Non-destructive Detection of Defects in Carbon Fiber-Reinforced Carbon Matrix Composites Using SQUID

Naoko KASAI
Yoshimi HATSUKADE
Hiroshi TAKASHIMA

Publication
IEICE TRANSACTIONS on Electronics   Vol.E88-C    No.2    pp.180-187
Publication Date: 2005/02/01
Online ISSN: 
DOI: 10.1093/ietele/e88-c.2.180
Print ISSN: 0916-8516
Type of Manuscript: Special Section INVITED PAPER (Special Section on Superconducting Electronic Devices and Their Applications)
Category: 
Keyword: 
NDT,  C/C,  pulse tube cryocooler,  deep-lying defect,  breaking process,  

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Summary: 
Carbon fiber composites are increasingly used as structural materials because of their unique and advantageous characteristics. Carbon fiber reinforced carbon matrix composite (C/C) has the characteristics of high fatigue resistance, fracture toughness and heat resistance up to 3000 K, and is an important component of refractory tiles and nozzles in space shuttles. Useful nondestructive testing methods for C/C are now required. We have developed a SQUID-NDT system based on a non-magnetic coaxial pulse tube cryocooler (PTC), a HTS-SQUID gradiometer and a field generator with ferrite cores that induces high currents in specimens with low electric conductivity. The cryostat with the PTC is compact, at 50 mm in diameter and 400 mm in height. It weighs a total of 4 kg. The system noise is 80 µ0/Hz1/2 corresponding to 1.3 nT/m/Hz1/2 at 100 Hz. We used the system to investigate the usefulness of the SQUID-NDT in detecting flaws in C/C composites. Hidden cracks in C/C multi-layered specimens were detected up to depth of 15 mm. Hidden cracks in C/C-Al stacked sample was also clearly detected. In addition, we magnetically detected the mechanical breaking process of a C/C specimen under tensile load using the current injection method. For this study, a technique for visualizing current detouring defects was developed for detection of deteriorating areas in the specimen. The deteriorating area, identified from the current map, expands during breaking process and agrees with the results obtained by the microscopic observation of the breaking process. The interrupted current Iint, estimated by summing the detour current, clearly changed depending on the stage of the breaking process, suggesting that Iint may be applicable as good index for distinguishing each stage in the breaking process. It is concluded that a SQUID-NDT is applicable to C/C composites and advanced complex materials with low electric conductivity in addition to metallic materials.