Abstract
We review stationary and mobile systems that are used for the nondestructive evaluation of room temperature objects and are based on superconducting quantum interference devices (SQUIDs). The systems are optimized for samples whose dimensions are between 10 micrometers and several meters. Stray magnetic fields from small samples (10 µm–10 cm) are studied using a SQUID microscope equipped with a magnetic flux antenna, which is fed through the walls of liquid nitrogen cryostat and a hole in the SQUID’s pick-up loop and returned sidewards from the SQUID back to the sample. The SQUID microscope does not disturb the magnetization of the sample during image recording due to the decoupling of the magnetic flux antenna from the modulation and feedback coil. For larger samples, we use a hand-held mobile liquid nitrogen minicryostat with a first order planar gradiometric SQUID sensor. Low-Tc DC SQUID systems that are designed for NDE measurements of bio-objects are able to operate with sufficient resolution in a magnetically unshielded environment. High-Tc DC SQUID magnetometers that are operated in a magnetic shield demonstrate a magnetic field resolution of ~4 fT/√Hz at 77 K. This sensitivity is improved to ~2 fT/√Hz at 77 K by using a soft magnetic flux antenna.
Highlights
Nondestructive evaluation (NDE) describes the characterization of the structure and/or functionality of an object without compromising its usability
We briefly review stationary and mobile low-Tc and high-Tc Superconducting quantum interference devices (SQUIDs) systems that have been developed in Forschungszentrum Jülich (FZJ) and the Kotel’nikov Institute of Radio Engineering and Electronics (IRE) for the NDE of room temperature objects, in the context of those developed elsewhere
The operation of SQUIDs is based on the dependence of the phase shift of the wave-function of Cooper pairs on the magnetic flux passing through the SQUID loop, similar to the phase shift of the wave-function of a charged particle in the Aharonov-Bohm effect
Summary
Nondestructive evaluation (NDE) describes the characterization of the structure and/or functionality of an object without compromising its usability. Successful applications of SQUID-based NDE systems from the last thirty years for monitoring materials and structures have been described and assessed elsewhere [5]. The disadvantages of such systems include their operation at cryogenic temperatures and, the expense of performing routine measurements. SQUID-based NDE systems have been developed and employed for the detection of defects in steel plates [7], the study of stress–strain states in ferromagnetic materials [8], the detection of ruptures in steel ropes on bridge structures [9], and the detection of cracks in turbine blades of aircraft engine turbine blades [10].
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