An eddy current test (ECT) is a common nondestructive test to detect flaws in metal structures. Many ECTs use both a detection coil and an induction coil. The application of this method to ferromagnetic materials can be challenging because the applied magnetic field is composed of not only the eddy current but also the magnetization signal. Therefore, we developed two types of miniaturized ECT probes, namely, single- and dual-channel tunneling magnetoresistance (TMR) probes, by using a TMR sensor instead of a detection coil. To evaluate the performance of crack detection in steel with regard to the length, width, and depth of cracks, the sensor probe was line-scanned above the crack position. The output signal was detected using a lock-in amplifier and analyzed using the real and imaginary components of a magnetic vector. The detected signal was then divided into the magnetization and eddy current components. The dual TMR sensor probe installed inside the induction coil could yield a stable crack signal without the influence of magnetization fluctuation. The line-scanned differential signal intensity and phase show a signal change with a high signal-to-noise ratio at the crack position. These signals depend on the width and length of the crack. A 2-D scanning could provide a magnetic map corresponding to the crack shape.

In order to ensure the safety of infrastructure, it is important to be able to detect both surface and inner cracks in the thick steel plates used. However, it is difficult to use conventional magnetic measurement techniques for inspecting inner cracks in steel because of its high permeability as well as owing to the variations in it. To solve this problem, we developed a method called unsaturated ac magnetic flux leakage (MFL) testing based on a magnetic resistance gradiometer for analyzing the inner cracks in steel. The proposed method uses a sensor probe consisting of a semicircular yoke with induction coils at each end, and a gradiometer with two anisotropic magnetic resistance sensors for detecting the components perpendicular to the steel surface. The conventional MFL testing method requires a strong power source for observing the MFL in the magnetic saturation region. In contrast, in this paper, a gradiometer-based method for detecting the differential intensity and phase is used to detect low levels of magnetic leakage using a weak power source when the test steel sample is unsaturated. During the measurements, a deep inner crack could be detected by decreasing the frequency. The line-scanned differential signal exhibited a peak just above the crack position and was observed to be depth dependent. Based on these results, the method could be used to determine the position of the crack as well as its depth.

Reinforcing steel bars (Rebar) in the concrete need to be inspected to prevent the structure from collapsing. Magnetic non-destructive testing methods can inspect the rebar without the influence of concrete. However, since the inspection is performed at a high lift-off on concrete, a strong applied magnetic field is required in order to measure a sufficient magnetic signal generated from the rebar. In addition, if a single sensor is used to measure the rebar, this requires the scanning of extensive area along the invisible rebar covered with the concrete. To overcome these problems, we have developed a method to detect and image the defect signal using unsaturated AC magnetic flux leakage (USAC-MFL) testing combining a magnetic sensor array probe. The USAC-MFL is a method that uses an alternating magnetic field in the initial magnetization characteristics of steel, which does not require a strong applied magnetic field compared to the conventional magnetic flux leakage method. By measuring the magnetic flux from the rebar in a wide area using the magnetic sensor array probe, the defect signal was able to be detected and imaged. The defect signal was able to be confirmed up to the lift-off of 100 mm even when the defect was half length of rebar or located on the back side.

To measure an impedance magnetocardiogram (I-MCG) signal, we have developed a superconducting quantum interference device (SQUID) magnetometer with a room-temperature pickup coil. The pickup coil (30-mm diameter) is set outside the cryostat and is connected to the input coil in the SQUID. The noise level of the magnetometer is 90 fT Hz^-1/2 at 10 kHz. Using the magnetometer, we measured I-MCG signals (below 15 pT)–which were obtained by applying an ac current (10 kHz) with constant amplitude (7 mA, peak-to-peak) to a subject–from the heart.