CN112765820B - Multi-axis notch fatigue life prediction method based on plastic region - Google Patents
Multi-axis notch fatigue life prediction method based on plastic region Download PDFInfo
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Abstract
The invention discloses a multiaxial notch fatigue life prediction method based on a plastic region, which comprises the following steps: calculating the critical tensile stress amplitude based on the relation between the critical tensile stress amplitude of the plastic region of the test piece and the theoretical stress concentration coefficient in uniaxial stretching under different notches; calculating the radius of the notch characterization plastic region under the semicircular assumption of the notch plastic region, and calculating the area of the notch plastic region and the area of the critical plastic region; defining a notch plastic region influence factor and a fatigue damage control parameter; substituting the fatigue damage control parameters into a uniaxial fatigue S-N curve of the smooth piece, and predicting the fatigue life of the multiaxial notch. The fatigue life prediction precision of the notch piece is higher; compared with a critical distance method and a stress field method with higher precision, the method has simpler calculation process and is convenient for engineering use.
Description
Technical Field
The invention belongs to the technical field of aviation systems, and particularly relates to a multiaxial notch fatigue life prediction method based on a plastic region.
Background
Fatigue refers to the change in mechanical properties of a material or structure that occurs under repeated loading. Unlike static load failure, fatigue failure is a progressive process that gradually builds up on a microscopic scale and over a longer period of time, and thus is inevitably one of the main causes of structural failure. In engineering practice, structures are often subjected to multiaxial loads during service due to the complexity of the loads. In addition, due to the notch effect such as openings, grooves, bosses, bends, and forks, which often exist in the engineering component, the structural part area is likely to be in a multiaxial stress strain state even when the engineering component is subjected to unidirectional loads. Therefore, the research on the fatigue performance of the notch piece under multiaxial load has important significance for developing fatigue theory and improving the fatigue resistance of engineering structures.
At present, a unified theory is not formed in the field of multiaxial fatigue research of notch parts, and various fatigue life prediction theories are proposed from different perspectives. According to the explanation of the notch effect and the fatigue damage control parameters adopted by various theories, the notch effect can be divided into: the nominal stress method, the local stress strain method and the critical domain method can be further divided into a critical distance method and a stress field intensity method. The nominal stress method and the local stress strain method are simpler, the engineering application is more, but the fatigue life prediction accuracy is poorer because the consideration of the notch effect is insufficient; the critical distance method and the stress field intensity method have higher precision, but the calculation process is complex, and the engineering application is inconvenient.
The current research mainly improves the multiaxial fatigue life prediction precision of the notch piece and simplifies the calculation process, in recent years, some students at home and abroad put forward a new multiaxial fatigue life prediction method of the notch piece on the basis of previous research results, and good results are obtained. However, the fatigue life analysis research of the notch piece under the multiaxial load is still not comprehensive and deep enough, and a great deal of theoretical and experimental research is still needed.
Disclosure of Invention
In order to overcome the defects in the prior art, the invention aims to provide a multiaxial notch fatigue life prediction method based on a plastic region, so as to solve the problem that the fatigue life prediction precision of a notch piece is lower by using a traditional fatigue life prediction method in the prior art.
In order to achieve the above purpose, the invention adopts the following technical scheme:
The invention discloses a multiaxial notch fatigue life prediction method based on a plastic region, which comprises the following steps:
(1) Calculating the critical tensile stress amplitude based on the relation between the critical tensile stress amplitude of the plastic region of the test piece and the theoretical stress concentration coefficient in uniaxial stretching under different notches;
(2) Calculating the radius of the notch characterization plastic region under the semicircular assumption of the notch plastic region, and calculating the area of the notch plastic region and the area of the critical plastic region;
(3) Defining a notch plastic region influence factor and a fatigue damage control parameter;
(4) Substituting the fatigue damage control parameters into a uniaxial fatigue S-N curve of the smooth piece, and predicting the fatigue life of the multiaxial notch.
Further, the step (1) includes: the relation between the critical tensile stress amplitude sigma a0 of the plastic region of the test piece and the theoretical stress concentration coefficient K T:
Wherein, K T is the theoretical stress concentration coefficient of the test piece in uniaxial tension under different notches, and the calculation formula of K T is as follows:
Wherein σ max is the maximum local elastic stress and σ 0 is the nominal stress; the theoretical stress concentration factor K T can be found experimentally, by engineering drawing, by empirical formula or by finite element analysis, and is considered to be a known quantity.
Further, the step (2) includes:
For the area of the notch plastic region, the shape and the area of the notch plastic region change along with the geometric size of the notch, the load level and the load path, and the actual shape of the notch plastic region gradually tends to be semicircular along with the reduction of the load level; assuming that the plastic region of the notch is a semicircle with the root of the notch as the center and R p as the radius, the radius R p is calculated:
Wherein R p is the radius of the notch characterization plastic region, The phase difference is the pull-torsion load phase difference;
further, the area S p of the notch plastic region can be obtained:
Since the boundary between the plastic region and the elastic region is not clear on a microscopic scale, this tends to cause the internal stress-strain field of the material near the boundary of the plastic region to become extremely complex, which also promotes the propagation of fatigue cracks objectively; in order to characterize the "contribution" of the plastic region near the notch to the accumulation of fatigue damage, the concept of a critical plastic region is proposed to describe the region of the material inside affected by the plastic region; the radius of the critical plastic region is:
The critical plastic area is:
further, the step (3) includes: defining a notch plastic region influence factor f by using a characterization method of the notch plastic region and the critical plastic region:
Defining fatigue damage control parameters
Wherein f is a notch plastic region influencing factor; The fatigue damage control parameter is as follows; σ Mises is the maximum von Mises stress at the root of the notch, obtained by finite element analysis, which is a known quantity.
Further, the step (4) includes:
Substituting the fatigue damage control parameter into an S-N curve analysis type of the single-shaft smooth piece, and predicting the fatigue life of the notch piece under the multiaxial load; wherein the S-N curve of the single axis slip can be obtained by experiment or looking up an engineering chart, of known magnitude.
The invention has the beneficial effects that:
Compared with the traditional fatigue life prediction method, such as a nominal stress method and a local stress strain method, the fatigue life prediction method for the notch piece has higher fatigue life prediction precision; compared with a critical distance method and a stress field method with higher precision, the method has simpler calculation process and is convenient for engineering use.
Drawings
FIG. 1a is parameters of a smooth test piece;
FIG. 1b is the parameters of the V notch test piece;
FIG. 1c is the parameters of the R2 notch test piece;
FIG. 1d is the parameters of the R5 notch test piece;
FIG. 2 is a schematic S-N curve of a uniaxial fatigue test slider;
FIG. 3a is a schematic diagram of a predicted V-notch life;
FIG. 3b is a schematic diagram of a predicted R2 notch life;
FIG. 3c is a schematic diagram of a predicted R5 notch life;
fig. 4 is a schematic diagram of the method of the present invention.
Detailed Description
The invention will be further described with reference to examples and drawings, to which reference is made, but which are not intended to limit the scope of the invention.
Referring to FIG. 4, the method for predicting the fatigue life of the multiaxial notch based on the plastic region comprises the following steps:
(1) Calculating the critical tensile stress amplitude based on the relation between the critical tensile stress amplitude of the plastic region of the test piece and the theoretical stress concentration coefficient in uniaxial stretching under different notches;
(2) Calculating the radius of the notch characterization plastic region under the semicircular assumption of the notch plastic region, and calculating the area of the notch plastic region and the area of the critical plastic region;
(3) Defining a notch plastic region influence factor and a fatigue damage control parameter;
(4) Substituting the fatigue damage control parameters into a uniaxial fatigue S-N curve of the smooth piece, and predicting the fatigue life of the multiaxial notch.
The material adopted in the example is 316L stainless steel prepared by a laser selective melting technology, and fatigue tests of various load paths, namely single-axis, proportional and 90-degree non-proportional multi-axis loads, are carried out. The test pieces are divided into a smooth test piece and a notch test piece, wherein the notch test piece is divided into a V-shaped notch test piece (the radius of curvature of the notch root is 0.07 mm), an R2 notch test piece and an R5 notch test piece. The test piece parameters are shown in FIGS. 1 a-1 d;
the S-N curve obtained by carrying out a uniaxial fatigue test on the smooth piece is shown in figure 2;
S1: stress concentration coefficients of each type of test piece in uniaxial stretching were obtained according to elastoplastic finite element analysis, as shown in table 1:
TABLE 1
Substituting the relation:
obtaining critical tensile stress amplitude sigma a0, wherein table 2 is the critical tensile stress amplitude of each type of test piece; the following are provided:
TABLE 2
S2: substituting the functional relation:
Obtaining the areas of a notch plastic region and a critical plastic region; table 3 shows the areas of notch plastic regions and critical plastic regions corresponding to different axial stress amplitudes of test pieces of different notch types under multiaxial proportional loading and multiaxial non-proportional loading conditions, as follows:
TABLE 3 Table 3
S3: defining notch plastic region influencing factors:
Defining fatigue damage control parameters
Calculating to obtain a notch plasticity influence factor f; table 4 is the corresponding notch plastic zone impact factors for the test pieces of different notch types under multiaxial proportional and non-proportional loading, as follows:
TABLE 4 Table 4
Calculating to obtain fatigue damage control parameters; table 5 shows the maximum von Mises stress and fatigue damage control parameters of the notch root corresponding to different axial stress amplitudes under multiaxial proportional and non-proportional loading of test pieces of different notch types, as follows:
TABLE 5
S4: substituting the fatigue damage control parameter into a uniaxial fatigue S-N curve (figure 2) of the smooth piece, predicting the fatigue life of the multiaxial notch, and comparing the fatigue life with the fatigue life obtained by the test; the results are shown in FIGS. 3 a-3 c;
the method is used for estimating the multiaxial fatigue life of the notch piece, the overall trend of the predicted result is consistent with experimental data, and most of the predicted result is within three times of errors.
The present invention has been described in terms of the preferred embodiments thereof, and it should be understood by those skilled in the art that various modifications can be made without departing from the principles of the invention, and such modifications should also be considered as being within the scope of the invention.
Claims (4)
1. A multiaxial notch fatigue life prediction method based on a plastic region is characterized by comprising the following steps:
(1) Calculating the critical tensile stress amplitude based on the relation between the critical tensile stress amplitude of the plastic region of the test piece and the theoretical stress concentration coefficient in uniaxial stretching under different notches;
(2) Calculating the radius of the notch characterization plastic region under the semicircular assumption of the notch plastic region, and calculating the area of the notch plastic region and the area of the critical plastic region;
(3) Defining a notch plastic region influence factor and a fatigue damage control parameter;
(4) Substituting the fatigue damage control parameter into a single-axis fatigue S-N curve of the smooth piece, and predicting the fatigue life of the multi-axis notch;
The step (1) comprises: the relation between the critical tensile stress amplitude sigma a0 of the plastic region of the test piece and the theoretical stress concentration coefficient K T:
Wherein, K T is the theoretical stress concentration coefficient of the test piece in uniaxial tension under different notches, and the calculation formula of K T is as follows:
Wherein σ max is the maximum local elastic stress and σ 0 is the nominal stress; the theoretical stress concentration factor K T can be found experimentally, by engineering drawing, by empirical formula or by finite element analysis, and is considered to be a known quantity.
2. The method for predicting multiaxial notch fatigue life based on plastic regions of claim 1 wherein step (2) comprises:
Assuming that the plastic region of the notch is a semicircle with the root of the notch as the center and R p as the radius, the radius R p is calculated:
Wherein R p is the radius of the notch characterization plastic region, For the pull-torsion load phase difference, σ a is the axial stress amplitude;
further, the area S p of the notch plastic region is obtained:
The critical plastic region radius R p,c is:
the critical plastic area S p,c is:
3. the method for predicting multiaxial notch fatigue life based on plastic regions of claim 2 where step (3) comprises: defining a notch plastic region influence factor f by using a characterization method of the notch plastic region and the critical plastic region:
Sp＝0
Defining fatigue damage control parameters
Wherein f is a notch plastic region influencing factor; The fatigue damage control parameter is as follows; σ Mises is the maximum von Mises stress at the root of the notch, which is a known quantity.
4. The method of multi-axis notch fatigue life prediction based on plastic regions according to claim 1, wherein step (4) comprises: substituting the fatigue damage control parameter into an S-N curve analysis type of the single-shaft smooth piece, and predicting the fatigue life of the notch piece under the multiaxial load; wherein the S-N curve of the single axis slip can be obtained by experiment or looking up an engineering chart, of known magnitude.
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Citations (2)
| Publication number | Priority date | Publication date | Assignee | Title |
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| CN106596255A (en) * | 2016-11-18 | 2017-04-26 | 北京航空航天大学 | Hole-edge crack diagnosis method based on optical fiber spectrum image analysis |
| CN108627328A (en) * | 2018-05-24 | 2018-10-09 | 电子科技大学 | A kind of Notched specimen Multiaxial Fatigue Life Prediction method of coupling stress gradient modification |
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| Publication number | Priority date | Publication date | Assignee | Title |
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| CN106596255A (en) * | 2016-11-18 | 2017-04-26 | 北京航空航天大学 | Hole-edge crack diagnosis method based on optical fiber spectrum image analysis |
| CN108627328A (en) * | 2018-05-24 | 2018-10-09 | 电子科技大学 | A kind of Notched specimen Multiaxial Fatigue Life Prediction method of coupling stress gradient modification |
Non-Patent Citations (2)
| Title |
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| 一种新的多轴缺口疲劳寿命预测方法;吴文涛等;飞机设计;第35卷(第4期);第41-46页 * |
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