High-stress volume determination method under gap-defect interaction

Abstract

The invention discloses a high stress volume determination method under notch-defect interaction, and belongs to the technical field of damage tolerance evaluation and fatigue strength prediction.The method comprises the steps that stress distribution data of notch roots in a notch component structure containing defects are calculated through finite element analysis, and the ratio of axial stress to nominal stress is output; calculating the virtual defect size of the notch component structure; calculating a stress concentration parameter of the notch root part in the component structure containing the defect notch by utilizing a critical distance method and combining the ratio; and calculating the high-stress volume area by analyzing the competition effect of the gap and the defect and combining the virtual defect size and the stress concentration parameter. The method comprehensively considers the interaction between the gap and the defect, accurately calculates the high-stress volume of the structure, and facilitates the prediction of a fatigue failure part, thereby improving the accuracy of fatigue evaluation. And the method is universal for gap structural members of any size and shape, and is simple and convenient to operate and wide in application range.

Description

Technical Field

[0001] The invention relates to the technical field of damage tolerance evaluation and fatigue strength prediction, and more particularly to a method for determining a high stress volume under notch-defect interaction. Background Art

[0002] In mechanical structure design, strength verification is often the last hurdle before it is put into actual production and application. In the early days, structural strength design was mainly based on static strength design, and the whole set of methods for structural strength analysis, design and testing has matured. Since the end of the 20th century, with the application and promotion of computer-aided design technology, in-depth research on structural optimization algorithms, iterative upgrades of material manufacturing processes, and continuous improvement of basic testing levels, the basic design capabilities in the mechanical industry have been significantly improved. At this stage, structural failures due to insufficient static strength can basically be avoided. As the service environment of mechanical structures becomes increasingly harsh and complex, fatigue caused by alternating stress has become one of the most common failure modes of engineering structures.

[0003] Fatigue failure usually occurs at geometric discontinuities (machined holes, notches, etc.) or defects. The actual failure mechanism can be driven by a combination of the failure modes of defects (local stress concentration) and notches (global stress concentration). Global geometric discontinuities and defects both have negative effects on the fatigue properties of materials, and these effects may produce synergistic effects. The coupling effect of the two is more obvious than either effect alone. Previous studies have studied the fatigue assessment of global geometric discontinuities (notches) and local (defect) stress concentrations by stress concentration coefficients and stress intensity factors, respectively. However, the mechanism of their coupling effects still needs further study. To perform fatigue assessment on real structures, it is first necessary to identify the weak parts of the structure, that is, the areas prone to fatigue failure. Usually, this area has the characteristics of high stress values. With the continuous efforts of scholars, the high stress volume method (also known as control volume or risk volume) has been continuously improved and developed. Directly using 80-90% of the high stress area for failure analysis is not in line with reality. The same high stress volume value method for different structures is not reliable, that is, for different structures, the selection of high stress volume is individual, rather than all taking a specific percentage in general.

[0004] Therefore, the existing fatigue assessment methods have certain limitations, which are mainly manifested in the lack of comprehensive and personalized analysis of the structure, especially the lack of in-depth research on geometric discontinuities and defect coupling effects, which may lead to inaccurate and unreliable fatigue failure prediction.

[0005] Based on the critical distance method and fracture mechanics, the present invention conducts notch-defect competition analysis, comprehensively considers factors such as geometric structure, material properties and defect distribution to determine the high stress volume stress threshold of complex structures, proposes a method for determining the high stress volume of structural parts with notches of any size, and clarifies the key areas for complex structures to be applied in life assessment and reliability analysis. Summary of the invention

[0006] In view of this, based on the advantages of the critical distance method and fracture mechanics in solving the problem of notches and defects, the present invention provides a method for determining the high stress volume under the interaction of notches and defects, which solves the problem that traditional fatigue assessment methods mostly consider the effects of notches and defects separately, resulting in inaccurate assessment. The present invention comprehensively considers the interaction between notches and defects, accurately calculates the high stress volume of the structure, helps to predict the fatigue failure site, and thus improves the accuracy of fatigue assessment.

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

[0008] A method for determining a high stress volume under notch-defect interaction comprises the following steps:

[0009] S1. Calculate the stress distribution data of the notch root in the defect notch component structure through finite element analysis, and output the ratio of axial stress to nominal stress K t,S ;

[0010] S2. Calculate the virtual defect size of the notched component structure

[0011] S3, using the critical distance method, combined with the ratio K t,S , calculate the stress concentration parameter K at the root of the notch in the structure containing the defect notch t,0.5L ;

[0012] S4. By analyzing the competitive effects of gaps and defects, combined with the virtual defect size and stress concentration parameter K t,0.5L , calculate the high stress volume area.

[0013] It can be seen from the above technical solutions that compared with the prior art, the present invention has the following technical advantages:

[0014] (1) The present invention compares the degree of weakening of structural fatigue strength by notches and defects and conducts competitive failure analysis, determines the high stress volume of complex structures by comprehensively considering factors such as geometric structure, material properties and defect distribution, establishes a high stress volume calculation process based on notch-defect interaction, clarifies the key areas of complex structures used in life assessment and reliability analysis, and has reference significance for the control volume used for size effect extrapolation and the judgment of failure locations of actual engineering structures. It has a wide range of applications, ranging from traditional casting and forging (a small number of processing defects or service defects) to emerging additive manufacturing technology (a large number of processing defects).

[0015] (2) The notch-defect competition model proposed in the present invention has a relatively simple calculation process; and the required parameters can be directly determined by finite element analysis, or directly obtained by a small number of experiments. It has universality for notched structural parts of any geometric shape, is easy to operate, and has a wide range of applicability;

[0016] (3) In the present invention, the high stress volume threshold stress varies with the defect and notch geometry, which is different from the general use of the 80% or 90% high stress volume concept (V 80％ or V 90％ , that is, the volume of material where stress exceeds 80% or 90% of the maximum stress), taking into account the actual geometry, stress and defect distribution of the structure, a practical method for calculating high stress volume is given;

[0017] (4) The application of the proposed notch-defect competition model is not limited to the materials used in the present invention. The high stress volume of the notch structure is calculated by failure competition analysis and probability method. It is suitable for the evaluation of the failure site of the notch structure under random defect distribution or a small amount of processing / service defects, and the prediction result is highly accurate. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] 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.

[0019] Figure 1 A flow chart of a method for determining a high stress volume under notch-defect interaction provided by an embodiment of the present invention;

[0020] Figure 2 A schematic diagram of the dimensions of a notched specimen for a GH4169 fatigue test provided in an embodiment of the present invention;

[0021] Figure 3 Schematic diagram of critical distances of different notched parts;

[0022] Figure 4 It is a schematic diagram of the high stress volume threshold stress of GH4169 notched specimen changing with defect size and the comparison of test results;

[0023] Figure 5 This is a schematic diagram showing the comparison between the failure position and high stress volume of the notched specimen in the GH4169 fatigue test using the method of the present invention. DETAILED DESCRIPTION

[0024] 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.

[0025] The embodiment of the present invention discloses a method for determining a high stress volume under notch-defect interaction, referring to Figure 1 As shown, the following steps are included:

[0026] S1. Calculate the stress distribution data of the notch root in the defect notch component structure through finite element analysis, and output the ratio of axial stress to nominal stress K t,S ;

[0027] S2. Calculate the virtual defect size of the notched component structure

[0028] S3, using the critical distance method, combined with the ratio K t,S , calculate the stress concentration parameter K at the root of the notch in the structure containing the defect notch t,0.5L ;

[0029] S4. By analyzing the competitive effects of gaps and defects, combined with the virtual defect size and stress concentration parameter K t,0.5L , calculate the high stress volume area.

[0030] The present invention aims at the problem that the weak parts of the structure under the action of defects (inclusions, holes, scratches and corrosion points) are difficult to judge the fatigue failure position (notch or defect), based on the competition relationship between notch and defect (notch-defect relationship is not only competition, but also mutual promotion-stress concentration superposition, and the judgment of failure position depends more on competition, so the present invention is collectively referred to as competition, and the actual proposed model also considers the promotion relationship), and proposes a method for calculating the high stress volume under the coupling of defects and notch effects, and clarifies the key areas for complex structures to be applied to life assessment and reliability analysis. The present invention quantifies the notch effect with the critical distance method and the equivalent stress concept, quantifies the influence of defects through the fatigue strength model based on the stress intensity factor and the defect size, conducts competitive failure analysis on the degree of weakening of the fatigue strength of the structure by notches and defects, and comprehensively determines the high stress volume of the complex structure based on factors such as geometric structure, material properties and defect distribution. The proposed method flow is universal for structural parts with notches of any size and shape, easy to operate, and has a wide range of applications.

[0031] Take the failure location prediction process of the defective notch component to be analyzed as an example. The specimen is made of GH419 material. Figure 2 As shown, there are three different geometric shapes including quasi-smooth parts, C-notches, and V-notches, and their respective related size data are displayed; the specimen size is analyzed through the competitive failure analysis between notches and defects, with the critical distance and the virtual defect size (El Haddad parameter) as the main parameters to realize the failure location prediction process of the defective notch component to be analyzed, which specifically includes the following steps:

[0032] 1. Calculate the stress distribution at the notch root based on finite element analysis and output the ratio of axial stress to nominal stress K t,S

[0033] Linear elastic finite element analysis is performed on the component to be analyzed (the Young's modulus of GH4169 in the present invention is 182GPa, and the Poisson's ratio is 0.325), the stress distribution at the root of the notch is determined, and the stress value of the root of the notch with the distance from the notch (the direction of the maximum stress gradient) is output. In the present invention, the maximum stress gradient direction of the notch part is the direction of the line connecting the roots of the notches on both sides. Through the finite element analysis software Output axial stress (fatigue test and finite element analysis are both uniaxial tensile load) and nominal stress ratio K t,S , as shown in Table 1, and the data in Table 1 is used as input for step 3. Among them, K t Represents the stress concentration factor.

[0034] Table 1 Stress gradient of specimens with different notches

[0035]

[0036]

[0037] 2. Fatigue strength assessment of defective materials and output of virtual defect size (i.e. El Haddad parameter)

[0038] The present invention adopts the El Haddad model to calculate the fatigue strength of materials under the action of defects, and its specific formula is as follows:

[0039]

[0040] where Δσ w0 is the fatigue strength of defect-free material, the square root of the defect projected area Indicates the defect size, is the El-Haddad parameter, which is calculated by the following formula:

[0041]

[0042] Among them, ΔK th,lc is the long crack extension threshold, Y is the defect geometry correction coefficient (related to the shape, position and other uncertain parameters of the defect). Generally, the Y value for surface defects is 0.65, while that for internal defects is 0.5. The experimental results show that both low- and high-cycle failures are caused by surface defects. The Y value of the present invention is 0.65. ΔK th,lc With Δσ w0 The parameter data are shown in Table 2 (the material parameter acquisition test method is a prior art and will not be repeated here). Due to the existence of material dispersion, ΔK th,lc With Δσ w0 The parameter values ​​are all ranges and not fixed values. As shown in Table 2.

[0043] It is worth noting that in step 2 only The calculation result is used as the input of step 4, and the fatigue strength Δσ in formula (1) w This is a non-essential calculation item. It is listed to introduce equation (2) and to derive the final notch-defect competition model in step 4, which reflects the degree of weakening of the notch fatigue strength by defects.

[0044] Table 2 Parameters related to high stress volume calculation

[0045]

[0046] 3. Calculate the fatigue strength of the notched part based on the critical distance method and output the stress concentration parameter K at the notch root t,0.5L

[0047] The critical distance is an effective tool to solve the effect of the notch. The stress at a point a certain distance from the notch root (the half length of the critical distance L / 2) is taken as the effective stress Δσeff , expressed as:

[0048] Δσ eff =Δσ(L / 2) (3)

[0050] Δσ represents the stress magnitude, L is the critical distance. For high cycle fatigue problems, the calculation of the critical distance is the same as the virtual crack length in the El Haddad model. The parameter Y is also considered. The calculation is based on the surface defect Y = 0.65, that is, the critical distance L is equal to the El Haddad parameter When Δσ eff =Δσ w0 , Δσ w0 is the fatigue strength of defect-free material. It is assumed that the notched part has reached the fatigue failure condition. The fatigue limit Δσ of the notched part without defect is w '(the minimum nominal stress for fatigue failure of notched parts) can be expressed as:

[0051]

[0052] K t,0.5L is the ratio of the maximum (tensile) stress to the nominal stress at the half-length critical distance (K t Only the stress concentration at the root of the notch is described. The present invention expands its concept, such as K t,n％ , K t,0.5L With K t,L , convenient for high stress volume calculation), K t,0.5L According to Table 1 and the critical distance half length L / 2, specifically, through the software tool The K value is obtained by fitting the data in Table 1 using interpolation method. t,S As the distance S from the root of the notch changes, the K corresponding to the critical distance of L / 2 calculated in this step is obtained. t,0.5L Values, as shown in Table 2 and Figure 3 As shown, Figure 3 Schematic diagram of critical distance of different notched parts. t,0.5L The calculation result is used as the input of step 4, and the fatigue strength Δσ of the notched part w 'It reflects the degree of weakening of notch fatigue strength caused by notch, and is not a necessary calculation item.

[0053] 4. Perform notch-defect competition analysis and calculate the K corresponding to the high stress volume t,n％ With high stress volume V n％ Based on the above analysis, the competition between defects can screen out larger defects through extreme value statistics, while the competition between defects and notches requires comparative analysis of the degree of weakening of material strength by both. In summary, assuming that the fatigue strength of the material is the same under the influence of defects (step 2) and notches (step 3), the following formula can be obtained:

[0054] Δσ w =Δσ w ' (5)

[0056]

[0057] Among them, Δσ w ' is the fatigue limit of the part without defect notch, K t,n％ Defined as the stress concentration factor K t The nth percentile value (i.e. K t ×n%), which is related to the notch shape and critical defect characteristics and is used to represent the high stress volume V n％ Boundary stress conditions.

[0058] Combining equation (4) in step 3 with equation (6) in step 4, the stress concentration factor threshold for determining the high stress volume (the product of this threshold and the nominal stress is the high stress volume threshold stress) is:

[0059]

[0060] The result of step 2 and K obtained in step 3 t,0.5L (as shown in Table 2) into formula (7), we can get K t,n％ With defects Functional relationship between them, it is worth noting that K t,0.5L and All are ranges. The obtained function relationship should be a family of curves, not a single curve. Substitute the boundary values ​​of the range into the calculation, take the upper and lower boundary curves and the middle range, and the obtained boundary and the middle shaded part are as follows Figure 4 As shown. The expression of high stress volume (percentage) of notched parts is:

[0061]

[0062] Finite element software Filter out the unit set with stress exceeding the maximum stress n% ​​(select the maximum defect size of 0.2mm and Figure 4 Intersection point K of lower boundary of middle dispersion zone t,n％ used for n% calculation), which is the high stress volume V of the structure n％ Based on the high stress volume V n％ The fatigue failure location of the structure containing defect notches can be predicted.

[0063] The present invention uses the uniaxial fatigue test data of the additive GH4169 material to verify the above method. The specimen size is as follows: Figure 2 As shown; Figure 4 and Figure 5The comparison chart of the predicted results and the actual test data of the additive GH4169 with different notches under uniaxial load is given in FIG. Figure 4 It can be seen that the maximum defect value and the limit failure position obtained by computer tomography (CT) as the test data point fall within the dispersion band, which shows that the proposed model achieves accurate evaluation. In addition, the failure positions of the quasi-smooth parts obtained by scanning electron microscope (SEM) cross-section analysis are 1mm and 2.5mm away from the root, both located at the V 95％ It is far from the predicted smooth part dispersion zone. But it is worth noting that Figure 4 The middle dispersion zone is caused by the dispersion of the material itself. The stress concentration factor threshold of the high stress volume should fall within this range, and the actual test point position should fall on the upper right side of the lower boundary of the range, because not all test sample points (when the high stress volume is selected) can reach the limit state. As the sample size increases, the maximum point of the high stress volume range of all samples should gradually approach Figure 4 The lower boundary of each dispersion zone. Figure 5 It can be seen that the predicted high stress volume (i.e., the range where fatigue failure may occur) covers all failure locations obtained from the tests, and the predicted boundary is close to the farthest point of the failure location, making an accurate judgment on the failure location boundary.

[0064] Based on the competition relationship between notches and defects, this paper proposes a new method for calculating high stress volume. This method comprehensively considers the influence of notches and defects on the fatigue performance of the structure, and quantitatively analyzes it through the critical distance method and fatigue strength model, and finally determines the threshold stress of the high stress volume. This method has the following advantages:

[0065] Targetedness: It can determine the high stress volume according to the actual conditions of different structures, avoiding the limitations of the fixed percentage method.

[0066] Accuracy: Competitive failure analysis allows for more accurate prediction of the size and location of high stress volumes.

[0067] Versatility: This method can be applied to fatigue life assessment of different materials and structures.

[0068] 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 device 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.

[0069] 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 method for determining high stress volume under notch-defect interaction, characterized in that: The following steps are involved: S1. Calculate the stress distribution data of the notch root in the defect notch component structure through finite element analysis, and output the ratio of axial stress to nominal stress K t,S ; S2. Calculate the virtual defect size of the notched component structure S3, using the critical distance method, combined with the ratio K t,S , calculate the stress concentration parameter K at the root of the notch in the structure containing the defect notch t,0.5L ; S4. By analyzing the competitive effects of gaps and defects, combined with the virtual defect size and stress concentration parameter K t,0.5L , calculate the high stress volume area.

2. The method for determining high stress volume under notch-defect interaction according to claim 1, characterized in that: The step S2 comprises: The virtual defect size of the component structure is calculated by the following formula Among them, ΔK th,lc is the long crack growth threshold, Δσ w0 is the fatigue strength of defect-free material, and Y is the defect geometry correction factor.

3. The method for determining high stress volume under notch-defect interaction according to claim 1, characterized in that: The step S3 comprises: Based on the critical distance method, the critical distance L is set as the El Haddad parameter When the effective stress Δσ at the critical distance eff Equal to the fatigue strength of defect-free material Δσ w0 When the fatigue limit of the notched part is w ' is expressed as: K t,0.5L is the ratio of the maximum stress to the nominal stress at the critical distance half length L / 2; through the ratio K t,S Obtained with the critical distance half length L / 2.

4. The method for determining high stress volume under notch-defect interaction according to claim 2, characterized in that: In step S4, analyzing the competitive effects of gaps and defects includes: Assuming that the fatigue strength of the material under the influence of defects and notches is the same, the formula is as follows: Board w =Ds w (5) Among them, Δσ w ' is the fatigue limit of the notched part without defects, K t,n％ Defined as the stress concentration factor K t The nth percentile value is related to the notch shape and critical defect characteristics; is the actual defect size.

5. The method for determining high stress volume under notch-defect interaction according to claim 4, characterized in that: In the step S4, calculating the high stress volume area includes: Formula (4) and formula (6) are combined to obtain the stress concentration factor threshold for judging the high stress volume: The result obtained in step S2 and K obtained in step S3 t,0.5L Substituting into formula (7), we get K t,n％ With defects Functional relationship between t,0.5L and All are ranges, and the obtained function relationship is a family of curves. Substitute the boundary values ​​of the range into the calculation, and take the upper and lower boundary curves and the middle range; The high stress volume expression of the notched part is: By screening out the unit set whose stress exceeds the maximum stress n%, the high stress volume V of the structure is obtained. n％ .

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