Method for acquiring circumferential mechanical property parameters of pipe material

Abstract

The present invention belongs to the field of manufacturing engineering / digital information, and provides a method for acquiring circumferential mechanical property parameters of a pipe material. In order to obtain a more accurate and comprehensive description of a strain hardening behavior of a material, it is required to use a true stress-strain curve, such that the present invention uses a flexible tensile test fixture to perform a tensile test on a circumferential tensile pipe material sample. The tensile test process comprises: using a non-contact video extensometer and a universal tensile testing machine to obtain tensile measurement data of the pipe material; then preprocessing the tensile measurement data of the pipe material; using the preprocessed data for calculating true stress-true strain data of the circumferential tensile pipe material sample; and setting initial and end strains of a plastic section, and using a linear regression fitting method for calculating a strength coefficient and a strain hardening exponent in the plastic section. The strength coefficient and the strain hardening exponent in the plastic section and the true stress-true strain data of the circumferential tensile sample are used for finite element analysis of pipe systems.

Description

Method for obtaining circumferential mechanical properties parameters of pipes Technical Field

[0001] This invention belongs to the field of manufacturing engineering and digital information, and more particularly relates to a method for obtaining circumferential mechanical property parameters of pipes. A flexible tensile test fixture has been developed to perform tensile tests on pipes of various specifications. This method efficiently and accurately obtains circumferential mechanical test data and calculates circumferential mechanical property parameters suitable for finite element simulation. Background Art

[0002] As an important component of the power unit, the safety and reliability of the piping system directly affect the overall safety of the structure. Hydraulic, fuel, lubricating oil, environmental control, oxygen and other piping systems are commonly distributed inside complex product structures. These piping systems have different shapes and are intricately distributed. They play the role of pressure transmission, fuel delivery, cable protection, etc. For example, the rupture of high-pressure pipelines is the main cause of engine failure. Traditional methods such as fluid-solid coupling vibration and fatigue life failure analysis have the problems of complex parts and difficult processing. Finite element analysis of the strength of the piping system can provide a basis for the design and optimization of the system. The accuracy of the finite element analysis depends on the accuracy of the material parameters. There are relevant standards for the axial mechanical property test of pipes, but there are no relevant standards for the circumferential mechanical property test. Therefore, it is necessary to quickly and accurately obtain the circumferential performance of the pipe and calculate the parameters required for the finite element analysis.

[0003] Traditional mechanical property testing typically uses strain-gauge extensometers, which have a very limited range. Contact measurement methods can affect the test piece, and each extensometer requires individual calibration. Wear and tear can also affect measurement accuracy as the number of tests increases. In destructive tensile testing, it's impossible to track specimens until they reach significant deformation or rupture. Data obtained after removing the extensometer is extrapolated, resulting in limited accuracy.

[0004] Using digital image correlation to measure specimen deformation effectively circumvents these shortcomings. This method tracks speckle patterns on the material surface, tracking features at specific locations to measure material strain. By performing correlation matching on the grayscale features surrounding a point in the speckle pattern before and after deformation, the position of that point can be tracked under different deformation conditions. The measurement is non-contact, ensuring the specimen is unaffected by any external forces. High measurement accuracy and a wide range allow tracking until the specimen is completely destroyed, providing accurate data throughout the entire test.

[0005] Accurately and comprehensively describing a material's strain hardening behavior requires a true stress-strain curve. This is because the stress and strain in an engineering stress-strain curve are measured using the original cross-sectional area and original gauge length of the specimen's gauge section and do not represent the actual instantaneous stress and strain. Hoop performance testing, however, cannot directly provide true stress-strain data; instead, data processing and calculation are required to generate the stress-strain data used in finite element simulations.

[0006] Summary of the Invention

[0007] The present invention aims to solve the problems existing in the prior art and proposes a method for obtaining the circumferential mechanical property parameters of a pipe.

[0008] The technical solution of the present invention:

[0009] The method for obtaining the circumferential mechanical properties parameters of the pipe is as follows:

[0010] Step (1), using a flexible tensile test fixture for detecting the circumferential mechanical performance parameters of the pipe, a tensile test is performed on the circumferential tensile specimen of the pipe, so that the circumferential tensile specimen is subjected to tension until it breaks; during the tensile test, based on the digital image correlation method, a non-contact video extensometer is used to obtain the deformation of the circumferential tensile specimen in the entire tensile process in real time, and then obtain the engineering strain of the circumferential tensile specimen, and then combine with a universal tensile testing machine to obtain the time and force of the tensile test; the four types of data, namely the deformation of the circumferential tensile specimen, the engineering strain of the circumferential tensile specimen, the time of the tensile test and the force of the tensile test, are collectively referred to as pipe tensile measurement data;

[0011] Step (2) preprocessing the pipe tensile measurement data; preprocessing refers to the following steps: clearing all abnormal data before sampling, force verification and conversion, data cleaning, and data screening; the preprocessed data only contains two types of data: the force of the tensile test and the engineering strain of the circumferential tensile specimen;

[0012] Step (3), calculating the true stress and true strain data of the circumferential tensile test specimen of the pipe based on the data obtained in step (2);

[0013] Step (4): Based on the true stress and true strain data of the circumferential tensile specimen, the starting strain and ending strain of the plastic section are set, and the strength coefficient K and strain hardening exponent β in the plastic section are calculated by linear regression fitting method. The strength coefficient K and strain hardening exponent β in the plastic section and the true stress-true strain data of the circumferential tensile specimen are used for finite element analysis of the pipeline system.

[0014] Furthermore, the flexible tensile test fixture comprises two sets, an upper set and an lower set, each set comprising a chuck, a D-block and a pin;

[0015] The chuck includes a clamping part, a gradually expanding part and a D-block mounting part; the clamping part is a thin cylindrical part and is used to adapt to the V-shaped clamping block of the universal tensile testing machine; the upper and lower ends of the gradually expanding part are respectively fixed to the bottom end of the clamping part and the top end of the D-block mounting part; the side of the D-block mounting part is penetrated by a D-block mounting groove, and the length of the groove of the D-block mounting groove is less than the length of the circumferential tensile specimen; the circumferential tensile specimen is placed on the arc segment of the D-block, and during the tensile test, the circumferential tensile specimen and the D-block are in the groove of the D-block mounting groove; the pin transversely penetrates the D-block mounting part of the chuck and the side of the D-block to connect the D-block to the D-block mounting part of the chuck, and a clearance fit is adopted between the D-block mounting part of the chuck, the D-block and the pin.

[0016] Furthermore, the D-block is a flexible clamping part, and the diameter of the D-block is 1mm to 2mm smaller than the diameter of the circumferential tensile specimen to be tested; D-blocks of different diameters are designed to achieve circumferential stretching of circumferential tensile specimens of pipes with different diameters.

[0017] Furthermore, the circumferential tensile specimen is cut from the pipe to be tested, and the circumferential tensile specimen has a symmetrical structure, and the gauge length sections of the circumferential tensile specimen are located on both sides of the middle of the circumferential tensile specimen.

[0018] Furthermore, assuming that the length of the D-block mounting groove is B, the width is A, the pipe diameter is D, and the width of the circumferential tensile specimen is L0, then B>D / 2+L0, A is not less than L0, the pipe thickness δ / the pipe diameter D is less than 0.05, the length of the gauge section of the circumferential tensile specimen is L=π·D / 12, and the transition portion of the gauge section is smoothed.

[0019] Furthermore, the steps of the tensile test are as follows:

[0020] 1) Perform speckle treatment on the gauge length section of the circumferential tensile specimen: first, evenly spray white matte developer, and after natural air drying, spray black matte paint dots to form speckles;

[0021] 2) Select a D-type block that matches the specifications of the circumferential tensile specimen and clamp the circumferential tensile specimen on a flexible tensile test fixture; then clamp the flexible tensile test fixture on a universal tensile testing machine and lubricate the contact area between the D-type block and the circumferential tensile specimen with polytetrafluoroethylene;

[0022] The speckle-treated gauge section is placed on the side so that it is within the measurement range of the non-contact video extensometer. The distance is set to ensure that the entire gauge section is within the field of view of the high-speed camera of the non-contact video extensometer. The focus is adjusted and the distance calibration is performed.

[0023] 3) Start the universal tensile testing machine and the non-contact video extensometer to conduct tensile test measurements and obtain the tensile measurement data of the pipe.

[0024] Further, the specific process of step (2) is as follows:

[0025] Take the tensile measurement data of the pipe as the output data set, including force F, deformation ΔL, engineering strain ε , , 12 , T , , 12 ,

[0029] , p , T , 0p ,

[0027] ,

[0028] , 1m , 0(p+1) , 11 ,

[0026] , 1m , 11 , p , , eng , time t, read the force F data column and record it as column A0 = [A 01 , A 02 , ……, A 0n T , read the deformation ΔL data column and record it as column B0 = [B 01 , B 02 , ……, B 0n T , read the engineering strain ε eng data column and record it as column C0 = [C 01 , C 02 , ……, C 0n T , read the time t data column and record it as D0 = [D 01 , D 02 , ……, D 0n T ; [[ID=​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​11 ,C 12 ,……,C 1m ] T , the time t data column is recorded as D1=[D 11 ,D 12 ,……,D 1m ] T ;m <n;

[0030] (2.2) Force calibration and conversion;

[0031] Read all the data in column A1 [A 11 ,A 12 ,……,A 1m ] T , obtain the maximum force Fmax;

[0032] Fmax≤200, then [A 11 ,A 12 ,……,A 1m ] T ×1000=[A' 11 ,A' 12 ,……,A' 1m ] T , [A' 11 ,A' 12 ,……,A' 1m ] T Overwrite[A 11 ,A 12 ,……,A 1m ] T , get the new A1 column;

[0033] If Fmax>200, go directly to the next step;

[0034] (2.3) Data cleaning;

[0035] Read all the data in column C1 [C 11 ,C 12 ,……,C 1m ] T , judge C 1q Is ≤0, q∈{1,2,3,……,m}; record the last C 1q For positions with zero or negative values, all the data [A1, B1, C1, D1] before that position are cleared, and the deformation ΔL data column and the time t data column are deleted, finally forming a new data set [A2, C2];

[0036] (2.4) Data screening;

[0037] Perform periodic sampling data screening on the [A2, C2] dataset to reduce data and calculation volume while ensuring a smooth transition of the true stress-true strain curve.

[0038] Furthermore, the data screening includes the following three screening methods:

[0039] Method 1: Fixed-interval sampling;

[0040] Specify a single value as the periodic sampling interval, perform fixed-interval extraction on the [A2, C2] dataset, arrange the extracted data in order, denoted as [A3, C3], where A3 is the force F dataset and C3 is the engineering strain ε eng dataset;

[0041] Method 2: Variable-interval sampling;

[0042] Calculate the sampling interval based on the total data volume to obtain the dataset;

[0043] Let the data volume required to plot the true stress-true strain curve be X, the total data volume be Y, and the sampling interval Δs = Y / X. Extract data from the [A2, C2] dataset every Δs rows, arrange the extracted data in order, denoted as [A4, C4], where A4 is the force F dataset and C4 is the engineering strain ε eng dataset;

[0044] Method 3: Fluctuation sampling;

[0045] Let the priority sampling number series be 0, 2, 5, 8. Perform pre-screening on the [A2, C2] dataset, that is, extract the rows with the last digits of 0, 2, 5, 8. The total number of rows of the pre-screened data is denoted as N, and it is required that N ≥ 150. The generated dataset is denoted as [A5, C5]; perform data verification on the first 70% of the [A5, C5] dataset;

[0046] u = C 5w -C 5(w-1) , w = 2, 3,..., 0.7N, perform row-by-row verification. When u < 0, it is recorded as a fluctuation, and accumulate one by one until the verification is completed. Finally, obtain the accumulated fluctuation number v;

[0047] Let the tolerance coefficient be k1, 0 < k1 < 0.05. When v < 0.7 * k1 * N, the screening is completed; otherwise, increase the sampling period or adjust the tolerance coefficient, repeat the sampling and verification process until the requirements are met. Increasing the sampling period means reducing the total number of rows N by reducing the last digits of the extracted rows;

[0048] Select a sampling method according to the actual working conditions for data screening, arrange the extracted data in order, denoted as [A6, C6], where A6 is the force F dataset and C6 is the engineering strain ε eng dataset.

[0049] Furthermore, the specific process of step (3) is as follows:

[0050] (3.1) Calculate the cross-sectional area S0 of the gauge length segment:

[0051] Assume that the diameter of the circumferential tensile test specimen of the pipe is D, the width is L0, the thickness of the pipe is δ, the milling width of one side of the gauge section (4) is P1, and the milling width of the other side of the gauge section is P2;

[0052] Then the width of the gauge section M = L0-P1-P2;

[0053] The cross-sectional area of ​​the gauge section S0 = M·δ = (L0-P1-P2)·δ;

[0054] (3.2) Assume that the data obtained from the tensile test are: force F and engineering strain ε eng ;

[0055] Then the engineering stress σ eng for:

[0056] Then the true stress σ is:

[0057] Then the true strain ε is: ε=ln(1+ε eng );

[0058] (3.3) Draw the true stress-true strain curve.

[0059] Furthermore, the process of calculating the strength coefficient K and strain hardening exponent β in the plastic section by the linear regression fitting method is as follows: the plastic section range is selected on the true stress-true strain curve image, and linear regression calculation is performed after taking the logarithm to obtain the strength coefficient K value and strain hardening exponent β value of the plastic section area:

[0060] σ=Kε β ;

[0061] logσ=logK+β·logε.

[0062] Beneficial effects of the present invention: The present invention proposes a method for obtaining parameters of the circumferential mechanical properties of pipes, which is suitable for large-diameter pipes subjected to finite element analysis. The present invention is based on a digital image correlation method and uses a non-contact video extensometer, which can achieve real-time and high-precision acquisition of engineering strains during the entire tensile process of the sample. By applying the present invention, the circumferential mechanical property data of the pipe can be accurately and comprehensively measured, and the real stress-strain data and curves can be obtained through data processing algorithms to more accurately describe the strain hardening behavior of the material. By defining the range of the plastic segment, the strength coefficient K and strain hardening exponent β within the plastic region can also be calculated, providing accurate material data for subsequent finite element analysis of pipeline systems in military fields such as aviation, aerospace, engines, satellites, missiles, and civilian fields such as automobiles, ships, and high-speed railways. BRIEF DESCRIPTION OF THE DRAWINGS

[0063] Figure 1 is the design diagram of the flexible tensile test fixture.

[0064] Figure 2 is the design diagram of the circumferential tensile specimen.

[0065] Figure 3 shows the calculation of the cross-sectional area of ​​the gauge section.

[0066] Figure 4 shows the true stress-true strain curve.

[0067] In the figure: 1-chuck; 2-pin; 3-D ​​block; 4-gauge length section. DETAILED DESCRIPTION

[0068] This embodiment proposes a method for obtaining circumferential mechanical property parameters of pipes. A flexible tensile test fixture for detecting the circumferential mechanical property parameters of large-diameter pipes is provided. A tensile test is performed on circumferential tensile specimens of the pipes. By separating two D-shaped blocks 3 within the flexible tensile test fixture, upward and downward tension is applied to the circumferential tensile specimens until they break. During the tensile test, a non-contact video extensometer, based on digital image correlation, is used to accurately and in real time obtain the displacement between speckles and the deformation of the circumferential tensile specimen throughout the entire tensile process. This allows the deformation of the circumferential tensile specimen to be determined, and further, the engineering strain of the circumferential tensile specimen to be determined. This data, combined with a universal tensile testing machine, is used to obtain the time and force data of the tensile test. The four types of data—the deformation and engineering strain of the circumferential tensile specimen, and the time and force of the tensile test—are collectively referred to as pipe tensile measurement data. The pipe tensile measurement data is preprocessed, which involves the following steps: clearing all abnormal data before sampling, force verification and conversion, data cleaning, and data filtering. The preprocessed data only contains force and engineering strain data. Then, the true stress-true strain data of the circumferential tensile specimen of the pipe is calculated based on the preprocessed data. By setting the starting strain and ending strain of the plastic section, the strength coefficient K and strain hardening exponent β in the plastic section can be calculated by the linear regression fitting method.

[0069] This invention utilizes a flexible tensile test fixture and employs a symmetrical tensile test method for circumferential tensile testing of pipes, employing a circumferential tensile specimen with gauge lengths 4 positioned on either side. This method minimizes the effects of work hardening associated with conventional transverse specimens and also mitigates friction between the flexible tensile test fixture's D-block 3 and gauge length 4, facilitating data acquisition using a non-contact video extensometer.

[0070] The flexible tensile test fixture described in this embodiment includes two sets, an upper set and a lower set, each set including a clamp 1, a D-shaped block 3 and a pin 2, see Figure 1.

[0071] 1) The chuck 1 includes a clamping part, a gradually expanding part and a D-block mounting part; the clamping part is a thin cylindrical part with a length of 60mm, which is used to match the V-type clamping block of the universal tensile testing machine. The two ensure that the clamping force is large enough to prevent slipping. The upper and lower ends of the gradually expanding part are respectively fixed to the bottom end of the clamping part and the top end of the D-block mounting part. The side of the D-block mounting part is penetrated by a D-block mounting groove. The groove of the D-block mounting groove is 70mm long and 20mm wide, which is suitable for circumferential tensile testing of pipes with a diameter of 40-100mm. The groove length of the D-block mounting groove needs to be suitable for circumferential tensile testing of pipes. The material of the chuck 1 is 40Cr tempered to ensure that the pin 2 part will not be broken by shear force, while improving the service life of the flexible tensile test fixture.

[0072] 2) The circumferential tensile specimen is placed on the arc segment of the D-block 3. During the tensile test, the circumferential tensile specimen and the D-block 3 are located in the notch of the D-block mounting groove; the pin 2 laterally penetrates the D-block mounting portion of the chuck 1 and the side of the D-block 3 to connect the D-block 3 to the D-block mounting portion of the chuck 1.

[0073] Furthermore, the D-block 3 is a flexible clamping component. Its diameter is 1mm to 2mm smaller than the diameter of the hoop tensile specimen to be tested, making it easier to clamp the hoop tensile specimen. By designing D-blocks 3 with different diameters, hoop tensile tests can be performed on hoop tensile specimens of pipes with different diameters.

[0074] 3) The pin 2 is used to connect the D-block mounting portion of the chuck 1 and the D-block 3, with a clearance fit between the three.

[0075] As shown in FIG2 , the circumferential tensile specimen of this embodiment is used as an example to conduct mechanical property testing and parameter extraction on a certain type of pipe. The circumferential tensile specimen has a symmetrical structure and is made of 1Gr18Ni10Ti. Measurements show that the diameter D is 50 mm and the wall thickness δ is 1.1 mm.

[0076] The specific implementation steps of this embodiment are as follows:

[0077] Step 1: Prepare the circumferential tensile specimen:

[0078] A circumferential tensile specimen was cut from the pipe to be tested using the laser cutting method. The width of the circumferential tensile specimen, L0, was measured to be 12.7 mm, P1 was 2.95 mm, and P2 was 3.12 mm. The gauge section 4 was processed symmetrically. The length L of the gauge section 4 of the circumferential tensile specimen was calculated as follows:

[0079] The length of the gauge section 4 is L = π·D / 12 = 3.14*50 / 12 = 13 mm, and the transition portion of the gauge section 4 is smoothed;

[0080] Perform speckle treatment on gauge section 4 of the circumferential tensile specimen and evenly spray a white matte developer as a background to prevent reflections from the circumferential tensile specimen from interfering with image recognition. The white developer should evenly cover gauge section 4 of the circumferential tensile specimen, ensuring that the original color is not exposed. It should also not be sprayed too thickly to prevent collapse during the tensile process, which could affect recognition. After air drying, spray black matte paint dots as spots to form speckles. The black matte paint dots should be evenly distributed in small dots.

[0081] Step 2: Tensile test preparation:

[0082] Select a D-type block 3 with a diameter of 48mm to 49mm and clamp the circumferential tensile specimen to the flexible tensile test fixture. The gauge section 4 of the circumferential tensile specimen should be positioned to the side so that it is within the measurement range of the non-contact video extensometer. Adjust the non-contact video extensometer's position to ensure that the entire gauge section 4 is within the field of view of the non-contact video extensometer's high-speed camera. Adjust the focal length to complete distance calibration, and enable the light source for fill light and real-time calculation. Then, clamp the flexible tensile test fixture for the pipe to the universal tensile testing machine. Use polytetrafluoroethylene to lubricate the contact area between the D-type block 3 and the circumferential tensile specimen.

[0083] Step 3: Tensile test:

[0084] Start the universal tensile testing machine and non-contact video extensometer to conduct a tensile test: Complete the relevant tensile test settings in the software of the universal tensile testing machine, set the universal tensile testing machine beam displacement rate to 5mm / min, turn on the data acquisition function to obtain the deformation data measured by the non-contact video extensometer, and start the tensile test after the force and displacement data of the universal tensile testing machine are cleared. The image captured by the high-speed camera in the non-contact video extensometer is transmitted back to the computer, and the deformation of the circumferential tensile specimen is calculated using the digital image correlation method. The real-time engineering strain during the tensile test is further obtained. The time and force data of the tensile test are obtained through the universal tensile testing machine. After the test is broken, the universal tensile testing machine automatically stops the tensile test, exports the relevant test data, and the test ends. Some of the original data of the circumferential tensile test are shown in Figure 4.

[0085] Step 4: Test data preprocessing:

[0086] The preprocessing of the circumferential mechanical properties test data of pipes mainly involves cleaning and screening the force data obtained by the universal tensile testing machine and the engineering strain data calculated by the non-contact video extensometer, removing unreasonable values, and using a periodic sampling algorithm to sample and screen the data to obtain force and engineering strain data sets.

[0087] Table 1 Raw data of some circumferential tensile test

[0088] The output data sets of the universal tensile testing machine and the non-contact video extensometer include force (F), deformation (Δ L), engineering strain (ε eng ), time (t), see Table 1. Read force F data column is recorded as column A0 = [A 01 , A 02 ,……,A 0n ] T =[-0.63374,0,7.462829,2.109,0.000948,……] T , read the deformation ΔL data column and record it as column B0=[B 01 , B 02 ,……,B 0n ] T =[7.579771,0,7.522981,2277,-0.01622,……] T , read the engineering strain ε eng The data column is recorded as column C0 = [C 01 , C 02 ,……,C 0n ] T= [0.1515954, 0, 0.1504596, 45.54, -0.000324, ……] T , the read time t data column is recorded as D0 = [D 01 , D 02 , ……, D 0n T = [0, 0, 152.4487, 2277, 0, ……] T .

[0089] (1) Clearing all abnormal data before starting sampling:

[0090] Read column D0, D 0(p+1) - D 0p = T p , p ∈ {1, 2, 3, 4, ……, n - 1}; T p represents the time difference between the (p + 1)-th row and the p-th row;

[0091] If (where 1.5 < k < 2 and f is the sampling frequency), then mark the (p + 1)-th row data as normal, otherwise mark the (p + 1)-th row and the p-th row as abnormal.

[0092] When 5 consecutive normal marks appear, stop reading and clear all the data in the rows with abnormal marks. Obtain the data sets, A1 = [0.001, 0.001, 0.001, 0.001, 0.001, ……] T , B1 = [-0.00738, -0.0036, 0, -0.00486, 0.00145, ……] T , C1 = [-0.00049, -0.00024, 0, -0.00032, 0.0001, ……] T ， D1 = [0.067, 0.134, 0.201, 0.27, 0.337, ……] T . The results are shown in Table 2.

[0093] Table 2 Circumferential tensile test data after partially clearing abnormal data

[0094] (2) Force (F) calibration and conversion:

[0095] Read all the data in column A1:

[0096] [A 11 , A 12 , ……, A 1m T = [0.001, 0.001, 0.001, 0.001, 0.001, ……] T ​​, the maximum force value Fmax=7.463 is obtained.

[0097] If Fmax≤200, then [A 11 ,A 12 ,……,A 1m ] T ×1000=[A' 11 ,A' 12 ,……,A' 1m ] T =[1,1,1,1,1……] T , [A' 11 ,A' 12 ,……,A' 1m ] T Overwrite[A 11 ,A 12 ,……,A 1m ] T , get the new A1 column [1,1,1,1,1……] T The results are shown in Table 2.

[0098] (3) Data cleaning:

[0099] Read all the data in column C1 [C 11 ,C 12 ,……,C 1m ] T =[-0.00049,-0.00024,0, -0.00032,0.0001,……] T , judge C 1q Whether ≤0, q∈{1,2,3,……,m}.

[0100] Read all the data in column C1 [C 11 ,C 12 ,……,C 1m ] T , judge C 1q Is ≤0; q∈{1,2,3,……,m}; record the last C 1q For positions with zero or negative values, all data [A1, B1, C1, D1] before that position are cleared, and the deformation ΔL data column and time t data column are deleted, finally forming a new data set [A2, C2], A2 = [231, 231, 232, 233, 234, ...] T , C2=[0.00018,0.00035,0.00085,0.00043,0.00077,……] T The results are shown in Table 3.

[0101] Table 3 Partial data of the hoop tensile test after cleaning

[0102] (4) Data screening:

[0103] The present invention proposes three screening methods for periodic sampling data screening of the [A2, C2] data set.

[0104] Method 1: Fixed interval sampling:

[0105] Specify 10 as the periodic sampling interval, extract every 10 rows of the data set [A2, C2], and arrange the extracted data in order, recorded as [A3, C3], where A3 is the force F data set and C3 is the engineering strain ε eng Dataset: A3 = [231, 240, 256, 278, 300, ...] T , C3=[0.00018,0.00052,0.00085,0.00052,0.00052……] T The results are shown in Table 4.

[0106] Method 2: Variable interval sampling:

[0107] The sampling interval is calculated based on the total amount of test data to obtain the data set.

[0108] Assuming that the amount of data required to draw the true stress-true strain curve is 150 and the total amount of data is 1742, then:

[0109] The sampling interval Δs = 1742 / 150 = 11.6, so the data set [A2, C2] is sampled every 11 rows, and the sampled data are arranged in order and recorded as [A4, C4], where A4 is the force F data set and C4 is the engineering strain ε eng Dataset:

[0110] A4=[231,242,260,285,312,……] T ,

[0111] C4=[0.00018,0.00111,0.00077,0.00094,0.00119,……] T The results are shown in Table 4.

[0112] Method 3: Fluctuation Sampling:

[0113] Assume that the priority sampling numbers are 0, 2, 5, and 8. Pre-screen the data set [A2, C2] to extract rows ending in 0, 2, 5, and 8. The total number of rows after pre-screening is N = 697, and the resulting data set is [A5, C5]. A5 = [231, 234, 237, 239, 242, ...] T, C5=[0.00035,0.00077,0.00077,0.00085,0.00111,…] T The first 70% of the dataset [A5, C5] will be subjected to data validation. The results are shown in Table 4.

[0114] Table 4 Partial data obtained by three screening methods

[0115] u=C 5w -C 5(w-1) , (w=2,3,……,0.7N), check row by row. When u<0, it is recorded as one fluctuation. Accumulate one by one until the verification is completed, and finally get the fluctuation number v=48.

[0116] Assume that the tolerance coefficient is k1=0.04, 0.7*k1*N=0.7*0.04*697=19.52. Since 48>19.52, it is necessary to increase the sampling period (reduce the tail digit of the extracted row) or adjust the tolerance coefficient. After multiple samplings, it is finally determined that the row data with the tail digit 2 is extracted. The tolerance coefficient k1=0.04 can meet the verification requirements.

[0117] The third sampling method is selected, and the extracted data are arranged in order, recorded as [A6, C6], where A6 is the force F data set and C6 is the engineering strain ε eng Dataset: A6 = [231, 242, 258, 280, 303, ...] T , C6=[0.00035,0.00111,0.00136,0.00094,0.00102,……] T The results are shown in Table 5.

[0118] Table 5

[0119] Step 5: Obtain performance parameters:

[0120] For the force F and engineering strain ε obtained in step 4 eng The data is calculated to calculate the true stress σ and true strain ε, and the true stress-true strain curve is output.

[0121] Given: See Figure 3, the diameter D of the circumferential tensile test specimen of the pipe is 50 mm, the wall thickness δ is 1.1 mm, the width L0 of the circumferential tensile test specimen after processing is 12.7 mm, P1 is 2.95 mm, and P2 is 3.12 mm, then:

[0122] The width of the gauge section 4 is M = L0 - P1 - P2 = 12.7 - 2.95 - 3.12 = 6.63 mm;

[0123] The cross-sectional area of ​​the gauge section 4 is S0 = (L0 - P1 - P2) δ = 6.63 * 1.1 = 7.293 mm;

[0124] Calculate the engineering stress σ eng :

[0125] The true stress σ is calculated as:

[0126] σ=σ eng (1+ε eng );

[0127] The true strain ε is calculated as:

[0128] ε=ln(1+ε eng );

[0129] Enter the plastic segment range: the starting strain of the plastic segment is 0.08, and the ending strain of the plastic segment is 0.2. Calculate the strength coefficient K value and strain hardening exponent β value within the plastic region.

[0130] σ=Kε β ;

[0131] logσ=logK+β·logε;

[0132] Where: σ is the true stress and ε is the true strain.

[0133] The calculated strength coefficient K is 1427 and the strain hardening exponent β is 0.4386.

Claims

1. A method for obtaining the circumferential mechanical properties parameters of a pipe, characterized in that: The steps are as follows: Step (1): Use a flexible tensile test fixture for detecting the circumferential mechanical property parameters of the pipe to conduct a tensile test on the circumferential tensile specimen of the pipe until the specimen breaks under tension. During the tensile test, based on the digital image correlation method, use a non-contact video extensometer to obtain the deformation of the circumferential tensile specimen throughout the entire tensile process in real time, thereby obtaining the engineering strain of the circumferential tensile specimen. Then, combined with a universal tensile testing machine, obtain the time of the tensile test and the force of the tensile test. The deformation of the circumferential tensile specimen, the engineering strain of the circumferential tensile specimen, the time of the tensile test, and the force of the tensile test are collectively referred to as pipe tensile measurement data; Step (2): Preprocess the pipe tensile measurement data; the preprocessing refers to sequentially performing: clearing all abnormal data before starting sampling, calibrating and converting the force, data cleaning, and data screening; the data after preprocessing only includes two types of data, namely the force of the tensile test and the engineering strain of the circumferential tensile specimen; the specific process is as follows: The pipe tensile measurement data is used as the output data set, including force F, deformation ΔL, engineering strain ε eng , time t, the reading force F data column is recorded as column A0 = [A 01 ,A 02 ,……,A 0n ] T , read the deformation amount ΔL data column and record it as column B0 = [B 01 ,B 02 ,……,B 0n ] T , read the engineering strain ε eng The data column is recorded as column C0 = [C 01 ,C 02 ,……,C 0n ] T , the reading time t data is recorded as D0 = [D 01 ,D 02 ,……,D 0n ] T ; (2.1) Clear all abnormal data before starting sampling; Read D0 data column, D 0(p+1) -D 0p =T p , p∈{1,2,3,4,…,n-1}; T p It represents the time difference between row p+1 and row p; when Mark the p + 1 row of data as normal, otherwise mark the p + 1 row and the p row as abnormal, where 1.5 < k < 2 and f is the sampling frequency; When 5 normal marks appear continuously, the reading stops and all rows with abnormal marks are cleared to obtain a new set of data: the force F data column is recorded as column A1 = [A 11 ,A 12 ,……,A 1m ] T , the deformation amount ΔL data column is recorded as column B1 = [B 11 ,B 12 ,……,B 1m ] T , engineering strain ε eng The data column is recorded as column C1 = [C 11 ,C 12 ,……,C 1m ] T , the time t data column is recorded as D1 = [D 11 ,D 12 ,……,D 1m ] T ;m <n; (2.2) Calibrate and convert the force; Read all the data in column A1 11 ,A 12 ,……,A 1m ] T , obtain the maximum force Fmax; If Fmax ≤ 200, then [A 11 , A 12 , ……, A 1m T × 1000 = [A' 11 , A' 12 , ……, A' 1m T , and overwrite [A' 11 , A' 12 , ……, A' 1m T on [A 11 , A 12 , ……, A 1m T , and obtain a new column A1;​​​​ If Fmax > 200, then directly proceed to the next step; (2.3) Data cleaning; Read all the data in column C1 11 ,C 12 ,……,C 1m ] T , judge C 1q Is it ≤0, q∈{1,2,3,……,m}; record the last A C 1q For positions with zero or negative values, all data [A1, B1, C1, D1] before the position are cleared, and the deformation ΔL data column and the time t data column are deleted, and finally a new data set [A2, C2] is formed; (2.4) Data screening; Perform periodic sampling data screening on the [A2, C2] dataset to reduce data and calculation amount while ensuring a smooth transition of the true stress - true strain curve; Step (3): Calculate the true stress and true strain data of the circumferential tensile specimen of the pipe based on the data obtained in Step (2); Step (4): Based on the true stress and true strain data of the circumferential tensile specimen, by setting the starting strain and ending strain of the plastic section and using the linear regression fitting method, calculate the strength coefficient K and strain hardening index β within the plastic section. The strength coefficient K and strain hardening index β within the plastic section, as well as the true stress - true strain data of the circumferential tensile specimen, are used for the finite element analysis of the pipeline system.

2. The method for obtaining the circumferential mechanical properties parameters of a pipe according to claim 1, characterized in that: The flexible tensile test fixture described above includes two sets, upper and lower. Each set includes a chuck (1), a D - shaped block (3), and a pin (2); The chuck (1) comprises a clamping portion, a gradually expanding portion and a D-block mounting portion; the clamping portion is a thin cylindrical shape and is used to match the V-block of a universal tensile testing machine; the upper and lower ends of the gradually expanding portion are respectively fixed to the bottom end of the clamping portion and the top end of the D-block mounting portion; the side of the D-block mounting portion penetrates through a D-block mounting groove, and the length of the notch of the D-block mounting groove is less than the length of the annular tensile specimen; the annular tensile specimen is placed on the arc segment of the D-block (3), and during the tensile test, the annular tensile specimen and the D-block (3) are located in the notch of the D-block mounting groove; the pin (2) transversely penetrates the D-block mounting portion of the chuck (1) and the side of the D-block (3), and connects the D-block (3) with the D-block mounting portion of the chuck (1), and a clearance fit is adopted between the D-block mounting portion of the chuck (1), the D-block (3) and the pin (2).

3. The method for obtaining the circumferential mechanical properties parameters of a pipe according to claim 2, characterized in that: The D-shaped block (3) is a flexible clamping part, and the diameter of the D-shaped block (3) is 1 mm to 2 mm smaller than the diameter of the circumferential tensile test specimen to be tested; D-shaped blocks (3) of different diameters are designed to realize circumferential stretching of circumferential tensile test specimens of pipes of different diameters.

4. The method for obtaining the circumferential mechanical properties parameters of a pipe according to claim 1, characterized in that: The annular tensile specimen is cut from the pipe to be tested, and the annular tensile specimen is a symmetrical structure, and the gauge section (4) of the annular tensile specimen is located on both sides of the middle of the annular tensile specimen.

5. The method for obtaining the circumferential mechanical properties parameters of a pipe according to claim 2, characterized in that: Assuming that the length of the D-block installation groove is B, the width is A, the pipe diameter is D, and the width of the circumferential tensile test specimen is L0, then B>D / 2+L0, A is not less than L0, the pipe thickness δ / the pipe diameter D is less than 0.05, the length of the gauge section (4) of the circumferential tensile test specimen is L=π·D / 12, and the transition portion of the gauge section (4) is smoothed.

6. The method for obtaining the circumferential mechanical properties parameters of a pipe according to claim 2, characterized in that: The steps of the tensile test are as follows: 1) Performing speckle treatment on the gauge section (4) of the circumferential tensile specimen: first, spraying white matte developer evenly, and after natural air drying, spraying black matte paint dots to form speckles; 2) selecting a matching D-type block (3) according to the specifications of the circumferential tensile specimen, and clamping the circumferential tensile specimen on a flexible tensile test fixture; then clamping the flexible tensile test fixture on a universal tensile testing machine, and using polytetrafluoroethylene to lubricate the contact portion between the D-type block (3) and the circumferential tensile specimen; The speckle-processed gauge section (4) is placed on the side so that the gauge section (4) is within the measurement range of the non-contact video extensometer, the distance is set to ensure that the gauge section (4) is completely within the field of view of the high-speed camera of the non-contact video extensometer, the focus is adjusted and the distance is calibrated; 3) Start the universal tensile testing machine and the non-contact video extensometer to perform tensile tests and obtain the tensile measurement data of the pipe.

7. The method for obtaining the circumferential mechanical properties parameters of a pipe according to claim 1, characterized in that: The data screening is method 1 or method 2: Method 1: fixed interval sampling; Specify a single value as the periodic sampling interval, extract the data set [A2, C2] at fixed intervals, and arrange the extracted data in order, recorded as [A3, C3], A3 is the force F data set, C3 is the engineering strain ε eng Datasets; Method 2: variable interval sampling; Calculate the sampling interval according to the total amount of data to obtain the data set; Assume that the amount of data required to draw the true stress-true strain curve is X, the total amount of data is Y, the sampling interval Δs = Y / X, and the data set [A2, C2] is extracted once every Δs interval. The extracted data are arranged in order and recorded as [A4, C4], where A4 is the force F data set and C4 is the engineering strain ε eng Dataset.

8. The method for obtaining the circumferential mechanical properties parameters of a pipe according to claim 1, characterized in that: The data screening is fluctuation sampling; Assume that the priority sampling number system is 0, 2, 5, 8, and pre-screen the data set [A2, C2], that is, extract the row data with the last digits of 0, 2, 5, 8. The total number of rows after pre-screening is recorded as N, and N is required to be ≥ 150. The generated data set is recorded as [A5, C5]. The first 70% of the data set [A5, C5] will be verified. u=C 5w -C 5(w-1) , w=2,3,……,0.7N, check row by row, when u<0, it is recorded as one fluctuation, accumulate one by one until the verification is completed, and finally get the accumulated fluctuation number v; Assume the allowable coefficient is k1, 0<k1<0.05, when v<0.7*k1*N, the screening is completed; otherwise, increase the sampling period or adjust the allowable coefficient, repeat the sampling and verification process until the requirements are met, and increase the sampling period, that is, reduce the total number of data rows N by reducing the tail number of the extracted rows; According to the actual working conditions, a sampling method is selected to screen the data, and the extracted data are arranged in order, recorded as [A6, C6], where A6 is the force F data set ， C6 is the engineering strain ε eng Dataset.

9. The method for obtaining the circumferential mechanical properties parameters of a pipe according to claim 1, characterized in that: The specific process of step (3) is as follows: (3.1) Calculate the cross-sectional area S0 of the gauge section (4): Assume that the diameter of the circumferential tensile test specimen of the pipe is D, the width is L0, the thickness of the pipe is δ, the milling width on one side of the gauge section (4) is P1, and the milling width on the other side of the gauge section (4) is P2; Then the width of the gauge section (4) is M = L0-P1-P2; The cross-sectional area of ​​the gauge section (4) is S0=M·δ=(L0-P1-P2)·δ; (3.2) Assume that the data obtained from the tensile test are: force F and engineering strain ε eng ; Then the engineering stress σ eng for: Then the true stress σ is: Then the true strain ε is: ε=ln(1+ε eng ); (3.3) Draw the true stress-true strain curve.

10. The method for obtaining the circumferential mechanical properties parameters of a pipe according to claim 1, characterized in that: The process of calculating the strength coefficient K and strain hardening exponent β in the plastic section by linear regression fitting method is as follows: the plastic section range is selected on the true stress-true strain curve image, and linear regression calculation is performed after taking the logarithm to obtain the strength coefficient K value and strain hardening exponent β value of the plastic section area: σ=Kε β ; logσ=logK+β·logε.

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