Method and system for measuring ultra-high cycle fatigue performance
阅读说明：本技术 一种超高周疲劳性能测定方法及测定系统 (Method and system for measuring ultra-high cycle fatigue performance ) 是由 张洪源 曹珺 廖云飞 于 2021-09-03 设计创作，主要内容包括：本发明属于力学性能测试方法领域,尤其是一种预估条件疲劳极限强度和超高周疲劳性能测定系统。上述预估条件疲劳极限强度的测定方法,包括如下步骤,以经验拉伸极限强度σ-(b)的40％～60％的作为试验载荷进行第一次疲劳试验；采用步进法升载荷继续后续疲劳试验,所述第一试样在指定寿命下至少完成两级载荷；计算预估条件疲劳极限强度。以预估条件疲劳极限强度逐级递增值或逐级递减值为加载载荷,另选取试样开始新的疲劳试验；重复所述新的疲劳试验步骤至满足配成对子的数量和闭合条件,计算指定寿命下的条件疲劳极限强度。本方法试验周期短、可靠度高、试验用料少,可以准确地获得高可靠度的材料超高疲劳性能。(The invention belongs to the field of mechanical property testing methods, and particularly relates to a system for measuring fatigue limit strength under a pre-estimation condition and ultrahigh cycle fatigue property. The method for measuring the fatigue limit strength under the estimated condition comprises the following steps of empirically measuring the tensile limit strength sigma b 40 to 60 percent of the total weight of the steel is used as a test load to carry out a first fatigue test; adopting a stepping method to increase the load to continue a subsequent fatigue test, wherein the first sample at least completes two stages of loads under the specified service life; and calculating the fatigue limit strength of the estimated condition. Selecting a sample to start a new fatigue test by taking the fatigue limit strength of the pre-estimated condition as a loading load in a step-by-step increasing or step-by-step decreasing mode; and repeating the new fatigue test steps until the number of pairs and the closing condition are met, and calculating the conditional fatigue limit strength under the specified service life. The method has the advantages of short test period, high reliability, less test material consumption and capability of accurately obtaining the ultrahigh fatigue performance of the material with high reliability.)
1. A method for determining ultra-high cycle fatigue performance comprises the following steps:
step 1: acquiring the empirical tensile ultimate strength sigma b of a first sample, performing a first fatigue test by taking 40-60% of sigma b as a test load, increasing the load by adopting a stepping method to continue a subsequent fatigue test, wherein the load increase step difference of each test is 5-10% until the first sample is damaged, and calculating the fatigue ultimate strength under the estimated condition;
step 2: obtaining an initial value after the fatigue limit strength of the estimated condition is rounded, and taking the initial value as a loading load by gradually increasing or decreasing the value step by step;
and step 3: selecting another sample to start a new fatigue test, in the new fatigue test, if the sample is damaged when the sample does not reach the specified service life, selecting another sample to reduce the first-level loading load to continue the test, and if the sample does not damage when the sample reaches the specified service life in the new fatigue test, selecting another sample to increase the first-level loading load to continue the test; matching data points with opposite loading test results of every two adjacent levels into pairs;
and 4, step 4: and (3) repeating the step (3) until the number of the matched pairs and the closing condition are met, and calculating the conditional fatigue limit strength under the specified service life according to the total number of the matched pairs, the number of the pairs under the same load and the load average value of the matched pairs.
2. The method of claim 1, wherein the fatigue property of the sample is measured,
in step 1, the first sample is subjected to at least two stages of loads under the specified service life, and the maximum stress sigma of the first sample without damage is obtained0Stress interval delta sigma, failure cycle number N under last stage stressfailThe number of cycles N corresponding to the fatigue limit to be determinedlife；
The calculation formula of the fatigue limit strength under the estimation condition is as follows:
in the formula: sigmaeEstimating the fatigue limit of the condition;
σ0the maximum stress at which no test piece failure occurred;
Δ σ is the stress interval;
Nfailthe failure cycle times under the last stage of stress;
Nlifethe cycle number corresponding to the fatigue limit required to be solved;
the number of cycles corresponding to the fatigue limit is 3 x 107Or 108Or 109。
3. The method of claim 1, wherein the fatigue property of the sample is measured,
in step 3, in the continuous test of reducing the first-stage loading load or increasing the first-stage loading load, the grade difference of reducing the first-stage loading load or increasing the first-stage loading load is 3% -5% of the estimated conditional fatigue limit strength, and the grade difference of the loading load is kept unchanged in the whole test process.
4. The method of claim 1, wherein the fatigue property of the sample is measured,
in step 3, the data points forming the pairs are effective data points, the effective data points comprise overtaking and destroying, the pair which appears for the first time comprises a first data point and a second data point, the pair which appears for the last time comprises an ith data point and an ith +1 data point, an undetermined data point of the next-stage load can be calculated according to the ith +1 data point, and if the undetermined data point is the same as the first data point in load level, a closing condition is formed.
5. The method of claim 1, wherein the fatigue property of the sample is measured,
in step 4, the calculation formula of the conditional fatigue limit strength at the specified life is as follows:
wherein n' is the total number of pairs, ni' is the number of pairs under the same load,load mean value for pairing pairs;
in the formula, σi、σi+1To make pairs of stress values.
6. The method of claim 1, wherein the fatigue property of the sample is measured,
and the number of the pairs which are matched is 3-7 pairs.
7. An ultra-high cycle fatigue performance measurement system, comprising:
the stepping method testing unit is used for acquiring the empirical tensile ultimate strength sigma b of the first sample, performing a first fatigue test by taking 40-60% of sigma b as a test load, adopting a stepping method to raise the load to continue a subsequent fatigue test, and calculating the fatigue ultimate strength under the estimated condition until the first sample is damaged, wherein the load increase step difference of each test is 5-10%;
the initial load obtaining unit obtains an initial value after the fatigue limit strength of the estimated condition is rounded, and the initial value is used as a loading load by gradually increasing or decreasing values;
the lifting method test unit is used for alternatively taking a sample to start a new fatigue test, in the new fatigue test, if the sample is damaged when the sample does not reach the specified service life, another sample is selected to reduce the first-stage loading load to continue the test, and if the sample does not reach the specified service life in the new fatigue test, the other sample is selected to increase the first-stage loading load to continue the test; matching data points with opposite loading test results of every two adjacent levels into pairs;
and the fatigue limit strength calculating unit repeatedly runs the lifting method test unit until the number of the matched pairs and the closing condition are met, and calculates the conditional fatigue limit strength under the specified service life according to the total number of the matched pairs, the number of the pairs under the same load and the load average value of the matched pairs.
8. The ultra-high cycle fatigue performance measurement system of claim 1,
in the stepping method test unit, in the process of lifting the load by adopting the stepping method, the first sample at least completes two stages of loads under the specified service life, and the maximum stress sigma of the first sample which is not damaged is obtained0Stress interval delta sigma, failure cycle number N under last stage stressfailThe number of cycles N corresponding to the fatigue limit to be determinedlife；
The calculation formula of the fatigue limit strength under the estimation condition is as follows:
in the formula: sigmaeEstimating the fatigue limit of the condition;
σ0the maximum stress at which no test piece failure occurred;
Δ σ is the stress interval;
Nfailthe failure cycle times under the last stage of stress;
Nlifethe cycle number corresponding to the fatigue limit required to be solved;
the number of cycles corresponding to the fatigue limit is 3 x 107Or 108Or 109。
9. The ultra-high cycle fatigue performance measurement system of claim 1,
in the lifting method test unit, in the continuous test of reducing the first-stage loading load or increasing the first-stage loading load, the grade difference of reducing the first stage or increasing the first stage is 3-5% of the fatigue limit strength of the estimated condition, and the grade difference of the loading load is kept unchanged in the whole test process;
the data points which are matched into pairs are effective data points, the effective data points comprise overtaking and destroying, the pairs which appear for the first time comprise a first data point and a second data point, the pairs which appear for the last time comprise an ith data point and an ith +1 data point, the data points to be determined of the next-stage load can be calculated according to the ith +1 data point, and if the load level of the data points to be determined is the same as that of the first data point, a closed condition is formed.
10. The ultra-high cycle fatigue performance measurement system of claim 1,
in the fatigue limit strength calculation unit, the calculation formula of the conditional fatigue limit strength at the specified life is as follows:
wherein n' is the total number of pairs, ni' is the number of pairs under the same load,load mean value for pairing pairs;
load mean of said pairing pairsThe calculation formula of (2) is as follows:
in the formula, σi、σi+1Stress values for pairing pairs;
the number of the pairs which are matched is 3-7 pairs.
Technical Field
The invention belongs to the field of mechanical property testing methods, and particularly relates to a method and a system for testing ultrahigh cycle fatigue property.
Background
Fatigue failure is a common failure mode of modern engineered materials. Fatigue performance of engineering material, design of engineering structural member and life prediction toolHas very important significance. Typically, the maximum number of cycles for the high cycle fatigue performance test is 107The design of fatigue is often 10 times7Fatigue life is taken as the design life. However, with the development of modern industry, under the higher pursuit of long service life and high reliability in the national important technical fields of aeroengines, high-speed rails, gas turbines and the like, the fatigue life of a plurality of key structural components is required to reach 108Or even 1010Next, the process is carried out. Particularly in the field of aeroengines, the American aviation Engine structural integrity Specification (MIL-HDBK-1783B) issued in 2002 requires that the high cycle fatigue life of all engine components should reach 109The cycle is repeated, and the non-ferrous metal alloy part 3 multiplied by 10 is also proposed in the GJB/Z101-97 aviation Engine structural integrity guide of China7The requirement of the secondary cycle life. Therefore, the safe and reliable ultrahigh cycle fatigue performance data can be efficiently obtained, and the method plays a crucial role in the development of important mechanical equipment in China.
At present, in the field of high-cycle fatigue testing, the 10 th of material is generally measured by adopting an elevating method in engineering7And (3) the fatigue strength is conditioned, 5-7 pairs of data are usually obtained by the method, the precision and the reliability are high, and the test can be generally completed within 2-3 weeks. In the 90 s of the 20 th century, considering that in the field of aviation, particularly core parts, the materials are scarce, the manufacturing cost is high, and the blanking is difficult, so that the number of the materials for testing is small, a test method named as a stepping method is provided in the American high-cycle fatigue cycle plan, and only 4-6 samples are usually needed. However, the test time of the method is slightly higher or basically equivalent to that of a lifting method, but the method has the disadvantages of larger dispersion and low reliability, cannot meet the safety requirement of engineering design, and is mainly used for research purposes.
In the field of ultra-high cycle fatigue testing, in consideration of ultra-high time cost and economic cost, a single-point method or a grouping method is generally adopted in the field of ultra-high cycle fatigue research to test 3-5 samples, but the result is high in dispersity and low in reliability. If the method of the lifting method is used for carrying out the ultra-high cycle fatigue performance test (corresponding to the service life of 3 multiplied by 10)7～1010) To obtain 109The conditional fatigue strength takes at least 200 weeks (about 3.8 years), plus in the longer life regionThe larger internal dispersion causes the fatigue strength range to be difficult to determine, more samples and machines are possibly needed, and the whole test time and the economic cost are high and cannot be implemented. Thus conventional 107The test method of the cycle life is not suitable for measuring the ultra-high cycle fatigue performance. A reliable and uniform ultrahigh cycle fatigue test method is not established in engineering.
Disclosure of Invention
The invention aims to solve the problems of more test materials, long test period, low reliability and the like in the conventional method, and provides a comprehensive method for measuring the fatigue limit strength and the ultra-high cycle fatigue performance of a material under the estimation condition based on the combination of a stepping method and a lifting method. The testing method has the advantages of short testing period, high reliability and less testing materials, can accurately obtain the ultrahigh fatigue performance data of the material with high reliability, and can solve the problem that the fatigue strength range is difficult to determine due to larger dispersity in a longer service life interval.
In order to achieve the purpose, the invention mainly provides the following technical scheme:
a method for determining ultra-high cycle fatigue performance comprises the following steps:
step 1: acquiring the empirical tensile ultimate strength sigma b of a first sample, performing a first fatigue test by taking 40-60% of sigma b as a test load, increasing the load by adopting a stepping method to continue a subsequent fatigue test, wherein the load increase step difference of each test is 5-10% until the first sample is damaged, and calculating the fatigue ultimate strength under the estimated condition;
step 2: obtaining an initial value after the fatigue limit strength of the estimated condition is rounded, and taking the initial value as a loading load by gradually increasing or decreasing the value step by step;
and step 3: selecting another sample to start a new fatigue test, in the new fatigue test, if the sample is damaged when the sample does not reach the specified service life, selecting another sample to reduce the first-level loading load to continue the test, and if the sample does not damage when the sample reaches the specified service life in the new fatigue test, selecting another sample to increase the first-level loading load to continue the test; matching data points with opposite loading test results of every two adjacent levels into pairs;
and 4, step 4: and (3) repeating the step (3) until the number of the matched pairs and the closing condition are met, and calculating the conditional fatigue limit strength under the specified service life according to the total number of the matched pairs, the number of the pairs under the same load and the load average value of the matched pairs.
In the above-mentioned ultra-high cycle fatigue performance measuring system, in step 1, the first specimen is subjected to at least two stages of loading for a predetermined life, and the maximum stress σ of the first specimen without failure is obtained0Stress interval delta sigma, failure cycle number N under last stage stressfailThe number of cycles N corresponding to the fatigue limit to be determinedlife；
The calculation formula of the fatigue limit strength under the estimation condition is as follows:
in the formula: sigmaeEstimating the fatigue limit of the condition;
σ0the maximum stress at which no test piece failure occurred;
Δ σ is the stress interval;
Nfailthe failure cycle times under the last stage of stress;
Nlifethe number of cycles corresponding to the fatigue limit to be determined.
The number of cycles corresponding to the fatigue limit is 3 x 107Or 108Or 109。
In the method for measuring the ultra-high cycle fatigue performance, in the step 3, in the continuous test of reducing the first-stage loading load or increasing the first-stage loading load, the grade difference of reducing the first-stage loading load or increasing the first-stage loading load is 3% -5% of the estimated conditional fatigue limit strength, and the grade difference of the loading load is kept unchanged in the whole test process.
In the method for measuring the ultra-high cycle fatigue performance, in step 3, the data points which are paired into pairs are effective data points, the effective data points comprise overtaking and breaking, the pairs which appear for the first time comprise a first data point and a second data point, the pairs which appear for the last time comprise an ith data point and an (i + 1) th data point, the undetermined data points of the next-stage load can be calculated according to the (i + 1) th data point, and if the undetermined data points are the same as the first data point in load level, a closing condition is formed.
In the above method for measuring ultra-high cycle fatigue performance, the formula for calculating the conditional fatigue limit strength at the specified life is:
wherein n' is the total number of pairs, ni' is the number of pairs under the same load,is the load mean of the pairing pairs.
In the method for determining the ultra-high cycle fatigue performance, the load mean value of the pair is preparedThe calculation formula of (2) is as follows:
in the formula, σi、σi+1To make pairs of stress values.
In the method for determining the ultra-high cycle fatigue performance, the number of the pairs meeting the requirement of matching is 3-7 pairs.
An ultra-high cycle fatigue performance measurement system comprising: the stepping method testing unit is used for acquiring the empirical tensile ultimate strength sigma b of the first sample, performing a first fatigue test by taking 40-60% of sigma b as a test load, adopting a stepping method to raise the load to continue a subsequent fatigue test, and calculating the fatigue ultimate strength under the estimated condition until the first sample is damaged, wherein the load increase step difference of each test is 5-10%; the initial load obtaining unit obtains an initial value after the fatigue limit strength of the estimated condition is rounded, and the initial value is used as a loading load by gradually increasing or decreasing values; the lifting method test unit is used for alternatively taking a sample to start a new fatigue test, in the new fatigue test, if the sample is damaged when the sample does not reach the specified service life, another sample is selected to reduce the first-stage loading load to continue the test, and if the sample does not reach the specified service life in the new fatigue test, the other sample is selected to increase the first-stage loading load to continue the test; matching data points with opposite loading test results of every two adjacent levels into pairs; and the fatigue limit strength calculating unit repeatedly runs the lifting method test unit until the number of the matched pairs and the closing condition are met, and calculates the conditional fatigue limit strength under the specified service life according to the total number of the matched pairs, the number of the pairs under the same load and the load average value of the matched pairs.
In the stepping method test unit, in the process of lifting the load by adopting the stepping method, the first sample at least completes two stages of loads under the specified service life, and the maximum stress sigma of the first sample which is not damaged is obtained0Stress interval delta sigma, failure cycle number N under last stage stressfailThe number of cycles N corresponding to the fatigue limit to be determinedlife；
The calculation formula of the fatigue limit strength under the estimation condition is as follows:
in the formula: sigmaeEstimating the fatigue limit of the condition;
σ0the maximum stress at which no test piece failure occurred;
Δ σ is the stress interval;
Nfailthe failure cycle times under the last stage of stress;
Nlifethe cycle number corresponding to the fatigue limit required to be solved;
the number of cycles corresponding to the fatigue limit is 3 x 107Or 108Or 109。
In the lifting method test unit, in the continuous test of reducing the first-stage loading load or increasing the first-stage loading load, the grade difference of reducing the first stage or increasing the first stage is 3-5% of the fatigue limit strength of the estimated condition, and the grade difference of the loading load is kept unchanged in the whole test process;
the data points which are matched into pairs are effective data points, the effective data points comprise overtaking and destroying, the pairs which appear for the first time comprise a first data point and a second data point, the pairs which appear for the last time comprise an ith data point and an ith +1 data point, the data points to be determined of the next-stage load can be calculated according to the ith +1 data point, and if the load level of the data points to be determined is the same as that of the first data point, a closed condition is formed.
In the fatigue limit strength calculation unit, the calculation formula of the conditional fatigue limit strength at the specified life is as follows:
wherein n' is the total number of pairs, ni' is the number of pairs under the same load,load mean value for pairing pairs;
load mean of said pairing pairsThe calculation formula of (2) is as follows:
in the formula, σi、σi+1Stress values for pairing pairs;
the number of the pairs which are matched is 3-7 pairs.
By the technical scheme, the invention at least has the following advantages:
1) the method for measuring the ultra-high cycle fatigue performance provided by the invention preliminarily determines the fatigue strength range of the material to be measured under the specified service life by continuously lifting one sample, solves the problem that the fatigue limit strength of the fatigue estimation condition is difficult to determine due to the large dispersion of the fatigue strength result in a longer service life interval in the ultra-long fatigue life test, reduces the time and economic cost of the early stress exploration phase, and obtains estimation through a lifting method.
2) The method for measuring the ultrahigh-cycle fatigue performance combines a stepping method and a lifting method, has short test period, high reliability and less test materials, and can accurately obtain the ultrahigh-cycle fatigue performance of the material with high reliability.
The foregoing description is only an overview of the technical solutions of the present invention, and in order to make the technical solutions of the present invention more clearly understood and to implement them in accordance with the contents of the description, the following detailed description is given with reference to the preferred embodiments of the present invention and the accompanying drawings.
Drawings
FIG. 1 is a schematic view of a fatigue specimen of the present invention;
FIG. 2 is an example of a fatigue lift map of the present invention;
fig. 3 is another fatigue lift map example of the present invention.
Detailed Description
To further explain the technical means and effects of the present invention adopted to achieve the predetermined object, the following detailed description of the embodiments, structures, features and effects according to the present invention will be made with reference to the accompanying drawings and preferred embodiments.
The embodiment provides a method for measuring ultra-high cycle fatigue performance, which comprises the following steps: step 1, acquiring the empirical tensile ultimate strength sigma b of a first sample, performing a first fatigue test by taking 40-60% of sigma b as a test load, increasing the load by adopting a stepping method to continue a subsequent fatigue test, and calculating the fatigue ultimate strength under the estimated condition until the first sample is damaged, wherein the load increase step difference of each test is 5-10%; step 2, obtaining an initial value after the fatigue limit strength of the estimated condition is rounded, and taking the initial value as a loading load by gradually increasing or decreasing the value step by step; step 3, alternatively taking a sample to start a new fatigue test, in the new fatigue test, if the sample is damaged when the sample does not reach the specified service life, selecting another sample to reduce the first-stage loading load to continue the test, and if the sample does not damage when the sample reaches the specified service life in the new fatigue test, selecting another sample to increase the first-stage loading load to continue the test; matching data points with opposite loading test results of every two adjacent levels into pairs; and 4, repeating the step 3 until the number of the pairs and the closing condition are met, and calculating the conditional fatigue limit strength under the specified service life according to the total number of the pairs, the number of the pairs under the same load and the load average value of the pairs.
In this embodiment, the estimated conditional fatigue limit strength obtained in step 1 can be very close to the conditional fatigue limit strength of the sample to be tested in the specified life, and the estimated conditional fatigue limit strength is in the set region of the conditional fatigue limit strength in the specified life, so that it can be ensured that the loading load after adding one level or reducing one level is in the set range in a new fatigue test, so that the data is more accurate.
In the lifting method, the test is stopped when the specified service life is reached no matter whether the sample is damaged or not, namely, the fatigue limit strength can be repeatedly verified, and finally, the reliable conditional fatigue limit strength under the specified service life is obtained.
An ultra-high cycle fatigue performance measurement system comprising: the stepping method testing unit is used for acquiring the empirical tensile ultimate strength sigma b of the first sample, performing a first fatigue test by taking 40-60% of sigma b as a test load, adopting a stepping method to raise the load to continue a subsequent fatigue test, and calculating the fatigue ultimate strength under the estimated condition until the first sample is damaged, wherein the load increase step difference of each test is 5-10%; the initial load obtaining unit obtains an initial value after the fatigue limit strength of the estimated condition is rounded, and the initial value is used as a loading load by gradually increasing or decreasing values; the lifting method test unit is used for alternatively taking a sample to start a new fatigue test, in the new fatigue test, if the sample is damaged when the sample does not reach the specified service life, another sample is selected to reduce the first-stage loading load to continue the test, and if the sample does not reach the specified service life in the new fatigue test, the other sample is selected to increase the first-stage loading load to continue the test; matching data points with opposite loading test results of every two adjacent levels into pairs; and the fatigue limit strength calculating unit repeatedly runs the lifting method test unit until the number of the matched pairs and the closing condition are met, and calculates the conditional fatigue limit strength under the specified service life according to the total number of the matched pairs, the number of the pairs under the same load and the load average value of the matched pairs.
The ultrahigh cycle fatigue performance measuring system can obtain the estimated condition fatigue limit strength through the stepping method test unit, and the estimated condition fatigue limit strength is very close to the condition fatigue limit strength under the specified service life, so that a sample to be tested can be subjected to repeated verification experiments in a set area of the condition fatigue limit strength under the specified service life in the test process of the lifting method test unit, and the condition fatigue limit strength under the reliable specified service life can be obtained through a plurality of times of fatigue experiments. The test system can greatly improve the test efficiency and reduce the labor cost of the test.
Preferably, in step 1, the first sample is subjected to at least two-stage loading under a specified life, and the maximum stress sigma of the first sample without damage is obtained0Stress interval delta sigma, failure cycle number N under last stage stressfailThe number of cycles N corresponding to the fatigue limit to be determinedlife；
The calculation formula of the fatigue limit strength under the estimation condition is as follows:
in the formula: sigmaeEstimating the fatigue limit of the condition;
σ0the maximum stress at which no test piece failure occurred;
Δ σ is the stress interval;
Nfailthe failure cycle times under the last stage of stress;
Nlifethe number of cycles corresponding to the fatigue limit to be determined.
Preferably, the fatigue limit corresponds to a cycle number of 3 × 107Or 108Or 109。
In step 3, in the continuous test of reducing the first-stage loading load or increasing the first-stage loading load, the grade difference of reducing the first-stage loading load or increasing the first-stage loading load is 3% -5% of the estimated conditional fatigue limit strength, and the grade difference of the loading load is kept unchanged in the whole test process.
In step 3, the data points forming the pairs are effective data points, the effective data points comprise overtaking and destroying, the pair which appears for the first time comprises a first data point and a second data point, the pair which appears for the last time comprises an ith data point and an ith +1 data point, an undetermined data point of the next-stage load can be calculated according to the ith +1 data point, and if the undetermined data point is the same as the first data point in load level, a closing condition is formed.
The calculation formula of the conditional fatigue limit strength under the specified service life is as follows:
wherein n' is the total number of pairs, ni' is the number of pairs under the same load,is the load mean of the pairing pairs.
Load mean of said pairing pairsThe calculation formula of (2) is as follows:
in the formula, σi、σi+1To make pairs of stress values.
The number of the pairs which are matched is 3-7 pairs.
Example 1
The schematic structure of the sample according to this example is shown in FIG. 1.
In the embodiment, a titanium alloy which is an aircraft engine material is taken as a test material, and the titanium alloy material has the tensile ultimate strength of 993MPa at room temperature according to experience, and the stress ratio of 10 of 0.1 at room temperature is measured8Secondary condition fatigue limit strength. The specific method comprises the following steps:
the titanium alloy material is processed into M14 multiplied by 70(mm) bar-shaped test pieces, and the schematic diagram of the test pieces is shown in figure 1. After the completion of the mounting of the specimens, a fatigue test (993MPa × 40.28%) was carried out by applying a maximum stress of 400MPa to a first specimen of the titanium alloy material according to the method for measuring the fatigue limit strength under the estimated conditions described in example 1, and the test of the fatigue limit strength under the estimated conditions was completed with 20MPa (maximum stress 400MPa × 5% ═ 20MPa) as the load-lifting interval, and the test results are shown in table 1.
TABLE 1 fatigue test results of smooth specimen stepping method for certain titanium alloy with stress ratio of 0.1 at room temperature
According to the method for determining the fatigue limit strength under the estimation condition, the fatigue strength range of the material to be measured under the specified service life is preliminarily determined by continuously increasing the load of a sample, the problem that the fatigue strength range is difficult to determine in the ultra-long fatigue life test is solved, and the time and the economic cost in the early stress exploration stage are reduced.
Specifically, in the new fatigue test, the following steps are included:
step 2.1: starting the test by using a second sample, and observing the test result if the second sample is under the condition that the initial value is the loading load;
step 2.2 includes two results:
if the result of the step 2.1 is that the second sample is damaged before reaching the specified service life (namely the result is damaged), a third sample is selected to reduce the first-stage loading load to continue the test, and the test result is observed;
the result 2 is that if the result of the step 2.1 is that the second sample does not damage under the loading load until the specified service life is reached (namely the result is an overtopping result), a third sample is selected to increase the first-stage loading load for continuous test, and the test result is observed;
step 2.3 is also illustrated in two ways, further illustrated below by taking the result 1 of step 2.2 as an example:
mode 1: in the result 1 of the step 2.2, if the third sample is damaged before reaching the specified service life, a fourth sample is selected to reduce the first-level loading load for continuous test, and the test result is observed;
mode 2: in the result 1 of the step 2.2, if the third sample does not damage under the loading load until the specified service life is reached, selecting a fourth sample to increase the first-stage loading load (namely the loading load is the same as that of the second sample in the step 1) to continue the test, and observing the test result;
step 2.4: repeating the step 2.3, and if the test sample reaches the specified service life under the loading load and is not damaged, namely the test sample exceeds the specified service life, selecting an alternative test sample, increasing the first-stage loading load and continuing the test; and matching the data points with opposite test results of every two adjacent loading loads into pairs until the number of the matched pairs and the closing condition are met, and calculating the conditional fatigue limit strength under the specified service life according to the total number of the matched pairs, the number of the pairs under the same load and the load average value of the matched pairs.
That is, in this step, if no damage occurs at the specified life under the load, the test is continued with increasing the primary load in the next step; on the contrary, in this step, if the failure occurs without reaching the specified life under the load, the primary load is lowered in the next step to continue the test.
In the continuous test of reducing the first-stage loading load or increasing the first-stage loading load, the grade difference of reducing the first-stage loading load or increasing the first-stage loading load is 3% -5% of the fatigue limit strength of the estimated condition, and the grade difference of the loading load is kept unchanged in the whole test process.
The data points which are matched into pairs are effective data points, the effective data points comprise overtaking and destroying, the pairs which appear for the first time comprise a first data point and a second data point, the pairs which appear for the last time comprise an ith data point and an ith +1 data point, the data points to be determined of the next-stage load can be calculated according to the ith +1 data point, and if the load level of the data points to be determined is the same as that of the first data point, a closed condition is formed.
Wherein the first data point and the ith data point are the data points which are firstly made, and the second data point and the (i + 1) th data point are the data points which are then made. The undetermined data point is a data point to be made after the i +1 th data point, and whether the sample in the undetermined data point is damaged or not is irrelevant to the experiment as long as the load of the undetermined data point is the same as the load of the first data point.
The ultrahigh cycle fatigue performance measuring system disclosed in the embodiment provides a comprehensive method for measuring the ultrahigh cycle fatigue performance of a material based on the combination of a stepping method and a lifting method, the method firstly uses a sample to continuously lift and load to preliminarily determine the fatigue strength range (namely the estimated condition fatigue limit) of the material to be measured under the specified service life, then uses a plurality of samples to repeatedly test in the specified strength range near the estimated condition fatigue limit, and finally obtains ultrahigh cycle fatigue performance data of the material to be measured with certain reliability.
In the embodiment, the test is stopped after the specified service life is reached, namely, the test is stopped as long as the specified service life is reached regardless of whether the sample is broken, so that the influence on the overall test result due to the special change of a certain sample material is avoided; in addition, the test is stopped when the specified service life is reached, the test is not required to be stopped after the fracture is reached, and the test time can be saved, so that the test period in the conventional method is long.
In the embodiment, the number of the pairs to be matched is set according to the number of the samples or the test requirement, and the reliability of the test can be ensured when the pairs to be matched are 3 pairs, so that the problem of low reliability caused by a special material in a conventional test method is solved.
As shown in figure 2, when the method meets the requirement of matching three pairs, the method can obtain the reliable ultrahigh-cycle fatigue strength by only 6 samples, and compared with the traditional method, the method can reduce the number of the required samples by two thirds, and the method can reduce the fatigue strength by at least 40% when being used in a testing machine.
Fig. 3 is a lifting diagram obtained in another embodiment of the method, and in fig. 3, 5 pairs are satisfied.
The calculation formula of the conditional fatigue limit strength under the specified service life is as follows:
wherein n' is the total number of pairs, ni' is the number of pairs under the same load,is the load mean of the pairing pairs.
Load mean of said pairing pairsThe calculation formula of (2) is as follows:
in the formula, σi、σi+1To make pairs of stress values.
The number of the pairs which are matched is 3-7 pairs.
And determining the number of pairs to be matched in the test according to the test reliability requirement, and stopping the test when the number of pairs to be matched and the closing condition are met in the test.
According to the test result of the fatigue limit strength under the estimation condition, the fatigue limit strength under the estimation condition is preliminarily determined to be 445 MPa.
Then, in the ultra-high cycle fatigue performance measurement system, a test was first performed with a load slightly greater than the estimated fatigue limit. The fatigue test is carried out by selecting 450MPa as the maximum stress, and the lifting method test is completed by using 10MPa (450 multiplied by 3 percent integer, namely 10MPa) as the stress level difference, the test result is shown in table 2, and the lifting graph is shown in table 2 and table 3.
TABLE 2 fatigue test results of smooth specimen stepping method for certain titanium alloy with stress ratio of 0.1 at room temperature
Sample number
Maximum stress/MPa
Life/time
A03
440
>1×108
A07
440
>1×108
A02
450
16369300
A04
450
>1×108
A06
450
42468100
A05
460
31808300
TABLE 3 evaluation chart of the titanium alloy room temperature, stress ratio 0.1 smooth specimen lifting method fatigue test
According to the test result of the lifting method, the titanium alloy is determined to be 10 under the conditions of room temperature and stress ratio of 0.18The fatigue limit strength under the second condition was 448MPa, and the calculation results are shown in Table 4.
TABLE 4 fatigue limit of certain titanium alloy by lifting method for smooth specimen with stress ratio of 0.1 at room temperature (1X 10)8) Calculated value
Calculating the subsample standard deviation S and the coefficient of variation Cv of the conditional fatigue limit according to the test result, determining that 0.013 is less than 0.0201 according to a table 5, and the fatigue limit data meets the 95% confidence requirement.
Table 5 minimum test piece number criterion (confidence α 95%, relative error δ 5%)
Minimum number of test pieces n
Range of coefficient of variation
3
<0.0201
4
0.0201～0.0314
5
0.0314～0.0403
6
0.0403～0.0476
7
0.0476～0.0541
The test example is used for testing the ultrahigh-cycle fatigue performance of a certain titanium alloy material of an aeroengine, and 10 of the material is preliminarily determined by a stepping method8The next condition fatigue limit strength range, then using lifting method to accurately obtain 10 of material8The test method has short test period, high reliability, less test material consumption, and high yieldAnd (4) ultra-high fatigue performance data of the material with reliability.
The ultra-high cycle fatigue performance measuring system disclosed in the embodiment has the following advantages:
compared with the conventional method, the test method combining the stepping method and the lifting method has the advantages of short test period, high reliability, less test material consumption and capability of accurately obtaining the ultrahigh fatigue performance of the material with high reliability.
Example 2
The tensile ultimate strength of a certain titanium alloy is 906MPa, the tensile ultimate strength is 65 percent according to experience and is taken as the first-stage load (maximum stress 590MPa) of the lifting method by taking the whole, and the principle test data according to the lifting method are as follows
The same titanium alloy material is taken as the first-stage load (maximum stress 450MPa) of the stepping method by taking the tensile ultimate strength of 50% according to experience and rounding, and the data of the stepping method are as follows:
TABLE 6 stepping method test data of certain titanium alloy under room temperature and R-1 condition
Maximum stress/MPa
Life/cycle
First stage
450
＞3×107
Second stage
470
＞3×107
Third stage
490
12597700
The fatigue limit estimated by the stepping method is 478MPa, the maximum stress 490MPa is selected as the first-stage load of the lifting method test, and the test data is as follows:
TABLE 7 fatigue limit elevation graph of certain titanium alloys under room temperature and R-1 conditions
TABLE 8 fatigue test data of certain titanium alloys under the conditions of room temperature and R-1
Maximum stress/MPa
Life/cycle
510
10895400，608100
490
23146400，＞3×107，＞3×107
470
＞3×107
Finally, the method of the invention is used for obtaining 3 x 10 of the material7The fatigue limit is 493 MPa.
Comparative example
Comparison of booksExample the difference from example 2 is that 3X 10 is accomplished by conventional lifting method7Fatigue limit data.
The tensile ultimate strength of a certain titanium alloy is 906MPa, the tensile ultimate strength is 65% according to experience and is taken as the first-stage load (maximum stress 590MPa) of the lifting method in an integral mode, and the principle test data according to the lifting method are as follows:
TABLE 9 fatigue limit elevation graph of certain titanium alloys under room temperature and R-1 conditions
TABLE 10 fatigue test data of certain titanium alloys under the conditions of room temperature and R-1
Maximum stress/MPa
Life/cycle
590
175000
570
5393700，464800
550
15354527，4768464，＞3×107
530
27036946，＞3×107
510
3966589，2690146，6427800，10895400
490
175795，＞3×107，＞3×107，＞3×107，＞3×107，
470
＞3×107，
Finally, the 3X 10 of the material is obtained by using a lifting method7The fatigue limit is 496 MPa.
Comparing example 2 with the comparative example, it can be seen that:
1) the fatigue limit finally obtained by the two methods is almost not different, which shows that the accuracy and the reliability of the method in the invention are equivalent to those of the traditional method.
2) The test was carried out by the lifting method in the comparative example using 18 specimens for about 800 hours (machine-clean time) to complete 3X 107Ultra-high cycle fatigue test.
3) The test method of the present invention was completed in about 550 hours (machine-finished) with 7 specimens7Ultra-high cycle fatigue test.
4) Compared with the traditional lifting method, the method of the invention has the advantages that the material consumption is reduced by 157 percent, and the time consumption is reduced by about 45 percent.
The above description is only a preferred embodiment of the present invention, and is not intended to limit the present invention in any way, and any simple modification, equivalent change and modification made to the above embodiment according to the technical spirit of the present invention are still within the scope of the technical solution of the present invention.
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