Kei Urabe

Characterization of Long-period Grating Refractive Index Sensors and Their Applications

The influence of grating length and bend radius of long-period gratings (LPGs) on refractive index sensing was examined. Sensitivity to refractive indexes smaller than that of silica could be enhanced by bending LPGs. Bent LPGs lost sensitivity to refractive indexes higher than that of silica, whereas a 20-mm-long LPG arranged in a straight line had considerable sensitivity. These experimental results demonstrated that the sensitivity characteristics of LPGs to refractive index could be controlled by grating length and bend radius.

Impact damage detection in CFRP using fiber Bragg gratings

A fiber-optic ultrasonic sensing system was constructed with fiber Bragg gratings based on the intensity modulation of light reflected from fiber Bragg grating sensors. This fiber-optic system consists of fiber Bragg gratings for sensing and filtering, a broadband light source as well as a photo detector. The feasibility of damage monitoring using the fiber-optic system was investigated. Transient ultrasonic Lamb waves generated with a piezoceramic pulser were propagated in a cross-ply CFRP with visible impact damage. The response of the fiber Bragg grating sensor to the Lamb wave propagated through the damaged area was compared with a reference response in an intact area. Frequency characteristics of the response signal were analyzed in order to evaluate Lamb wave interaction with damage. Moreover, Lamb wave detection using a piezoceramic sensor was performed to compare with the response of the fiber Bragg grating sensor. The experimental results demonstrated that fiber Bragg grating sensors are comparable to piezoceramic sensors in ultrasonic detection and the fiber-optic system proved to be effective in impact damage detection of CFRP.

Monitoring of resin flow and cure with an electromagnetic wave transmission line using carbon fiber as conductive elements

The authors previously presented a new method for monitoring resin flow and cure with a high-frequency electromagnetic wave transmission line constructed inside a structure, where information on discontinuity or distribution can be obtained by using frequency characteristics or time domain response. In this paper, we propose the use of carbon fiber for conductive elements constructing the transmission line, which were constructed from metal in the previous works, so as to use material reinforcements (i.e., carbon fiber) as sensing probes. The experimental results demonstrated the possibility of carbon fiber as transmission line elements.

Analysis of coaxial-to-circular waveguide transition

The coaxial-to-circular waveguide transition is analyzed using the Galerkins method of moments. Because of the axisymmetric structure as well as the assumption of the TEM-mode incident wave, which is an axisymmetric mode, the scattered waves from the discontinuities in the coaxial and circular waveguides can be expressed as axisymmetric modes, and therefore, the analysis can be extremely simplified. The reflection characteristics are calculated as a function of frequency, and excellent agreement with the result presented in the reference literature is confirmed. This technique targets not only the nondestructive inspection of wall thinning and cracks inside conductive pipes but also the slot array antenna in circular waveguides.

VISUALIZATION OF ULTRASONIC WAVES SCATTERED FROM REAR DEFECTS BY USING A LASER‐BASED IMAGING TECHNIQUE

We proposed a method for visualizing wave propagation in ultrasonic fields. This method generates thermal‐excitation ultrasonic waves on a specimen through pulsed laser scanning and detects the propagating signals via a reception transducer mounted on a fixed point. Dynamic images of ultrasonic waves propagated from a fixed point are created using the reciprocity principle of wave propagation. This method has the excellent feature that enables a visible and quick flaw inspection for actual objects. However, since this method visualizes all waves propagating on an object, it is disadvantageous in that defect echoes are often hidden in the forward traveling wave. In this research, we developed a data processing method for image enhancement of defect echoes. In the ultrasonic tests with aluminum plates or pipes with rear defects, we could visualize the defect echoes as they scatter in a radial manner, like water rings, on the surface of the specimen, which proved the validity of this data processing method. ...

Detection of missing fastener based on vibration mode analysis using fiber Bragg grating (FBG) sensors

We propose a damage detection method based on frequency responses measured by FBG sensors during vibration tests. When random noise vibrates a laminated composite panel and a stiffener fastened with bolts, the peak gain of the resonance frequencies can be obtained. We calculated the correlation coefficient of the normalized gain for the frequency responses, and then predicted the location of a missing bolt. This method will make possible to predict the location of damage with a limited number of FBG sensors while a structure is vibrating even if the excitation point changes.

Cure monitoring of matrix resin with high-frequency electromagnetic wave transmission line

A new monitoring method for the curing process of matrix resin is proposed. In this method, a high-frequency electromagnetic wave transmission line is constructed inside or on the surface of a material and changes in the characteristics of the line during the curing process are measured. The use of a high-frequency electromagnetic wave mainly aims at getting information on the position of the discontinuity or distribution of material properties, making use of its short wavelength. As the first step of the investigations, an experimental setup of a microstrip line and a parallel-wire line was constructed, and changes in the frequency characteristics of reflectance from each of the lines were monitored during the curing of epoxy resin between the pair of conductors of the line. Changes in the characteristics when the resin is partly filled were also investigated to examine the possibility of detecting local distributions or discontinuities. The experimental results are successfully explained by theoretical calculations. Results obtained in this study suggest the potential of the transmission line of a high-frequency electromagnetic wave as a tool for in-situ monitoring of curing and other properties.

Transmission characteristics of circular waveguide with coaxial-to-circular waveguide transitions

The reflection and transmission characteristics of a circular waveguide with coaxial-to-circular waveguide transitions are evaluated using Galerkins method of moments. Because of the axisymmetric structure as well as the assumption of a TEM-mode incident wave, which is an axisymmetric mode, the scattered waves from the structural discontinuities can be expressed as axisymmetric modes; therefore, the analysis can be considerably simplified. In addition, by making the most of the structural symmetry for both sides (ports), a further reduction in the calculation can be performed. The characteristics are calculated as a function of frequency, and reasonable and interesting results are obtained. The circumferential slot antenna in the circular waveguide as well as the nondestructive inspection of circumferential cracks and wall-thinning inside conductive pipes are promising candidates for this technique.

Characteristics of circular waveguide with step discontinuities including coaxial feeders

The reflection and transmission characteristics of a circular waveguide with step discontinuities including coaxial feeders are evaluated using Galerkins method of moments. Two sizes of cylindrical step discontinuities of large and small diameters are considered. This cylindrical discontinuity is placed so that it aligns the center axis with the circular waveguide. Because of the axisymmetric structure as well as the assumption of a TEM-mode incident wave, which is an axisymmetric mode, the scattered waves from the structural discontinuities can be expressed as axisymmetric modes. Therefore, the analysis can be considerably simplified. The condition to cancel reflected waves by placing two discontinuities at the proper distance is confirmed.

Measured performances of coaxial-to-circular waveguide transitions

The reflection and transmission characteristics of coaxial-to-circular waveguide transitions are measured and investigated. Two identical transitions are directly connected so that they faced each other and evaluated using a vector network analyzer. The parameters of this transition are optimized in advance at the 5 GHz band using the method of moments. As a result, good agreement can be confirmed between the calculated and measured results. This transition targets not only the slot array antenna in circular waveguides but also the nondestructive inspection of wall thinning and cracks inside conductive pipes.