Showing papers by "Jung-Ju Lee published in 2005"
TL;DR: In this article, the actual crack front evolution is predicted and considered in estimating the fatigue life by implementing the successive analysis technique based on the three-dimensional finite element scheme for single-sided repairs.
Abstract: Experimental and analytical studies are conducted to characterize the fatigue crack growth behavior of precracked aluminum plates repaired with asymmetric bonded composite patch. For single-sided repairs, due to the asymmetry and the presence of out-of-plane bending, crack front shape would become skewed curvilinear starting from a uniform through crack profile, as observed from previous studies. As the stress intensity factor (SIF) calculated at the crack tip is much influenced by crack front shape, it is necessary to predict and consider the actual crack front shape evolution for the accurate analysis of fatigue behavior. Nevertheless, in most previous studies, crack front has been modeled as a uniform straight one, which may cause inaccuracy. In this study, the actual crack front evolution is predicted and considered in estimating the fatigue life by implementing the successive analysis technique based on the three-dimensional finite element scheme. Experimental tests with various plate thicknesses (2,...
14 citations
TL;DR: In this paper, a light source was modulated in triangular form at 1 MHz with an electro-optic modulator, and a simple signal processing scheme was used to demodulate the received signals into independent sensor signals.
Abstract: Fiber Bragg grating sensors are very useful and attractive for strain or temperature measurement, and they can measure at multiple positions through the use of multiplexing techniques. This paper presents the development of a novel multiplexing technique using intensity modulation of a light source. A light source was modulated in triangular form at 1 MHz with an electro-optic modulator. Respective sensor signals had different time-delay and power levels due to the sensor positions with respect to the modulated source and their spectra. A simple signal processing scheme could be used to demodulate the received signals into independent sensor signals. One of the major advantages of this technique is the enhancement of the signal-to-noise ratio by a high-power source in comparison with conventional time division multiplexing using a pulsed source. A five-sensor system, which was operated over two different Bragg wavelength ranges, was demonstrated.
10 citations
TL;DR: In this article, the effect of the fiber orientation distribution on tensile strength of fiber reinforced polymeric composite material was investigated, and it was shown that the increasing range of the value of fiber-reinforced polymeric composites' strength in the direction of fiber orientation is getting wider as the fiber content ratio increases.
Abstract: The study for strength calculation of one way fiber-reinforced composites and the study measuring precisely fiber orientation distribution were presented. However, because the DB that can predict mechanical properties of composite material and fiber orientation distribution by the fiber content ratio was not constructed, we need the systematic study for that. Therefore, in this study, we investigated what effect the fiber content ratio and fiber orientation distribution have on the strength of composite sheet after making fiber reinforced polymeric composite sheet by changing fiber orientation distribution with the fiber content ratio. The result of this study will become a guide to design data of the most suitable parts design or fiber reinforced polymeric composite sheet that uses the fiber reinforced polymeric composite sheet in industry spot, because it was conducted in terms of developing products. We studied the effect the fiber orientation distribution has on tensile strength of fiber reinforced polymeric composite material and achieved this results below. We can say that the increasing range of the value of fiber reinforced polymeric composite’s tensile strength in the direction of fiber orientation is getting wider as the fiber content ratio increases. It shows that the value of fiber reinforced polymeric composite’s tensile strength in the direction of fiber orientation 90° is similar with the value of polypropylene’s intensity when fiber orientation function is J= 0.7, regardless of the fiber content ratio. Tensile strength of fiber reinforced polymeric composite is affected by the fiber orientation distribution more than by the fiber content ratio.
8 citations
TL;DR: In this paper, the Tsai-Pagano equation was used to evaluate the tensile strength and modulus of MWNT/PMMA composites and the experimental results were not in agreement with what they estimated.
Abstract: 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.
3 citations
TL;DR: In this paper, a new Structural Health Monitoring (SHM) technique for composite laminates through the use of embedded FBG sensors is presented, which monitors the ply stress states of a laminate and compares them with failure criteria continuously during structures' service time.
Abstract: In this paper, a new Structural Health Monitoring (SHM) technique for composite laminates through the use of embedded FBG sensors is presented. This method monitors the ply stress states of a laminate and compares them with failure criteria continuously during structures’ service time. The ply stress state of each ply composing the composite laminate can be obtained by embedding three FBG sensors in the laminate based on the classical lamination theory. In this study FBG sensor embedded graphite/epoxy composite laminate specimens were fabricated. With ply stress states being monitored, tension and fatigue tests were performed until laminates’ failure. Experimental results show that laminates experience fracture when the ply stress states are beyond the boundaries of failure criteria. Embedded FBG sensors had good fracture strain and reliability. Therefore, critical damage can be detected by the ply stress states which are close to the boundaries of failure criteria.
2 citations
TL;DR: In this article, the authors proposed a sensor coefficient in the linear combination signal for a hybrid extrinsic Fabry-Perot interferometric (TRHEFPI) optical fiber sensor.
Abstract: A transmission/reflection-type hybrid extrinsic Fabry–Perot interferometric (TRHEFPI) optical fiber sensor presents transmission-type and reflection-type sensor signals simultaneously, and measurement directions can be robustly distinguished with the phase lead or lag of the linear combination signal from the original sensor signal. This sensor principle compensates for the ambiguous distinction of the measurement directions and direction changes of conventional interferometric optical fiber sensors, including extrinsic Fabry–Perot interferometric optical fiber sensors, due to the exclusive use of fringe counting in signal processing. Designing a sensor coefficient in the linear combination signal is one of the most important works in signal processing for the TRHEFPI optical fiber sensor because the magnitude of the phase shifts depends on the sensor coefficient. A design method of the sensor coefficient is presented in this research. The method was verified with experimental sensor signals and applied to strain measurement experiments. A method of absolute measurement using the TRHEFPI optical fiber sensor is also presented.
1 citations
01 Jun 2005
TL;DR: In this article, a sliding mode control with a time delay estimation (SMCTE) is proposed for SMA actuators whose quantitative model is difficult to obtain, and the gain tuning method for the SMCTE is developed based on the previous study on the TDC.
Abstract: We deal with the sliding mode control using the time delay estimation. The time delay estimation is able to weaken the need for obtaining a quantitative plant model analogous to the real plant so the sliding mode control with a time delay estimation (SMCTE) is very suitable for plant such as SMA actuators whose quantitative model is difficult to obtain. We have already studied the application of the time delay control (TDC) to SMA actuators in other literature. Based on the previous study on the TDC, we developed the gain tuning method for the SMCTE, which results were nearly the same as the TDC. With respect to the step response, the SMCTE proved its predominance in a comparison with other control schemes such as the PID control and the relay control. As well as the contribution of the above control methodology, the model identification for SMA actuators has also been studied. The dynamics for an SMA actuator was newly derived using the modified Liang's model. The derived dynamics showed a continuity at the change of the phase transformation process but the original Liang's model could not.