A cyclic creep life prediction method based on ductility exhaustion method

Abstract

This invention discloses a method for predicting cyclic creep life based on the ductility exhaustion method. Specifically, it involves conducting cyclic creep tests at the same test temperature, including at least two valley stresses and two valley holding times; and obtaining the steady-state creep rate and cyclic creep fracture strain ε under different conditions. f Establish the steady-state creep rate and cyclic creep fracture strain ε during cyclic creep. f The functional relationship between them; calculate the peak and valley strain rates of cyclic creep, based on the peak and valley strain rate expression and the cyclic creep fracture strain ε. f Functional calculation of cyclic creep damage d under different conditions c According to the cyclic creep damage d c Predicting cyclic creep life N c This invention establishes a cyclic creep fracture strain expression, incorporates the influence of viscoelastic recovery on creep ductility, and performs cyclic creep damage calculations in peak and valley stages, enabling accurate prediction of the fracture life of materials under cyclic creep.

Description

Technical Field

[0001] This invention belongs to the field of material life prediction technology, specifically relating to a method for predicting cyclic creep life based on the ductility exhaustion method. Background Technology

[0002] High-temperature energy equipment often experiences cyclic creep loads due to routine maintenance, aging repairs, and deep peak shaving. These loads have a low cycle frequency, long peak-to-valley load retention time, and exhibit deformation recovery at low stress points, leading to accelerated creep and decreased ductility in high-temperature structural components. The applicability and accuracy of traditional creep-fatigue life prediction methods are questionable. Therefore, developing methods that can accurately predict structural lifespan under cyclic creep loads is crucial.

[0003] The ductile exhaustion method, which uses inelastic deformation as the main parameter, is adopted by the high-temperature structural integrity assessment code R5 and is widely used for the structural integrity evaluation of equipment in high-temperature fields. However, viscoelastic recovery affects the accumulation of inelastic deformation of materials. Therefore, it is necessary to modify the traditional ductile exhaustion method to calculate the cumulative damage under cyclic creep, thereby predicting the service life. Summary of the Invention

[0004] The purpose of this invention is to provide a method for predicting cyclic creep life based on the ductility exhaustion method, which can realize the prediction of the life of materials under cyclic creep load.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A method for predicting cyclic creep life based on the ductility exhaustion method includes the following steps:

[0007] S1: A cyclic creep test is conducted at the same temperature, and the cyclic creep test includes at least two valley stresses and two valley holding times;

[0008] S2: Based on the results of the cyclic creep test, obtain the steady-state creep rate under different conditions. and cyclic creep fracture strain ε f ;

[0009] S3: Establish steady-state creep rate for cyclic creep. With cyclic creep fracture strain ε f The functional relationship between them;

[0010] S4: Calculate the peak and valley strain rates of cyclic creep, based on the peak and valley strain rate expression and the cyclic creep fracture strain ε. f Functional calculation of cyclic creep damage d under different conditions c ;

[0011] S5: Based on the above calculation of cyclic creep damage d c Predicting cyclic creep life N c .

[0012] Furthermore, the specific operation process of step S3 includes the following steps:

[0013] S31: The steady-state creep rate of cyclic creep The average steady-state creep rate under peak load conditions in the cyclic creep curves under different conditions, excluding the first and last cycles, is shown in Equation (1):

[0014]

[0015] In the formula, This represents the peak steady-state creep rate in the i-th cycle, where N is the cycle number;

[0016] S32: The established steady-state creep rate of the cyclic creep. With cyclic creep fracture strain ε f The functional relationship between them is

[0017]

[0018] In the formula, A and n are material constants related to temperature.

[0019] Furthermore, the specific operation process of step S4 includes the following steps:

[0020] S41: Peak strain rates of cyclic creep under different conditions Represented as:

[0021]

[0022] In the formula σ p The peak stress is σ, where m1 is the stress exponent and σ is the peak stress. v For the valley stress, t v0 The holding time for each cycle valley value is given, N is the number of cycles, and A1, A, m, l are temperature-dependent material constants.

[0023] S42: Strain rates at various cyclic creep valleys under different conditions Represented as:

[0024]

[0025] In the formula t p0 The peak load holding time for the first cycle is given by E, where E is the elastic modulus of the material, t is time, and B and b are temperature-dependent material constants.

[0026] S43: Calculate the peak-valley stage damage accumulation separately, as shown in equation (5):

[0027]

[0028] In the formula, t p For peak load holding time, t v To determine the valley load holding time, substitute equations (2) to (4) into equation (5) to obtain the cyclic creep damage d. c :

[0029]

[0030] Furthermore, the cyclic creep life N obtained in step S5 under different conditions c for:

[0031]

[0032] Substituting equation (6) into equation (7), we get:

[0033]

[0034] Compared with the prior art, the present invention has the following advantages:

[0035] 1) This invention calculates the cumulative cyclic creep damage in peak and valley stages, which can accurately predict the cyclic creep life under different conditions;

[0036] 2) This invention takes into account the influence of viscoelastic deformation recovery on creep ductility, which can improve the accuracy of life prediction. Attached Figure Description

[0037] Figure 1 This is a flowchart illustrating the prediction method of the present invention;

[0038] Figure 2 The steady-state creep rate of cyclic creep in a double logarithmic coordinate system is shown in this embodiment of the invention. With cyclic creep fracture strain ε f A graph showing the functional relationship between them;

[0039] Figure 3 This is a graph showing the prediction results of an embodiment of the present invention. Detailed Implementation

[0040] The technical solutions of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. Obviously, the following embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0041] Example

[0042] like Figure 1As shown, a method for predicting cyclic creep life based on the ductility exhaustion method includes the following steps:

[0043] S1: Conduct cyclic creep tests, which include at least two valley stresses and two valley holding times, and are performed at the same temperature. In this example, AlCoCrFeNi is subjected to cyclic creep at 800℃. 2.1 Seven cyclic creep tests were conducted on eutectic high-entropy alloy (EHEA). The peak stress was fixed at 50 MPa and the peak holding time was fixed at 20 h. In four of the tests, the valley stress was held for 5 h, with valley stresses of 0 MPa, 10 MPa, 15 MPa, and 20 MPa, respectively. In the other three tests, the valley stress was 10 MPa, with valley holding times of 2.5 h, 7.5 h, or 10 h, respectively.

[0044] S2: Based on the results of the cyclic creep test, obtain the steady-state creep rate under different conditions. and cyclic creep fracture strain ε f .

[0045] S3: Establish steady-state creep rate With cyclic creep fracture strain ε f The functional relationship between them

[0046] S31: The steady-state creep rate of cyclic creep The average steady-state creep rate at peak load in all cyclic creep curves under different conditions, excluding the first and last cycles:

[0047]

[0048] In the formula, This represents the peak steady-state creep rate in the i-th cycle, where N is the cycle number.

[0049] S32: The established steady-state creep rate of the cyclic creep. With cyclic creep fracture strain ε f The functional relationship between them is:

[0050]

[0051] In the formula, A and n are temperature-dependent material constants. In this embodiment, A = 31.62 and n = 0.65 are obtained.

[0052] In the examples AlCoCrFeNi 2.1 The EHEA describes the functional relationship between steady-state creep rate and cyclic creep fracture strain in a double logarithmic coordinate system as follows: Figure 2 As shown.

[0053] S4: Calculate the peak and valley strain rates of cyclic creep, based on the peak and valley strain rate expression and the cyclic creep fracture strain ε. f Functional calculation of cyclic creep damage d under different conditions c .

[0054] S41: Peak strain rates of cyclic creep under different conditions Represented as:

[0055]

[0056] In the formula σ p The peak stress is σ, where m1 is the stress exponent and σ is the peak stress. v For the valley stress, t v0 The holding time for each cycle valley value is defined as follows: N is the cycle number, and A1, A, m, and l are temperature-dependent material constants. In this embodiment, A1 = 2.75 × 10⁻⁶. -10 , m1=4.10, A=236.95, m=-3.97, l=1.74.

[0057] S42: Strain rates at various cyclic creep valleys under different conditions Represented as:

[0058]

[0059] In the formula t p0 Here, E is the peak hold-up time for the first cycle, E is the elastic modulus of the material, and B and b are temperature-dependent material constants. In this embodiment, B = 6.56 × 10⁻⁶. -4 b = 3.88 × 10 17 .

[0060] S43: Calculate the peak-valley stage damage accumulation separately, as shown in equation (5):

[0061]

[0062] Substituting equations (2)-(4) into equation (5), we obtain the cyclic creep damage d. c :

[0063]

[0064] S5: Based on the above calculation of cyclic creep damage d c Predicting cyclic creep life N c .

[0065]

[0066] Substituting equation (6) into equation (7), we get:

[0067]

[0068] In the examples, AlCoCrFeNi was subjected to the following conditions at 800℃ with a valley stress holding time of 5 hours, valley stresses of 0 MPa, 10 MPa, 15 MPa, and 20 MPa, and a valley stress of 10 MPa with valley stress holding times of 2.5 hours, 7.5 hours, and 10 hours. 2.1 EHEA lifetime assessment results are as follows: Figure 3 As shown. By Figure 3 The results show that the lifetime prediction accuracy is within 1.5 times the error band. Therefore, it can be concluded that the cyclic creep lifetime prediction method of this invention can accurately predict AlCoCrFeNi. 2.1 EHEA cyclic creep rupture life at 800°C.

Claims

1. A method for predicting cyclic creep life based on the ductility exhaustion method, characterized in that... Includes the following steps: S1: A cyclic creep test is conducted at the same temperature, and the cyclic creep test includes at least two valley stresses and two valley holding times; S2: Based on the results of the cyclic creep test, obtain the steady-state creep rate under different conditions. and cyclic creep fracture strain ; S3: Establish steady-state creep rate for cyclic creep. With cyclic creep fracture strain The functional relationship between them; S4: Calculate the peak and valley strain rates of cyclic creep, based on the peak and valley strain rate expression and the cyclic creep fracture strain. Functional calculation of cyclic creep damage under different conditions ; S5: Cyclic creep damage calculated above Predicting cyclic creep life ; The specific operation process of step S3 includes the following steps: S31: The steady-state creep rate of cyclic creep The average steady-state creep rate under peak load conditions in the cyclic creep curves under different conditions, excluding the first and last cycles, is shown in Equation (1): （1） In the formula, Indicates the first Peak steady-state creep rate of each cycle, It is a repeating number; S32: The established steady-state creep rate of the cyclic creep. With cyclic creep fracture strain The functional relationship between them is （2） In the formula It is a material constant that is related to temperature; The specific operation process of step S4 includes the following steps: S41: Peak strain rates of cyclic creep under different conditions Represented as: (3) In the formula Peak stress, Stress index For valley stress, The load retention time for each cycle valley value. For the cycle number, These are temperature-dependent material constants; S42: Strain rates at various cyclic creep valleys under different conditions Represented as: (4) In the formula The peak load time for the first cycle. Let t be the elastic modulus of the material, and t be time. These are temperature-dependent material constants; S43: Calculate the peak-valley stage damage accumulation separately, as shown in equation (5): (5) In the formula, For peak load holding time, To determine the valley load holding time, substitute equations (2) to (4) into equation (5) to obtain the cyclic creep damage. : (6)。 2. The method for predicting cyclic creep life based on the ductility exhaustion method as described in claim 1, characterized in that... The cyclic creep life obtained under different conditions in step S5 for: (7) Substituting equation (6) into equation (7), we get: (8)。

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