Advanced Materials Research Vols. 123-125

Abstract: Nuclear power plant (NPP) is commonly categorized as a harsh environment that gives rise to age-related degradation on the structures of plant and eventually leads to radiation leakage that threatens human. Integrated structural health monitoring (ISHM) technology is a strong candidate for NPP accidents. The emergence of optical fiber technology into SHM system for NPP was greatly interested by researchers, and also prior research works have shown that the fiber Bragg grating (FBG) was able to retain its reflectivity under radiation exposure. In this paper, a metal-coated fiber was newly used to develop a FBG acoustic sensor for ISHM of NPP. The 7 mm length of aluminum, copper/carbon coatings were successfully removed with sodium hydroxide and nitric acid solutions. A 5 mm FBG was successfully inscribed in the silica core through the 7 mm long coating-removed silica section of copper/carbon-coated fiber and the initial reflectivity was 71%. Then, the FBG acoustic sensor was developed in one-end-free FBG configuration on the stainless steel vessel using a high temperature metallic adhesive. The reflective power of the sensor was stabilized at 345°C during the high temperature elevation cyclic process. The FBG acoustic sensor showed good response to the acousto-ultrasonic waves during pencil-lead breaking and laser ultrasonics tests. The high temperature FBG acoustic sensor written in the cost-effective metal/carbon is feasible to be used for ISHM of the NPP.

Abstract: Most accelerometers are composed of a mass, a spring, and a damper. The quantification of the acceleration could be obtained from the equation of dynamic motion expressed in terms of the relative moving displacement. Therefore, if the moving displacement of a seismic mass is inferred from the output signal of a single-degree-of-freedom (SDOF) system, the acceleration could theoretically be obtained by a deformation response factor expressed in the relation between relative displacement and excited acceleration. A reflective grating panel-optical fiber sensor can measure the relative displacement by the movement of the grating panel that is attached to the seismic mass. Ultimately, this research is intended for application to grating panel-optical fiber sensors. However, in order to fabricate the adequate probe for this sensing principle, microscopic rotation (< 0.007 degree) should be prevented for stable reflectivity. Therefore, this paper describes the optimum design of the mass-spring structure which features a larger linear motion range of the leaf spring and no rotation of the seismic mass. Accordingly, finite element analyses (FEA) were accomplished for reflecting the grating panel-optical fiber sensor probe. The leaf spring was employed for the strict linear motion, and some parametric studies on the design of the leaf spring was conducted. First of all, in order to have flexibility in the leaf spring, the leaf spring is divided into N equal parts (N= 3, 4, 5). Furthermore, some parametric studies were carried out, including the length of the seismic mass, arm width, and length of the leaf spring, as well as the moment balancing design. Through the comparisons of each FEA result, the best design of the leaf spring was determined to obtain the optimum features over the measurement range of acceleration and frequency within the linear elastic of the leaf spring.

Abstract: It is highly desirable to increase the sampling rate of a fiber Bragg grating (FBG) interrogator in other to sense dynamic strains caused by impulsive acoustic wave. We have developed a wavelength interrogator featuring 100k samplings per second that consists of a solid-state spectrometer, a photodiode array and fully parallel read-out circuits. Central wavelengths on the reflected partial spectra corresponding to FBGs are calculated by the centroid method with the selected groups of the consecutive photodiodes at which each FBG spectrum is imaged. The centroid calculation is simple to be implemented in a field-programmable gate array (FPGA) and fast enough to capture impulsive strain waveforms in real time. Short-term noise on the interrogated wavelengths is estimated to be around 0.5 in terms of stain within the sampling bandwidth.

Abstract: A series of amorphous polyphosphazenes containing carbazole-based multifunctional chromophores and two hyper-structured photorefractive molecular glasses with a cyclotriphosphazene core were synthesized. These photorefractive polymers and molecular glasses show low glass transition temperature (20-90oC) and can easily be fabricated into optically transparent films with long-term stability. Results of two-beam-coupling and four-wave-mixing experiments demonstrated that high gain coefficient and diffraction efficiency were achieved at zero electric field for samples fabricated with these materials.

Abstract: Two novel copolymers combining the electron-transporting group (Alq3), hole-transporting group (carbazole and phenothiazine) and chromophore had been obtained. The copolymers were characterized by 1H NMR, IR and UV-vis absorption spectrum, and exhibited good solubility in a wide range of organic solvents such as toluene, THF, chloroform, DMF and DMSO. GPC shows moderate molecular masses of the copolymers around 15000 and a narrower weight distribution (PDI≈1.4). The DSC and TGA measurements indicated that the resulting copolymers have excellent thermal stability and higher glass transition temperatures than the commonly used light-emitting conjugated polymers. The PL properties of the copolymers were investigated in solution and in the solid state. Only the Alq3 emission (520 nm) was observed at high concentration (10 g/L) or in solid state. However, with the decrease in concentration of the copolymer solution, the characteristic emission of carbazole appears. This means the energy transfer taken place in higher concentration is more efficient than in lower concentration solution.

Abstract: Non-destructive evaluation (NDE) and structural health management (SHM) with the ability to evaluate the severity of a damage are important to ensure the reliability of a structure. We propose a local non-destructive evaluation (NDE) system based on Anomalous Wave Propagation Imaging (AWPI) method. When possible damage is flagged during the lifecycle of the structure, the proposed system will be launched for automatic damage evaluation. This technology was demonstrated on a CFRP skin-spar-stringers wingbox integrated with an AE sensor. 17 mm diameter impact damage was made between the stringers using hammer strike from outer surface of the skin. Based on the impact location determined by other global structural health monitoring system, the AWPI automatically inspects an area 400×400 mm2 with the impacted location enclosed. Anomalous Wave Propagation Movie (AWPM) was generated as inspection result. As contrast to its predecessor, the AWPM shows only the damage induced ultrasonic wave (anomalous wave), making the damage detection an intuitive decision making process. Precise damage localization was performed by identifying the location of area with anomalous wave propagation in the AWPM. Besides, the size of the area with anomalous wave agreed well with the size of impact damage, which demonstrated that damage size quantification is possible using the proposed system. Being sensitive only to anomalous wave, it is expected that this NDE system is exceptionally suitable not only for aircraft structures such as wingbox with stiffeners, but also for other complex engineering structures.

Abstract: In this study, effective computational procedures are introduced and used to characterize the dynamic behavior of cylindrical panels with circular cross-section having the single delamination between laminate layers. Based on the computed results it is possible to determine the effect of delamination on the overall structural dynamic behavior. Those results are used to quantify the difference between the results of the relevant parameters in the cases of perfect and defected structures. Usually, the wave propagation can be observed with the use of piezoelectric sensors. Therefore, in the next step of our analysis we modeled delaminated structures with a finite number of PZT sensors to consider also their influence on the structural dynamic response. The numerical analysis have been conducted with the use of 3D finite elements. A lot of numerical results allow us to understand better the influence of various parameters on the form of wave propagation in cylindrical multilayered shells.

Abstract: Time Domain Reflectometry has been applied to detect damage on sandwich structure. Face plated need to be copper plated to embed sensor on surface. Core also needs to be conductive to send signal back. Total 6 lines of sensor were tested with varying impact energy and impact location. TDR were able to locate damage with high accuracy. Damage degrees were detectable for critical hit, but not clear enough to predict impact energy. However TDR was sensitive enough to detect damage that wasn’t visible by visual inspection.

Abstract: We carried out experiments to detect impact locations on a composite plate using two types of composite plates, a composite flat plate with a constant thickness of 5 mm and a composite stiffened panel with stringers. Four multiplexed FBG sensors were attached to the bottom surface of the composite plates to acquire impact signals. The FBG sensor wavelength shift data were collected at a sampling frequency of 40 kHz using a high-speed FBG interrogator (SFI-710, Fiberpro Inc., Korea). The arrival times of the impact signals at each FBG sensor were obtained using a signal processing procedure. The arrival times were affected by noise level and signal-to-noise ratio. In order to overcome this weakness, signal processing techniques such as wavelet decomposition, normalization using each noise level and filtering with a moving average were adopted. To calculate the impact locations of the composite plate, a neural network algorithm was applied.

Abstract: Signals propagate on plate-like structures as ultrasonic guided waves, and analysis of Lamb waves has been widely used for on-line monitoring. In this study, the wave velocities of symmetric and anti-symmetric modes in various directions of propagation were investigated. Since the wave velocities of these two modes are different, it is possible to compute the difference in their arrival times when these waves propagated the distance from the vibration source to sensor. This paper presents an evaluation formulation of wave velocity and describes a generalized algorithm for locating a vibration source on a thin, laminated plate. With the different velocities of two modes based on Lamb wave dispersion, the method uses two sensors to locate the source on a semi-infinite interval of a plate. The experimental procedure supporting this method employs pencil lead breaks to simulate vibration sources on quasi-isotropic and unidirectional laminated plates. The transient signals generated in this way are transformed using a wavelet transform. The vibration source locations are then detected by utilizing the distinct wave velocities and arrival times of the symmetric and anti-symmetric wave modes. The method is an effective technique for identifying impact locations on plate-like structures.