Pipeline steel ring welding seam toughness evaluation method based on fracture morphology fractal reconstruction

Abstract

The invention relates to a pipeline steel ring welding seam toughness evaluation method based on fracture morphology fractal reconstruction. The method comprises the following steps: firstly, preparing an SENB / CT sample according to ASTME 1820 and other standards, completing a fracture toughness test, and calculating to obtain JIC or KIC; performing three-dimensional point cloud acquisition on the fracture of the fractured sample by using a non-contact laser scanning system with the resolution of less than or equal to 1 mu m, denoising, splicing and registering to obtain the three-dimensional shape of the fracture, and extracting the arithmetic mean roughness Sa; and performing fractal analysis on the reconstructed morphology by adopting an improved box counting method, and calculating fracture fractal dimension Df. And constructing a multivariate nonlinear prediction model by taking a toughness parameter as a dependent variable and taking fracture morphology characteristic characterization parameters Sa and Df as independent variables, and accurately evaluating the fracture toughness of the pipeline steel ring welding seam when a decision coefficient R2 is greater than or equal to 0.90. The method has the advantages of less sample demand and short analysis period, can quantitatively evaluate the fracture toughness of the material from the morphological characteristics of the fracture, obviously reduces the detection cost, improves the in-service pipeline integrity evaluation efficiency, and is suitable for rapid toughness prediction of circumferential weld materials of different steel grades and welding processes.

Description

Technical Field

[0001] The present invention relates to the technical field of pipeline fracture mechanics and failure analysis, and particularly to a method for rapidly evaluating and predicting the fracture toughness of pipeline steel girth weld materials by using three-dimensional reconstruction of fracture surface morphology and quantification of fractal features. Background Art

[0002] The fracture toughness of pipeline steel girth welds is crucial for the safe operation of oil and gas pipeline networks. With the wide application of high-grade pipelines under complex geological conditions, girth welds have become the main failure hazards in pipeline systems. Once a fracture occurs, it will lead to serious safety and environmental accidents. Traditional toughness evaluation mainly obtains fracture parameters through standard mechanical tests. However, such methods are not only time-consuming and laborious, but also face difficulties in sampling and limited representativeness of results for in-service pipelines.

[0003] Although morphology analysis methods such as metallography and scanning electron microscopy can provide microstructural information, they lack a unified quantitative standard and are difficult to objectively evaluate the overall toughness of welds. Nondestructive testing techniques have advantages in real-time monitoring. However, the quantitative relationship between signals and material properties has not been established, and the actual application effect is restricted by various factors.

[0004] Recent studies have shown that there is an inherent connection between fracture surface morphology and material fracture behavior, and the fractal theory provides a mathematical description framework for this connection. The fractal dimension of the fracture surface is closely related to the energy dissipation mechanism and can be used as an index to evaluate toughness. However, existing fractal analysis is mostly limited to simple specimens and two-dimensional characterization, and there is insufficient research on the three-dimensional fracture surface characteristics of girth welds in complex environments, lacking a reliable toughness prediction model.

[0005] Facing these technical challenges, there is an urgent need to develop a fractal analysis method based on three-dimensional fracture surface morphology reconstruction, establish a quantitative relationship between the fracture surface morphology characteristics and toughness parameters of girth welds, and provide a more efficient and reliable technical means for pipeline safety assessment. Summary of the Invention

[0006] To overcome the defects of the prior art, the present invention provides a method for evaluating the toughness of pipeline steel girth welds based on fractal reconstruction of fracture surface morphology.

[0007] To achieve the above object, the technical solution of the present invention is as follows:

[0008] A method for evaluating the toughness of pipeline steel girth welds based on fractal reconstruction of fracture surface morphology, comprising the following steps:

[0009] 1) Prepare three-point bend (SENB) or compact tension (CT) standard specimens in accordance with fracture toughness test standards such as ASTM E1820, and complete the fracture toughness test at a specified temperature and loading rate to obtain load - crack propagation data;

[0010] 2) Process the raw data obtained in step 1 according to the calculation procedures of the test standards to obtain the fracture toughness parameter J of the material IC or K IC ;

[0011] 3) Use a non-contact laser scanning system with a resolution not higher than 1 μm to globally scan the fracture surface of the fractured specimen to obtain point cloud data; denoise, splice, and register the point cloud, reconstruct the three-dimensional morphology of the fracture surface, and calculate the arithmetic mean roughness S of the fracture surface according to ISO 25178 a ;

[0012] 4) Apply the improved box-counting method to perform fractal analysis on the three-dimensional fracture morphology reconstructed in step 3 to obtain the fractal dimension D of the fracture surface f ;

[0013] 5) Take the fracture toughness parameter J IC or K IC obtained in step 2 as the dependent variable, and take the surface roughness S a obtained in step 3 and the fractal dimension D f obtained in step 4 as the independent variables to establish a multivariate non-linear model. When the coefficient of determination R 2 of the model is ≥ 90%, it is determined that the model can accurately predict the fracture toughness of the pipeline steel girth weld material based on the fracture surface roughness and fractal dimension

[0014] 2. The method for evaluating the toughness of pipeline steel girth welds based on fractal reconstruction of fracture morphology according to claim 1, wherein the calculation method of the fracture toughness value is:

[0015]

[0016]

[0017] J = 3.75R m (Δa - 0.2)(4)

[0018] where P is the indenter load during the test, N; S is the span, m; B is the specimen thickness, m; B N is the net thickness of the specimen, m; ν is the Poisson's ratio; E is the elastic modulus, Pa; U p is the plastic part of the area of the load-displacement curve, J; R m is the tensile strength, Pa.

[0019] 3. The method for evaluating the toughness of pipeline steel girth welds based on fractal reconstruction of fracture morphology according to claim 1, wherein the calculation equation of the relevant fractal dimension of the box-counting method is:

[0020]

[0021] Among them, D f is the fractal dimension; N total (δ) is the total number of boxes used in the box-counting method, and δ is the box size used in different box-counting methods.

[0022] The present invention has the following beneficial effects:

[0023] (1) The method for evaluating the toughness of the girth weld of pipeline steel based on the fractal reconstruction of the fracture morphology provided by the present invention provides a rapid method for evaluating the toughness of the girth weld based on the three-dimensional fracture morphology, significantly reducing the test cost and cycle and facilitating practical engineering applications.

[0024] (2) By establishing a quantitative relationship model between the multi-scale fractal characteristics of the fracture and the toughness parameters, the limitation that traditional morphology analysis can only qualitatively evaluate is overcome, and reliable prediction of the fracture toughness of the girth weld is realized.

[0025] (3) Combining high-precision three-dimensional reconstruction technology and fractal theory, an effective correlation between the fracture morphology and fracture mechanics parameters is realized, providing a more scientific and accurate technical method for evaluating the toughness of pipeline welds. Description of the Drawings

[0026] Figure 1 is a schematic diagram of the box-counting method used for calculating the fractal dimension of the present invention;

[0027] Figure 2 is a fitting result diagram of the fracture toughness of the girth weld, the fractal dimension, and the surface roughness in the present invention. Detailed Embodiments

[0028] The following further describes the present invention in detail with reference to the drawings:

[0029] As Figure 1 shown, the present invention provides a method for evaluating the toughness of the girth weld of pipeline steel based on the fractal reconstruction of the fracture morphology, which is characterized by including the following steps:

[0030] 1) Prepare three-point bend (SENB) or compact tension (CT) standard specimens according to fracture toughness test standards such as ASTM E1820, and complete the fracture toughness test at a specified temperature and loading rate to obtain load–crack propagation data;

[0031] 2) Process the original data obtained in step 1 according to the calculation procedures of the test standard to obtain the fracture toughness parameter J IC or K IC , where the fracture toughness calculation formula is;

[0032]

[0033] J = 3.75Rm (Δa - 0.2)(4)

[0034] 3) Use a non - contact laser scanning system with a resolution not higher than 1 μm to globally scan the fracture surface of the fractured specimen to obtain point - cloud data; denoise, splice, and register the point - cloud, reconstruct the three - dimensional morphology of the fracture surface, and calculate the arithmetic mean roughness S of the fracture surface according to ISO 25178 a ;

[0035] 4) Apply the improved box - counting method to the three - dimensional fracture morphology reconstructed in step 3 for fractal analysis to obtain the fractal dimension D of the fracture surface f , where the fractal - dimension calculation formula is;

[0036]

[0037] 5) Take the fracture toughness parameter J IC or K IC obtained in step 2 as the dependent variable, and take the surface roughness S a obtained in step 3 and the fractal dimension D f obtained in step 4 as independent variables, establish a multiple - nonlinear model, and the model formula is, for example, J IC = f(D f , S a ). When the coefficient of determination R 2 of the model is ≥ 90%, it is determined that the model can accurately predict the fracture toughness of the pipeline - steel girth - weld material based on the fracture - surface roughness and fractal dimension

[0038] The following is illustrated with specific examples:

[0039] (1) Three - point - bend specimens are respectively taken from the base metal, heat - affected zone, and weld. The J IC calculated according to relevant calculation standards is shown in Table 1

[0040] Table 1 Fracture toughness J of girth weld IC Calculation results

[0041] Region Base metal Heat affected zone Weld seam <![CDATA[Fracture toughness value J IC (kJ / m 2 )]]> 797.93 775.32 66.32

[0042] (2) Determine the numerical values of the surface roughness and fractal dimension of the fracture surfaces of different specimens as shown in Table 2

[0043] Table 2 Calculated values of fracture - surface roughness and fractal dimension

[0044] Region Base metal Heat affected zone Weld seam <![CDATA[Surface roughness S a (um)]]> 190.23 178.65 120.34 <![CDATA[Fractal dimension D f > 2.243 2.176 2.124

[0045] (3) According to the fracture - toughness values and the measured fracture - surface roughness and fractal dimension, use the multiple - regression equation to fit relevant formulas (such as Figure 2As shown, the goodness of fit is greater than 90%, providing a new prediction method for the rapid assessment of the fracture toughness of circumferential welds.

Claims

1. A method for evaluating the toughness of the girth weld of pipeline steel based on the fractal reconstruction of fracture morphology, characterized in that, Including the following steps: 1) Prepare three-point bend (SENB) or compact tension (CT) standard specimens in accordance with fracture toughness test standards such as ASTM E1820, and complete the fracture toughness test at a specified temperature and loading rate to obtain load-crack propagation data; 2) Process the raw data obtained in step 1 according to the calculation procedure of the test standard to obtain the fracture toughness parameter J IC or K IC ; 3) Use a non-contact laser scanning system with a resolution no higher than 1 μm to globally scan the fracture surface of the fractured specimen to obtain point cloud data; denoise, splice, and register the point cloud, reconstruct the three-dimensional morphology of the fracture surface, and calculate the arithmetic mean roughness S of the fracture surface according to ISO 25178 a ; 4) Apply the improved box-counting method to the three-dimensional fracture morphology reconstructed in step 3 for fractal analysis to obtain the fractal dimension D of the fracture surface f ; 5) Using the fracture toughness parameter J IC or K IC obtained in step 2 as the dependent variable, and using the surface roughness S a obtained in step 3 and the fractal dimension D f obtained in step 4 as the independent variables, a multiple nonlinear model is established. When the coefficient of determination R 2 of the model is ≥ 90%, it is determined that the model can accurately predict the fracture toughness of the pipeline steel girth weld material based on the fracture surface roughness and the fractal dimension.

2. The method for evaluating the toughness of the girth weld of pipeline steel based on the fractal reconstruction of the fracture morphology according to claim 1, characterized in that, The calculation method of the fracture toughness value is: J = 3.75R m (Δa - 0.2)(4) Among them, P is the indenter load during the test, in N; S is the span, in m; B is the specimen thickness, in m; B N is the net thickness of the specimen, in m; ν is the Poisson's ratio; E is the elastic modulus, in Pa; U p is the plastic part of the area of the load-displacement curve, in J; R m is the tensile strength, in Pa.

3. The method for evaluating the toughness of the girth weld of pipeline steel based on the fractal reconstruction of the fracture morphology according to claim 1, characterized in that The relevant fractal dimension calculation equation of the box-counting method is: Among them, D f is the fractal dimension; N total (δ) is the total number of boxes used in the box-counting method, and δ is the box size used in different box-counting methods.

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