CN114912307A - High-cycle fatigue life prediction method for nickel-based single crystal superalloy - Google Patents

Abstract

The invention discloses a high-cycle fatigue life prediction method of a nickel-based single crystal superalloy, which comprehensively considers the shape and position of the defect and the influence of the tip plastic zone, and constructs the shape factor and the position factor of the defect, and considers the tip plastic zone. D parameters, and build a life prediction model based on the D parameters. This method can realize the effective prediction of high cycle fatigue life of nickel-based single crystal superalloys containing defects.

Description

A high-cycle fatigue life prediction method for nickel-based single crystal superalloys

technical field

The invention belongs to the field of fatigue life prediction, and particularly relates to a high-cycle fatigue life prediction method of a nickel-based single crystal superalloy.

Background technique

Nickel-based single crystal superalloy has the advantages of high high temperature strength, strong oxidation resistance and excellent fatigue resistance, and is the main material of choice for advanced aero-engine turbine blades. High cycle fatigue is a common failure mode during turbine blade service. The stress is concentrated at the defects of nickel-based single crystal casting, and fatigue cracks are thus initiated in the defects.

The degree of stress concentration at the defect is not only related to the size of the defect, but also to the shape of the defect and the position to the surface. According to the finite element analysis results, for the same defect, the closer to the surface, the more serious the degree of stress concentration.

A defect can be viewed as a type of crack. When subjected to an applied load, the crack tip will form a plastic zone, and the appearance of the plastic zone reduces the stiffness of the material, which is equivalent to the growth of the crack. Therefore, when considering the effect of defect size on the degree of stress concentration, the plastic zone needs to be taken into account.

The previous high-cycle fatigue life prediction model for defect-containing nickel-based single crystal only considers the influence of the size of the defect. Therefore, a stress concentration parameter is constructed, which reflects the shape, size, location of the defect and the effect of the plastic zone on the stress concentration at the defect. It is very meaningful to predict the high-cycle fatigue life of nickel-based single crystals with defects through this parameter.

SUMMARY OF THE INVENTION

In order to achieve the above purpose, the present invention proposes a new parameter D, which is used for life prediction. This parameter comprehensively considers the influence of the shape, size and location of the defect, as well as the influence of the plastic zone at the crack tip. The present invention adopts following technical scheme:

A high-cycle fatigue life prediction method for nickel-based single crystal superalloy, the life prediction method is based on D parameter, and the expression of D parameter is:

In the formula, L is the position stress intensity factor, S is the roundness constituting shape factor, a is the defect size, σ _y is the yield strength, and σ is the stress.

Further, include the following steps:

(1) Carry out high cycle fatigue test to obtain life, stress and fracture;

(2) Measure the defect area area _defect , defect perimeter C _defect , defect size a, defect-to-surface distance d _defect , and fracture diameter d of the defect at the initiation of the crack;

(3) Determine the relationship between the position stress intensity factor L and d _defect /d through finite element analysis:

L=f(d _defect /d);

(4) Constructing the roundness to form the shape factor S:

(5) according to step (1)～step (4), construct D parameter;

(6) According to the parameter D, a life prediction model is established.

Further, the life prediction model is:

lgN _f =α-βlg(I)

式中，N_f为疲劳寿命；σ_max为最大拉应力；σ_min为最小拉应力；当应力为负时，σ_min＝0；ΔD_th表示各试件ΔD参数值中的最小值，Δσ＝σ_max-σ_min，α、β为拟合参数。ΔD= _Dmax - _Dmin

In the formula, N _f is the fatigue life; σ _max is the maximum tensile stress; σ _min is the minimum tensile stress; when the stress is negative, σ _min =0; ΔD _th represents the minimum value of the ΔD parameters of each specimen, Δσ = σ _max -σ _min , α and β are fitting parameters.

The beneficial effects of the present invention relative to the prior art:

Aiming at the problem of high-cycle fatigue life prediction of nickel-based single crystals, the present invention proposes a new life prediction parameter D, which takes into account the influence of the shape, location and plastic zone of the defect, and establishes a life prediction model based on this parameter to realize the Efficient prediction of low-cycle fatigue life of nickel-based single crystals.

Description of drawings

Fig. 1 is [001] orientation DD6 nickel-based single crystal superalloy high cycle fatigue fracture at 850 ℃;

Figure 2 shows the comparison between the predicted life and the test life of the DD6 nickel-based single crystal superalloy under the high-cycle fatigue of the method of the present invention at 850°C.

Detailed ways

The idea, specific structure and technical effects of the present invention will be clearly and completely described below with reference to specific embodiments, so as to fully understand the purpose, features and effects of the present invention.

A method for predicting the high cycle fatigue life of a nickel-based single crystal superalloy, using the following technical solutions:

First of all, a defect can be regarded as a kind of crack. According to the principle of fracture mechanics, there is a plastic zone at the tip of the crack, and the appearance of the plastic zone reduces the stiffness of the material, so it needs to be equivalent to the growth of the crack length, and the growth amount r _y is:

Among them, σ _y is the yield strength, K is the stress intensity factor, and its expression is:

where Y is the geometry factor, σ is the stress, and a is the defect size.

According to formulas (1) and (2), we can get:

Secondly, there is a stress concentration near the defect, and the degree of stress concentration is related to the shape of the defect. Therefore, the roundness is used to form the shape factor, as shown in formula (4):

Among them, S is the roundness constituting the shape factor, area _defect is the defect area, and C _defect is the defect perimeter.

Furthermore, the degree of stress concentration near the defect is also related to the location of the defect. The defect location is represented by the ratio of the distance from the defect to the surface and the diameter of the fracture, and through the finite element analysis, the mathematical relationship between the stress concentration factor and the distance from the defect to the fracture edge is determined, and the positional stress intensity factor L (that is, the stress concentration factor at the defect) is obtained. expression, as shown in formula (5):

L=f(d _defect /d) (5)

Among them, L is the position stress intensity factor, d _defect is the distance from the defect to the surface, and d is the fracture diameter.

Introducing the roundness shape factor S and the position stress intensity factor L to replace Y in equation (3), a new stress intensity factor expression can be obtained, that is, the parameter D:

The specific implementation steps are as follows:

Step 1: Design and process high-cycle fatigue specimens, and test them at different stress levels to obtain fatigue data and fractures of the specimens.

Step 2: Measure the area area _defect , the perimeter C _defect , the size a, the defect-to-surface distance d _defect and the fracture diameter d of the defect causing the crack initiation. The definition of defect size here is: the maximum value of the distance between any two points in the defect.

Step 3: Determine the mathematical relationship between the position stress intensity factor L, the defect-to-surface distance d _defect and the fracture diameter d through finite element analysis.

Step 4: Calculate the parameter D of each specimen as described above.

Step 5: Use the parameter D calculated in Step 4 to build a life prediction model, so as to predict the high cycle fatigue life.

Example 1

Taking 850 ℃ [001] oriented nickel-based single crystal superalloy DD6 as an example, the specific application process of the present invention is as follows:

(1) The fatigue test of [001] oriented nickel-based single crystal superalloy DD6 was carried out at 850°C. The test loads were 900MPa, 800MPa, 850MPa, 680MPa, 675MPa, 600MPa, 580MPa and 575MPa, respectively, and the stress ratio was 0.05. The test results are shown in Table 1.

Table 1 Fatigue test results

(2) Obtain the macroscopic image of the fracture and the microscopic morphology of the defect at the initiation of the crack by scanning electron microscope, such as Figure 1. The perimeter, area, length, distance to the surface, and diameter of the fracture at the source region of the fracture of different specimens were measured. The test results are shown in Table 2.

Table 2 Fracture measurement results

(3) Through the finite element analysis, it is determined that the relationship between the position stress intensity factor L and the defect position is:

(4) Calculate the parameter D of each test piece. The results are shown in Table 3.

Table 3 Parameter calculation results

(5) Build a life prediction model.

Based on the parameter D, the following life prediction model is constructed here

lgN _f =α-βlg(I) (10)

In the formula, N _f is the fatigue life; σ _max is the maximum tensile stress; σ _min is the minimum tensile stress; when the stress is negative, σ _min =0; ΔD _th represents the minimum value of the ΔD parameters of each specimen, Δσ = σ _max -σ _min , α and β are fitting parameters.

The comparison between the predicted fatigue life and the actual fatigue life is shown in Figure 2. It can be seen that the predicted life is basically within the triple line, with high accuracy, indicating that the D parameter is used to build a life prediction model is effective.

The above descriptions are only specific embodiments of the present invention, and are not intended to limit the protection scope of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included in the present invention. within the scope of protection of the invention.

Claims (3)

1. a nickel-based single crystal superalloy high cycle fatigue life prediction method, is characterized in that, the life prediction method is based on D parameter, and the expression of D parameter is:

In the formula, L is the position stress intensity factor, S is the roundness constituting shape factor, a is the defect size, σ _y is the yield strength, and σ is the stress. 2. The high-cycle fatigue life prediction method of nickel-based single crystal superalloy according to claim 1, characterized in that, comprising the following steps: (1) Carry out high cycle fatigue test to obtain life, stress and fracture; (2) Measure the defect area area _defect , the defect perimeter C _{def ect} , the defect size a, the distance from the defect to the surface d _defect , and the fracture diameter d at the crack initiation point; (3) Determine the relationship between the position stress intensity factor L and d _defect /d through finite element analysis: L=f(d _defect /d); (4) Constructing the roundness to form the shape factor S:

(5) according to step (1)～step (4), construct D parameter; (6) According to the parameter D, a life prediction model is established. 3. The high-cycle fatigue life prediction method of nickel-based single crystal superalloy according to claim 2, wherein the life prediction model is: lgN _f =α-β1g(I)

ΔD= _Dmax - _Dmin

In the formula, N _f is the fatigue life; σ _max is the maximum tensile stress; σ _min is the minimum tensile stress; when the stress is negative, σ _min =0; ΔD _th represents the minimum value of the ΔD parameters of each specimen, Δσ = σ _max -σ _min , α and β are fitting parameters.

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