Research On Detection Technology Of Aramid Paper Honeycomb Material Based On Air-coupled Ultrasonic

As a new type of composite material,aramid paper honeycomb is widely used in aviation and aerospace fields due to its excellent properties such as high strength,light weight,high temperature resistance,acid and alkali resistance,good heat and sound insulation,and long life cycle.However,aramid paper honeycomb materials are prone to delamination and debonding defects during production and service.If water vapor enters the honeycomb cells,freezing expansion will destroy the bonding of adjacent honeycomb cells under low temperature environment.This will reduce the performance of the honeycomb sandwich material,leaving potential safety hazards,and even causing sudden accidents.Therefore,it is of great significance to adopt effective methods for material defect detection.In view of the above problems,This paper analyzes and compares the commonly used defect detection techniques,and adopts the ultrasonic detection method finally.Aramid paper honeycomb is a kind of porous composite material.The fluid coupling agent used in conventional ultrasonic testing will soak into the paper honeycomb and pollute the material,which will seriously affect the mechanical strength and stability of the material.In addition,the non-metallic aramid paper honeycomb has low acoustic impedance.Based on this,this paper uses air-coupled ultrasound without coupling agent to detect defects in aramid paper honeycomb,and studies the key technologies.First,this paper introduces the characteristics of ultrasonic waves and the influence of medium acoustic impedance on the propagation path,and gives the theoretical basis for using ultrasonic waves to detect defects in materials.On this basis,the article analyzes the common methods of ultrasonic testing,combined with the display method of defect results,finally decides to use vertical penetration and C-scanning to complete air-coupled ultrasonic defect detection,and gives the working principle of the whole system.Then this paper selects the aircoupled ultrasonic probe based on the parameters of the aramid paper honeycomb material,and designs a low-noise small-signal conditioning circuit according to the center frequency of the probe.The circuit amplifies the μV-level electrical signal converted by the receiving probe at the receiving end to several volts,and then passes through a band-pass filter to reduce noise.Then,for waveform display and signal processing,this paper designs a signal acquisition system,which converts the analog ultrasonic signal into a digital signal and sends it to a PC through a serial port.Finally,aiming at the shortcomings of low signal-to-noise ratio and prone to top distortion of the collected digital signal,combined with the advantages of wavelet analysis "signal microscope" and the superiority of variational modal decomposition(VMD)in narrowband harmonic signal extraction,the method of wavelet analysis combined with VMD is used to process the ultrasonic signal.After algorithm processing,the ultrasonic signal is closer to the ideal real situation.In this paper,ultrasonic C-scanning is performed on circular defect areas with diameters of 15 mm,12mm,9mm,and 6mm on aramid paper honeycomb panels.By analyzing the imaging results,it can be known that the designed air-coupled ultrasonic imaging system can realize the defect detection of materials.A 200 kHz ultrasonic probe pair can detect circular debonding defects with a diameter of more than 9mm inside a 30 mm thick aramid paper honeycomb material.After using the algorithm to process the ultrasonic signal,the debonding position in the C-scan image is more obvious,and the detailed information in the map increases,and the outline of the honeycomb inside the material is also highlighted in the image.