Method for calculating service life of material under action of thermal shock load based on KAM value growth model

Abstract

The invention discloses a method for calculating the service life of a material under the action of a thermal shock load based on a KAM value growth model, and belongs to the technical field of high-temperature structural strength. Comprising a high-temperature thermal shock experiment of the material, a KAM value growth model after the thermal shock load action and a service life calculation method of the material under the thermal shock load action based on the KAM value growth model. Adopted test instruments comprise a multi-stage high-temperature heating furnace, a metallographic microscope and a scanning electron microscope. The method is simple in design test and low in cost; the method for calculating the service life of the material under the action of the thermal shock load based on the KAM value growth model is excellent in performance in a long crack stage or a short crack stage, and can be used for calculating the service life of the thermal fatigue crack in the whole process.

Description

Technical Field

[0001] The present invention relates to a method for calculating the life of a material under thermal shock load based on the KAM value growth model, belonging to the technical field of high-temperature structural strength. Background Technique

[0002] As a core component of modern aircraft, the high-temperature components of aeroengines (such as turbine blades, combustion chambers, etc.) frequently experience severe thermal shock loads during operation. When these components start up, shut down, or undergo sudden changes in operating conditions, they will bear rapidly changing temperature fields and thermal stresses, resulting in significant microstructural evolution and damage accumulation within the material. Traditional life prediction methods are mainly based on macroscopic stress-strain analysis, such as the Manson-Coffin model, but these methods are difficult to accurately capture the micro-damage mechanism of materials under thermal shock loads, especially the local strain accumulation within grains and the initiation and propagation of microcracks.

[0003] Therefore, how to more accurately describe the micro-damage mechanism of materials under thermal shock loads, so as to achieve life prediction from micro to macro, has become a problem that needs to be solved by us. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for calculating the life of a material under thermal shock load based on the KAM value growth model to solve the problems mentioned in the above background technique.

[0005] To solve the above technical problems, the present invention provides the following technical solutions:

[0006] A method for calculating the life of a material under thermal shock load based on the KAM value growth model, the method includes the following steps:

[0007] Step 101: Conduct high-temperature thermal shock experiments on the material test pieces under different temperature conditions, and perform EBSD observations every certain number of cycles to obtain the changes in KAM values under different temperature conditions and different numbers of cycles;

[0008] Step 102: Based on the change trend of the KAM values obtained by observation, establish the relationship between the KAM values, temperature difference, maximum thermal stress, and number of cycles, and construct a KAM value prediction model;

[0009] Step 103: Based on the experimental results of the high-temperature thermal shock experiment in Step 101, statistically analyze the relationship between the crack length and the number of cycles for different temperature cycles, and draw a relationship diagram; establish the relationship between the stress intensity factor and the crack length according to the relationship diagram, and calculate the relationship between the stress intensity factor threshold and the crack length;

[0010] Step 104: Calculate the notch thermal stress under different temperature cycles according to the finite element software, and calculate the thermal stress concentration factor by the finite element method;

[0011] Step 105: Relate the crack length to the thermal shock temperature, the heating and cooling rate ratio, and the number of cycles to obtain a multi-scale thermal fatigue crack growth model; based on the KAM value prediction model, correct the multi-scale thermal fatigue crack growth model:

[0012]

[0013] Among them, represents the thermal shock crack growth rate, a represents the crack length, N represents the number of cycles, C 1 , n and m represent material constants, R represents the thermal stress ratio, v represents the Poisson's ratio of the material, R V represents the heating and cooling rate ratio, E' represents the elastic modulus, σ ys represents the yield stress of the material, ΔK represents the stress intensity factor, ΔK th represents the stress intensity factor threshold, and β represents the correction factor;

[0014] Step 105: Integrate Equation 1 to obtain a multi-scale thermal fatigue crack life prediction model corrected based on the KAM value.

[0015] As a preferred embodiment of the present invention, for the high-temperature thermal shock test of the material, the KAM value growth model after the action of the thermal shock load, and the life calculation method of the material under the action of the thermal shock load based on the KAM value growth model, the experimental apparatuses used are a multi-stage high-temperature heating furnace, a metallographic microscope, and a scanning electron microscope.

[0016] As a preferred embodiment of the present invention, the high-temperature thermal shock experiment of the material is used to obtain the relationship between the number of cycles and the crack length at different upper limit temperatures and the change of the KAM value.

[0017] As a preferred embodiment of the present invention, the expression for establishing the connection between the KAM value, the temperature difference, the maximum thermal stress, and the number of cycles to construct the KAM value prediction model is as follows:

[0018]

[0019] Among them, a * represents the scanning step size, ζ * represents the coefficient independent of the slip system within the grain, represents the characteristic size, α represents the linear thermal expansion coefficient of the material, ΔT represents the temperature difference, C represents the influence coefficient of the thermal stress amplitude and the material properties, σ a represents the stress amplitude, which is embodied as the maximum thermal stress in this article, and n represents the hardening index of the material.

[0020] As a preferred embodiment of the present invention, the expression for establishing the relationship between the stress intensity factor and the crack length is as follows:

[0021]

[0022] where k t represents the stress concentration factor at the notch of the test piece; σ max represents the maximum thermal stress in the test area of the test piece; σ cl represents the closing stress of the thermal shock crack; represents the thermal fatigue crack growth influence factor; ρ represents the notch root radius of the test piece; Q represents the shape correction factor; F represents the boundary condition; c represents the depth of the thermal shock crack; t represents the thickness of the test piece; b represents the width of the test piece; Φ represents the angular function at the tip of the elliptical crack.

[0023] As a preferred embodiment of the present invention, the expression for calculating the thermal stress concentration factor by the finite element method is as follows:

[0024]

[0025] where σ max represents the maximum stress at the stress concentration, σ 0 represents the nominal stress.

[0026] As a preferred embodiment of the present invention, the relationship expression between the calculated stress intensity factor threshold ΔK th and the crack length a is as follows:

[0027]

[0028] where d represents the limit of the microscopic crack size of the material; σ eR represents the ordinary fatigue limit of the material.

[0029] As a preferred embodiment of the present invention, the expression of the multi-scale thermal fatigue crack life prediction model based on the KAM value correction is as follows:

[0030]

[0031] where, N i represents the crack initiation life; a i represents the crack initiation size; a t represents the crack length; ΔK|a t represents the termination crack length; ΔK|a i represents the initial crack length.

[0032] As a preferred embodiment of the present invention, by substituting the KAM value in formula (2), the stress intensity factor in formula (3), the thermal stress concentration factor in formula (4), and the stress intensity factor threshold in formula (5) into formula (6) for integration, a multi-scale thermal fatigue crack life prediction model corrected based on the KAM value can be obtained.

[0033] As a preferred embodiment of the present invention, based on the influence of the microstructure on the propagation of small cracks and in combination with the change of the KAM value in some regions of the specimen after the action of thermal shock load, a method for calculating the life of materials under thermal shock load based on the KAM value growth model is established. This method can more comprehensively reflect the crack propagation behavior during the action of thermal shock load.

[0034] Based on the influence of the microstructure on the propagation of small cracks and in combination with the change of the KAM value in some regions of the specimen after the action of thermal shock load, a method for calculating the life of materials under thermal shock load based on the KAM value growth model is established. This method can more comprehensively reflect the crack propagation behavior during the action of thermal shock load.

[0035] Compared with the prior art, the beneficial effects achieved by the present invention are as follows: A method for calculating the life of materials under thermal shock load based on the KAM value growth model provided by the present invention. As an important parameter characterizing the local strain accumulation of materials, the KAM value can effectively reflect the damage evolution of the microstructure of materials during thermal shock. By establishing the quantitative relationship between the KAM value, the thermal shock temperature, and the number of cycles, the present invention can more accurately describe the microscopic damage mechanism of materials under thermal shock load, thereby realizing the life prediction from the microscopic to the macroscopic. This method not only makes up for the deficiencies of traditional macroscopic models but also provides a new technical means for the life assessment of high-temperature components of aeroengines, having important engineering application value and theoretical significance. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The drawings are used to provide a further understanding of the present invention and constitute a part of the specification. They are used together with the embodiments of the present invention to explain the present invention and do not constitute a limitation to the present invention.

[0037] Figure 1 is the flowchart of the method of the present invention;

[0038] Figure 2 is the comparison of prediction data in the 20°C - 1100°C - 20°C of the embodiment of the present invention;

[0039] Figure 3 is the comparison of prediction data in the 600°C - 1100°C - 600°C of the embodiment of the present invention;

[0040] Figure 4It is the comparison of the predicted data at 800°C - 1100°C - 800°C in the embodiments of the present invention. Specific embodiments

[0041] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0042] The purpose of the high-temperature thermal shock experiment on the material is to obtain the relationship between the grain size of the material and the number of thermal shock cycles, so as to later relate the time step of the CA system to the corresponding number of thermal shock cycles and the upper limit temperature of thermal shock.

[0043] The high-temperature thermal shock experiment on the material is carried out under a series of different upper limit temperature conditions T i until a certain number of cycles is reached, then it is taken down for observation, and the changes in grain size, δ phase, and precipitation of carbides are recorded.

[0044] The method for calculating the life of a material under the action of a thermal shock load based on the KAM value growth model is specifically as follows:

[0045] Step 101: Conduct high-temperature thermal shock experiments under different temperature conditions T i and prepare samples of the test piece for EBSD observation every certain number of cycles, and obtain the changes in the number of cycles and KAM values under different temperature conditions T i by observing the KAM map.

[0046] Step 102: Based on the observed change trend of the KAM value, establish a KAM value prediction model, which establishes a connection between the complex and difficult-to-calculate and non-intuitive KAM value and the intuitive temperature difference, maximum thermal stress, and number of cycles:

[0047]

[0048] Among them, a * represents the scanning step size, ζ * represents the coefficient independent of the slip system within the grain, represents the characteristic size, α represents the linear thermal expansion coefficient of the material, C represents the influence coefficient of the thermal stress amplitude and material properties, σ a represents the stress amplitude, which is reflected as the maximum thermal stress in this article, and n represents the hardening index of the material.

[0049] Step 103: Use the proposed KAM value prediction model to correct the multi-scale thermal fatigue crack propagation model.

[0050]

[0051] Among them, represents the thermal shock crack growth rate, a represents the crack length, N represents the number of cycles, C 1 , n and m represent material constants, R represents the thermal stress ratio, v represents the Poisson's ratio of the material, R V represents the heating and cooling rate ratio, E' represents the elastic modulus, σ ys represents the yield stress of the material, ΔK represents the stress intensity factor, ΔK th represents the stress intensity factor threshold, and β represents the correction factor.

[0052] Step 104: For the thermal shock test conducted in Step 101, specimens are taken out every certain number of cycles for polishing, and the growth and expansion of each crack are traced under a metallurgical microscope. The relationship between the crack length and the number of cycles for different temperature cycles is statistically analyzed, and an a-N relationship diagram is plotted. And based on the a-N relationship diagram, the relationship between the stress intensity factor ΔK and the crack length a is established:

[0053]

[0054] Among them, k t represents the stress concentration coefficient at the notch of the specimen; σ max represents the maximum thermal stress in the test area of the specimen; σ cl represents the closing stress of the thermal shock crack; represents the thermal fatigue crack growth influence factor; ρ represents the root radius of the notch of the specimen; Q represents the shape correction factor; F represents the boundary condition; c represents the depth of the thermal shock crack; t represents the thickness of the specimen; b represents the width of the specimen; Φ represents the angular function at the tip of the elliptical crack;

[0055] Step 105: Use the Transient Thermal module and Transient Structural module in the finite element software ANSYS to conduct a thermo-mechanical coupling analysis on the thermal shock test piece, and the simulation conditions are strictly consistent with the experimental conditions. Calculate the notch thermal stress σ under different temperature cycles, and calculate the thermal stress concentration coefficient k t . The specific expression of the thermal stress concentration coefficient is:

[0056]

[0057] In the formula, σ max is the maximum stress at the stress concentration, and σ 0 is the nominal stress.

[0058] Step 106: Calculate the stress intensity factor threshold ΔK according to the experimental results in Step 101 th The relationship with the crack length a is as follows:

[0059]

[0060] In the above formula, d is the limit of the microscopic crack size of the material; σ eR is the ordinary fatigue limit of the material;

[0061] Step 107: Integrating the formula in Step 102, a multi-scale thermal fatigue crack life prediction model corrected based on the KAM value can be obtained:

[0062]

[0063] Among them, N i represents the crack initiation life; a i represents the crack initiation size; a t represents the crack length; ΔK|a t represents the final crack length; ΔK|a i represents the initial crack length;

[0064] Step 108: By substituting the KAM value in formula (2), the stress intensity factor in formula (3), the thermal stress concentration factor in formula (4), and the stress intensity factor threshold in formula (5) into formula (6) for integration, a multi-scale thermal fatigue crack life prediction model corrected based on the KAM value can be obtained.

[0065] Next, the present invention will be further described in combination with specific implementation cases. The present invention can be better understood according to the implementation cases. The specific material ratios, process conditions and their results described in the implementation cases are only used to illustrate the present invention, and should not and will not limit the present invention described in detail in the claims.

[0066] In this implementation case, the life calculation method of the material under thermal shock load proposed by the present invention based on the KAM value growth model is applied to the thermal shock life prediction of DD6 single crystal alloy, including the following steps:

[0067] Step 101: Three groups of thermal shock tests were carried out with temperature cycles of 20°C - 1100°C - 20°C, 600°C - 1100°C - 600°C, and 800°C - 1100°C - 800°C respectively. At the initial stage, crack initiation was observed after 150, 100, and 50 cycles respectively. The thermal fatigue crack initiation life was defined as the number of cycles required to form a main crack with a length of 0.1 mm. Before each observation, the specimen was polished to 2500 mesh, the cracks were observed using a metallurgical microscope, and the crack length was analyzed using Origin software. At 1000 cycles, the cracks were observed uniformly once, and then the specimens under 20°C - 1100°C - 20°C, 600°C - 1100°C - 600°C, and 800°C - 1100°C - 800°C were observed for cracks every 1000, 1500, and 2000 cycles respectively. And for each observation, the test piece was sampled for EBSD observation, and the changes of different cycle numbers and KAM values under different temperature conditions T i were obtained by observing the KAM map.

[0068] Step 102: Based on the observed change trend of KAM values, a KAM value prediction model was established, which established a connection between the complex and difficult-to-calculate and non-intuitive KAM values and the intuitive temperature difference, maximum thermal stress, and number of cycles. The established KAM value prediction model is as follows:

[0069]

[0070] Step 103: Using the proposed KAM value prediction model, the multi-scale thermal fatigue crack propagation model was corrected:

[0071]

[0072] Among them, represents the thermal shock crack propagation rate, a represents the crack length, N represents the number of cycles, C 1 , n and m represent material constants, R represents the thermal stress ratio, v represents the Poisson's ratio of the material, R V represents the heating and cooling rate ratio, E' represents the elastic modulus, σ ys represents the yield stress of the material, ΔK represents the stress intensity factor, ΔK th represents the stress intensity factor threshold, and β represents the correction factor.

[0073] Step 104: For the thermal shock tests carried out in Step 101, the test pieces were taken out at certain intervals of the number of cycles for polishing, and the growth and propagation of each crack were traced under a metallurgical microscope. The relationship between the crack length and the number of cycles under different temperature cycles was statistically analyzed, and an a-N relationship diagram was plotted. And based on the a-N relationship diagram, the relationship between the stress intensity factor ΔK and the crack length a was established:

[0074]

[0075] Among them, k t represents the stress concentration coefficient at the notch of the test piece; σ max represents the maximum thermal stress in the test area of the test piece; σ cl represents the closing stress of the thermal shock crack; represents the thermal fatigue crack growth influence factor; ρ represents the notch root radius of the test piece; Q represents the shape correction factor; F represents the boundary condition; c represents the depth of the thermal shock crack; t represents the thickness of the test piece; b represents the width of the test piece; Φ represents the angular function at the tip of the elliptical crack;

[0076] Step 105: Use the Transient Thermal module and Transient Structural module in the finite element software ANSYS to perform thermo-mechanical coupling analysis on the thermal shock test piece, and the simulation conditions are strictly consistent with the experimental conditions. Calculate the notch thermal stress σ under different temperature cycles, and calculate the stress concentration coefficient k through the finite element method t . The specific expression of the stress concentration coefficient is:

[0077]

[0078] In the formula, σ max is the maximum stress at the stress concentration, and σ 0 is the nominal stress.

[0079] The calculated stress concentration coefficients are shown in Table 1:

[0080] Table 1 Thermal stress concentration coefficients of standard thermal fatigue specimens

[0081]

[0082] Step 106: Calculate the relationship between the stress intensity factor threshold ΔK th and the crack length a according to the experimental results in Step 101:

[0083]

[0084] In the above formula, d is the limit of the microscopic crack size of the material; σ eR is the ordinary fatigue limit of the material;

[0085] Step 107: Integrating the formula in Step 102, a multi-scale thermal fatigue crack life prediction model based on the KAM value correction can be obtained:

[0086]

[0087] Among them, N idenotes the crack initiation life; a i denotes the crack initiation size; a t denotes the crack length; ΔK|a t denotes the final crack length; ΔK|a i denotes the initial crack length;

[0088] Step 108: By substituting the KAM value in formula (2), the stress intensity factor in formula (3), the thermal stress concentration factor in formula (4), and the stress intensity factor threshold in formula (5) into formula (6) for integration, a multi-scale thermal fatigue crack life prediction model corrected based on the KAM value can be obtained.

[0089] Step 109: It can be seen from Figures 1 - 3 that in the small crack stage, the prediction data coincides well with the experimental data, indicating that the model in this paper has high prediction accuracy in the small crack stage. It can be seen from Figure 1 that during the 20°C - 1100°C - 20°C cyclic thermal shock, as the number of cycles increases, the predicted crack length is basically near the mean value of the measured data, and the two are in good agreement. It can be seen from Figure 2 that during the 600°C - 1100°C - 600°C cyclic thermal shock, as the number of cycles increases, the prediction result is greater than the measured data. It can be seen from Figure 3 that during the 800°C - 1100°C - 800°C cyclic thermal shock, as the number of cycles increases, the predicted crack length is lower than the test data, and the maximum deviation between the predicted data and the average value of the experimental data is 9.64%.

[0090] Those skilled in the art of this technology can understand that, unless otherwise defined, all terms (including technical terms and scientific terms) used here have the same meaning as the general understanding of those of ordinary skill in the art to which this invention belongs. It should also be understood that terms such as those defined in a general dictionary should be understood to have a meaning consistent with their meaning in the context of the prior art, and will not be interpreted with an idealized or overly formal meaning unless defined as here.

[0091] The specific embodiments described above have further elaborated on the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only the specific embodiments of the present invention and is not used to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for calculating the life of a material under thermal shock load based on a KAM value growth model, characterized in that: The method comprises the following steps: Step 101: performing a high temperature thermal shock test on the material test piece under different temperature conditions, and performing EBSD observation at a certain cycle interval to obtain changes in KAM values ​​under different temperature conditions and different cycle numbers; Step 102: by observing the change trend of the KAM value obtained, the KAM value is linked with the temperature difference, the maximum thermal stress and the number of cycles to construct a KAM value prediction model; Step 103: based on the experimental results of the high temperature thermal shock experiment in step 101, the relationship between the crack length and the number of cycles of different temperature cycles is statistically analyzed and a relationship diagram is drawn; according to the relationship diagram, a relationship between the stress intensity factor and the crack length is established, and a relationship between the stress intensity factor threshold and the crack length is calculated; Step 104: Calculate the notch thermal stress under different temperature cycles according to finite element software, and calculate the thermal stress concentration factor by finite element method; Step 105: Correlate the crack length with the thermal shock temperature, the heating and cooling rate ratio, and the number of cycles to obtain a multi-scale thermal fatigue crack growth model; based on the KAM value prediction model, modify the multi-scale thermal fatigue crack growth model: in, represents the thermal shock crack growth rate, a represents the crack length, N represents the number of cycles, C1, n and m represent material constants, R represents the thermal stress ratio, v represents the Poisson's ratio of the material, R V represents the heating and cooling rate ratio, E' represents the elastic modulus, σ ys represents the yield stress of the material, ΔK represents the stress intensity factor, ΔK th represents the stress intensity factor threshold, β represents the correction factor; Step 105: Integrate Formula 1 to obtain a multi-scale thermal fatigue crack life prediction model based on KAM value correction.

2. The method for calculating the life of a material under thermal shock load based on a KAM value growth model according to claim 1, characterized in that: High temperature thermal shock test of materials, KAM value growth model after thermal shock load, and life calculation method of materials under thermal shock load based on KAM value growth model. The experimental equipment used are multi-stage high temperature heating furnace, metallographic microscope and scanning electron microscope.

3. The method for calculating the life of a material under thermal shock load based on a KAM value growth model according to claim 2, characterized in that: The high temperature thermal shock test of the material is used to obtain the relationship between different cycle numbers and crack length at different upper limit temperatures and the change of KAM value.

4. The method for calculating the life of a material under thermal shock load based on a KAM value growth model according to claim 3, characterized in that: The KAM value is linked to the temperature difference, the maximum thermal stress and the number of cycles to construct the KAM value prediction model as follows: Among them, a * represents the scanning step length, ζ * represents the coefficient in the grain that is independent of the slip system, represents the characteristic size, α represents the linear thermal expansion coefficient of the material, ΔT represents the temperature difference, C represents the influence coefficient of the thermal stress amplitude and material properties, σ a It represents the stress amplitude, which is the maximum value of thermal stress in this paper, and n represents the hardening index of the material.

5. The method for calculating the life of a material under thermal shock load based on a KAM value growth model according to claim 4, characterized in that: The expression for establishing the relationship between stress intensity factor and crack length is as follows: Among them, k t Indicates the stress concentration factor at the notch of the test piece; σ max Indicates the maximum thermal stress in the test area of ​​the test piece; σ cl represents the closing stress of the thermal shock crack; θ represents the influencing factor of thermal fatigue crack growth; ρ represents the root radius of the notch of the specimen; Q represents the shape correction factor; F represents the boundary condition; c represents the depth of the thermal shock crack; t represents the thickness of the specimen; b represents the width of the specimen; Φ represents the angular function of the elliptical crack tip.

6. The method for calculating the life of a material under thermal shock load based on a KAM value growth model according to claim 4, characterized in that: The expression for calculating the thermal stress concentration factor by the finite element method is as follows: Among them, σ max represents the maximum stress at the stress concentration point, and σ0 represents the nominal stress.

7. The method for calculating the life of a material under thermal shock load based on a KAM value growth model according to claim 6, characterized in that: The calculated stress intensity factor threshold ΔK th The relationship between the crack length a is as follows: Where d represents the microscopic crack size limit of the material; σ eR Indicates the normal fatigue limit of the material.

8. The method for calculating the life of a material under thermal shock load based on a KAM value growth model according to claim 7, characterized in that: The expression of the multi-scale thermal fatigue crack life prediction model based on KAM value correction is as follows: Among them, N i Indicates the crack initiation life; a i Indicates the crack initiation size; a t represents the crack length; ΔK|a t represents the termination crack length; ΔK|a i Indicates the initial crack length.

9. The method for calculating the life of a material under thermal shock load based on the KAM value growth model according to claim 8, characterized in that: By bringing the KAM value in formula (2), the stress intensity factor in formula (3), the thermal stress concentration factor in formula (4) and the stress intensity factor threshold in formula (5) into formula (6) for integration, a multi-scale thermal fatigue crack life prediction model based on KAM value correction is obtained.

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