A method for predicting fatigue crack growth threshold and fatigue life based on martensite content

Abstract

The present invention provides a method for predicting fatigue crack growth threshold and fatigue life based on martensite content, which includes the following steps: S1, constructing a martensite content calculation model; S2, constructing a fatigue crack growth threshold prediction model; S3, constructing a fatigue life prediction model; S4, setting different strain amplitudes and cycle times, taking test duplex steel samples and performing multiple cyclic loading under strain control at different strain amplitudes; S5, calculating the martensite content of the duplex steel; S6, calculating the fatigue crack growth threshold of the duplex steel; S7, predicting the fatigue life of the duplex steel based on the fatigue life prediction model. The present invention establishes a fatigue crack growth threshold prediction formula based on martensite content, which is simple to operate. The strain amplitude and cycle times of the duplex steel to be tested and the inherent parameters can be used to calculate the martensite content generated by the material under cyclic loading conditions, and then the fatigue crack growth threshold and fatigue life of the duplex steel can be predicted.

Description

Technical Field

[0001] The present invention relates to the technical field of fatigue life prediction of metal materials, and in particular to a method for predicting fatigue crack growth threshold and fatigue life based on martensite content. Background Art

[0002] According to previous statistics, 50% to 90% of mechanical component failures are caused by fatigue. Especially in the past 30 years, as machinery has advanced toward higher temperatures, higher speeds, and larger sizes, the stresses required of mechanical parts have become increasingly higher, and operating conditions have become increasingly harsh, leading to a proliferation of fatigue failures. Many mechanical components and structures, such as shafts, crankshafts, connecting rods, gears, springs, bolts, pressure vessels, offshore platforms, turbine blades, and welded structures, are subject to fatigue failure. Therefore, structural components and systems subjected to cyclic loading require the highest and most reliable fatigue life assessments. Fatigue strength is not only crucial in cutting-edge industries such as aerospace, aviation, shipbuilding, and nuclear energy, but is also a key factor affecting the reliability and service life of general mechanical products. Therefore, research on fatigue strength is imperative for the machinery industry. Fatigue crack failure often occurs in metallic materials, and fatigue life depends on the interplay between crack initiation and propagation, which is inextricably linked to the microstructure surrounding the crack. Therefore, microstructural analysis of the fatigue crack perimeter and the prediction of fatigue life are crucial.

[0003] Given the enormous potential for TRIP duplex stainless steel applications and the repeated loads its structural components endure during service, the low-cycle fatigue behavior of duplex stainless steel is of particular interest. This behavior is influenced by the material's cyclic hardening and softening behavior, as well as the microstructural evolution during cyclic loading. The transformed martensite and untransformed austenite exhibit a Kurdjumov-Sachs (KS) orientation relationship, which enhances the cooperative deformation capacity between the two phases. However, with continued cycling, more martensite forms. These newly formed martensite grains, due to their brittleness, are more susceptible to microvoids / cracks, reducing the crack initiation life and acting as a rapid crack propagation pathway, thereby deteriorating the crack growth life. This suggests that fatigue life may be closely related to the interplay between crack initiation and propagation induced by mechanically induced martensite from retained austenite. However, the mechanism of the effect of strain-induced martensite on crack growth in duplex stainless steels during cyclic loading is poorly understood, and the influence of martensite generated in economized TRIP duplex steels during cyclic loading on fatigue crack growth and low-cycle fatigue life remains unclear. Therefore, systematically studying the effects of martensite on the low-cycle fatigue life of duplex stainless steels and establishing accurate fatigue life assessment and prediction models are particularly important for promoting the industrial application of these high-performance metastable duplex stainless steels. The fatigue crack growth threshold is a key predictor of the safety of cracked components during their expected service life. It reflects the material's fatigue resistance and is of great significance for the design of long-life and infinite-life components in engineering. The fatigue crack growth threshold is a crucial basis for damage tolerance design. Fatigue crack growth generally progresses through five stages: slip (planar or rippled), crack nucleation, microcrack propagation, macrocrack propagation, and fracture. While these stages are difficult to strictly distinguish, the mechanisms of initiation and propagation in each stage are distinct. Among the theoretical hypotheses describing fatigue crack growth, the Paris formula is the most widely used. Practice has shown that when the stress intensity factor at the crack tip is below a certain value, the fatigue crack will no longer propagate or will propagate very slowly. This value is referred to as the fatigue crack growth threshold. Two definitions are commonly used in engineering: 1. When the material undergoes 10 7 ~10 8 After fatigue, when the crack no longer expands or the expansion does not exceed 0.05mm, the corresponding stress intensity factor value is the fatigue expansion threshold. 2. Define the fatigue expansion rate as 10 -7 ~10 -6 The stress intensity factor value corresponding to mm / c or the condition threshold rate proposed based on the actual component life requirement is 10 -9 ~10 -8 The corresponding value when mm / c.

[0004] Experimental acquisition of fatigue crack growth threshold is the most basic means, but the test process is cumbersome, the operation time is long, and many parameters are required, which makes it difficult to promote engineering production and application. Therefore, it is urgent to study a fatigue crack growth threshold prediction method that is simple, easy to operate and has accurate prediction results to accurately evaluate and predict the fatigue crack growth threshold and fatigue life of the test steel. Summary of the Invention

[0005] In view of the above-mentioned deficiencies in the prior art, the purpose of the present invention is to provide a method for predicting fatigue crack growth threshold and fatigue life based on martensite content, which can provide a fatigue crack growth threshold prediction method with simple method, high accuracy and easy acquisition of relevant parameters.

[0006] In order to achieve the above object, the present invention specifically provides a method for predicting fatigue crack growth threshold and fatigue life based on martensite content, characterized in that it comprises the following steps:

[0007] S1. Establishing a martensite content database for dual-phase steel at different strain amplitudes and cycle times, and constructing a martensite content calculation model based on the martensite content database;

[0008] S2. Constructing a fatigue crack growth threshold prediction model, wherein the fatigue crack growth threshold prediction model is specifically:

[0009]

[0010] Where ΔKth is the fatigue growth threshold; E is Young's modulus, Tm is the melting point of the test steel, δb is the tensile strength, δs is the yield strength, R is the stress ratio, and f M is the martensite content, n' is the cyclic hardening coefficient, and B is the fatigue crack growth threshold coefficient;

[0011] S3. Construct a fatigue life prediction model. The fatigue life prediction model is as follows:

[0012]

[0013] In the formula, x, y and z are intermediate parameters, and f M is the martensite content, ε a is the strain amplitude, ΔKth is the fatigue crack growth threshold;

[0014] S4. Setting different strain amplitudes and cycle times, taking the test duplex steel specimens and performing multiple cyclic loading under strain control at different strain amplitudes, and recording the strain amplitudes and cycle times;

[0015] S5. Substituting the strain amplitude and cycle number obtained in step S4 into the martensite content calculation model constructed in step S1, and calculating the martensite content of the dual-phase steel using the martensite content calculation model;

[0016] S6. Substituting the martensite content obtained in step S5 into the fatigue crack growth threshold prediction model in step S2 to calculate the fatigue crack growth threshold of the dual-phase steel;

[0017] S7. Substituting the fatigue crack growth threshold, martensite content, and strain amplitude of the dual-phase steel into the fatigue life prediction model in step S3, the fatigue life of the dual-phase steel is predicted.

[0018] Preferably, the fatigue life prediction model in step S3 is specifically as follows:

[0019]

[0020] Preferably, the calculation formula of the fatigue crack growth threshold coefficient B in step S3 is as follows:

[0021] B=F 试 *V 加 *Q 强 *L 延 *b*T 轴

[0022] Where, F 试 is the test frequency, V 加 is the sample loading rate, Q 强 is the yield strength ratio, which is the ratio of yield strength to tensile strength, L 延 is the elongation of dual-phase steel, b is the Burgers vector, T 轴 is the axial strain control parameter.

[0023] Preferably, the martensite content calculation model in step S2 is as follows:

[0024] f M =-kε a [1-exp(-q4Nε a )] m

[0025] Where, f M is the martensite content, ε a is the total strain amplitude, and N is the number of cycles.

[0026] Preferably, in the above formula

[0027]

[0028]

[0029] Among them, a, b, c, d, e, f, g, h, i, j, k, and m are fitting parameters.

[0030] Preferably, wherein:

[0031]

[0032]

[0033]

[0034] Preferably, the dual-phase steel is a cost-effective TRIP dual-phase steel.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] (1) The present invention establishes a martensite content calculation model, which can calculate the martensite content of the test steel based on the strain amplitude and the number of cycles. The operation is simple and does not require obtaining a large number of parameters through experimental measurements. The content of martensite generated during the cyclic loading of the dual-phase steel can be quickly calculated, thereby greatly saving the time and operating costs required for the test and being easy to use in various work scenarios.

[0037] (2) The present invention establishes a fatigue crack growth threshold prediction formula based on the calculated martensite content, which can quickly predict the fatigue crack growth threshold of the test steel. Without causing damage to the test piece, the fatigue crack growth threshold can be quickly and accurately obtained based on the martensite content, thereby guiding the subsequent use of the test piece.

[0038] (3) The present invention establishes a fatigue life prediction model based on strain amplitude, fatigue crack growth threshold and martensite content. By substituting strain amplitude, fatigue crack growth threshold and martensite content into the fatigue life prediction model, the output calculation result is the fatigue life of the dual-phase steel. The fitting parameters involved in the fatigue life prediction model are accurate parameters that are easy to calculate and are obtained based on a large amount of data. They can accurately evaluate the life of the dual-phase steel in practical applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 This is a flow chart of a method for predicting fatigue crack growth threshold and fatigue life based on martensite content in an embodiment of the method of the present invention;

[0040] Figure 2 A comparison diagram of the predicted and experimentally measured variables of martensite content at different strain amplitudes in the embodiment of the method of the present invention;

[0041] Figure 3 A comparison chart of the fatigue crack growth threshold prediction value of the present invention and the value calculated by the empirical model;

[0042] Figure 4 This is a comparison chart of the actual fatigue life and predicted fatigue life of the economical TRIP dual-phase steel in an embodiment of the present invention. DETAILED DESCRIPTION

[0043] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

[0044] The present invention provides a method for predicting fatigue crack growth threshold and fatigue life based on martensite content, such as Figure 1 As shown, the method specifically includes the following steps:

[0045] S1. Establish a martensite content database for dual-phase steel at different strain amplitudes and cycle times, and construct a martensite content calculation model based on the martensite content database; the martensite content database is obtained and established based on a large amount of test data of dual-phase steel at different strain amplitudes and cycle times to ensure the accuracy of the calculation results of the martensite content calculation model.

[0046] S2. Construct a fatigue crack growth threshold prediction model. The fatigue crack growth threshold prediction model is specifically as follows:

[0047]

[0048] Where ΔKth is the fatigue crack growth threshold; E is Young's modulus, Tm is the melting point of the test steel, δb is the tensile strength, δs is the yield strength, R is the stress ratio, and f M is the martensite content, n' is the cyclic hardening coefficient, and B is the fatigue crack growth threshold coefficient.

[0049] S3. Construct a fatigue life prediction model. The fatigue life prediction model is as follows:

[0050]

[0051] In the formula, x, y and z are intermediate parameters, and f M is the martensite content, ε a is the strain amplitude and ΔKth is the fatigue crack growth threshold.

[0052] S4. Set different strain amplitudes and cycle times, take the dual-phase steel specimens tested with different strain amplitudes and perform multiple cyclic loading under strain control, and record the strain amplitude and cycle times.

[0053] S5. Substitute the strain amplitude and cycle number obtained in step S4, as well as the dual-phase steel parameters, into the martensite content calculation model constructed in step S1, and calculate the martensite content of the dual-phase steel using the martensite content calculation model. The dual-phase steel parameters involved are all obtainable without destroying the test piece.

[0054] S6. Substituting the martensite content obtained in step S5 into the fatigue crack growth threshold prediction model to calculate the fatigue crack growth threshold of the dual-phase steel;

[0055] S7. Substitute the fatigue crack growth threshold, martensite content, and strain amplitude of the dual-phase steel into the fatigue life prediction model to predict the fatigue life of the dual-phase steel.

[0056] Preferably, the calculation formula of the fatigue crack growth threshold coefficient B in step S3 is as follows:

[0057] B=F 试 *V 加 *Q 强 *L 延 *b*T 轴

[0058] Among them, F 试 The test frequency of this test steel is 0.03-0.1 Hz. In this embodiment, the upper limit is 0.1 Hz. V 加 is the sample loading rate. The loading rate of this test steel is 0.2*10 -2 / s,Q 强 The yield strength ratio is the ratio of yield strength to tensile strength. The yield strength ratio of this test steel is 0.58. 延 is the elongation of the test steel, and the elongation of this test steel is 0.5871. b is the Burgers vector, and the value of this test steel is 0.25. 轴 is the axial strain control parameter. The axial strain control parameter T of the test steel is 轴 Take 0.60*10 -6 According to the data of this test steel, B=1.0*10 -1 , which is dimensionless.

[0059] S4. Based on the fatigue growth threshold prediction model and the martensite content calculation model, a fatigue life prediction model for duplex steel is derived. The fatigue life prediction model for duplex steel can be used to predict the fatigue life of the test steel. The fatigue life prediction model is as follows:

[0060]

[0061] In the formula, x, y, and z are all intermediate parameters; M is the martensite content, ε a is the strain amplitude and ΔKth is the fatigue crack growth threshold.

[0062] Calculate the intermediate parameter values ​​of the fitting and substitute the intermediate parameter values ​​into the following formula:

[0063]

[0064] In actual work, the fatigue life of steel can be predicted by the above formula. The fatigue life prediction model can accurately predict the fatigue life of duplex steel based on strain amplitude, fatigue crack growth threshold and martensite content, providing a reference for the engineering application of duplex steel. Its fatigue life can be accurately predicted without destroying the sample.

[0065] This embodiment provides a method for predicting fatigue crack growth threshold and fatigue life based on martensite content. The overall process diagram is as follows: Figure 1 As shown, the test material used in this embodiment is an economical duplex stainless steel alloyed with Mn-N, hereinafter referred to as economical TRIP duplex steel.

[0066] The specific steps of the whole method are as follows:

[0067] S1. Take multiple pieces of economical TRIP dual-phase steel and perform strain-controlled cyclic loading at different strain amplitudes and record the relevant data. Based on this large amount of data, establish a martensite content database of dual-phase steel at different strain amplitudes and cycle times, and construct a martensite content calculation model based on the martensite content database.

[0068] Specifically, the calculation model of the martensite content of this test steel is as follows:

[0069] f M =-kε a [1-exp(-q4Nε a )] m

[0070] in

[0071]

[0072]

[0073] S2. Construct a fatigue crack growth threshold prediction model. The fatigue crack growth threshold prediction model is specifically as follows:

[0074]

[0075] Where ΔKth is the fatigue crack growth threshold; E is Young's modulus, Tm is the melting point of the test steel, δb is the tensile strength, δs is the yield strength, R is the stress ratio, and f M is the martensite content, n' is the cyclic hardening coefficient, and B is the fatigue crack growth threshold coefficient.

[0076] The calculation formula of B is: B=F 试 *V 加 *Q 强 *L 延 *b*T轴

[0077] Where, F 试 is the test frequency, which is 0.1Hz, V 加 is the sample loading rate, which is 0.2*10 -2 / s,Q 强 The yield strength ratio is the ratio of yield strength to tensile strength. The yield strength ratio is 0.58. 延 is the elongation of the test steel, which is 0.5871; b is the Burgers vector, which is 0.25; T 轴 is the axial strain control parameter, and its value is 0.60*10 -6 Substituting the above values ​​into the calculation formula, the fatigue crack growth threshold coefficient B of the test steel is calculated to be 1.0*10 -11 , which is dimensionless.

[0078] S3. Construct a fatigue life prediction model. The fatigue life prediction model is as follows:

[0079]

[0080] In the formula, x, y and z are fitting parameters, and f M is the martensite content, ε a is the strain amplitude and ΔKth is the fatigue crack growth threshold.

[0081] Substituting the relevant fitting parameters, we get:

[0082]

[0083] S4. Select a piece of test steel as the prediction object again, set different strain amplitudes and cycle times, take the test dual-phase steel specimens and perform multiple cyclic loading under strain control at different strain amplitudes, and record the strain amplitude and cycle times.

[0084] S5. Substitute the strain amplitude and cycle number obtained in step S4 into the martensite content calculation model constructed in step S1, and calculate the martensite content of the dual-phase steel using the martensite content calculation model.

[0085] S6. Substituting the martensite content obtained in step S5 into the fatigue crack growth threshold prediction model to calculate the fatigue crack growth threshold of the dual-phase steel.

[0086] S7. Substitute the fatigue crack growth threshold, martensite content, and strain amplitude of the dual-phase steel into the fatigue life prediction model to predict the fatigue life of the dual-phase steel.

[0087] Based on the above formula, the martensite content at different strain amplitudes was calculated. The specific martensite content is as follows: Figure 2 As shown, Figure 2 The figure shows a comparison between the experimental value and the predicted value of the martensite content at different cycles under strain amplitudes of 0.7, 0.9 and 1.1. It can be seen from the figure that under strain amplitudes of 0.7, 0.9 and 1.1, the martensite content at different cycles calculated using the martensite content calculation formula in this patent, that is, the predicted value, is very close to the experimental value obtained through the experiment.

[0088] The material constant fitting values ​​under the four strain amplitudes are shown in Table 1 below:

[0089] Table 1

[0090] strain amplitude k q m 0.5 -372.613 0.51857 1.13238 0.7 -473.926 1.01661 2.51472 0.9 -432.791 0.67634 1.55887 1.1 -446.587 0.55144 1.26126

[0091] The fatigue crack growth threshold * martensite content / strain amplitude is used as the horizontal coordinate X and the fatigue life Y is used as the vertical coordinate to perform polynomial fitting, and the fitting curve is obtained as follows: Figure 3 As shown, the relationship between fatigue crack growth threshold, martensite content, strain amplitude and life is expressed as:

[0092] Y=258949.66601-139474.40055*X+24849.9213*X 2 -1460.70955*X 3

[0093] Right now:

[0094]

[0095] The above model can be used to calculate fatigue life and apply it to engineering practice. A fatigue life model has been established based on the fatigue crack growth threshold, which can predict the fatigue life of steel. The fatigue life prediction model can accurately predict the fatigue life of dual-phase steel based on strain amplitude, fatigue crack growth threshold, and martensite content, providing a reference for the practical application of dual-phase steel in engineering. The fatigue life prediction model proposed in this invention is simple, easy to operate and apply, and can ensure the accuracy of the prediction results.

[0096] The present invention calculates the crack growth threshold value by the empirical formula to obtain the approximate crack growth threshold value, namely Figure 3 The approximate extension threshold in is used as a reference. At the same time, the martensite content under different cycles is substituted into the fatigue crack extension threshold prediction model to obtain the predicted fatigue crack extension threshold, namely Figure 3The predicted growth threshold in [ 1 ] is compared with the approximate growth threshold to measure the prediction accuracy of the fatigue crack growth threshold prediction model. The figure shows that the predicted growth threshold and the approximate growth threshold are very close, demonstrating that the fatigue crack growth threshold obtained by the fatigue crack growth threshold prediction model has a very high accuracy. The data related to the approximate growth threshold of the test steel and the fatigue crack growth threshold calculated by the fatigue crack growth threshold prediction model are shown in Table 2.

[0097] Table 2 Comparison of two threshold values ​​under different strain amplitudes

[0098]

[0099] Afterwards, the fatigue crack growth threshold, martensite content and strain amplitude of the dual-phase steel are substituted into the fatigue life prediction model to obtain the fatigue life prediction results, thereby predicting the fatigue life of the dual-phase steel.

[0100] Figure 4 This is a schematic diagram of the comparison between the fatigue life prediction results and the actual life. It can be seen from the figure that the results predicted by the fatigue life prediction model, i.e., the predicted fatigue life values ​​in the figure, and the actual life, i.e., the actual fatigue life values ​​in the figure, are almost completely consistent with each other, indicating that the fatigue life prediction model can well predict the fatigue crack growth threshold of TRIP dual-phase steel and predict the fatigue life based on the fatigue crack growth threshold, providing a reference for the engineering application of dual-phase steel.

[0101] Comparisons verified the accuracy and practicality of the proposed fatigue crack growth threshold prediction model and fatigue life prediction model in predicting crack growth threshold and fatigue life. Verification results demonstrated the successful quantitative characterization of the effect of martensite content on fatigue crack growth threshold. The proposed fatigue crack growth threshold prediction model accurately predicted the fatigue crack growth threshold over a wide strain range, while the fatigue life prediction model effectively predicted life. Furthermore, both models require only a small number of material parameters for prediction, facilitating engineering applications. The models do not involve meaningless fitting parameters and can be calculated without extensive testing, thus reducing the computational complexity and parameter acquisition difficulty, facilitating their application in multiple fields.

[0102] In addition, fatigue testing poses a daunting challenge to the durability and reliability of equipment, requiring enormous time and costs. The model proposed in this method only requires the fatigue crack growth threshold at a reference martensite content and the martensite content at different strain amplitudes to make predictions. The reference martensite content is usually selected as the martensite content data at room temperature, where experimental data is easily available. Compared with traditional fatigue testing, the fatigue crack growth threshold and fatigue life at different martensite contents can be quickly and easily predicted. In particular, the martensite-related Young's modulus E and the material melting point Tm can be obtained in a non-destructive manner to characterize the martensite content.

[0103] The embodiments described above are merely descriptions of preferred implementations of the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should fall within the scope of protection determined by the claims of the present invention.

Claims

1. A method for predicting fatigue crack growth threshold and fatigue life based on martensite content, characterized by: It includes the following steps: S1. Establishing a martensite content database for dual-phase steel at different strain amplitudes and cycle times, and constructing a martensite content calculation model based on the martensite content database; S2. Constructing a fatigue crack growth threshold prediction model, wherein the fatigue crack growth threshold prediction model is specifically: Where ΔKth is the fatigue crack growth threshold; E is Young's modulus, Tm is the melting point of the test steel, δb is the tensile strength, δs is the yield strength, R is the stress ratio, and f M is the martensite content, n' is the cyclic hardening coefficient, and B is the fatigue crack growth threshold coefficient; S3. Construct a fatigue life prediction model, which is as follows: Where N f is the fatigue life, x, y and z are intermediate parameters, f M is the martensite content, ε a is the strain amplitude, ΔKth is the fatigue crack growth threshold; S4. Setting different strain amplitudes and cycle times, taking the test duplex steel specimens and performing multiple cyclic loading under strain control at different strain amplitudes, and recording the strain amplitudes and cycle times; S5. Substituting the strain amplitude and cycle number obtained in step S4 into the martensite content calculation model constructed in step S1, and calculating the martensite content of the dual-phase steel using the martensite content calculation model; S6. Substituting the martensite content obtained in step S5 into the fatigue crack growth threshold prediction model in step S2 to calculate the fatigue crack growth threshold of the dual-phase steel; S7. Substituting the fatigue crack growth threshold, martensite content, and strain amplitude of the dual-phase steel into the fatigue life prediction model in step S3, the fatigue life of the dual-phase steel is predicted.

2. The method for predicting fatigue crack growth threshold and fatigue life based on martensite content according to claim 1, characterized in that: The fatigue life prediction model in step S3 is specifically as follows:

3. The method for predicting fatigue crack growth threshold and fatigue life based on martensite content according to claim 1, characterized in that: The calculation formula of the fatigue crack growth threshold coefficient B in step S3 is as follows: B=F 试 *V 加 *Q 强 *L 延 *b*T 轴 Where, F 试 is the test frequency, V 加 is the sample loading rate, Q 强 is the yield strength ratio, which is the ratio of yield strength to tensile strength, L 延 is the elongation of dual-phase steel, b is the Burgers vector, T 轴 is the axial strain control parameter.

4. The method for predicting fatigue crack growth threshold and fatigue life based on martensite content according to claim 1, characterized in that: The martensite content calculation model in step S1 is as follows: favorite M =-kε a [1-exp(-q4Nε a )] m Where, f M is the martensite content, ε a is the total strain amplitude, and N is the number of cycles.

5. The method for predicting fatigue crack growth threshold and fatigue life based on martensite content according to claim 4, characterized in that: In the above formula: Among them, a, b, c, d, e, f, g, h, i, j, k, and m are fitting parameters.

6. The method for predicting fatigue crack growth threshold and fatigue life based on martensite content according to claim 5, characterized in that: In the above formula:

7. The method for predicting fatigue crack growth threshold and fatigue life based on martensite content according to claim 1, characterized in that: The duplex steel is economical TRIP duplex stainless steel.

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