Fracture analysis method and system applied to SHPB impact test

Abstract

The invention provides a fracture analysis method and system applied to an SHPB impact test, and belongs to the technical field of SHPB tests.A shooting module is arranged in the pulse direction to obtain an impact image, and time history images of a frozen moraine soil sample at different impact rates are recorded; the method comprises the steps of performing resolution optimization on an impact image based on an image interpolation method to obtain a processed image, enhancing a difference region of a fracture in an undamaged sample based on a histogram equalization method, extracting features of the difference region, summarizing time-history features through a time-history image, inputting the time-history features into a target tracking model, and outputting a topological structure of the sample fracture. And quantifying to obtain the geometrical characteristics of the sample fragments.

Description

Technical Field

[0001] This specification relates to the technical field of crack analysis, and in particular to a crack analysis method and system applied to SHPB impact testing. Background Art

[0002] The SHPB (Hopkinson Pressure Bar) has become a key experimental method for studying the dynamic mechanical properties of materials across various engineering fields. Currently, existing SHPB tests typically analyze specimen failure statistics after the impact. Even with high-speed photography, analysis often only captures the specimen's failure process; real-time analysis through time alignment with test data is not currently available. Glacial till, a product of glacial activity, is found in many parts of the world, including the East European Plain, the Great Lakes region of North America, and China's Qinghai-Tibet Plateau and surrounding areas. In the Qinghai-Tibet Plateau and its surrounding areas, due to climatic conditions, the till often exists as perennially or seasonally frozen ground, known as frozen till.

[0003] Numerous infrastructure construction projects, including railways, highways, and airports, are underway on and around the Qinghai-Tibet Plateau. Blasting, an economical and efficient construction method, is widely used during the construction of these projects. Frozen moraine, one of the primary support structures of these projects, is often subjected to the impact loads imposed by blasting. Understanding the dynamic mechanical properties of frozen moraine and clarifying its impact damage and failure characteristics is crucial for accurately predicting the impact of blasting on it and its overlying infrastructure, thereby ensuring the safety, controllability, and efficiency of blasting. However, the complex composition and structure of frozen moraine pose significant challenges to studying its dynamic mechanical properties. Limited by the current lack of research on the dynamic mechanical properties of frozen moraine, the impact of blasting cannot be accurately predicted, resulting in significant uncertainty in blasting operations in areas covered by frozen moraine.

[0004] An existing Chinese patent, publication number CN110608956A, discloses a system and method for measuring the dynamic Poisson's ratio of brittle materials, which includes a SHPB horizontal impact device, a high-speed camera, a synchronous trigger, a hyper-dynamic strain acquisition device and a digital image receiving device. The hyper-dynamic strain acquisition device is connected to the SHPB horizontal impact device so as to be able to acquire the dynamic strain signals of the incident wave, reflected wave and transmitted wave output by the SHPB horizontal impact device in real time; the synchronous trigger is electrically connected to the hyper-dynamic strain acquisition device and the high-speed camera respectively so as to be able to control the hyper-dynamic strain acquisition device and the high-speed camera to achieve synchronous triggering and real-time measurement; the digital image receiving device is electrically connected to the high-speed camera.

[0005] The above technology aims to effectively improve the authenticity and reliability of the dynamic Poisson's ratio measurement of brittle materials; it provides a new adaptable, accurate and reliable method for the dynamic Poisson's ratio measurement of brittle materials such as rock under impact loads, but without real-time analysis after time alignment.

[0006] Therefore, it is necessary to provide a crack analysis method and system for SHPB impact testing to extract and analyze the characteristic information of the entire process of sample surface crack evolution, penetration, and sample crushing during the sample impact process. Summary of the Invention

[0007] The embodiments of this specification provide a crack analysis method and system for application in SHPB impact testing. Through the above technology, the evolution of cracks, changes in fragment size, and changes in broken fracture surface morphology during the impact process can be automatically counted, and then the real-time impact of different initial or loading conditions on material damage can be analyzed. Therefore, this quantitative impact analysis is not just a quantitative analysis after the loading is completed in the traditional sense, but a quantitative analysis of the entire loading process.

[0008] In some embodiments, a crack analysis method applied to a SHPB impact test comprises a camera module arranged in a pulse direction to obtain impact images and record time history images of a frozen moraine soil sample at different impact rates; The resolution of the impact image is optimized based on the image interpolation method to obtain the processed image. The difference area between the crack and the undamaged sample is enhanced based on the histogram equalization method, and the features of the difference area are extracted. The time history features are summarized through the time history image and input into the target tracking model. The topological structure of the sample crack is output, and the geometric features of the sample fragments are quantified.

[0009] Based on this technical approach, permafrost and frozen moraine soils are brittle before impact loading, but soon after impact, due to the ambient temperature and the impact temperature rise, the ice melts and they quickly become loose, making it difficult to accurately count their fragmentation morphology. This is especially true for moving soil and frozen moraine soils, where fragments cannot be found like hard objects such as rocks. Therefore, this application is particularly necessary to record high-speed time-scale images of frozen moraine soil samples under SHPB impact. This overcomes the defect in the existing technology that cannot accurately determine the crack morphology of such samples under high-speed impact, and achieves a breakthrough in experimental data collection in this field, namely, studying the instantaneous morphological changes of materials such as frozen moraine soil under different impact rates.

[0010] Furthermore, the fill light module is activated according to the time of the impact.

[0011] Furthermore, in the image difference method, the grayscale values ​​of adjacent pixels in a single initial image are used to generate the grayscale values ​​of unknown pixels.

[0012] Furthermore, histogram equalization improves contrast by stretching the range of pixel values.

[0013] Furthermore, the topological structure includes a crack branching structure and a crack through-structure.

[0014] Furthermore, the geometric characteristics include equivalent diameter and edge angle.

[0015] Furthermore, it includes SHPB test system, shooting module, and fill light module; The SHPB test system includes an impact rod, an incident rod, an output rod, and a strain module. The shooting module and the fill light module are symmetrically arranged on both sides of the sample with the sample as the center of symmetry.

[0016] The beneficial effects of the present invention are: 1. Aiming at the low contrast and high noise characteristics of frozen moraine, a customized interpolation method and histogram equalization combination strategy were used to solve the image degradation problem in high-speed photography; 2. Utilize target tracking + CNN algorithm to automatically identify crack topology (such as branching and penetration) and quantify the geometric features of fragments (such as equivalent diameter and edge angle), replacing traditional manual measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] This specification will be further described in the form of exemplary embodiments, which will be described in detail with reference to the accompanying drawings. These embodiments are not limiting, and in these embodiments, like numbers represent like structures, wherein: Figure 1 is a schematic diagram of the working principle shown in some embodiments of this specification; Figure 2 Schematic diagram of fragment cracks after impact according to some embodiments of this specification.

[0018] Explanation of reference numerals: 1020: waveform shaper; 1021: incident rod; 1022: strain module; 1023: transmission rod. DETAILED DESCRIPTION

[0019] To more clearly illustrate the technical solutions of the embodiments of this specification, the following briefly describes the drawings required for describing the embodiments. Obviously, the drawings described below are merely examples or embodiments of this specification. Those skilled in the art can apply this specification to other similar scenarios based on these drawings without inventive effort. Unless otherwise apparent from the context or otherwise noted, the same reference numerals in the figures represent the same structure or operation.

[0020] It should be understood that the terms "system," "device," "unit," and / or "module" used herein are a method for distinguishing different components, elements, parts, portions, or assemblies at different levels. However, if other terms can achieve the same purpose, the terms may be replaced by other expressions.

[0021] As used in this specification and claims, unless the context clearly indicates otherwise, the words "a," "an," "an," and / or "the" do not refer to the singular but also include the plural. Generally speaking, the terms "comprises" and "include" only indicate the inclusion of the steps and elements specifically identified, and these steps and elements do not constitute an exclusive list. A method or apparatus may also include other steps or elements.

[0022] Flowcharts are used throughout this specification to illustrate the operations performed by systems according to embodiments of this specification. It should be understood that preceding or following operations do not necessarily need to be performed in exact order. Instead, the steps may be processed in reverse order or simultaneously. Furthermore, other operations may be added to these processes, or one or more operations may be removed from these processes.

[0023] Example: The difficulties of the existing technology are: 1) image data processing. The frame rate required for impact testing is too high. Most high-speed photography images are dark and not clear enough. In this case, crack analysis itself is very difficult, so image processing is required (this application solves this problem through super-resolution and contrast enhancement); 2) manual analysis is prone to errors. This embodiment uses the computer itself after contrast enhancement to perform feature extraction and extract crack information in each frame; 3) the problem of corresponding crack evolution to macroscopic mechanical properties. This embodiment uses at least two cameras to cover and shoot images of the sample surface, and uses time alignment to combine the high-speed images with the SHPB test signals to correspond the crack propagation process with the evolution of macroscopic mechanical properties, thereby understanding the real-time impact of crack propagation on the mechanical properties of the material; 4) fragmentation analysis. In the past, fragmentation analysis often involved manually picking up the fragments and then analyzing them. However, it is inevitable that the fragments are too small or fly into corners and cannot be found. At the same time, manual analysis is very time-consuming and labor-intensive. For many materials, their morphology may have changed during this time. This embodiment uses target tracking + CNN algorithm to analyze images, which can quickly obtain data such as the size, morphology, geometric characteristics, and quantity of fragments after impact, greatly improving analysis efficiency and avoiding many changes caused by time.

[0024] Please refer to Figure 1, use two or more high-speed cameras (one in front and one behind) to shoot the outside of the sample to ensure that the shooting angle can cover most of the surface of the sample (at least 80% of the surface of the sample should be photographed). The frame rate of the high-speed camera should not be less than 500,000 fps; Among them, the target tracking + CNN algorithm is an improved convolutional neural network (CNN) algorithm, which combines optical flow and feature point matching for target tracking. The optical flow in this application obtains its optical flow information by connecting the fill light module. In this embodiment, the feature point is set in the gravel soil mixed particle body between the strain module 1022 (strain gauge 1 and strain gauge 2). It should be noted that the volume compression coefficient of the material is related to the dry density and fine particle content. The glacial soil with a high fine particle content has a higher density and greater soil stiffness. Therefore, the fine particle content and optical flow information are both input features of the algorithm. The algorithm extracts the characteristic details of the broken image based on the content in the image, as well as the input fine particle content and optical flow information, and obtains the crack topology structure. The optical flow information includes the light flux and the illumination size. The input optical flow information and fine particle content are used to eliminate interference factors in the image (such as shadows, reflective areas, etc.), and retain the complete crack topology structure. The elimination method is a conventional technology in image optimization and will not be repeated here.

[0025] Due to the high speed of impact loading and the high resolution of high-speed photography, fill lights are needed to increase exposure. LED array fill lights are used to provide 45° oblique or circular fill lights on both sides.

[0026] The trigger condition for high-speed shooting is dynamic self-triggering. When the captured image changes, storage starts automatically. The storage time is usually no more than 1ms.

[0027] Super-resolution technology: Due to the high impact velocity, even with fill light, it's difficult to avoid the problem of low image quality in high-speed photography. Therefore, super-resolution technology is used to improve image quality and better prepare for further crack identification. Given that the images in this experiment represent a single specimen surface, the color and structure are relatively simple. Therefore, the super-resolution technology used is image interpolation. Specifically, the grayscale values ​​of known neighboring pixels are used to generate the grayscale values ​​of unknown pixels, thereby regenerating an image with higher resolution from the original image. This method only references the current single low-resolution image and achieves image quality improvement by changing the number of image pixels.

[0028] Contrast enhancement algorithm: Contrast enhancement is intended to highlight the difference between cracks and intact specimen surfaces, making them more suitable for machine analysis and processing, and simplifying crack identification. This technique can be implemented in many ways, such as linear transformation, gamma transformation, and histogram equalization. This example uses histogram equalization, which primarily enhances contrast by stretching the range of pixel values.

[0029] Feature extraction: Refers to the characteristics of the surface points of the specimen during the impact process, such as deformation, displacement, cracking, etc.

[0030] Quantitative impact: Currently, existing SHPB tests basically count the damage to the specimen after the impact is completed. Even with the cooperation of high-speed photography, it is often only to analyze the damage process of the specimen. Real-time analysis after time alignment with the test data has not yet been carried out. The above technology can automatically count the evolution of cracks during the impact process, the changes in fragment size, and the changes in the broken fracture surface morphology, and then analyze the real-time impact of different initial or loading conditions on material damage. Therefore, this quantitative impact analysis is not just a quantitative analysis after the loading is completed in the traditional sense, but a quantitative analysis of the entire loading process. The time history characteristics are the parameters in the above quantitative analysis, including time parameters, position parameters, crack evolution parameters, etc.

[0031] Please refer to Figure 2 Taking the typical frozen moraine soil in Linzhi area of ​​Qinghai-Tibet Plateau as the research object, the impact test of frozen moraine soil was carried out using large diameter SHPB equipment combined with environmental chamber. The stress-strain curves of frozen moraine soil with different strain rates, different freezing temperatures and different initial moisture contents were obtained, and the differences in their failure modes were analyzed. Figure 2 The results of crack growth calculations of representative volume elements of frozen moraine soil under impact loading are shown in the figure. Figure 2 (a) shows crack growth at a freezing temperature of -25°C, (b) at a freezing temperature of -15°C, and (c) at a freezing temperature of -5°C. Based on this, a dynamic constitutive model for frozen moraine soil was constructed using peridynamic theory, debonding damage theory, and the ZWT model. This model is used to predict the macroscopic mechanical properties and mesoscopic damage evolution of frozen moraine soil under impact loading. This model helps to understand the impact damage mechanism and dynamic mechanical properties of frozen moraine soil. It also provides a tool for calculating the macroscopic dynamic mechanical properties and mesoscopic damage of frozen moraine soil under impact loading.

[0032] Because the failure strain of brittle materials like frozen moraine is very small, direct impact loading makes it difficult to ensure that the stress equilibrium requirements of the SHPB experiment are met before the specimen fails. To this end, this embodiment utilizes waveform shaping technology. A waveform shaper 1020 is added, and an 18×8×1.5mm (outer diameter×inner diameter×thickness) copper spacer is placed between the impact rod and the incident rod 1021. This extends the pulse rise time and reduces high-frequency oscillations of the incident wave front. The resulting shaped incident wave has a rise time of approximately 150°, which is sufficient to meet the requirements of dynamic stress equilibrium. A smaller amplitude also yields more stable and clear sample image videos.

[0033] The specific parameters of the SHPB equipment used are as follows: 1) The incident rod, transmission rod 1023, and impact rod are made of 35CrMn alloy steel, with a wave velocity of 5190 m / s; 2) the incident and transmission rods are 75 mm in diameter; 3) the impact rod is 25 cm long, the incident rod is 120 cm long, and the transmission rod is 120 cm long. Because the failure strain of brittle materials such as frozen moraine is very small, direct impact loading would be difficult to ensure that the stress equilibrium requirements of the SHPB test are met before the specimen fails.

[0034] In addition to the above examples, to ensure the integrity and uniformity of frozen moraine soil samples, pre-treatment of the samples is also required. Traditional frozen soil (frozen clay or clayey soil) samples are typically prepared by mixing dry soil and water, allowing it to stand, and then freezing it in a mold. However, due to moraine soil's poor water-holding capacity, the ice produced by freezing the mixture directly after standing is insufficient to bind the moraine soil particles together to form frozen soil. To this end, the sample preparation method used is as follows: 1) Place the moraine soil in a drying oven and dry it at 105°C for 12 hours; 2) Remove the soil, seal it, cool it for 2 hours, and then weigh it; 3) Calculate the mass of water required to prepare the sample with the desired initial moisture content; 4) Use purified water to make some ice cubes and crush them into very fine ice chips; 5) Mix 60% of the ice chips with the dried moraine soil; 6) Add 40% of water to the ice chips and dry moraine soil mixture and mix thoroughly; 7) Place the mixture into a mold (with internal dimensions of 75 mm diameter and 25 mm height), compact it with a compactor, and freeze it in a set temperature environmental chamber for 24 hours. The dimensions are 75 × 45 mm (diameter × height).

[0035] The sample preparation described above achieves a uniform offset of the transmitted wave by matching the sample dimensions with the wave shaper. Specifically, the sample dimensions, incident rod, transmission rod, and wave shaper cross-section ratios are 75:75:75:18, and the ratio of the inner and outer diameters of the transmitted wave shaper is 18:8. This optimization achieves optimal image quality and sample stress-strain response.

[0036] In summary, high-speed photography was used to capture videos of frozen moraine soil specimens undergoing impact testing. Computer super-resolution technology was then used to process the high-speed video footage during the impact process, improving its resolution and clarity. Utilizing an automated algorithm for enhancing the spectral contrast between surface cracks and the specimen itself, target tracking techniques from computer image processing, and a convolutional neural network algorithm, characteristic information was extracted from the entire process of surface crack evolution, penetration, and sample fragmentation during the impact test. This information was then time-aligned with the SHPB impact test results to generate time-course data on the fragmentation process of frozen moraine soil. Artificial intelligence analysis was used to autonomously analyze the failure characteristics (number of fragments, size distribution, fracture surface morphology, and fracture characteristics) of frozen moraine soil specimens under different impact loading rates and initial conditions (temperature, initial moisture content, gradation, etc.), quantifying the effects of loading rate and initial conditions on the failure characteristics of frozen moraine soil.

Claims

1. A crack analysis method for SHPB impact testing, characterized in that: The shooting module is set in the pulse direction to obtain impact images and record the time history images of the frozen moraine soil sample at different impact rates; The resolution of the impact image is optimized based on the image interpolation method to obtain the processed image. The difference area between the crack and the undamaged sample is enhanced based on the histogram equalization method, and the features of the difference area are extracted. The time history features are summarized through the time history image and input into the target tracking model. The topological structure of the sample crack is output, and the geometric features of the sample fragments are quantified to obtain the dynamic model of the destruction characteristics of the frozen moraine soil sample under the corresponding impact rate.

2. A crack analysis method for SHPB impact testing according to claim 1, characterized in that: The fill light module is activated according to the impact time.

3. A crack analysis method for SHPB impact testing according to claim 2, characterized in that: In the image interpolation method, the grayscale values ​​of adjacent pixels in a single initial image are used to generate the grayscale values ​​of unknown pixels.

4. A crack analysis method for SHPB impact testing according to claim 3, characterized in that: Histogram equalization increases contrast by stretching the range of pixel values.

5. A crack analysis method for SHPB impact testing according to claim 4, characterized in that: The topological structure includes the fracture branching structure and the fracture through-structure.

6. A crack analysis method for SHPB impact testing as claimed in claim 5, characterized in that: Geometric characteristics include equivalent diameter and edge angle.

7. A crack analysis system for SHPB impact testing, comprising: a SHPB testing system applied to a crack analysis method for SHPB impact testing according to claim 6, characterized in that: It also includes shooting module and fill light module; The SHPB test system includes an impact rod, an incident rod, an output rod, and a strain module. The shooting module and the fill light module are symmetrically arranged on both sides of the sample with the sample as the center of symmetry.

8. A crack analysis system for SHPB impact testing as claimed in claim 7, characterized in that: The frame rate of the shooting module is not less than 500,000 fps.

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