Method for obtaining critical stress intensity factor of hydrogen environment material

Abstract

The invention discloses a method for obtaining a critical stress intensity factor of a hydrogen environment material, and belongs to the technical field of stress intensity factor measurement. According to the method disclosed by the invention, a hollow notched sample is prepared from a suitable pipeline material according to hydrogen embrittlement research, then the two ends of the sample are filled with hydrogen, tensile test loading is carried out, and the test is stopped after the sample is failed and fractured due to hydrogen embrittlement. The method is more in line with the actual stress state of the pipeline, can provide the material stress intensity factor value and the critical fracture stress intensity factor of the material under different loads, and has important significance for evaluating the fracture toughness and crack resistance of the material in the hydrogen environment.

Description

Technical Field

[0001] The present invention belongs to the technical field of stress intensity factor measurement, and particularly relates to a method for obtaining the critical stress intensity factor of materials in a hydrogen environment. Background Art

[0002] The critical stress intensity factor refers to the minimum stress intensity factor at which a crack undergoes unstable propagation under an external load. Existing specimens for testing the critical stress intensity factor are compact tension (CT) specimens or three-point bending specimens, and CT specimens or three-point bending specimens have a certain applicability in evaluating the fracture properties of pipeline materials. However, when considering hydrogen transmission pipelines, it is also necessary to consider hydrogen permeation and hydrogen embrittlement problems. The diffusion of hydrogen in metals and its reaction with metals may lead to brittle fracture of pipeline materials. The evaluation of these problems requires special tests and analyses. Hydrogen environment CT or three-point bending experiments are usually carried out in an autoclave (hydrogen environment chamber), and the specimen is completely surrounded by hydrogen. However, in actual hydrogen transmission pipelines, only the inner surface is in contact with hydrogen. Therefore, the geometric shape and loading method of hydrogen environment CT or three-point bending specimens are difficult to accurately simulate the stress state in actual pipelines, and cannot accurately reflect the mechanical properties of materials in the actual service environment, and cannot provide a direct evaluation of hydrogen permeation and hydrogen embrittlement. Moreover, the preparation of CT and three-point bending specimens is complex, especially the prefabrication of cracks, and the entire experiment is expensive and time-consuming. Summary of the Invention

[0003] In order to overcome the above-mentioned disadvantages of the prior art, the purpose of the present invention is to provide a method for obtaining the critical stress intensity factor of materials in a hydrogen environment, which is used to solve the technical problem that the geometric shape and loading method are difficult to accurately simulate the actual stress state in a hydrogen transmission pipeline when using CT or three-point bending specimens for simulation tests.

[0004] To achieve the above purpose, the present invention adopts the following technical solutions:

[0005] The present invention discloses a method for obtaining the critical stress intensity factor of materials in a hydrogen environment, including the following steps:

[0006] S1: Cut and process materials from the material to be tested to prepare a hollow specimen; then pre-set a crack in the hollow specimen to obtain a hollow notched specimen for the test;

[0007] S2: Charge hydrogen into the hollow notched specimen and simultaneously conduct a tensile test. When the hollow notched specimen fails and fractures, stop charging hydrogen and break the specimen;

[0008] S3: Collect tensile test parameters and calculate the critical stress intensity factor.

[0009] Furthermore, the shape of the hollow notched specimen is a cylindrical specimen with a hollow center in the middle;

[0010] Preparing a notch on the hollow specimen means introducing a notch around the center circumference of the inner surface of the hollow specimen.

[0011] Furthermore, the hollow structure of the hollow notched specimen is formed by electric discharge machining for perforation, three - time wire cutting, and sandstone grinding in sequence; the surface roughness of the inner surface of the hollow structure is less than 1;

[0012] The outer diameter of the smooth section of the middle experimental part of the hollow notched specimen is 11 mm, the length of the experimental part is 44 mm, and the

[0013] Furthermore, the depth is 2.34 mm, and the diameter of the middle hollow part is 2 mm. Before performing step S2, the outer surface of the hollow notched specimen needs to be treated; the outer surface treatment includes surface cleaning, electrochemical polishing, and pickling.

[0014] Furthermore, in S2, when hydrogen is charged into the hollow notched specimen, the injection pressure of hydrogen is determined according to the actual service conditions (usually between 0.1 and 100 MPa), the injection concentration of pure hydrogen refers to the hydrogen for experiments, and it should meet the technical requirements of high - purity hydrogen in GB / T 3634.2, and the hydrogen - doped gas is determined according to the gas in the actual pipeline.

[0015] Furthermore, in S3, the fracture parameters collected from the tensile test include the area enclosed by the boundary between the slow crack growth region and the fast crack growth region, the axial load at fracture, the pressure at the core of the hollow specimen, the thickness of the crack root from the specimen surface, and the radius of the middle hole.

[0016] Furthermore, in S3, the tensile test parameters are collected and the critical stress intensity factor is calculated, which is calculated by the following formula:

[0017]

[0018] Among them, K IC is the critical stress intensity factor in the hydrogen environment, F is the axial load at fracture, p is the pressure at the core of the hollow specimen, t is the crack depth, a is the thickness of the crack root from the specimen surface, and r is the radius of the middle hole.

[0019] Furthermore, the crack depth t is obtained by analyzing the fracture surface of the specimen after tensile fracture;

[0020] The steps of the analysis are as follows: Check the fracture surface of the specimen after tensile fracture, determine the boundary between the slow crack growth region and the fast crack growth region through an optical microscope, and take the distance from the outside of the specimen to this boundary as the crack depth t of the critical stress intensity factor. The calculation formula is as follows:

[0021]

[0022] Among them, S is the area enclosed by the boundary between the slow crack growth region and the fast crack growth region.

[0023] Furthermore, the tensile test is a uniaxial loading carried out using a tensile testing machine.

[0024] Furthermore, when conducting the tensile test, the axis of the hollow notched specimen is kept consistent with the direction of the tensile test loading.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] The present invention discloses a method for obtaining the critical stress intensity factor of a hydrogen environment material. A hollow notched specimen is prepared according to a suitable pipeline material for hydrogen embrittlement research, and then hydrogen is filled at both ends of the specimen and tensile test loading is carried out. The test is stopped after the specimen fails and fractures after hydrogen embrittlement. The fracture surface of the cross-section of the broken hollow notched specimen is inspected, and the critical stress intensity factor conforming to the material is calculated. This method uses a hollow notched specimen to guide crack propagation under an external load, applies hydrogen pressure to make it absorb hydrogen. Compared with other complex methods, the preparation and measurement processes of the hydrogen embrittlement-specific hollow specimen are relatively easy. Since it is more in line with the stress state of the actual pipeline, the method of measuring the stress intensity factor with the hydrogen embrittlement-specific hollow specimen is a relatively accurate measurement method, which can provide the material stress intensity factor values under different loads and the critical stress intensity factor (fracture toughness) of the material, and is of great significance for evaluating the fracture toughness and crack resistance of the material. Moreover, this method sets a high-pressure hydrogen environment in the core of the specimen, requires very little hydrogen, and does not require an additional hydrogen pressure maintaining device. Compared with the experimental method in an autoclave (hydrogen environment chamber), the equipment cost and maintenance cost are reduced by 1 / 5 to 1 / 2. The method of measuring the stress intensity factor with the hydrogen embrittlement-specific hollow specimen proposed in this patent provides specific details in aspects such as specimen preparation, test conditions, and data analysis used in this method, making the hydrogen embrittlement-specific hollow specimen a reliable and reproducible measurement method. The method of measuring the stress intensity factor with the hydrogen embrittlement-specific hollow specimen is applicable to various materials, including metals, ceramics, and composite materials, etc. Description of the Drawings

[0027] Figure 1 is a flowchart of the method of the present invention;

[0028] Figure 2 is a partial schematic view of the hollow notched specimen of the present invention;

[0029] Figure 3 is a typical specimen drawing of the hollow notched specimen of the present invention;

[0030] Figure 4Schematic diagram of the area enclosed by the boundary between the slow crack growth region and the fast crack growth region;

[0031] Figure 5 Comparison diagram of the critical stress intensity factor measured by the method of the present invention and CT specimens. Detailed implementation manners

[0032] In order to enable those skilled in the art to better understand the solution of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0033] It should be noted that the terms "first", "second", etc. in the description and claims of the present invention and the above-mentioned drawings are used to distinguish similar objects, and do not have to be used to describe a specific order or sequence. It should be understood that such data can be interchanged under appropriate circumstances so that the embodiments of the present invention described here can be implemented in an order different from those illustrated or described here. In addition, the terms "comprising" and "having" and any variations thereof are intended to cover non-exclusive inclusion. For example, a process, method, system, product or device including a series of steps or units does not necessarily have to be limited to those steps or units clearly listed, but may include other steps or units not clearly listed or inherent to these processes, methods, products or devices.

[0034] As Figure 1 shown, the present invention discloses a method for obtaining the critical stress intensity factor of a hydrogen environment material, including the following steps:

[0035] Material preparation and specimen preparation: Pre-cut materials from the structure or material blank to be tested, and process the blank by machining. For hollow specimens, electrical discharge machining, three-pass wire cutting, and sandstone grinding should be used in sequence to process to the specified intermediate hollow size.

[0036] According to Figure 2 the shape, prepare a hollow specimen; then pre-set a notch on the hollow specimen to obtain a hollow notched specimen for testing. Figure 3 is a recommended typical specimen drawing; the outer diameter of the smooth section in the middle experimental part of the hollow notched specimen is 11 mm, the length of the experimental part is 44 mm, the notch depth is 2.34 mm, and the diameter of the middle hollow part is 2 mm. The processing of the middle hollow structure of the specimen includes electrical discharge machining, three-pass wire cutting, and sandstone grinding. The inner surface roughness of the hollow specimen should be less than 1 specimen; the depth of the pre-set notch is 2.38 mm.

[0037] Surface treatment of specimens: The surface of the hollow notched specimens is cleaned to remove impurities, oil stains and oxides, and finally electrochemically polished or pickled.

[0038] Experimental loading: After hydrogen is introduced into the middle notched specimen from both ends of the specimen, uniaxial loading is applied to the specimen.

[0039] Crack detection: During the loading process, the propagation state of the crack at the notch is determined. When the specimen fails and fractures, hydrogen charging is stopped and the specimen is pulled apart.

[0040] Data processing: It includes the area S ( Figure 4 as shown) enclosed by the boundary between the slow crack growth region and the fast crack growth region, the axial load F at fracture, the pressure p at the core of the hollow specimen, the thickness a of the crack root from the specimen surface, and the radius r of the middle hole.

[0041] Calculation of the critical stress intensity factor: The critical stress intensity factor K is calculated by the formula Ic .

[0042] Examples

[0043] For 4 types of pipeline steel materials, namely X60M straight seam submerged arc welded (SAWL) steel pipe, X65M SAWL steel pipe, X70M SAWL steel pipe, and X80M SAWL steel pipe, the yield strengths are 475.8 MPa, 517.1 MPa, 558.5 MPa, and 592.9 MPa respectively.

[0044] The critical stress intensity factor is tested using traditional CT specimens and the hollow notched specimens proposed in the present invention. The dimensions of the CT specimens are thickness B = 12.7 mm, width W = 25.4 mm. The specimens have 5% side grooves and prefabricated fatigue cracks, a (crack depth) / W = 0.55, and the hollow notched specimens adopt the specimen form proposed in the present invention.

[0045] The CT specimens are tested in a high-pressure autoclave in a hydrogen environment (pressure is 15 MPa). The test is carried out in accordance with GB / T21143, using the rising displacement method for control, and the rising rate of K is 0.005 N·mm -3 / 2 / s. The critical stress intensity factor K is obtained by conversion through CTOD or J integral. In the hollow notched test, the hydrogen pressure in the hollow part of the specimen is 15 MPa, and the stress intensity factor is calculated using the method proposed in the present invention. The components of hydrogen are: 99.999% pure H 2 , H 2 O < 3 ppm; O 2 < 2 ppm and N 2 < 6 ppm; There are 3 specimens for parallel specimens under the same conditions.

[0046] The calculated results of the critical stress intensity factors of the two testing methods are as follows Figure 5 shown. The results of the two methods for the same material are similar, indicating that the hollow notched specimen is also a reliable method for testing the critical stress intensity factor in a hydrogen environment.

[0047] In the present invention, a hollow specimen is used for experimental simulation and calculation of the critical stress intensity factor. The preparation is relatively simple. Usually, a hydrogen pressure is applied to the specimen to make it absorb hydrogen. This can introduce hydrogen embrittlement in the specimen, and sufficient pretreatment is required to measure the stress intensity factor. Compared with other complex methods, the preparation and measurement process of the hollow specimen dedicated to hydrogen embrittlement is relatively easy; since it is more in line with the stress state of the actual pipeline, the method of measuring the stress intensity factor with the hollow specimen dedicated to hydrogen embrittlement is a relatively accurate measurement method, which can provide the values of the stress intensity factor of the material under different loads and the critical fracture of the material, and is of great significance for evaluating the fracture toughness and crack resistance of the material; the present patent proposes a method for measuring the stress intensity factor with a hollow specimen dedicated to hydrogen embrittlement, providing specific details such as specimen preparation, test conditions, and data analysis used in this method, making the hollow specimen dedicated to hydrogen embrittlement a reliable and repeatable measurement method; the method of measuring the stress intensity factor with the hollow specimen dedicated to hydrogen embrittlement is applicable to various materials, including metals, ceramics, and composite materials, etc. This wide applicability makes the hollow specimen dedicated to hydrogen embrittlement a common means for studying the fracture properties of materials; by measuring the stress intensity factor with a hydrogen embrittlement specimen, information on crack propagation behavior can be obtained. This information is of great significance for the study of the fracture mechanism of materials and engineering applications, and can help us better understand the life and durability of materials under the action of hydrogen.

[0048] The hollow notched specimen used in the present invention usually adopts a hollow cylindrical design, and the shape is as shown in Figure 2 and Figure 3 shown, with an initial notch to generate stress concentration, thus guiding crack propagation under an external load. The geometric dimensions of the specimen need to be designed according to specific experimental requirements, including specimen length, diameter, and crack length, etc. The material of the specimen needs to conform to the material characteristics of the hydrogen transmission pipeline; hydrogen is injected into the central hole of the specimen to generate hydrogen embrittlement. The injection pressure and concentration of hydrogen need to be controlled according to experimental requirements to ensure the observability and measurability of crack propagation; a tensile load is applied to the hollow notched specimen in a hydrogen environment, which can be controlled by a tensile testing machine. The rate and amplitude of specimen stress loading need to be set according to experimental requirements to obtain accurate measurement results of the stress intensity factor.

[0049] The above content is only to illustrate the technical idea of the present invention, and the protection scope of the present invention cannot be limited thereby. Any modification made on the basis of the technical solution according to the technical idea proposed by the present invention falls within the protection scope of the claims of the present invention.

Claims

1. A method for obtaining the critical stress intensity factor of a hydrogen environment material, characterized in that, it includes the following steps: S1: Cut a processing material from the material to be tested and prepare a hollow specimen; then pre-set a crack in the hollow specimen to obtain a hollow notched specimen for the test; S2: Hydrogenate the hollow notched specimen while performing a tensile test. When the hollow notched specimen fails and fractures, stop hydrogenation and break the specimen; S3: Collect the tensile test parameters and calculate the critical stress intensity factor.

2. The method for obtaining the critical stress intensity factor of a hydrogen environment material according to claim 1, characterized in that, the shape of the hollow notched specimen is a cylindrical specimen with a hollow center in the middle; The pre-setting of the notch for the hollow specimen is to introduce a notch in a circle on the central circumference of the inner surface of the hollow specimen.

3. The method for obtaining the critical stress intensity factor of a hydrogen environment material according to claim 2, characterized in that, the hollow structure of the hollow notched specimen is formed by electric discharge machining, three - wire cutting and sandstone grinding in sequence; the surface roughness of the inner surface of the hollow structure is less than 1; The outer diameter of the smooth section of the middle experimental part of the hollow notched specimen is 11 mm, the length of the experimental part is 44 mm, the notch depth is 2.34 mm, and the diameter of the middle hollow part is 2 mm.

4. The method for obtaining the critical stress intensity factor of a hydrogen environment material according to claim 1, characterized in that, before performing step S2, the outer surface of the hollow notched specimen needs to be treated; the outer surface treatment includes surface cleaning, electrochemical polishing and pickling.

5. The method for obtaining the critical stress intensity factor of a hydrogen environment material according to claim 1, characterized in that, in S2, when hydrogenating the hollow notched specimen, the injection pressure of hydrogen is 0.1 - 100 MPa, and the injection concentration meets the technical requirements of high - purity hydrogen in GB / T 3634.

2.

6. The method for obtaining the critical stress intensity factor of a hydrogen environment material according to claim 1, characterized in that, in S3, the collected tensile test fracture parameters include the area enclosed by the boundary between the slow crack growth region and the fast crack growth region, the axial load at fracture, the pressure at the core of the hollow specimen, the thickness of the crack root from the specimen surface, and the radius of the middle hole.

7. The method for obtaining the critical stress intensity factor of a hydrogen environment material according to claim 6, characterized in that, in S3, collect the tensile test parameters and calculate the critical stress intensity factor, which is calculated by the following formula: Among them, K IC is the critical stress intensity factor in a hydrogen environment, F is the axial load at fracture, p is the pressure at the core of the hollow specimen, t is the crack depth, a is the thickness from the crack root to the specimen surface, and r is the radius of the middle hole.

8. The method for obtaining the critical stress intensity factor of a hydrogen environment material according to claim 7, characterized in that, the crack depth t is obtained by analyzing the fracture surface of the broken specimen; the steps of the analysis are: check the fracture surface of the broken specimen, determine the boundary between the slow crack growth region and the fast crack growth region through an optical microscope, and take the distance from the outside of the specimen to this boundary as the crack depth t of the critical stress intensity factor. The calculation formula is as follows: where S is the area enclosed by the boundary between the slow crack growth region and the fast crack growth region.

9. A method for obtaining the critical stress intensity factor of a hydrogen environment material according to claim 1, characterized in that, the tensile test is a uniaxial loading carried out using a tensile testing machine.

10. A method for obtaining the critical stress intensity factor of a hydrogen environment material according to claim 9, characterized in that, when performing the tensile test, the axis of the hollow notched specimen is kept consistent with the direction of the tensile test loading.

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