Service life evaluation method for whole corrosion fatigue crack propagation process of metal material

Abstract

The invention relates to the field of test methods, in particular to a method for evaluating the service life of a metal material in the whole corrosion fatigue crack propagation process, which comprises the following specific steps: S1, designing a corrosion environment device; s2, preparing an etchant solution; s3, selecting and measuring the size of a sample; s4, prefabricating fatigue cracks; s5, setting test parameters and performing a test; s6, calculating the opening displacement variation (delta delta) of the crack tip; s7, establishing a mathematical model; and S8, evaluating the service life. According to the novel method for simultaneously describing the processes of the three stages of corrosion fatigue crack propagation, the processes of the three stages of corrosion fatigue crack propagation are well described by taking the crack tip opening displacement variable quantity containing elastic and plastic displacement superposition as a driving parameter; the method can provide a technical basis for whole-process life evaluation of corrosion fatigue crack propagation, and meets the requirements of fatigue life evaluation of ocean engineering components.

Description

Technical Field

[0001] The present invention relates to the field of test methods, and particularly relates to a method for evaluating the life of the whole process of corrosion fatigue crack propagation of metal materials. Background Art

[0002] When evaluating and checking the fatigue life of offshore engineering structures, it is necessary to master the law of the whole process of fatigue crack propagation in a corrosive environment. The whole process of fatigue crack propagation of metal materials generally includes the first, second, and third stages. Generally, when the stress intensity factor range ΔK describing the stress-strain field at the crack tip is greater than its crack propagation threshold value ΔK th , the fatigue crack begins to propagate in the first stage, and the propagation rate gradually increases from 10 -7 mm / week and starts to increase gradually. The crack propagation in this stage is very slow and is controlled by shear stress. After propagating for a period of time, due to the continuous hindrance of grain boundaries, the crack propagation begins to gradually change direction and enters the second stage. The propagation rate in this stage is about 10 -5 mm / week and is controlled by normal stress. When the crack propagation reaches the critical value of the crack length, the crack propagation enters the third stage. At this time, the crack propagation rate reaches 10 -3 mm / week to 10 -2 mm / week, and then rapidly becomes unstable and fractures. At present, due to the slow propagation rate and long time consumption in the first stage, and the fast propagation rate and close-to-unstable stage in the third stage, the second stage has received more attention in the scientific and engineering communities. In this stage, the Paris formula is most commonly used for crack propagation rate in air. This formula uses the stress intensity factor range ΔK at the crack tip as the driving parameter, and there are already international standards such as ASTM E647-2015 Standard Test Method for Measurement of Fatigue Crack Growth Rates and national standards such as GB / T 6398-2017 Metallic Materials - Fatigue Testing - Fatigue Crack Growth Method. After adding a corrosion ring, the fatigue crack growth rate test in the second stage can be carried out according to the above standards. Considering the crack propagation laws of the first, second, and third stages simultaneously, the improved Forman formula is most mentioned in the literature in an air environment. This formula also uses the stress intensity factor range ΔK at the crack tip as the driving parameter, comprehensively considering the crack propagation rate threshold value and fracture toughness, and can describe the three stages of fatigue crack propagation simultaneously. However, experimental studies have found that although the improved Forman formula considers the three stages of fatigue crack propagation, since it still uses ΔK as the driving parameter, and ΔK can only describe the mechanical state of linear elasticity or small-scale yielding near the crack tip, and when the crack propagates to the later stage, for most metal materials, the crack tip often exceeds the small-scale yielding condition.

[0003] Publication No.: CN117935997B A dynamic measurement method for the plastic zone at the fatigue crack tip of a metal material, comprising the following steps: S1: Conduct a da / dN test to establish the mathematical relationship between r0 and V; S2: Observe the change of the r0 value in real time according to the V value; S3: Continuously monitor the change amount V of the opening displacement at the crack mouth of the specimen during the test, and at the same time start the program compiled in step S2, and stop the test when the ratio of the plastic zone size to the crack length reaches a preset value; S4: According to the obtained test data, in accordance with the fitting form of the Paris formula, obtain the effective equation of da / dN and △K. However, when the plastic zone size at the crack tip does not meet the small-scale yielding condition, the test is stopped, and the problem of accurately describing the full-life assessment of corrosion fatigue crack propagation cannot be solved.

[0004] Therefore, improving the Forman formula is actually no longer applicable, and it is obviously even less able to describe the three stages of fatigue crack propagation in a corrosive environment. There are also no other literature reports on the study of simultaneously describing the propagation process of the three stages of fatigue crack propagation in a corrosive environment. Summary of the Invention

[0005] In view of this, the present invention aims to propose a life assessment method for the entire process of corrosion fatigue crack propagation of metal materials, to solve the problem in the prior art that the full-life assessment of corrosion fatigue crack propagation cannot be accurately described.

[0006] Based on the need for using the full-process rate data of corrosion fatigue crack propagation when conducting corrosion fatigue life assessment and verification of offshore engineering components, and the current situation that there is no suitable model describing the full-process law of corrosion fatigue crack propagation, the present invention proposes a new method for simultaneously describing the propagation laws of the three stages of corrosion fatigue crack propagation. This method uses the change amount of the crack tip opening displacement, which is the superposition of elastic and plastic displacements, as the driving parameter, and can better describe the process of the three stages of corrosion fatigue crack propagation; at the same time, the relationship between the propagation rates of the three stages of corrosion fatigue crack propagation and the change amount of the crack tip opening displacement is established, which can provide a technical basis for the full-life assessment and verification of corrosion fatigue crack propagation, and meet the needs of safety and reliability assessment of offshore engineering components.

[0007] The technical solution of the present invention is implemented as follows:

[0008] The present invention discloses a life assessment method for the entire process of corrosion fatigue crack propagation of metal materials, including the following specific steps:

[0009] S1: Design a corrosion environment device: fabricate a container suitable for containing the corrosive liquid and facilitating the observation of cracks;

[0010] S2: Configure the corrosive solution: Prepare a corrosive liquid according to the standard to simulate the marine corrosion environment;

[0011] S3: Select and measure the specimen size: Prepare the specimen and measure the key dimensions such as thickness, width, and notch length;

[0012] S4: Pre - fabricate fatigue cracks: Pre - fabricate initial cracks on the specimen according to the design parameters;

[0013] S5: Set test parameters and conduct tests: Use an electro - hydraulic servo material testing machine to conduct da / dN tests and threshold value tests under a corrosive environment to obtain a series of da / dN values;

[0014] S6: Calculate the change in crack - tip opening displacement (Δδ): Obtain the crack length a value based on the compliance method and calculate Δδ in combination with the test data;

[0015] S7: Establish a mathematical model: Analyze the data distribution characteristics of (da / dN, Δδ) and construct a mathematical model of da / dN - Δδ under a corrosive environment;

[0016] S8: Life assessment: Use the obtained model, combined with the critical crack length dimension a c and the initial crack length a0, to evaluate the life of the three - stage corrosion fatigue crack propagation of metallic materials for ocean engineering structures.

[0017] Furthermore, in step S2, prepare artificial seawater or 3.5% NaCl aqueous solution according to the standard to simulate the ocean corrosion environment.

[0018] Furthermore, in step S3, set at least two specimens, one for the da / dN test in a corrosive environment and one for the △K th test in a corrosive environment.

[0019] Furthermore, in step S3, the specimen adopts a three - point bending SEB specimen.

[0020] Furthermore, in step S5, the test parameters include the applied test load range △P, test frequency f, stress ratio R, the initial stress intensity factor K value, and its decreasing gradient C g .

[0021] Furthermore, in step S5, in the series of da / dN tests, use the seven - point increasing polynomial method or the secant method to calculate the da / dN value.

[0022] Furthermore, in step S6, the crack - tip opening displacement δ includes the component δ caused by elastic deformation e and the component δ caused by plastic deformation p , according to the relevant mechanical theory and the principle of similar triangles, the expression is as follows:

[0023] ……………………(1)

[0024] …………………………… (2)

[0025] ……………………………………………… (3)

[0026] According to the above three equations, it can be known that the crack tip opening displacement δ is:[[]]

[0027] … (4)

[0028] In the above formula, δ e is the elastic component of the crack tip opening displacement, in mm; K is the stress intensity factor at the crack tip, in MPa·m 0.5 ; E is the elastic modulus of the test material, in MPa; R p0.2 is the yield strength of the test material, in MPa; a is the crack length, in mm; z is the thickness of the knife edge used for clamping the clip-on extensometer (when the clamping knife edge is directly machined on the specimen, z is 0), in mm; W is the specimen width, in mm; r(W - a) is the distance between the crack tip and the rotation center, in mm; r is the rotation factor, and the Chinese standard stipulates that r is 0.44; V e is the elastic component of the displacement at the clamping knife edge of the extensometer, in mm; δ p is the plastic component of the crack tip opening displacement, in mm; V p is the plastic component of the displacement at the clamping knife edge of the extensometer, in mm; V is the displacement at the clamping knife edge of the extensometer, in mm;

[0029] During the entire da / dN test process, the crack length a is measured according to the compliance method, and the normalized crack length is expressed as:[[]]

[0030] …………………………… (5)

[0031] In the formula, a / W is the normalized crack length, C0, C1, C2, C3, C4, C5 are compliance coefficients (constants), U x is the dimensionless compliance, which is related to the elastic modulus of the test material, specimen size, external load, etc., and is expressed as follows:[[]]

[0032] ……………………………………………… (6)

[0033] In the formula, B is the specimen thickness, in mm; V, E are the same as defined above; P is the external load, in N.

[0034] According to the above formula (4), it can be obtained that:[[]]

[0035] …………………………………… (7)

[0036] ……………………………………(8)

[0037] It is defined here that:

[0038] ………………………………………………………(9)

[0039] ……………………………………………………(10)

[0040] Equation (7) - Equation (8) is:

[0041] …………………………………………(11)

[0042] In the above formula, Δδ is the change in the crack tip opening displacement, in mm; ΔV is the change in the opening displacement at the clamping edge of the extensometer, in mm; δ max and δ min are the maximum and minimum values of the crack tip opening displacement respectively, in mm; V max and V min are the maximum and minimum values of the displacement at the clamping edge of the extensometer respectively, in mm, and can be directly read and obtained during the test.

[0043] Furthermore, in step S7, the method for constructing the mathematical model of da / dN - Δδ under the corrosion environment is as follows:

[0044] S71: According to the obtained series of da / dN values and △δ values, obtain the data set (da / dN, △δ);

[0045] S72: Place the data set (da / dN, △δ) of the specimen in the coordinate system and observe its distribution law;

[0046] S73: According to the distribution characteristics of the data set (da / dN, △δ), obtain the full - process expression of the corrosion fatigue crack growth rate da / dN with Δδ as the driving parameter through the fitting method:

[0047] …………………………………………(12)

[0048] In the above formula, f is the function expression symbol.

[0049] Furthermore, in step S8, the initial crack length a0 is obtained by non - destructive testing method, and the critical crack length a c is obtained by fracture toughness test.

[0050] Compared with the prior art, a method for evaluating the life of a metal material during the entire process of corrosion fatigue crack propagation has the following advantages:

[0051] 1. A new method proposed by the present invention for simultaneously describing the three-stage process of corrosion fatigue crack propagation, with the change in the crack tip opening displacement that includes the superposition of elastic and plastic displacements as the driving parameter, can better describe the three-stage process of corrosion fatigue crack propagation. At the same time, the relationship between the propagation rates of the three stages of corrosion fatigue crack propagation and the change in the crack tip opening displacement and the life evaluation model for the entire process of corrosion fatigue crack propagation are established, which can provide a technical basis for the life evaluation of the entire process of corrosion fatigue crack propagation and meet the needs of fatigue life evaluation of marine engineering components. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] The drawings constituting a part of the present invention are used to provide a further understanding of the present invention. The schematic embodiments and descriptions thereof of the present invention are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0053] Figure 1 is a graph showing the data distribution and fitting of da / dN~△δ in the three stages of corrosion fatigue crack propagation of a certain titanium alloy. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0054] In order to make the technical means, objectives and effects of the present invention easy to understand, the embodiments of the present invention will be described in detail below with reference to specific drawings.

[0055] It should be noted that all the terms for indicating directions and positions in the present invention, such as: "upper", "lower", "left", "right", "front", "rear", "vertical", "horizontal", "inner", "outer", "top", "bottom", "lateral", "longitudinal", "center", etc., are only used to explain the relative position relationship and connection situation between components in a specific state, and are only for the convenience of describing the present invention, rather than requiring the present invention to be constructed and operated in a specific orientation. Therefore, it should not be construed as a limitation of the present invention. In addition, the descriptions involving "first", "second", etc. in the present invention are only for descriptive purposes and cannot be construed as indicating or implying their relative importance or implicitly indicating the quantity of the indicated technical features.

[0056] In the description of the present invention, unless otherwise clearly defined and limited, the terms "mounted", "connected", "connected" shall be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection; it can be directly connected, or indirectly connected through an intermediate medium, and it can be the communication inside two components. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific situations.

[0057] In the description of this specification, the description referring to terms such as "one embodiment", "some embodiments", "schematic embodiments", "examples", "specific examples", or "some examples" means that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in a suitable manner in any one or more embodiments or examples.

[0058] The present invention discloses a method for evaluating the life of the whole process of corrosion fatigue crack propagation of metal materials, including the following specific steps:

[0059] S1: Design a corrosion environment device: fabricate a plexiglass container suitable for holding the corrosive liquid and facilitating the observation of cracks;

[0060] According to the size and shape of the specimen, design a suitable container to hold the corrosive liquid. Usually made of transparent plexiglass, it ensures that the crack propagation can be clearly observed during the experiment and that there is no leakage. The design of the container takes into account the convenience of installing fatigue testing machine fixtures and extensometers and other equipment, providing a stable and visual corrosion environment, which is conducive to real-time monitoring of the crack development.

[0061] S2: Configure the corrosive solution: prepare a corrosive liquid according to the standard to simulate the marine corrosion environment;

[0062] According to relevant standards (such as ASTM G48 or GB / T 6398 - 2017), prepare a corrosive liquid, ensure that the purity of the chemical reagents used is high enough to avoid the influence of impurities on the experimental results, and the conditions such as the pH value and temperature of the solution also need to be adjusted according to the standard. Simulate the real marine corrosion environment to improve the authenticity and representativeness of the experimental results.

[0063] S3: Select and measure the specimen size: prepare the specimen and measure the key dimensions such as thickness, width, and notch length;

[0064] Select a suitable specimen and use precise measuring tools (such as micrometers) to measure its thickness (B), width (W), notch length (a m ) and other key dimensions, and record these data for subsequent calculation of the change in crack tip opening displacement (Δδ), which helps to provide basic data support for accurately calculating the crack propagation rate and ensures the consistency and accuracy of the experimental data.

[0065] S4: Pre - fabricate a fatigue crack: pre - fabricate an initial crack on the specimen according to the design parameters;

[0066] By controlling the stress intensity factor range ΔK or the number of loading cycles, an initial crack is prefabricated on the specimen. Using a specific loading procedure, such as gradually increasing the stress until the desired crack length is formed. This setting creates a standardized starting point, making the test results of different specimens more comparable and reducing errors caused by differences in initial conditions.

[0067] S5: Set test parameters and conduct tests: Use an electro-hydraulic servo material testing machine to conduct da / dN tests and threshold value tests under corrosive environments to obtain a series of da / dN values.

[0068] Set the working parameters of the electro-hydraulic servo material testing machine, including the applied test load range ΔP, test frequency f, stress ratio R, etc. Then place the specimen in the corrosive environment to start the test. For da / dN tests, keep the load amplitude constant; for threshold value tests, use the K-decreasing method. This setting can accurately simulate the load conditions under actual working conditions, thus obtaining reliable fatigue crack growth rate data.

[0069] S6: Calculate the change in crack tip opening displacement (Δδ): Obtain the crack length a value based on the compliance method and calculate Δδ in combination with test data.

[0070] Measure the change in crack length a using the compliance method and calculate Δδ in combination with known mechanical relationships. This setting provides a more comprehensive description method than the traditional stress intensity factor and is applicable to different stages of crack propagation.

[0071] S7: Establish a mathematical model: Analyze the data distribution characteristics of (da / dN, Δδ) and construct a mathematical model of da / dN - Δδ under corrosive environments.

[0072] Collect the (da / dN, Δδ) data set, plot a scatter diagram, and apply an appropriate fitting function (such as polynomial regression analysis) to construct a mathematical model of da / dN - Δδ under corrosive environments. This setting provides a scientific basis for predicting the fatigue life of metal materials under corrosive environments.

[0073] S8: Life assessment: Use the obtained model, combined with the critical crack length dimension a c and the initial crack length a0, to evaluate the life of the three stages of corrosion fatigue crack propagation of metal materials for offshore engineering structures.

[0074] Based on the established mathematical model, combined with the principles of fracture mechanics, obtain the initial crack length a0 and the critical crack length a c and use this to evaluate the fatigue life of the metal material. This setting realizes the effective evaluation of the fatigue life of metal materials for offshore engineering structures and improves the safety and reliability of the structure.

[0075] This setting uses the change in crack tip opening displacement (Δδ) as the driving parameter, which includes elastic and plastic deformations. A mathematical relationship between the corrosion fatigue crack growth rate da / dN and Δδ is established, which can more accurately describe the three stages (I, II, and III) of fatigue crack growth, solve the problem that the improved Forman formula is no longer applicable in the later stage of crack growth due to exceeding the small-scale yielding condition, and consider the influence of the corrosive environment on the fatigue crack growth of metal materials.

[0076] Specifically, in step S2, artificial seawater or a 3.5% NaCl aqueous solution, etc., is prepared according to the standard to simulate the marine corrosion environment.

[0077] According to the required simulated marine environmental conditions, for artificial seawater, high-purity NaCl (sodium chloride) and other necessary components are selected to ensure that the chemical reagents used do not contain impurities that may affect the experimental results. According to the standard-specified ratio (such as a 3.5% NaCl aqueous solution), the required mass of NaCl is accurately weighed and completely dissolved in a certain amount of deionized water. Stir until the salt is completely dissolved to form a uniform solution. According to the need, adjust the pH value and temperature of the solution to the specified range. To more realistically reflect the actual marine environmental conditions, a buffer or other chemical substances can be added to maintain the pH stability of the solution.

[0078] By configuring artificial seawater similar to the composition of natural seawater or a NaCl aqueous solution with a specific concentration, this setting can effectively simulate the actual corrosion conditions that metal materials may encounter in the marine environment, which helps to ensure the consistency of experimental conditions, thereby improving the reliability and comparability of experimental results.

[0079] Specifically, in step S3, at least two specimens are set up, one for the da / dN test in the corrosive environment and one for the △K th test.

[0080] Select a suitable metal material according to the research purpose, such as titanium alloy, and process it into a standard SEB (Single Edge Bend) specimen. Accurately measure the key dimensions of each specimen, including thickness (B), width (W), and the machined notch length (a m ) to calculate the change in crack tip opening displacement (Δδ) later. This setting can ensure the consistency and accuracy of the experiment. A pre-crack is fabricated on each specimen by controlling the stress intensity factor range (△K) or the number of loading cycles to achieve the required initial crack length.

[0081] da / dN test: Using an electro-hydraulic servo material testing machine, conduct a fatigue crack growth rate (da / dN) test in a specified corrosive environment, record the change in crack length after each cycle, and calculate the da / dN value using the seven-point incremental polynomial method or the secant method, aiming to evaluate the relationship between the crack growth rate and the stress intensity factor under specific stress conditions.

[0082] △K th Test: Conduct a crack growth threshold value (△K th ) test in a corrosive environment, gradually reduce the stress intensity factor until no crack growth is observed, and determine the critical stress intensity factor difference △K th , which helps to determine the ability of the material to resist the initiation of crack growth, that is, to find the lowest threshold of the stress intensity factor below which no significant crack growth occurs.

[0083] This setup can obtain comprehensive information about the fatigue performance of the material in a corrosive environment by conducting two types of tests, da / dN and △K th , including both the crack growth rate and the starting point of crack growth, and can more accurately predict the behavior of the material during its entire service life.

[0084] Specifically, in step S3, the specimen uses a three-point bending SEB specimen.

[0085] The three-point bending SEB specimen can well simulate the bending stress state that a structural component may encounter during actual use, making the loading method similar to the stress situation in many engineering structures. Therefore, it can provide data close to the actual situation and can accurately measure the fatigue crack growth rate (da / dN) and the threshold value (△K th ) of metallic materials in a corrosive environment.

[0086] This setup uses a three-point bending loading method, which can more easily control parameters such as the load magnitude, frequency, and stress ratio applied to the specimen, thus ensuring the consistency and accuracy of the experimental conditions.

[0087] Specifically, in step S5, the test parameters include the external test load range △P, test frequency f, stress ratio R, the initial stress intensity factor K value, and its descent gradient C g .

[0088] Control the difference between the maximum and minimum loads applied to the specimen, thereby affecting the stress state at the crack tip. Different load ranges will result in different ranges of stress intensity factors (ΔK), which in turn affect the fatigue crack growth rate. The test frequency affects the thermal effect during crack growth and the changes in the internal microstructure of the material, and may have an impact on the crack growth rate. Different test frequencies can help understand the response characteristics of the material under different loading speeds, which is helpful for a more comprehensive evaluation of the fatigue performance of the material. The stress ratio determines the proportion of the tensile and compressive parts during the loading cycle and affects the crack tip closure behavior. By changing the stress ratio, different stress states under actual working conditions can be simulated, such as pure tension (R = 0), tensile-compressive cycle (R = -1), etc., which helps to reveal the influence mechanism of the stress state on crack growth. The initial stress intensity factor K value describes the stress field intensity near the crack tip and is one of the key factors determining whether the crack starts to grow. The selection of the initial stress intensity factor K value directly affects the study of crack growth behavior in the initial stage of the test and is also of great significance for determining the threshold value △K th is of great significance. The decreasing gradient C g is used to describe the rate at which the stress intensity factor gradually decreases as the test progresses, which helps to determine at what stress intensity factor level the crack stops growing. By adjusting the decreasing gradient, the critical stress intensity factor difference △K th can be found under fine control conditions.

[0089] Preferably, for the da / dN test under a corrosive environment: it is controlled by a constant external load range △P = 4.14 kN, the load ratio is 0.1, the test frequency is 5 Hz, and the sine waveform is used.

[0090] Preferably, for the △Kth test: the initial value of the initial stress intensity factor K is 13.3 MPa·m 0.5 , the load ratio is also 0.1, the test frequency is also 5 Hz, and the decreasing gradient Cg of K is -0.08.

[0091] Preferably, in step S5, the series of da / dN values are calculated using the seven-point increasing polynomial principle.

[0092] When conducting a fatigue crack growth test, record the change in crack length after each loading cycle. This typically involves using high-precision measurement tools, such as clip-on COD gauges (Crack Opening Displacement Gauges), to accurately monitor the change in crack tip opening displacement. Select seven consecutive data points from the data obtained during the experiment. These data points represent the crack length values (a) at specific cycle numbers. Selecting seven points is to ensure there is sufficient data for polynomial fitting while avoiding the problem of overfitting caused by too many data points. Using the selected seven data points, apply the method of incremental polynomial regression analysis to fit a curve. By differentiating the polynomial equation obtained from the above fitting, an expression for the crack growth rate da / dN can be obtained.

[0093] This seven-point incremental polynomial setting can more accurately capture the non-linear characteristics during crack growth, thereby providing a more accurate da / dN value, avoiding the influence of measurement errors of individual data points on the final result, and improving the reliability of the result.

[0094] Preferably, in step S5, the da / dN value can also be calculated using the secant method to obtain a series of da / dN values.

[0095] During the fatigue test, record the change in crack length after each loading cycle. Usually, crack length data at multiple cycle numbers will be recorded. Select two adjacent data points from the experimental data, which represent the crack lengths at different cycle numbers respectively. Use these two points to calculate the average crack growth rate within this interval. Repeat the above steps for all adjacent data points to obtain a series of da / dN values.

[0096] Compared with the complex polynomial fitting method, the secant method only requires simple difference operations to complete the calculation, greatly simplifying the data analysis process and being able to more intuitively reflect the local behavior of crack growth in a specific interval.

[0097] In step S6, when calculating the change in crack tip opening displacement and its mathematical model relationship with the corrosion fatigue crack growth rate, taking the three-point bending SEB specimen commonly used in the da / dN test of fatigue crack growth in air as an example, in a corrosive environment, only an external corrosion environment device needs to be added, which does not affect the following mechanical analysis. The da / dN test generally requires the specimen thickness to satisfy the plane stress state. The crack tip opening displacement δ includes a component δ e caused by elastic deformation and a component δ p caused by plastic deformation. According to relevant mechanical theories and the principle of triangle similarity, the expression is as follows:

[0098] ……………………(1)

[0099] …………………………… (2)

[0100] ……………………………………………… (3)

[0101] According to the above three equations, it can be known that the crack tip opening displacement δ is:[[]]

[0102] … (4)

[0103] In the above formula, δ e is the elastic component of the crack tip opening displacement, in mm; K is the stress intensity factor at the crack tip, in MPa·m 0.5 ; E is the elastic modulus of the test material, in MPa; R p0.2 is the yield strength of the test material, in MPa; a is the crack length, in mm; z is the thickness of the knife edge used for clamping the clip-on extensometer (when the clamping knife edge is directly machined on the specimen, z is 0), in mm; W is the specimen width, in mm; r(W - a) is the distance between the crack tip and the rotation center, in mm; r is the rotation factor, and the Chinese standard stipulates that r is 0.44; V e is the elastic component of the displacement at the clamping knife edge of the extensometer, in mm; δ p is the plastic component of the crack tip opening displacement, in mm; V p is the plastic component of the displacement at the clamping knife edge of the extensometer, in mm; V is the displacement at the clamping knife edge of the extensometer, in mm.

[0104] By separately considering the elastic component and the plastic component of the crack tip opening displacement, the mechanical behavior near the crack tip can be more comprehensively described, including different stages of crack propagation, considering the influence of the stress intensity factor K within the linear elastic range and the influence of plastic deformation on crack propagation, and improving the prediction ability of crack propagation behavior under actual working conditions.

[0105] Specifically, in step S6, during the entire da / dN test process, the normalized crack length of the crack length a calculated by the compliance method is:[[]]

[0106] ……………… (5)

[0107] In the formula, a / W is the normalized crack length, C0, C1, C2, C3, C4, C5 are compliance coefficients (constants), U x is the dimensionless compliance, which is related to the elastic modulus of the test material, specimen size, external load, etc., and is expressed as follows:[[]]

[0108] ……………………………………………… (6)

[0109] In the formula, B is the thickness of the specimen, in mm; V and E are the same as defined previously; P is the externally applied load, in N.

[0110] The compliance method will not cause damage to the specimen during the measurement process, ensuring the integrity of the specimen so that it can be continued to be used in subsequent fatigue tests. By establishing a relationship model between the crack length and the compliance, the complex data analysis process is simplified, making the conversion from experimental data to the crack growth rate more direct and effective.

[0111] Specifically, in step S6, a calculation program is compiled to calculate the value of △δ. The calculation method of the △δ value: According to the above formula (4), we can get:

[0112] …………………………………(7)

[0113] …………………………………(8)

[0114] Here, it is defined that:

[0115] ………………………………………………(9)

[0116] ………………………………………………(10)

[0117] Formula (7) - formula (8) is:

[0118] …………………………………(11)

[0119] In the above formula, Δδ is the change in the crack tip opening displacement, in mm; ΔV is the change in the opening displacement at the extensometer clamping knife edge, in mm; δ max and δ min are the maximum and minimum values of the crack tip opening displacement, in mm; V max and V min are the maximum and minimum values of the displacement at the extensometer clamping knife edge, in mm, which can be directly read and obtained during the test.

[0120] Equations (7) and (8) describe the relationship between the crack tip opening displacement (δ) and the displacement (V) at the extensometer clamping edge. By calculating Δδ, the change in the crack tip opening displacement can be accurately captured, the different stages of fatigue crack growth can be accurately judged, and the complex mechanical state near the crack tip can be better described. Especially in the later stage of crack growth, when the material enters the elastic-plastic deformation stage, the advantage is obvious. The mathematical model constructed based on Δδ can more accurately reflect the true behavior of the material under different conditions, thereby improving the reliability and accuracy of the entire evaluation process.

[0121] During the entire corrosion fatigue test process (including the second + third stages under constant stress amplitude control and the first stage using the decreasing K method), a series of Δδ data can be obtained by calculating according to Equation (11) based on the relevant test data collected.

[0122] Specifically, in step S7, the method for constructing the mathematical model of da / dN - Δδ in a corrosive environment is as follows:

[0123] S71: Obtain the data set (da / dN, Δδ) based on the obtained series of da / dN values and Δδ values;

[0124] S72: Place the data set (da / dN, Δδ) of the specimen in a coordinate system and observe its distribution pattern;

[0125] S73: According to the distribution characteristics of the data set (da / dN, Δδ), obtain the full-process expression of the corrosion fatigue crack growth rate da / dN with Δδ as the driving parameter through the fitting method:

[0126] ………………………………………… (12)

[0127] In the above formula, f is the function expression symbol.

[0128] This setting can capture the change trend of the fatigue crack growth rate (da / dN) with the change in the crack tip opening displacement (Δδ), can comprehensively describe the fatigue behavior of the material, can more accurately capture the non-linear relationship, helps to improve the accuracy of the prediction model, and is used for the fatigue life assessment and verification of ocean engineering structures.

[0129] Preferably, the principle is to make the square of the fitting coefficient R 2 as close to 1 as possible, and the data deviates from the fitting curve as little as possible.

[0130] Specifically, in step S8, the initial crack length a0 is obtained by non-destructive testing methods, and the critical crack length a c is obtained through fracture toughness tests.

[0131] Perform inspections according to the operating procedures of the selected non-destructive testing method. For example, if ultrasonic testing is used, the position and angle of the probe need to be set to ensure that crack signals can be accurately captured; if magnetic particle testing is used, a magnetic field needs to be applied and magnetic powder sprayed to observe the aggregation of magnetic powder at the crack, analyze the detected data or images, determine the starting position and length of the crack, and record them as the initial crack length a0.

[0132] Commonly adopted standards include ASTM E399 "Standard Test Method for Plane-Strain Fracture Toughness of Metallic Materials" or GB / T 4161 "Test Method for Plane-Strain Fracture Toughness K IC of Metallic Materials", etc. Prepare specimens that meet the standard requirements, such as compact tension (CT) specimens or three-point bending (SEB) specimens, ensure that the geometric dimensions and machining accuracy of the specimens meet the standard requirements, apply a load to the specimens under appropriate loading conditions until crack propagation occurs. Record the load-displacement curve and determine the critical load P Q when the crack begins to propagate, calculate the fracture toughness, and according to the fracture toughness test results and relevant formulas, the critical crack length a c can be calculated.

[0133] Example 1

[0134] Design a corrosion environment device for containing the corrosive liquid; configure a 3.5% NaCl aqueous solution to simulate the marine environment.

[0135] Take a certain titanium alloy as the test material and machine a set of 2 SEB specimens. The specimen thickness B is 12 mm, the width W is 24 mm, and the specimen length is 120 mm.

[0136] The test is carried out according to GB / T 6398-2017 "Metallic Materials - Fatigue Testing - Fatigue Crack Propagation Method" on a 10t electro-hydraulic servo material testing machine. First, prefabricate a section of fatigue crack for each of the two specimens to eliminate the influence of the notch on the fatigue crack propagation. Then one specimen is used for the da / dN test under the corrosion environment: controlled by a constant external load range △P = 4.14 kN, the load ratio is 0.1, the test frequency is 5 Hz, and the sine waveform; the other specimen is used for the △K th test under the corrosion environment: the initial value of the stress intensity factor K is 13.3 MPa·m 0.5 , the load ratio is also 0.1, the test frequency is also 5 Hz, and the decline gradient C g of K is -0.08 to conduct the threshold value test of corrosion fatigue crack propagation. da / dN is calculated using the seven-point increasing polynomial principle.

[0137] Use a clip-on COD gauge with a precision of 0.001 mm to measure the crack length a by the compliance method. The measurement principles are shown in Equations (5) and (6) respectively.

[0138] According to Equation (11), a calculation program is compiled to calculate the value of △δ. At the same time, according to Equations (5) and (6), a series of a values can be obtained, and the fitting equation of △δ - a is:

[0139] ……… (13)

[0140] It can be seen that the relationship between △δ and a can be described by a cubic polynomial of one variable.

[0141] Put the data groups (da / dN, △δ) of the two specimens in the coordinate system and observe their distribution law, as shown in Figure 1 .

[0142] According to the distribution characteristics of the data groups (da / dN, △δ), a quintic increasing polynomial of one variable is used to fit the da / dN - △δ data, and the corresponding fitting equation is obtained as:

[0143] ……… (14)

[0144] Figure 1 Among them, it includes the first, second, and third stages of corrosion fatigue crack growth. The entire process does not need to consider fracture toughness and fatigue crack growth threshold values. According to the obtained da / dN - △δ curve equation, combined with the relationship between △δ and a, the evaluation formula for the three-stage life of corrosion fatigue crack growth can be obtained as:

[0145]

[0146] In the evaluation model of Equation (15), the initial crack length a0 can be obtained through non-destructive testing, and a c can be obtained through fracture toughness tests. Based on this, the life of the three stages of corrosion fatigue crack growth of metal materials for offshore engineering structures can be evaluated and verified, achieving the invention purpose. In addition, if the life of the entire process (three stages) of fatigue crack growth of metal materials in air is evaluated, when conducting fatigue crack growth rate da / dN tests and its threshold value △K th tests, a corrosion environment device does not need to be added.

[0147] The above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A life assessment method for the entire process of corrosion fatigue crack growth of metal materials, characterized in that: The specific steps are as follows: S1: Design a corrosion environment device: Make a container suitable for holding the corrosive liquid and convenient for observing cracks; S2: Prepare corrosive solution: Prepare corrosive liquid according to the standard to simulate the marine corrosion environment; S3: Select and measure specimen dimensions: Prepare specimens and measure key dimensions such as thickness, width, and notch length; S4: Prefabricated fatigue crack: Prefabricate initial cracks on the specimen according to the design parameters; S5: Set test parameters and conduct tests: Use an electro-hydraulic servo material testing machine to conduct da / dN tests and threshold value tests in a corrosive environment to obtain a series of da / dN values; S6: Calculate the crack tip opening displacement change (Δδ): Based on the flexibility method, the crack length a value is obtained, and Δδ is calculated in combination with the test data; S7: Establish mathematical model: Analyze the distribution characteristics of (da / dN, Δδ) data and construct a mathematical model of da / dN-Δδ under corrosion environment; S8: Life assessment: Using the obtained model, combined with the critical crack length size a c and initial crack length a0, the life of the three stages of corrosion fatigue crack growth of marine engineering structure metal materials is evaluated.

2. The life assessment method for the entire process of corrosion fatigue crack growth of metal materials according to claim 1 is characterized in that: In step S2, artificial seawater or 3.5% NaCl aqueous solution is prepared according to standards to simulate a marine corrosion environment.

3. The life assessment method for the entire process of corrosion fatigue crack growth of metal materials according to claim 1 is characterized in that: In step S3, at least two specimens are provided, one for the da / dN test in a corrosive environment and one for the ΔK test in a corrosive environment. th test.

4. The life assessment method for the whole process of corrosion fatigue crack growth of metal materials according to claim 1 is characterized in that: In step S3, a three-point bending SEB specimen is used as the specimen.

5. The life assessment method for the whole process of corrosion fatigue crack growth of metal materials according to claim 1 is characterized in that: In step S5, the test parameters include the applied test load range ΔP, the test frequency f, the stress ratio R, the initial stress intensity factor K value and its descending gradient C g .

6. The life assessment method for the whole process of corrosion fatigue crack growth of metal materials according to claim 1 is characterized in that: In step S5, the da / dN value is calculated using the seven-point incremental polynomial method or the secant method in the series of da / dN tests.

7. The life assessment method for the whole process of corrosion fatigue crack growth of metal materials according to claim 1 is characterized in that: In step S6, the crack tip opening displacement δ includes a component δ caused by elastic deformation e and the component δ caused by plastic deformation p , according to relevant mechanics theory and triangle similarity principle, the expression is as follows: ……………………（1） ……………………………（2） ………………………………………………（3） According to the above three equations, the crack tip opening displacement δ is: …（4） In the above formula, δ e is the elastic component of the crack tip opening displacement, mm; K is the crack tip stress intensity factor, MPa·m 0.5 ; E is the elastic modulus of the test material, MPa; R p0.2 is the yield strength of the test material, MPa; a is the crack length, mm; z is the thickness of the blade used for the clamping extensometer (when the clamping blade is directly processed on the specimen, z is 0), mm; W is the specimen width, mm; r (Wa) is the distance between the crack tip and the rotation center, mm; r is the rotation factor, and the Chinese standard stipulates that r is 0.44; V e is the elastic component of the displacement at the extensometer clamping edge, mm; δ p is the plastic component of the crack tip opening displacement, mm; V p is the plastic component of the displacement at the clamping edge of the extensometer, mm; V is the displacement at the clamping edge of the extensometer, mm; During the entire da / dN test, the crack length a is measured according to the compliance method, and the normalized crack length is expressed as: ……………………………（5） Where a / W is the normalized crack length, C0, C1, C2, C3, C4, C5 are flexibility coefficients (constants), and U x It is dimensionless flexibility, which is related to the elastic modulus of the test material, the specimen size, the applied load, etc., and is expressed as follows: ………………………………………………（6） Where, B is the specimen thickness, mm; V and E are the same as defined above; P is the applied load, N. According to the above formula (4), we can get: ……………………………………（7） ……………………………………（8） Here is the definition: ………………………………………………………（9） ……………………………………………………（10） Formula (7)-Formula (8) is: …………………………………………（11） In the above formula, Δδ is the change in the crack tip opening displacement, mm; ΔV is the change in the opening displacement at the extensometer clamping edge, mm; δ max and δ min are the maximum and minimum values ​​of the crack tip opening displacement, mm; V max and V min They are the maximum and minimum displacements of the extensometer’s mounting edge, mm, and can be directly read during the test.

8. The life assessment method for the whole process of corrosion fatigue crack growth of metal materials according to claim 1 is characterized in that: In step S7, the method for constructing a mathematical model of da / dN-Δδ in a corrosive environment is as follows: S71: Obtain a data set (da / dN, △δ) according to the obtained series of da / dN values ​​and △δ values; S72: Place the data set (da / dN, △δ) of the sample in a coordinate system and observe its distribution pattern; S73: According to the distribution characteristics of the data set (da / dN, △δ), the full process expression of the corrosion fatigue crack growth rate da / dN with △δ as the driving parameter is obtained by fitting method: …………………………………………（12） In the above formula, f is a function expression symbol.

9. The life assessment method for the whole process of corrosion fatigue crack growth of metal materials according to claim 1 is characterized in that: In step S8, the initial crack length a0 is obtained by nondestructive testing, and the critical crack length a c Obtained through fracture toughness test.

Images