The Nanoelectronics Research Institute (NeRI) of the National Institute of Advanced Industrial Science and Technology (AIST), in collaboration with the Graduate School of Science and Engineering at Waseda University and the Institute of Space and Aeronautical Science, have used a superconducting quantum interference device (SQUID) to develop the first ever non-destructive testing technology for carbon fiber-reinforced, carbon composite materials (C/C composites). To date, no useful non-destructive test method was available to detect flaws in C/C composites.

C/C composites are used in the space shuttle’s heat-resistant tiles and are of increasing importance as a high-temperature material for such applications as heat insulation. Use of such general-purpose non-destructive test methods as X-rays and ultrasound have proved difficult with C/C composites and, to date, no non-destructive method has been developed to detect C/C composite flaws. The AIST applied a non-destructive testing technology that used SQUID—a new technology for metal materials that has been the subject of much research in Japan and overseas in recent years—and imaged electric current flow in C/C composites. The location of flaws could be determined from the current distribution. The AIST team also developed a new SQUID non-destructive test method that can infer the state of these flaws. This is the first ever non-destructive test method that can detect minute flaws in C/C composites.

With a view to the practical implementation of this method, the AIST team developed a prototype that included a small, lightweight, low magnetic noise, and low-vibration refrigerator (in collaboration with Iwatani Gas). By incorporating this refrigerator with a high-temperature superconductor SQUID (HTS-SQUID)—developed by the AIST—the team produced a low-noise SQUID non-destructive test system that does not require the use of difficult-to-handle liquid nitrogen and that can be easily operated by anybody anywhere as long as an electric power outlet is available. Moreover, by incorporating into the system an applied magnetic field generator, which uses ferrite to induce a current in highly resistive C/C composites, the team produced a prototype system that has the potential for use as a non-contact non-destructive test system for C/C composites on site, at testing and other locations.

First ever non-destructive method uses a SQUID to test for flaws in C/C composites
Previously, it was not possible to test for flaws in C/C composites using a non-destructive test method. In this research, the AIST team has developed a new SQUID non-destructive testing technology that involves visualizing the electric current flowing through C/C composites and identifying flaw locations from the current distribution. This is the first ever non-destructive method that can test for minute flaws in C/C composites. The key breakthroughs in this research were the use of the highly sensitive magnetic sensor SQUID—which is in a different league to other magnetic sensors—and the development of a method to visualize current flow using a SQUID gradiometer that measures the spatial gradient of the magnetic field generated by the current flowing through the test material. These developments enabled the development of a non-destructive method to test flaws in C/C composites. Moreover, the visualization of the current allowed the researchers to not only identify flaw location, but also to gain an understanding of flaw state. This flaw characterization is also expected to have applications to diagnosis of C/C composite material deterioration and assessment of expected lifespan.

Prototype low-noise, SQUID non-destructive test system using a refrigerator
The key breakthroughs in this research were (1) the use of a HTS-SQUID gradiometer as a sensor, as it can operate stably even in a poor magnetic environment, and (2) the inclusion of a coaxial pulse-tube refrigerator made out of nonmagnetic materials that is lightweight and generates little magnetic noise and vibration. In addition the refrigeration (cooling) stage was separated from the stage on which the SQUID was mounted, with the two stages connected by a flexible copper wire, which made the system thermally stable and enabled the generation of little magnetic noise and vibration. Moreover, by incorporating into the system an applied magnetic field generator, which uses ferrite to induce a current in highly resistive C/C composites, the team produced a prototype system that has the potential for use as a non-contact, non-destructive testing system, for example to test space shuttle heat-resistant tiles.

Previously, it was not possible to test for flaws in C/C composites using a non-destructive test method. In this research, the AIST team has developed a new SQUID non-destructive testing technology that involves visualizing the electric current flowing through C/C composites and identifying flaw locations from the current distribution. This is the first ever non-destructive method that can test for minute flaws in C/C composites. The key breakthroughs in this research were the use of the highly sensitive magnetic sensor SQUID—which is in a different league to other magnetic sensors—and the development of a method to visualize current flow using a SQUID gradiometer that measures the spatial gradient of the magnetic field generated by the current flowing through the test material. These developments enabled the development of a non-destructive method to test flaws in C/C composites. Moreover, the visualization of the current allowed the researchers to not only identify flaw location, but also to gain an understanding of flaw state. This flaw characterization is also expected to have applications to diagnosis of C/C composite material deterioration and assessment of expected lifespan.

The key breakthroughs in this research were (1) the use of a HTS-SQUID gradiometer as a sensor, as it can operate stably even in a poor magnetic environment, and (2) the inclusion of a coaxial pulse-tube refrigerator made out of nonmagnetic materials that is lightweight and generates little magnetic noise and vibration. In addition the refrigeration (cooling) stage was separated from the stage on which the SQUID was mounted, with the two stages connected by a flexible copper wire, which made the system thermally stable and enabled the generation of little magnetic noise and vibration. Moreover, by incorporating into the system an applied magnetic field generator, which uses ferrite to induce a current in highly resistive C/C composites, the team produced a prototype system that has the potential for use as a non-contact, non-destructive testing system, for example to test space shuttle heat-resistant tiles.