickness of 5 inches. Both a ring support around the equator and the 18-point Hindle-type support would lead to the best optical quality under both self weight and thermal loading.

I materiali compositi sono, con ogni probabilità, il futuro delle strutture aeronautiche, poichè garantiscono ottime prestazioni strutturali con pesi minimi; in particolare, il Carbon Fiber Reinforced Polymer (CFRP) è tra i più diffusi nel settore. Sebbene siano conosciuti fin dalla Seconda Guerra Mondiale, essi non sono stati ancora esaurientemente studiati, poiché le loro caratteristiche meccaniche sono strettamente legate al processo di produzione, che può essere molto diversificato; inoltre, l’evolversi del loro comportamento è spesso difficilmente prevedibile e osservabile, a causa del fenomeno della propagazione interna dei danni. Questi motivi tengono aperti ancora molti interrogativi circa la caratterizzazione meccanica dei materiali compositi in particolari condizioni operative. Il presente elaborato, pertanto, ha lo scopo di rispondere ad alcune importanti domande su come cambiano le caratteristiche meccaniche interne del CFRP in seguito ad un impatto a bassa energia (BVID - Barely Visible Impact Damage) e, soprattutto, su come si caratterizza il materiale al sopraggiungere anche dell’inevitabile invecchiamento nel tempo (hygro-thermal aging). Dopo un’introduzione generale sui compositi e sul CFRP, si cercherà di ripondere ad esse tramite l’analisi di risultati sperimentali ottenuti in laboratorio.

I CFRP laminati posseggono molteplici vantaggi rispetto ai più comuni materiali metallici, in primis, le elevate proprietà meccaniche associate ad un peso ridotto. Tuttavia, presentano due problematiche in grado di minacciare l’affidabilità a lungo termine del manufatto: la delaminazione e il basso damping, ovvero la scarsa capacità di smorzare le vibrazioni. Lo scopo del presente elaborato di tesi è stato quello di produrre tessuti nanofibrosi elastomerici da integrare nei CFRP laminati al fine di contrastare il fenomeno della delaminazione e migliorarne le capacità smorzanti. I tessuti nanofibrosi sono stati prodotti attraverso la tecnica dell’electrospinning e successivamente caratterizzati morfologicamente tramite analisi SEM, termicamente mediante analisi DSC e meccanicamente tramite prove di trazione. I nanotessuti integrati all’interno del laminato sono stati poi sottoposti a test DCB ed ENF, utili per valutare la tenacità a frattura interlaminare in Modo I e Modo II.

In the common design and certification process, a large number of experimental tests, going from small specimens to the full structure, are performed. In these tests, the environmental conditions, especially temperature and humidity, which the structural component will be submitted are considered. This multiplies the number of tests to be performed. With the aim of reducing the economic cost and the time needed for the design and the certification of components, numerical tools that allow a reduction of experimental tests and help in the analysis of results, are developed. In this framework, the present thesis tackles the topic of hygrothermal effects in composite materials in both fields: experimental tests and numerical tools. Specifically, the thesis focuses its attention on two analysis levels which are seldom present in the scientific literature: the optimization of structural components and the experimental characterization of the translaminar failure of the material, both considering environmental changes
En el proceso habitual de diseño y certificación de componentes aeronáuticos se realiza un elevado número de ensayos experimentales que van desde pequeñas probetas hasta la estructura final. La consideración de las condiciones ambientales, especialmente temperatura y humedad, a las que el componente se verá sometido, incrementará el número de ensayos a realizar. Para disminuir el elevado coste y el tiempo de diseño y certificación, se desarrollan herramientas numéricas que permiten sustituir parte de los ensayos experimentales y facilitan el análisis de resultados. En este ámbito, la presente tesis enfoca la problemática de la consideración de los efectos ambientales tanto en los ensayos experimentales como en el desarrollo de herramientas numéricas. Concretamente, se centra en dos niveles de análisis con escasa o nula presencia en la literatura científica sobre efectos higrotérmicos: optimización de elementos estructurales considerando cambios ambientales y la caracterización experimental del fallo translaminar del material con variaciones higrotérmicas

Few decades ago, there were no guidelines for torsion design of reinforced concrete (RC) beams. Hence, many existing beams in older buildings have a lack of adequate torsional strength since they were not properly designed for torsion. One way to regain/rehabilitate adequate torsional strength is through application of externally bonded carbon fiber reinforced polymers (CFRP). To date, American Concrete Institute (ACI) code, as well as other building codes, do not have recommendations or provisions for strengthening RC beams for torsion using fiber-reinforced polymer (FRP) composites due to the inexistence of conclusive experimental and analytical data. Of the very limited works on this behavior, the majority of the focus has been devoted to experimental works. Realistic spandrel beams in a building that lack torsional strength were modelled in this research, and strengthened to examine various behaviors such as load capacity, deflection, torque, twist, crack propagation, ductility, and failure modes. For this purpose, six RC beams were tested: four reference beams and two strengthened beams were used to observe additional capacity through the use of carbon fiber-reinforced polymer (CFRP) sheets. To strengthen the beams, one layer of sheets was completely wrapped around them. Results show an additional torsional capacity of 63% and 178% relative to their respective reference beams. Through strengthening, modes of failure of the beams changed from brittle torsion-dominated failure to shear-flexure failure in both beams. The study also included crack pattern and ductility of test beams. Cracks became smaller in width and more evenly distributed across the torsion-loaded area, and torsional ductility was enhanced by 266% and 165% respectively. Flexural ductility was also greatly enhanced by more than five folds. Finally, using ACI 318-14, ACI 440.2R-02, and available formulae in the literature, the beams were analyzed and the respective values were compared.

The introduction of Fibre Reinforced Polymers (FRP) to the civil engineering market in the late 1980s resulted in the emergence of a range of new tools for rehabilitating and strengthening concrete structures. Strengthening using FRPs is typically accomplished using non-prestressed externally bonded FRPs. The technical and economic benefits of such strengthening could be further increased by prestressing the FRPs, especially when dealing with concrete structures. Prestressing concrete structures suppresses the appearance and growth of cracks in the serviceability limit state. This in turn increases the structure’s stiffness and resistance to degradation. Prestressing also increases the structure’s yield load but does not change its failure load relative to that of an analogous non-prestressed structure, provided that all other parameters are kept constant. In 2004, a pilot study was carried out at the Luleå University of Technology (LTU) to investigate the scope for using unbonded Carbon Fibre Reinforced Polymer (CFRP) strengthening systems, particularly those involving prestressing. In the early stages of this project, a number of difficulties were encountered in anchoring the CFRP rods to concrete structures: the conical wedge anchorages that were used tended to either cause premature failure of the rods or allowed the rod to slip out of the anchorage. It was therefore decided to study the mechanisms at work within these anchorages in more detail. The goal of the project was to develop a small, practical, reliable, and userfriendly anchorage for use in unbonded external CFRP strengthening systems. On the basis of a thorough literature review, which is described in Paper 1, it was concluded that despite the difficulties encountered, the conical wedge anchorages used with steel reinforcing rods were the most promising starting point for the design of a new anchorage for use with CFRPs. Importantly, the conical wedge anchorage can be made small in size and easy to mount while retaining a high degree of versatility; this is not true of bonded, sleeve, and clamping anchorages. Analytical and numerical models were used to investigate the distribution of radial stress within these highly pressurized anchorages. Paper 2 describes an evaluation of the capability of three types of models - an analytical axisymmetric model based on the thick-walled-cylinder-theory and two Finite Element (FE) models, one axisymmetric and one three-dimensional - to predict the behaviour of a conical wedge anchorage. It was concluded that the axisymmetric models were incapable of modelling the stress distribution within the anchorage with sufficient accuracy, and so 3D FE models were used exclusively in subsequent studies. Paper 3 describes the development of a new anchorage for CFRP rods. The design process involved conducting pull-out studies on a series of prototypes, in conjunction with computational studies using a basic FE model, to identify and understand the prototypes’ failure modes. Between the computational data and experimental results, a good understanding of the factors affecting the interaction between the CFRP rod and the anchorage was obtained. The new anchorage design employs a one-piece wedge which effectively incorporates the three wedges and the inner sleeve from more conventional wedge anchorages into a single unit. This increases the reliability and user-friendliness of the anchorage because it eliminates the need to check the alignment of individual wedges. The new design has been patented; the published Swedish patent is included in the thesis as Paper 6. The newly-developed anchorage was then incorporated into a prestressing system and its performance was evaluated using a series of test beams. In parallel with the planning of these tests, a series of pull-out tests was conducted using the new anchorage. The strain measurements obtained in these experiments were compared to predictions made using a new, more advanced FE model, and used to refine the design of the new anchorage. Paper 4 describes this new FE model, the most important parameters affecting anchorage behaviour, and the final anchorage design. Paper 5 focuses on the possibilities provided by the new anchorage. Tests were performed using seven three meter long concrete beams prestressed with external unbonded CFRP tendons. One beam was unstrengthened; the other six were strengthened in different ways, with different prestressing forces, initial tendon depths, and with or without the use of a midspan deviator for the tendons. The results of these tests were compared to those obtained using otherwise identical beams prestressed with steel tendons and to the predictions of an analytical beam model developed for use with steel tendons. These tests showed that the prestressing works as intended and that the behaviour of beams prestressed with external unbonded CFRP tendons is fully comparable to that of beams prestressed with steel tendons. It was also found that the predictions of the analytical model were in good agreement with experimental observations, although there were some differences between the measured and predicted tendon stresses. The development of a functional anchorage represents a fulfilment of the objectives laid out at the start of this project, and represents an important step towards the practical use of prestressed unbonded external CFRP tendons in strengthening concrete structures. However, a number of outstanding questions remain to be addressed. Little is known about the safety of this kind of system, and the benefits of using CFRP tendons should be quantified. Furthermore, there are a number of potential technical issues that must be addressed. These include the risk of creep-rupture in the CFRP, the effects of thermal contraction and expansion on the anchorage, and the scalability of the anchorage as the tendon diameter is increased. Finally, the long-term behaviour of the anchorage and prestressing system should be investigated.
I och med introduktionen av fiberkompositer i byggbranschen under slutet av 80-talet har en rad nya verktyg för förstärkning och underhåll av betongkonstruktioner utvecklats. Förstärkning har oftast utförts med pålimmade kompositer utan förspänning. För att ytterligare öka verkningsgraden, både den tekniska och ekonomiska, kan förspänning vara en möjlighet. Särskilt för betongkonstruktioner. Förspänning av en betongkonstruktion medför att man i bruksgränstillståndet begränsar uppkomsten av sprickor och deras storlek. Det ger i sin tur en ökad styvhet hos konstruktionen. Därutöver höjs lasten för när det slakarmerade stålet flyter. I jämförelse med ospända konstruktioner är dock brottlasten densamma, så länge övriga parametrar behålls. Under 2004 genomfördes en pilotstudie vid Luleå tekniska universitet (LTU) för att undersöka framtida möjligheter och utmaningar med förspända, icke vidhäftande kolfiberkompositkablar. I det läget upptäcktes svårigheter att förankra kompositkabeln mot betongen. De koniska killås som användes orsakade antingen brott på kabeln redan vid låga belastningar eller glidning hos kabeln, som omöjliggjorde fullgod kraftöverföring. Ett beslut togs då att tills vidare fokusera på förankringen och genomföra en mer ingående studie kring denna. Som mål sattes upp att arbetet skulle resultera i en liten, tillförlitlig och användarvänlig förankring. Den skulle sen i en förlängning kunna användas för att slutföra pilotstudien och därefter i större tillämpningar. Trots de förhållandevis nedslående resultaten från pilotförsöken visade den grundliga litteraturstudien som presenteras i Artikel 1 att koniska killås trots allt verkar vara den mest lovande typen av förankring för kolfiberkablar. Den bör därför användas som utgångspunkt för fortsatt utveckling. I motsats till vidhäftande, hyls och klämmande förankringar kan killåset göras litet, lätt att montera och också användas i många praktiska tillämpningar. För att undersöka hur de höga radiella tryckspänningarna i ett sådant killås fördelas är olika former av beräkningsmodeller nödvändiga verktyg. I Artikel 2 jämförs tre olika modeller med avseende på hur väl de kan beskriva komplexiteten hos ett koniskt killås. Det är dels en analytisk axisymmetrisk modell, som också härleds i artikeln, dels en axisymmetrisk Finita Element (FE) modell och dels en 3D FE modell. Undersökningen visade att ingen av de axisymmetriska modellerna har kapacitet nog att tillförlitligt modellera killåset. I fortsatta undersökningar har därför endast 3D FE använts. Resultaten från en enkel FE modell ligger också, tillsammans med tidiga laboratorieförsök, som grund för Artikel 3. Däri beskrivs hur ett nytt killås via prototyper och nya lösningar utvecklats, och hur arbetet för att få fram det nya låset också gett en bättre förståelse för interaktionen mellan kolfiberkompositkabel och lås. Som avslutning presenteras en innovativ design där de tre kilarna och den inre hylsan sammanfogats till en enhet. Med den nyutvecklade designen blir förankringen såväl mer tillförlitlig som användarvänlig. Alla kilar har då redan från början rätt position i förhållande till varandra. Den utvecklade förankringslösningen har också lett fram till ett beviljat svenskt patent, bifogat i avhandlingen som Artikel 6. Efter utvecklingen av den nya förankringen var nästa steg i de uppsatta målen implementering av densamma i ett förspänningssystem och nya balkförsök i konstruktionslabbet. Parallellt med planeringen för balkförsöken pågick ett arbete med att ytterligare förbättra låsdesignen. Bland annat användes en mer detaljerad FE modell som sedan jämfördes med mätningar från en ny serie med dragprov. Den nya FE modellen tillsammans med en utvärdering av viktiga parametrar och den slutliga förankringsdesignen presenteras i Artikel 4. Artikel 5 sammanfattar och avslutar forskningsstudien med en testserie om sju stycken, tre meter långa, betongbalkar förspända med utanpåliggande kolfiberkompositstavar. En av balkarna provades utan förstärkning. Förstärkningen hos de övriga varierades med avseende på förspänningsgrad, förspänningens effektiva höjd och användandet av deviator vid balkmitt. Resultaten har jämförts mellan de provade balkarna, med identiska balkar förspända med stålkablar samt med en analytisk modell utvecklad för förspänning med stålkablar. Från resultaten kan utläsas att förspänningen fungerar bra och att beteendet hos balkarna förspända med utanpåliggande kolfiberkablar är fullt jämförbart med det hos balkarna förspända med stålkablar. Likaså visar jämförelsen med de modellerade beteendena på god överensstämmelse, även om vissa skillnader finns mellan uppmätta och modellerade spänningar i kolfiberkabeln. Med målen för forskningen uppfyllda och en ny fungerande förankring framtagen så har vägen till praktiska tillämpningar kortats betydligt, ändå finns några frågetecken kvar att räta ut. Ett är säkerheten hos den här typen av system och nyttan av att använda kolfiberkomposit istället för stål. Innan systemet används i praktiken bör därför följande frågeställningar belysas: Risk för krypbrott i kolfiberarmeringen, inverkan av temperaturförändringar (och temperaturrörelser) i förankringen samt eventuella storlekseffekter vid förankring av kablar med större diametrar. De här frågorna tillsammans med långtidsförsök på förankringen och förspänningssystemet bör ses som viktiga framtida forskningsfrågor.
Godkänd; 2011; 20110128 (ysko); DISPUTATION Ämnesområde: Konstruktionsteknik/Structural Engineering Opponent: PhD Chris Burgoyne, Dep of Engineering, University of Cambridge, UK Ordförande: Professor Björn Täljsten, Institutionen för samhällsbyggnad och naturresurser, Luleå tekniska universitet Tid: Fredag den 18 februari 2011, kl 10.00 Plats: F1031, Luleå tekniska universitet

A recent innovation for the shear strengthening of reinforced concrete (RC) beams is to externally bond Carbon Fibre Reinforced Polymer (CFRP) composite plates or strips. This technique has become popular because of the many advantages of CFRP composites such as: high strength-to-weight ratio, good corrosion resistance, and versatility in coping with different sectional shapes and corners. This study focuses on shear strengthening of structural members using CFRP. The understanding of concrete structures designed for strengthening in shear is still an area where uniform design rules do not exist or are treated very briefly. The research programme to study the shear contribution of externally bonded CFRP sheets/strips of RC beams includes laboratory tests of more than twenty-nine beam samples (of an original conceptual model incorporating a shear plane) with beams of different materials: nine aluminium beams, twenty concrete beams, and some timber beams for initial studies. There are twenty-six pure tensile laboratory tests to study bond behaviour between the parent material and CFRP. In addition, there are six pure shear specimens and tests to determine other material properties. The numerical analyses employ the finite element method and many numerical models are developed for simulation of the contribution of the CFRP for shear strengthening and bond strength. On completion of the experimental programme and FE analyses, the resulting information is used to formulate a new proposal for shear strengthening of concrete beams using CFRP. The bond strengths predicted using existing methods and new proposals from this study are compared with experimental data of this study and previous studies, demonstrating that the new proposals are valid and offer improvement over existing methods.

This Thesis investigates the damage sensing capabilities of the electrical potential measurement technique in carbon fibre reinforced polymer composites. Impact damage was introduced in multidirectional laminates and its effect on potential distribution studied. It was found that delaminations and fibre breakages within the laminate can be detected and located by measuring potential changes on the external composite surface. The extent and size of potential changes were significantly affected by the position of the current electrodes in relation to the potential measurement probes. A numerical model was developed investigating the effect of different size delaminations, located in various positions within the lamina, on electrical potential distributions on the external ply, and a quantitative analysis of the numerical results is presented. The numerical simulations demonstrated that the measured potential changes on the external ply were in proportion to the delamination size. The numerical and experimental results were compared and the optimum configuration of current electrodes and potential probes for damage detection selected. The response of electrical potential to mechanical strain, in unidirectional and multidirectional samples was also investigated. It was found that the conductive medium, used for introducing the current, defines the piezo-resistance performance of the composite. A finite element model was developed able to predict the effect of inhomogeneous current introduction in unidirectional specimens on electrical potential and piezo-resistance. The effects of temperature and water absorption on potential measurements were also presented.

Advanced composites have become one of the most popular methods of repairing and/or strengthening civil infrastructure in the past couple of decades. While the use of Fiber Reinforced Polymer laminates and sheets for the repair and strengthening of reinforced concrete structures is well established, research on the application of FRP composites to steel structures has been limited. The use of FRP material for the repair and rehabilitation of steel members has numerous benefits over the traditional methods of bolting or welding of steel plates. Carbon FRPs (CFRPs) have been preferred over other FRP material for strengthening of steel structures since CFRPs tend to posses higher stiffness. The emergence of high modulus CFRP plates, with an elastic modulus higher than that of steel, enables researchers to achieve substantial load transfer in steel beams before the steel yields. This research investigates both analytically and experimentally, the bond characteristics between ultra high modulus CFRP strengthened steel members and the flexural behavior of these members. A series of double strap joint tests with two different CFRP strip widths are carried out to evaluate the development length of the bond. Both ultra high modulus and normal modulus CFRP laminates are used to compare strengthened member performance. Steel plates reinforced with CFRP laminates on both sides are loaded in tension to evaluate the load transfer characteristics. Debonding under flexural loads is also studied for ultra high modulus CFRP strengthened steel girders. Flexural tests are carried out under 4-point bending on several small scale wide flange beams. This study also introduces the novel ultra high modulus CFRP plate strip panels for strengthening of steel bridge girders. The first field application of ultra high modulus CFRP laminates in strengthening steel bridge girders in the United States is also carried out as part of the research. Full scale load tests carried out before and after the strengthening are utilized to measure the degree of strengthening achieved and checked against the expected results. A finite element model is developed and calibrated using data obtained from the field testing of the bridge. The model is then used to evaluate the behavior of the bridge under different conditions before and after the strengthening process.

The research relates to a study on the routing/slotting of CFRP composites of the type used in aerospace applications. Following a literature review, 3 phases of experimental work were undertaken to evaluate the effects of key process variables on the machinability of CFRP. The influence of varying operating parameters, tool material and cutting environment were initially investigated in Phase 1 work. The results showed that use of PCD was critical and highlighted the importance of chilled air in maintaining adequate tool life and acceptable workpiece integrity. Delivery of chilled air through a single-nozzle arrangement generally led to an increase in forces and delamination with the twin-nozzle configuration showing superior workpiece surface roughness. Phase 2 work detailed the effect of workpiece lay-up configuration on cutting forces, temperature and surface integrity following slotting and routing. Plies in the 45 direction generally exhibited the highest level of surface damage following machining. Experiments in Phase 3 showed that relatively small helix angles (± 3) had a negligible effect on tool life, forces and temperature. In addition, cutters with a single relief angle were found to have lower stability in operation compared to tools with a secondary clearance angle, with detrimental effects on surface roughness.

Numerous studies have been carried out on confined concrete columns and externally strengthened concrete beams. The main concern, throughout these studies, was the static and short-term response. However, little has been done concerning the creep behaviour of such elements. This investigation presents a series of experiments that were carried out at the University of Sherbrooke in an attempt to clarify the time-dependent behaviour of FRP-strengthened concrete elements. Forty-two concrete cylinders, unconfined and confined with carbon fibre reinforced polymer (CFRP) wraps, in addition to 26 concrete beams without and with external FRP laminates, were investigated for their creep behaviour. Also, FRP specimens were examined under different levels of sustained load. The main parameters of this study are (i) the level of sustained stress and (ii) the strengthening scheme. The major objectives of this study may be classified into two parts: (i) to illustrate the effects of the confining pressure and the applied stress on the time-dependent behaviour of FRP-confined concrete columns, and (ii) to evaluate the effect of applied sustained load on the immediate and the long-term deflection of strengthened beams and examine the efficiency of the FRP laminate as a strengthening scheme. It was confirmed that FRP materials are effective in increasing the capacity of the strengthened members. Consequently, safety is guaranteed in increasing the magnitude of the applied sustained loads. Despite this fact, when beams and columns are subjected to higher sustained load levels, beam deflections and/or column deformations are inevitable. Under such levels, FRP strengthening has proven to yield satisfactory results in increasing the member capacities. Yet, the long-term deflection performance for beams as well as the long-term axial column deformation were barely enhanced. As regards FRP-confined columns, the obtained results have confirmed the effectiveness of FRP confinement in increasing the bearing capacity and the ductility of concrete columns. The enhancement of the long-term creep performance of FRP-confined columns, which is the focal objective of this study, has been demonstrated through the systems' capability of sustaining higher loads. However, such high sustained load levels resulted in a large amount of strain. The magnitude of this strain may be considered the criterion that will dictate the level of sustained load, in accordance to the design guides' permissible deformation. Strengthening concrete beams by means of FRP laminates bonded to the beam tension surface has proven to increase the resisting moment of beams. Likewise the confinement of columns, external strengthening of beams has permitted the application of higher sustained loads that were not allowed in the absence of FRP laminates. Yet under high levels of sustained loading, the added laminates which have not significantly increased the beam stiffness did not contribute significantly to decrease the long-term deflection. Similar to the discussion on columns, the permissible deflections listed in the codes may dictate the limits of the applied load on FRP-strengthened concrete beams"--Résumé abrégé par UMI

Master of Science
Department of Architectural Engineering
Donald J. Phillippi
For this thesis, research was performed on CFRP bonded composite lap-joints with one and two continuous laminas through the lap. Composite wraps used to retrofit existing structures use lap joints to maintain their integrity. The use of composites for retrofitting structures has many advantages over traditional methods, such as steel jacketing, and is becoming more widely accepted in the structural engineering industry. While much literature exists documenting the performance of composite wraps as a whole when applied to concrete columns, less information is available on the behavior of the lap-joint of the wrap. Developing a better understanding of how the lap-joint behaves will help researchers further understand composite column wraps. This research sought to determine what affect continuous middle laminas may have on the stiffness of lap joints and whether or not stress concentrations exist in the lap-joint due to a change in stiffness.

Mestrado em Engenharia de Materiais
VII Abstract In the present work “smart” nanocontainers were synthesized in order to incorporate them into an organic coating and protect against corrosion of the aluminum alloy (AA2024) galvanically coupled with carbon fiber reinforced plastic (CFRP). The containers were loaded with organic (2-mercaptobenzothiazole and 1,2,3 – benzotriazole) and inorganic (metavanadate, tungstate and molybdate) inhibitors in the case of Mg/Al and Zn/Al LDH nanoreservoirs. In the case of the bentonite nanocontainers, the containers were loaded with Ce(NO3)3. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses were performed in order to characterize the obtained nanocontainers. The nanocontainers were enbeded into epoxy coating on the surface of model multi-material system (AA2024 galvanically coupled with CFRP). The LDHs loaded with different inhibitors and combined with bentonite loaded with Ce3+, increase the anticorrosion protection properties of the coating. The analyses of the anticorrosion properties of the coatings were performed using zero resistance ammetry (ZRA), electrochemical impedance spectroscopy (EIS), optical microscopy, salt spray test (SST) and scanning vibrating electrode technique (SVET) measurements. The laboratory work was realized in University of Aveiro in collaboration with industrial environment of Airbus group.
No presente trabalho revestimentos "inteligentes" foram sintetizados com a finalidade de proteger contra a corrosão a liga de alumínio AA2024 acoplada galvanicamente com a fibra de carbono reforçado com plástico (CFRP). Os nanocontentores LDH Mg/Al LDH e Zn/Al LDH foram carregados com os inibidores orgânicos 2-mercaptobenzotiazole e 1,2,3-benzotriazole, e com inibidores inorgânicos metavanadato, tungstato e molibdato. No caso dos nanocontentores de bentonite o inibidor incorporado foi o Ce(NO3)3. A análise por difração de raios-X (DRX) e microscopia eletrónica de varrimento (MEV) foram realizadas a fim de caracterizar os nanocontentores obtidos. Os nanocontentores foram aplicados em revestimento epóxi na superfície do sistema modelo (AA2024 galvanicamente acoplado com CFRP), os LDH’s preenchidos com inibidores em mistura com Ce3+ carregado na bentonite foram usados com o objetivo de aumentar as propriedades de proteção do revestimento contra a corrosão. As análises das propriedades anticorrosivas dos revestimentos foram realizadas utilizando o ZRA (Zero resistance ammeter), espectroscopia de impedância eletroquímica (EIS), microscopia ótica, teste de nevoeiro salino (SST) e SVET (scanning vibrating electrode technique). O trabalho foi realizado em ambiente laboratorial e posteriormente em ambiente industrial (Airbus group).

Drilling on fibre reinforced composites is a crucial process in fabrication of airframes in aircraft industry. In this research, an extensive experimental investigation on drilling and machining CFRP laminates using different tools is carried out to analyse effects of processing parameters on drilling performance. Drilling performance and quality of circular holes on a commercial aircraft CFRP laminate are investigated, using drill bit with three different configurations made of solid carbide, namely GT50 dagger drill, GT15 reamer drill, and twist drill. Back support of different geometry, as full support, partial support and no support, is employed during drilling at spindle speeds of 500, 1000, and 2000 rpm, and feed rate of 50 mm/min. Thrust force and torque, are measured. Quantification of the quality and holes integrity is accomplished by evaluating surface roughness, heat distribution, drilled hole roundness or circularity, chip size, and damage factor. The second major study is an energy-based analysis based on the energy balance model established by William’s on cutting polymers is presented by addressing Mode I fracture as a key mechanism in different cutting directions in a unidirectional CFRP laminate, induced by orthogonal cutting. Then, tool wear and tool life of dagger and reamer drill bits are investigated, evaluating blunting and wear of the tools. With that, assessment on tool wear and tool life are made by addressing their significant influence on thrust force and torque during drilling, delamination factor in the CFRP laminates, fibre peel-up and push-down mechanisms, surface roughness and temperature increase. Lastly, finite element analysis is added to explore and predict the drilling mechanism and chip removal mechanism as a function of failure criteria. With all that has been addressed above, this study plays a critical role for selection of the optimal drilling conditions for minimising production cost and maximising productivity.

The use of fiber reinforced polymers (FRP's) for structural repair or retrofit has increased significantly in the last decade, with adoption for civil infrastructure occurring only in the last 20 years. These products are most often used to increase the capacity of damaged or deteriorated structures. Much research has been performed in the arena of testing of various FRP's bonded to both concrete and masonry substrates, the majority of which focusing on three areas; flexural strengthening, in-plane shear strengthening, and mechanical anchoring. Anchorage is commonly the limiting factor in the application of FRP's, due to the inability of the edge of the polymer matrix to reliably extend beyond a point of zero-interfacial stress. Where interfacial stresses exist and the FRP is terminated localized disbondment often occurs, these localized failures then propagate across the entire bond of the structural system. Various mechanical termination details have been tested to mitigate the potential failure modes near the ends of the fabric. There, however, has been very limited research performed on the behavior of dowels which are installed parallel to the FRP fabric and splayed onto the FRP fabric matrix. In this research the mechanical properties of carbon fiber reinforced polymer (CFRP) dowels with a parallel orientation to uniaxial carbon fabric are experimentally tested to determine the tensile capacity of "dowel to splay" CFRP connections and to discover any dominant failure modes.

Esse trabalho apresenta a validação do método numérico dos elementos finitos para estimar a resistência ao cisalhamento da interface entre camadas de um compósito polimérico de fibra de carbono. Foram realizados ensaios de Resistência ao Cisalhamento Interlaminar (ILSS, interlaminar shear strength) para validação do modelamento numérico. O método numérico consistiu no desenvolvimento de dois modelos em elementos finitos utilizando um programa comercial (ANSYS Rev. 10). O primeiro usando elementos finitos de casca tridimensional e o segundo, usando elementos finitos planos para simular o ensaio ILSS. O modelo numérico que apresentou resultados mais próximos aos experimentais, o modelo tridimensional de casca, apresentou um erro de apenas 5,6%, indicando uma aproximação bastante satisfatória.
This work presents the validation of the numerical method of finite elements to estimate the interlaminar shear strength in a carbon fiber reinforced polymeric composite. ILSS (interlaminar shear strength) tests were performed to validate the numerical modeling. The numerical method consisted of two different finite element models using a commercial software (ANSYS Rev. 10.0). The first model uses tridimensional shell finite elements and the second model, plane finite elements to simulate the ILSS test. The numerical method that presented the closest results to those from the experimental method was the tridimensional shell model, with an error deviation of only 5.6%, which indicates very good precision

Acoustic emissions are small vibration pulses, elastic waves, emitted from damage processes such as crack growth inside a material. Acoustic emission (AE) is also the name of the test method in which theses emissions are recorded and analysed and the method is used in materials research and the testing and inspection of structures. At Scania, a large manufacturer of trucks and buses, previous attempts to implement this technique has been unsuccessful due to the fact that the hydraulic rigs in which the material typically is tested, produce a high background noise level, that covers the interesting emissions from the material.In this thesis two materials, a grey iron and a carbon fiber reinforced polymer were tested in a hydraulic rig at Scania. Since the material signal was buried in the noise, the entire waveform was recorded, which is an unusual approach, since it generates large amounts of data. It was shown that using frequency analysis, it is possible to extract the material emissions in spite of the hydraulic noise. That fact makes it possible to follow the internal processes of the material leading up to failure, which means new interesting opportunities in materials testing at Scania.
Akustisk emission är små elastiska vågor som bland annat kommer från processer i ett material, såsom spricktillväxt. Akustisk emission (AE) är namnet på den testmetod där dessa vibrationer registreras och analyseras. Metoden används i materialprovning och för att testa och inspektera komponenter, såsom tryckkärl. På Scania, en stor tillverkare av lastbilar och bussar, har tidigare undersökningar för att implementera denna teknik på utmattning inte lyckats. Anledningen ligger i att de hydrauliska riggarna som testningen vanligtvis sker i, typiskt sett genererar ett bakgrundsljud som skymmer den intressanta signalen från materialet. I detta examensarbete testades två typer av material, gråjärn och en kolfiberarmerad komposit, i en hydraulisk rigg på Scania. Eftersom de akustiska emissionerna från materialet gömdes i bakgrunden användes metoden att spara ner hela vågformen för signalen, vilket är ovanligt eftersom detta innebär att mycket stora mängder data måste sparas. Det visade sig genom frekvensanalys vara möjligt att extrahera de akustiska emissionerna från materialet, trots det hydrauliska bruset. Det faktum att det är möjligt att följa de processerna inuti materialet, som föregår brottet, öppnar upp nya intressanta möjligheter för materialprovning på Scania.

Valutazione degli effetti ambientali sulle caratteristiche meccaniche di materiali compositi avanzati. Laminati in fibra di carbonio e resina epossidica, sottoposti a cicli termici, sono stati analizzati per determinare le resistenze strutturali degli stessi. I valori ottenuti sono stati comparati con materiale non invecchiato.

Nella tesi viene caratterizzato meccanicamente un giunto incollato acciaio-CFRP usato nella produzione di un componente automotive. In particolare viene analizzata l'influenza data da trattamenti termici di invecchiamento e sono messe a confronto diverse tipologie di adesivi.

Composite material parts are typically produced in the near-net-shape, i.e. very close to the finished product. However, additional machining processes are often required to meet dimensional and tolerance requirements. Particularly drilling stands out as the most widespread machining process of carbon fibre reinforced plastic (CFRP) composite parts, primarily in the aerospace industry, due to the widespread use of mechanical joints, such as rivets, rather than welded or bonded joints. CFRP drilling is noticeably challenging due to material abrasiveness, inhomogeneity and anisotropic properties; tool wear rates are inherently high leading to superior cutting forces and detrimental effects on part surface quality and material integrity. Damages such as delamination, cracks or matrix thermal degradation are often observed as the result of uncontrolled tool wear or improper machining conditions. The development of effective non-destructive control techniques, such as optical inspection for drilled hole quality assessment for process, tool and product quality evaluation is dealt with in this PhD thesis with the aim to contribute to the reduction of scraps and tool costs as well as to the improvement of process productivity in the drilling of CFRP composite material parts for aeronautical assembly. In this thesis work, the most effective parameters for hole quality evaluation in drilling of CFRP/CFRP and Al/CFRP stacks for aeronautical assembly were studied and selected. An automatic technique based hole image processing was developed for the evaluation of these quality parameters for holes drilled with a traditional tool and an innovative tool purposely developed. Tool wear was finally evaluated to verify its relationship with the hole quality.

碩士
國立臺北科技大學
土木與防災研究所
98
CFRP (Carbon Fiber Reinforced Polymer) with high tensile strength, high durability and reinforcement components do not change the appearance of retrofitted member and so on. Results of previous studies show that the RC beams shear retrofitted by externally wrapping CFRP sheets were often unable to take full advantage of the expected CFRP high strength cause of the spalling due to interface failure (Debonding) between the RC surface of the beams and the CFRP sheets. Therefore, this paper proposes the use of anchored mechanism to overcome this problem. In this study, a total of six full-scaled rectangular RC beam specimens were designed for the four-point bending tests in National Center for Research on Earthquake Engineering (NCREE) and the advanced “image-based measurement” was used in this research to obtain the global strain field of the external wrapping CFRP sheets. One of these specimens is the bench mark specimen was designed to subject the brittle shear failure whiling the monotonic loading tested. The other specimens retrofitted by different externally CFRP U-shaped wrapping were divide into three groups. The first one group is pure CFRP U-shaped wrapping specimen, the second group specimen is CFRP retrofitted specimen with adhesive anchors and the third group includes three specimens retrofitted by CFRP sheets with the different use amounts of CFRP anchors. Test results indicated that the debonding between concrete surface of the beam and CFRP sheet occurred at the above of central line of beam section in the shear region caused by the shear crack of the specimen effect the strength of CFRP wrapping retrofitted. The strength of the retrofitted specimen with the use of adhesive anchors was increased but the failure behavior was still brittle shear failure. The CFRP retrofit specimens using CFRP anchors not only increase the strength but also the ductility. The failure behaviors of these three specimens used different amounts of CFRP anchors were flexural failure with huge deformation. The local strain data from the traditional strain gauge use on the surface of CFRP sheets was similar to the strain data from image-based measurement.The research of the CFRP anchor design procedures, the proposed working method of the RC beam retrofitted demonstrated effect and the used of advanced image-based measurement in this study is acceptable.

碩士
國立臺北科技大學
土木與防災研究所
96
External confinement by the wrapping of CFRP sheets (or CFRP jacketing) provides a very effective method for the retrofit of reinforced concrete (RC) columns subject to either static or seismic loads. Research result showed that retrofitting of rectangular columns by CFRP wrapping was ineffective because of crushed column concrete cause of bulging of column sides, thus, this study propose “CFRP wrapping conjugate CFRP anchors” rehabilitate method. The objective of this study was to research, develop the CFRP Anchor and to evaluate the efficacy of seismic retrofit of rectangular columns by CFRP wrapping and CFRP Anchors. Five specimens of retangular reinforced concrete columns were tested by cyclic lateral force under reversed bending and constant axial load (0.2f’cAg.). Two different techniques of using CFRP wrapping and CFRP Anchors to improve concrete confinement, shear strength and moment strength in these specimens. These specimens were divided into two groups. Two specimens of group 1 were retrofitted shear strength and ductility by CFRP wrapping and CFRP anchors and two specimens of group2 were retrofitted moment strength by longitudinal CFRP sheets and CFRP anchors and shear strength, ductility by CFRP wrapping. From test result of group 1, the behavior and effectiveness indicated that comparing with test conducted on the specomen wrapping by CFRP without anchor, anchoring techniques used were effective in improving the column confinement and in increasing the ability of energy disspation. From test result of group 2, the initial elastic modulus increase cause of logngitudinal CFRP sheets linked the foundation by CFRP anchor. But it’s failure in linking mechanism of CFRP anchors by low cycle fatigue to do justice to moment strength. The research and develop of CFRP anhor in this study of it’s used methods and theorem in rehabilation had reinforcement effect.The study of CFRP anchor is referable for seismic retrofit engineerings.

Il lavoro di tesi si pone come obiettivo quello di realizzare un modello numerico del processo di foratura mediante tecnica agli elementi finiti. L’obiettivo è quindi riuscire a prevedere il comportamento di un generico utensile di foratura durante il processo, prevedere il comportamento dell’utensile con il progredire dell’usura. Una volta realizzato il modello analitico sarà possibile inoltre prevedere anche il comportamento di un utensile con geometria non convenzionale, al fine di valutare in modo previsionale il suo livello di performance prima della produzione. Per la realizzazione del modello FEM è stato utilizzato un solutore di tipo esplicito e sono state studiate le diverse tipologie di contatti (contatto tra i diversi layer di materiale) e formulazione dell’elemento, le principali discriminanti di un modello analitico di questo genere. Sono stati settati i parametri di processo, quelli convenzionalmente utilizzati dai produttori in campo aeronautico, e già testati in alcune sperimentazioni ed il software ha restituito delle curve caratteristiche forza assiale (N) / tempo (s), caratteristiche del processo di foratura. È stata valutata l’influenza dell’usura sulla utensile, variando alcuni parametri caratteristici del materiale, in particolare i valori di failure, ossia quei parametri definiti sulla base di criteri energetici mediante i quali un elemento del modello collassa, e quindi non partecipa più alla rigidezza del sistema, se sottoposto ad un carico superiore a quello stabilito. Per la taratura del modello è stato eseguito un confronto numerico-sperimentale sulla base di prove effettuate su stacks in carboresina (CFRP/CFRP) e stacks ibridi Al/CFRP realizzati con una punta di riferimento. La stessa punta è stata riprodotta in virtuale con un software di modellazione geometrica CAD ed utilizzata nel modello matematico.