Laser-induced plasma shock wave transient pressure measurement system and method

Abstract

The invention discloses a laser-induced plasma shock wave transient pressure measuring system which comprises a cushion, a PVDF sensor is attached to the upper surface of the cushion, a PDV probe is attached to the lower surface of the cushion, the PVDF sensor and the PDV probe are jointly connected with an oscilloscope, the PVDF sensor is further connected with a resistor R in parallel, and the resistor R is connected with the oscilloscope in parallel. The invention further discloses a shock wave transient pressure measuring method. According to the laser-induced plasma shock wave transient pressure measurement system and method, PDV and PVDF are cooperatively used, the particle speed is accurately measured in a non-contact mode, and the shock wave pressure is obtained.

Description

Technical Field

[0001] The present invention belongs to the technical field of laser shock detection devices, and particularly relates to a transient pressure measurement system for laser-induced plasma shock waves, and also relates to a method for measuring the transient pressure of shock waves. Background Art

[0002] Traditional transient pressure measurement methods, such as piezoelectric sensors, shadow imaging, laser interferometry, etc., have problems such as limited measurement range, limited spatial distribution rate, susceptibility to environmental influences, and high cost. In the paper "Physical study of laserproduced plasma in confined geometry" published on March 15, 1990, Fabbro et al. proposed using synthetic X-cut quartz crystals as piezoelectric sensors to measure the transient pressure generated by laser-induced plasma, that is, placing the gold-plated electrodes of the quartz crystal between the glass cover and the aluminum foil, using the piezoelectric effect of quartz to convert the pressure signal into current output, and then calculating the pressure value through a formula. However, due to the limited linear response range of quartz and the possibility of not being able to fully capture the rapid changes in the pressure waveform under ultrashort pulses, this method limits its applicability and accuracy under high pressure or extreme conditions. Lai Guihua et al. proposed in their paper Early Dynamics of A Laser-Induced Underwater Shock Wave published in January 2022 that the transient pressure of laser-induced underwater shock waves can be measured by combining a high-time-resolution shadow imaging system with a numerical simulation method, and the evolution of the shock wave radius over time can be captured by using double-exposure shadow images. The shock wave velocity is calculated by measuring the displacement and time interval, and the velocity is converted into pressure based on the Rice-Walsh empirical formula. However, due to the limited time and space resolution and the reliance on model assumptions, large errors may occur. In their paper Experimental Study on the Dynamic Response of Bulk Metallic Glass under High-Speed Impact published in June 2014, Li Tao et al. proposed to measure the particle velocity wave profile at the sample / window interface based on a high-precision DPS laser interferometer, and to indirectly calculate the transient pressure by combining the impact Hugoniot relationship. However, the laser interferometer is highly sensitive to the experimental environment and is more dependent on the accuracy of the material Hugoniot parameters. If there is a deviation in the parameters, the pressure calculation results may be inaccurate. The applicant Chengdu Taice Technology Co., Ltd. proposed a dynamic pressure measurement method based on fiber interferometer in patent CN108844621B. By embedding a micro-fiber pressure sensor into the shock wave propagation path, the laser Doppler effect is used to capture the nano-scale displacement of the sensor diaphragm caused by the plasma shock wave in real time, and then the transient pressure waveform is obtained by interferometric signal demodulation. However, the micro-processing precision of the sensor is extremely high, resulting in high manufacturing costs, and the diaphragm is prone to plastic deformation under extreme transient pressure, thus affecting the measurement repeatability.

[0003] With the development of technology, due to its advantages such as high spatio-temporal resolution and non-contact measurement, PVDF piezoelectric sensors have gradually become an important means for transient pressure measurement. The traditional method for transient pressure measurement is to use PVDF sensors. In the current mode, the output of PVDF sensors is a charge or current signal, which needs to be converted into a voltage signal through a resistor. At this time, the PVDF sensor will affect the resistor, resulting in signal distortion or measurement deviation. The applicant, Central South University, proposed in Patent CN114878372B that in the current mode, the PVDF piezoelectric sensor will affect the resistor, and the PVDF piezoelectric sensor is also vulnerable to interference from external environmental factors; when using the PDV system to measure the back particle velocity of materials and inversely calculate the front pressure through formulas, errors will be introduced, resulting in inaccurate pressure calculation results. Summary of the Invention

[0004] The first object of the present invention is to provide a transient pressure measurement system for laser-induced plasma shock waves, which accurately measures the particle velocity and obtains the shock wave pressure in a non-contact manner by using PDV and PVDF in cooperation.

[0005] The second object of the present invention is to provide a method for measuring the transient pressure of shock waves.

[0006] The first technical solution adopted by the present invention is a transient pressure measurement system for laser-induced plasma shock waves, including a cushion. One end face of the cushion is attached with a PVDF sensor, and a PDV probe is arranged near the other end face of the cushion. The PVDF sensor and the PDV probe are jointly connected to an oscilloscope. The PVDF sensor is also connected in parallel with a resistor R, and the resistor R is connected in parallel with the oscilloscope.

[0007] The characteristics of the first technical solution of the present invention also lie in that The cushion is made of any one of an acrylic plate with a thickness of 3 mm, an aluminum plate, or 304 stainless steel.

[0008] A silicone oil layer is coated on the end face of the cushion where the PVDF sensor is attached.

[0009] The second technical solution adopted by the present invention is a method for measuring the transient pressure of shock waves. Using the transient pressure measurement system for laser-induced plasma shock waves, the specific steps are as follows: Step 1, adjust the PVDF sensor to the current mode; Step 2, apply a shock wave pressure to the PVDF sensor using a variable pulse width nanosecond pulsed laser. At this time, the PVDF sensor will release a charge quantity Q ( t ), and the charge quantity Q ( t ) passes through the resistor R to form a current loop for dischargingI ( t ); Step 3: Collect the voltage signal released by resistor R throughout the process using an oscilloscope U ( t ), and calculate the total charge released by the PVDF sensor Q ( t ); Step 4: Calculate the shock wave pressure based on the back surface particle velocity signal and the pad parameters obtained by the PDV probe; Step 5: Generate a pressure-time graph of the calculation results for comparison to complete the detection.

[0010] The second technical solution of the present invention is further characterized in that The total charge Q ( t ) in Step 3 is specifically expressed as: (1) Wherein, t is time, U(t) is voltage, R is the resistance value; Then the transient stress measured by the PVDF sensor is: (2) In the formula, K represents the dynamic calibration coefficient of the PVDF sensor, which is , A represents the working area of the sensor The shock wave pressure in Step 4 is specifically expressed as: (3) In the formula, P represents the pressure of the shock wave, ρ is the density of the pad, v is the back surface particle velocity, c is the sound speed in the pad.

[0011] The specific parameters of the variable pulse width nanosecond pulse laser body are: wavelength 1053 mm, laser pulse width range 10 - 300 ns, laser energy range 1 - 50 J, and spot diameter range 2 - 15 mm.

[0012] The calculation results in Step 5 are the transient stress measured by the PVDF sensor and the shock wave pressure.

[0013] Compared with the prior art, the beneficial effects of the present invention are: (1) The laser-induced plasma shock wave transient pressure measurement system and method provided by the present invention accurately measure the particle velocity in a non-contact manner and indirectly deduce the pressure. Its high time resolution and spatial resolution can capture the dynamic characteristics of the shock wave; the PVDF sensor directly outputs the pressure signal using the piezoelectric effect and has a wide frequency response range and fast response ability. By using the PDV probe and the PVDF sensor in cooperation, high precision and high reliability of transient pressure measurement are achieved.

[0014] (2) The laser-induced plasma shock wave transient pressure measurement system and method provided by the present invention complement each other with the non-contact characteristic of the PDV probe and the direct measurement advantage of the PVDF sensor, not only expanding the applicability of the system in complex environments and enhancing the ability of the system to work stably in different scenarios, but also providing an efficient solution for measuring transient pressure under laser-induced conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 is a schematic flow chart of the shock wave transient pressure measurement method of the present invention; Figure 2 is a schematic structural diagram of the laser-induced plasma shock wave transient pressure measurement system in Embodiment 1 of the present invention; Figure 3 is a schematic diagram of shock wave pressure-time after fitting of PVDF and PDV of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0016] The present invention will be described in detail below in conjunction with the drawings and specific embodiments. The described embodiments are only part of the embodiments of the present invention, not all of the embodiments.

[0017] Embodiment 1 The present invention provides a laser-induced plasma shock wave transient pressure measurement system, as Figure 2 shown, including a pad. One end face of the pad is attached with a PVDF sensor. A PDV probe is placed 2-3 mm away from the other end face of the pad. The PVDF sensor and the PDV probe are jointly connected to an oscilloscope. The PVDF sensor is also connected in parallel with a resistor R, and the resistor R is connected in parallel with the oscilloscope.

[0018] Among them, the pad is made of any one of an acrylic board with a thickness of 3 mm, an aluminum plate, or 304 stainless steel. The upper surface of the pad is coated with a silicone oil layer.

[0019] More specifically, an acrylic plate with a thickness of 3 mm is selected as the cushion, deionized water is used as the constraint layer, and black tape is used as the absorption layer. The radius of the black tape is 2.5 mm. Silicone oil is applied to the surface of the acrylic plate and a PVDF sensor (piezoelectric film sensor) is placed. In this embodiment, the current mode of the PVDF sensor is selected. The black tape is pasted on the surfaces of the acrylic plate and the PVDF sensor. The PDV probe (photon Doppler velocimeter) is placed at the rear end of the cushion, and is synchronously connected to the PVDF sensor and the oscilloscope, and as Figure 2 shown, the PVDF sensor is connected in parallel with a 5W, 50Ω resistor and an oscilloscope.

[0020] Furthermore, through comparative tests on 3-mm-thick acrylic plates, aluminum plates, and 304 stainless steels, it is found that acrylic plates have more advantages in reducing reflection and improving measurement accuracy. Secondly, the selection of the resistor should be adapted to the current mode of the PVDF sensor. Selecting an appropriate resistor with a certain resistance and power can make the measured shock wave pressure time distribution characteristics consistent with the actual shock wave characteristics. The resistance value is determined through comparative tests on resistors with different resistance values and powers. Through the comparison results, under the condition of a 5W, 50Ω resistor, the waveform consistency of the PDV and PVDF sensors is better. Therefore, in this embodiment, a 5W, 50Ω resistor R is selected.

[0021] Embodiment 2 This embodiment provides a method for measuring the transient pressure of a shock wave. As Figure 1 shown, the laser-induced plasma shock wave transient pressure measurement system provided in Embodiment 1 is adopted, and the specific steps are as follows: Step 1, adjust the PVDF sensor to the current mode; because under the action of an external force, the charge generated by the PVDF sensor can be collected through two circuit modes, namely the charge mode and the current mode. In the charge mode, the charge released by the sensor is directly read through an external capacitor Q(t) , but this mode reflects the average stress and has a relatively low time resolution. In contrast, the current mode can capture transient signals more accurately, provide a higher time resolution, and is more suitable for the laser-induced plasma shock wave method because the generation and propagation time of the shock wave is in the nanosecond range.

[0022] Step 2, use a variable pulse width nanosecond pulse laser to apply shock wave pressure to the PVDF sensor. At this time, the PVDF sensor will release a charge Q ( t ), and the charge Q ( t ) is discharged through the resistor R to form a current loop I ( t ); Among them, the parameters of the variable pulse width nanosecond pulse laser are as follows: the laser pulse width is selected as 30 ns, the laser energy is selected as 2.248 J, and the spot radius is 0.25 mm.

[0023] Step 3: Collect the voltage signal released by the resistor R throughout the process through an oscilloscope U ( t ), and calculate the total charge released by the PVDF sensor Q ( t ); Step 4: Calculate the shock wave pressure based on the back surface particle velocity signal and the pad parameters obtained by the PDV probe; Step 5: Compare the transient stress with the shock wave pressure to generate a pressure-time graph to complete the detection.

[0024] Example 3 Based on Example 2, the total charge in Step 3 Q ( t ) is specifically expressed as: (1) Among them, t is time, U ( t ) is voltage, R is the resistance value; Then the transient stress measured by the PVDF sensor is: (2) In the formula, K represents the dynamic calibration coefficient of the PVDF sensor, which is , and A represents the working area of the sensor. Usually, it can be approximately equivalent to the area of the laser spot. From this formula, the relationship between the shock wave pressure and time can be obtained.

[0025] Example 4 Based on Example 3, Step 4 is specifically as follows: The acoustic impedance parameter of PVDF needed is 0.25, and the acoustic impedance parameter of the acrylic plate is 0.32; and the material parameters of the acrylic plate itself are , .

[0026] Among them, the shock wave pressure is specifically expressed as: (3) In the formula, P represents the pressure of the shock wave, ρ is the density of the pad, v is the back surface particle velocity, c is the sound speed in the pad.

[0027] Example 5 Based on Example 4, Step 5 is specifically as follows: Compare the pressure data processed by the PDV probe and the PVDF sensor, and plot them on one graph using ORIGIN software. From this, the relationship between the pressure and time of PVDF and PDV can be obtained, and it can be intuitively seen that the shock wave waveforms of PVDF and PDV show high consistency in terms of the peak position, shape, and attenuation characteristics. Also, by adjusting the laser parameters and repeating Step 4 and Step 5, multiple sets of verification data can be obtained. Example 6 Using the shock wave transient pressure measurement method provided in the above example, as Figure 3 shown, it is the pressure-time curve measured by the PVDF sensor and the PDV probe under the condition of the same energy of 2.248 J.

[0028] Among them, the horizontal axis: time (unit: nanosecond, ns), the range is 0 to 50 ns, reflecting the time process of the measurement. The vertical axis: pressure (unit: gigapascal, GPa), the range is 0 to 200 GPa, reflecting the intensity of the pressure. The solid line (30ns - 2.248J - PVDF): represents the pressure change directly measured by the PVDF sensor. Its characteristic is that it rapidly rises to the peak (reaching the highest pressure at about 10 ns), then rapidly drops, and there are fluctuations in the later stage, showing the dynamic attenuation process of the pressure. The dashed line (30ns - 2.248J - PDV) represents the pressure change obtained through the PDV probe (photon Doppler velocimeter, indirectly deriving the pressure). The curve trend is similar to that of PVDF, and the consistency is good. The system and method for measuring the transient pressure of a laser-induced plasma shock wave provided by the present invention combines the use of PDV and PVDF, which not only expands the applicability of the system in complex environments and improves the ability of the system to work stably in different scenarios, but also provides an efficient innovative solution for measuring transient pressure under laser-induced conditions.

Claims

1. A transient pressure measurement system for laser-induced plasma shock waves, characterized in that It includes a cushion. One end face of the cushion is attached with a PVDF sensor, and a PDV probe is arranged near the other end face of the cushion. The PVDF sensor and the PDV probe are jointly connected to an oscilloscope. The PVDF sensor is also connected in parallel with a resistor R, and the resistor R is connected in parallel with the oscilloscope.

2. The laser-induced plasma shock wave transient pressure measurement system according to claim 1, characterized in that The cushion is made of any one of acrylic plates, aluminum plates or 304 stainless steels with a thickness of 3 mm.

3. The laser-induced plasma shock wave transient pressure measurement system according to claim 1, wherein A silicone oil layer is coated on the end face of the cushion where the PVDF sensor is attached.

4. The method for measuring the transient pressure of a shock wave, characterized in that, When using the laser-induced plasma shock wave transient pressure measurement system according to any one of claims 1-2, the specific steps are as follows: Step 1, adjust the PVDF sensor to the current mode; Step 2: Apply shock wave pressure to the PVDF sensor using a variable pulse width nanosecond pulsed laser. At this time, the PVDF sensor will release a charge quantity. Q ( t ) The charge quantity generated by it Q ( t ) passes through the resistor R to form a current loop through discharging I ( t ); Step 3, collect the voltage signal released by the resistor throughout the process using an oscilloscope R and U ( t ) calculate the total charge released by the PVDF sensor Q ( t ); Step 4, calculate the shock wave pressure based on the back particle velocity signal obtained by the PDV probe and the cushion parameters; Step 5, generate a pressure-time graph from the calculation results for comparison to complete the detection.

5. The shock wave transient pressure measurement method according to claim 4, characterized in that, The total charge amount described in Step 3 Q ( t ) is specifically expressed as: （1） wherein, t is time, U(t) is voltage, R is the resistance value; Then the transient stress measured by the PVDF sensor is: （2） In the formula, K represents the dynamic calibration coefficient of the PVDF sensor, and is , A represents the working area of the sensor.

6. The shock wave transient pressure measurement method according to claim 4, characterized in that, The shock wave pressure described in Step 4 is specifically expressed as: （3） In the formula, P represents the pressure of the shock wave, ρ is the density of the cushion, v is the particle velocity on the back surface, c is the sound velocity in the cushion.

7. The method for measuring the transient pressure of a shock wave according to claim 4, characterized in that The specific parameters of the variable pulse width nanosecond pulse laser body are: wavelength 1052 mm, laser pulse width range 10 - 300 ns, laser energy range 1 - 50 J, and spot diameter range 2 - 15 mm.

8. The shock wave transient pressure measurement method according to claim 4, wherein The calculation results in Step 5 are the transient stress and the shock wave pressure.

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