SHPB experimental device for honeycomb porous structure and data processing method

Abstract

The invention discloses an SHPB (split Hopkinson pressure bar) experimental device for a honeycomb porous structure and a data processing method, belongs to the technical field of dynamic mechanical property testing of material structures, and aims to solve the problem of dynamic parameter errors caused by bridge unbalance, signal noise and waveform interception deviation in an SHPB experiment in a traditional method. The bridge is ensured to be in a balanced state through calibration before an experiment, a time-voltage curve obtained through data acquisition is subjected to waveform return-to-zero adjustment, local weighting polynomial regression noise suppression, accurate recognition and accurate extraction of wave head and wave tail features of three waves (incident waves, reflected waves and transmitted waves), and the accuracy and accuracy of the wave head and wave tail features are improved. And analyzing the dynamic parameters based on a two-wave method to calculate an engineering stress-strain curve, a real stress-strain curve, a real stress-time curve, a real strain-time curve and a strain rate-time curve so as to realize full-process error control. Multiple experiments prove that the method has the remarkable advantages of high precision and high reliability.

Description

Technical Field

[0001] This invention belongs to the field of dynamic mechanical property testing technology for material structures, specifically relating to an SHPB experimental device and data processing method for honeycomb porous structures. Background Technology

[0002] Strain rate is a core physical quantity describing the rate of deformation of a material under external force. It is the linear or shear strain occurring per unit time and directly affects the mechanical behavior and failure mode of the material. When the strain rate exceeds 10... -2 s -1 When materials are subjected to extreme dynamic mechanical loads, the deformation process will exhibit significant dynamic mechanical response characteristics. With the rapid development of high-tech methods such as explosive welding, laser shot peening, and high-speed manufacturing, there is an urgent need to understand and master the relevant laws and mechanisms of metallic materials under extreme dynamic loads with high or even ultra-high strain rates.

[0003] Due to their high specific stiffness and strength, honeycomb porous structures are increasingly used in aerospace, transportation, and engineering protection fields. Precise understanding of their mechanical properties under dynamic loads is essential for their application. The mechanical behavior and failure modes of materials differ from those under static and quasi-static loading conditions. At high strain rates, the dynamic yield strength of most metallic materials is several times that of their quasi-static yield strength. Due to the adiabatic temperature rise and dislocation motion resistance under dynamic loading, work hardening of most metallic materials decreases with increasing strain rate, but the yield strength increases and exhibits positive strain rate sensitivity. Polymer materials (such as PC plastics) can exhibit a brittle-ductile transition under high-speed impact. Therefore, the study of dynamic mechanical properties of materials is a crucial link between basic science and engineering practice, especially significant in the design of equipment for extreme service environments and the development of new materials.

[0004] In the field of dynamic mechanical property testing of materials, the split Hopkinson bar (SHPB) device, with its unique physical mechanism and engineering adaptability, has become a preferred method for testing high strain rates (10⁻¹⁰). 2 ~10 4 s -1 The SHPB (Strain Rate Purpose Test) is the core equipment for testing. Compared to other power devices such as drop hammer impact, light gas gun loading, and explosion-driven methods, the SHPB apparatus has significant advantages in strain rate control and testing range, data acquisition and signal fidelity, sample applicability and cost-effectiveness, theoretical completeness and standardization, and technological expansion and intelligent upgrades. Commonly used SHPB apparatuses include... Figure 1As shown, it is mainly composed of driving device, impact rod, speed measuring device, input rod, strain gauge, sample, output rod, absorption rod, damper, data acquisition and storage device, dynamic strain gauge and other devices, the strain gauges attached to the input rod and the output rod can obtain the waveform signals of the incident pulse, reflected pulse and transmitted pulse, and through the three pulse waveform signals, the time-voltage curve of the incident rod, the time-voltage curve of the transmitted rod, the reflected wave waveform curve, the transmitted wave waveform curve, the engineering stress-strain curve, the true stress-strain curve, the true stress-time curve, the true strain-time curve and the strain rate-time curve can be obtained.

[0005] At present, in the process of converting the time-voltage curve recorded by the data acquisition and storage device into the stress-strain curve and the like, the following problems mainly exist:

[0006] (1) Compared with the traditional homogeneous continuous medium small cylindrical sample and the cap-shaped sample, the honeycomb porous structure under the loading of dynamic load shows the characteristics of multiple reflections, high-frequency noise dominance and nonlinear amplitude attenuation;

[0007] (2) Overall error of test data caused by initial unbalance of bridge circuit: The split Hopkinson bar relies on the change of strain gauge resistance to change the balanced bridge circuit and then output the voltage difference, but if the initial bridge circuit of the device is unbalanced, the overall error of the test data will occur;

[0008] (3) Inaccuracy of the method for selecting the position of the wave head and tail of the three waves: In the voltage-time curve, the selection of the wave head and tail of the incident wave, reflected wave and transmitted wave has a great influence on the calculated stress-strain curve. The current technology mainly uses 1 / 3h, 2 / 3h (h is the amplitude of the wave) to fit the curve and calculate the intercept to determine the wave shape, or uses the point-by-point comparison method and the principle of constant elastic wave velocity to select the first reflected wave and transmitted wave shape, but the difference between the two methods for determining the position of the wave head and tail of the honeycomb porous structure and the true position is large, especially when facing complex waveforms, which will seriously interfere with the calculated stress-strain curve;

[0009] (4) Abnormal data points exist in the time-voltage curve and the overall volatility is large: The time-voltage curve is composed of data points at different times, and in the SHPB experiment test, the effective data points usually far exceed 100, and each curve is obtained by calculation. However, the honeycomb porous structure time-voltage curve recorded by the data acquisition and storage device and the curve in the processing process are prone to abnormal data points, and combined with noise interference and other factors, the overall curve has large volatility. SUMMARY

[0010] This invention provides an SHPB experimental apparatus and data processing method for honeycomb porous structures, which can more realistically and accurately reflect their dynamic mechanical properties.

[0011] To achieve the above objectives, the present invention adopts the following technical solution:

[0012] A SHPB experimental apparatus for honeycomb porous structures includes: SHPB, dynamic strain gauge, and data acquisition and storage device. The SHPB includes a driving device, an impact rod, a velocity measuring device, an input rod, a strain gauge, an output rod, and an absorption rod. The strain gauge is attached to the middle position of the input rod and the output rod and connected to the dynamic strain gauge. The dynamic strain gauge is connected to the data acquisition and storage device.

[0013] The driving device applies an initial velocity to the impact rod, ensuring that the input and output rods are in an elastic state during the loading process, and that the velocity of the impact rod does not exceed the calculation result of equation (1):

[0014]

[0015] In the formula: R e ρ b and c b These are the yield strength, density, and longitudinal wave velocity of the input rod and output rod, respectively.

[0016] The impact rod, input rod, and output rod are made of high-strength materials with a diameter of 50mm. The length-to-diameter ratio of the input rod and output rod is not less than 40. They can be designed according to equations (2) and (3) respectively:

[0017] L I ≥40d b (2)

[0018] L T ≥40d b (3)

[0019] In the formula: L I For the length of the input rod, L T The length of the output rod, d b The diameters of the input and output rods.

[0020] The impact rod is made of the same material and has the same diameter as the input rod and the output rod. The length of the impact rod is greater than 10 times its diameter and less than 0.5 times the length of the input rod.

[0021] The above-mentioned device is used for SHPB experimental data processing of honeycomb porous structures, including the following steps:

[0022] (1) Pre-experiment calibration: Before the experiment begins, the bridge circuit is precisely calibrated to ensure that the bridge is in a balanced state;

[0023] (2) Zero adjustment: The zero adjustment of the waveform of the time-voltage curve obtained by the data acquisition device is performed, that is, the initial voltage value is kept as zero;

[0024] (3) Curve smoothing: The abnormal data points and noise appearing in the time-voltage curve recorded by the data acquisition and storage device are removed, and the curve is smoothed;

[0025] (4) Waveform selection: The head and tail positions of the incident wave, reflected wave and transmitted wave are accurately selected, and the incident wave, reflected wave and transmitted wave data intercepted are curve fitted;

[0026] (5) Obtaining of each curve: The time-voltage curve of the incident rod, the time-voltage curve of the transmitted rod, the reflected wave and transmitted wave waveform curve, and the engineering stress-strain curve are obtained through the intercepted incident wave, reflected wave and transmitted wave, the obtained engineering stress-strain curve is converted into the real stress-time curve through the conversion relationship between the engineering stress-strain and the real stress-strain, and then the real strain-time curve and the strain rate-time curve are obtained;

[0027] (6) Saving data: The waveforms, data and fitted curves obtained through the experiment and calculation are saved.

[0028] In the above steps, the specific process of step (1) is: connecting the bridge circuit to the data acquisition system, starting the speed measuring device, preheating the strain signal acquisition and storage device to the equipment stability, setting the parameters of the data acquisition system to ensure the integrity of the recorded signal; under the conditions of no load or known load, the output voltage of the bridge is recorded, the variable resistor in the bridge is adjusted to make the output voltage reach the minimum value or the theoretical equilibrium value, and the adjustment process is repeated until the output of the bridge is stable in the equilibrium state;

[0029] The specific process of step (2) is: the zero adjustment of the waveform of the time-voltage curve obtained by the data acquisition device is performed, that is, the initial voltage value is measured to determine whether it is zero and adjusted by hardware or software method to ensure that the initial voltage is zero, and the zero adjustment effect is verified and the adjustment process and parameters are recorded, and finally the accurate and reliable time-voltage curve is obtained;

[0030] The specific process of step (3) is: the abnormal data points and noise appearing in the time-voltage curve recorded by the data acquisition and storage device are removed, and the time-voltage curve data is smoothed by the method of polynomial regression of local data to obtain the smoothed time-voltage curve;

[0031] The specific process of accurately determining the head and tail positions of the incident wave, reflected wave and transmitted wave in step (4) is as follows:

[0032] (I) Waveform baseline determination: symmetrically paste strain gauges, respectively, take the average value of symmetric strain gauge test signal as the incident wave, reflected wave and transmitted wave in data processing, take the average value of the flat section data before the start of the incident wave as the waveform baseline value of the incident wave and reflected wave, take the average value of the flat section data before the start of the transmitted wave as the waveform baseline value of the transmitted wave, make the incident wave, reflected wave and transmitted wave waveform baseline zero in data processing;

[0033] (II) Waveform start point determination: according to the waveform of incident wave, reflected wave and transmitted wave, the corresponding waveform start point is determined, the determined start point should be on the baseline of the corresponding waveform and close to the take-off point of the waveform, the maximum value of the incident wave is determined according to the waveform of the incident wave, the time when the incident wave rises to one tenth of its maximum value and the corresponding discrete data point are determined, a straight line is fitted by taking one tenth of the maximum value of the incident wave corresponding time and a certain amount of data points (the number is determined according to the sampling frequency) backward, the intersection point of the straight line and the baseline is taken as the start point n1 of the incident wave, after determining the start point of the incident wave, the start points of the reflected wave and the transmitted wave are determined by formula (4) and formula (5):

[0034]

[0035] In the formula: int[] is the integer function; n1, n2 and n3 are the data point serial numbers corresponding to the start points of incident wave, reflected wave and transmitted wave respectively; a1 and a2 are the distances from the center of strain gauge in input rod and output rod to the sample end; b c s is the longitudinal wave velocity in input rod and output rod; c i is the elastic longitudinal wave velocity in the sample; t I is the sampling time interval.

[0036] (III) Aligning the waveform start point: move the incident wave, reflected wave and transmitted wave so that their start points are at the same time, and take this time as the starting time in data processing, the incident wave, reflected wave and transmitted wave after aligning the start point should satisfy the homogenization assumption, that is, formula (6):

[0037] e I (t)+e R (t)=e T (t) (6)

[0038] In the formula: e I , e R and e T respectively represent the elastic strain generated by the incident wave, reflected wave and transmitted wave.

[0039] (IV) Determination of stress wave action time: select the last maximum value point before the waveform enters the falling edge as the end point of the incident wave to determine the action time of the stress wave.

[0040] The specific method for obtaining each curve in step (5) by intercepting incident waves, reflected waves and transmitted waves is as follows:

[0041] According to the calculation formula of the Hopkinson bar reflected strain and the transmitted strain, the time-voltage curve is converted into the reflected strain-time curve and the transmitted strain-time curve, and the specific strain theoretical calculation formula (7) is as follows:

[0042]

[0043] In the formula: U i (i = 1, 2, 3...) represents voltage, K1 is the sensitivity coefficient of the strain gauge, K2 is the amplification coefficient of the dynamic strain meter, V 0 is the power supply voltage of the bridge, and finally the time-voltage curve is converted into the reflected strain-time curve and the transmitted strain-time curve.

[0044] The engineering stress is calculated according to formula (8):

[0045]

[0046] In the formula: R s is the engineering stress; E b is the elastic modulus of the Hopkinson pressure bar; r b and r0 respectively represent the diameters of the pressure bar and the sample; e T represents the elastic strain generated by the transmitted wave, and finally the engineering stress-time curve can be obtained.

[0047] The engineering strain is calculated according to formula (9):

[0048]

[0049] In the formula: c b is the longitudinal wave velocity in the Hopkinson pressure bar; l0 is the initial sample length; t represents time; e R represents the elastic strain generated by the reflected wave; τ represents the time integral variable, and the engineering strain-time curve can be obtained.

[0050] The strain rate is calculated according to formula (10):

[0051]

[0052] In the formula: c b is the longitudinal wave velocity in the Hopkinson pressure bar; l0 is the initial sample length; e R represents the elastic strain generated by the reflected wave, and the strain rate-time curve can be obtained.

[0053] The engineering stress-strain curve can be obtained by combining the engineering strain-time curve and the engineering stress-time curve. According to the conversion between the engineering stress-strain curve and the true stress-strain curve in the compression mechanical performance experiment, the final true stress-time curve, true strain-time curve, and true stress-strain curve can be obtained. The specific conversion formulas are calculated according to equations (11) and (12):

[0054] R c =R s (1-e s (11)

[0055] e c =|ln(1-e s (12)

[0056] In the formula: R c R represents the actual stress; s For engineering stress; e c For realistic response; e s To adapt to engineering contingencies.

[0057] Beneficial effects: This invention provides an SHPB experimental apparatus and data processing method for honeycomb porous structures, which has the following advantages compared with the prior art:

[0058] This invention can be used not only for homogeneous continuous medium small cylindrical samples and cap-shaped samples, but also for porous structures with characteristics such as multiple reflections, high-frequency noise dominance, and nonlinear amplitude attenuation. By ensuring the bridge is in a balanced state through pre-experiment calibration, adjusting the waveform of the time-voltage curve obtained from data acquisition to zero, suppressing noise through local weighted polynomial regression, and accurately identifying and extracting the characteristics of the three wave heads and tails, this invention solves the problems of dynamic parameter errors caused by bridge imbalance, signal noise, and waveform truncation deviation in the SHPB experiment in traditional methods. The obtained curves have high precision and high reliability. Attached Figure Description

[0059] Figure 1 This is a schematic diagram of the SHPB experimental setup.

[0060] Figure 2 This is a time-voltage curve of the incident rod obtained after SHPB dynamic mechanical testing of the Ti-6Al-4V titanium alloy honeycomb porous structure in an embodiment of the present invention.

[0061] Figure 3 This is a time-voltage curve of the transmission rod obtained after SHPB dynamic mechanical testing of the Ti-6Al-4V titanium alloy honeycomb porous structure in the embodiment of the present invention.

[0062] Figure 4 Yes Figure 2Time-voltage curve of the incident rod after zeroing and smoothing.

[0063] Figure 5 Yes Figure 3 Time-voltage curve of the transmission rod after zeroing and smoothing.

[0064] Figure 6 This is a waveform diagram of the first incident wave and reflected wave selected in an embodiment of the present invention.

[0065] Figure 7 This is the first transmitted wave waveform selected in this embodiment of the invention.

[0066] Figure 8 This is the engineering stress-strain curve obtained in the embodiments of the present invention.

[0067] Figure 9 This is the actual stress-strain curve obtained in the embodiments of the present invention.

[0068] Figure 10 This is the actual stress-time curve obtained in the embodiments of the present invention.

[0069] Figure 11 This is the true strain-time curve obtained in the embodiments of the present invention.

[0070] Figure 12 This is the strain rate-time curve obtained in the embodiments of the present invention.

[0071] In the figure: 1-driving device, 2-impact rod, 3-velocity measuring device, 4-input rod, 5-strain gauge, 6-sample, 7-output rod, 8-absorption rod, 9-damper, 10-data acquisition and storage device, 11-dynamic strain gauge. Detailed Implementation

[0072] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments:

[0073] like Figure 1 As shown, an SHPB experimental device for honeycomb porous structures includes: a driving device 1, an impact rod 2, a velocity measuring device 3, an input rod 4, a strain gauge 5, an output rod 7, an absorption rod 8, and a damper 9; a sample 6 is placed between the input rod 4 and the output rod 7, and the strain gauge 5 is attached to the middle position of the input rod 4 and the output rod 7 respectively and connected to a dynamic strain gauge 11, which is connected to a data acquisition and storage device 10.

[0074] The driving device 1 applies an initial velocity to the impact rod 2. During the loading process, it is ensured that the input rod 4 and the output rod 7 are in an elastic state, and the velocity of the impact rod 2 does not exceed the calculation result of equation (1):

[0075]

[0076] wherein: R e , p b and c b are the yield strength, density and longitudinal wave velocity of the input rod and the output rod, respectively.

[0077] The impact rod 2, the input rod 4 and the output rod 7 are made of high-strength material, and the diameter is 50 mm. The length-diameter ratio of the input rod 4 and the output rod 7 is not less than 40, which can be designed according to formula (2) and formula (3) respectively:

[0078] L I ≥ 40d b (2)

[0079] L T ≥ 40d b (3)

[0080] wherein: L I is the length of the input rod, L T is the length of the output rod, and d b is the diameter of the input rod and the output rod.

[0081] The material and diameter of the impact rod are the same as those of the input rod and the output rod, the length of the impact rod is greater than 10 times the diameter and less than 0.5 times the length of the input rod.

[0082] In use, the bridge circuit is connected with the data acquisition system, the speed measuring device and the strain signal acquisition and storage device are turned on, and preheating is performed until the equipment reaches a stable state. The data acquisition system is parameterized to ensure that the signal can be recorded completely. Under the conditions of no load or known load, the output voltage of the bridge is recorded, and the variable resistor in the bridge is adjusted to make the output voltage drop to the minimum value or the theoretical equilibrium value. This adjustment process is repeated until the bridge output is stable in the equilibrium state.

[0083] During the experiment, the compressed gas drives the impact rod 2 to impact the incident rod 4, and a compressed incident pulse consistent with the impact direction is generated in the incident rod 4. When the incident pulse propagates to the position of the sample 6, the sample 6 will deform at high speed under the action of the pulse. At this time, part of the pulse energy will be absorbed by the deformation of the sample 6, and the remaining energy will be divided into two parts. One part propagates into the output rod 7 to form a transmitted pulse, and the other part returns to the input rod 4 to form a reflected pulse. The incident wave, the reflected wave and the transmitted wave are amplified by the dynamic strain meter, and then the data acquisition and storage device collects the data;

[0084] The data acquisition system is opened, the initial voltage is adjusted by addition and subtraction operation, and the time-voltage curve is smoothed and denoised, so as to eliminate initial error and noise interference, and make the obtained stress-strain curve more real and accurate.

[0085] The specific process of accurately determining the positions of the wave heads and wave tails of the incident wave, the reflected wave and the transmitted wave is as follows:

[0086] (I) Determination of the waveform baseline: symmetrically pasting strain gauges, respectively taking the average values of the symmetric strain gauge test signals as the incident wave, the reflected wave and the transmitted wave in data processing, taking the average value of the data of the straight section before the starting point of the incident wave as the waveform baseline value of the incident wave and the reflected wave, taking the average value of the data of the straight section before the starting point of the transmitted wave as the waveform baseline value of the transmitted wave, and making the waveform baselines of the incident wave, the reflected wave and the transmitted wave zero in data processing;

[0087] (II) Determination of the waveform starting point: according to the waveforms of the incident wave, the reflected wave and the transmitted wave, the corresponding waveform starting points are determined, the determined starting points should be on the baseline of the corresponding waveform and close to the take-off point of the waveform, the maximum value of the incident wave is determined according to the waveform of the incident wave, the time when the incident wave rises to one tenth of its maximum value and the corresponding discrete data point are determined, a straight line is fitted by taking the time corresponding to one tenth of the maximum value of the incident wave and a certain amount of data points (such as 100 data points) backward, the time corresponding to the intersection point of the straight line and the baseline is taken as the starting point n1 of the incident wave, after the starting point of the incident wave is determined, the starting points of the reflected wave and the transmitted wave are determined through formula (4) and formula (5):

[0088]

[0089] In the formula, int[] is an integer function; n1, n2 and n3 are respectively the data point serial numbers corresponding to the starting points of the incident wave, the reflected wave and the transmitted wave; a1 and a2 are respectively the distances from the center of the strain gauge in the input rod and the output rod to the sample end; c b is the longitudinal wave velocity in the input rod and the output rod, c s is the elastic longitudinal wave velocity in the sample; l0 is the initial sample length; t i is the sampling time interval.

[0090] (III) Aligning the waveform starting points: moving the incident wave, the reflected wave and the transmitted wave so that their starting points are located at the same time, and taking this time as the starting time in data processing, the incident wave, the reflected wave and the transmitted wave after the starting points are aligned should satisfy the homogenization assumption, that is, formula (6):

[0091] e I (t)+e R (t)=e T (t) (6)

[0092] In the formula, eI , e R and e T represent the elastic strain produced by the incident wave, reflected wave and transmitted wave, respectively.

[0093] (IV) Determination of the action time of stress wave: the last maximum point before the falling edge of the selected wave form is taken as the end point of the incident wave to determine the action time of stress wave.

[0094] Each curve is calculated by the above-mentioned intercepted incident wave, reflected wave and transmitted wave, and the specific steps are as follows:

[0095] According to the calculation formula of Hopkinson bar reflected strain and transmitted strain, the time-voltage curve is converted into the reflected strain-time curve and the transmitted strain-time curve, and the specific strain theoretical calculation formula (7) is as follows:

[0096]

[0097] In the formula: U i (i=1, 2, 3...) represents voltage, K1 is the sensitivity coefficient of strain gauge, K2 is the amplification coefficient of dynamic strain meter, V 0 is the power supply voltage of the bridge, and finally the time-voltage curve is converted into the reflected strain-time curve and the transmitted strain-time curve.

[0098] The engineering stress is calculated according to formula (8):

[0099]

[0100] In the formula: R s is the engineering stress; E b is the elastic modulus of Hopkinson pressure bar; r b and r0 represent the diameters of the pressure bar and the sample, respectively; e T represents the elastic strain produced by the transmitted wave, and finally the engineering stress-time curve can be obtained.

[0101] The engineering strain is calculated according to formula (9):

[0102]

[0103] In the formula: c b is the longitudinal wave velocity in Hopkinson pressure bar; l0 is the initial sample length; t represents time; e R represents the elastic strain produced by the reflected wave; τ represents the time integral variable, and the engineering strain-time curve can be obtained.

[0104] The strain rate is calculated according to formula (10):

[0105]

[0106] wherein c b is the longitudinal wave velocity in the Hopkinson pressure bar; l0is the initial specimen length; e R represents the elastic strain generated by the reflected wave, and the strain rate-time curve can be obtained.

[0107] The engineering stress-strain curve can be obtained from the engineering strain-time curve and the engineering stress-time curve. According to the conversion of the engineering stress-strain curve and the true stress-strain curve in the compression mechanical property experiment, the final true stress-time curve, the true strain-time curve and the true stress-strain curve can be obtained, and the specific conversion formula is calculated according to formula (11) and formula (12):

[0108] R c = R s (1-e s ) (11)

[0109] e c = |ln(1-e s )| (12)

[0110] wherein R c is the true stress; R s is the engineering stress; e c is the true strain; e s is the engineering strain.

[0111] Experimental verification

[0112] The Ti-6Al-4V titanium alloy honeycomb porous structure manufactured by the additive manufacturing laser selective melting technology was selected as the specimen, and the SHPB dynamic compression mechanical property test data of the specimen at 600 s -1 was studied by using the above method. The specific process is as follows:

[0113] (1) The time-voltage data of the incident bar and the time-voltage data of the transmission bar obtained after the SHPB dynamic mechanical test of the Ti-6Al-4V titanium alloy honeycomb porous structure were read respectively, and the curves were drawn, as shown in Figure 2 and Figure 3

[0114] (2) The time-voltage data of the incident bar and the time-voltage data of the transmission bar shown in Figure 2 and Figure 3 were zeroed and smoothed, as shown in Figure 4 and Figure 5

[0115] (3) The wave shape starting point and the end point were determined by using the determination method, and the time-voltage data of the incident bar and the time-voltage data of the transmission bar were obtained, as shown in Figure 4 ​​The positions of the wave head and wave tail of the incident wave and reflected wave of the first waveform are determined, and the positions are adjusted smoothly to obtain the time-voltage curve of the incident rod and the time-voltage curve of the reflected rod of the first waveform, as shown in FIG. 2A and FIG. 2B. Figure 6 Similarly, the time-voltage curve of the transmitted rod can also be obtained, as shown in FIG. 2C. Figure 7

[0116] (4) The time-voltage curve is converted into the reflected strain-time curve and the transmitted strain-time curve by using the calculation model of formula (7). The sensitivity coefficient K1 of the strain gauge is 2, the dynamic strain gauge amplification coefficient K2 is 100, and the bridge power supply voltage is 5V. Finally, the time-voltage curve is converted into the reflected strain-time curve and the transmitted strain-time curve by using formula (13):

[0117] e i = 5.0 × 10 -3 · U i (13)

[0118] In the formula, e i and U i represent strain and voltage, respectively.

[0119] The strain rate can be obtained according to formula (10):

[0120]

[0121] In the formula, c represents the strain rate; c b is the longitudinal wave velocity in the Hopkinson pressure rod; l0 is the initial sample length; e R represents the elastic strain generated by the reflected wave.

[0122] Suppose that the loading starts at time 0, and the engineering stress of the sample at time t can be obtained according to formula (15):

[0123]

[0124] In the formula, R s represents the engineering stress; E b and r b are the elastic modulus and radius of the pressure rod; w s and w s represent the width and height of the honeycomb porous structure sample, respectively; and e T represents the elastic strain generated by the transmitted wave.

[0125] Suppose that the loading starts at time 0, and the engineering strain of the sample at time t can be obtained according to formula (16):

[0126]

[0127] In the formula,​ and e s1 respectively the strain rate and engineering strain at the previous time, e s2 represents the engineering strain at the next time, and Δt is the time difference;

[0128] The true stress-time curve, the true strain-time curve are calculated according to formula (11) and formula (12), and then the true stress-strain curve is obtained by simultaneous solution; finally, the engineering stress-strain curve (formula (13)), the true stress-strain curve (formula (14)), the true stress-time curve (formula (15)), the true strain-time curve (formula (16)) and the strain rate-time curve (formula (17)) of the sample in the compression process are obtained. Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Although the honeycomb porous structure has the characteristics of multiple reflections, high frequency noise dominance, nonlinear amplitude attenuation and the like under impact, the experimental curves obtained by the experimental device and the data processing method have high precision and high reliability.

[0129] (5) modifying, smoothing, saving and the like of all the curves.

[0130] The above only describes the preferred embodiments of the present application, and it should be noted that those skilled in the art can make several improvements without departing from the principles of the present application, and these improvements should also be considered as the protection scope of the present application.​​​​

Claims

1. A method for processing SHPB experimental data of a cellular porous structure, characterized in that, The method comprises the following steps: (1) pre-experiment calibration: before the experiment, the bridge circuit is accurately calibrated to ensure that the bridge is in a balanced state; (2) zero adjustment: the zero adjustment of the waveform of the time-voltage curve obtained by the data acquisition device is performed, that is, the initial voltage value is kept as zero; (3) curve smoothing: the abnormal data points and noise appearing in the time-voltage curve recorded by the data acquisition and storage device are removed, and the curve is smoothed; (4) waveform selection: the head and tail positions of the incident wave, the reflected wave and the transmitted wave are accurately selected, and the incident wave, the reflected wave and the transmitted wave data intercepted are subjected to curve fitting; (5) obtaining of each curve: the time-voltage curve of the incident bar, the time-voltage curve of the transmitted bar, the reflected wave and the transmitted wave waveform curve, and the engineering stress-strain curve are obtained through the intercepted incident wave, the reflected wave and the transmitted wave, the obtained engineering stress-strain curve is converted into the real stress-time curve through the conversion relationship between the engineering stress-strain and the real stress-strain, and then the real strain-time curve and the strain rate-time curve are obtained.

2. The data processing method for SHPB experiment of cellular porous structure according to claim 1, characterized in that, The specific process of accurately determining the head and tail positions of the incident wave, the reflected wave and the transmitted wave is as follows: (I) determination of the waveform baseline: the average values of the symmetric strain gauge test signals are taken as the incident wave, the reflected wave and the transmitted wave during data processing, the average values of the data of the flat section before the start of the incident wave and the reflected wave are taken as the waveform baseline values of the incident wave and the reflected wave, the average values of the data of the flat section before the start of the transmitted wave are taken as the waveform baseline values of the transmitted wave, and the waveform baselines of the incident wave, the reflected wave and the transmitted wave are zeroed during data processing; (II) determination of the waveform start point: the waveform start points are determined according to the waveforms of the incident wave, the reflected wave and the transmitted wave, the determined start points should be on the baseline of the corresponding waveform and close to the take-off point of the waveform, the maximum value of the incident wave is determined according to the waveform of the incident wave, the time when the incident wave rises to one-tenth of its maximum value and the corresponding discrete data point are determined, a straight line is fitted by taking the time corresponding to one-tenth of the maximum value of the incident wave and a certain amount of data points backward, and the time corresponding to the intersection point of the straight line and the baseline is taken as the start point n1 of the incident wave, after the start point of the incident wave is determined, the start points of the reflected wave and the transmitted wave are determined by the following formula: wherein: int[] is an integer function; n1, n2 and n3 are the data point serial numbers corresponding to the starting points of the incident wave, the reflected wave and the transmitted wave respectively; a1 and a2 are the distances from the center of the strain gauge to the sample end in the input rod and the output rod respectively; c b is the longitudinal wave velocity in the input rod and the output rod, c s is the longitudinal wave velocity in the sample; l0 is the initial sample length; t i is the sampling time interval; (III) alignment of the waveform start points: the incident wave, the reflected wave and the transmitted wave are moved so that their start points are located at the same time, and the time is taken as the starting time in data processing, the incident wave, the reflected wave and the transmitted wave after the start points are aligned should satisfy the homogenization assumption, that is, the following formula is satisfied: e I (t)+e R (t)=e T (t) wherein: e I , e R , and e T respectively represent the elastic strain generated by the incident wave, the reflected wave, and the transmitted wave; (IV) determination of the stress wave action time: the last maximum value point before the waveform enters the falling edge is selected as the end point of the incident wave to determine the action time of the stress wave.

3. The data processing method for SHPB experiment of cellular porous structure according to claim 1 or 2, characterized in that, The time-voltage curve of the incident rod and the time-voltage curve of the transmission rod are obtained by calculating the intercepted incident wave, reflected wave and transmission wave; the time-voltage curve is converted into the reflected strain-time curve and the transmission strain-time curve by using the formula e i = 5.0 x 10 -3 ·U i , wherein e i and U i respectively represent the strain and the voltage; Using the formula The strain rate-time curve is obtained, where represents the strain rate, c b is the longitudinal wave velocity in the split Hopkinson pressure bar, lo is the initial specimen length, e R represents the elastic strain due to the reflected wave.

4. The data processing method for SHPB experiment of cellular porous structure according to claim 3, characterized in that, The engineering stress-time curve is obtained according to the formula where R s represents the engineering stress, E b and r b is the elastic modulus and radius of the compression rod, w s and h s represent the width and height of the honeycomb porous structure, respectively, e T represents the elastic strain generated by the transmitted wave; The engineering strain-time curve is obtained according to the formula wherein and e s1 are the strain rate and the engineering strain, respectively, at the previous time, e s2 denotes the engineering strain at the next time, and Δt is the time difference.

5. The data processing method for SHPB experiment of cellular porous structure according to claim 4, characterized in that, The engineering stress-strain curve is obtained from the engineering strain-time curve and the engineering stress-time curve, the final real stress-time curve, the real strain-time curve and the real stress-strain curve are obtained according to the conversion of the engineering stress-strain curve and the real stress-strain curve in the compression mechanical property experiment, and the specific conversion formula is as follows: R c = R s (1-e s ) e c = |ln(1-e s )| where: R c is true stress; R s is engineering stress; e c is true strain; e s is engineering strain.

6. The data processing method for SHPB experiment of cellular porous structure according to claim 1, characterized in that, The specific process of step (1) is: connecting the bridge circuit to the data acquisition system, starting the speed measuring device, preheating the strain signal acquisition and storage device to the equipment stability, setting the parameters of the data acquisition system to ensure the integrity of the recorded signal; Under the conditions of no load or known load, record the output voltage of the bridge, adjust the variable resistor in the bridge to make the output voltage reach the minimum value or the theoretical equilibrium value, repeat the adjustment process until the output of the bridge is stable in the equilibrium state.

7. The data processing method for SHPB experiment of cellular porous structure according to claim 1, characterized in that, The specific process of step (2) is: adjusting the waveform zero of the time-voltage curve obtained by the data acquisition device, that is, measuring the initial voltage value to determine whether it is zero and adjusting it by hardware or software method to ensure that the initial voltage is zero, and verifying the zero effect and recording the adjustment process and parameters to obtain accurate and reliable time-voltage curve.

8. The data processing method for SHPB experiment of cellular porous structure according to claim 1, characterized in that, The specific process of step (3) is: removing the abnormal data points and noise of the time-voltage curve recorded by the data acquisition and storage device, smoothing the time-voltage curve data by using the method of polynomial regression on local data to obtain the smoothed time-voltage curve.

9. A SHPB experimental device for a cellular porous structure, comprising: SHPB, dynamic strain meter, data acquisition and storage device, the SHPB includes driving device, impact rod, speed measuring device, input rod, strain gauge, output rod, the strain gauge is pasted in the middle position of the input rod and the output rod and is connected to the dynamic strain meter, the dynamic strain meter is connected with the data acquisition and storage device; characterized in that the driving device applies initial speed to the impact rod, ensures that the input rod and the output rod are in the elastic state during the loading process, and the speed of the impact rod does not exceed: where: R e , p b and c b are the yield strength, density and longitudinal wave speed of the input and output rods, respectively.

10. The SHPB experimental device for cellular porous structure according to claim 9, characterized in that, The length-diameter ratio of the input rod and the output rod is not less than 40.

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