Study On The Detection Technique Of Bolt Holes Defect With Planar Electromagnetic Sensor

Because of the influence of a variety of external stress and environmental corrosion during the long-term employment course,the aircraft’s bolt holes for fixing are prone to generate fatigue cracks,serious threatening to the flight safety of the aircraft.The traditional nondestructive testing methods for detection of wing bolt holes are mainly in crack detection of surface and near surface,and the deep defect detection sensitivity of bolt holes tends to be low.In response to these problems,this paper applys the principle of low-frequency eddy current testing,proposed to carry out the bolt hole defect detection method based on planar magnetic sensor,which the crack depth around the bolt hole defect detection can reach more than 5 mm.The main works of this thesis are as follows:(1)A novel type of planar electromagnetic sensors based on TMR element is designed.Based on the skin effect eddy effects on vortex penetration depth,a layer stagger differential exciting coil structure is designed to suppress the surface vortex density and improve eddy penetration depth,and the optimized design of combined planar electromagnetic sensor is achieved with the finite element simulation software COMSOL Multiphysics,which is composed of the exciting coil and TMR components and be beneficial to the increase of detection depth and sensitivity.(2)The weak signal detection and digital signal processing of the sensor are carried out.The weak output signals of TMR components are amplified with low noise preamplifier,the ARM microprocessor with 12 bit D/A and A/D is used for signal generation and acquisition,and the acquired signal are processed with lock-in amplifier algorithms,and 4-way sensor signal’s calibration and defect discrimination are achieved based on Borland C ++ Builder software development platform,thus the detection accuracy are improved effectively.(3)The experiment platform for bolt hole defect detection is developed.The experimental scheme is prepared and the crack defects in bolt hole are detected with the combined planar electromagnetic sensor.Experimental results show that the planar electromagnetic sensors have the detection capability for bolt hole with 5mm depth cracks.