Research On Novel Grating Acoustic Emission Sensor

In the fields of aviation,aerospace and other engineering fields,it is necessary to conduct real-time online monitoring and assessment of the health status of large equipment such as aircraft and engine blades,and predict the degree of structural damage and changes in structural properties,so as to avoid major accidents and improve the operation safety of equipment.Therefore,the structural health monitoring of aviation equipment is of great significance and has become an urgent need in the aviation field.Acoustic emission(AE)technology is a real-time dynamic online monitoring technology,and the health status of the equipment under test can be monitored through the analysis of the acoustic emission signal.The traditional acoustic emission sensor is made based on the principle of piezoelectric ceramics and belongs to the resonant sensor,which has the advantages of good stability,high conversion efficiency and stable waveform.However,its volume is slightly large,and it is difficult to adapt to some special detection requirements.A fiber Bragg grating(FBG)acoustic emission sensor is proposed to solve the problem of component damage detection in small spaces.The fiber grating itself has the characteristics of light weight,anti-electromagnetic interference,small size,corrosion resistance and good insulation,especially for a single sensor,the cost is very low.Therefore,the research of fiber grating acoustic emission sensors has gradually become a hot research direction.However,the sensitivity of the bare grating is low,which is about 1.22 pm/με,and the material is silicon dioxide,which is small and brittle,easy to break,and difficult to effectively detect the acoustic emission signal.According to the requirements for acoustic emission detection in engineering applications,the acoustic emission sensor needs to have the characteristics of high sensitivity,miniaturization,and easy installation and arrangement.Therefore,aiming at the sensitivity and mechanical strength of the grating sensor,this paper designs the sensitization structure of the new grating acoustic emission sensor,builds the grating acoustic emission detection system,and conducts the acoustic emission detection research of lead breaking and tensile test of the specimen.The test results show that the sensitivity and mechanical strength of the designed grating acoustic emission sensors have been greatly improved.The main contents are as follows:Aiming at the problems that grating acoustic emission sensors need fast demodulation and real-time detection in practical applications,this paper studies narrow-band laser demodulation,matched grating demodulation,spectral analysis,and tunable F-P filtering multiple demodulation methods,select narrow-band laser demodulation method to achieve accurate demodulation of acoustic emission signal.The output wavelength of the narrow-band laser is set at the 3 d B bandwidth point of the FBG,so that the grating acoustic emission sensor obtains high sensitivity and is in the linear region of the grating reflection sideband.The practical application of grating acoustic emission sensor provides the necessary conditions.Aiming at the problem of low sensitivity and easy breakage of bare gratings in structural health monitoring,a four-loop hollow-out sensitive grating acoustic emission sensor was designed,a narrow-band grating acoustic emission demodulation system was constructed,and the acoustic emission signal was detected.Based on the basic I-shaped package structure,it is proposed to add a loop-shaped hollow structure,so that the strain area is concentrated in the loop-shaped hollow part,which increases the strain sensitivity of the grating,provides protection for the bare grating,and increases the mechanical strength of the grating acoustic emission sensor.The static stress and dynamic response of the package structure were simulated and analyzed by using COMSOL software,and the structural parameters of the sensor were optimized,and the grating acoustic emission sensor was fabricated by the additive manufacturing technology.The acoustic emission signal detection experiment was carried out by using the grating acoustic emission sensor.The experimental study shows that the structure greatly improved the sensitivity and stability of the grating,and played an obvious sensitization effect.The response sensitivity of the acoustic emission signal was increased by 1.47 times.Aiming at the problem that the direct coupling method of fiber grating is insufficient(low acoustic-solid coupling efficiency),and it is easily affected by the fluctuation of the external environment,a grating acoustic emission sensor sensitization structure with axially coupled cone structure is proposed.The sensitization effect of the grating uses additive manufacturing technology to make the sensor housing to prevent the influence of external clutter on the grating.The COMSOL software was used to optimize the design of different materials and different cone angles of the axially coupled cone,and the sensor was used to conduct tensile experiments.The experimental results show that the sensitization structure has good response characteristics to the acoustic emission signal,the sensitization effect is obvious,the sound-solid coupling efficiency is improved,and the grating is effectively protected to reduce the influence of external environment changes on the measurement accuracy.Aiming at the requirement of multi-sensor system for the health monitoring of the operation state of large equipment and the location of structural damage,based on the advantage of fiber grating itself that it is easy to realize multi-point measurement,this paper proposes a lateral coupling cone sensitization structure that is convenient for multi-grating series multi-point measurement.The sensitization effect can be enhanced,and the multi-point measurement and structural damage location of the grating acoustic emission sensor array can be realized.In the structure,the grating and the coupling cone are connected laterally.During the packaging process,a prestress value is applied to the grating for quantification to ensure the consistency of the sensor parameters.The sensor is used to form a sensor array,and the acoustic emission source signal is obtained through the grating acoustic emission sensor array,and the narrowband acoustic emission signal is extracted by Shannon wavelet transform as the input signal.The MUSIC algorithm is used to estimate the direction of arrival,and then the acoustic emission source is calculated accurately.The sensitization effect of the sensitization structure of the grating acoustic emission sensor and the consistency of the sensing parameters among multiple sensors are verified by simulation analysis and localization experimental research,and the accurate localization of the AE source is realized.In summary,three new grating acoustic emission sensors are designed and developed in this paper.Through lead breaking and tensile test acoustic emission detection research,it is shown that the designed sensor structure can effectively improve the sensitivity of the grating sensor and realize the detection of acoustic emission signals;The research results of positioning detection of acoustic emission sources can provide important reference value for large-scale equipment health monitoring acoustic emission technology.