A prefabricated fatigue crack method for testing the fracture toughness of corrosion-resistant metal materials

Abstract

The application provides a prefabricated fatigue crack method for corrosion-resistant metal material fracture toughness test, and relates to the technical field of metal material fracture performance detection, and at least comprises the following steps: S1, providing a metal material sample with a certain size; S2, taking a conventional sample, establishing a finite element model of the conventional sample, analyzing a hypothetical crack, and determining a notch structure; S3, machining the notch structure of step S2 on the metal material sample, and then performing fatigue loading, so that the center of the metal material sample and the front end of the notch on the sample surface can simultaneously initiate and expand fatigue cracks, and then a straight prefabricated fatigue crack is obtained. The method is simple, a notch structure capable of realizing a uniform stress field is obtained through finite element analysis, and a straight prefabricated fatigue crack can be obtained when the sample is subjected to fatigue loading.

Description

Technical Field

[0001] This invention relates to the field of fracture performance testing technology for metallic materials, and more specifically, to a method for pre-fabricated fatigue cracks for fracture toughness testing of corrosion-resistant metallic materials. Background Technology

[0002] Fracture toughness reflects the ability of a cracked material to resist crack propagation when subjected to load. During the production, processing and service of materials, defects (such as inclusions, welding defects, cracks, etc.) are inevitably generated. These defects gradually induce microcracks and form macrocracks during service. Further propagation of macrocracks may lead to the fracture failure of the component. Therefore, it is necessary to test and evaluate the fracture toughness of load-bearing materials.

[0003] Corrosion-resistant metal materials are commonly used in marine, chemical, and energy fields. During their service, they must not only resist corrosion damage in complex environments but also withstand the risk of cracking under load. In the research and development and selection of corrosion-resistant metal materials, not only are corrosion-resistant properties required, but also excellent fracture properties. Linking fracture toughness indicators with failure modes such as corrosion cracking and corrosion fatigue can provide a basis for the service life of materials and provide assurance for equipment design and safety assessment.

[0004] Existing fracture toughness testing standards (GB / T 21143-2014, ISO 12135:2021, ASTM E1820-24, ASTM E399-24) all test the fracture toughness of materials by loading specimens with pre-existing fatigue cracks. Pre-existing cracks in notched specimens through fatigue loading is a crucial step in fracture toughness testing. The basic principle of fracture toughness testing is to approximate a three-dimensional specimen as a two-dimensional model and then calculate the fracture toughness parameters. The crack in the specimen is approximated as a two-dimensional crack (i.e., a through-crack). Therefore, the testing standards have strict requirements for the straightness of the pre-existing crack.

[0005] In actual fatigue crack induction, because the material at the center of the specimen is in a plane strain state, the stress is greater than that on the sides. Therefore, the crack propagates faster at the center and slower on the sides, resulting in a curved leading edge of the pre-induced fatigue crack. The degree of curvature at the leading edge of the pre-induced fatigue crack varies depending on the material and specimen thickness. Generally, the thicker the specimen, the more pronounced the curvature of the leading edge and the lower the straightness of the crack. Studies have shown that lower crack straightness affects the accuracy of test results; excessively low straightness can even lead to failure in crack validity testing, rendering the test results invalid. For example, GB / T 21143-2014 requires that the difference between the crack length at any of the seven points at the center of the specimen and the average value of the nine points should not exceed 0.1 times the initial crack length, while GB / T 4161-2007 requires that the difference between the crack length and the average crack length on the two surfaces of the specimen should not exceed 15%.

[0006] The prior art disclosed in CN103323297B is a method for pre-fabricating fatigue cracks in fracture specimens of high-strength steel welded joints, comprising: Step 1, providing a specimen having length, width, and thickness, wherein a mechanical notch is formed on a first specimen surface defined by the length and thickness; Step 2, locally compressing the tip of the mechanical notch, wherein the load applied during the local compression is along the thickness direction; Step 3, subjecting the specimen to fatigue testing until a fatigue crack appears at the tip of the mechanical notch; Step 4, subjecting the specimen to reverse bending loading; Step 5, again locally compressing the tip of the mechanical notch, and again subjecting the specimen to fatigue testing until the fatigue crack propagates to a first preset length, and again subjecting the specimen to reverse bending loading; Step 6, repeating Step 5 multiple times. The prior art method is complex and cumbersome, and its core is to intervene in the propagation of fatigue cracks through local plastic deformation. This method mainly addresses the problem of uneven crack propagation at the thickness center of the welded joint specimen and is not suitable for base material specimens of large, thick plates. Therefore, this method is difficult to reliably and consistently obtain pre-existing fatigue cracks that meet the strict flatness requirements of the standard, and the accuracy of its test results still faces challenges, especially for base material samples with greater thickness, where its limitations are more prominent. Summary of the Invention

[0007] In view of this, the present invention aims to propose a method for pre-fabricated fatigue cracks for fracture toughness testing of corrosion-resistant metallic materials, in order to solve the problem that the flatness of pre-fabricated fatigue cracks in thick specimens in fracture toughness testing is low, which affects the accuracy of fracture toughness test results or causes the test results to be invalid due to failure of crack validity verification.

[0008] To achieve the above objectives, the technical solution of the present invention is implemented as follows:

[0009] A method for pre-inducing fatigue cracks for fracture toughness testing of corrosion-resistant metallic materials, comprising at least the following steps:

[0010] S1. Provide a metal material sample of a certain size;

[0011] S2. Take a conventional sample, establish a finite element model of the conventional sample, analyze the assumed crack, and determine the notch structure.

[0012] S3. Process the notch structure of step S2 on the metal material sample, and then perform fatigue loading so that the notch front end of the sample center and the surface can initiate and expand fatigue cracks at almost the same time, and finally obtain straight pre-existing fatigue cracks.

[0013] The method in this setup is simple, and through finite element analysis, a notch structure that can achieve a uniform stress field is obtained. This allows for the formation of straight pre-existing fatigue cracks during fatigue loading of the specimen, resulting in more accurate test results.

[0014] Furthermore, in step S2, the conventional sample is provided with a straight notch structure.

[0015] Furthermore, in step S2, the finite element model is established using Abaqus software.

[0016] Further, in step S2, the finite element model includes a model notch, and positions d1, d2, d3...dn are sequentially selected along the thickness direction to obtain the corresponding stress intensity factors. K Values ​​were analyzed to determine the force intensity factor corresponding to the notch in different models. K Values ​​are used to determine the gap structure.

[0017] Furthermore, in step S2, in order to keep the stress intensity factor of the model notch consistent, the notch structure is set as an arc-shaped notch.

[0018] Furthermore, step S2 also includes establishing stress intensity factors with different arc angles. K Distance from the sample surface d The curve was further analyzed to determine the optimal arc-shaped notch angle.

[0019] Furthermore, the central angle 2 of the arc-shaped notch α The range is from 20° to 76°.

[0020] Furthermore, the central angle 2 of the arc-shaped notch α The range is from 20° to 40°.

[0021] Furthermore, the central angle 2 of the arc-shaped notch α It is 20°.

[0022] Furthermore, when the metallic material specimen is a compact tensile specimen, the specimen thickness is... B The width of the sample is W ,in, W =2 B The maximum notch length at the center of the thickness of the metal material is a max The minimum notch length at the surface is a min The radius of the arc is R The arc angle is 2 α It satisfies the following formula:

[0023] c / B =1 / (2 sin α (1-(1-sin) 2 α ) 1 / 2 (4)

[0024] c = a max- a min (2)

[0025] Furthermore, in step S1, the dimensions of the metal material sample include its thickness, which is ≥30mm.

[0026] Compared with existing technologies, the pre-inducing fatigue crack method for fracture toughness testing of corrosion-resistant metallic materials described in this invention has the following advantages:

[0027] (1) The method of the present invention is simple and the principle is clear. Through finite element analysis, a notch structure that can achieve a uniform stress field is obtained, so that when the sample is subjected to fatigue loading, straight pre-existing fatigue cracks can be obtained, thus improving the accuracy of the test results.

[0028] (2) Based on the analysis of the stress field distribution of the fracture toughness specimen, the present invention establishes an arc-shaped notch specimen and finds that as the arc angle 2 α As the radius increases, the stress distribution at the front end of the notch changes significantly. When the arc angle is 2... α At around 20°, the stress intensity factor distribution at the crack tip is relatively consistent, and fatigue cracks can be initiated and propagated simultaneously at the notch tips of the sample center and surface when pre-fabricating fatigue cracks.

[0029] (3) Based on numerical calculation and analysis of the fatigue crack propagation law of metallic materials, this invention found that when pre-inducing fatigue cracks, the Δ value in region II of the fatigue crack propagation rate curve should be selected. KIt can reduce the difference between the amount of crack propagation at the center of the specimen and the surface of the specimen, and further obtain a straighter crack shape when pre-inducing fatigue cracks;

[0030] (4) The selection of the arc-shaped notch specimen and the pre-existing fatigue crack stress intensity factor parameters designed in this invention is universal. It can be used not only on compact tensile specimens, but also on other specimen types such as three-point bending specimens. It can be used for other test processes that require pre-existing fatigue cracks. Attached Figure Description

[0031] Figure 1 This is a diagram of a 40mm thick compact tensile specimen of the present invention, where the left is the front view and the right is the left view;

[0032] Figure 2 This is a half-view of the symmetrical model of the compact tensile specimen of the present invention;

[0033] Figure 3 This is a schematic diagram of the shape of the arc-shaped notch in the compact tensile specimen of the present invention.

[0034] Figure 4 This is a stress intensity factor distribution diagram at the notch tip of the present invention;

[0035] Figure 5 For the present invention c / B Follow α The functional relationship graph;

[0036] Figure 6 This is a typical fatigue crack propagation rate curve of metallic materials according to the present invention.

[0037] Figure 7 This is a model diagram of the compact tensile specimen of the present invention;

[0038] Figure 8 This is a model diagram of a compact tensile specimen with a straight notch according to the present invention;

[0039] Figure 9 This is a model diagram of a compact tensile specimen with an arc-shaped notch according to the present invention;

[0040] Figure 10 The image shows the crack leading edge contour of specimens with different arc-shaped notches according to the present invention.

[0041] Figure 11 This is a crack front profile diagram for different maximum stress intensity factor ranges according to the present invention.

[0042] Explanation of reference numerals in the attached figures:

[0043] 1-Notch front end, 11-Straight notch front end on the sample surface, 12-Straight notch front end at the center of the sample, 13-Arched notch front end on the sample surface, 14-Arched notch front end at the center of the sample, 2-Pre-existing fatigue crack, 21-Pre-existing fatigue crack leading edge on the straight notch surface of the sample, 22-Pre-existing fatigue crack leading edge at the center of the straight notch of the sample, 23-Pre-existing fatigue crack leading edge on the arc-shaped notch surface of the sample, 24-Pre-existing fatigue crack leading edge at the center of the arc-shaped notch of the sample, 3-Pre-existing crack surface, 4-Pre-existing fatigue crack leading edge outline, 5-Arched notch. Detailed Implementation

[0044] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0045] This invention relates to a method for pre-inducing fatigue cracks in fracture toughness testing of corrosion-resistant metallic materials, comprising at least the following steps:

[0046] S1. Provide a metal material sample of a certain size;

[0047] Specifically, in step S1, the dimensions of the metal material sample include its thickness, which is 30~80mm.

[0048] S2. Take a conventional sample, establish a finite element model of the conventional sample, analyze the stress field distribution of the assumed crack, i.e., the notch front end 1, and determine the notch structure.

[0049] S3. Process the notch structure of step S2 on the metal material sample, and then perform fatigue loading so that fatigue cracks can be initiated and extended simultaneously at the center of the metal material sample and the front end of the notch on the surface of the metal material sample, thereby obtaining straight pre-existing fatigue cracks.

[0050] Specifically, such as Figure 8 As shown, in a conventional specimen model, before fatigue loading, the model includes a notch front end 1, which includes a straight notch front end 11 on the specimen surface and a straight notch front end 12 at the specimen center. Correspondingly, after fatigue loading, the model includes a pre-existing fatigue crack 2, which includes a pre-existing fatigue crack leading edge 21 on the surface of the straight notch of the specimen and a pre-existing fatigue crack leading edge 22 at the center of the straight notch of the specimen.

[0051] For conventional fracture toughness specimens, the specimen notch is straight. Due to stress concentration at the notch, the pre-existing fatigue crack 2 is initiated at the notch tip 1. Therefore, in step S2, the conventional specimen is provided with a straight notch structure.

[0052] Specifically, in step S2, according to relevant fracture toughness test standards, the conventional specimen adopts a standard size such as... Figure 1As shown, a 40mm thick compact tensile specimen was used.

[0053] Specifically, in step S2, the finite element model is established using Abaqus software, and the finite element model is half of the structure of a compact tensile specimen.

[0054] In step S2, the finite element model includes a model notch. Assuming the front end of the model notch is a crack, positions d1, d2, d3...dn are sequentially selected on the model notch along the thickness direction, and the corresponding stress intensity factors are obtained simultaneously. K The values ​​are analyzed to determine the notch structure by identifying the notch locations in different models and their corresponding force intensity factor K values. Positions d1, d2, d3...dn represent the edges of the notches in the models.

[0055] More specifically, at a distance of 0.5 mm from the front of the notch, the stress intensity factor reflects the magnitude of the stress field at the crack tip. K As the distance d from the sample surface changes, such as Figure 4 As shown in Figure 40-0, where 40-0 refers to a through-type straight crack, it can be seen that the stress intensity factor value is the largest at the center of the sample thickness, the crack propagation rate is fast at the center, and the crack propagation rate is slow on both sides.

[0056] Specifically, in step S2, in order to keep the stress intensity factor of the model notch consistent, the notch structure is set as an arc-shaped notch 5.

[0057] More specifically, when the notch is arc-shaped, the pre-existing fatigue crack 2 includes a pre-existing fatigue crack leading edge 23 at the surface of the arc-shaped notch of the specimen and a pre-existing fatigue crack leading edge 24 at the center of the arc-shaped notch of the specimen, such as... Figure 9 As shown.

[0058] More specifically, the thickness of the metal material sample is B The unit is mm, and the sample width is... W The unit is mm, where, W =2 B The maximum notch length at the center of the thickness of the metal material is a max The unit is mm, and the minimum notch length at the surface is... a min The unit is mm, and the radius of the arc is... R Unit: mm, arc angle: 2 α The unit is °, and it satisfies the following formula:

[0059] R 2 =( B / 2) 2 +( R - c) 2 (1)

[0060] c = a max- a min (2)

[0061] sin α =( B / 2 ) / R (3)

[0062] According to equations (1) and (3), we know that:

[0063] c / B =1 / (2 sin α (1-(1-sin) 2 α ) 1 / 2 (4)

[0064] More specifically, the angle of the arc-shaped notch is obtained through finite element analysis.

[0065] More specifically, such as Figure 4 As shown, finite element analysis revealed that as the arc angle 2... α As the pressure increases, the stress distribution at the front end 13 of the arc-shaped notch on the sample surface and at the front end 14 of the arc-shaped notch at the center of the sample changes significantly. Figure 4 The image shows different arc angles 2 α The corresponding crack intensity factor distribution changes are shown in the figure. 40-20, 40-40, 40-60, and 40-76 represent the arc angles 2... α The specimens have arc-shaped notches of 20°, 40°, 60°, and 76°. When 2 α When the angle is 20°, the stress intensity factor distribution at the crack tip is most uniform; when 2 α When the angle is ≥40°, the stress intensity factor at the center of the specimen thickness is lower than that at the specimen surface, making it easier for cracks to initiate from the surface; when 2 α When the size is too large, even if cracks can initiate on both sides of the sample before the center, the flatness of the fatigue crack will be significantly reduced because the initial crack lengths at the center and on the surface of the sample are different due to the arc-shaped notch.

[0066] In formula (4) c / B Follow α The functional relationship is as follows Figure 5 As shown, the sample thickness B At a certain time, c along with α Monotonically increasing, whenα When the notch is too large, the arc-shaped notch causes a difference between the notch length at the center of the sample and the notch length at the sample surface. c It has increased significantly.

[0067] More specifically, when the arc angle is 2 α At 20°, c / B ≈0.04, assuming the length of the notch at the center of the sample is approximately 0.45. W That is, 0.90 B The length of the notch at the center of the sample differs from the length of the notch at the surface of the sample by only about 4%, which has little effect on the straightness of the crack. When 2 α At 76°, c / B ≈0.17, the length of the notch tip alone does not meet the standard requirement that the difference between the surface crack length and the average crack length should not exceed 15%, which has a significant impact on the straightness of the crack.

[0068] Therefore, when the arc angle is 2 α At an angle of approximately 20°, the stress intensity factor distribution at the crack tip is relatively uniform. During pre-existing fatigue crack initiation, the notch tips at the center and surface of the specimen can initiate and propagate fatigue cracks almost simultaneously, resulting in a relatively straight pre-existing fatigue crack. In specimen size design, the maximum notch length at the center of the arc-shaped notch specimen... a cen-n More specifically, for arc-shaped notch specimens, a cen-n and a max The values ​​are the same. a cen-n It reflects geometric relationships. a max It reflects the size relationship. But for a straight notch, a cen-n , a sur-n , a max , a min They are all equal. a cen-n Comparable to the notch length of conventional specimens a n Consistent.

[0069] This invention provides a method for pre-inducing fatigue cracks for fracture toughness testing of corrosion-resistant metallic materials. Step S3 further includes the following steps:

[0070] S31. Select the corresponding target stress intensity factor range Δ according to the type of metallic material to be tested. K ;

[0071] S32. Process the notch structure of step S2 on the metallic material specimen, and then apply fatigue loading so that fatigue cracks can simultaneously initiate and propagate at the notch tip at the center of the metallic material specimen and on the surface of the metallic material. At the same time, control the load parameters to ensure that the actual Δ at the crack tip is within the range of Δ. K At the selected target Δ K Within the range, straight pre-induced fatigue cracks are obtained; wherein, the actual Δ K Located in region II of the material fatigue crack propagation rate curve and biased to the left of this region, region II represents the stable stage of fatigue crack propagation. The material fatigue crack propagation rate d... a / d N This refers to the amount of crack propagation during each fatigue cycle. a The length of the crack. N The fatigue cycle number is related to the stress intensity factor range Δ at the crack tip. K Related. Typical fatigue crack propagation rate curves for metallic materials are as follows: Figure 6 As shown, the fatigue crack propagation process can be divided into three regions: Region I, Region II, and Region III. Region I is the initial stage of fatigue crack propagation, with a crack propagation rate d. a / d N With the range of stress intensity factor Δ K The increase is rapid; region II is the stable stage of fatigue crack propagation, and the d in this stage... a / d N With Δ K Satisfying the Paris formula means:

[0072] d a / d N = C (Δ K ) n (5)

[0073] In formula (5) C , n All are constants.

[0074] Zone III represents the final stage of fatigue crack propagation. a / d N Fracture occurs in cycles with larger crack values ​​and less crack propagation.

[0075] The range of stress intensity factor Δ for pre-existing fatigue cracks K Region III cannot be selected, as crack propagation is rapid, the crack propagation surface is coarser, and the Δ value is larger. K This will increase the plastic zone at the crack tip, making it impossible to simulate fine cracks in actual engineering, and will not meet the requirements for crack preload in the standard, thus affecting the fracture toughness test results.

[0076] Integrating equation (5) yields

[0077] a ( N )= a 0+ C (Δ K ) n N (6)

[0078] Δ a = a ( N )- a 0

[0079] = C (Δ K ) n N (7)

[0080] In equation (6), a 0 represents the initial crack length, Δ a This represents the amount of crack propagation.

[0081] When a straight notch is used, the surface of the sample can be considered to be parallel to the center. a 0 is the same, both are the length of the gap. a n The crack propagation amounts at the sample surface and center are Δ a sur Δ a cen The stress intensity factors at the surface and center of the specimen range Δ K sur、 Δ K cen Combining equation (7), we have:

[0082] Δ a sur / Δ a cen =(Δ K sur / Δ K cen ) n (8)

[0083] Depend on Figure 4 It can be seen that the stress intensity factor is the largest at the center of the specimen thickness when the notch is straight. K sur / Δ K cen Less than 1, combined with formula (4). n The smaller the value, the more Δ a sur / Δ a cenThe closer to 1, the smaller the difference in crack propagation between the sample surface and the center. From Figure 6 It can be seen that the fatigue crack propagation rate curve in region II... n The value is significantly smaller than that of region I, therefore region II is chosen. K It helps to reduce Δ a sur Δ a cen The difference between them. Considering that the rate of pre-induced fatigue cracks should not be too fast, the left side of region II is selected. ΔK It can achieve fine cracks with a smaller plastic region at the crack tip while promoting the straightness of fatigue cracks. Based on previous experimental experience, steel materials Δ K The pressure should be selected at (32~38) MPa·m 1 / 2 Nearby, titanium alloy material Δ K The pressure should be selected at (20~28) MPa·m 1 / 2 Nearby, aluminum alloy material Δ K The pressure should be selected at (12~18) MPa·m 1 / 2 Nearby, the appropriate Δ can be selected based on the results of fatigue crack propagation rate tests on the material. K .

[0084] When a 20° arc-shaped notch is used, Δ K sur / Δ K cen Approaching 1, combined with equation (4), at this time Δ a sur / Δ a cen It is also close to 1, and the difference in crack propagation between the sample surface and the center is small.

[0085] The arc angle 2 of the present invention α The stress distribution at the notch tip is significantly affected when 2 α Around 20°, the stress intensity factor distribution at the crack tip is relatively uniform. During pre-existing fatigue cracking, fatigue cracks simultaneously initiate and propagate at the notch tips on the sample center and surface, resulting in relatively straight pre-existing fatigue cracks. When the stress intensity factor range during pre-existing fatigue cracking is selected from the Δ range of region II... K It can reduce the gap between the amount of crack propagation at the center and the surface of the sample, and further obtain a straighter pre-existing fatigue crack.

[0086] A method for pre-inducing fatigue cracks in fracture toughness testing of corrosion-resistant metallic materials has been applied in fracture toughness testing of various metallic materials. Especially when conventional notched specimens cannot effectively propagate surface cracks during pre-inducing fatigue cracks, the method of this invention can significantly improve the straightness of the pre-inducing fatigue cracks, which is beneficial for obtaining accurate fracture toughness test results that meet standard requirements. The technical solutions in the embodiments of this invention will be clearly and completely described below.

[0087] Example 1

[0088] Taking a compact tensile specimen as an example, the model diagram of the arc-notch specimen is as follows: Figure 7 As shown, the straight-notch specimen and the arc-shaped notch specimen are not easily distinguishable from each other in appearance. The conventional specimen has a straight notch. Figure 8 The sample designed in this invention has an arc-shaped notch. Figure 9 The thickness of the compact tensile specimen is 40 mm, the arc angle is 20°, and the arc radius is... R Approximately 115 mm, sample size as follows Figure 1 As shown, the unit is mm, and the sample material is 6082 aluminum alloy. By comparing and analyzing the pre-existing fatigue crack profiles with straight notches and arc notches, the results show that the method of the present invention can make the leading edge of the pre-existing fatigue crack straighter, which is beneficial to obtaining accurate fracture toughness test results.

[0089] 1) Test material: 40mm thick 6082 aluminum alloy

[0090] 2) Ambient temperature: (23~28)℃; Ambient relative humidity: (32~47)%

[0091] 3) The specimen type is a compact tensile specimen, and the notch types are a straight notch and an arc notch, with the arc angle of the arc notch being 2. α The arc radii of the notched specimens are 115 mm, 58 mm, and 40 mm, respectively, with angles of 20°, 40°, and 60°. The specimen shapes and dimensions are as follows: Figure 1 As shown, the unit is mm.

[0092] 4) Test item: KIC test for plane strain fracture toughness

[0093] 5) Experimental procedure:

[0094] ①The fatigue crack propagation rate test results of 6082 aluminum alloy are as follows: Figure 6 As shown, it can be seen that Δ K At (12~22) MPa·m 0.5 Within this range, the specimen is in the stable crack propagation stage (stage II).

[0095] ② Adopt a reductionK The maximum stress intensity factor range for pre-induced fatigue cracks in arc-notched specimens was set to 13 MPa·m. 0.5 The maximum stress intensity factor range for the straight-notched specimen was set to 9 MPa·m. 0.5 and 13 MPa·m 0.5 The waveform is a sine wave, and the stress ratio is... R s 0.1, frequency f s 8Hz, crack length a Reaching 0.5 W The pre-inducing fatigue cracks are completed.

[0096] ③ The specimen is loaded at a rate of 1.8 kN / s, and the loading load is recorded by the testing machine. F and the displacement of the crack mouth opening V The test should be stopped when the sample breaks.

[0097] ④ Record the fracture morphology and compare the influence of notch shape and pre-existing fatigue crack parameters on crack straightness.

[0098] 6) Test results:

[0099] Equation (9) is used to reflect the straightness of the crack, as shown below:

[0100] f= ( a max- a min ) / a 0 (9)

[0101] In the formula, a max The maximum crack length of the fatigue crack is measured. a min The minimum crack length measured for fatigue cracks. a 0 represents the average fatigue crack length measured by the nine-point method. f A higher value indicates a lower fatigue crack flatness.

[0102] ① The maximum stress intensity factor ranges from 13 MPa·m 0.5 At that time, the number of pre-existing fatigue crack cycles for specimens with different notch shapes was approximately (13000~16000). The fatigue crack straightness index values ​​corresponding to different notch structures are shown in Table 1; among them, the notch tip 1, pre-existing crack surface 3, and pre-existing fatigue crack leading edge contour 4 are as follows: Figure 10 As shown in Table 1, Figure 10 It can be seen that the straightness of the crack front edge is optimal when the arc angle is 20°.

[0103] ② The maximum stress intensity factor range of the straight-notch specimen is 9 MPa·m 0.5 and 13 MPa·m 0.5 The corresponding pre-existing fatigue crack cycles are approximately 55,000 and 14,000, respectively, and the corresponding fatigue crack straightness index values ​​are shown in Table 2; among them, the notch tip 1, pre-existing crack surface 3, and pre-existing fatigue crack leading edge contour 4 are shown in Table 2. Figure 11 As shown in Table 2, Figure 11 It can be seen that the maximum stress intensity factor ranges from 13 MPa·m. 0.5 The straightness of the crack front at that time was better than the maximum stress intensity factor range of 9 MPa·m. 0.5 The straightness of the leading edge of the crack at that time.

[0104] Table 1. Fatigue crack straightness index values ​​corresponding to different notch structures.

[0105]

[0106] Table 2. Fatigue crack straightness index values ​​corresponding to different maximum stress intensity factor ranges.

[0107]

[0108] pass Figure 10 and Figure 11 A comparison of the crack leading edge contours shows that, according to the method of the present invention, an arc angle of 2 is used. α For specimens with a 20° arc-shaped notch, the straightness of the pre-existing fatigue crack tip increases significantly. The maximum stress intensity factor range is selected in the second stage of the fatigue crack propagation rate test curve, which is beneficial to obtaining a straighter fatigue crack. For fracture toughness specimens with greater thickness, the method of this invention can effectively improve the straightness of the pre-existing fatigue crack.

[0109] While the present invention has been disclosed above, it is not limited thereto. Any person skilled in the art can make various modifications and alterations without departing from the spirit and scope of the invention; therefore, the scope of protection of the present invention should be determined by the scope defined in the claims.

Claims

1. A method for pre-inducing fatigue cracks for fracture toughness testing of corrosion-resistant metallic materials, characterized in that, It should include at least the following steps: S1. Provide a metal material sample of a certain size; the size of the metal material sample includes its thickness, which is ≥30mm; when the metal material sample is a compact tensile sample, the sample thickness is... B The width of the sample is W ,in, W =2 B The maximum notch length at the center of the thickness of the metal material is a. max The minimum notch length at the surface is a min The radius of the arc is R The arc angle is 2 α It satisfies the following formula: c / B =1 / (2 sin α )(1-(1-sin2 α )1 / 2) c =a max -a min ; S2. Take a conventional specimen, establish a finite element model of the specimen, analyze the hypothetical crack, and determine the notch structure; the conventional specimen has a straight notch structure; the hypothetical crack is the notch tip; the finite element model includes the model notch, and on the model notch, positions d1, d2, d3...dn are sequentially taken along the thickness direction, and the corresponding stress intensity factors are obtained. K Values ​​were analyzed to determine the location of the notch in different models and its corresponding force intensity factor. K Values ​​determine the gap structure; S3. Process the notch structure of step S2 on the metal material sample, and then perform fatigue loading so that fatigue cracks can be initiated and extended simultaneously at the notch front end of the metal material sample center and the sample surface, thereby obtaining straight pre-existing fatigue cracks. Step S3 also includes the following steps: S31. Select the corresponding target stress intensity factor range Δ according to the type of metallic material to be tested. K ; S32. Process the notch structure of step S2 on the metal material sample, and then perform fatigue loading so that fatigue cracks can be initiated and extended simultaneously at the center of the metal material sample and the front end of the notch on the surface of the metal material. At the same time, control the load parameters so that the actual ΔK of the crack tip is within the selected target ΔK range to obtain a straight pre-induced fatigue crack.

2. The method according to claim 1, characterized in that, The finite element model was built using Abaqus software.

3. The method according to claim 1, characterized in that, In step S2, in order to keep the stress intensity factor of the model notch consistent, the notch structure is set as an arc-shaped notch.

4. The method according to claim 3, characterized in that, The central angle of the arc-shaped notch is 2. α The range is from 20° to 76°.

5. The method according to claim 3, characterized in that, The central angle of the arc-shaped notch is 2. α The range is from 20° to 40°.

6. The method according to claim 3, characterized in that, The central angle of the arc-shaped notch is 2. α It is 20°.

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