Impedance Measuring to Detect Fractures in Steel Frames Using Resonance Circuit on Fire Resistive Covering

In order to detect the presence or absence of fractures in constructional members for steel buildings caused by external forces such as earthquakes without the removal of any fire resistive covering materials, we are developing a new nondestructive inspection (NDI) system. This system detects the presence or absence of fractures in a steel frame by measuring the impedance of a coil which exposes the test material to a magnetic field. The system is easy to handle and detection takes only a short time. In this system, the coil is connected to a compensating capacitor with a series-resonant circuit to reduce the impedance and multiply the impedance change by increasing the quality factor. First, the rates of change of impedance and phase angles were measured with the impedance meter of the proposed NDI system. Two types of test samples were investigated. One type of sample had a fracture in the steel plate. The others imitated the beam to column connection with or without a fracture. In both types of samples, the differences in the rates of change of impedance and phase caused by the presence or absence of fractures were measured. Second, in order to confirm the experimental result and establish the design method for optimizing the arrangement and shape of the coil, we calculated the fractional deviation of inductance between presence or absence of the fracture in the steel frame for two types of test samples through a 3-D magnetic field analysis taking eddy current into account. Since the analyzed the fractional deviations of inductance were in good agreement with those measured, the validity of our analysis was confirmed.