Gelişmiş ve nano takviyeli kompozit malzemeler için karbon nanotüp içerikli akıllı boyanın yapısal sağlık izleme için geliştirilmesi

Fiber reinforced polymer composites are widely used in aerospace, automotive, civil and marine applications, through their excellent mechanical properties, and low density. With improving technology, the demand for lighter, stronger and reliable materials increases day by day. To satisfy these demands, deeply researches are being carried out on advanced and nano-enhanced composite structures. Since the composites are laminated structures, most of the damage types generate under the surface of structures and that is why composites are more vulnerable to failure during operation. Therefore, structural health monitoring (SHM) is an essential application to provide reliability and cost saving for aerospace operations. There are many SHM methods like strain gauges, fiber optic sensors and lamb waves; however, because of their multifunctionality, such as high mechanical properties, electrical and thermal conductivity, carbon nanotubes (CNTs) are novel applications for SHM methods. There are mainly two types for adding CNTs into the system, one of them is embedding CNTs within the composite and the other one is applying onto the surface of structure. In this study, both growth CNTs which are synthesized using thermal chemical vapor deposition (th-CVD) method and commercially provided CNTs from Sigma Aldrich are used for CNT based SHM application. CNT embedded polymer nanocomposites (CNT-PNCs) are applied onto the surface of carbon fiber reinforced polymer (CFRP) composites as a smart paint for strain sensing. In addition to SHM application, nano-enhanced cylindrical composite structures are fabricated and torsional tests are performed in this study to determine the failure mode of these structures before applying CNT based SHM method. To synthesize CNTs, using th-CVD method Si substrate covered with Alumina oxide and Ferrous catalyst are used and C2H4/H2 gases are deposited at atmospheric pressure with specified recipe. Growth and commercially provided CNTs are added into the epoxy resin and shear mixing and homogenization are applied for dispersion of CNTs. For CNT based SHM applications percolation threshold is a critical parameter to obtain an effective system. So, the first step of this study is determining percolation threshold for CNT-PNCs. 0.1, 0.25 and 0.5 wt% growth CNTs and 0.25, 0.5 and 1 wt% Sigma-CNTs are used to fabricate CNT-PNCs. 2-probe electrical conductivity measurement method is used to determine electrical conductivity of CNT-PNCs. According to electrical conductivity measurements, electrical conductivity of growth CNT-PNCs (0.3x10-3-5x10-1 S/m) are beyond the percolation threshold even at low CNTs loading, however Sigma CNT-PNCs have six order of magnitude lower electrical conductivity than growth CNT-PNCs. As a result, growth CNT-PNCs are applied onto the surface of CFRP composites, which are fabricated by vacuum infusion method, with specified dimensions as a smart paint for strain sensing. Elastic mode, plastic mode and flextural tests are applied for CFRP composites and resistance change of CNT smart paint is measured simultaneously with keithly source meter. When is compaired with base specimen, which does not containe CNTs, CNT smart paint displays an important effect on resistance change. According to results, resistance change of 0.25 and 0.5 wt% CNT embedded smart paints show a decisive increament related to strain changes. While the resistance change is higher for 0.25 wt% CNT smart paint (120%) at elastic mode tension tests, 0.5 wt% CNT smart paint displays higher resistance change (300%) for plastic mode tension and flextural tests where the composite specimens are totally fractured. This would be the result of tunneling effect and for high strain conditions, tunneling effect lose its importance. Since the carbon fibers are conducting materials and they also carry electrons when the CNT smart paint is fractured, CFRP composite structure have to be eliminated from CNT smart paint application to determine real effectiveness of CNT smart paint as a strain sensor. That is why, at the last step of SHM application, insulator acrylic paint is applied between CNT smart paint and CFRP composites. Flextural test is performed on these specimens and a sudden and huge increment is obtained for resistance change of CNT smart paint. Nano-enhancement is an ongoing research area for novel applications, so at the second part of this study, nano-enhanced cylindrical composite structures are fabricated to determine their failure modes, which is important for investigation of effectiveness of SHM systems on these structures. Electrospinning method is used to fabricate electrospun carbon nanofibers (ECNs) mats. Since CNT agglomeration is a critical problem for application of electrospinning and aspect ratio of CNTs is effective on agglomeration, Sigma-CNTs with 5 µm length are used to obtaine well dispersed CNTs in ECNs mats. To prepare polymer solutions, 10 wt% polyvinyl butyral (PVB) is added in methanol and Sigma CNTs are also added with different weight fractions (0.5, 1 and 2 wt%). Polymer solutions are magnetically stirred for 24 hours to provide homogenious dispersion of CNTs and electrospinning is directly applied on carbon fiber prepreg. For fabrication of carbon fiber prepreg reinforced composites (CFp-RC) 10 plies of carbon fiber prepreg are wrapped on a cylindrical Teflon mold continuously and a Teflon hose is used as an outer mold. The specimen fabrication of the CFp-RC cylinder with embedded ECNs mats includes three types of specimens, such as base specimen, middle interphase specimen and separate interphase specimen. Base specimens without ECNs mats are fabricated as reference materials. For middle interphase specimens the ECNs mats are applied to middle four surfaces and for separate interphase specimens ECNs mats are applied to separate four surfaces. Torsion tests are applied on composite specimens and shear stress vs. shear strain data are obtanined to determine effect of ECNs mats on interlaminar shear strength (ILSS) properties of composites. 0.5 and 1 wt% CNT ECNs mats specimens show higher shear stress results when compared with base specimens, however 2 wt% CNT ECNs mats specimens have lower shear strength results. The worse results of 2 wt% CNT-ENCs mats specimens might be attributed to CNTs agglomeration and decreased curing degree that causes decrease on mechanical properties of structures. Since cylindrical composites are hard to fabricate by hand, the mechanical test results are affected from fabrication conditions. To eliminate this problem, Mode-I Mode-II mixture test specimens are also fabricated in the second part of this study to determine effectiveness of ECNs mats on ILSS properties of laminated composites. To fabricate test specimens, 16 plies carbon fiber prepreg are used and only one layer of ECNs mat is used for each specimens. ECNs mat coated carbon fiber prepreg is placed to the middle surface of laminates. ECNs mat is applied upto the crack initiation points and a release film is placed after the ECNs mats to obtaine delamination at the middle surface. Mode-I Mode-II mixture tests will be performed on these specimens and effect of ECNs mats on ILSS of composite structures will be determined. After the investigation of failure modes of nano-enhanced composites, CNT based SHM method will be applied to the nano-enhanced and complex shaped structures.