We have developed a novel nondestructive inspection technique for infrastructures using spintronics technologies. Since the tunnel magneto-resistance (TMR) effect has become dramatically more sensitive in recent years, TMR sensors can be applied to the highly sensitive nondestructive inspection named magnetic hammering test (MHT). The proposed MHT technique is based on the principle of detecting slight fluctuations in the spatial magnetic field caused by the vibration of steel materials. Since the fluctuations in the magnetic field occur with the natural vibration frequency of the steel materials, their condition can be detected from the change of natural frequency using highly sensitive TMR sensors. In this work, we have demonstrated that the size of steel plates was determined with high accuracy using the MHT technique.

A new nondestructive inspection method, the magnetic hammer test (MHT), which uses a compact and highly sensitive tunnel magnetoresistance (TMR) sensor, is proposed. This method complements the magnetic flux leakage method and eliminates the issues of the hammer test. It can therefore detect weak magnetic fields generated by the natural vibration of a pipe with a high signal-to-noise ratio. In this study, several steel pipes with different wall thicknesses were measured using a TMR sensor to demonstrate the superiority of MHT. The results of the measurement show that wall thickness can be evaluated with the accuracy of several tens of microns from the change in the natural vibration frequency of the specimen pipe. The pipes were also inspected underwater using a waterproofed TMR sensor, which demonstrated an accuracy of less than 100 μm. The validity of these results was by simulating the shielding of magnetic fields and vibration of the pipes with the finite element method (FEM) analysis. The proposed noncontact, fast, and accurate method for thickness testing of long-distance pipes will contribute to unmanned, manpower-saving nondestructive testing (NDT) in the future.

The magnetic hammer test (MHT) technique was developed using tunnel magnetoresistive (TMR) sensors that detect very weak magnetic signals generated by the natural vibration of steel plates. For the non-destructive testing (NDT) of infrastructures, which requires high accuracy and energy conservation, TMR sensors with high sensitivity have the potential to meet the NDT requirements. TMR sensors detected weak signals of nanotesla due to natural vibration and accurately evaluated the size of steel plates from the natural frequency. In addition, a finite element method (FEM) simulation analysis was conducted to elucidate the mechanism of the magnetic signals generated by the MHT. We concluded that the MHT technique using TMR sensors can be applied as a novel NDT method for detecting small fractures and corrosion in steel materials.