In-situ measurement method for corrosion fatigue crack growth rate based on synchrotron radiation light source

Abstract

The application discloses a kind of in-situ corrosion fatigue crack propagation rate measurement methods based on synchrotron radiation source, which includes carrying out in-situ corrosion fatigue test based on three-dimensional imaging of synchrotron radiation source;After the sample is subjected to a predetermined number of cycle stress loading, the projection image of the coaxial phase contrast CT of the different fault of the sensitive area of the sample is collected using the synchrotron radiation source;Determine whether the sample is fractured and failed, if so, go to the next step, otherwise return to the previous step;The projection image is preprocessed, and the maximum length crack in all preprocessed projection images corresponding to the same cycle is extracted as the fatigue crack;According to the length of the corresponding fatigue crack and the interval cycle number when the adjacent two projection images are collected, the average crack propagation rate during the adjacent two projection image collection periods is calculated.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of fatigue crack testing, and particularly relates to a method for measuring in-situ corrosion fatigue crack growth rate based on a synchrotron radiation source. BACKGROUND

[0002] When metal components are used in important fields such as marine machinery, aerospace, oil and gas exploitation, and rail transportation, they usually undergo the combined action of multiple forms of load and corrosion environment. These loads include static and dynamic stress, and the service environment includes atmospheric corrosion, seawater corrosion, salt spray and other conditions. Under such complex service conditions, metal components not only have to bear mechanical stress, but also have to resist damage caused by environmental media. Fracture mechanics parameters, such as fatigue crack growth rate and crack propagation threshold, are key indicators for metal material engineering applications, and corrosion fatigue crack growth rate is one of the important parameters for evaluating the corrosion fatigue fracture resistance of metal materials.

[0003] In a salt spray or salt solution environment, the fatigue crack problem of metal materials becomes extremely complex, mainly because the reaction of these corrosion environments with the crack tip and crack surface of the metal material produces a large amount of corrosion products and hydrogen bubbles, etc. The crack closure effect caused by corrosion products may cause the crack propagation rate to decrease, while anodic dissolution may cause the crack propagation rate to increase, and the hydrogen atoms extending to the crack tip may reduce the plasticity of the crack tip and thus increase the corrosion fatigue crack propagation rate. In addition, due to the existence of secondary cracks (secondary cracks), the crack propagation model becomes more complex, and the appearance of secondary cracks makes the crack propagation path complex, even forms a crack network, and increases the difficulty of measuring the crack propagation length. At the same time, secondary cracks will cause the stress field in the material to redistribute, thereby changing the stress concentration state near the main crack, affecting the crack propagation rate and direction. The superposition of these factors affects the accuracy of the corrosion fatigue crack propagation rate measurement.

[0004] At present, in the laboratory and engineering applications, often use such as potential difference method, replication method (or called copy method) and infrared thermal imaging technology to measure fatigue crack propagation rate. Potential difference method by applying a constant current to the sample, and using the principle of conductor cross-sectional area change caused by current field change to monitor the crack length. This method requires very high requirements for insulation measures, otherwise the leakage of the fixture will cause the parallel resistance between the test piece, which will seriously affect the accuracy of the test results. In addition, the potential difference method can only provide discrete point data, and cannot obtain the information of the whole crack propagation area, so it is difficult to fully describe the occurrence and development mechanism of the crack. Some researchers measure the fatigue crack propagation rate under salt spray conditions by direct current potential difference method, but this method deposits salt on the sample surface by salt printing and salt transfer methods, but the uniformity and accuracy of salt deposition by these two methods are difficult to guarantee, which directly affects the consistency of the experimental results. Replication method can provide higher measurement accuracy, but it needs to interrupt the experiment several times at different stages of the experiment to measure the crack length, which not only prolongs the total length of the experiment, but also affects the corrosion rate during the experiment, thereby affecting the accuracy of the corrosion fatigue crack propagation rate measurement. As a non-contact detection means, infrared thermal imaging technology can estimate the position of the crack tip by analyzing the temperature distribution image of the sample surface, and then calculate the crack growth. However, this method requires a series of complex pretreatment of the sample, and the temperature fluctuation of the external environment may significantly affect the test results. Especially in the case of small temperature difference on the sample surface and low contrast of the thermal image, the accuracy of the measurement will be greatly reduced, which is obviously not suitable for the measurement of fatigue crack propagation rate in corrosive environment.

[0005] In summary, the existing technical means are still difficult to observe and record the crack propagation behavior in real time, and also cannot accurately capture the growth of the crack and the measurement of its length. SUMMARY

[0006] In view of the above problems in the prior art, the in-situ corrosion fatigue crack propagation rate measurement method based on the synchrotron radiation source provided by the present application solves the problem that the prior art cannot accurately capture the growth of the crack and the measurement of its length.

[0007] In order to achieve the above-mentioned purpose of the application, the technical scheme adopted by the present application is:

[0008] The present application provides an in-situ corrosion fatigue crack propagation rate measurement method based on a synchrotron radiation source, which comprises the following steps:

[0009] S1, install the in-situ corrosion fatigue testing machine on the three-axis displacement turntable of the synchrotron radiation source, and install the sandglass-shaped sample on the in-situ corrosion fatigue testing machine, so that it is coaxial with the three-axis displacement turntable;

[0010] S2, set the environment of the sample to be a corrosive medium environment, control the ceramic heating sheet of the in-situ corrosion fatigue testing machine to heat, and keep the temperature constant to the target temperature;

[0011] S3, after the sample is subjected to stress loading for a preset number of cycle times, the preset loading force is maintained, and the projection images of the in-line phase contrast CT of different sections of the sensitive area of the sample are collected under sample rotation using a synchrotron radiation source;

[0012] S4, determine whether the sample has failed, if yes, go to step S5, otherwise return to step S3;

[0013] S5, pre-process the projection images of the in-line phase contrast CT, and extract the maximum length crack in all pre-processed projection images corresponding to the same cycle time as the fatigue crack;

[0014] S6, according to the length of the fatigue crack corresponding to the adjacent two times of collecting projection images and the number of cycle times of the interval, calculate the average crack propagation rate during the adjacent two times of collecting projection images.

[0015] Further, the method for extracting the maximum length crack corresponding to the same cycle time comprises:

[0016] S51, for all pre-processed projection images corresponding to the same cycle time, the gray value of the pixel point of the projection image is sequentially stored in each row of a three-dimensional matrix according to the projection image collection order;

[0017] S52, use an edge detection algorithm to obtain the pixel points containing the sample outline in the three-dimensional matrix row by row, and form a plurality of outline matrices;

[0018] S53, for each outline matrix, respectively extract the pixel points with the same horizontal coordinate and the pixel points with the same vertical coordinate, and calculate the horizontal coordinate mean value of the pixel points with the same horizontal coordinate and the vertical coordinate mean value of the pixel points with the same vertical coordinate;

[0019] S54, select the pixel points in each outline matrix whose horizontal and vertical coordinates are equal to the maximum horizontal coordinate mean value and the maximum vertical coordinate mean value as the center of the circle;

[0020] S55, calculate the distance between the outline pixel points in each outline matrix and their corresponding center of the circle, and perform probability statistics, and use the distance with the highest occurrence probability as the radius r of the circle;

[0021] S56, for the circle corresponding to each pre-processed projection image, assign the pixel points with a distance from the center of the circle exceeding the radius to 0, and assign the remaining pixel points to 1, to obtain matrix one;

[0022] S57, threshold segmentation is performed on the preprocessed projection image, pixel points higher than a preset threshold in the same projection image are assigned as 0, and the rest of the pixel points are assigned as 1, to obtain matrix two;

[0023] S58, the matrix one and the matrix two corresponding to the same projection image are multiplied element by element to obtain a crack region, and then edge detection is performed on the crack region, the gray value of the pixel point of the crack edge is retained, and the rest of the pixel points are assigned as 0;

[0024] S59, the distance l of the edge pixel point in each projection image to the center of the circle is calculated, the crack length a = r-l, and the maximum length crack in all projection images corresponding to the same cycle is selected as the fatigue crack.

[0025] Further, the method for extracting the maximum length crack corresponding to the same cycle comprises:

[0026] A1, all preprocessed projection images corresponding to the same cycle are imported into a three-dimensional reconstruction software, and points with brightness greater than a preset value in the projection image are filtered;

[0027] A2, by means of brightness inversion command, the matrix and the crack region are respectively assigned with a first gray value and a second gray value, and the crack and the pore part are selected by means of gray threshold, wherein the first gray value is less than the second gray value;

[0028] A3, numerical analysis is performed on the extracted crack and pore to obtain the volume, surface area, length and width of the crack and the pore, the crack data with a size greater than a size threshold is retained, and a crack entity model is generated;

[0029] A4, the length of the crack entity model is obtained by using a ruler tool in the three-dimensional reconstruction software, and the maximum length crack in all projection images corresponding to the same cycle is selected as the fatigue crack.

[0030] Further, the method for preprocessing the projection image of the on-axis phase contrast CT comprises:

[0031] The projection image of the on-axis phase contrast CT is input into an image algorithm software PITER or PITRE_BM, FlatDark+ is selected to perform interval background sampling, and parameters are adjusted to realize phase recovery of the projection image;

[0032] A single projection image is selected to generate a single sine image, and the sample rotation axis is adjusted;

[0033] The value of Ring's Width is set to correct the ring-shaped artifacts of all projection images, and then the 32-bit projection image is converted into 8-bit data to obtain the preprocessed projection image.

[0034] Further, each cycle refers to a complete stress variation process from minimum stress to maximum stress and back to minimum stress during stress loading.

[0035] Further, the average crack propagation rate is calculated as follows:

[0036]

[0037] Wherein, is the average crack propagation rate between the i-1 time and the i time; the i time corresponds to the i time of completing the preset number of cycle loading; a i and a i-1 are the fatigue crack lengths at the i-1 time and the i time, respectively; N i and N i-1 are the cumulative cycle numbers completed by the sample at the i time and the i-1 time, respectively.

[0038] Further, the preset loading force is the average of the maximum stress and the minimum stress of each cycle.

[0039] Compared with the prior art, the beneficial effects of the present application are:

[0040] 1、The present scheme can collect projection images of the coaxial phase contrast CT of the sensitive area of the sample by using the existing in-situ corrosion fatigue testing machine and the synchrotron radiation source. Compared with the commonly used potential difference method, the replication method and the infrared thermal imaging measurement method, the method saves the construction of the observation equipment in the measurement process, and the method has wide applicability and can be popularized to the measurement of crack propagation rate of metals and other materials in complex environments.

[0041] 2、The present scheme collects images under the rotating sample when collecting projection images. The three-dimensional imaging data obtained by this method can calculate the crack propagation rate of the sample in multiple directions.

[0042] In addition, the present scheme uses the distance from the edge pixel point to the center of each projection image to obtain the crack length. The whole process does not need manual annotation or software adjustment, but can automatically process and analyze, which can greatly reduce manual intervention and improve accuracy.

[0043] 3、The present scheme applies the synchrotron radiation source technology to the corrosion fatigue crack propagation test, and uses the X-ray phase contrast imaging method to accurately obtain the crack propagation data. BRIEF DESCRIPTION OF DRAWINGS

[0044] Figure 1 It is a schematic diagram of the structure of an hourglass-shaped sample.

[0045] Figure 2A flow chart of the in-situ corrosion fatigue crack propagation rate measurement method based on the synchrotron radiation source.

[0046] Figure 3 A schematic diagram of the in-situ corrosion fatigue forming principle.

[0047] Figure 4 A three-dimensional image formed by stacking all projection images corresponding to the same cycle.

[0048] Figure 5 A schematic diagram of the center and radius of the contour matrix.

[0049] Figure 6 A crack length schematic diagram.

[0050] Figure 7 A curve of the corrosion fatigue crack length and propagation rate evolution with cycle.

[0051] Figure 8 A corrosion fatigue crack propagation rate versus stress intensity factor diagram.

[0052] 1, sample; 11, sensitive area; 2, synchrotron radiation source; 21, monochromator; 22, detector; 3, in-situ corrosion fatigue testing machine; 4, corrosion medium. DETAILED DESCRIPTION

[0053] The specific embodiments of the present application are described below to facilitate the understanding of the present application for those skilled in the art, but it should be clear that the present application is not limited to the scope of the specific embodiments, and for those skilled in the art, any changes that are obvious within the spirit and scope of the present application as defined and determined by the appended claims are obvious, and all applications utilizing the concept of the present application are within the scope of protection.

[0054] REFERENCE Figure 2 , Figure 2 A flow chart of the in-situ corrosion fatigue crack propagation rate measurement method based on the synchrotron radiation source 2 is shown; as shown in Figure 1 , the method S includes steps S1-S6.

[0055] In step S1, the in-situ corrosion fatigue testing machine 3 is installed on the three-axis displacement turntable of the synchrotron radiation source 2, and the sandglass-shaped sample 1 is installed on the in-situ corrosion fatigue testing machine 3, coaxial with the three-axis displacement turntable. The specific structure of the in-situ corrosion fatigue testing machine 3 of the present scheme can refer to patent application CN202310083984.3. When installing the sample 1, the automatic centering function of the in-situ corrosion fatigue testing machine 3 is used to apply a preset load to ensure that the sample 1 is coaxial with the shaft.

[0056] As shown in Figure 1As shown, the hourglass-shaped specimen 1 of this embodiment is designed with dimensions and surface roughness according to the national standard GB / T 6398-2017, "Metallic Materials Fatigue Test - Fatigue Crack Growth Method." The hourglass-shaped specimen 1 used in the present invention has a length of F, a clamping end diameter of G, a straight end diameter of C, and a length of E. Figure 1 The areas with larger diameters at both ends are the clamping sections of the specimen 1, and the area with smaller dimensions in the middle is the sensitive area 11, which is the area where cracks are generated and eventually break during the test.

[0057] like Figure 3 As shown, the sensitive area 11 of the sample 1 is located on the same straight line as the monochromator 21 and the detector 22 of the synchrotron radiation light source 2, to ensure that the X-ray beam is focused on the sensitive area 11 of the sample 1 so as to accurately collect image information.

[0058] Before the test begins, the test parameters and imaging parameters need to be determined. The test parameters include the diameter d of the sample 1, the stress amplitude σ a , stress ratio R and fatigue frequency f, through the formula S=π(d / 2) 2 , T=1 / f, σ max =2σ m / (1-R),σ min =Rσ max Calculate cross-sectional area S, cycle period T, and stress peak σ max and stress valley value σ min ; The imaging parameters involve the settings of light source exposure time and sampling frequency.

[0059] In step S2, the environment of the sample 1 is set to the corrosive medium 4 environment (specifically salt spray solution / salt spray), the ceramic heating plate of the in-situ corrosion fatigue testing machine 3 is controlled to heat (specifically, the corrosive medium 4 in which the sample 1 is located is heated), and the temperature is kept constant to the target temperature.

[0060] In step S3, after stress loading specimen 1 for a preset number of cycles, the preset loading force is maintained and, while specimen 1 rotates, coaxial phase-contrast CT projection images of different slices of sensitive region 11 of specimen 1 are collected using synchrotron radiation source 2. During each image acquisition, the height of specimen 1 is adjusted using a three-axis translation turntable to acquire projection images of each slice, layer by layer, from the top of sensitive region 11 downward. Once the entire sensitive region 11 is acquired, the process proceeds to the next step.

[0061] During implementation, this solution preferably defines each cycle as a complete stress change process from minimum stress to maximum stress and then back to minimum stress during stress loading; the preset loading force is the average value of the sum of the maximum stress and the minimum stress of each cycle.

[0062] In step S4, it is judged whether the sample 1 is fractured and failed, if yes, step S5 is entered, otherwise, step S3 is returned;

[0063] In step S5, the projection images of the coaxial phase contrast CT are preprocessed, and the maximum length crack in all the preprocessed projection images corresponding to the same cycle is extracted as the fatigue crack;

[0064] In an embodiment of the present application, the method for preprocessing the projection images of the coaxial phase contrast CT comprises:

[0065] The projection images of the coaxial phase contrast CT are input into the image algorithm software PITER or PITRE_BM, the FlatDark+ is selected for interval background sampling, and the parameters are adjusted (here, the parameters can be manually adjusted to appropriate parameters) to realize the phase recovery of the projection images.

[0066] A single projection image in the middle position of the sensitive area 11 of the sample 1 is selected to generate a single sine image, and the rotation axis of the sample 1 is adjusted; this is mainly due to the technical problems of light source line station and the stability problems of the testing machine, which cause semicircular artifacts and ring artifacts in the image, affecting the imaging effect, and setting appropriate rotation axis parameters can effectively eliminate the semicircular artifacts and display the internal defect characteristics of the sample 1.

[0067] The value of the Width of the Ring is set to correct the ring artifacts of all the projection images, and then the 32-bit projection images are converted into 8-bit data to obtain the preprocessed projection images.

[0068] In step S6, according to the length of the fatigue crack corresponding to the adjacent two times of collecting projection images and the interval cycle number, the average crack propagation rate during the adjacent two times of collecting projection images is calculated:

[0069]

[0070] Wherein, is the average crack propagation rate between the i-1 time and the i time; the i time corresponds to the i time of completing the preset number of cycle loading; a i and a i-1 are the fatigue crack lengths at the i-1 time and the i time, respectively; i and N i-1 are the cycle numbers of the sample 1 accumulated at the i time and the i-1 time, respectively.

[0071] In the present scheme, two methods for extracting the maximum length crack are provided, which are specifically introduced as follows:

[0072] The first method for extracting the maximum length crack corresponding to the same cycle comprises:

[0073] S51, for all pre-processed projection images corresponding to the same cycle, in the order of projection image acquisition, the gray value corresponding to the pixel point of the projection image is stored in each row of a three-dimensional matrix, and the three-dimensional matrix is equivalent to a three-dimensional image formed by stacking, which can be referred to in detail Figure 4 .

[0074] S52, the edge detection algorithm is used to obtain the pixel points containing the sample 1 outline in the three-dimensional matrix row by row, and a plurality of outline matrices are formed;

[0075] S53, for each outline matrix, the pixel points with the same horizontal coordinate and the pixel points with the same vertical coordinate are extracted respectively, and the horizontal coordinate mean value of the pixel points with the same horizontal coordinate and the vertical coordinate mean value of the pixel points with the same vertical coordinate are calculated respectively;

[0076] S54, the pixel point with the horizontal and vertical coordinates equal to the maximum horizontal coordinate mean value and the maximum vertical coordinate mean value in each outline matrix is selected as the center of the circle;

[0077] S55, the distance between the outline pixel points in each outline matrix and the corresponding center of the circle is calculated, and probability statistics is performed, and the distance with the highest occurrence probability is used as the radius r of the circle. The center of the circle and the radius measurement principle diagram corresponding to the outline matrix can be referred to Figure 5 .

[0078] S56, for the circle corresponding to each pre-processed projection image, the pixel points with a distance from the center of the circle exceeding the radius are assigned a value of 0, and the remaining pixel points are assigned a value of 1, to obtain matrix one;

[0079] S57, the pre-processed projection image is subjected to threshold segmentation processing, the pixel points higher than the preset threshold in the same projection image are assigned a value of 0, and the remaining pixel points are assigned a value of 1, to obtain matrix two;

[0080] S58, the matrix one and the matrix two corresponding to the same projection image are subjected to element-by-element multiplication operation to obtain the crack region, and then the crack region is subjected to edge detection, the gray value of the pixel points of the crack edge is retained, and the remaining pixel points are assigned a value of 0;

[0081] S59, the distance l of the edge pixel points in each projection image to the center of the circle is calculated, and the crack length a = r-l. The principle diagram of the crack length can be referred to Figure 6 ; then the maximum length crack in all projection images corresponding to the same cycle is selected as the fatigue crack.

[0082] The second method for extracting the maximum length crack corresponding to the same cycle comprises:

[0083] A1, import all pre-processed projection images corresponding to the same cycle into the three-dimensional reconstruction software (data processing in Avizo), filter the points in the projection image with brightness greater than the preset value to obtain a suitable gray scale range, i.e. select the matrix part of sample 1 without defects.

[0084] A2, by brightness inversion command, the matrix, crack zone is respectively given first gray scale value and second gray scale value, and the crack and pore part is selected by gray scale threshold, wherein the first gray scale value is less than the second gray scale value;

[0085] A3, numerical analysis is carried out on the extracted crack and pore to obtain the volume, surface area, length and width of the crack and pore, the crack data with size greater than the size threshold is retained, and the crack entity model is generated;

[0086] A4, the length of the crack entity model is obtained by using the ruler tool in the three-dimensional reconstruction software, and the maximum length crack in all projection images corresponding to the same cycle is selected as the fatigue crack.

[0087] The following takes 7050-T7451 aluminum alloy as an example, and the corrosion fatigue crack propagation rate method proposed in the scheme is used to successfully measure the fatigue crack propagation rate of pre-corrosion 7050 aluminum alloy in 3.5wt.% NaCl solution, and the specific test conditions are as follows:

[0088] Based on the 2BL16U2 imaging line station of Shanghai Synchrotron Radiation Facility, the test is carried out by using in-situ corrosion fatigue testing machine 3, the sample 1 is 1mm in diameter, the corrosion environment is 3.5wt.% NaCl solution, and the stress amplitude is 320MPa.

[0089] The in-situ imaging data of sample 1 is obtained by using steps S2-S4, and then the data processing method of steps S51-S59 is used to obtain the evolution process of corrosion fatigue crack length and corrosion fatigue crack propagation rate with cycle, and the results are shown in Figure 7 and Figure 8 As can be seen from the two figures, the crack propagation rates of four corrosion fatigue cracks on the same sample are measured simultaneously by the scheme, and the lengths of the corrosion fatigue cracks are positively correlated with the cycle.

Claims

1. In-situ corrosion fatigue crack growth rate measurement method based on synchrotron radiation source, characterized in that: Including steps: S1. Install the in-situ corrosion fatigue testing machine on the three-axis displacement turntable of the synchrotron radiation source, and install the hourglass-shaped specimen on the in-situ corrosion fatigue testing machine so that it is coaxial with the three-axis displacement turntable; S2. Set the sample environment to a corrosive medium environment, control the heating of the ceramic heating plate of the in-situ corrosion fatigue testing machine, and keep the temperature constant to the target temperature; S3. After the sample is subjected to a preset number of cyclic stress loading cycles, the preset loading force is maintained, and while the sample is rotating, coaxial phase-contrast CT projection images of different sections of the sensitive area of ​​the sample are collected using a synchrotron radiation light source; S4, determine whether the sample is broken and failed, if so, go to step S5, otherwise return to step S3; S5. Preprocessing the projection images of the coaxial phase contrast CT, and extracting the maximum length crack in all preprocessed projection images corresponding to the same cycle as the fatigue crack; Methods for extracting the maximum length crack corresponding to the same cycle include: S51. For all pre-processed projection images corresponding to the same cycle, sequentially store the grayscale values ​​corresponding to the pixels of the projection images into each row of a three-dimensional matrix according to the order in which the projection images are acquired; S52, using an edge detection algorithm to obtain pixel points containing the sample contour in a three-dimensional matrix row by row to form multiple contour matrices; S53. For each contour matrix, extract the pixel points with the same abscissa and the pixel points with the same ordinate, and calculate the abscissa mean of the pixel points with the same abscissa and the ordinate mean of the pixel points with the same ordinate; S54, selecting the pixel point whose horizontal and vertical coordinates of the pixel point in each contour matrix are equal to the maximum horizontal coordinate mean and the maximum vertical coordinate mean as the center of the circle; S55. Calculate the distance between each contour pixel in the contour matrix and its corresponding circle center, and perform probability statistics, using the distance with the highest probability of occurrence as the radius of the circle. r ; S56. For each circle corresponding to the pre-processed projection image, assign the pixel points whose distance from the center of the circle exceeds the radius to 0, and assign the remaining pixel points to 1, to obtain a matrix 1; S57, performing threshold segmentation processing on the pre-processed projection image, assigning a value of 0 to the pixels above the preset threshold in the same projection image, and assigning a value of 1 to the remaining pixels, to obtain a second matrix; S58, performing an element-by-element multiplication operation on the matrix 1 and the matrix 2 corresponding to the same projection image to obtain a crack area, then performing edge detection on the crack area, retaining the grayscale values ​​of the pixels at the crack edge, and assigning the remaining pixels to 0; S59, calculate the distance from the edge pixel point to the center of the circle in each projected image l , crack length a=r-l , select the maximum length crack in all projection images corresponding to the same cycle as the fatigue crack; Alternatively, the method for extracting the maximum length crack corresponding to the same cycle includes: A1, importing all pre-processed projection images corresponding to the same cycle into 3D reconstruction software, and filtering points in the projection images with brightness greater than a preset value; A2. Use the brightness inversion command to assign the first grayscale value and the second grayscale value to the matrix and the crack area respectively, and select the crack and pore parts using the grayscale threshold, where the first grayscale value is smaller than the second grayscale value; A3. Perform numerical analysis on the extracted cracks and pores to obtain the volume, surface area, length, and width of the cracks and pores, retain the crack data with a size greater than the size threshold, and generate a crack solid model; A4. Use the ruler tool in the 3D reconstruction software to obtain the length of the crack solid model, and select the crack with the maximum length in all projection images corresponding to the same cycle as the fatigue crack; S6. Calculate the average crack growth rate during the period between two adjacent projection image acquisitions based on the lengths of the fatigue crack corresponding to the two adjacent projection image acquisitions and the number of cycles between the two intervals.

2. The in-situ corrosion fatigue crack growth rate measurement method according to claim 1, characterized in that: The method for preprocessing the projection image of the in-line phase contrast CT includes: Input the projection images of the in-line phase contrast CT into the image algorithm software PITER or PITRE_BM, select FlatDark+ for interval background sampling, and adjust the parameters to achieve phase recovery of the projection images; Select a projection image in the middle of the sensitive area of ​​the sample to generate a single sinusoidal image, and adjust the rotation axis of the sample; The Ring Width value is set to correct the ring artifacts of all projection images. Then, the 32-bit projection images are converted to 8-bit data to obtain the preprocessed projection images.

3. The in-situ corrosion fatigue crack growth rate measurement method according to claim 1, characterized in that: Each cycle refers to the complete stress change process from minimum stress to maximum stress and then back to minimum stress during the stress loading process.

4. The in-situ corrosion fatigue crack growth rate measurement method according to claim 1, characterized in that: The calculation expression of the average crack growth rate is: in, For the i -1 time to i The average crack growth rate between moments; i The moment corresponds to the i Complete the preset number of cycles of loading; a i and a i-1 Respectively i -1 moment and i Fatigue crack length at time; and N i-1 Respectively i Moment and i -1 is the cumulative number of cycles completed by the sample at time 1.

5. The in-situ corrosion fatigue crack growth rate measurement method according to claim 1, characterized in that: The preset loading force is the average value of the sum of the maximum stress and the minimum stress of each cycle.

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