Tatsuro Kosaka

Detection of transverse cracks in CFRP composites using embedded fiber Bragg grating sensors

In the present research, fiber Bragg grating (FBG) sensors were applied for the detection of transverse cracks, which cause strain distribution within the gage length, in carbon fiber reinforced plastic (CFRP) cross-ply laminates. An uncoated FBG sensor was embedded in 0° ply on the border of 90° ply in a CFRP cross-ply laminate. The reflection spectra from the FBG sensor were measured at various tensile stresses. As a result, the reflection spectrum became broad and had some peaks with an increase of the transverse crack density in the 90° ply. After the crack density was saturated, the spectrum became narrow and had one large peak again. In order to confirm that the change in the spectrum was caused by transverse cracks, the spectra were calculated theoretically. The calculated result reproduced the change in the measured spectrum very well. These results show that the occurrence of transverse cracks can be detected from the change in the form of the reflection spectrum, and that the spectrum width at the half maximum is a good indicator for the quantitative evaluation of the transverse crack density in real time.

Hard x-ray telescope to be onboard ASTRO-H

The new Japanese X-ray Astronomy satellite, ASTRO-H will carry two identical hard X-ray telescopes (HXTs), which cover 5 to 80 keV. The HXT mirrors employ tightly-nested, conically-approximated thin-foil Wolter-I optics, and the mirror surfaces are coated with Pt/C depth-graded multilayers to enhance hard X-ray effective area by means of Bragg reflection. The HXT comprises foils 450 mm in diamter and 200 mm in length, with a focal length of 12 m. To obtain a large effective area, 213 aluminum foils 0.2 mm in thickness are tightly nested confocally. The effective area is expected to be ~ 310 cm2 at 30 keV and the image quality to be ~1.′7 in half-power diameter.

Strain monitoring of braided composites by using embedded fiber-optic strain sensors

Recently, fiber optic strain sensors have been applied to internal strain and damage monitoring of composites because of their small size, light weight and flexibility. Braided fiber reinforced plastics (FRP) are compatible with fiber optic sensors because optical fibers can be integrated directly and easily into fabrics. In the present paper, the strain monitoring of braided glass fiber reinforced plastics (GFRP) was conducted by using embedded fiber Bragg grating (FBG) and extrinsic Fabry–Perot interferometric (EFPI) sensors during the cure process, tensile tests and fatigue tests. From the experimental results of cure monitoring, it was found that both sensors can be used only for monitoring of thermal residual strain during cooling process. From the results of tensile tests, it was found that both sensors could measure strain correctly until damage initiation of braided GFRP. It also appeared that FBG sensors could monitor damage to FRP by observing the reflected spectral shape. From the fatigue tests, it appeared that the strain measured by embedded FBG sensors was affected by fatigue damage. Therefore, it is concluded that internal strain monitoring of braided FRP using fiber optic strain sensors is very useful for cure and health monitoring.

Current status of ASTRO-H Hard X-ray Telescopes (HXTs)

ASTRO-H is an international X-ray mission of ISAS/JAXA, which will be launched in 2014. One of the main characteristics of ASTRO-H is imaging spectroscopy in the hard X-ray band above 10 keV. ASTRO-H will carry two identical Hard X-ray telescopes (HXTs), whose mirror surfaces are coated with Pt/C depth-graded multilayers to enhance hard X-ray effective area up to 80 keV. HXT was designed based on the telescope on board the SUMIT balloon borne experiment. After feasibility study of the HXT design, the FM design has been deteremined. Mass production of the mirror shells at Nagoya University has been going on since August 2010, and production of mirror shells for HXT-1 was completed in March 2012. After the integation of X-ray mirrors for HXT-1, we measured hard X-ray performance of selected mirror shells for HXT-1 at a synchrotron radiation facility, SPring-8 beamline BL20B2. We will perform environment tests and ground calibarations at SPring-8 for HXT-1. In HXT-2, foil production is going on.

Detection of transverse cracks in composites by using embedded FBG sensors

In the present research, fiber Bragg grating (FBG) sensors were applied for the detection of transverse cracks, which cause strain distribution within the gage length, in carbon fiber reinforced plastic (CFRP) cross-ply laminates. An uncoated FBG sensor was embedded in 0° ply on the border of 90° ply in a CFRP cross-ply laminate. The reflection spectra from the FBG sensor were measured at various tensile stresses. As a result, the reflection spectrum became broad and had some peaks with increase of the transverse crack density in the 90° ply. After the crack density was saturated, the spectrum became narrow and had one large peak again. For confirming that the change in the spectrum form was caused by transverse cracks, the spectra were calculated theoretically. The calculated result reproduced the change in the measured spectrum form very well. These results show that the occurrence of transverse cracks can be detected from the change in the form of the reflection spectrum, and the spectrum width at the half-maximum is a good indicator for the quantitative evaluation of the transverse crack density on real-time.

Report on a simultaneous ion viscosity, strain and impedance measurement technique using a novel integrated dielectric, optical fiber and piezoelectric sensing element for the online characterization of smart structures

This paper reports on a simultaneous ion viscosity, strain and impedance (SISI) system in order to measure the physical and chemical properties of composites during their curing process. The SISI system uses an integrated multi-sensing element, entitled DOP, that is comprised of dielectric (D), optical fiber (O) and piezoelectric (P) sensors. This system was used to measure several data simultaneously in real time and in situ. The results clearly show that there is a direct relationship between the ion viscosity, impedance and strain changes during the curing process. It was found that dielectric sensor is very sensitive to physical and chemical changes of the composite both in the heating and cross-linking periods. The piezoelectric proved to be a useful element during the heating period with a very sensitive and surprising behavior during the cooling period. The optical fiber also demonstrated a very striking profile in strain variations during cooling.

High-Resolution Ultrasonic Detection of Subsurface Transverse Cracks in CFRP Laminates. Simulation and Experiments

Scanning acoustic microscopy (SAM) has proven to be an effective nondestructive evaluation method for damage detection in composite laminates. A new high-number-of-aperture (HA) 30MHz pulse lens (100-degree aperture angle) can detect microcracks such as transverse cracks which cannot be detected by a conventional 30MHz pulse lens (30-degree aperture angle), in addition to delaminations in CFRP laminates. A picture of SAM images results from elastic characteristics of CFRP. Therefore, an ultrasonic wave propagation analysis in anisotropic media is necessary to understand SAM images. Two types of simulations are conducted in this paper to analyze the effect of waves propagating in CFRP on SAM images. One is a ray tracing simulation to demonstrate the energy flow in the media, and the other is an image construction using the method of angular spectrum. They show that SAM images of transverse cracks are constructed by quasi-longitudinal waves and faster quasitransverse waves. Vol. 5, Nos. 3-4, 1996 High-Resolution Ultrasonic Detection of Subsurface Transverse Cracks in CFRP Laminates

Performance of ASTRO-H hard x-ray telescope (HXT)

The Japanese X-ray Astronomy Satellite, Hitomi (ASTRO-H) carries hard X-ray imaging system, covering the energy band from 5 keV to 80 keV. The hard X-ray imaging system consists of two hard X-ray telescopes (HXT) and the focal plane detectors (HXI). The HXT employs tightly-nested, conically-approximated thin foil Wolter-I optics. The mirror surfaces of HXT were coated with Pt/C depth-graded multilayers. We carried out ground calibrations of HXTs at the synchrotron radiation facility SPring-8/ BL20B2 Japan, and found that total effective area of two HXTs was about 350 cm2 at 30 keV, and the half power diameter of HXT was about 1.’9. After the launch of Hitomi, Hitomi observed several targets during the initial functional verification of the onboard instruments. The Hitomi software and calibration team (SCT) provided the Hitomi’s data of G21.5-0.9, a pulsar wind nebula, to the hardware team for the purpose of the instrument calibration. Through the analysis of the in-flight data, we have confirmed that the X-ray performance of HXTs in orbit was consistent with that obtained by the ground calibrations.

ASTRO-H Hard X-ray Telescope (HXT)

The new Japanese X-ray Astronomy satellite, ASTRO-H will carry two identical hard X-ray telescopes (HXTs), which cover 5 to 80 keV, in order to provide new insights into frontier of X-ray astronomy. The HXT mirror surfaces are coated with Pt/C depth-graded multilayers to enhance hard X-ray effective area by means of Bragg reflection, and 213 mirror reflectors with a thickness of 0.22 mm are tightly nested confocally in a telescope. The production of FM HXT-1 and HXT-2 were completed in 2012 and 2013, respectively. The X-ray performance of HXTs were measured at the synchrotron radiation facility SPring-8/ BL20B2 Japan. The total effective area of two HXTs is about 350 cm2 at 30 keV and the angular resolution of HXT is about 1.’9 in half power diameter at 30 keV. The HXTs are in the clean room at ISAS for waiting the final integration test.

Ray-tracing simulation and in-orbit performance of the ASTRO-H hard x-ray telescope (HXT)

A ray-trace simulation code for the Hard X-ray Telescope (HXT) on board the Hitomi (ASTRO-H) satellite is being developed. The half power diameter and effective area simulated based on the code are consistent with ground measurements within 10%. The HXT observed the pulsar wind nebula G21.5-0.9 for 105 ksec. We confirmed that the encircled energy function and the half power diameter obtained from the data are consistent with the ground measurements.