Determination of stress wave head of Hopkinson bar based on image analysis

Abstract

The present invention discloses a method for determining the wave head of a Hopkinson bar stress wave based on image analysis, comprising: 1) collecting an image sequence of the contact ends of the incident bar and the transmission bar with the impact specimen during a separated Hopkinson bar impact test; 2) performing image analysis on the image sequence to obtain the velocity-time curves of the incident bar end and the transmission bar end; 3) combining the velocity-time curves of the incident bar end and the transmission bar end, and using a target optimization algorithm to determine the wave head interval of the stress wave signal. The key to the present invention is to introduce a digital image analysis method to assist in determining the relative intervals of the wave heads of the strain-time curves corresponding to the three stress waves. The method mentioned in the present invention has higher accuracy and objectivity, and is more suitable for soft material impact experiments.

Description

Technical Field

[0001] The present invention relates to the technical field of Split Hopkinson Pressure Bar (SHPB) experiment, and in particular to a new method for accurately locating the wave head interval of the strain-time curve of the incident wave, the transmitted wave and the reflected wave in the SHPB experiment based on the traditional SHPB experiment technology and combined with the image analysis method. Background Art

[0002] SHPB experimental technology is an experimental method for testing the dynamic mechanical properties of materials. It has been widely used in mechanical property testing of engineering materials at high strain rates. In traditional SHPB experiments, strain gauges need to be attached to the incident rod and the transmission rod before the experiment. During the impact process, the strain gauges are used to collect the strain-time curves of the stress wave signals on the incident rod and the transmission rod, and finally the stress-strain relationship of the material is calculated. The strain-time curve of the stress wave usually has a certain volatility, and the wave head of the curve is not clear, so the interval of the wave head needs to be determined manually. Different combinations of wave heads will have a certain impact on the SHPB experimental results. Therefore, determining the interval of the stress wave signal wave head is one of the keys to SHPB experimental data processing.

[0003] At present, most scholars use the method of directly determining the wave head of the stress wave signal. There are several methods: (1) Select a certain time point on the strain-time curve. If the strain deviation of multiple consecutive points after this point exceeds 1 / 20 of the signal amplitude, and the strain deviation of no point before this point exceeds 1 / 20 of the amplitude, then this point is considered to be the wave head of the stress wave signal; (2) Move the signal curves of the transmitted wave and the reflected wave so that the time points corresponding to the characteristic points of the strain-time curves corresponding to the two waves are the same as the time of the characteristic points of the incident wave. The characteristic point can be the point where the slope of the strain-time curve changes suddenly or the point with the largest strain. The intersection of the three curves (if there is no intersection, the closest point is taken) is the wave head of the incident wave. According to the distance between the wave head and the characteristic point, the wave heads of the reflected wave and the transmitted wave can be calculated. Both methods only process stress wave data. Stress wave data has a certain volatility. When actually processing SHPB experimental data, it is necessary to smooth the stress wave data. The smoothing operation may cause the characteristic value to shift, thereby causing errors. For soft material impact experiments, the transmission wave signal is smaller and the error is more obvious. The absolute position of the wave head of the stress wave signal has little effect on the stress-strain curve obtained in the experiment, but only affects the starting point of the data. The wave head interval of several stress waves is one of the key influencing factors. Therefore, it is necessary to propose a more accurate method to determine the wave head interval of the SHPB experimental stress wave signal. Summary of the invention

[0004] The purpose of the present invention is to solve the problem of how to determine the wave head interval of stress wave signal in SHPB experiment, and proposes a method for determining the wave head of Hopkinson pressure bar stress wave based on image analysis. The method can accurately locate the wave head interval of three stress waves in SHPB experiment, avoid the interference of human factors, and improve the accuracy of experimental results.

[0005] To achieve the above object, the technical solution provided by the present invention is: a method for determining the Hopkinson bar stress wave head based on image analysis, comprising the following steps:

[0006] 1) During the split-Hopkinson pressure bar impact test, the image sequence of the incident bar, the transmission bar and the contact end of the impact specimen is collected;

[0007] 2) Perform image analysis on the image sequence to obtain the velocity-time curves of the incident rod end and the transmitted rod end;

[0008] 3) Combining the velocity-time curves of the incident rod end and the transmitted rod end, the wave head interval of the stress wave signal is determined using the target optimization algorithm.

[0009] Further, in step 1), the image sequence refers to a pattern with an identification mark.

[0010] Further, in step 2), the image analysis refers to an analysis method that can calculate the position change of the same pattern at different times. The displacement-time curve of the rod end is first obtained by image analysis, and then the displacement-time curve is numerically derived to obtain the velocity-time curve of the rod end.

[0011] Further, the step 3) comprises the following steps:

[0012] 3.1) Applying the three-wave method or the two-wave method, using the strain-time curve of the collected stress wave signal, another set of velocity-time curves of the incident rod end and the transmitted rod end are calculated;

[0013] 3.2) Check the degree of convergence of the velocity-time curves of the two rod ends. The degree of convergence refers to the degree of dispersion that is less than the specified threshold, which is expressed by formula (1), where x i represents the i-th time point, i = 1, 2, ..., n, f and g represent the velocity-time curve of the rod end calculated by using stress wave data and the velocity-time curve of the rod end obtained by image analysis, respectively, δ is the threshold, and ∏ is the function corresponding to the index for measuring the degree of discreteness;

[0014]

[0015] 3.3) If the indicator measuring the degree of discreteness is less than the threshold, the wave head interval is the desired result; otherwise, adjust the intervals between the wave heads of the incident wave, reflected wave and transmitted wave signals, repeat steps 3.1) to 3.3), and accurately locate the wave head intervals of the three stress wave strain-time curves; wherein, the wave head refers to the time point corresponding to the starting point of the main rising edge or the main falling edge of the strain-time curve of the stress wave.

[0016] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0017] 1. The present invention proposes a new experimental method based on image analysis, which provides more objective and accurate rod end velocities of the incident rod and the transmitted rod, and can be used to correct experimental data.

[0018] 2. The standard of the wave head interval determined by the present invention is that the velocity-time curve of the rod end calculated by the stress wave approaches the velocity-time curve of the rod end obtained by the image analysis method. Compared with the general method of judging a single feature point, it has higher robustness and can be used for stress wave data with greater discreteness.

[0019] 3. For soft material impact experiments, the transmitted wave signal is relatively weak, and the smoothing operation in the traditional SHPB experiment is more likely to affect the determination of the relative time interval of the stress wave signal wave head; the interval of the stress wave signal wave head determined by the present invention has higher accuracy and is more suitable for soft material impact experiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 is a schematic diagram of the experimental shooting area; in the figure, 1 is the incident rod of the SHPB device, 2 is the transmission rod of the SHPB device, 3 is the specimen, 4 and 5 are strain gauges, 6 and 7 are speckle patterns, and 8 is the shooting area of ​​the high-speed camera.

[0021] Figure 2 This is a picture taken by a high-speed camera in an actual experiment; in the picture, the black frame is the calculation area for digital image analysis.

[0022] Figure 3 It is the stress wave curve diagram obtained from the SHPB experiment; in the figure, 1 is the incident wave, 2 is the reflected wave, and 3 is the transmitted wave; a is the incident wave head, b is the reflected wave head, and c is the transmitted wave signal head; d1 is the time interval of the reflected wave head relative to the incident wave head, and d2 is the time interval of the transmitted wave signal head relative to the incident wave head.

[0023] Figure 4 It is a comparison chart of the nominal stress-nominal strain curve of the specimen obtained by applying the digital image method to correct the stress wave signal head and the traditional method; among them, the nominal stress refers to the force per unit area of ​​undeformed, and the nominal strain refers to the elongation per unit undeformed length. DETAILED DESCRIPTION

[0024] The present invention is further described in detail below in conjunction with embodiments and drawings, but the embodiments of the present invention are not limited thereto.

[0025] In this embodiment, the split Hopkinson pressure bar used is a polycarbonate bar specially used for impacting soft materials. Figure 1 As shown, the rod used is a polycarbonate rod with a diameter of 20 mm, and the lengths of the bullet, incident rod, and transmission rod are 200 mm, 600 mm, and 300 mm respectively; the high-speed camera system uses a Phantom v1610 system with a shooting rate of 100,000 frames / s. The experiment includes the following steps:

[0026] 1) Before the experiment, the speckle pattern is sprayed on the surfaces of the incident rod and the transmission rod in contact with the specimen for subsequent digital image analysis (such as Figure 2 As shown in the figure, the specimen used is a cylindrical silicone specimen with a diameter of 8 mm and a thickness of 1 mm. Lubricant needs to be applied to the surface of the specimen before the experiment.

[0027] 2) The driving mode of the Hopkinson pressure bar is electromagnetic drive, and the speed of the bullet is proportional to the voltage of the driving device. During the experiment, the voltage of the driving device is first adjusted to a fixed value, and the bullet will hit the incident bar at a fixed speed (2m / s), and then the incident bar will hit the specimen, and the specimen will hit the transmission bar to complete an impact process. During the whole process, the strain gauge is used to collect the corresponding stress wave signal, and the high-speed camera is used to collect the digital image sequence of the speckle area of ​​the incident bar and the transmission bar. The image sequence refers to a pattern with identifiable marks.

[0028] 3) Processing the acquired digital image sequence includes the following steps:

[0029] 3.1) Perform image analysis on the image sequence captured by the high-speed camera to obtain more accurate rod end velocity-time curves of the incident rod and the transmitted rod; image analysis refers to an analysis method that can calculate the position change of the same pattern at different times (such as digital image correlation method), first obtain the displacement-time curve of the rod end through image analysis, and then numerically differentiate the displacement-time curve to obtain the velocity-time curve of the rod end.

[0030] 3.2) Using the two-wave method or the three-wave method, the strain-time curve of the stress wave signal (such as Figure 3 The velocity-time curves of the rod ends are converted into another set of velocity-time curves of the rod ends.

[0031] 3.3) Check the degree of convergence of the velocity-time curves of the two rod ends. The degree of convergence refers to the degree of dispersion that is less than the specified threshold, which can be expressed by formula (1), where x irepresents the i-th time point, i = 1, 2, ..., n, f and g represent the velocity-time curve of the rod end calculated by using stress wave data and the velocity-time curve of the rod end obtained by image analysis, respectively, δ is the threshold, and ∏ is the function corresponding to the index for measuring the degree of discreteness;

[0032]

[0033] If the indicator measuring the degree of discreteness is less than the threshold, the wave head interval is the desired result; otherwise, adjust the intervals between the wave heads of the incident wave, reflected wave and transmitted wave signals, repeat steps 3.2) to 3.3), and accurately locate the wave head intervals of the three stress wave strain-time curves; wherein the wave head refers to the time point corresponding to the starting point of the main rising edge or the main falling edge of the strain-time curve of the stress wave. Figure 4 In order to compare the nominal stress-nominal strain curve of the specimen obtained by using the digital image method to correct the stress wave signal head and the traditional method, there are obvious differences between the two sets of curves for soft materials, and it is necessary to use the digital image method to correct the experimental data.

[0034] In summary, the present invention uses the rod end velocity-time curve obtained by image analysis as a reference, and adjusts the wave head interval of the three stress wave curves obtained by strain gauges, so that the velocity-time curves of the incident rod end and the transmitted rod end calculated according to the stress wave curve approach the velocity-time curve obtained based on image analysis. Through this method, the intervals between the wave heads of the three stress waves can be accurately located. The degree of convergence can be characterized by the sum of the squares of the velocity differences of the two velocity-time curves at the same time, and the smaller the value, the closer the two groups of curves are. Through this method, the wave head intervals of the three stress wave strain-time curves can be accurately located, and the accuracy of analyzing the dynamic stress-strain relationship of the material can be improved.

[0035] The experimental results show that the interval determination method of the stress wave signal head in the SHPB experiment based on image analysis can accurately locate the relative wave heads of the three stress waves, and assist in obtaining more objective and accurate experimental results.

[0036] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be equivalent replacement methods and are included in the protection scope of the present invention.

Claims

1. A method for determining the head of a Hopkinson bar stress wave based on image analysis, characterized in that: The following steps are involved: 1) During the split-Hopkinson pressure bar impact test, the image sequence of the incident bar, the transmission bar and the contact end of the impact specimen is collected; 2) Perform image analysis on the image sequence to obtain the velocity-time curves of the incident rod end and the transmitted rod end; 3) Combining the velocity-time curves of the incident rod end and the transmitted rod end, the wave head interval of the stress wave signal is determined using the target optimization algorithm, including the following steps: 3.1) Apply the three-wave method or the two-wave method, and use the strain-time curve of the collected stress wave signal to calculate another set of velocity-time curves of the incident rod end and the transmitted rod end; 3.2) Check the degree of convergence of the velocity-time curves of the two groups of rod ends. The degree of convergence refers to the degree of dispersion that is less than the specified threshold, which is expressed by formula (1), where: represents the i-th time point, i=1,2,…,n, f and g represent the velocity-time curve of the rod end calculated by stress wave data and the velocity-time curve of the rod end obtained by image analysis, respectively. is the threshold value, The function corresponding to the index measuring the degree of dispersion; （1）； 3.3) If the index measuring the degree of discreteness is less than the threshold, the wave head interval is the desired result; otherwise, adjust the intervals between the wave heads of the incident wave, reflected wave and transmitted wave signals, and repeat steps 3.1) to 3.3) to accurately locate the wave head intervals of the three stress wave strain-time curves; wherein the wave head refers to the time point corresponding to the starting point of the main rising edge or the main falling edge of the strain-time curve of the stress wave.

2. The method for determining the Hopkinson bar stress wave head based on image analysis according to claim 1, characterized in that: In step 1), the image sequence refers to a pattern with identification marks.

3. The method for determining the Hopkinson bar stress wave head based on image analysis according to claim 1, characterized in that: In step 2), the image analysis refers to an analysis method that can calculate the position change of the same pattern at different times. The displacement-time curve of the rod end is first obtained by image analysis, and then the displacement-time curve is numerically derived to obtain the velocity-time curve of the rod end.

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