Method for obtaining critical stress intensity factor of hydrogen environment material

Abstract

The application 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, a hollow notch sample is prepared according to hydrogen embrittlement research on a suitable pipeline material, then hydrogen is filled into two ends of the sample, a tensile test is loaded, and the test is stopped after the sample fails and breaks due to hydrogen embrittlement. The method is more in line with the stress state of an actual pipeline, can provide stress intensity factor values of the material under different loads and the critical fracture stress intensity factor of the material, and has important significance for evaluating the fracture toughness and crack resistance of the material in a hydrogen environment.

Description

Technical Field

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

[0002] The critical stress intensity factor (CSI) is the minimum stress intensity factor at which a crack propagates unstably under an external load. Existing specimens used to test CSIs are compact tension (CT) specimens or three-point bend tests. CT specimens and three-point bend tests have some applicability in evaluating the fracture properties of pipeline materials. However, when considering hydrogen pipelines, hydrogen permeation and hydrogen embrittlement must also be considered. Hydrogen diffusion and reaction with metals can lead to brittle fracture in pipeline materials, and evaluating these issues requires specialized testing and analysis. CT or three-point bend tests in hydrogen environments are typically conducted in an autoclave (hydrogen environmental chamber), completely enveloping the specimen in hydrogen. In reality, only the inner surface of a hydrogen pipeline is exposed to hydrogen. Therefore, the geometry and loading methods of these CT or three-point bend specimens in hydrogen environments make it difficult to accurately simulate the stress state in actual pipelines. They cannot accurately reflect the mechanical properties of the material in real-world service environments and cannot provide a direct assessment of hydrogen permeation and hydrogen embrittlement. Furthermore, the preparation of CT and three-point bend specimens is complex, especially the crack prefabrication, making the entire experiment expensive and time-consuming. Summary of the Invention

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

[0004] In order to achieve the above object, the present invention adopts the following technical solutions:

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

[0006] S1: Cut the processed material from the material to be tested to prepare a hollow specimen; then pre-crack the hollow specimen to obtain a hollow notch specimen for testing;

[0007] S2: The hollow notch specimen is charged with hydrogen and a tensile test is performed simultaneously. When the hollow notch specimen fails and breaks, the hydrogen charging is stopped and the specimen is broken;

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

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

[0010] The prefabricated notch in the hollow sample is a notch introduced around the central circumference of the inner surface of the hollow sample.

[0011] Furthermore, the hollow structure of the hollow notch specimen is formed by sequentially adopting electric spark perforation, three-way wire cutting and sand and stone grinding; the inner surface roughness of the hollow structure is less than 1;

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

[0013] Furthermore, the depth is 2.34 mm and the diameter of the middle hollow part is 2 mm. Before performing step S2, the hollow notch sample needs to be subjected to external surface treatment, which includes surface cleaning, electrochemical polishing and pickling.

[0014] Furthermore, in S2, when the hollow gap specimen is filled with hydrogen, 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 used in the reference test should meet the technical requirements of high-purity hydrogen in GB / T 3634.2, and the hydrogen-blended gas is determined according to the gas in the actual pipeline.

[0015] Furthermore, in S3, the collected tensile test fracture parameters include the area enclosed by the boundary between the slow crack growth zone and the fast crack growth zone, the axial load at fracture, the pressure at the center 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, tensile test parameters are collected and the critical stress intensity factor is calculated, which is obtained by the following formula:

[0017]

[0018] Among them, K IC is the critical stress intensity factor in hydrogen environment, F is the axial load at fracture, p is the pressure at the center 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.

[0019] Furthermore, the crack depth t is obtained by analyzing the fracture surface of the sample after being pulled apart;

[0020] The analysis steps are as follows: inspecting the fracture surface of the sample after being pulled apart, determining the boundary between the slow crack growth zone and the fast crack growth zone by optical microscopy, and taking the distance from the outside of the sample to the boundary as the crack depth t of the critical stress intensity factor, and the calculation formula is as follows:

[0021]

[0022] Where S is the area enclosed by the boundary between the slow crack growth zone and the fast crack growth zone.

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

[0024] Furthermore, when conducting a tensile test, the axis of the hollow notch specimen is kept consistent with the direction of 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 material in a hydrogen environment. According to hydrogen embrittlement research, a hollow notch specimen is prepared from a suitable pipeline material. Subsequently, hydrogen is filled at both ends of the specimen and a tensile test is performed. The test is stopped after the specimen fails and breaks due to hydrogen embrittlement. The fracture of the cross section of the hollow notch specimen after being pulled apart is inspected, and the critical stress intensity factor of the material is calculated. The method uses a hollow notch specimen to guide crack propagation under an external load, and applies hydrogen pressure to absorb hydrogen. Compared with other complex methods, the preparation and measurement process of the hollow specimen specifically for hydrogen embrittlement is relatively easy. Since it is more in line with the actual stress state of the pipeline, the method of measuring the stress intensity factor of the hollow specimen specifically for hydrogen embrittlement is a relatively accurate measurement method, which can provide the stress intensity factor value of the material under different loads and the critical stress intensity factor (fracture toughness) of the material, which is very useful for evaluating the fracture toughness and resistance of the material. The method is of great significance for the crack performance. In addition, the method sets up a high-pressure hydrogen environment in the core of the sample, and the amount of hydrogen required is very small. No additional hydrogen pressure maintaining device is required. Compared with the experimental method in the autoclave (hydrogen environment box), the equipment cost and maintenance cost are reduced by 1 / 5 to 1 / 2; the hollow sample specially used for hydrogen embrittlement proposed in this patent measures the stress intensity factor, and provides specific details of the sample preparation, test conditions and data analysis used in this method, making the hollow sample specially used for hydrogen embrittlement a reliable and repeatable measurement method; the method of measuring the stress intensity factor of the hollow sample specially used for hydrogen embrittlement is applicable to various materials, including metals, ceramics and composite materials. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0028] Figure 2 It is a partial schematic diagram of the hollow notch specimen of the present invention;

[0029] Figure 3 This is a typical sample drawing of the hollow notch sample of the present invention;

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

[0031] Figure 5 The figure is a comparison diagram of the critical stress intensity factors of the method of the present invention and the CT specimen test. DETAILED DESCRIPTION

[0032] In order to enable those skilled in the art to better understand the solutions 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 drawings in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts should fall within the scope of protection 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 are not necessarily used to describe a specific order or sequence. It should be understood that the numbers used in this way can be interchanged where appropriate, so that the embodiments of the present invention described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions. For example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

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

[0035] Material preparation and sample preparation: Pre-cut materials from the structure or material blank to be tested, and process the blank by machining. The hollow specimen should be processed in sequence by electric spark perforation, three-way wire cutting and sand and stone grinding to the specified middle hollow size.

[0036] according to Figure 2 shape, prepare a hollow specimen; then pre-notch the hollow specimen to obtain a hollow notch specimen for testing. Figure 3 This is a recommended typical specimen drawing. The outer diameter of the smooth section of the hollow notch specimen's central test section is 11mm, the test section length is 44mm, the notch depth is 2.34mm, and the diameter of the central hollow section is 2mm. The hollow structure of the specimen is processed using EDM drilling, three-stage wire cutting, and sand and gravel grinding. The inner surface roughness of the hollow specimen should be less than that of the specimen 1. The depth of the pre-set notch is 2.38mm.

[0037] Sample surface treatment: Clean the surface of the hollow notch sample to remove impurities, oil stains and oxides, and finally perform electrochemical polishing or pickling.

[0038] Experimental loading: After hydrogen is injected into the middle notch sample from both ends of the sample, the sample is subjected to uniaxial loading.

[0039] Crack detection: Determine the expansion state of the crack at the notch during the loading process. When the sample fails and breaks, stop hydrogen charging and pull the sample apart.

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

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

[0042] Example

[0043] For four types of pipeline steel materials, namely X60M straight seam submerged arc welded (SAWL) steel pipe, X65MSAWL steel pipe, X70MSAWL steel pipe, and X80M SAWL steel pipe, the yield strengths are 475.8MPa, 517.1MPa, 558.5MPa, and 592.9MPa, respectively.

[0044] Critical stress intensity factor (CSIF) tests were conducted using conventional CT specimens and hollow notch specimens proposed in this invention. The CT specimens had a thickness of B = 12.7 mm and a width of W = 25.4 mm. The specimens had a 5% side groove and a prefabricated fatigue crack, with a (crack depth) / W = 0.55. The hollow notch specimens used the specimen configuration proposed in this invention.

[0045] The CT specimens were tested in a hydrogen atmosphere autoclave (pressure of 15 MPa). The test was carried out in accordance with GB / T21143, using the rising displacement method, and the rising rate of K was 0.005 N·mm -3 / 2 / s, and the critical stress intensity factor K is obtained by CTOD or J-integral conversion. In the hollow notch test, the hydrogen pressure in the hollow portion of the specimen is 15 MPa, and the stress intensity factor is calculated using the method proposed in this invention. The hydrogen composition is: 99.999% pure H2, H2O <3 ppm; O2 <2 ppm; and N2 <6 ppm. Three parallel specimens are tested under the same conditions.

[0046] The calculation results of critical stress intensity factors of the two test methods are as follows: Figure 5As shown in the figure, the results of the two methods for the same material are similar, which shows that hollow notch specimens are 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 to perform experimental simulation calculations of the critical stress intensity factor. The preparation is relatively simple, and hydrogen pressure is usually applied to the specimen to make it absorb hydrogen. This can introduce hydrogen embrittlement into 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 specifically for hydrogen embrittlement is relatively easy; because it is more consistent with the stress state of the actual pipeline, the method of measuring the stress intensity factor of the hollow specimen specifically for hydrogen embrittlement is a relatively accurate measurement method, which can provide the stress intensity factor value of the material under different loads and the critical fracture of the material, which is of great significance for evaluating the fracture toughness and crack resistance of the material; the method of measuring the stress intensity factor of the hollow specimen specifically for hydrogen embrittlement proposed in this patent provides specific details of the specimen preparation, test conditions and data analysis used in this method, making the hollow specimen specifically for hydrogen embrittlement a reliable and repeatable measurement method; the method of measuring the stress intensity factor of the hollow specimen specifically for hydrogen embrittlement is applicable to various materials, including metals, ceramics and composite materials. This wide range of applicability makes hydrogen embrittlement-specific hollow specimens a common tool for studying material fracture properties. Measuring the stress intensity factor (SIF) on hydrogen embrittlement specimens can provide information on crack growth behavior. This information is crucial for studying fracture mechanisms and engineering applications, helping us better understand the lifespan and durability of materials exposed to hydrogen.

[0048] The hollow notch specimen used in the present invention is usually designed as a hollow cylindrical specimen, with a shape such as Figure 2 and Figure 3 As shown, the initial notch produces stress concentration, thereby guiding crack propagation under external load. The geometric dimensions of the specimen need to be designed according to specific experimental requirements, including specimen length, diameter, and crack length. The material of the specimen needs to conform to the material properties of the hydrogen transmission pipeline; hydrogen is injected into the center hole of the specimen to produce 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; the hollow notch specimen is subjected to a tensile load 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 stress intensity factor measurement results.

[0049] The above content is only for explaining the technical idea of ​​the present invention and cannot be used to limit the protection scope of the present invention. Any changes made on the basis of the technical solution in accordance with the technical idea proposed by the present invention shall fall within the protection scope of the claims of the present invention.

Claims

1. A method for obtaining the critical stress intensity factor of a material in a hydrogen environment, characterized in that: The following steps are involved: S1: Cut the processed material from the material to be tested to prepare a hollow specimen; then pre-fabricate a notch in the hollow specimen to obtain a hollow notch specimen for testing; S2: The hollow notch specimen is charged with hydrogen and a tensile test is performed simultaneously. When the hollow notch specimen fails and breaks, the hydrogen charging is stopped and the specimen is broken; S3: Collect the fracture parameters of the tensile test and calculate the critical stress intensity factor; The shape of the hollow notch specimen is a cylindrical specimen with a hollow center; The prefabricated notch in the hollow specimen is a notch introduced around the central circumference of the inner surface of the hollow specimen; The hollow structure of the hollow notch specimen is formed by sequentially using electric spark perforation, three-way wire cutting and sand and stone grinding; The outer diameter of the smooth section of the middle experimental part of the hollow notch 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 hole is 2 mm; In S3, the collected fracture parameters of the tensile test include the area enclosed by the boundary between the slow crack growth zone and the fast crack growth zone, the axial load at fracture, the pressure at the center of the hollow specimen, the thickness of the crack root from the specimen surface, and the radius of the middle hole; In S3, the fracture parameters of the tensile test are collected and the critical stress intensity factor is calculated, which is obtained by the following formula: ； in, is the critical stress intensity factor in hydrogen environment, F is the axial load at fracture, p is the pressure at the center 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.

2. The method for obtaining the critical stress intensity factor of a material in a hydrogen environment according to claim 1, characterized in that: Before performing step S2, the hollow notch sample needs to be subjected to external surface treatment; the external surface treatment includes surface cleaning, electrochemical polishing and pickling.

3. The method for obtaining the critical stress intensity factor of a material in a hydrogen environment according to claim 1, wherein: In S2, when the hollow gap sample is filled with hydrogen, the injection pressure of hydrogen is 0.1~100MPa, and the injection concentration meets the technical requirements of high-purity hydrogen in GB / T 3634.

2.

4. The method for obtaining the critical stress intensity factor of a material in a hydrogen environment according to claim 1, characterized in that: The crack depth t is obtained by analyzing the fracture surface of the sample after being pulled apart; The analysis steps are as follows: inspecting the fracture surface of the sample after being pulled apart, determining the boundary between the slow crack growth zone and the fast crack growth zone by optical microscopy, and taking the distance from the outside of the sample to the boundary as the crack depth t of the critical stress intensity factor, and the calculation formula is as follows: ； Where S is the area enclosed by the boundary between the slow crack growth zone and the fast crack growth zone.

5. The method for obtaining the critical stress intensity factor of a material in a hydrogen environment according to claim 1, characterized in that: The tensile test is performed using a tensile testing machine with uniaxial loading.

6. The method for obtaining the critical stress intensity factor of a material in a hydrogen environment according to claim 5, characterized in that: When conducting a tensile test, keep the axis of the hollow notch specimen consistent with the direction of tensile test loading.

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