Dissertations / Theses on the topic 'CNF/Epoxy Glass Fiber'

Dissertation (Ph. D.)--University of Akron, Dept. of Polymer Engineering, 2006.
"May, 2006." Title from electronic dissertation title page (viewed 10/11/2006) Advisor, Erol Sancaktar; Committee members, James L. White, Kyonsuku Min, Darrell H. Reneker, Wieslaw Binienda; Department Chair, Sadhan C. Jana; Dean of the College, Frank N. Kelley; Dean of the Graduate School, George R. Newkome. Includes bibliographical references.

Kinetic equation parameters for the curing reaction of a commercial glass fiber reinforced high performance epoxy prepreg composed of the tetrafunctional epoxy tetraglycidyl 4,4-diaminodiphenyl methane (TGDDM), the tetrafunctional amine curing agent 4,4'-diaminodiphenylsulfone (DDS) and an ionic initiator/accelerator, are determined by various thermal analysis techniques and the results compared. The reaction is monitored by heat generated determined by differential scanning calorimetry (DSC) and by high speed DSC when the reaction rate is high. The changes in physical properties indicating increasing conversion are followed by shifts in glass transition temperature determined by DSC, temperature-modulated DSC (TMDSC), step scan DSC and high speed DSC, thermomechanical (TMA) and dynamic mechanical (DMA) analysis and thermally stimulated depolarization (TSD). Changes in viscosity, also indicative of degree of conversion, are monitored by DMA. Thermal stability as a function of degree of cure is monitored by thermogravimetric analysis (TGA). The parameters of the general kinetic equations, including activation energy and rate constant, are explained and used to compare results of various techniques. The utilities of the kinetic descriptions are demonstrated in the construction of a useful time-temperature-transformation (TTT) diagram and a continuous heating transformation (CHT) diagram for rapid determination of processing parameters in the processing of prepregs. Shrinkage due to both resin consolidation and fiber rearrangement is measured as the linear expansion of the piston on a quartz dilatometry cell using TMA. The shrinkage of prepregs was determined to depend on the curing temperature, pressure applied and the fiber orientation. Chemical modification of an epoxy was done by mixing a fluorinated aromatic amine (aniline) with a standard aliphatic amine as a curing agent for a commercial Diglycidylether of Bisphenol-A (DGEBA) epoxy. The resulting cured network was tested for wear resistance using tribological techniques. Of the six anilines, 3-fluoroaniline and 4-fluoroaniline were determined to have lower wear than the unmodified epoxy, while the others showed much higher wear rates.

Thesis (Master)--İzmir Institute of Technology, İzmir, 2006
Keywords: polymer composites, Nanoparticles, glass fiber, mechanical properties, thermal properties. Includes bibliographical references (leaves 75-79).

The tensile strength of the fiber/matrix interface was determined through the development of an innovative test procedure. A miniature tensile coupon with a through-thickness oriented, embedded single fiber was designed. Tensile testing was conducted in a scanning electron microscope (SEM) while the failure process could be observed. Finite element stress analysis was conducted to determine the state of stress at the fiber/matrix interface in the tensile loaded specimen, and the strength of the interface. Test specimens consisting of dry E-glass/epoxy and dry and seawater saturated carbon/vinylester 510A were prepared and tested. The load at the onset of debonding was combined with the radial stress distribution near the free surface of the specimen to reduce the interfacial tensile strength (σ_i). For glass/epoxy, σ_i was 36.7±8.8 MPa. For the dry and seawater saturated carbon/vinylester specimens the tensile strengths of the interface were 23.0±6.6 and 25.2±4.1 MPa, respectively. The difference is not significant.

This thesis includes the study of the mechanical performance of two different types of fibers reinforced hybrid composites. Two kinds of fibers, natural fiber (flax) and synthetic fiber (E-glass), are used to reinforce epoxy resin. To evaluate the effective properties of the hybrid composites, a micromechanical analysis of the structure genome (SG) of a unidirectional fiber hybrid composites is performed using nite element analysis (FEA). Both fibers are assumed to be circular and packed in a hexagonal pattern. The effects of varying volume fractions and fiber locations, of the two fibers, on the elastic properties of the hybrid composites are studied using FEA. Rule of hybrid mixtures (RoHM) and Halpin-Tsai equations, which are analytical equations, are used as a preliminary prediction of the elastic constants of the hybrid composites. Then, the comparison is made between FEA and analytical results. The predicted elastic constants through numerical homogenization are in good a agreement with analytical results. The effect of changing fiber locations on the tensile strength of hybrid composite is investigated using tensile tests. Impact strength of single fiber composites and ax/glass fiber hybrid composites, in which various stacking sequences of ax and glass fibers are used, are obtained using Charpy impact tests. Moisture absorption test was performed by immersing single fiber composites and various stacking sequences of hybrid composites in deionized water at room temperature for a week. To investigate the effect of water absorption on the tensile properties of composite, tensile test was done on various stacking sequences of the hybrid composite. FEA and analytical equations showed that Young's and shear moduli increased and the axial Poison's ratio decreased linearly with the glass fiber content. Also, FEA showed that changing fiber locations have no effect on the effective properties of the hybrid composite. However, changing fiber stacking sequences showed a significant effect on tensile strength, impact strength, and water absorption properties of the hybrid composites. It was concluded that better design of the hybrid composite was achieved when glass fibers placed on the extreme positions and flax fibers in the middle. Positive hybrid effect is achieved from hybridization of E-glass fiber with flax fiber.

Metal implants are a type of hard tissue replacement currently used. Metals used for implants include: stainless steel, titanium, chrome, and cobalt alloys. Such implants often fail at the interface with bone. Metal implants fail when the surface of the implant is coated with an osteoconductive material. An osteoconductive material provides scaffolding for cellular migration, cellular attachment, and cellular distribution. A reason for metal implant failure could be the vastly different material properties than bone. Motivation for the research was to find a suitable bone substitute other than metal. Materials considered were: zirconia toughened alumina, carbon fiber reinforced epoxy, and glass fiber reinforced epoxy. Those materials have been used in previous biological applications and can be cast into complex configurations. Objectives of the study were to compare material properties of the composites to bone. A method to create porosity was then tested in the material that was similar to bone in critical material property. Some of the materials were statistically similar to bone in yield strength. Method to create interconnected porosity in those materials resulted in 49% void space.

Compression strength after impact tests were conducted on unidirectional composite rods with sleeves. These elements represent local members of open three-dimensional composite lattice structures (e.g., based on isogrid or IsoTruss® technologies). The unidirectional cores composed of carbon, glass, or basalt fiber/epoxy composites were co-cured in aramid sleeves. Sleeve patterns included both bi-directional (unsymmetric) braids and unidirectional spiral wraps with sleeve coverage ranging from nominally half to full. The diameters were nominally 8 and 11 mm (5/16 and 7/16 in). The larger diameter had nominally twice the cross-sectional area, to quantify the effects of scaling. The specimens were long enough to encourage local buckling failure as expected in members of typical composite lattice structures. The unsupported lengths varied from 127 mm (5.0 in) to 160 mm (6.3 in). Specimens were radially impacted at mid-length with energy levels ranging from 0 to 20 J (0 to 14.8 ft-lbs) and tested in longitudinal compression to quantify the effects of local impact damage on the buckling strength. In undamaged specimens, sleeve type and sleeve coverage have no effect on the ultimate compression strength of carbon, glass, or basalt composites (7% or less standard deviation for each material). When impacted, the influence of sleeve type and sleeve coverage varies with the type of fiber in the unidirectional core. Sleeve type and coverage did not affect the compression strength after impact for fiberglass composites. On the other hand, both carbon and basalt composites exhibited improved performance with braided (vs. spiral) sleeves (up to 34% stronger) and full (vs. half) coverage (up to 38% stronger). The compression strength of carbon configurations decreases with increasing impact energy regardless of sleeve type or coverage. The higher flexibility of glass and basalt composites, however, allowed some configurations to maintain the same compression strength after impact as their undamaged counterparts, at lower impact energy levels. Doubling cross-sectional area of basalt composites significantly improves the stiffness and compression strength after impact, more than doubling the impact energy required to achieve the same compression strength.

Essais d'identification d'une loi locale d'endommagement de fatigue basée sur la théorie de Kachanov-Rabotnov à partir d'essais mécaniques. Intégration de cette loi dans un calcul de structure. Introduction de la notion de dispersion par l'application d'une loi de Weibull. Identification de la dispersion locale à partir de la dispersion globale par la méthode de simulation de Monte-Carlo.

碩士
國立勤益科技大學
化工與材料工程系
99
In this study, novel nanocomposites are preparation and characterization. The Taguchi experimental design methodology is used to optimize the composition of a nanopowders/glass fiber epoxy resin material comprising nano-alumina, nano-silica, carbon black nanoparticle, epoxy resin, glass fiber and diluent. The effect of nanopowders and diluent addition on the corrosion resistance, thermal properties, and mechanical properties of the various samples are then observed. The results show that the addition of nanopowders will affect the properties of glass fiber/epoxy composites, resulting in thermal stability, corrosion resistance, glass transition temperature, hardness, storage modulus increased. Overall, that nanocomposite comprising 2 wt.% nano-Al2O3, 2 wt.% nano-SiO2, and 2 wt.% carbon black nanoparticle reduced the thermal expansion coefficient (α1) by 17.55% and increased the thermal decomposition temperature by 5.84% compared to that of the sample with no nanopowders. That nanocomposite comprising 2 wt.% nano-Al2O3, 2 wt.% nano-SiO2, 2 wt.% carbon black nanoparticle , and 3.75 wt.% diluent has the best storage modulus ,water and corrosion resistance. The experimental data generated in the Taguchi trials are processed using a regression analysis technique in order to derive analytical formulae relating the composition of the composite samples to their mechanical, thermal and corrosion properties. It is shown that the results obtained using the analytical formulae are in good agreement with the experimental observations. Thus, the derived formulae provide a quick and convenient means of predicting the mechanical and thermal response of glass fiber/ epoxy nanocomposites with a known composition without the need for experimental investigation.

碩士
中國文化大學
材料科學與製造研究所
87
ABSTRACT The reserch present a proprietary process developed to manufacture pultruded blocked polyurethane modified epoxy resin composites in order to improve the impact strength of pultruded epoxy composites. The effect of processing parameters on the mechanical properties of pultruded glass fiber reinforced epoxy/blocked PU composites has been studied. Processing parameters include die temperature, pulling rate, postcure temperature and time, filler type and content, and fiber content. From the viscosity test, the optimum temperature of impregnation tank is set between in 40 and 50℃. The pre- polymer has at least 1hr pot life in order to let the fiber having enough time for processing. From the result of SEM fracture surface, and longiltudinal , the fiber have good wet-out. From above discussion, the epoxy/blocked PU have excellent process feasibility for pultrusion. From the DSC and mechanical properties tests, the optimum die temperature range is between 220℃and 240℃. From the experiment results, the range of pulling rate is set at 20 ~ 60 cm/min, and the best pulling rate is 20 cm/min. It is found that the mechanical properties increase with filler content at 3 ~ 9 phr for calcium carbonate and kaolin,respectively.The optimum postcure temperature and time is 100℃and 1.5hr.The suitable fiber glass content is between 67.3 and 74.2 wt％.

碩士
國立交通大學
機械工程系所
94
This research is aimed to investigate the organoclay effect on mechanical behaviors of the fiber/epoxy/organoclay nanocomposites. Tensile, flexure and fracture behaviors were considered in this study. To demonstrate the organoclay effect, three different loadings, 2.5, 5 and 7.5 wt% of organoclay were dispersed in the epoxy resin using mechanical mixer followed by sonication. The corresponding glass/epoxy nanocomposites were prepared by impregnating the organoclay epoxy mixture into the dry glass fiber through a vacuum hand lay-up process. For the tensile behaviors, the coupon specimens were tested in MTS machine in both longitudinal and transverse directions. The flexural properties were characterized using three point bending tests. In addition, the fracture behaviors of the fiber composites were determined from the double cantilever beam specimens. From the tensile tests, it was revealed that the longitudinal tensile strength decrease as the organoclay loading increases, on the other hand, the transverse tensile strength as well as the transverse tensile modulus increases with the increase of the organoclay. SEM observation on the transverse failure specimens indicates that the enhanced mechanism is due to the interfacial bonding between the fibers and the surrounding matrix modified by organoclay. The similar tendency was also found in the transverse flexural strength of the composites. From the mode I fracture tests It was indicated that with the increase of the organoclay, the corresponding fracture toughness of the composites decreases appreciably. In addition to the unidirectional lamina, the quasi-isotropic laminates with organoclay were prepared and tested in tension. Experimental results depict that the strength of the laminates is not affected appreciably by the organoclay.

碩士
中華大學
機械工程學系碩士在職專班
99
The demand for high degree of strength, corrosion-resistant, and lightweight materials has been rapidly increased in the field of aerospace, automotive and engineering. Due to their good resistance to bending, high strength, and more flexible to be redesigned, composite materials have very wide range of applications. This study focus on how the composite materials are utilized in the sandwich-structure, and to be analyzed for their performance. To construct the sandwich-structure with composite materials, glass fiber/epoxy layer are stacked up as both the top and bottom layer of the structure, while the 30k-ps high-density polystyrene foam is used as the core layer the structure. The experiment on sandwich-structure was conducted by changing several variables, such as the stacking layers of glass fiber, and the thickness of polystyrene foam in the middle layer. Furthermore, the same test was performed again with adding new variables, such as single I beam and double I beam, in order to do the comparison between those two tests. The bending tests were performed to evaluate the performance of sandwich-structure. The results indicate that, when the stacking layers of glass fiber material increase, the specimens can correspondingly withstand greater loading, bending moment, and stress. In addition, the specimens can also withstand greater loading and bending moment when the thickness of core material increases, but not the stress. It is because the same material with the same stack of layers of glass fiber can generate the same value of stress. Furthermore, the sandwich-structure with single I beam and double I beam can tolerate greater loading, bending moment and stress as well. Keywords: glass fiber, epoxy, composites, sandwich structures, bending test.

碩士
長庚大學
化工與材料工程研究所
91
High voltage transformer insulation have been designed and manufactured using glass fiber reinforced epoxy composite materials by winding techniques. This class of materials offers excellent mechanical property, high electrical insulation,good thermal stability, easy processing in design. Some structure-property relationship issues were characterized and discussed in this paper. The results show that the Tg of this material is about 120℃, the modulus is 44 GPa at room temperature, and the breakdown voltage is 32 kV in the longitudinal direction.Those property in the transverse direction is much lower than longitudinal direction.This paper deals with two different directions of composite insulation materials for HV outdoor insulation technology.The interfacial property between epoxy and fiber is the most important factor to influence its mechanical and electrical property.

碩士
逢甲大學
紡織工程所
97
In this study, we first discuss the different layers of laminations via the resistance to compression test. It was determined that the best laminations consisted of 25 layers, with thermal conduction coefficient value of 0.069 W/m℃, compressive strength of 477.65 N/mm2 and bending strength of 427.808 N/mm2. We increased the concentrations of SiO2, Clay and TiO2 by adding these three powders separately to the Glass fiber/Epoxy laminations. Then, we investigated their influence on the ratios of heat insulation, resistance to compression and bending. According to the results, thermal conduction coefficient didn’t improve significantly by adding the three kinds of powders. In the resistance to compression aspect, the compressive strength of composites increased about 15.4 % and 2.7% by adding 1 wt% of SiO2 and 3% of TiO2, respectively, and decreased about 1.5% by adding 4 wt% of Clay. In the resistance to bending test aspect, the bending strength decreased about 3.5%, 13.4% and 18% by adding 3 wt% of SiO2, 2 wt% of Clay and 3 wt% of TiO2, respectively. We used 2K factorial designs to explore and determine the selection sector that we wanted. That selection sector was used to research the effects of heat insulation, resistance to compression and bending when the three kinds of powders were added to the Glass fiber/Epoxy laminations at the same time. The research was exhibited that the most influencing factor in heat insulation aspect was the single factor. When less TiO2 and more SiO2 and Clay were added to the selection sector thermal conduction coefficient value decreased. In resistance to compression aspect, the most influencing factor was the TiO2 single factor. When less SiO2 and Clay and more TiO2 were added to the selection sector added compressive strength value increased. In resistance to bending aspect, the most influencing factor was SiO2-Clay di-factor. When less SiO2 and Clay and more TiO2 were added to the selection sector bending strength value increased.

Fiber-reinforced composite (FRC) materials are ideal for the aerospace and automotive industries which require high-strength structures with exceptional specific properties. The unfortunate reality is composite materials are susceptible to complex failure modes and difficult-to-predict damage growth as a result of their heterogeneity and anisotropy. Thus, robust structural health monitoring (SHM) for in-operation tracking of damage formation and accumulation is important for these materials. Self-sensing materials are a strong candidate to replace traditional composite SHM because they do not suffer from the disadvantages of point-based sensing. The piezoresistive effect in nanofiller-modified materials is a common approach to material self-sensing. Research to date in piezoresistivity has predominantly focused on the direct current (DC) response of such materials. This is an important limitation because alternating current (AC) has important advantages – it inherently possesses more information (AC data can relate both impedance magnitude and phase to damage), AC effects can be leveraged for improved damage sensitivity, and AC interrogation can reduce power requirements. Therefore, to develop knowledge that will facilitate the transition to AC, this work explores the effect of high-cycle
fatigue loading on the AC response of carbon nanofiber (CNF)-modified glass fiber/epoxy laminates. In this study, impedance magnitude and phase angle are measured along the length and through the thickness of composite specimens with an open-hole stress concentration
subjected to tension fatigue-loading up to 10 MHz. The collected impedance data is fit to an equivalent circuit model as a function of cycle. These results show that high-cycle fatigue loading does indeed have an appreciable effect on the equivalent circuit behavior of the material. However, clear and definitive trends were not observed thereby suggesting that further research is needed into the basic mechanisms of AC transport in nanocomposites if frequency-dependent transport is to be used to track fatigue loading.

A thesis submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science in Engineering. Johannesburg, May 2016
In this study, hybrid multiscale epoxy composites were developed from woven glass fabrics and PAN nanofibers embedded with short ECNFs (diameters of ~200nm) produced via electrospinning. Unlike VGCNFs or CNTs which are prepared through bottom-up methods, ECNFs were produced through a top-down approach; hence, ECNFs are much more cost-effective than VGCNFs or CNTs. Impact absorption energy, tensile strength, and flexural strength of the hybrid multiscale reinforced GFRP composites were investigated. The control sample was the conventional GFRP composite prepared from the neat epoxy resin. With the increase of ECNFs fiber volume fraction up to 1.0%, the impact absorption energy, tensile strength, and flexural strength increased. The incorporation of ECNFs embedded in the PAN nanofibers resulted in improvements on impact absorption energy, tensile strength, and flexural properties (strength and modulus) of the GFPC. Compared to the PAN reinforced GRPC, the incorporation of 1.0% ECNFs resulted in the improvements of impact absorption energy by roughly 9%, tensile strength by 37% and flexural strength by 29%, respectively. Interfacial debonding of matrix from the fiber was shown to be the dominant mechanism for shear failure of composites without ECNFs. PAN/ECNFs networks acted as microcrack arresters enhancing the composites toughness through the bridging mechanism in matrix rich zones. More energy absorption of the laminate specimens subjected to shear failure was attributed to the fracture and fiber pull out of more ECNFs from the epoxy matrix. This study suggests that, the developed hybrid multiscale ECNF/PAN epoxy composite could replace conventional GRPC as low-cost and high-performance structural composites with improved out of plane as well as in plane mechanical properties. The strengthening/ toughening strategy formulated in this study indicates the feasibility of using the nano-scale reinforcements to further improve the mechanical properties of currently structured high-performance composites in the coming years. In addition, the present study will significantly stimulate the long-term development of high-strength high-toughness bulk structural nanocomposites for broad applications.
MT2016

Yes
This paper investigates the strength and ductility of concrete confined by Glass/Epoxy ±55° Filament Wound Pipes (GFRP) under axial compression. A total of 24 cylinderical specimens were prepared with expansive and Portland cements, properly compacted and un-compacted for different composite fresh concrete matrix. Test results showed that compressive strength and axial deformation at failure of concrete confined with GFRP tubes increased by an average of 2.85 and 5.57 times these of unconfined concrete, respectively. Macro and micro analyses of GFRP pipes after failure were also investigated. Debonding, whitening, matrix/transfer cracking, delamination and splitting mechanisms were detected at failure, respectively. The experimental results were also employed to assess the reliability of design models available in the literature for confined concrete compressive strength.

碩士
中華科技大學
飛機系統工程研究所
107
In this study, the synergistic effect on mechanical properties of graphene nanoplatelets (GNPs)/multi-walled carbon nanotube (MWCNTs) reinforced epoxy/glass fiber composite (GFRP) laminates via VARTM process were investigated. The mechanical properties of MWCNTs reinforced GFRP laminate were investigate in the preliminary study in order to understand the reinforce behavior of single carbon nanomaterials reinforced GFRP. After discuss the above experiment result, the GNP/CNT hybrids (0.1, 0.25, 0.5 and 1.0 wt%) with different mixing ratios (i.e., 9:1, 5:5, and 1:9) were dispersed in epoxy resin to prepare GNP/CNT/GFRP laminates. The mechanical properties such as ultimate tensile strength, flexural strength, flexural modulus, and interlaminar shear strength (ILSS) of these nanocomposite laminates were investigated. Additionally, the fracture surfaces of the specimens examined using field-emission scanning electron microscopy to determine the dispersal mechanisms and reinforce behavior of the GNP/CNT hybrids in the GFRP laminate. The experimental results indicate that the mechanical properties of GNP/CNT/GFRP laminates are optimized by reinforcement through the addition of GNP/CNT hybrids. A synergistic effect was noticed when the GNP/CNT hybrids were added. The mechanical properties of the GNP/CNT/GFRP laminates, such as the tensile strength, flexural strength, and ILSS, were superior to those of the GFRP laminate containing a single carbon nanomaterial and neat GFRP laminate. However, the Young’s modulus and flexural modulus of the GNP/CNT/GFRP laminate show negligible improvements compared to that of the n containing a single carbon nanomaterial and neat GFRP laminate.

Dissertação de Mestrado Integrado em Engenharia Mecânica apresentada à Faculdade de Ciências e Tecnologia
O tema da presente dissertação, “Estudo de compósitos estruturais no processo de produção de pás eólicas”, decorreu em ambiente industrial através da colaboração institucional da Ria Blades, SA, Vagos, Aveiro. A grande competitividade e a concorrência dos mercados incentivam as empresas à adaptação de novos desafios, promovendo uma constante evolução para serem bem sucedidas.Os objetivos gerais propostos para este trabalho consistiram na i) otimização do processo de produção de laminados de reforços de acabamento a aplicar na linha de colagem de uma pá eólica e ii) a sua implementação na linha de produção.Para tal, foram utilizadas várias abordagens, destacando-se diferentes processos de impregnação das fibras (manual e mecânica), compactação a vácuo com folhas poliméricas distintas e ainda o uso ou não de uma intercamada designada por folha desmoldante.Os resultados obtidos permitem concluir, genericamente, que a impregnação mecânica conduz a uma poupança de resina epoxídica e a uma molhabilidade mais uniforme do laminado. Na moldação a vácuo, a utilização de sacos à base de nylon compactam melhor que os de polietileno, tendo-se obtido valores de espessura e de porosidade menores no compósito final. A presença ou não da camada de folha desmoldante parece não afetar o processo de produção.Face a esses resultados, para otimizar a linha de produção, a empresa irá manter o saco de vácuo à base de nylon na linha de produção e será retirada a camada de folha desmoldante, sendo a impregnação das fibras feita mecanicamente.
The theme of this dissertation, "Study of structural composites in the wind turbine production process", was carried out in an industrial environment through the institutional collaboration of the Ria Blades, SA, Vagos, Aveiro. The high market competitiveness and the rivalry in the business sector, encourages the companies to adapt to new challenges and constantly evolve to survive to be successful.The general objectives proposed for this work were: i) optimization of the production process of finishing reinforcement laminates to be applied in the glue line of a wind blade and ii) its implementation in the production line.For this, several approaches were used, highlighting different processes of fiber impregnation (manual and mechanical), vacuum compaction by using different polymer sheets and the use or not of an interlayer called release sheet.The experimental results obtained, allow concluding that the mechanical impregnation of the glass fibers by epoxy resin is a both economical and efficient process. In vacuum molding, it appears that the nylon bag compacts better than the polyethylene one, resulting in lower values of thickness and porosity in the composite. The presence or not of the release film layer does not appear to affect the process as a whole.Faced with these results, the company will keep the vacuum nylon-based bag and discharge the release-foil layer to optimize the production line of the wind turbine blade, with the impregnation of the fibers done mechanically.

Dissertação de Mestrado Integrado em Engenharia Mecânica apresentada à Faculdade de Ciências e Tecnologia
O uso de materiais compósitos em todas as áreas de desenvolvimento tem sido cada vez mais abundante devido à robustez e às características variadas que se podem obter através da mistura de diferentes tipos de materiais.A indústria naval é um grande setor no qual a utilização de materiais compósitos prevalece sobre outros tipos de materiais, pelo que é natural desenvolver estudos e investigações em materiais para este setor em específico. Neste setor os compósitos são sujeitos a solicitações agressivas em simultâneo com a inserção num ambiente corrosivo.Esta dissertação tem como objetivo principal entender o efeito de um ambiente salino em compósitos de epóxi reforçado com fibras de vidro, bem como os efeitos que o pré-carregamento pode ter na vida à fadiga de um provete deste tipo de compósito.Foram ensaiados 16 provetes do material compósito referido no parágrafo anterior cuja imersão em ambiente salino durou 900 dias, observando-se após este período uma percentagem de absorção de água de cerca de 1% do peso do provete. Destes, 9 provetes foram pré-carregados em grupos de 3, com 25%, 50% e 75% de pré-carga.Aquando da solicitação à fadiga, os provetes foram fotografados com duas câmaras calibradas para posterior análise com o programa de correlação de imagem digital VIC3D®. Após obtenção dos valores de deformação principal e sobre o eixo y (direção de solicitação), estes foram analisados graficamente com o programa Microsoft Excel®.Com o aumento do teor de água no provete há a perda de resistência à fadiga do mesmo, como é comprovado na curva S-N: a curva dos provetes imersos durante 900 dias no ambiente salino apresentam piores resultados do que as curvas S-N dos provetes de controlo e do que os provetes imersos durante 229 dias, o que confirma a redução de resistência à fadiga do provete com a exposição a ambientes corrosivos salinos.Relativamente à perda de resistência do provete causada pelo pré-carregamento, não se obtiveram resultados totalmente conclusivos, uma vez que não há uma linearidade entre o valor da pré-carga e o número de ciclos completos pelo provete.
The use of composite materials has been more abundant due to its robustness and to the variety of characteristics that can result from the mixture of different types of materials.The naval industry is a major sector in wich composite materials play a significant role. This motivates the investigation and development of composites for this sector specifically. In this type of industry the materials are subject to various forces while being in contact with a corrosive environment.This dissertation has the objective of understanding the effect of a seawater environment in glassfiber reinforced epoxi composites, as well as the effects that a pre-load can have on the fatigue life of this type of composite.16 specimens were immersed in saltwater for 900 days, a period after wich the mass of the specimens had increased by 1% of its weight due to water infiltration. From these, 9 of specimens were pre-loaded in groups of 3, with the loads being 25%, 50% and 75%.While under fatigue testing, the specimens were photographed using two calibrated cameras paired with the digital image correlation program VIC3D®. After extracting the principal strain and the y axis strain values, these were graphically analysed using Microsoft Excel®.With the rise of water contente in the specimen, there is a loss of properties of fatigue resistance, wich is proved with S-N curve values: the 900 days immersed specimens showed worse results than the 229 days immersed specimens and the control specimens, wich confirms the reduction of fatigue resistance due to corrosive saltwater environments.The loss of fatigue resistance due to pre-load is not fully conclusive because the results of the testing are not linear between the pre-load and the number of cycles complete.

In der vorliegenden Arbeit wird die Frage untersucht, ob effektive Eigenschaften von Verbunden auch nach dem Auftreten einer Dehnungslokalisierung aufgrund von entfestigendem Materialverhalten noch durch numerische Homogenisierungsmethoden berechnet werden können. Ihr Nutzen für diesen Anwendungsfall wird in der Literatur kritisch beurteilt. Aus diesem Grund werden hier systematisch alle Teilaufgaben betrachtet, die zu diesem Zweck gelöst werden müssen. Die erste dieser Aufgaben ist die Charakterisierung der einzelnen Verbundbestandteile. Zur Demonstration einer experimentell gestützten Charakterisierung wird ein glasfaserverstärktes Epoxidharz als Beispielmaterial gewählt. Neben der Beschreibung von Faser- und Matrixmaterial wird besonderes Augenmerk auf die Charakterisierung der Grenzschicht zwischen beiden gelegt. Die für die Glasfasern vorliegenden Festigkeitsmessungen entsprechen nicht der Kettenhypothese. Daher werden zahlreiche Verallgemeinerungen der Weibull-Verteilung untersucht, um störende Effekte zu erfassen. Schließlich werden Wahrscheinlichkeitsverteilungen hergeleitet, die Faserbrüche im Bereich der Einspannung einbeziehen. Die Messwerte können von diesen Verteilungen gut wiedergegeben werden. Zusätzlich macht ihre Anwendung das aufwändige Aussortieren und Wiederholen jener Experimente unnötig, bei denen der Faserbruch im Klemmbereich auftritt. Zur Modellierung der Grenzfläche wird ein Kohäsivzonengesetz entwickelt. Die Bestimmung seiner Parameter erfolgt anhand von Daten aus Pullout- und Einzelfaserfragmentierungsversuchen. Aus diesen ermittelte Festigkeiten und Energiefreisetzungsraten weisen eine sehr gute Übereinstimmung zwischen beiden Versuchen auf. Dabei erfolgt die Parameteridentifikation mithilfe von Finite-Elemente-Modellen anstatt der häufig genutzten vereinfachten analytischen Modelle, welche üblicherweise eine schlechtere Übereinstimmung erreichen. Sobald eine Dehnungslokalisierung auftritt, ist neben der Materialmodellierung auch das Homogenisierungsschema zu verallgemeinern. Zu diesem gehören die Generierung repräsentativer Volumenelemente, Randbedingungen (RB) und ein Mittelungsoperator. Anhand des aktuellen Standes der Literatur werden die Randbedingungen als ein signifikanter Schwachpunkt von Homogenisierungsverfahren erkannt. Daher erfolgt die Untersuchung periodischer RB, linearer Verschiebungsrandbedingungen und minimal kinematischer RB sowie zweier adaptiver RB, nämlich Lokalisierungspfad-ausgerichteter RB und generalisiert periodischer RB. Unter der Bezeichnung Tesselationsrandbedingungen wird ein weiterer Typ adaptiver RB vorgeschlagen. Zunächst erfolgt der Beweis, dass alle drei adaptiven RB die Hill-Mandel-Bedingung erfüllen. Des Weiteren wird mittels einer Modifikation der Hough-Transformation ein systematischer Fehler derselben bei der Bestimmung der Richtung von Lokalisierungszonen eliminiert. Schließlich werden die Eigenschaften aller Randbedingungen an verschiedenen Beispielen demonstriert. Dabei zeigt sich, dass nur Tesselationsrandbedingungen sowohl beliebige Richtungen von Lokalisierungszonen erlauben als auch fehlerhafte Lokalisierungen in Eckbereichen ausschließen. Zusammengefasst können in der Literatur geäußerte grundlegende Einschränkungen hinsichtlich der Anwendbarkeit numerischer Homogenisierungsverfahren beim Auftreten von Dehnungslokalisierungen aufgehoben werden. Homogenisierungsmethoden sind somit auch für entfestigendes Materialverhalten anwendbar.
The thesis at hand is concerned with the question if numerical homogenization schemes can be of use in deriving effective material properties of composite materials after the onset of strain localization due to strain softening. In this case, the usefulness of computational homogenization methods has been questioned in the literature. Hence, all the subtasks to be solved in order to provide a successful homogenization scheme are investigated herein. The first of those tasks is the characterization of the constituents, which form the composite. To allow for an experimentally based characterization an exemplary composite has to be chosen, which herein is a glass fiber reinforced epoxy. Hence the constituents to be characterized are the epoxy and the glass fibers. Furthermore, special attention is paid to the characterization of the interface between both materials. In case of the glass fibers, the measured strength values do not comply with the weakest link hypothesis. Numerous generalizations of the Weibull distribution are investigated, to account for interfering effects. Finally, distributions are derived, that incorporate the possibility of failure inside the clamped fiber length. Application of such a distribution may represent the measured data quite well. Additionally, it renders the cumbersome process of sorting out and repeating those tests unnecessary, where the fiber fails inside the clamps. Identifying the interface parameters of the proposed cohesive zone model relies on data from pullout and single fiber fragmentation tests. The agreement of both experiments in terms of interface strength and energy release rate is very good, where the parameters are identified by means of an evaluation based on finite element models. Also, the agreement achieved is much better than the one typically reached by an evaluation based on simplified analytical models. Beside the derivation of parameterized material models as an input, the homogenization scheme itself needs to be generalized after the onset of strain localization. In an assessment of the current state of the literature, prior to the generation of representative volume elements and the averaging operator, the boundary conditions (BC) are identified as a significant issue of such a homogenization scheme. Hence, periodic BC, linear displacement BC and minimal kinematic BC as well as two adaptive BC, namely percolation path aligned BC and generalized periodic BC are investigated. Furthermore, a third type of adaptive BC is proposed, which is called tesselation BC. Firstly, the three adaptive BC are proven to fulfill the Hill-Mandel condition. Secondly, by modifying the Hough transformation an unbiased criterion to determine the direction of the localization zone is given, which is necessary for adaptive BC. Thirdly, the properties of all the BC are demonstrated in several examples. These show that tesselation BC are the only type, that allows for arbitrary directions of localization zones, yet is totally unsusceptible to spurious localization zones in corners of representative volume elements. Altogether, fundamental objections, that have been raised in the literature against the application of homogenization in situations with strain localization, are rebutted in this thesis. Hence, the basic feasibility of homogenization schemes even in case of strain softening material behavior is shown.

Adhesive bonding which has been in use for long as a traditional joining method has gained ground in the last couple of decades due to the introduction of advanced composite materials into the aerospace industry. Bonded structures have advantages such as high corrosion and fatigue resistance, ability to join dissimilar materials, reduced stress concentration, uniform stress distribution, good damping characteristics etc. They also have certain limitations like environmental degradation, existence of defects like pores, voids and disbonds, difficulty in maintenance and repair etc. A serious drawback in the use of adhesively bonded structures has been that there are no established comprehensive non-destructive testing (NDT) techniques for their evaluation. Further, a reliable evaluation of the effect of the existing defects on strength and durability of adhesive joints is yet to be achieved. This has been a challenge for the research and development community over several decades and hence, been the motivation behind this piece of research work. Under the scope of the work carried out in the thesis, some of the primary factors such as the existence of defects, degradation of the adhesive, stress and strain distribution in the bonded region etc., have been considered to study the bond integrity in composite to composite lap shear joints. The problem becomes complex if all the parameters affecting the adhesive joint are varied simultaneously. Taking this into consideration, one of the key parameters affecting the bond quality, viz., the adhesive layer degradation was chosen to study its effect on the bonded joint. The epoxy layer was added with different, definite amount of Poly vinyl alcohol (PVA) to arrive at sets of bonded joint specimens with varied adhesive layer properties. A thorough review of different non destructive testing methods applied to this particular problem showed that ultrasonic wave based techniques could be the right choice. To start with, preliminary experimental investigations were carried on unidirectional glass fiber reinforced plastic (GFRP-epoxy) lap joints. The adhesive joints were subjected to non destructive evaluation (NDE) using ultrasonic through transmission and pulse echo techniques as also low energy digital X-ray techniques. The results obtained showed a variation in reflected and transmitted ultrasonic pulse amplitude with bond quality. Digital X-Ray radiography technique showed a variation in the intensity of transmitted x-rays due to variation in the density of adhesive. Standard mechanical tests revealed that the addition of PVA decreased the bond strength. A plot of coefficient of reflection from the first interface and the bond strength showed a linear correlation between them. After obtaining a cursory feel and understanding of the parameters involved with the preliminary experiments on GFRP adhesive joints which yielded interesting and encouraging results, further work was carried on specimens made out of autoclave cured carbon fiber reinforced plastic (CFRP)-epoxy bonded joints. Normal incidence ultrasound showed a similar trend. Analyses of the Acoustic Emission (AE) signals generated indicate early AE activity for degraded joints compared to healthy joints. Literary evidences suggest that the ultrasonic shear waves are more sensitive to interfacial degradation. An attempt was made to use oblique incidence ultrasonic interrogation using shear waves. The amplitude of reflected shear waves from the interface increased with an increase in degradation. Further, a signal analysis approach in the frequency domain revealed a shift in the frequency minimum towards lower range in degraded samples. This phenomenon was verified using analytical models. An inversion algorithm was used to determine the interfacial transverse stiffness which decreased significantly due to increase in degradation. Conventional ultrasonic evaluation methods are rendered ineffective when a direct access to the test region is not possible; a different approach with guided wave techniques can be explored in this scenario. Investigations on CFRP-epoxy adhesive joints using Lamb waves showed a decrease in the amplitude of ‘So’ mode in degraded samples. Theoretical dispersion curves exhibited a similar trend. Frequency domain studies on the received modes using Gabor wavelet transform showed a negative shift in frequency with increased degradation. It was also observed that the maximum transmission loss for the most degraded sample with 40 percent PVA occurred in the range of 650 – 800 kHz. Non linear ultrasonic (NLU) evaluation revealed that the nonlinearity parameter (β) increased with increased degradation. Kissing bonds are most commonly occurring type of defects in adhesive joints and are very difficult to characterize. A recent non-contact imaging technique called digital image correlation (DIC) was tried to evaluate composite adhesive joints with varied percentage of inserted kissing bond defects. The results obtained indicate that DIC can detect the kissing bonds even at 50 percent of the failure load. In addition, to different experimental approaches to evaluate the bonded joint discussed above, the effect of degradation on the stresses in the bond line region was studied using analytical and numerical approach. A linear adhesive beam model based on Euler beam theory and a nonlinear adhesive beam model based on Timoshenko beam theory were used to determine the adhesive peel and shear stress in the joint. Digital image correlation technique was used to experimentally obtain the bond line strains and corresponding stresses were computed assuming a plane strain condition. It was found that the experimental stresses followed a similar trend to that predicted by the two analytical models. A maximum peel stress failure criterion was used to predict failure loads. A failure mechanism was proposed based on the observations made during the experimental work. It was further shown that the critical strain energy release rate for crack initiation in a healthy joint is much higher compared to a degraded joint. The analytical models become cumbersome if a larger number of factors have to be taken into account. Numerical methods like finite element analysis are found to be promising in overcoming these hurdles. Numerical investigation using 3D finite element analysis was carried out on CFRP-epoxy adhesive joints. The adherend – adhesive interface was modeled using connector elements whose stiffness properties as well as the bulk adhesive properties for joints with different amounts of PVA were determined using ultrasonic inspection method. The peel and shear stress variation along the adhesive bond line showed a similar trend as observed with the experimental stress distribution (DIC) but with a lesser magnitude. A parametric study using finite element based Monte-Carlo simulation was carried out to assess the effect of variation in various joint parameters like adhesive modulus, bondline thickness, adherend geometrical and material properties on peel and shear stress in the joint. It was found that the adhesive modulus and bond line thickness had a significant influence on the magnitude of stresses developed in the bond line. Thus, to summarize, an attempt has been made to study the bond line integrity of a composite epoxy adhesive lap joint using experimental, analytical and numerical approaches. Advanced NDE tools like oblique incidence ultrasound, non linear ultrasound, Lamb wave inspection and digital image correlation have been used to extract parameters which can be used to evaluate composite bonded joints. The results obtained and reported in the thesis have been encouraging and indicate that in specific cases where the bond line thickness and other relevant parameters if can be maintained or presumed reasonably non variant, it is possible to effectively evaluate the integrity of a composite bonded joint.