Research On PRFECT Method For Defect Of Aircraft Multi-layer Metal

As an important bearing component of the aircraft,the multi-layer metal fastening structure of the aircraft is subjected to continuous ground-air-ground cyclic loading to cause stress concentration around the hole of fasteners such as rivets and high-lock bolts to initiate fatigue cracks.The traditional non-destructive testing method is difficult to detect under in-service conditions,and the remote field eddy current testing technology breaks through the skin effect limitation in principle,which has great advantages for deep hidden defect detection.In this paper,the structure parameters of the probe are optimized by using a multi-layer metal hidden defect detection simulation model,and experiments are performed to optimize the structure of the plane far-field eddy current probe.Experiment to optimize detection parameters.Through the finite element simulation,a plane far-field eddy current detection model of multi-layer metal screwed components of an aircraft was established,and the influence of the size of the excitation coil on the distribution of the magnetic field on the metal component and the relationship between the normalized sensitivity and the inner diameter of the excitation coil were studied.When the inner diameter of the excitation coil is12.4mm,it has the highest normalized sensitivity at a depth of 4mm.When the magnetic field shielding structure is composed of oriented silicon steel+T2 copper+oriented silicon steel,its magnetic field shielding effectiveness reaches 21.96 dB,which is an increase of 191.24%.The presence of the air layer inside the probe will reduce the shielding effectiveness of the magnetic field.The different distribution of the air layer position can cause the magnetic shielding effectiveness to decrease by 33.56%.When the magnetic field enhancement structure is cylindrical,the amplitude of the detection signal is caused by defects.The perturbation amount reached the highest value of 4.14×10^-4V,thereby improving the detection sensitivity of the probe.Combined with the results of finite element simulation,a plane far-field eddy current probe suitable for the detection of multi-layer metal screwed components of aircraft is developed.The magnetic induction intensity at the center of the excitation coil is inversely proportional to its diameter,and its magnetic field permeability is proportional to the diameter of the excitation coil;A single shielding layer,or a double-layer shielding layer,and a three-layer shielding structure can more effectively prevent the primary magnetic field from propagating to the center of the probe.After experimental research,using cylindrical silicon steel as the detection part of the magnetic field enhancement module has the best detection effect;compared with The detection coil with a height of4mm and a height of 2mm is more conducive to the extraction of the secondary magnetic field with defect information.At the same time,the eddy current detection system and the software of the host computer are set up using modular components and Labview graphical programming software.An aircraft multi-layer metal screw joint with artificial defects was designed and tested,and the developed plane far-field eddy current probe was used to test it.When the probe is placed coaxially with the high lock bolt,the eddy current field caused by the defect is more significant.The effects of the excitation frequency and the buried depth of the defect on the far-field eddy current detection signal are studied.The test results show that the detection effect of hidden crack defects is best when the excitation frequency is0.3 k Hz.As the depth of the defect increases,the signal amplitude decreases.;At the same time,the effects of defect length and probe lift-off height on far-field eddy current detection signals were studied.As the defect length decreased or the paint layer thickness increased,the detection signals showed a decreasing trend,and the aircraft multi-layer metal screw was realized.High-precision characterization of hidden defects in joint members.