CN108572115B - Test method for testing fatigue strength of material - Google Patents
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Abstract
The invention relates to a test method for testing the fatigue strength of a material, which is used for efficiently measuring the fatigue strength of the material through a parallel test. The invention divides a large batch of test pieces into a plurality of groups, completes the test on different testing machines in parallel and generatesMultiple purposeAnd carrying out graphic analysis and data processing on the stress value lifting graph of the group of samples to obtain a synthesized stress value lifting graph, and then calculating the mean value and the variance of the fatigue strength according to a conventional method. When a plurality of groups of parallel tests are carried out, a large amount of time cost is saved, only few effective data are wasted, the method is practical and feasible, and the method can be used for the fatigue strength test of metal materials when the number of samples is large.
Description
The application has the following application numbers: 201510263521.0 entitled fatigue testing Material
Test methods for Strength ", filing date: divisional application of the invention patent application on 5/21/2015.
Technical Field
The invention relates to the field of fatigue strength limit data processing of measured materials, in particular to a parallel lifting test method for fatigue strength testing.
Background
The fatigue strength of a material at a given life is often not directly measurable. Since the development of the method for measuring fatigue strength has been proposed, the method is widely used for measuring random characteristics of fatigue strength of materials, such as widely used metallic materials. The conventional lift test method generally adopts the following procedures:
(1) the fatigue strength of the test piece at the predetermined life is estimated. The initial stress level and grade difference of the test were determined. The initial stress level is usually determined by reference to existing relevant test data or pre-test data, and the step difference is taken to be 5% of the estimated fatigue strength.
(2) Fatigue life tests were conducted at the initial stress level.
(3) If the test piece fails fatigue before the predetermined life. At this stress level, denoted "failure", the next test piece was tested for fatigue life at a stress level one step lower. If the test piece reaches the preset service life and is not damaged, the test of the test piece is stopped, the test is recorded as overflow under the stress level, the next test piece is subjected to the fatigue life test under the stress level higher by one level, and the test of each test piece depends on the test result of the previous test piece. And (4) raising or lowering the stress level of the test according to the rule, and repeating the previous process until the number of the test pieces meets the requirement of the minimum number of the test pieces and the requirement of closing the lifting chart. And the stress differences at each stage are kept constant in the test. The key for measuring the fatigue strength by adopting the lifting method lies in the stress increment
And (4) selecting. In general, the stress increment is preferably selected so that the test is conducted at a stress level of class 4. When the fatigue strength measured by the conventional fatigue test method is known, a fatigue strength within 5% can be taken as the stress increment.
A complete lifting diagram can be obtained through the process. The loading stress value of the next sample must be referred to whether the previous sample is 'broken' or 'overflowed', and the experiment is generally carried out continuously by a single machine. Therefore, the traditional lifting test method is long in time consumption and very low in efficiency when the sample size is large.
Disclosure of Invention
The invention aims to provide a test method for testing the fatigue strength of a material, which is short in time consumption and high in efficiency.
The technical scheme for realizing the aim of the invention is to provide a test method for testing the fatigue strength of a material, which comprises the following steps:
dividing the samples according to the test intoMThe number of the groups is set to be,M≥2；
② group 1 samples, the number of cycle bases selected
Stress ratiorSelecting a stress value close to the fatigue limit estimated by the material with reference to the material property to test the 1 st test piece; if the number of cycles reaches the selection base number
Before failure, part 2 was tested at a first lower stress level, if the selection base number is still reached at the cycle number
Before the failure occurs, the first-order stress level is reduced to carry out the 3 rd test, and if the selection base number is passed
If the cycle is not broken, stopping the test of the 3 rd part, and increasing the primary stress level, namely, performing the 4 th test under the stress level of the 2 nd part; by analogy, a stress value lifting graph can be obtained, and the lifting graph meets the closing condition;
(iii) the restM- Group 1 samples the test was started from the effective stress values obtained in group 1, the same cycle base was chosen
Stress ratiorRepeating the operation of the first step to obtain a stress value lifting graph;
to the generation ofMAnd carrying out graphic analysis and data processing on the stress value lifting graph of the group of samples to obtain a synthesized stress value lifting graph, and then calculating the mean value and the variance of the fatigue strength according to a conventional method.
Further, in step (iv), on the productMThe stress value elevation graph of the group of samples is subjected to graphic analysis and data processing, and the specific process of obtaining the synthesized stress value elevation graph is divided into 2 conditions: (1) the two groups of stress value lifting graphs are closed, and the respective first effective stress values are on the same stress level; at the moment, the synthesis of the stress value lifting graph can be completed only by removing the last data in the previous group of data and connecting the last data with the second group of effective data; (2) the two groups of stress value lifting graphs are closed, and the respective first effective stress values are not on the same stress level; and at the moment, the effective data of the second group is connected to a stress value position on the first group, which is at the same level with the first effective stress value of the second group, and the stress value of the position in the first group is cut off, so that the synthesis of the stress value lifting graph is completed.
Further, in the step iv, the specific method for calculating the mean value and the variance of the fatigue strength is as follows: the mean value of the endurance fatigue strength limit is found using the following formula:
in the formula (I), the compound is shown in the specification,
the lowest stress level used;dis the stress increment;N-the number of successes or failures in the test piece, whichever is smaller;
in the formulaiIs the serial number of the stress level, the serial number of the lowest stress level is 0;
the number of successes or failures that occur at the corresponding stress level depends on which event is less;Hthe number of the highest stress level; the "+" sign in the formula is used when there are fewer success events, and the "-" sign is used when there are fewer failed test pieces;
the standard deviation of the endurance fatigue strength limit distribution is estimated as follows:
in the formula (I), the compound is shown in the specification,
。
further, in step (II), the reference material property is obtained by referenceS- NThe curves are empirical in fatigue strength values to select stress values close to the estimated fatigue limit of the material. Further, stress values within ± 5% of the estimated fatigue limit of the material are selected.
The invention has the positive effects that: according to the test method for testing the fatigue strength of the material, a large number of test pieces are divided into a plurality of groups, and tests are completed on different testing machines in parallel. The multi-group test piece parallel test saves a large amount of time and cost, only wastes few effective data, effectively reduces the generation of random errors of the test, is practical and feasible, and can be used for the fatigue strength test of metal materials when the number of samples is large.
Drawings
Fig. 1 is two sets of data lift graphs, namely A, B two sets of test data lift graphs, obtained by the invention applied to practical tests and obtaining the first equivalent effective stress value.
Fig. 2 is a lift diagram obtained by synthesizing two sets of data in fig. 1, namely A, B two sets of synthesized data lift diagrams, according to the present invention applied to practical experiments.
Fig. 3 is two sets of data lift graphs, namely C, D two sets of test data lift graphs, obtained by applying the invention to practical tests and obtaining the first unequal effective stress values.
Fig. 4 is a lift diagram obtained by synthesizing the two sets of data in fig. 3, namely C, D two sets of synthesized data lift diagrams, according to the present invention applied to practical experiments.
Detailed Description
(example 1)
The test method for testing the fatigue strength of the material comprises the following steps:
dividing the samples according to the test intoMThe number of the groups is set to be,M≥2。
② 1 group of samples, selecting cycle base number and stress ratiorReference to material properties (e.g. reference toS- NEmpirical fatigue strength values on curves)) a stress value close to the material's estimated fatigue limit was selected for testing of the 1 st test piece, in this example a stress value within ± 5% of the material's estimated fatigue limit was selected. If the number of cycles reaches the selection base number
Before failure, part 2 was tested at a first lower stress level, if the selection base number is still reached at the cycle number
Before the failure occurs, the first-order stress level is reduced to carry out the 3 rd test, and if the selection base number is passed
The cycle did not fail and the test was stopped for part 3 and the primary stress level was increased, i.e. the 4 th test was run at the stress level of part 2. By analogy, a stress value lifting graph can be obtained, and the lifting graph meets the closing condition.
(iii) the restM- Group 1 samples the test was started from the effective stress values obtained in group 1, the same cycle base was chosen
Stress ratiorAnd repeating the operation of the step I to obtain a stress value lifting graph.
To the generation ofMAnd carrying out graphic analysis and data processing on the stress value lifting graph of the group of samples to obtain a synthesized stress value lifting graph, and then calculating the mean value and the variance of the fatigue strength according to a conventional method.
The specific process of obtaining the synthesized stress value lifting graph is divided into 2 cases: (1) the two groups of stress value lifting graphs are closed, and the respective first effective stress values are on the same stress level. At the moment, the synthesis of the stress value lifting graph can be completed only by removing the last data in the previous group of data and connecting the last data with the second group of effective data; (2) the two groups of stress value lifting graphs are closed, and the respective first effective stress values are not on the same stress level. And at the moment, the effective data of the second group is connected to a stress value position on the first group, which is at the same level with the first effective stress value of the second group, and the stress value of the position in the first group is cut off, so that the synthesis of the stress value lifting graph is completed.
The present invention will be described in further detail with reference to the accompanying drawings. In the attached drawings of the specification "
"represents" destruction "and" O "represents" overflow ".
In fig. 1, the group a test pieces 1 and 2 have opposite results, that is, effective data appears at the beginning of the experiment until the test piece 13 is completed, and a closed stress value lifting graph is formed. The opposite result occurs for the group B test pieces 14 and 15, i.e. valid data starts to appear until the test piece 24 is completed, and a closed stress value lifting graph is formed. At this time, the first effective stress values of the two sets (the loading stresses of test piece 1 and test piece 14) are equal.
The data of the test piece 13 is discarded, and simultaneously the test piece 14 and the subsequent data are connected to the position, so that a stress value lifting graph (shown in figure 2) formed by synthesizing two sets of data A and B is obtained.
In fig. 3, the test pieces 1 and 2 in group C have opposite results, that is, effective data appears at the beginning of the experiment until the test piece 13 is completed, and a closed stress value lifting graph is formed. And D, the test piece numbers 15 and 16 have opposite results, namely valid data begin to appear until the test piece 25 is completed, and a closed stress value lifting graph is formed. The same result occurs for test piece numbers 14 and 15, and the data for test piece 14 is discarded due to the invalidity. At this time, the first effective stress values of the two sets (the loading stresses of test piece 1 and test piece 15) are not equal.
Fig. 4 is a stress value lifting graph after the two sets of data of C and D are synthesized. And finding a proper position in the lifting graph of the group C, wherein the stress value corresponding to the position is the same as that of the first effective test piece 15 of the group D, namely the position of the test piece 2, deleting the data of the position, and inserting the data of the test piece 15 of the group D and the subsequent data to form a closed stress value lifting graph.
In determining the endurance fatigue limit distribution, it is assumed to be a normal distribution. The mean value of the endurance fatigue strength limit is found from the stress value lifting graph synthesized in fig. 2:
in the formula (I), the compound is shown in the specification,
the lowest stress level used;dis the stress increment;N-the number of successes or failures in the test piece, whichever is smaller.
In the formulaiIs the serial number of the stress level, the serial number of the lowest stress level is 0;
the number of successes or failures that occur at the corresponding stress level depends on which event is less;Hthe highest stress level is numbered. The "+" sign in the formula is used for less success events and the "-" sign is used for less failed test pieces.
The standard deviation of the endurance fatigue strength limit distribution is estimated as follows:
in the formula (I), the compound is shown in the specification,
。
table 1 based on the data of fig. 2:
TABLE 1 ultimate distribution parameters of permanent strength in a Normal distribution
Mean value:
，
standard deviation:
。
table 2 based on the data of fig. 4:
TABLE 2 ultimate distribution parameters of the permanent strength in a normal distribution
Mean value:
，
standard deviation:
。
in the conventional lifting method, the stress value of the latter test piece must be set according to the stress value of the former test piece, and the test must be continuously carried out. This wastes a lot of time, resulting in inefficient operation. Compared with the traditional lifting method, the parallel lifting test method for the fatigue strength test can reduce the generation of random errors of the test and save a large amount of time.
It should be understood that the above examples are only for clearly illustrating the present invention and are not intended to limit the embodiments of the present invention. Other variations and modifications will be apparent to persons skilled in the art in light of the above description. And are neither required nor exhaustive of all embodiments. And such obvious variations or modifications which fall within the spirit of the invention are intended to be covered by the scope of the present invention.
Claims (2)
1. A test method for testing fatigue strength of a material, characterized by comprising the steps of:
dividing the samples according to the test intoMThe number of the groups is set to be,M≥2；
② group 1 samples, the number of cycle bases selected
Stress ratiorSelecting a stress value close to the fatigue limit estimated by the material with reference to the material property to test the 1 st test piece; if the number of cycles reaches the selection base number
Before failure, part 2 was tested at a first lower stress level, if the selection base number is still reached at the cycle number
Before the failure occurs, the first-order stress level is reduced to carry out the 3 rd test, and if the selection base number is passed
If the cycle is not broken, stopping the test of the 3 rd part, and increasing the primary stress level, namely, performing the 4 th test under the stress level of the 2 nd part; by analogy, a stress value lifting graph can be obtained, and the lifting graph meets the closing condition;
(iii) the restM-1 group of samples the effective stress values obtained from the 1 st group are tested in parallel on different testing machines, and the same cycle base number is selected
Stress ratiorRepeating the operation of the step II to obtain a stress value lifting graph;
to the generation ofMCarrying out graphic analysis and data processing on the stress value lifting graph of the group sample to obtain the synthesized stress value lifting graphLowering the graph, and then solving the mean value and the variance of the fatigue strength according to a conventional method; to what is generatedMThe stress value elevation graph of the group of samples is subjected to graphic analysis and data processing, and the specific process of obtaining the synthesized stress value elevation graph is divided into 2 conditions: (1) the two groups of stress value lifting graphs are closed, and the respective first effective stress values are on the same stress level; at the moment, the synthesis of the stress value lifting graph can be completed only by removing the last data in the previous group of data and connecting the last data with the second group of effective data; (2) the two groups of stress value lifting graphs are closed, and the respective first effective stress values are not on the same stress level; and at the moment, the effective data of the second group is connected to a stress value position on the first group, which is at the same level with the first effective stress value of the second group, and the stress value of the position in the first group is cut off, so that the synthesis of the stress value lifting graph is completed.
2. A test method for testing the fatigue strength of a material according to claim 1, characterized in that: in the second step, the reference material property is obtained by referenceS- NThe curves are empirical in fatigue strength values to select stress values close to the estimated fatigue limit of the material.
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| CN106092787A (en) * | 2016-08-23 | 2016-11-09 | 中国航空工业集团公司西安飞机设计研究所 | A kind of Metal Material Fatigue curve characterizing method |
| CN111413492A (en) * | 2020-04-30 | 2020-07-14 | 山东大学 | Method and system for detecting novel coronavirus COVID-2019 pneumonia |
| CN111855446B (en) * | 2020-07-14 | 2023-07-04 | 天津钢管制造有限公司 | Method for predicting fatigue limit of titanium alloy |
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| CN108562504A (en) | 2018-09-21 |
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| CN108562504B (en) | 2020-10-09 |
| CN104833600A (en) | 2015-08-12 |
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