CN111693387A - Method for determining minimum number of rapid fatigue tests - Google Patents
Method for determining minimum number of rapid fatigue tests Download PDFInfo
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- CN111693387A CN111693387A CN202010541222.XA CN202010541222A CN111693387A CN 111693387 A CN111693387 A CN 111693387A CN 202010541222 A CN202010541222 A CN 202010541222A CN 111693387 A CN111693387 A CN 111693387A
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- 238000009661 fatigue test Methods 0.000 title claims abstract description 32
- 238000000034 method Methods 0.000 title claims abstract description 27
- 238000001228 spectrum Methods 0.000 claims abstract description 33
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- 230000007774 longterm Effects 0.000 abstract description 6
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
- G01N3/32—Investigating strength properties of solid materials by application of mechanical stress by applying repeated or pulsating forces
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
- G01N2203/0001—Type of application of the stress
- G01N2203/0005—Repeated or cyclic
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
- G01N2203/0058—Kind of property studied
- G01N2203/0069—Fatigue, creep, strain-stress relations or elastic constants
- G01N2203/0073—Fatigue
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Abstract
The invention provides a method for determining the minimum times of rapid fatigue tests, which comprises the following steps: determining the reliability R of the specified service life of the part, selecting a logarithmic value lg (sigma) of the load sigma and the fatigue life N corresponding to the reliability RRLogarithmic value lg (N)R) And a double-logarithmic straight line is formed between the two, and the load spectrum is converted into the total equivalent cycle number of the single-stage load of the maximum one-stage load according to the slope of the double-logarithmic straight line, namely the minimum number of rapid fatigue tests. The minimum number of rapid fatigue tests obtained by the method does not need to be multiplied by a multiple t like the fatigue times calculated according to the current theory, and the t is an empirical value obtained by long-term groping and can be used as the minimum number of rapid fatigue tests. Therefore, for the new material parts, the time cost for long-term exploration for obtaining t is greatly saved.
Description
Technical Field
The invention belongs to the technical field of fatigue reliability tests, is suitable for fatigue tests of metal material parts on automobiles and other machinery, and particularly relates to a method for determining the minimum times of rapid fatigue tests.
Background
The Miner theory that the fatigue boundary dominates, the core content is: and measuring the damage degree of each stage of load cycle number in the load spectrum by using the median life (reliability R is 50%) of each stage of load, wherein the sum of the damage degrees is the accumulated damage of the load spectrum, and when the accumulated damage is equal to 1, the part is damaged.
According to the theory, the load spectrum under the actual working condition of the component (the load spectrum corresponds to a certain service life, the service life can be expressed by working time, mileage and the like, and the service life is called as the specified service life for short), the load cycle times of all stages except the maximum one-stage load are respectively converted into the equivalent cycle times of the maximum one-stage load, and then the equivalent cycle times are accumulated, namely the load spectrum is converted into the total equivalent cycle number sigma N of the single-stage load of the maximum one-stage load. The load spectrum borne by the component is equal to the cumulative damage to the component caused by the component bearing the maximum first-order load for a single-stage cycle of Σ N times.
Obviously, the test time consumed by examining the components by the maximum one-stage load single-stage cycle sigma N times is greatly shortened in the bench test compared with the test time consumed by examining the components by adopting the load spectrum, so that the test efficiency can be effectively improved, and in the test process of examining the components by the maximum one-stage load single-stage cycle sigma N times, the load is fixed, and the load is not changed like the load spectrum, so the test is easier to realize.
However, the fact shows that the total equivalent cycle number Σ N of the single-stage load converted according to the Miner theory is severely small, resulting in insufficient examination. The sampled sample is subjected to a single-stage load total equivalent cycle sigma N bench test, and the reliability R1 of the bench test is more than or equal to the target reliability R. But the reliability R2 of the sample at the specified lifetime does not meet the standard. Namely R1 is more than or equal to R but R2 is less than or equal to R. This formal Miner theory is inherently deficient (poor precision and severely biased toward insecurity).
In order to overcome the above-mentioned drawbacks of the Miner theory, in the prior art, generally, based on the variation of various factors such as the reliability R of the component, the specific material of the component, and the test loading manner, the minimum number of rapid fatigue tests can be obtained by multiplying Σ N by a multiple t (t needs to be obtained after a long-term trial). So that it can ensure: if the verification result R1 of the product reliability is larger than or equal to R in the bench test passing the times, the corresponding reliability R2 of the product in the specified service life is larger than or equal to R.
Based on objective rules, all metal materials, dynamic load (hereinafter referred to as load) sigma takes logarithmic value lg (sigma), and fatigue life N corresponding to reliability RRLogarithmic value lg (N)R) And the two are in linear relation. The slope of the line is different for different R. Hereinafter referred to simply as a log-log line. As shown in FIG. 1, generalizing the general rules reflected in FIG. 1 are:
①NRis a decreasing function of R.
the expression is that the formed double logarithmic straight line series presents a bell mouth shape from top to bottom along with different R, the smaller R is, the corresponding lg (N) is smallerR) The more severe the lg (σ) straight line inclination, i.e. closer to horizontal; conversely, the larger R, the corresponding lg (N)R) The closer the lg (σ) line is to vertical.
Therefore, the equivalent times of each stage of small load cycle times after conversion to large load is determined, and the equivalent times become larger along with the increase of the reliability R; becomes smaller as the reliability R becomes smaller.
The Miner theory considers that the fatigue life corresponding to a certain level of load is fixed, so that the reliability R in figure 1 is 50% of the slope of a straight line to equivalently convert the cycle times among different loads, and the reliability R of a part to be actually checked by engineering is more than 50%. In this case, the number of cycles of equivalently converting a small load to a large load is fixed according to the Miner's theory with the slope of the line R being 50% in FIG. 1, and is significantly smaller than the number of cycles of converting the slope of the line R (> 50%) required for the rope, according to the above 2-point rule. This is where Miner's theory is unreasonable and is the root cause of the aforementioned inherent deficiencies (poor accuracy and severe bias towards insecurity).
Disclosure of Invention
Aiming at the defects in the existing theory and algorithm, the invention discloses a method for determining the minimum times of rapid fatigue tests on the basis of theoretical innovation. The single-stage load total equivalent cycle number of the maximum first-stage load obtained by the method can be directly used as the minimum times of the rapid fatigue test to perform bench test examination on the sample without multiplying by a multiple t.
The technical scheme of the invention is as follows by combining the attached drawings of the specification:
a method for determining the minimum number of rapid fatigue tests, said method and specific steps are as follows:
s1: determining a reliability target R of the part under the corresponding service life of the specified load spectrum, wherein R is more than or equal to 50%;
s2: the dynamic load sigma takes a logarithmic value lg (sigma), and the corresponding service life N of the specified load spectrum with the corresponding reliability RRLogarithmic value lg (N)R) The two are in linear relation; forming a double-logarithmic line by different reliability degrees R and different slopes of the line, and selecting a corresponding double-logarithmic line according to the reliability degree R determined in the step S1;
s3: and respectively converting the cycle times of each stage of dynamic load of the load spectrum into the single-stage cycle number of the maximum one stage of dynamic load according to the double-logarithmic straight line selected by the S2, summing the single-stage cycle numbers, and calculating the corresponding reliability R, wherein the load spectrum is equivalent to the total equivalent cycle number of the maximum one stage of dynamic load, namely the minimum number of rapid fatigue tests.
In step S1, the reliability R of the component may be any value of 50% or more.
In step S3, according to the log-log straight line corresponding to R selected in step S2, the dynamic load σ cycle is calculated n times, and the equivalent times converted into the maximum dynamic load is:
(n/NR)╳NR，max
said N isR，maxThe specified load spectrum corresponding life of the maximum dynamic load corresponding reliability R;
thus, the cycle times N of different dynamic loads ∑ in the load spectrum are respectively converted into the cycle times of the maximum first-stage dynamic load corresponding to the reliability R, then the sum is carried out to calculate the corresponding reliability R, and the load spectrum is equivalent to the total equivalent cycle number ∑ N of the maximum first-stage dynamic loadRThe minimum number of rapid fatigue tests of the part sample is obtained;
the minimum number of rapid fatigue tests thus calculated can ensure that: when the bench test passes the times, if the verification result of the product reliability R1 is that R1 is larger than or equal to R, the corresponding reliability R2 of the product is always larger than or equal to R when the corresponding service life of the load spectrum is specified.
Compared with the prior art, the invention has the beneficial effects that:
according to the method (the method for determining the minimum number of times of the rapid fatigue test), the calculated total equivalent cycle number of the single-stage load of the maximum one-stage load can be used as the minimum number of times of the rapid fatigue test, and the multiplication of a multiple t (t is an empirical value obtained by long-term groping) is not needed as in the background art, so that the time cost for obtaining t for long-term groping for novel material parts is greatly saved.
Drawings
FIG. 1 is a diagram of the method for determining the minimum number of rapid fatigue tests according to the present invention, wherein the load σ is logarithmic lg (σ) and corresponding to the fatigue life N with the reliability RRLogarithmic value lg (N)R)，
The two are shown in a linear series with different reliability R.
FIG. 2 is a block diagram of a method for determining a minimum number of rapid fatigue tests according to the present invention.
Detailed Description
For clearly and completely describing the technical scheme and the specific working process thereof, the specific implementation mode of the invention is as follows by combining the attached drawings of the specification:
the present example discloses a method for determining the minimum number of rapid fatigue tests, as shown in fig. 1 and fig. 2, the specific process of the method for determining the minimum number of rapid fatigue tests is as follows:
s1: determining a reliability target R (R is more than or equal to 50%) of the part under the specified service life;
in step S1, the reliability target R of the component is determined at the stage of designing the component product. And the minimum number of rapid fatigue tests is calculated to ensure that: if the verification result R1 of the product reliability is larger than or equal to R in the bench test passing the times, the corresponding reliability R2 is larger than or equal to R when the product has the designated service life.
In this example, the reliability R of the component is determined to be 90%, i.e., the minimum number of rapid fatigue tests is calculated, and is to ensure that: through the bench test of the times, if the reliability of the product is more than or equal to 90%, the sample can be judged to be qualified, namely R1 is more than or equal to 90%, and the corresponding reliability R2 of the sample in the specified service life can be ensured to be more than or equal to 90%.
S2: the logarithmic value of the dynamic load σ lg (σ), corresponding to a specified load spectrum with a corresponding reliability of R, corresponding to a lifetime of NRLogarithmic value lg (N)R) The two are in linear relation; forming a double-logarithm straight line by different reliability degrees R and different slopes of the straight line, and selecting the double-logarithm straight line according to the determined reliability degree R of the part;
in step S2, according to S1, a log-log straight line with R of 90% is selected.
S3, converting the corresponding reliability R of the load spectrum to 90% according to the selected double-logarithmic straight line S2, and calculating the total equivalent cycle number sigma N of the single-stage load of the maximum one-stage load90；
In step S3, according to the log-log line corresponding to 90% of R selected in step S2, the σ cycle of a certain level of load in the load spectrum is calculated N times, and the number of times of conversion into the maximum load is (N/N) corresponding to 90% of R90)╳N90，max，
Wherein N is90This load stage itself corresponds to a lifetime of 90%. N is a radical of90，maxLife corresponding to 90% for maximum load;
in this way, the cycle numbers N of different loads σ are each converted to the cycle number corresponding to the maximum first-order load with R being 90%, and then summed to calculate the total equivalent cycle number ∑ N for the maximum load with respect to the load spectrum with R being 90%, and the corresponding R being 90%, respectively90Namely, the method is the minimum number of rapid fatigue tests of the part sample.
verified ∑ N90the ratio of ∑ N (the number of cycles of the maximum primary load calculated from R ═ 50% linear regression according to Miner's theory) is highly consistent with the empirical factor t in the background art90the method can be used as the minimum times of rapid fatigue tests without experience times t, when the sample passes through the maximum load, the times sigma N90After the bench test, if the reliability is more than or equal to 90%, the sample can be judged to be qualified, namely R1 is more than or equal to 90%, and the corresponding reliability R2 of the sample in the specified service life can be ensured to be more than or equal to 90%. Therefore, the invention greatly saves the cost for carrying out long-term test comparison on new materials or parts to obtain t.
The principle of the method for determining the minimum number of rapid fatigue tests is as follows:
the core content of the existing Miner theory is as follows: and measuring the damage degree of each stage of load cycle number in the load spectrum by using the median life (reliability R is 50%) of each stage of load, wherein the sum of the damage degrees is the accumulated damage of the load spectrum, and when the accumulated damage is equal to 1, the part is damaged.
The method of the invention innovatively develops the Miner theory based on the objective rule embodied by the log-log linear series: and measuring the damage degree of the reliability R-X corresponding to the load cycle times of each stage in the load spectrum by using the service life of the reliability R-X corresponding to each stage of load (0< X <1), wherein the sum of the damage degrees is the accumulated damage of the reliability R-X corresponding to the load spectrum. When this cumulative damage is equal to 1, the reliability of the part is X. It can be seen that the Miner theory is a special case of the theory of the method of the present invention when R is 50%.
Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention, and that variations, modifications, substitutions and alterations can be made to the above embodiments by those of ordinary skill in the art within the scope of the present invention.
The above-described embodiments of the present invention should not be construed as limiting the scope of the present invention. Any other corresponding changes and modifications made according to the technical idea of the present invention should be included in the protection scope of the claims of the present invention.
Claims (3)
1. A method of determining a minimum number of rapid fatigue tests, characterized by:
the method comprises the following specific steps:
s1: determining a reliability target R of the part under the corresponding service life of the specified load spectrum, wherein R is more than or equal to 50%;
s2: the dynamic load sigma takes a logarithmic value lg (sigma), and the corresponding service life N of the specified load spectrum with the corresponding reliability RRLogarithmic value lg (N)R) The two are in linear relation; the slope of the line having different reliability ROtherwise, forming a double-logarithmic straight line, and selecting a corresponding double-logarithmic straight line according to the reliability R determined in the step S1;
s3: and respectively converting the cycle times of each stage of dynamic load of the load spectrum into the single-stage cycle number of the maximum one stage of dynamic load according to the double-logarithmic straight line selected by the S2, summing the single-stage cycle numbers, and calculating the corresponding reliability R, wherein the load spectrum is equivalent to the total equivalent cycle number of the maximum one stage of dynamic load, namely the minimum number of rapid fatigue tests.
2. The method of determining a minimum number of rapid fatigue tests of claim 1, wherein:
in step S1, the reliability R of the component may be any value of 50% or more.
3. The method of determining a minimum number of rapid fatigue tests of claim 1, wherein:
in step S3, according to the log-log straight line corresponding to R selected in step S2, the dynamic load σ cycle is calculated n times, and the equivalent times converted into the maximum dynamic load is:
(n/NR)╳NR，max
said N isR，maxThe specified load spectrum corresponding life of the maximum dynamic load corresponding reliability R;
thus, the cycle times N of different dynamic loads ∑ in the load spectrum are respectively converted into the cycle times of the maximum first-stage dynamic load corresponding to the reliability R, then the sum is carried out to calculate the corresponding reliability R, and the load spectrum is equivalent to the total equivalent cycle number ∑ N of the maximum first-stage dynamic loadRThe minimum number of rapid fatigue tests of the part sample is obtained;
the minimum number of rapid fatigue tests thus calculated can ensure that: when the bench test passes the times, if the verification result of the product reliability R1 is that R1 is larger than or equal to R, the corresponding reliability R2 of the product is always larger than or equal to R when the corresponding service life of the load spectrum is specified.
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Application publication date: 20200922 |