Notch structure fatigue limit prediction method considering multi-defect features

Abstract

The invention discloses a notch structure fatigue limit prediction method considering a multi-defect feature, and the method comprises the following steps: calculating a virtual defect size # imgabs0 # considering a defect position and a shape feature, and carrying out the fatigue limit prediction of a smooth part under the action of the virtual defect size # imgabs1 # through an El Hadad model, and obtaining the fatigue limit delta sigma w of the smooth part; calculating a fatigue notch coefficient Kf and an ideal notch structure fatigue limit delta sigma w'in combination with the critical distance L; based on the fatigue limit delta sigma w of the smooth part and the fatigue limit delta sigma w'of the ideal notch structure, a probability K-T graph is constructed for notch-defect competition analysis, and the fatigue limit delta sigma w ''of the notch structure is obtained. According to the method, the interaction between the gap and the defect is comprehensively considered, a brand new gap-defect competition model is provided to quantify the competition relation between the gap and the defect, fatigue limit prediction is carried out by constructing the probability K-T graph, and the method has wide applicability and high prediction precision.

Description

Technical Field

[0001] The invention relates to the field of damage tolerance evaluation and complex structure strength analysis, and more particularly to a fatigue limit prediction method for a notch structure taking into account multi-defect characteristics. Background Art

[0002] Fatigue limit prediction is a key link in damage tolerance assessment and complex structural strength analysis, especially in industries with high reliability requirements such as aerospace, automobile manufacturing, and energy equipment. The materials and structures used in these fields are often subjected to complex stress environments, and may contain tiny defects such as inclusions, holes, scratches, and corrosion spots inside or on the surface. These defects may cause sudden failure of the material under long-term cyclic loads. Therefore, accurately predicting the fatigue limit of defective structures is crucial to ensure the safety of engineering structures and extend their service life.

[0003] However, existing technologies have certain limitations when dealing with fatigue limit prediction under notch-defect interactions. Traditional fatigue design methods are mainly based on safe life design, assuming that there are no defects inside the material. With the development of detection technology, it is gradually realized that materials are not "flawless" but have various forms of defects. However, most current methods study the effects of macroscopic geometric discontinuities (such as notches) and microscopic defects on fatigue performance separately, lacking a unified framework to simultaneously consider the coupling effects of the two. This separate analysis method is difficult to fully reflect the complex interaction between the two factors under actual working conditions, thereby limiting the accuracy of fatigue limit prediction results.

[0004] In addition, although some studies have evaluated global geometric discontinuities and local stress concentrations separately through stress concentration coefficients and stress intensity factors, they have not fully considered the synergistic effects between the two. Especially in the case of randomly distributed multiple defects, the existing models fail to effectively quantify the specific effects of factors such as defect location, size and shape on the fatigue limit of complex structures, which makes traditional models incapable of coping with the more diverse defect forms brought about by modern manufacturing processes (such as additive manufacturing).

[0005] Therefore, how to design a fatigue limit prediction method for notched structures that takes into account the characteristics of multiple defects, comprehensively consider the interaction between notches and defects, and improve the understanding and prediction accuracy of the fatigue behavior of complex defective structures is an urgent problem that technical personnel in this field need to solve. Summary of the invention

[0006] In view of this, the present invention provides a fatigue limit prediction method for notched structures taking into account multiple defect characteristics, which aims to solve the prediction deviation caused by single factor analysis in traditional models, and also achieves a more accurate and comprehensive evaluation of the fatigue limit of complex structures containing defects by introducing virtual defect size, critical distance method and probabilistic KT diagram.

[0007] In order to achieve the above object, the present invention adopts the following technical solution:

[0008] A method for predicting fatigue limit of notched structure considering multi-defect characteristics comprises the following steps:

[0009] S1. Calculate the virtual defect size considering the defect location and shape characteristics

[0010] S2. Using the El Haddad model, virtual defect size Prediction of the fatigue limit of smooth parts under the action of w ;

[0011] S3. Calculate fatigue notch factor K based on critical distance L f Compared with the ideal notch structure fatigue limit Δσ w ';

[0012] S4, fatigue limit Δσ based on smooth parts w And the fatigue limit Δσ of the ideal notched structure w ', construct a probabilistic KT diagram for notch-defect competition analysis and obtain the fatigue limit Δσ of the notch structure w ”.

[0013] Furthermore, in S1, the virtual defect size considering the defect position and shape characteristics is calculated. include:

[0014]

[0015] Among them, ΔK th,lc represents the long crack growth threshold, Y represents the defect geometry correction factor, Δσ w0 Expressed as the fatigue limit of an ideal smooth part.

[0016] Furthermore, the defect geometry correction coefficient Y is expressed as:

[0017]

[0018] Among them, a represents the half length of the major axis of the defect, b represents the half length of the minor axis of the defect, and h represents the distance from the defect center to the material surface.

[0019] Furthermore, in S2, the fatigue limit of the smooth part Δσ is obtainedw ,include:

[0020]

[0021] in, Indicates the defect size.

[0022] Furthermore, in S3, the ideal notch structure fatigue limit Δσ is calculated w ',include:

[0023] S31, define the half length of the critical distance of the notch root of the ideal notch structure The stress at eff ;

[0024]

[0025] Among them, Δσ represents the stress magnitude;

[0026] S32, set the critical distance L equal to the virtual defect size And determine when Δσ eff =Δσ w0 When the ideal notch structure reaches the fatigue failure condition, the fatigue limit Δσ of the ideal notch structure is w ' is expressed as:

[0027]

[0028] Among them, K f =K t,0.5L , K t,0.5L The critical distance half length The effective stress Δσ at eff Ratio to nominal stress.

[0029] Furthermore, in S4, the fatigue limit Δσ of the notch structure is obtained. w ",include:

[0030] If the defect is located at the notch root, the fatigue limit Δσ w ” is expressed as:

[0031]

[0032] If the defect is located at a position other than the notch root, the fatigue limit Δσ w ” is expressed as:

[0033]

[0034] Among them, K t Indicates the stress concentration factor at the notch root, K t,n％It indicates the stress concentration factor corresponding to different areas of the notch, and n% represents the specific location parameters of the notch.

[0035] It can be seen from the above technical solution that compared with the prior art, the technical solution of the present invention has the following advantages:

[0036] Beneficial effects:

[0037] 1. By introducing the defect position and shape characteristics to correct the virtual defect size, combined with the El Haddad model, this method can more accurately predict the mapping relationship between defect characteristics and material fatigue limit. By calculating the virtual size of the defect that causes specific structural failure and combining it with the fatigue limit prediction of smooth parts, it can more accurately reflect how the defects in the actual notch structure affect its fatigue life, thereby providing more reliable data support for engineering design.

[0038] 2. This method combines the critical distance L, sets the critical distance L equal to the virtual defect size, and calculates the fatigue notch coefficient and fatigue limit of the ideal notch structure by carefully analyzing the stress distribution, and determines the failure standard of the notch structure without defects. Then, combined with the mapping relationship between defects and material fatigue limits, a fatigue limit prediction method for notched parts under the combined action of defects and notches is given. This method can intuitively display the fatigue limit corresponding to the final failure of the material caused by defects and notches, allowing engineers to better understand and evaluate the potential impact of complex geometric shapes and defect distribution on structural integrity, and make evaluations on the structure and repair / replacement recommendations.

[0039] 3. This method also proposes a competitive analysis based on the position of defects and notches, and constructs a probabilistic KT diagram to comprehensively determine the fatigue limit of defective notched structures. It not only considers the impact of a single factor on fatigue performance, but also comprehensively considers the interaction between the uneven stress caused by macroscopic notches and the stress concentration caused by microscopic defects, and conducts a comprehensive competitive analysis. For defects at different positions (located in the notch), the probabilistic KT diagram constructed by this method shows the degree of influence of defects on the fatigue limit of the structure, and provides a practical method for evaluating the key areas and their strength for life assessment and reliability analysis of complex structures. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0041] Figure 1A flow chart of a method for predicting fatigue limit of a notched structure taking into account multiple defect characteristics provided by an embodiment of the present invention;

[0042] Figure 2 A schematic diagram of a notch component provided in an embodiment of the present invention;

[0043] Figure 3 The ratio K of the axial stress to the nominal stress provided in the embodiment of the present invention is t,S A graph showing the change in distance from the notch root;

[0044] Figure 4 A probability KT graph provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0045] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0046] like Figure 1 As shown, this embodiment provides a method for predicting fatigue limit of notched structure considering multiple defect characteristics, comprising the following steps:

[0047] S1. Calculate the virtual defect size considering the defect location and shape characteristics

[0048] S2. Use the E1 Haddad model to perform virtual defect size Prediction of the fatigue limit of smooth parts under the action of w ;

[0049] S3. Calculate the fatigue notch factor Kf and the fatigue limit Δσ of the ideal notch structure based on the critical distance L w ′;

[0050] S4, fatigue limit Δσ based on smooth parts w And the fatigue limit Δσ of the ideal notched structure w ′, construct the probability KT diagram to perform notch-defect competition analysis and obtain the fatigue limit Δσ of the notch structure w ".

[0051] This method corrects the size of virtual defects by introducing defect position and shape characteristics, and combines with the E1Haddad model to achieve a more accurate prediction of the fatigue performance of defective notch structures; combined with the critical distance and fatigue notch coefficient, the analysis of the influence of stress concentration on the fatigue limit of ideal notched parts is refined, providing an upper limit for the subsequent prediction of the fatigue limit of defective notched parts; and through notch-defect competition analysis and the construction of a probabilistic KT diagram, the effect of defects on the fatigue limit of the structure is comprehensively evaluated, thus providing a more reliable, more adaptable and more accurate fatigue assessment method. It significantly improves the safety and effectiveness of fatigue design of complex structures.

[0052] The following is a further detailed description of each step in the above fatigue limit prediction method;

[0053] First, the relevant concepts in this embodiment are specifically described here, as shown in the following table:

[0054] symbol name definition <![CDATA[Δσ w0 ]]> Fatigue limit of ideal smooth parts Fatigue limit of material without defects <![CDATA[Δσ w ]]> Fatigue limit of smooth parts Fatigue limit of material containing defects <![CDATA[Δσ w ]]> Fatigue limit of ideal notch structure Fatigue limit of parts without defect notches <![CDATA[Δσ w ]]> Fatigue limit of notched structure Fatigue limit of parts containing defects and notches

[0055] In this embodiment, S1 calculates the virtual defect size taking into account the defect position and shape characteristics. include:

[0056]

[0057] Among them, ΔK th,lc represents the long crack extension threshold, which can be obtained through crack extension test; Y represents the defect geometry correction factor, Δσ w0 Expressed as the fatigue limit of an ideal smooth part.

[0058] like Figure 2 As shown in the figure, in the elliptical defect notch component, the defect geometry correction coefficient Y is expressed as:

[0059]

[0060] Among them, a represents the half length of the major axis of the defect, b represents the half length of the minor axis of the defect, and h represents the distance from the defect center to the material surface.

[0061] Specifically, the relevant parameters required for the calculation of the defect geometry correction factor Y can be obtained by fatigue testing the target result until failure, and then carefully observing the fracture surface under a microscope to identify and determine the location and morphology of the fatal defect. Next, the image features of the relevant defects are digitally converted through image processing software (such as Matlab or other similar tools). The specific geometric parameters of the defect can be obtained, including key information such as surface depth (h), major axis half length (a) and minor axis half length (b).

[0062] The different value ranges of the defect geometry correction factor Y correspond to the different geometric forms of the defects and the degree of their influence on the fatigue limit of the material.

[0063] when When , the internal defect is relatively flat and far away from the surface, its influence is relatively small, and the Y value is small; when When , the subsurface defect shape becomes flat and gradually approaches the surface, the influence increases, and the Y value increases accordingly; when When the half length of the minor axis of the surface defect is at least equal to the distance from the defect center to the material surface, the defect influence reaches the maximum, and the Y value reaches the maximum. This piecewise function form of the correction coefficient Y can more accurately reflect the actual influence of defects in different positions and sizes, thereby improving the accuracy and reliability of fatigue limit prediction.

[0064] It is worth noting that the harm of subsurface defects may be greater than that of surface defects. On the one hand, subsurface defects interact with the outer surface, causing stress concentration greater than that of surface defects exposed to the outside world; on the other hand, if subsurface defects quickly break through and become surface defects, their equivalent defect size will be significantly larger, significantly reducing the fatigue resistance and fatigue life of the material.

[0065] Furthermore, for the ideal smooth part fatigue limit Δσ w0 , which can be obtained through experimental testing, for example, in a stable cyclic stress-strain curve, the stress value corresponding to the plastic strain reaching 0.05%.

[0066] In this step, by introducing the virtual defect size, combining the long crack extension threshold and the ideal smooth part fatigue limit, the fatigue limit of the smooth part is accurately predicted. In particular, the defect geometry correction factor Y defined in segments can accurately reflect the degree of influence of different defects on the material fatigue limit according to their geometric shapes and positions. It significantly improves the accuracy and reliability of fatigue limit prediction and ensures the safety and durability of complex structures in practical applications.

[0067] In this embodiment, S2 is used to obtain the fatigue limit Δσ of the smooth part. w ,include:

[0068]

[0069] in, Indicates the defect size.

[0070] In this embodiment, S3 calculates the fatigue limit Δσ of the ideal notch structure. w ',include:

[0071] S31, define the half length of the critical distance of the notch root of the ideal notch structure The stress at eff;

[0072]

[0073] Among them, Δσ represents the stress magnitude;

[0074] S32, set the critical distance L equal to the virtual defect size And determine when Δσ eff =Δσ w0 When the ideal notch structure reaches the fatigue failure condition, the fatigue limit Δσ of the ideal notch structure is w ' is expressed as:

[0075]

[0076] Among them, K f =K t,0.5L , K t,0.5L The critical distance half length The effective stress Δσ at eff Ratio to nominal stress.

[0077] Taking the notched component of GH4169 material as an example, t,0.5L The acquisition process is further explained:

[0078] Get K t,0.5L First, we rely on accurate linear elastic finite element analysis of the notch, using the GH4169 material model, setting the Poisson's ratio to 0.325 and the Young's modulus to 182GPa (considering it to be an isotropic material), and using Ansys software to simulate the stress distribution of the notch under uniaxial tensile load. The key to the analysis is to determine the stress concentration at the root of the notch, especially the stress change along the direction of the maximum stress gradient (i.e. the direction of the line connecting the notch roots on both sides). Through this step, the stress value of the notch root that changes with distance can be output.

[0079] Further, such as Figure 3 As shown in the figure, the axial stress to nominal stress ratio K output by Ansys is analyzed using Matlab software. t,S Perform interpolation processing and fit K t,S The curve changes with the distance from the notch root. This curve reveals the law of stress concentration changing with distance. In the curve, when the distance from the notch root is half the length, the corresponding K t,S The value is K t,0.5L . K t,0.5L As an important parameter to characterize the stress concentration degree at the root of the notched component, it directly reflects the influence of the notch on the fatigue limit of the material.

[0080] In this embodiment, S4 is based on the fatigue limit Δσ of the smooth part. w And the fatigue limit Δσ of the ideal notched structurew ', construct a probabilistic KT diagram for notch-defect competition analysis and obtain the fatigue limit Δσ of the notch structure w ″.

[0081] Obtain the fatigue limit Δσ of the notched structure w ” is expressed as:

[0082]

[0083] Among them, K t,n％ It indicates the stress concentration factor corresponding to different areas of the notch, and n% represents the specific location parameters of the notch.

[0084] Specifically, the location of the fatal defect of the cross section is obtained by microscopic observation after fatigue test, and then the ratio of the maximum stress at the defect location to the maximum stress value at the root of the notch is obtained by finite element analysis, which is n%. For example, when n=100%, K t,n％ Equal to the stress concentration factor K t , indicating that the defect is located at the root of the notch.

[0085] That is, the defect is located at the root of the notch, and the fatigue limit Δσ w ” is expressed as:

[0086]

[0087] Further, such as Figure 4 As shown in the figure, the construction process of the probability KT diagram includes: making the fatigue limit Δσ w ” and defect size Relationship curve family (K t,n％ n is 0-100%), n = 100% corresponding relationship curve and the corresponding fatigue limit Δσ of the ideal notched part w ”=Δσ w '(horizontal straight line) area ( Figure 4 Dark area, defect > notch) is used for fatigue limit prediction. The fatal defect is analyzed by section and its position (parameter n) and size (parameter ) is known, based on this, a data point is plotted on the probability KT graph. If the data point is located in this area ( Figure 4 Dark area, defect > notch), the corresponding ordinate is the fatigue limit of the notched part; if it is in Δσ w ”=Δσ w '(horizontal straight line), the fatigue limit of the notched part is equal to the fatigue limit of the ideal notched part Δσ w '.

[0088] Finally, through the probability KT diagram, we can intuitively see the impact of different defects and notches on fatigue limit, as well as the competitive relationship between them. Specifically, the KT diagram of defective notched parts can be expressed as two parts:

[0089]

[0090] In the figure, Δσ w ' value is used as the dividing line, and the KT diagram is divided into two areas, which reflects the competitive relationship between the notch and the defect. The area above this value indicates that the effect of the notch on fatigue strength is greater than that of the defect, which is defined as the virtual area. It is an extrapolated area with decreasing defect size, and the fatigue strength of the notch without defects is the maximum strength. The lower part shows that the effect of the defect is greater than that of the notch, that is, the KT diagram of the notch structure in the true sense. Different high stress volumes ( Figure 4 Only 50%-100% of the maximum stress is shown in the figure. The strength-defect size relationship under the high stress volume range is described according to the actual defect location. The lower limit of the KT diagram (100% maximum stress) indicates that the fatal defect is located near the root of the notch. The fatal defect is equivalent to the strength of the notch root in this stress range, and there is no defect in the high stress area that has a comparable effect and can compete with it.

[0091] In addition, the fatigue test data points are marked in the figure. The fatal defect (obtained by cross-section analysis) is located at the root of the notch, so it corresponds to n = 100%, and the corresponding defect size is The predicted fatigue limit value is 223MPa.

[0092] The fatigue limit prediction method of the notched structure in this embodiment corrects the virtual defect size by introducing defect position and shape characteristics, and combines with the El Haddad model to achieve accurate prediction of the fatigue performance of the defective structure; combines the critical distance and fatigue notch coefficient to refine the analysis of the influence of stress concentration and defect position on fatigue limit; and through notch-defect competition analysis and the construction of a probabilistic KT diagram, comprehensively evaluates the effect of defects on the fatigue limit of the structure, providing a more reliable, more adaptable and more accurate evaluation method for the fatigue design of complex structures.

[0093] In this specification, each embodiment is described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the embodiments can be referred to each other. For the system disclosed in the embodiment, since it corresponds to the method disclosed in the embodiment, the description is relatively simple, and the relevant parts can be referred to the method part.

[0094] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A fatigue limit prediction method for notched structures considering multi-defect characteristics, characterized in that: The following steps are involved: S1. Calculate the virtual defect size considering the defect location and shape characteristics S2. Using the El Haddad model, virtual defect size Prediction of the fatigue limit of smooth parts under the action of w ; S3. Calculate fatigue notch factor K based on critical distance L f Compared with the ideal notch structure fatigue limit Δσ w '; S4, fatigue limit Δσ based on smooth parts w And the fatigue limit Δσ of the ideal notched structure w ', construct a probabilistic KT diagram for notch-defect competition analysis and obtain the fatigue limit Δσ of the notch structure w ”.

2. A notch structure fatigue limit prediction method considering multiple defect characteristics according to claim 1, characterized in that: In S1, the virtual defect size considering the defect position and shape characteristics is calculated. include: Among them, ΔK th,lc represents the long crack growth threshold, Y represents the defect geometry correction factor, Δσ w0 Expressed as the fatigue limit of an ideal smooth part.

3. A notch structure fatigue limit prediction method considering multiple defect characteristics according to claim 2, characterized in that: The defect geometry correction coefficient Y is expressed as: Among them, a represents the half length of the major axis of the defect, b represents the half length of the minor axis of the defect, and h represents the distance from the defect center to the material surface.

4. A notch structure fatigue limit prediction method considering multiple defect characteristics according to claim 1, characterized in that: In S2, the fatigue limit Δσ of the smooth part is obtained w ,include: in, Indicates the defect size.

5. The method for predicting fatigue limit of notched structure considering multiple defect characteristics according to claim 1, characterized in that: In S3, the ideal notch structure fatigue limit Δσ is calculated w ',include: S31, define the half length of the critical distance of the notch root of the ideal notch structure The stress at eff ; Among them, Δσ represents the stress magnitude; S32, set the critical distance L equal to the virtual defect size And determine when Δσ eff =Δσ w0 When the ideal notch structure reaches the fatigue failure condition, the fatigue limit Δσ of the ideal notch structure is w ' is expressed as: Among them, K f =K t,0.5L , K t,0.5L The critical distance half length The effective stress Δσ at eff Ratio to nominal stress.

6. A notch structure fatigue limit prediction method considering multiple defect characteristics according to claim 1, characterized in that: In S4, the fatigue limit Δσ of the notched structure is obtained w ",include: If the defect is located at the notch root, the fatigue limit Δσ w ” is expressed as: If the defect is located at a position other than the notch root, the fatigue limit Δσ w ” is expressed as: Among them, K t Indicates the stress concentration factor at the notch root, K t,n％ It indicates the stress concentration factor corresponding to different areas of the notch, and n% represents the specific location parameters of the notch.

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