리벳 체결 단차부의 결함 검출에 관한 연구

Abstract

In aging aircrafts, flaws can appear in the fastener sites of the air-intakes (intakes) due to degradation of material or adverse operating conditions such as high pressure, concentrated stress, extreme vibrations, salty moisture, etc. Stepped layers are usually added while repairing the intake’s skin. Therefore, nondestructive testing is required for the early detection of flaws in the fastener sites, for both the normal layers and the stepped layers in the intakes. This thesis presents a finite element analysis (FEA) of the electromagnetic field distributions around the fastener in the stepped layers of the aging aircrafts’ intakes. The advantages and limitations of flaw detection are presented, based on the FEA simulation results. In addition, a nondestructive testing system designed based on the simulation results is developed. ANSYS (ver.11.0, EMAG), a commercialized FEA software, is used for the simulation. Two magnetic sources—a rectangular coil and a yoke-type magnetic source—are simulated and compared. A Hall sensor array is placed at the center of each magnetic source to measure the vertical component of the electromagnetic field. In case of using a rectangular coil, the magnetic flux distribution is not uniform at the Hall sensor position and is highly affected by the edge of the stepped layer; therefore, the flaw detection may be limited. However, when using a yoke-type magnetic source, the magnetic flux is distributed uniformly at the Hall sensor array position and is not affected much by the edge. In addition, the eddy current density and magnetic flux density produced by the yoke-type magnetic source are higher than that of the rectangular coil. Thus, the yoke-type magnetic source can provide a better flaw-detection capability than the rectangular coil. Based on the results of the simulation, the Hall sensor array (linearly integrated Hall sensor array, LIHaS) is fabricated and is integrated with a yoke-type magnetic source in a sensor probe. For a flat specimen, far-side corrosion with 5-mm diameter and 0.4-mm depth could be detected at the fastener site by the automatic flaw-detection algorithm. At the top layer on the stepped specimen, far-side corrosion with 5 mm diameter, 0.4 mm depth, and with 8 mm and 12 mm distances from the rivet center to step boundary could be detected. At the bottom layer on stepped specimen, the far-side corrosion at different locations (left, right, forward, backward) of fastener hole with 9 mm distance from the rivet center to step boundary could be detected. Right, forward, and backward corrosion with 5 mm distance from rivet center to step boundary could also be detected. However, the left side corrosion which is near to the stepped edge couldn’t be detected because the LIHaS could not access to the smaller area close to the stepped edge.