Method for testing the fracture toughness of girth welds of pipes

Abstract

The application discloses a pipeline girth weld fracture toughness testing method, which comprises the following steps: processing a notch and a fatigue pre-crack on a test piece with a predetermined size, wherein the length direction of the test piece is consistent with the axial direction of the pipeline; expanding the fatigue pre-crack at the front end of the notch of the test piece and performing fatigue pre-cracking; after the fatigue pre-cracking reaches a target value, processing a side groove on the test piece; continuously loading the test piece at a constant rate, while monitoring the load and displacement during the loading process; stopping after the test piece reaches a preset degree of crack expansion; during the continuous loading, measuring the crack expansion by using a regularization method; measuring the initial crack length and the final crack length; and inversing the crack length in the crack expansion process according to the measured initial crack length and the final crack length. The application can accurately obtain the fracture toughness of the pipeline girth weld, make the safety evaluation of the pipeline girth weld more accurate, and thus guarantee the safe operation of the pipeline.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of pipeline girth weld mechanical property testing, and particularly relates to a pipeline girth weld fracture toughness testing method. BACKGROUND

[0002] The current common fracture toughness testing standard is to use a deep crack single edge notched bend (SENB) or compact tension (CT) specimen, the purpose being to ensure that a higher constraint is generated at the crack tip. However, actual pipeline girth weld cracks are usually defects generated in the manufacturing or installation process, such as defects such as delamination, slag inclusion, incomplete penetration and the like generated in the weld, and stress corrosion cracks and the like, and have a lower constraint. The fracture toughness measured by using the high constraint specimen is used for fracture evaluation of the pipeline girth weld defects, which underestimates the ability of the pipe to resist fracture, resulting in conservative evaluation results, which has an important influence on the safe operation of the pipeline, and therefore a more reasonable evaluation method is needed to accurately obtain the fracture toughness of the pipeline girth weld.

[0003] In the prior art, the unloading compliance method is usually used in the case of high load rate, extreme temperature or corrosive environment. In the testing process using the unloading compliance method, the testing compliance is affected by the unloading loading, and there is a crack negative growth phenomenon in the initial stage of crack propagation, which affects the accuracy of the test; in addition, the test is expensive and time-consuming.

[0004] In view of the deficiencies of the existing testing methods, it is necessary to propose a new and effective testing technology for testing the fracture toughness of the pipeline girth weld, so that the safety evaluation of the pipeline girth weld is more accurate. SUMMARY

[0005] The present application provides a pipeline girth weld fracture toughness testing method, which can accurately obtain the fracture toughness of the pipeline girth weld, so that the safety evaluation of the pipeline girth weld is more accurate, thereby ensuring the safe operation of the pipeline.

[0006] The present application discloses a pipeline girth weld fracture toughness testing method, comprising:

[0007] The testing method comprises:

[0008] A notch and a fatigue pre-crack are processed on a test piece with a predetermined size, and the length direction of the test piece is consistent with the axial direction of the pipeline;

[0009] The fatigue pre-crack at the front end of the notch of the test piece is expanded, and fatigue pre-cracking is performed, and after the fatigue pre-cracking reaches a target value, a side groove is processed on the test piece;

[0010] The test piece is continuously loaded at a constant rate, and the load and displacement during the loading process are monitored, and the loading is stopped after the test piece reaches a preset degree of crack propagation, and the crack propagation is measured using a regularization method during the continuous loading;

[0011] The initial crack length and the final crack length are measured, and the crack length during crack propagation is inversely calculated according to the measured initial crack length and the final crack length.

[0012] In a preferred embodiment, the test piece is pre-cracked by fatigue under a three-point bending loading mode.

[0013] In a preferred embodiment, the crack mouth opening displacement of the test piece is measured by a COD gauge or the displacement is recorded by a loading device during the continuous loading of the test piece.

[0014] In a preferred embodiment, during the fatigue pre-cracking, the initial maximum fatigue load is P MAX = 0.8P M ;

[0015] Wherein,

[0016] W is the width of the test piece, B is the thickness of the test piece; σ YS is the yield strength of the test piece; h is the machining notch depth; S is the span.

[0017] In a preferred embodiment, the target value is a0 / W, wherein a0 is the original crack depth; W is the width of the test piece.

[0018] In a preferred embodiment, after the loading of the test piece is stopped, the test method further comprises:

[0019] The test piece is removed from the loading device and is subjected to thermal coloring;

[0020] The test piece after thermal coloring is cooled in liquid nitrogen;

[0021] The test piece cooled in liquid nitrogen is divided into two pieces to expose the fracture surface;

[0022] The crack propagation on the fracture surface of the test piece is measured by an optical measuring device to obtain the measured initial crack length and the final crack length.

[0023] In a preferred embodiment, a load-displacement curve or a load-crack mouth opening displacement curve is obtained during the loading process, and each load point on the load-displacement curve or the load-crack mouth opening displacement curve is subjected to regularization processing.

[0024] In a preferred embodiment, the regularization processing process is as follows:

[0025]

[0026] where P Ni is the normalized load at the ith load point, in units of stress; W is the specimen width; B is the specimen thickness; η pl is the J-integral plasticity factor; a bi is the blunted crack length for the ith load step, calculated by the equation:

[0027]

[0028] where a0is the initial crack length; σ Y is the effective yield strength or yield strength; J i is the J-integral for the ith load step, calculated by the J-integral equation.

[0029] In a preferred embodiment, the method further comprises, for each respective crack mouth opening displacement, normalizing to obtain a normalized plastic crack mouth opening displacement:

[0030]

[0031]

[0032] B e = B - (B - B N ) 2 / B

[0033] where C i is the compliance of the loading line based on the blunted crack a bi ; V i is the total crack mouth opening displacement; V pli is the plastic crack mouth opening displacement; the crack termination point P, V data is normalized by the final crack length a p ; a tangent is drawn from the crack termination point (P final , V final ) to the data point normalized by a0; the data points V pli / W < 0.001 and to the right of the tangent point are removed, but the crack termination point is included, and the (P Ni , V’ pli ) data is fitted;

[0034]

[0035] where a, b, c, d are coefficients, and after the coefficients are determined by fitting, the real-time crack length can be solved by back calculation.

[0036] In a preferred embodiment, the formula for calculating the J integral under plane strain crack tip constraint conditions is as follows:

[0037]

[0038] Among them, stress intensity factor K i The calculation formula is as follows:

[0039]

[0040]

[0041] The plastic part of the J integral J pl The calculation formula is as follows:

[0042]

[0043]

[0044]

[0045]

[0046]

[0047] Among them, A pl(i) -A pl(i-1) It is the increase in plastic area between the (i-1)th point and the ith point on the load-plastic crack opening displacement curve; J pl(i) The total crack propagation plasticity J integral at point i is represented by increasing J. pl(i-1) We obtain the plastic part J through integration J. pl(i) The calculation formula for J pl(i) A smaller crack propagation increment is required; A pl(i) The calculation formula is as follows:

[0048] A pl(i) =A pl(i-1) +[P (i) +P (i-1) ][V pl(i) +V pl(i-1) ] / 2

[0049] V pl(i) =Plastic portion of crack opening displacement=(V i -P i C i );

[0050] By drawing J i With Δa i The relationship between αJ and ηJ is used to obtain the J integral resistance curve, thereby determining αJ and ηJ; where, The data was fitted from a crack growth value of 0.5 mm to the last measurement using the least squares method.

[0051] The features and advantages of the present application are:

[0052] The pipeline girth weld fracture toughness test method provided in the embodiments of the present application can be tested under high load rate, extreme temperature or corrosive environment, etc., and can be used as an alternative method of the unloading compliance method, and the core advantages are as follows: the crack propagation length can be evaluated without repeating the unloading and reloading process in the test; the test specimen is loaded once in the test process, the crack length test is more convenient and accurate, the pipeline girth weld fracture toughness can be accurately obtained, the safety evaluation of the pipeline girth weld is more accurate, so as to ensure the safe operation of the pipeline; at the same time, the conservativeness of the pipeline girth weld fracture toughness is reduced, the test specimen has the characteristics similar to the pipeline, the obtained results can be well applied to the actual pipeline, and the test is economical and convenient.

[0053] Specific embodiments of the present application are disclosed in detail in the following description and claims, and are illustrated in the accompanying drawings. It should be understood that the embodiments of the present application are not limited in scope to the embodiments described herein, but extend at least to other embodiments with additional features and benefits.

[0054] Features described and / or illustrated with respect to one embodiment can be used in one or more other embodiments in the same or similar manner, in combination with or in place of features in other embodiments, or in combination with or in place of other features.

[0055] It should be emphasized that the term "comprises / comprising" when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components, or groups thereof. BRIEF DESCRIPTION OF DRAWINGS

[0056] Figure 1 A step flow chart of a novel pipeline girth weld fracture toughness test method provided in the embodiments of the present application;

[0057] Figure 2 A schematic diagram of a single-edge notched tensile specimen;

[0058] Figure 3 A schematic diagram of a single-edge notched tensile specimen;

[0059] Figure 4 A schematic diagram of a pipeline girth weld section;

[0060] Figure 5-1 A schematic diagram of a side groove processing;

[0061] Figure 5-2 A schematic diagram of a side groove processing; Figure 5-1Enlarged view of the middle side notch

[0062] Figure 6 Schematic view for the design of the specimen knife edge

[0063] Figure 7 Schematic view for the measurement of the crack length using the nine point method

[0064] Figure 8 Schematic view for the data processing of the regularization method

[0065] Figure 9 Schematic view for the data fitting of the regularization method

[0066] Figure 10 J-integral resistance curve

[0067] Figure 11-1 Front view of an X80 pipe girth weld

[0068] Figure 11-2 Top view of an X80 pipe girth weld

[0069] Figure 11-3 Schematic view of the specimen Figure 11-1 Schematic view of the cross section at A

[0070] Figure 11-4 Schematic view of the cross section at B Figure 11-2 Enlarged view of the middle side notch at B

[0071] Figure 12 Load-CMOD curve for the X80 girth weld single edge notched tension specimen

[0072] Figure 13 Regularization load curve for the X80 girth weld single edge notched tension specimen

[0073] Figure 14 Regularization method fitted data curve for the X80 girth weld single edge notched tension specimen

[0074] Figure 15 J-integral resistance curve for the X80 girth weld single edge notched tension specimen

[0075] BRIEF DESCRIPTION OF DRAWINGS

[0076] 1. Specimen

[0077] 10. Notch

[0078] 11. Clamped end

[0079] 12. Side notch

[0080] 2. Pipe girth weld DETAILED DESCRIPTION

[0081] The specific embodiments of the present application described herein are presented for purposes of illustration and description. They are not intended to be exhaustive of all of the possible embodiments of the present application. Indeed, it will be apparent to those of ordinary skill in the art that various modifications can be made to the specific embodiments described herein that fall within the scope of the present application. In some instances, well-known structures have not been described in detail in order to avoid obscuring the present application. In the drawings, like reference numerals refer to like elements throughout the several views. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items. As used herein, the term "includes" means includes but not limited to, and the term "including" means including but not limited to. As used herein, the term "based on" means based at least in part on. As used herein, the terms "vertical", "horizontal", "up", "down", "left", "right", and similar terms are used for illustration only and not intended to be limiting.

[0082] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the description of the application herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items. As used herein, the term "includes" means includes but not limited to, and the term "including" means including but not limited to. As used herein, the term "based on" means based at least in part on. As used herein, the terms "vertical", "horizontal", "up", "down", "left", "right", and similar terms are used for illustration only and not intended to be limiting.

[0083] Please refer to Figure 1 The present application provides a method for testing the fracture toughness of a girth weld of a pipeline for transporting oil and gas. The method can include the following steps:

[0084] Step S1: processing a notch and a fatigue pre-crack on a test piece with a predetermined size, the length direction of the test piece being consistent with the axial direction of the pipeline;

[0085] Step S2: extending the fatigue pre-crack at the front end of the notch of the test piece and performing fatigue pre-cracking, and after the fatigue pre-cracking reaches a target value, processing a side groove on the test piece;

[0086] Step S3: continuously loading the test piece at a constant rate while monitoring the applied load and displacement, and stopping after the test piece reaches a preset degree of crack propagation, and measuring the crack propagation using a regularization method during the continuous loading;

[0087] Step S4: measuring the initial crack length and the final crack length, and inversely calculating the crack length during the crack propagation according to the measured initial crack length and the final crack length.

[0088] The pipe girth weld fracture toughness testing method provided by the present application will be described in detail below in combination with specific drawings and implementation scenarios. Overall, the pipe girth weld fracture toughness testing method can include a pre-test preparation stage, a test stage, and a post-test data measurement and processing stage.

[0089] The pre-test preparation stage can specifically include a specimen preparation stage, a fatigue pre-crack stage, and a test device configuration stage.

[0090] 1. Specimen preparation stage

[0091] As shown in Figure 2 , the specimen 1 has a square (BxB) cross-section (i.e., B=W) and a length (H) of 10W between the clamping ends 11. The length of each clamping end 11 of the specimen is at least 4W, and the total length of the specimen 1 is at least 18W. The width (W) of the specimen 1 is as close as possible to the pipe wall thickness to make the measurement result more accurate.

[0092] The direction of the specimen 1 relative to the pipe girth weld 2 is shown in Figure 3 ; the axial direction of the specimen 1 corresponds to the axial direction of the pipe.

[0093] The specimen can be prepared using any of the following options:

[0094] a. Conventional mechanical machining is used, and then fatigue pre-cracking is performed.

[0095] b. An EDM (electrical discharge machining) notch 10 can be used, with a wire diameter of 0.15 mm or less.

[0096] To improve machining efficiency, the initial notch can be machined with a wire diameter of 0.25 mm. As shown in Figure 4 , the initial notch depth a should not exceed 2 / 3 of the original crack depth a0 during machining. The final notch is machined with a finer wire diameter to ensure that the notch width does not exceed 0.15 mm.

[0097] For specimens that require notch machining and fatigue pre-cracking, the minimum fatigue crack length should be at least the larger of 0.05W or 1.3 mm. To promote fatigue pre-cracking at low stress intensity levels, the root radius of the machined notch should be 0.08 mm or less.

[0098] For specimens that use EDM notch machining, the final EDM notch width should not exceed 0.15 mm and should not be less than the larger of 0.05W or 1.3 mm.

[0099] The ratio of the total size of the notch to the width of the specimen is in the range of 0.25≤a0 / W≤0.35.

[0100] The weld metal coupon is machined with a notch at the centerline of the weld. The heat affected zone coupon has the fatigue pre-crack or EDM notch tip located within 0.5 mm of the weld fusion line in the heat affected zone.

[0101] After the fatigue pre-crack or EDM machined notch, the coupon is machined to be in plane strain condition and to maintain a uniform and steady crack growth during testing. The side groove 12 root should be located at the center of the coupon length and perpendicular to the length of the coupon. The recommended depth of each side groove 12 is 5% of the coupon thickness, ensuring that the net coupon thickness (B N ) is equal to 0.90B. The root radius of the side groove 12 is in the range of 0.5 mm to 0.7 mm. The included angle of the side groove 12 is less than 90 degrees. The recommended side groove 12 is shown in Figure 5-1 and Figure 5-2 .

[0102] 2. Fatigue pre-crack stage

[0103] The fatigue pre-crack at the front end of the machined notch is at least greater than 0.05W or 1.3 mm.

[0104] The fatigue pre-crack can be performed under a three-point bending loading mode, which is simple. Specifically, when a bending test is performed in a three-point bending loading mode, a strip-shaped coupon is placed in a bending test fixture to form a simply supported beam, and the distance between two lower support points supporting the coupon can be adjusted according to the length of the coupon, and there is only one loading point above the coupon.

[0105] In the embodiment, the bending load fixture for the fatigue pre-crack can ensure uniform distribution of the thickness of the coupon and uniform expansion of the fatigue crack.

[0106] Before the formal test is performed, the coupon should be accurately positioned in the load fixture to ensure that the notch plane is consistent with the center of the loading point.

[0107] wherein the initial maximum fatigue load is P MAX = 0.8P M , wherein:

[0108]

[0109] W is the width of the coupon, B is the thickness of the coupon, σYS is the yield strength of the coupon, h is the machined notch depth, and S is the span.

[0110] The load ratio (P MIN / P MAX ) during the fatigue pre-crack should be in the range of 0 to 0.1.

[0111] After the fatigue pre-crack reaches the required a0 / W, the coupon should be as shown in Figure 5-1 andFigure 5-2 The machining side groove.

[0112] The present invention provides a method for testing the fracture toughness of pipe circumferential welds, which utilizes fatigue loading to pre-induce cracks and provides specific fatigue load settings and a formula for calculating the maximum fatigue load. Due to the limitations of electrical discharge machining (EDM) or wire cutting technology, the machined notch size is relatively large, resulting in a blunt crack that does not match the actual crack, leading to excessively high fracture toughness and inaccurate acquisition of the material's fracture toughness. The fatigue pre-induce crack and maximum fatigue load formula proposed in this invention ensure that no large plastic deformation occurs during crack propagation, affecting crack extension and ultimately producing a sharp crack, consistent with the effect of an actual crack, thus accurately acquiring the material's fracture toughness.

[0113] 3. Test setup stage

[0114] To conduct fracture toughness tests on pipe circumferential welds, a loading device should be provided for the test so that the applied load can be continuously measured during the test.

[0115] Among them, the force sensor and recording device shall comply with the requirements of GB / T 16825—1997.

[0116] The specimen directly includes a blade for holding the COD gauge, allowing direct measurement of CMOD (crack mouth opening displacement, crack mouth opening distance, opening displacement, or crack mouth opening displacement). The specimen blade design is as follows: Figure 6 As shown. Figure 6 The 2x dimension should be within the working range of the extensometer, the blade edge should be perpendicular to the specimen surface, and the parallel deviation should be within 0.5°. For the clamped COD gauge, it should be ensured that the extensometer and the blade contact point can rotate freely. Of course, other blades or extensometers that meet the test accuracy and measurement requirements can also be used to measure displacement.

[0117] After completing the preparatory work for the above tests (experiments), the formal testing phase can begin.

[0118] It should be noted that, compared to existing methods that use dual COD gauges to measure the crack opening displacement of SENT specimens, which require designing additional clamping tools and holding two additional COD gauges on the specimen, this invention can directly clamp a single COD gauge on the specimen to measure the crack opening displacement or use a loading device to record the displacement. The required measuring equipment is relatively simple and convenient. Furthermore, this invention can achieve its function under conditions such as large deformation, high load rate, extreme temperature or corrosive environment, or confined space, while existing equipment cannot function properly due to the influence of extreme conditions.

[0119] 4. Conducting the test

[0120] The overall objective of the test method is to load the specimen under displacement control while monitoring the applied load and the displacement of the COD gage. The test is stopped after the specimen has reached a certain degree of crack growth.

[0121] In the overloading of the crack growth using the compliance method, the specimen is continuously loaded, and the load-displacement curve (or load-CMOD curve) data of the specimen is recorded.

[0122] After each test is completed, the specimen is removed from the loading device, thermally colored, then cooled in liquid nitrogen, and then the specimen is divided into two parts to expose the fracture surface. The crack growth on the fracture surface of the specimen at the end of the test is measured by an optical measuring microscope, and the initial crack length and the final crack length are recorded and compared with the crack growth measured by the compliance method.

[0123] Before applying force to the specimen, the reading of the COD gage should be zeroed.

[0124] The specimen should be loaded at a constant rate in displacement control.

[0125] The present application continuously loads the specimen using the compliance method, avoiding the cyclic loading of the prior art of loading / unloading, thereby greatly saving the time cost, making the test process more convenient and fast.

[0126] 5. Crack measurement after the test

[0127] After the test is completed, the specimen is removed from the testing machine and thermally colored in an environment of about 300°C for 30 minutes to mark the tear zone. The thermally colored specimen can be cooled in liquid nitrogen and divided into two parts to expose the fracture surface. Some protective measures can be taken for this fracture surface, for example, a protective oil can be brushed on the fracture surface to prevent rusting. If light oil is used, the oil should be transparent so that the fracture surface can be optically measured later. After the test, the fracture surface should be photographed as soon as possible for permanent record.

[0128] The fracture surface of the specimen is examined and measured to determine the initial crack size and the final crack size. The measured crack growth can be compared with the crack growth predicted by the unloading compliance method or other techniques.

[0129] Generally, the unloading compliance method is usually used in the prior art to calculate the crack growth length. The unloading compliance method calculates the crack growth length by using the change of the remaining ligament length during crack growth, so that the compliance also changes, but due to the influence of crack closure effect during loading / unloading, there is a phenomenon of negative crack growth at the initial stage of crack growth.

[0130] The regularization method in the application utilizes the load separation principle, and inverses the crack length in the crack propagation process according to the measured initial crack length and the final crack length, thereby effectively avoiding the crack negative growth phenomenon.

[0131] The initial and final crack lengths are measured at nine equidistant points centered on the specimen centerline and to a distance of 0.005B from the root of the side groove (see Fig. 2). Figure 7 The original crack depth a0and the final crack depth a p (=a fp ) should be averaged by the measurement values close to the surface, and the result is added to the remaining seven crack depth measurement values and averaged, and the relationship is:

[0132]

[0133]

[0134] 6、Crack size calculation

[0135] The regularization processing is performed on each load point P i on the load-displacement curve or the load-CMOD curve:

[0136]

[0137] In the formula, P Ni is the normalized load at the i-th load point, which is the same as the stress unit; W is the specimen width; B is the specimen thickness; η pl is the J integral plastic factor; a bi is the blunting correction crack length of the i-th loading step, which is calculated by formula (4):

[0138]

[0139] In the formula, a0is the initial crack length; σ Y is the effective yield strength or yield strength; J i is the J integral of the i-th loading step, which is calculated by the seventh part.

[0140] For each corresponding CMOD regularization, the regularized plastic CMOD is obtained:

[0141]

[0142]

[0143] B e = B - (B - B N ) 2 / B (7)

[0144] In the formula, Ci a is the crack length at the crack termination point P, V bi is the loading line elastic compliance; V i is the total CMOD; V pli is the plastic CMOD. The crack termination point P, V data is regularized with the final crack length a p . The tangent line is drawn from the crack termination point (P final , V final ) to the data point regularized via a0, as shown in Fig. 6. The (P Figure 8 , V' pli ) data is fitted, excluding data points with V Ni / W < 0.001 and data points to the right of the tangent point (but including the crack termination point). pli

[0145]

[0146] where a, b, c, d are coefficients, after the coefficients are determined by fitting, the real-time crack length can be solved by back calculation, the fitted data is shown in Fig. 7. Figure 9

[0147] 7. Calculation of J integral

[0148] The formula for calculating J integral under the constraint condition of the plane strain crack tip is as follows:

[0149]

[0150] The formula for calculating the stress intensity factor K i is as follows:

[0151]

[0152]

[0153] The formula for calculating the plastic part J pl of J integral is as follows:

[0154]

[0155]

[0156]

[0157]

[0158]

[0159] A pl(i) -A pl(i-1) ​​is the plastic area increment between the i-1th point and the ith point on the load-plastic crack mouth opening displacement curve. J pl(i) represents the total crack propagation plastic J integral at the ith point, which is obtained by increasing J pl(i-1) CMOD and A pl ), and then considering the crack propagation increment correction result (using γ LLD ). J pl(i) needs a smaller crack propagation increment; A pl(i) is calculated as follows:

[0160] A pl(i) = A pl(i-1) +[P (i) +P (i-1) ][V pl(i) +V pl(i-1) ] / 2 (17)

[0161] Here V pl(i) = plastic part of crack mouth opening displacement = (V i -P i C i ).

[0162] Figure 10 is the J integral resistance curve. The J integral resistance curve is obtained by plotting J i versus Δa i , as shown in Figure 10 . The J integral resistance curve determined in Figure 10 conforms to the following power function formula, so that α J and η J can be determined.

[0163]

[0164] The data should be fitted from Δa = 0.5 mm to the last measured crack propagation value using the least squares method.

[0165] The pipeline girth weld fracture toughness test method proposed by the present application can be tested under high load rate, extreme temperature or corrosive environment, etc. It can be used as an alternative method of unloading flexibility method, and its core advantages are as follows:

[0166] ​The crack propagation length can be evaluated without repeating the unloading and loading process in the test; the test piece is loaded once during the test, the crack length test is more convenient and accurate, the fracture toughness of the pipe girth weld can be accurately obtained, the safety evaluation of the pipe girth weld is more accurate, so that the safe operation of the pipeline is ensured; meanwhile, the conservativeness of the fracture toughness of the pipe girth weld is reduced, the test piece has the characteristics similar to the pipeline, the obtained results can be well applied to the actual pipeline, and the test is economical and convenient.

[0167] In order to better illustrate the present application, further illustration will be made in combination with a specific scene.

[0168] In the embodiment, a natural gas pipeline girth weld is taken as an example, the pipeline material is X80 pipeline girth weld, the outer diameter of the pipeline is 1219mm, and the wall thickness of the pipeline is 18.4mm.

[0169] 1. Test piece preparation

[0170] The test piece is sampled from the X80 pipeline girth weld, the test piece has a square shape, the nominal thickness B of the test piece is 14mm, the nominal width W of the test piece is 14mm, and the length (H) between the clamping ends is 148mm, as shown in Figure 11-1 、 Figure 11-2 、 Figure 11-3 、 Figure 11-4 .

[0171] The test piece is machined by a conventional method, and then fatigue pre-cracking is performed.

[0172] The initial crack length of the test piece is 3.6mm, and the ratio of the initial crack depth size to the width of the test piece is 0.257, which meets 0.25≤a0 / W≤0.35.

[0173] The test piece is machined at the center line of the weld.

[0174] The depth of each side groove is 0.7mm, and the net thickness of the test piece is equal to 0.90B, that is, 12.6mm.

[0175] 2. Fatigue pre-cracking

[0176] The fatigue pre-crack length is 1.3mm.

[0177] The test piece is subjected to fatigue pre-cracking under the loading mode of three-point bending.

[0178] Since the nominal width W of the test piece is 14mm, the span during fatigue pre-cracking is 4W=56mm.

[0179] The initial maximum fatigue load is determined as 6kN according to formula (1).

[0180] 3. Test device

[0181] The test uses MTS-810 universal testing machine to apply load continuously during the test, and record the load, displacement and CMOD continuously during the load application.

[0182] The COD gauge is clamped at the notch of the specimen to measure CMOD directly.

[0183] 4. Test procedure

[0184] The specimen is loaded continuously at a constant rate of 0.2 mm / s in displacement control, and the load-CMOD curve is shown in Figure 12 .

[0185] 5. Crack measurement after test

[0186] After the test is completed, the specimen is removed from the loading device, and is then heat colored and cooled in liquid nitrogen. Then the specimen is divided into two pieces to expose the fracture surface. The crack propagation on the fracture surface of the specimen at the end of the test is measured by optical measuring microscope, and the initial and final crack lengths are measured at nine equidistant points centered on the centerline of the specimen and 0.07 mm from the root of the side groove. The initial crack depth a0 is determined to be 4.92 mm and the final crack depth a p is 5.91 mm by using formula (2), and the crack propagation is 0.99 mm.

[0187] 6. Calculation of crack size

[0188] Each load point P i and the corresponding CMOD on the load-CMOD curve are normalized by using formulas (3)-(7), and the results are shown in Figure 13 . Except for the data points of V pli / W <0.001 and the data points to the right of the tangent point (but including the crack termination point), the (P Ni , V’ pli ) data are fitted according to formula (8). After the coefficients are determined by fitting, the real-time crack length can be solved by back calculation, and the fitting data are shown in Figure 14 .

[0189] 7. Calculation of J integral

[0190] The stress intensity factor and J integral at each loading are calculated according to formulas (9)-(17).

[0191] 8. J integral resistance curve

[0192] The J integral resistance curve is plotted according to the crack size calculation in 6 and the calculation of J integral in 7, and is shown in Figure 15 . The J integral resistance curve formula is J = 1073.13(Δa) 0.55.

[0193] Any numerical values recited herein include all values from the lower value and up to the upper value in increments of one unit provided that there is a separation of at least 2 units between any lower value and any higher value. As an example, if it is stated that the amount of a component or a value of a process variable (for example, temperature, pressure, time and the like) is from 1 to 90, preferably from 20 to 80, more preferably from 30 to 70, it is intended that values such as 15 to 85, 22 to 68, 43 to 51, 30 to 32 etc. are expressly enumerated in this specification. For values which are less than one, one unit is considered to be 0.0001, 0.001, 0.01, 0.1 as appropriate. These are only examples of what is specifically enumerated herein and are not intended to limit the application in any way. Any numerical value, however, can be expressed as approximately or approximately.

[0194] All ranges are inclusive of the endpoints, and the endpoints themselves, unless otherwise indicated. "Approximately" or "about" used in connection with a range generally means that both endpoints are inclusive, and that the range includes all values between the two endpoints. Thus, "about 20 to 30" is intended to cover "about 20 to about 30," including at least the specified endpoints.

[0195] All articles and references, including patents and publications, cited herein are hereby incorporated by reference for all purposes. The term "consisting essentially of to describe a combination shall include the elements, ingredients, components or steps identified, and such other elements ingredients, components or steps that do not materially affect the basic and novel characteristics of the combination. The use of the term "comprising" or "including" to describe combinations of elements, ingredients, components or steps herein also is taken to cover embodiments consisting essentially of, as well as embodiments consisting of, those elements, ingredients, components or steps. The term "may" is intended to mean one or more possible values of a described attribute.

[0196] Plural elements, ingredients, components or steps can be provided by a single integrated element, ingredient, component or step. Alternatively, a single integrated element, ingredient, component or step might be divided into separate plural elements, ingredients, components or steps. To "comprise" or "comprising" as used in describing the components of the compositions of the present application means that the compositions comprise the named elements, ingredients, components or steps, but not to the exclusion of others. The term "consisting essentially of to describe a combination shall include the elements, ingredients, components or steps identified, and such other elements, ingredients, components or steps that do not materially affect the basic and novel characteristics of the combination. The use of the term "comprising" or "including" to describe combinations of elements, ingredients, components or steps herein also is taken to cover embodiments consisting essentially of, as well as embodiments consisting of, those elements, ingredients, components or steps. The term "may" is intended to mean one or more possible values of a described attribute.

[0197] It is to be understood that the above description is intended to be illustrative, and not restrictive. Many embodiments and applications other than the examples provided would be apparent to those of skill in the art upon reading the above description. The scope of the technology should be determined, not with reference to the above description, but should instead be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. The disclosures of all articles and references, including patent applications and publications, are incorporated by reference for the purpose of the present disclosure. Any portion of the subject matter described herein that is not expressly claimed in the claims is not abandoned by the statement of foregoing description.

[0198] The various embodiments in the specification are described in progressive manner, each embodiment focuses on the difference from other embodiments, and the same or similar parts between embodiments can be referred to each other.

[0199] The above embodiments are only for illustrating the technical concept and characteristics of the present application, and the purpose is to enable those skilled in the art to understand the content of the present application and implement it, and cannot limit the protection scope of the present application. Any equivalent changes or modifications made according to the spirit and principle of the present application shall be covered within the protection scope of the present application.

Claims

1. A method for testing the fracture toughness of a pipe circumferential weld, characterized in that, The testing method includes: Notches and fatigue pre-cracks are machined on specimens of predetermined dimensions, wherein the length direction of the specimen is consistent with the axial direction of the pipe; The fatigue precrack at the notch tip of the specimen is expanded and fatigue precrack is performed. After the fatigue precrack reaches the target value, a side groove is machined on the specimen. The specimen is continuously loaded at a constant rate, while the load and displacement are monitored during the loading process. Loading is stopped once the specimen reaches a predetermined crack propagation level. During continuous loading, crack propagation is measured using a regularization method. The regularization process for each load point during loading includes: In the formula, P Ni It is the first i The standardized load at each load point is in the same unit as the stress. W It is the width of the specimen; B It is the thickness of the specimen; η pl yes J Integral plasticity factor; a bi It is the first i The passivation correction crack length for each loading step is calculated by the following formula: In the formula, a 0 It is the initial crack length; σ Y Effective yield strength or yield strength; J i It is the first i Each loading step J The integral is obtained using the formula for J-integral; The method further includes: regularizing the crack opening displacement for each corresponding crack tip to obtain the regularized plastic crack tip opening displacement: In the formula, C i For crack correction based on passivation a bi The elastic flexibility of the loaded line; V i Is the general Crack mouth opening displacement ; V pli It refers to the displacement of the opening of the plastic crack mouth; the crack termination point. P , V Data using final crack length a p Perform regularization processing; starting from the crack termination point ( P final , V final Draw a tangent to the meridian a 0 Data points that undergo rule-based processing; remove V pli / W Data points <0.001 and data points to the right of the tangent point, but including the crack termination point, will be ( P Ni , V’ pli Fit the data; Where a, b, c, and d are coefficients. After the coefficients are determined by fitting, the real-time crack length can be calculated in reverse. Measure the initial crack length and the terminal crack length, and inversely calculate the crack length during the crack propagation process based on the measured initial crack length and terminal crack length.

2. The method for testing the fracture toughness of pipe circumferential welds as described in claim 1, characterized in that, The specimen was subjected to fatigue pre-cracking under a three-point bending loading condition.

3. The method for testing the fracture toughness of pipe circumferential welds as described in claim 1, characterized in that, During the continuous loading of the specimen, the displacement of the crack opening is measured by a COD gauge or the displacement is recorded using a loading device.

4. The method for testing the fracture toughness of pipe circumferential welds as described in claim 1, characterized in that, During the fatigue pre-cracking process, the initial maximum fatigue load is P MAX =0.8 P M ; in, W is the width of the specimen, and B is the thickness of the specimen; σ YS denoted as σy, where σy is the yield strength of the specimen; h is the depth of the machining notch; and S is the span.

5. The method for testing the fracture toughness of pipe circumferential welds as described in claim 4, characterized in that, The target value is a 0 / W ,in, a 0 denoted as the original crack depth; W represents the width of the specimen.

6. The method for testing the fracture toughness of a pipe circumferential weld as described in claim 1, characterized in that, After the loading of the specimen is stopped, the testing method further includes: The specimen was removed from the loading device and subjected to thermal coloring. The hot-colored specimens were cooled in liquid nitrogen; The specimen, after being cooled in liquid nitrogen, was split into two pieces, exposing the fracture surface. The crack propagation on the fracture surface of the specimen is measured using an optical measuring device to obtain the initial crack length and the final crack length.

7. The method for testing the fracture toughness of pipe circumferential welds as described in claim 3, characterized in that, During the loading process, load-displacement curves or load-crack opening displacement curves are obtained, and each load point on the load-displacement curve or load-crack opening displacement curve is regularized.

8. The method for testing the fracture toughness of a pipe circumferential weld as described in claim 1, characterized in that, Under plane strain crack tip confinement conditions J The formula for calculating integrals is as follows: Among them, stress intensity factor K i The calculation formula is as follows: J Plastic part of the integral J pl The calculation formula is as follows: in, A pl(i) - A pl(i-1) It is the first load-plastic crack opening displacement curve. i -1 point and the i The increase in plastic area between points; J pl(i) Indicates the first i Total crack propagation plasticity at the point J Points, by increasing J pl(i-1) Obtain; the plastic part obtained through J integral J pl(i) Calculation formula J pl(i) A smaller crack propagation increment is required; A pl(i) The calculation formula is as follows: V pl(i) =The plastic part of the crack opening displacement=( V i - P i C i ); By drawing J i and a i To obtain relationships J Integral resistance curve, to determine J and J; where, Use the least squares method to transfer the data from a =0.5mm was fitted to the final measured crack propagation value.

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