| Adhesive bonding is an advanced bonding technology which can improve the structural efficiency and structural damage safety of spacecraft.In recent years,the application scope of adhesive bonding technology in the aerospace field has shown a continuous growth trend.With the continuous improvement of aviation manufacturing level,the separation surface of the process has been greatly reduced,and mechanical connections of many parts have been replaced by co-curing,co-bonding and secondary bonding.Rubber curing easily affected by environmental factors,however,the external force of the load is very important for cementing quality and the effect of fitting the connection form of diverse and complex surface,including surface appearance of the traditional curing detection methods are mainly composed of offline form,cannot achieve the real time monitoring application field,so you need to develop a reliable online monitoring technology,in order to improve the reliability of the bonding process.The main purpose of this subject is to study a method for monitoring the solidification of adhesive layer based on laser ultrasonic technology,and to establish a set of flexible and widely applicable laser ultrasonic detection system for solidification monitoring,and to use this system to complete relevant theoretical and experimental research.The main achievements of the thesis include the following:(1)Based on the principle of two-wave mixed interference,a set of laser ultrasonic detection system suitable for non-contact motion detection is built,which can detect the target conveniently and quickly by moving the probe.In the ultrasonic detection system,BSO crystal photorefractive effect is used to detect ultrasonic vibration,and the parameters of the interferometer are optimized.A high speed signal acquisition platform for real time acquisition of ultrasonic signal was built by using high speed signal acquisition card.Using the Lab VIEW program of ni-scope and in the form of queue data storage,the fast acquisition of ultrasonic signal is realized.(2)The coupling efficiency of single-mode and multi-mode optical fibers is analyzed respectively according to the fiber structure in the two-wave mixing interferometer.Firstly,the structure of optical fiber is introduced,and the principle of total reflection condition and light propagation in optical fiber is explained.Secondly,the coupling efficiency of single-mode fiber is analyzed by using electromagnetic field theory,and the coupling efficiency of multi-mode fiber is considered by geometric optics analysis.The coupling efficiency of single-mode fiber and multi-mode fiber under axial migration,radial migration and Angle tilt of the end face is considered respectively.The influence of mode dispersion introduced by multimode fiber on signal detection is calculated.(3)Curing of epoxy resin will affect the characteristic parameters of ultrasonic,including sound speed,amplitude and acoustic impedance.According to the wave propagation path in the composite,the relation between amplitude and reflection coefficient in the lower aluminum plate is established by using far-field ultrasound.At the same time,the transmission wave is analyzed in time and frequency domain by wavelet transform,the signal is decomposed by complex Morlet wavelet,the amplitude and phase information of the signal are extracted,and the attenuation coefficient and phase velocity are calculated.The absorption attenuation of ultrasonic wave is related to the relaxation property of epoxy resin,the approximate linear relation between absorption attenuation and frequency is obtained by theoretical analysis,and the phase velocity is calculated to evaluate the dispersion of ultrasonic wave in epoxy resin.(4)The relationship between the ultrasonic attenuation coefficient and the phase velocity was derived by using the kramers-kronig relation,and the applicability of the Kramers-Kronig relation in the curing process of epoxy resin was verified based on the attenuation coefficient,and the dispersion curve with time was calculated during the curing process.Ultrasonic time-domain method,rheometer and differential scanning calorimetry were used to analyze the curing process of epoxy resin.The relaxation characteristics of epoxy resins have a direct effect on the dispersion.In this paper,the dispersion curve of epoxy resins during curing is used to establish a dispersion based curing degree calculation model,and the model is compared with that based on DSC method.The results show that,from the perspective of industrial production,DSC method is less sensitive than ultrasonic detection,and laser ultrasound is an online detection method for solidification degree of industrial production due to its non-contact and high sensitivity. |
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