Chun Gon Kim

Strain Monitoring and Damage Detection of a Filament Wound Composite Pressure Tank Using Embedded Fiber Bragg Grating Sensors

Composite pressure tanks are rapidly expanding in their range of use. However, for high pressure flammable or toxic gases, there is less confidence about their reliability. In this study, fiber Bragg grating (FBG) sensors embedded into a composite pressure tank monitored strain and detected damage. The sensor heads and fiber optic lines were protected with an acrylate recoating, adhesive films, PVC tubes and Teflon film to survive under the harsh environment of the filament winding process. During the quasi-static loading test, the internal strain was measured with embedded FBG sensor arrays. In order to detect damage occurred during the test, impacts were applied on three different positions in each side by an impact hammer. The difference between damaged and undamaged sides in impact response was analyzed. An interrogation system using a wavelength-swept fiber laser (WSFL) was used for measuring strain, and an erbium-doped fiber amplifier (EDFA) laser source with a tunable Fabry-Perot filter was used for measuring impact response.

Simultaneous Monitoring of Crack Signals and Strain of Composites Using a Stabilized EFPI Sensor System

An extrinsic Fabry-Perot interferometric (EFPI) sensor is a good transducer for the detection of ultrasonic fracture signals and for the measurement of structural strain. In this paper, we report on the development of a novel sensor system, used both to measure the strain of structures and to detect fracture signals of composites, simultaneously. The proposed system solved both the inherent ambiguity in the cosine response and the fade-out problem, which are serious obstacles to overcome before the application of the EFPI to fracture signal detection and to structural strain measurement. Introduction Optical fiber interferometric sensors are alternatives to robust piezoelectric transducers for the detection of acoustic emissions (AE), because they are more sensitive and have a wider dynamic range than other types of sensors. In addition, interferometric sensors can also be used to help predict the onset of a structural failure and to monitor the structural status of composite materials, because such sensors have outstanding compatibility with composites. A fiber Fabry-Perot interferometric (FFPI) sensor theoretically produces a sinusoidal signal under a quasi-static loading or under an external temperature change, as shown in Figure 1. As shown in Figure 1, the phase shift is greater than π if the external change is large enough. The phase shift induced by an AE, however, is smaller than π. Therefore, the sensitivity of an FFPI to the AE varies continuously, if the sensor is under external quasi-static change; that is, while it is easy to detect the AE signals that occur near the highest sensitivity range, those AE signals that occur near the lowest sensitivity range are barely detected at all. This is usually called a “fade-out” problem. The fade-out In te n si ty

Temperature-compensated strain measurement of full-scale small aircraft wing structure using low-cost FBG interrogator

Recently, health and usage monitoring systems (HUMS) are being studied to monitor the real-time condition of aircrafts during flight. HUMSs can prevent aircraft accidents and reduce inspection time and cost. Fiber Bragg grating (FBG) sensors are widely used for aircraft HUMSs with many advantages such as light weight, small size, easy-multiplexing, and EMI immunity. However, commercial FBG interrogators are too expensive to apply for small aircrafts. Generally the cost of conventional FBG interrogators is over

Comparison of Carbon-Based Nano Materials as Conductive Fillers for Single Layer Microwave Absorber

20,000. Therefore, cost-effective FBG interrogation systems need to be developed for small aircraft HUMSs. In this study, cost-effective low speed FBG interrogator was applied to full-scale small aircraft wing structure to examine the operational applicability of the low speed FBG interrogator to the monitoring of small aircrafts. The cost of the developed low speed FBG interrogator was about

Characteristics of Fiber Bragg Grating Sensors with Various Grating Lengths Embedded in Composite Materials

10,000, which is an affordable price for a small aircraft. 10 FBG strain sensors and 1 FBG temperature sensor were installed on the surface of the full-scale wing structure. Load was applied to the tip of the wing structure, and the low speed interrogator detected the change in the center wavelength of the FBG sensors at the sampling rate of 10Hz. To assess the applicability of the low-cost FBG interrogator to full-scale small aircraft wing structure, a temperature-compensated strain measurement algorithm was verified experimentally under various loading conditions of the wing structure with temperature variations.

Improvement of the Crack Resistance of a Carbon/Epoxy Composite at Cryogenic Temperature

In this paper, we have studied the permittivities of E-glass fabric/epoxy composite laminates containing three different types of carbon-based nano conductive fillers such as carbon black (CB), carbon nano fiber (CNF) and multi-wall carbon nano tube (MWNT). The measurements were performed for permittivities at the frequency band of 0.5 GHz ~ 18.0 GHz using a vector network analyzer with a 7 mm coaxial air line. The experimental results show that the complex permittivities of the composites depend strongly on the natures and concentrations of the conductive fillers. The real and imaginary parts of the complex permittivities of the composites were proportional to the filler concentrations. But, depending on the types of fillers and frequency band, the increasing rates of the real and imaginary parts with respect to the filler concentrations were all different. At the frequency of 10 GHz, the rates in the CNF filled composite and the MWNT filled composite were much larger then those of the CB filled composite. Between the CNF filled composite and MWNT filled composite, however, the former showed a little higher increasing rates than the other. These different rates can have great effect on the thickness in designing the single layer microwave absorbers. The effect of the different rates was examined by using Cole-Cole plots; the plot is composed of a single layer absorber solution line and permittivity lines of these three types of composites.

Mechanical Improvement of Multi-Walled Carbon Nanotube/Poly (Methyl Methacrylate) Composites

Because of their good multiplexing capabilities, fiber Bragg grating (FBG) sensors are being studied more actively than any of other fiber optic sensors. The application fields of FBG sensors have been mainly focused on composite materials through embedding rather than through surface attachments. However, there are many limitations on the embedding of FBG sensors into composite materials because of the birefringence that is induced when FBG sensors are not embedded parallel to the reinforcing fibers. This study investigates the fabrication of FBG sensors that have various grating lengths, good multiplexing capabilities, better stability from birefringence, and ease in production. The signal characteristics of FBG sensors are also verified through the cure monitoring of two composite laminates.

Design of Broad Band Radar Absorbing Composite Laminates

The improvement of crack resistance is essential to the application of composites for cryogenic use such as structures storing liquid oxygen or liquid hydrogen. In this study, an effort to improve the crack resistance of a carbon/epoxy composite was made by adding MWNTs (Multi-Walled Carbon Nanotubes) to the resin formulation. Ahead of the investigation of MWNT effect, an epoxy matrix system was developed by mixing two kinds of epoxy resins and adding additives for high toughness at cryogenic temperature. The MWNT-added carbon/epoxy unidirectional prepregs were fabricated by way of a filament winding method with different concentrations of MWNTs (0.2wt% and 0.7wt%). The mechanical tests were performed inside an environmental chamber at room temperature and -150°C. The developed material system has little influence on interlaminar shear strength but resulted in higher fracture toughness at -150°C than those of baseline material. Microcrack densities after thermo-mechanical cycles were measured through an optical microscope.

Finite Element Analysis of mass-spring system for measurement of relative moving displacement

Multi-walled carbon nanotube (MWNT)/poly (methyl methacrylate) composites were fabricated with the variation of the concentration rate of nanotubes by the solution casting. SEM images showed that the nanotubes were dispersed well throughout PMMA. Assuming that MWNTs in MWNT/PMMA composites were randomly oriented, the Tsai-Pagano equation, which can give the moduli of short fiber reinforced composites, was used to evaluate that of the MWNT/PMMA composite. For investigating mechanical properties of the MWNT/PMMA composite, tensile loading tests were performed, varying the concentration rate of the MWNTs. For each concentration rate of the MWNTs, at least 5 specimens of MWNT/PMMA composites were made and tested. As the concentration rate of the MWNTs increased from 0 to 0.15wt%, tensile strength and modulus of the MWNT/PMMA composites were improved by about 20% and 32%, respectively. However, the experimental results were not in agreement with what we estimated. Here are two reasons supposed. First, the MWNTs used in this research were not stretched straightly but entangled. It means that MWNTs cannot be assumed to be short fibers. Second, the concentration rate of the MWNTs is too small to be considered as short fiber composites.

Liquid Nitrogen Storing and Pressurization Test of a Type III Cryogenic Propellant Tank

The purpose of this study is to present the optimal design technology on the broad band radar absorbing composite laminates. The design concept is based on the 2-layer Dällenbach type radar absorber, in which the composite laminates act as the lossy layers. The radar absorbing function was achieved by controlling the electromagnetic property of the composite laminates by means of adding carbon nano materials into the matrix resin of the fiber-reinforced composite laminates. The laminates were fabricated with the E-glass fabric/epoxy prepregs and cured in an autoclave. The electromagnetic properties were measured for the frequency band of 0.5 ~ 18.0 GHz using a coaxial air line and a vector network analyzer. The radar absorbers were designed to have optimal absorbing performance in 10 GHz. The optimization was conducted to get the optimal number of the plies of the laminates and filler contents of the carbon nano materials in the respective layers. The performances of the fabricated absorbers were measured to be compared with the design results. The discrepancy between the design and the measurement was discussed.