Detection of deep-lying defects in carbon fiber composites using SQUID-NDE system cooled by a cryocooler

In this paper, we present our latest progress in the studies on SQUID-NDE for carbon fiber composites (CFCs), including carbon fiber reinforced plastics (CFRPs) and carbon fiber reinforced carbon matrix composites (C/Cs). We have developed a new SQUID-NDE system for the CFCs utilizing a pulse tube cryocooler for more practical and easy-handling use. Electromagnets using U-shaped ferrite cores have been introduced to the system in order to induce enough eddy current density in the relatively low-electric-conductivity CFCs. By using this system, we demonstrated the detection of deep-lying defects in thick and/or multi-layer CFCs specimens, partly including metal layer upon/below CFCs layer. CFRP plates and multi-layer C/C plates with total thickness of 20 mm having deep-lying slots at various depths were prepared. The slots at 15 mm below the surface in the CFRP plate and multi-layer C/C plate were successfully detected. Defect detections on more "real" multi-layer structures, including aluminum plates upon/below C/C plates, were also examined.