阅读说明：本技术 一种燃烧室机匣孔结构特征模拟试件设计方法 (Design method of combustion chamber casing hole structure characteristic simulation test piece ) 是由 王彦菊 贾旭 胡绪腾 沙爱学 于 2021-09-08 设计创作，主要内容包括：本发明公开了一种燃烧室机匣孔结构特征模拟试件设计方法,计算评估燃烧室机匣的孔结构特征部位影响边界,确定模拟该孔结构特征的试件局部和总体形貌；并与同规格虚拟裂纹前缘应力强度因子分布一致性条件,对孔结构特征模拟试件的局部尺寸和载荷模式进行优化设计。通过优化模拟试件中模拟孔结构特征段尺寸,使其虚拟裂纹的应力强度因子随裂纹尺寸的分布与实际燃烧室机匣中同规格裂纹前缘的应力强度因子分布一致,一方面将双变量空间的应力等效设计问题转变为一维单变量空间的应力强度因子等效设计问题,另一方面使模拟试件在足够的裂纹萌生与扩展区域内与实际结构的疲劳过程保持一致,进一步提高燃烧室机匣孔结构特征模拟试件设计的准确性。(The invention discloses a design method of a hole structure characteristic simulation test piece of a combustion chamber casing, which is used for calculating and evaluating the influence boundary of a hole structure characteristic part of the combustion chamber casing and determining the local and overall appearance of the test piece simulating the hole structure characteristic; and optimally designing the local size and the load mode of the hole structure characteristic simulation test piece according to the distribution consistency condition of the stress intensity factor of the virtual crack front edge with the same specification. By optimizing the size of the characteristic section of the simulated hole structure in the simulated test piece, the distribution of the stress intensity factor of the virtual crack along with the size of the crack is consistent with the distribution of the stress intensity factor of the front edge of the crack with the same specification in the actual combustion chamber casing, on one hand, the stress equivalent design problem of a bivariate space is converted into the stress intensity factor equivalent design problem of a one-dimensional univariate space, on the other hand, the simulated test piece is consistent with the fatigue process of the actual structure in an enough crack initiation and expansion area, and the accuracy of the design of the simulated test piece of the combustion chamber casing hole structure characteristic is further improved.)

1. A design method of a combustion chamber casing hole structure characteristic simulation test piece is characterized by comprising the following steps: the method comprises the following steps:

firstly, carrying out linear elastic finite element stress analysis on an actual hole structure on a combustion chamber casing, and extracting a maximum main stress normal plane of the characteristic part of the actual hole structure;

secondly, inserting a virtual crack into the maximum main stress part of the actual hole structure geometric model of the combustion chamber casing, performing elastic finite element analysis again to extract stress intensity factors of a crack front edge surface point and a deepest point, determining a virtual crack size minimum boundary and a virtual crack size maximum boundary on a normal plane by comparing the stress intensity factors with a crack propagation threshold value and fracture toughness of a material, calculating the stress intensity factors of cracks with different sizes in the range of the virtual crack size boundary, and obtaining the distribution of the stress intensity factors of the crack front edge point along with the size of the crack;

thirdly, calculating and evaluating the influence boundary of the hole structure characteristic part of the combustion chamber casing, and determining the local and overall appearance of a test piece simulating the hole structure characteristic;

and fourthly, optimally designing the local size and the load mode of the hole structure characteristic simulation test piece based on the condition of the stress intensity factor distribution consistency of the virtual crack front edge with the same specification in the second step.

2. The method of claim 1, wherein: the virtual crack in the second step is a crack assumed on the normal plane of the maximum principal stress, and the crack propagation direction is the direction of the maximum principal stress gradient; if the main stress point is located on the hole surface, the inserted virtual crack is a surface crack, and if the main stress point is located on the hole edge, the inserted virtual crack is an angle crack.

3. The method of claim 2, wherein: the virtual crack is a surface crack with a semi-ellipse front edge, or a quarter-ellipse corner crack or a straight penetrating crack; the length of the surface of the semi-elliptical surface crack is 2 times of the depth, and the lengths of the two surfaces of the quarter elliptical fillet crack are equal; the center of the ellipse of the semi-ellipse surface crack or the quarter ellipse fillet crack is positioned at the maximum main stress point of the characteristic part of the hole structure.

4. The method of claim 3, wherein: the minimum value of the virtual crack size boundary in the second step is determined by increasing the crack size of the characteristic part of the actual hole structure so that the stress intensity factor of the crack front edge reaches the material crack propagation threshold value;

the maximum value of the virtual crack size boundary is determined by increasing the crack size of the characteristic part of the actual hole structure so that the stress intensity factor of the crack front edge reaches the fracture toughness of the material crack;

if the size of the surface crack is increased to reach the structural boundary and the stress intensity factor does not reach the fracture toughness, the crack type is changed into a corner crack or a penetrating crack, and similarly, if the size of the quarter elliptical fillet crack reaches the structural boundary but does not reach the fracture toughness, the crack type is changed into a penetrating crack until the size of the crack is increased to enable the stress intensity factor of the front edge of the crack to be equal to the fracture toughness, and the size of the crack at the moment is the maximum boundary of the virtual crack size.

5. The method of claim 3, wherein: and in the second step, the stress intensity factors of the crack leading edge points are distributed along with the crack size as the stress intensity factors of the crack surface points and the deepest points which are changed along with the crack size, wherein the crack leading edge points of the semi-elliptical surface are two surface points and one deepest point, and the crack leading edge points of the quarter-elliptical fillet crack and the penetrating crack are two surface points.

6. The method of claim 1, wherein: in the third step, the influence boundary of the hole structure characteristic of the combustion chamber casing is the maximum value of 2 times of the virtual crack size boundary;

the initial appearance of the test section of the hole structure characteristic simulation test piece is that the loading sections at two ends of the hole structure characteristic simulation test piece with the actual hole structure characteristic geometric configuration on the cartridge receiver adopt a bolt form.

7. The method of claim 6, wherein: the loading mode of the hole structure characteristic simulation test piece is that loading shafts at two ends are collinear or non-collinear, the non-collinear loading is used for realizing the application of bending load, and the test section and the loading section adopt circular arc transition.

8. The method of claim 1, wherein: the optimization process in the fourth step comprises the following steps:

appointing each size of a test section simulating the hole structure characteristics in the test piece, the eccentricity of a loading shaft and the load size as design variables, establishing a geometric model of the test piece, inserting a virtual crack into the maximum principal stress part on the maximum principal stress normal plane of the test piece, calculating the stress intensity factor of a crack leading edge point, repeatedly inserting cracks with different sizes, calculating the stress intensity factor of the crack leading edge point with the same specification in the step two, and forming the stress intensity factor distribution changing along with the crack size; and comparing the stress intensity factor distribution with the stress intensity factor distribution in the step two, judging whether the design requirements are met, forming a final design if the design requirements are met, otherwise, modifying the design variables, recalculating and comparing the stress intensity factor distribution of the virtual cracks in the test piece until the conditions are met, and obtaining the final design.

9. The method of claim 8, wherein: the design requirements refer to: and whether the root mean square of the relative errors of the stress intensity factor distribution in the step four and the step two is less than 5 percent or not.

Technical Field

The invention relates to a design method of a structural feature simulation test piece of a combustion chamber casing hole, and belongs to the technical field of fatigue and damage tolerance of an aero-engine.

Background

The combustor case has significant stress concentrations in the hole structure areas such as oil holes, air holes, ignition holes and hole probing holes, which are dangerous characteristic parts affecting the fatigue life of the combustor case structure. In view of the complex structure and high price of the aeroengine combustion chamber, the fatigue test of combustion chamber components and even a complete machine is developed to evaluate the influence of the structural characteristics of the casing hole on the structural fatigue life, and the structure is long in period and high in price. By designing and developing a large number of elements for simulating the hole structure characteristics of the casing and developing a hole structure characteristic element-level fatigue test or a crack propagation test, the method is economical, short in period and more reliable in data, and becomes an important means for evaluating the fatigue life and the crack propagation life of the casing hole structure of the combustion chamber in engineering, and the accuracy of the design of the characteristic elements for simulating the hole structure becomes a key for determining the reliability of the evaluation result of the fatigue life and the crack propagation life of the combustion chamber structure.

Generally, the crack initiation point is the maximum point of the first principal stress point on the surface of the structure, also called the hazard point, and the crack propagation plane is the normal plane of the first principal stress. In addition to material performance parameters, the bi-directional stress gradient on the normal plane is a major factor affecting crack initiation and propagation at the structural risk points. Therefore, when designing an element (test piece) simulating the hole structure characteristics, firstly, the material, the ambient temperature and the like of the test piece and the casing hole structure are ensured to be the same, and in addition, the two-way stress gradient on the first main stress normal plane of the test piece and the casing hole structure is ensured to be the same, the former is generally easy to ensure, and the latter becomes the key point and the difficulty of the equivalent design of the simulated hole structure characteristic element and is the ultimate target of the design of the structure characteristic element (test piece). In practical design, the bidirectional stress gradient is often difficult to reference and describe, and the structural boundary of the casing hole which the element should comprise is difficult to define quantitatively, so that the prior published documents and patents have not been reported yet.

The existing document' Langshan, Wangchun, Chenjun, design method of any maximum stress gradient path wheel disc simulation piece [ J ]. aeronautical dynamics report, 2010(09): 2000-.

The design and test of a low-cycle fatigue simulator of a mortise and tenon groove of a certain compressor wheel disc [ J ] aeronautical dynamics report, 2008(10) 83-88 ], the design of the low-cycle fatigue simulator of a certain turbofan engine high-pressure turbine disc bolt hole [ J ] aeronautical dynamics report, 2018,33(10) 56-63' all adopt the equivalent design principle of the first principal stress and strain within 0.8mm (engineering crack length) of the maximum stress point part to design the simulator, but do not consider the stress equivalent after crack propagation.

The conventional patent CN201810796385.5 "a method for designing a simulation of a bolt hole of a compressor disc" proposes that the circumferential stress of the edge of the bolt hole of the wheel disc changes significantly within a certain distance, but is basically stable and unchanged outside the distance, and then the bolt hole simulation is designed by using the consistency of the circumferential stress distribution (stress value and stress gradient) of the edge of the inner hole as an optimization target, without considering the stress gradients in other directions.

The conventional patent CN201810808785.3 "a design method for turbine disc tongue-and-groove crack propagation simulation piece" discloses that the critical crack length is calculated by comparing the angular crack stress intensity factor and the fracture toughness value on a square simulation piece, and further, the thickness of the simulation piece is determined to be more than half of the critical crack length. On one hand, a manual calculation formula is adopted for calculating the stress intensity factor of the square section angle crack, and on the other hand, the purpose of the measure is only to ensure that the crack has enough expansion space and not to consider the expansion condition of the crack in the actual wheel disc.

According to the stress intensity factor weight function theory, two factors are used for determining the stress intensity factor of the crack front edge, namely the stress distribution along the original crack surface when the crack body is supposed to have no crack and the geometric factor (namely the weight function). Then, for two locally geometrically similar crack bodies, the geometrical factors of the two crack bodies are the same, and if the stress intensity factor distribution is also the same, it can be said that the stress distributions on the crack surfaces of the two crack bodies are similar. The consistency of the bivariate stress distribution on the crack propagation plane of the two structures can be indicated by the stress intensity factor distribution according to the above principle.

On the other hand, an excellent design should be able to maintain crack initiation and propagation of the structural feature element (specimen) consistent with the actual structural member over a sufficiently large area to accurately simulate the fatigue process in the actual structural member. The published documents often guarantee, within a small range, that the stress distributions of the two coincide, which is not sufficient to prove the consistency of the fatigue process. The existing theory shows that the material of the aeroengine combustion chamber casing is a typical high-strength material, the crack propagation rate of the aeroengine combustion chamber casing can be controlled by the parameter of the stress intensity factor, and the stress intensity factor of a simulated test piece and the stress intensity factor of an actual structure are ensured to be consistent in the design.

The invention provides a design method of a combustion chamber casing hole structure characteristic simulation test piece, which adopts a stress intensity factor parameter as a design target to ensure that the bidirectional stress gradient distribution of a combustion chamber casing hole structure part and the characteristic simulation test piece is consistent.

Disclosure of Invention

The purpose of the invention is: the design method of the combustion chamber casing hole structure characteristic simulation test piece is provided for meeting the design requirements of the combustion chamber casing hole structure characteristic simulation test piece of the aero-engine, and the problem that the bidirectional stress gradient of a dangerous characteristic part needs to be considered in the design process of the structure characteristic simulation piece is solved.

In order to solve the technical problem, the technical scheme of the invention is as follows:

a design method of a combustion chamber casing hole structure characteristic simulation test piece comprises the following steps:

firstly, carrying out linear elastic finite element stress analysis on an actual hole structure on a combustion chamber casing, and extracting a maximum main stress normal plane of the characteristic part of the actual hole structure;

secondly, inserting a virtual crack into the maximum main stress part of the actual hole structure geometric model of the combustion chamber casing, performing elastic finite element analysis again to extract stress intensity factors of a crack front edge surface point and a deepest point, determining a virtual crack size minimum boundary and a virtual crack size maximum boundary on a normal plane by comparing the stress intensity factors with a crack propagation threshold value and fracture toughness of a material, calculating the stress intensity factors of cracks with different sizes in the range of the virtual crack size boundary, and obtaining the distribution of the stress intensity factors of the crack front edge point along with the size of the crack;

thirdly, calculating and evaluating the influence boundary of the hole structure characteristic part of the combustion chamber casing, and determining the local and overall appearance of a test piece simulating the hole structure characteristic;

and fourthly, optimally designing the local size and the load mode of the hole structure characteristic simulation test piece based on the condition of the stress intensity factor distribution consistency of the virtual crack front edge with the same specification in the second step.

In the second step, the virtual crack is not a real crack of the characteristic part of the actual hole structure, but a crack assumed on the normal plane of the maximum principal stress, and the crack propagation direction is the direction of the maximum principal stress gradient; if the main stress point is located on the hole surface, the inserted virtual crack is a surface crack, and if the main stress point is located on the hole edge, the inserted virtual crack is an angle crack.

The virtual crack shape is a surface crack with semi-ellipse front edge, or a quarter-ellipse corner crack or a straight line penetrating crack with engineering universality; the length of the surface of the semi-elliptical surface crack is 2 times of the depth, and the lengths of the two surfaces of the quarter elliptical fillet crack are equal; the center of the ellipse of the semi-ellipse surface crack or the quarter ellipse fillet crack is positioned at the maximum main stress point of the characteristic part of the hole structure;

the minimum value of the virtual crack size boundary in the second step is determined by increasing the crack size of the characteristic part of the actual hole structure so that the stress intensity factor of the crack front edge reaches the material crack propagation threshold value;

the maximum value of the virtual crack size boundary is determined by increasing the crack size of the characteristic part of the actual hole structure so that the stress intensity factor of the crack front edge reaches the fracture toughness of the material crack;

if the size of the surface crack is increased to reach the structural boundary and the stress intensity factor does not reach the fracture toughness, the crack type is changed into a corner crack or a penetrating crack, and similarly, if the size of the quarter elliptical fillet crack reaches the structural boundary but does not reach the fracture toughness, the crack type is changed into a penetrating crack until the size of the crack is increased to enable the stress intensity factor of the front edge of the crack to be equal to the fracture toughness, and the size of the crack at the moment is the maximum boundary of the virtual crack size.

And in the second step, the stress intensity factors of the crack leading edge points are distributed along with the crack size as the stress intensity factors of the crack surface points and the deepest points which are changed along with the crack size, wherein the crack leading edge points of the semi-elliptical surface are two surface points and one deepest point, and the crack leading edge points of the quarter-elliptical fillet crack and the penetrating crack are two surface points.

In the third step, the influence boundary of the hole structure characteristic of the combustion chamber casing is the maximum value of 2 times of the virtual crack size boundary;

the initial morphology of the test section of the hole structure characteristic simulation test piece is the actual hole structure characteristic geometric configuration on the cartridge receiver; the loading sections at the two ends of the hole structure characteristic simulation test piece adopt a bolt form.

The loading mode of the hole structure characteristic simulation test piece is that loading shafts at two ends are collinear or non-collinear, the non-collinear loading is used for realizing the application of bending load, and the test section and the loading section adopt circular arc transition.

The optimization process in the fourth step comprises the following steps:

appointing each size of a test section simulating the hole structure characteristics in the test piece, the eccentricity of a loading shaft and the load size as design variables, establishing a geometric model of the test piece, inserting a virtual crack into the maximum principal stress part on the maximum principal stress normal plane of the test piece, calculating the stress intensity factor of a crack leading edge point, repeatedly inserting cracks with different sizes, calculating the stress intensity factor of the crack leading edge point with the same specification in the step two, and forming the stress intensity factor distribution changing along with the crack size; and comparing the stress intensity factor distribution with the stress intensity factor distribution in the step two, judging whether the design requirements are met, forming a final design if the design requirements are met, otherwise, modifying the design variables, recalculating and comparing the stress intensity factor distribution of the virtual cracks in the test piece until the conditions are met, and obtaining the final design.

Preferably, the design requirement is whether the root mean square of the relative errors of the stress intensity factor distributions in the fourth step and the second step is less than 5%.

The invention has the beneficial effects that: the invention provides a design method of a simulation piece considering bidirectional stress gradient for the structural characteristics of a casing hole of an aeroengine combustion chamber. According to the method, a large number of bidirectional stress values of the structural characteristic parts of the combustion chamber casing and the simulated test piece hole are not required to be directly compared, only normalized stress intensity factor distribution of the virtual crack leading edge point is required to be compared, design target data in geometric optimization is greatly simplified, design efficiency is improved, the stress intensity factor equivalent design is adopted to be more in line with a physical model, the fatigue process consistency of the simulated test piece and an actual component can be ensured, and the design accuracy of the simulated test piece is remarkably improved.

Drawings

FIG. 1 is a graph of stress distribution in a structural feature of a combustor casing bore;

FIG. 2 illustrates a virtual semi-elliptical surface crack, quarter-elliptical fillet crack, or straight line through crack that may be inserted into an actual pore structure feature;

FIG. 3 is a plot of the key points of virtual semi-elliptical surface cracks, quarter-elliptical fillet cracks, and through cracks;

FIG. 4 shows the main stress distribution at the fuel hole structure;

FIG. 5 is a schematic view of a virtual corner crack growing to a through crack;

FIG. 6 is a graph of normalized stress intensity factor with increasing size;

FIG. 7 shows a portion of the test piece similar to the structural feature of the case hole;

FIG. 8 test piece geometry;

FIG. 9 shows the dimensional optimization parameters of the test piece;

FIG. 10 is a flow chart of a method of the present invention.

Detailed Description

In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention. It is to be understood that the embodiments described are only a few embodiments of the present invention, and not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

Features of various aspects of embodiments of the invention will be described in detail below. The flow chart of the design method of the invention is shown in FIG. 10, and comprises the following steps:

firstly, carrying out linear elastic finite element analysis on an actual hole structure of the combustion chamber casing, and extracting a maximum principal stress normal plane of the characteristic part of the actual hole structure. The data required for the linear elastic finite element analysis of the actual pore structure of the combustor case include the combustor case geometric model, the operating pressure and constraints, the operating temperature field, and the elastic modulus and poisson's ratio of the material at the operating temperature. The structural characteristics of the combustion chamber casing hole comprise a fuel oil hole (with a larger aperture), an igniter opening hole, an air hole or a hole probing hole and the like, and the stress distribution diagram of the structural characteristic part of the hole is shown in figure 1. The present embodiment will be described by taking the fuel oil hole as an example.

And secondly, inserting a virtual crack into the maximum main stress part of the actual hole structure geometric model of the combustion chamber casing, performing elastic finite element analysis again to extract stress intensity factors of the surface point and the deepest point of the front edge of the crack, determining the minimum boundary and the maximum boundary of the virtual crack size on the normal plane by comparing the stress intensity factors with the crack propagation threshold value and the fracture toughness of the material, calculating the stress intensity factors of the cracks with different sizes in the range of the virtual crack size boundary, and obtaining the distribution of the stress intensity factors of the front edge point of the crack along with the size of the crack.

The virtual crack is not a real crack of an actual hole structure characteristic part, but a crack assumed on a maximum principal stress normal plane obtained in the first step, the crack propagation direction is the maximum principal stress gradient direction, and the shape of the virtual crack is a surface crack with engineering universality, a surface crack with a semi-ellipse front edge, or a corner crack with a quarter-ellipse front edge or a straight penetrating crack; the length of the surface of the semi-elliptical surface crack is 2 times of the depth, and the lengths of the two surfaces of the quarter elliptical fillet crack are equal; the center of the ellipse of the semi-ellipse surface crack or the quarter ellipse fillet crack is positioned at the maximum main stress point of the characteristic part of the hole structure; if the primary stress point is located on the hole surface, the inserted virtual crack is a surface crack, and if the primary stress point is located on the hole edge, the inserted virtual crack is an angular crack (see fig. 2). The surface crack surface length 2c is related to the crack depth a, and the hole edge surface length c and the flank surface length a of the corner crack as follows:

c＝a

minimum value a of virtual crack size boundary in the second step_minThe crack size of the characteristic part of the actual hole structure is increased to enable the stress intensity factor of the front edge of the crack to reach the material crack propagation threshold value delta K_thDetermining the maximum value a of the virtual crack size boundary_maxThe crack size of the characteristic part of the actual hole structure is increased to enable the stress intensity factor of the front edge of the crack to reach the fracture toughness K of the material crack_ICAnd (6) determining. If the size of the surface crack reaches the structural boundary and the stress intensity factor does not reach the fracture toughness yet, the crack type is changed into a corner crack or a penetrating crack, and similarly, if the size of the quarter elliptical fillet crack reaches the structural boundary but does not reach the fracture toughness, the crack type is changed into a penetrating crack until the size of the crack is increased so that the stress intensity factor of the front edge of the crack is equal to the fracture toughness, and the size of the crack at the moment is the maximum boundary of the virtual crack size. The leading points of surface cracks, corner cracks and through cracks are shown in fig. 3.

In the embodiment, the main stress distribution of the fuel oil pore structure part is shown in fig. 4, the main stress maximum point is positioned on the pore edge, so that a quarter ellipse fillet crack is inserted into the part, and the quarter ellipse fillet crack is determined to expand under the bidirectional gradient stress at least comprising a pore surface point tip c^-Point and side surface point tip a⁺Therefore, determining the crack size boundary takes into account the stress intensity factor of the crack leading edge point, namely:

when in useWhen a is_min＝a

When in useWhen a is_max＝a

Where P represents a load factor that determines a stress intensity factor, and F … represents a combustor case geometry factor that determines a stress intensity factor, the case hole feature crack stress intensity factor being a monotonically increasing function of crack sizes a and c. The stress intensity factor of each point under the increasing crack size is calculated step by step, then an expression of the stress intensity factor about the crack size c is fitted, and finally the two equations are solved. If the quarter-ellipse surface cracks increase to the geometric boundary without reaching the fracture toughness, a transition to through cracks occurs. And the value of a at which the fracture toughness is reached at the leading point of the through crack is taken as a_max. As illustrated in fig. 5, the equi-spaced, increased size corner cracks transition to equi-spaced, increased penetration cracks after reaching the geometric boundary until the stress intensity factor of the point leading the penetration cracks exceeds the fracture toughness.

In the second step, the distribution of the stress intensity factors of the crack front edge points along with the crack size is the distribution of the stress intensity factors of the crack surface points and the deepest points which change along with the crack size, and the distribution is used as the representation of the stress intensity distribution gradient. The normalized stress intensity factor for quarter ellipse corner cracks to penetration cracks increases with size in this example is shown schematically in fig. 6.

And thirdly, calculating and evaluating the influence boundary of the hole structure characteristic part of the combustion chamber casing, and determining the local and overall appearance of the test piece simulating the hole structure characteristic. The hole structure characteristic influence boundary of the combustion chamber casing is the maximum value of 2 times of the virtual crack size boundary, and the test section morphology of the test piece for simulating the casing hole structure characteristic is similar to the actual hole structure characteristic morphology on the casing in the influence boundary, for example, the hole structure characteristic influence boundary is spherical (as shown in fig. 7), and the spherical radius R is:

R＝2a_max

the loading sections at the two ends of the hole structure characteristic simulation test piece are in a bolt form, the threaded loading shafts at the two ends are designed in a collinear manner in the embodiment, and the test section and the loading section are in circular arc transition, as shown in fig. 8.

And fourthly, optimally designing the local sizes of the test sections of the hole structure characteristic simulation test piece, the eccentricity of a loading shaft and the size of the load based on the condition of normalized distribution consistency of the stress intensity factors of the virtual crack front edges with the same specification in the second step, wherein the embodiment comprises the following steps: designating each dimension of a test section simulating the hole structure characteristics in the test piece as a design variable (as shown in FIG. 9, L1 is the width of the test section, L2 is the width of notch reinforcement, L3 is the depth of notch, and L4 is the thickness of notch reinforcement), the eccentricity of a loading shaft is 0, the load is the axial force value transferred by the threads at two ends of the test piece, then establishing a geometric model of the test piece, inserting a virtual crack into the maximum main stress position on the maximum main stress normal plane of the test piece and calculating the stress intensity factor of the crack leading edge point, repeatedly inserting cracks with different dimensions to calculate the stress intensity factor of the crack leading edge point with the same specification in the second step, forming the stress intensity factor distribution changing along with the size of the crack, finally comparing with the stress intensity factor distribution in the second step, judging whether the root mean square of the relative error between the two is less than 5%, if so, forming the final design, otherwise, modifying the design variable to recalculate and comparing the normalized distribution of the stress intensity factor of the virtual crack in the test piece, until less than 5% gives the final design.

The foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.