| The explosive detonation produces powerful shock wave spreading outward,and the damage effect is especially strong at close range.In the field of new weapons development,explosive equipment development,protective engineering design,safety accidents assessment and so on,higher requirement has been put forward for the close-up shock wave pressure measurement.The prominent features of the close-up shock wave are large pressure amplitude,short rising edge,short pulse time,bad electromagnetic interference,therefore,sensors should have the characteristics of large range,high frequency response and strong anti-interference ability.Piezoelectric and piezoresistive pressure sensors are widely used in the field of explosion and shock measurement to measure the incident and reflected pressure of shock wave.However,the crystal of piezoelectric sensors is easy to be damaged under a large range of pressure and more susceptible to electromagnetic radiation.The piezoresistive sensors are sensitive to light and heat radiation which results in a large interference in the signal,moreover,the measuring ranges of such sensors are low.Thus,neither of the two sensors can meet the requirements of measuring close-up shock wave.Hopkinson Pressure Bar(HPB)is a sensor using cylindrical bar as an elastic sensitive element to convert pressure to mechanical quantities such as stress or strain,and then the mechanical quantities are converted to electrical signals with electrical or optical techniques.The prominent features of HPB sensor are that it has a large measuring range and its sensitive elements are not easy to be damaged.Consequently,HPB sensor can be applied to the measurement of close-range reflected pressure of shock wave.Existing HPB systems mainly paste strain gauges on the surface of pressure bar as sensitive elements,however,strain gauges are very susceptible to interference,and also suffers from poor performance of long distance transmission.In addition,due to the lateral inertia effect,the axial elastic stress wave in the bar will diffuse during its propagation.The longer the propagation distance along the bar,the greater the waveform distortion will be,which needs to be corrected.In this thesis,according to the actual requirements,a technical route combing Hopkinson Pressure Bar and Photonic Doppler Velocimetry(PDV)was selected to focus on solving engineering problems of system vulnerability to interference and poor long-term transmission performance as well as scientific problems of axial elastic stress wave dispersion correction in the bar,and several key techniques were tackled.The main work is summarized as follows:(1)In view of the inherent defects of classical Photonic Doppler Velocimetry,a side-axis heterodyne structure and a dual-laser mode PDV are developed.The compensation method of displacement drift caused by frequency fluctuation between two lasers in dual-laser mode PDV is studied based on optical fiber reflector.The results of verification experiment show that this method can reduce the root mean square error of displacement baseline by 4 orders of magnitude.The single lens probe is studied in detail and optimized by theoretical model and non-sequential model simulation.A computer-aided setting and tuning system is developed to realize the rapid installation and testing of the probe.A space-resolved probe is designed based on optical fiber dense array and double-centroid lens.The test results show that the probe design is reasonable and there is no overlap between the optical paths,so it can effectively avoid crosstalk between channels.Finally,the performance indexes of various structure PDVs and two probes are analyzed.(2)The elementary theory and accurate theory for one-dimensional stress wave propagation in the Hopkinson Pressure Bar are studied.The basic mechanical principle of measuring reflected pressure of shock wave by pressure bar is described based on the elementary theory of one-dimensional stress wave,and the formulas of transformation coefficient from strain,velocity to pressure are given,which can be used to guide the design of the system in principle.Subsequently,the Pochhamer-Chree frequency equation is solved numerically,and the phase velocity curves,group velocity curves and the relationship between the normalized frequency and propagation time of the first four order axial stress wave propagation modes are obtained.Based on the radial distribution function of the mechanical parameters in the bar,the formula of the conversion factor between the axial velocity at the center of the bar's free surface and the average stress at the loaded surface of the bar is deduced,which provides a theoretical basis for the subsequent inverse analysis of the measured velocity to the average stress.(3)The technical scheme and design principles of shock wave reflection pressure measurement system based on PDV are put forward to meet the requirements of measuring shock wave pressure in close range under strong electromagnetic,vibration,shock and fire-light interference.Static tension,dynamic tension and dynamic compression experiments are carried out on three candidate materials,and the elastic modulus,Poisson's ratio and dynamic strength of the materials are obtained.Based on the theoretical analysis,SKH51 was selected to make the bar,because this material has a small Poisson's ratio,high dynamic strength and excellent comprehensive performance,which can be used to make a pressure bar with measurement range up to 3GPa.The side-axis heterodyne structure PDV is used to measure the free surface velocity of the pressure bar,instead of the commonly used strain gauge pasted on the surface of the preesure bar.In this method,the pressure bar,lens and optical fiber that are not sensitive to electromagnetic radiation are close to the explosion products,which solves the problem that the strain system is vulnerable to electromagnetic interference.The probe for laser launching and receiving and the PDV are connected with single-mode fiber,which has high bandwidth,low loss and excellent long-line transmission characteristics.The structure of the pressure bar sensor is continuously improved and optimized.The sliding installation structure solves the additional error of the probe under the action of stress wave in the pressure bar's sleeve.Finally,the response characteristics,effective measurement time and lower measurement limit of the system are analyzed,and compared with the system with strain gauge.(4)The analysis method of the axial elastic stress wave propagation mode in the pressure bar and the local phase-amplitude joint correction algorithm are proposed to convert the free surface velocity signal into the average pressure of the front face of the pressure bar.Based on the velocity history data of the free surface of the pressure bar processed by the Hanning Window Short-time Fourier Transform,the arrival time of each frequency components are analyzed and compared with the theoretical results of the P-C equation to identify the propagation modes of elastic wave in the bar.Based on the analysis of propagation mode,a local phase-amplitude joint correction algorithm is proposed.This method only corrects the time-frequency points that conform to the dispersion law of the axial stress wave,avoiding the non-dispersive frequency components.The traditional frequency domain correction algorithms are phase correction or phaseamplitude joint correction,which will correct non-dispersive frequency components in the signal,thus introducing additional errors.Finally,validation experiments of several sets of bars are carried out.Comparing and analyzing the time-frequency diagrams and the theoretical curves of the free-surface velocity signals of the pressure bar 01 and 04 show that: The first-order mode axial elastic stress wave is propagated in bar 01,while the first-third-order modes axial elastic stress waves are propagated in bar 04.The local phase-amplitude joint correction is applied to the two bars' signal,and the average stress curves of the loaded surface are obtained.The results are compared with those given by elementary theory and global phase-amplitude correction algorithm,which verifies that the method achieves better dispersion correction and distribution correction.To sum up,a close-up shock wave reflected pressure measurement system is developed based on HPB and PDV techniques.The pressure range is 3 GPa,the resolution is 0.195 MPa,and the measurement uncertainty is 1.95 MPa.The free-surface velocity signal can be converted to the mean stress of the loaded surface using the propagation mode analysis method of the Hanning Window short-time Fourier transform and the local phase-amplitude joint correction algorithm.The signal rise time is corrected from4?s to 2.70?s,which improves the system frequency response. |
PDF Full Text Request