electromagnetic testing of bleed air duct tubes in aging
TRANSCRIPT
International Journal of Applied Electromagnetics and Mechanics 55 (2017) S211–S216 S211DOI 10.3233/JAE-172275IOS Press
Electromagnetic testing of bleed air ducttubes in aging aircrafts
Jungmin Kima, Minhhuy Lea, Wanpyo Honga and Jinyi Leea,b,∗aResearch Center for IT-based Real Time NDT, Chosun University, Gwangju 61452, KoreabDepartment of Control and Instrumentation Engineering, Graduate School of Chosun University,Gwangju 61452, Korea
Abstract. This paper presents a nondestructive testing method for bleed air duct tubes in aging aircrafts. The inspection tech-nique is based on eddy current testing. The sensor probe is constructed of a bobbin coil as a magnetic source and a bobbin-typeintegrated Hall sensor (BIHaS) array to measure the electromagnetic field distribution around cracks on the bleed air duct tube.The BIHaS has many Hall sensor elements arrayed in a circumference that can measure the radial component of the electro-magnetic field with high spatial resolution. For example, 45 Hall sensor elements were used for inspection of a bleed air ducttube having an inner diameter of 38 mm. To verify the effectiveness of the proposed system, tests were carried out on artificialcracks in bleed air duct tubes. The system was able to inspect 3 mm long cracks and their length was quantitatively evaluatedwith 1.57 mm of standard deviation.
Keywords: Control rod, encircling coil, eddy current, sensor array, small-bore piping system, magnetic camera, imaging
1. Introduction
Bleed air ducts are used to supply pressurized hot air from the engines and routed throughout theairframe to secondary systems of an aircraft. The bleed air ducts are thin titanium tubes which havehigh strength-to-weight ratio, good corrosion resistance and appropriate mechanical properties. Duringoperation, the tubes are subjected to high pressure and heating cycles that cause the development ofcracks. A number of titanium bleed air duct failures have been reported in previous researches [1–3].Thus, it is necessary to periodically perform nondestructive testing to confirm the integrity and safety ofthe bleed air ducts.
Eddy current testing (ECT) methods have been successfully developed for various tubing systems.Bobbin probes have been used for many years [4,5]. They simply use a single bobbin coil or differential-type bobbin coil as a sensing device. The bobbin probes are reliable and can be used to inspect frettingwear and pitting corrosion. However, they are limited in spatial resolution and cannot distinguish thelocation of cracks in the circumferential direction [5,6]. In addition, the eddy current flux produced bythe bobbin coil flows in the circumferential direction of the tube, thus, circumferential cracks could notbe inspected. To overcome this limitation, different designs of the exciting coil have been developed (e.g.
∗Corresponding author: Jinyi Lee, Research Center for IT-based Real Time NDT, Chosun University, Gwangju 61452, Korea.Tel.: +82 62 230 7101; E-mail: [email protected].
1383-5416/17/$35.00 c© 2017 – IOS Press and the authors. All rights reserved
S212 J. Kim et al. / Electromagnetic testing of bleed air duct tubes in aging aircrafts
Fig. 1. Inspection stage and a sensor probe.
plus-point coil and spiral coil) so that large eddy current in both the axial and circumferential directionsis produced [7]. For improving spatial resolution, eddy current array probes have been developed usingmultiple sensing coils arrayed in the circumferential direction [8,9]. The principle of the eddy currentarray probe is same as the bobbin probe, in which the variation of electromagnetic field in the tube dueto the existence of cracks will be detected by the impedance changes of the sensing coils. However,the probes are less sensitive at low frequencies. They are usually operated at several hundred kilohertzto megahertz. In addition, the spatial resolution is limited because of the limited size of the sensingcoil [10].
Therefore, this research presents an inspection system for bleed air duct using a bobbin-type integratedHall sensor (BIHaS) array based on eddy current testing technique. The exciting source is a bobbin coilplaced beside the BIHaS. When an alternating current is supplied to the bobbin coil, an eddy current willbe produced in the circumferential direction and concentrated around the crack in the tube. The radialcomponent of the secondary magnetic field, which is produced by the eddy current, will be capturedusing the BIHaS [11,12]. Thus, the crack can be inspected. The BIHaS has a large number of Hallsensor elements arrayed in the circumferential direction so that a high spatial resolution can be obtained.In this paper, 45 Hall sensor elements were used for inspection of titanium bleed air duct tubes with aninner diameter of 38 mm. Artificial cracks with lengths from 3 to 30 mm on the tubes were tested andtheir lengths were quantitatively estimated.
2. Experimental setup
Figure 1 shows the experimental setup of the inspection system. The sensor probe was scanned at aspeed of 30 mm/s inside the tube by the help of a motor and a pulling cable system. The cable wasattached to the front and end of the sensor probe so that the sensor probe is easily pulled and preventedfrom the rotating during scan. This helps to accurate evaluate the location of a crack on the circumfer-ential direction of the bleed air tube. The sensor probe includes a bobbin coil placed at a distance of10 mm from the BIHaS array. The bobbin coil has an outer diameter of 35 mm, a width of 5 mm, and
