CN111198140A - Method for rapidly predicting fatigue limit of welding joint based on fatigue damage entropy production rate - Google Patents
Method for rapidly predicting fatigue limit of welding joint based on fatigue damage entropy production rate Download PDFInfo
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Abstract
The invention discloses a method for quickly predicting the fatigue limit of a welding joint based on the entropy production rate of fatigue damage, which comprises the steps of carrying out a fatigue test on a PLG-200 high-frequency fatigue testing machine by using a butt joint test piece, measuring the temperature rise of a butt joint in the fatigue test process, establishing a cumulative fatigue damage entropy model of the butt joint, initially establishing a mathematical model for quickly predicting the fatigue limit of the welding joint based on the entropy production rate of the cumulative damage, obtaining an intersection point between two linear fitting functions, and predicting the welding joint by using a lifting method to obtain a fatigue limit test value; and comparing the fatigue limit predicted value with the fatigue limit test value to obtain an error value. The method establishes the functional relation between the entropy production rate of the accumulated fatigue damage entropy and the fatigue limit, finds the turning point of the variation degree of the entropy production rate of the accumulated fatigue damage entropy along with the load level, and takes the turning point as the predicted value of the fatigue limit so as to realize the rapid prediction of the fatigue limit from the angle of the entropy production rate of the fatigue damage entropy.
Description
Technical Field
The invention relates to the technical field of model construction, in particular to a method for quickly predicting fatigue limit of a welding joint based on a fatigue damage entropy production rate.
Background
Fatigue fracture is the primary form of structural failure, and many catastrophic failures in engineering are caused by structural fatigue failure. The fatigue problem of welded structures is of particular relevance due to the geometric discontinuity at the weld.
The traditional welding fatigue evaluation method based on the S-N curve comprises a nominal stress method, a hot spot stress method, a structural stress method and the like, wherein the nominal stress method is the earliest used evaluation method, and then the hot spot stress method solves the problem of stress concentration at a weld toe which cannot be considered by the nominal stress method, but the calculation result of the peak stress at the weld toe is different from one person to another due to the fact that finite element software is sensitive to grids. Professor dong a grid insensitive structural stress method, thoroughly solves the problem that the structural stress is sensitive to the grid, provides a new idea for fatigue research of a welded structure, and creates a new method.
In recent years, the fatigue problem of welded structures has been increasingly studied from the viewpoint of irreversible entropy. The degree of chaos of an isolated system always tends to increase, i.e. to progress towards the maximum entropy, according to the second law of thermodynamics. Weld fatigue is a typical process that progresses from ordered to disordered, involving microscopic interatomic dislocations and slips, and macroscopic crack initiation, propagation, and even fracture, with a consequent irreversible entropy increase. Therefore, theoretically, the welding fatigue problem can be evaluated from the point of view of the accumulated fatigue damage entropy.
Disclosure of Invention
The invention aims to provide a method for quickly predicting fatigue limit of a welding joint based on fatigue damage entropy production rate, which is based on the fact that accumulated fatigue damage in a test piece fatigue process under different load levels is a fixed value, establishes a functional relation between the accumulated fatigue damage entropy production rate and the fatigue limit, finds a turning point of the change degree of the accumulated fatigue damage entropy production rate along with the load level, and takes the turning point as a predicted value of the fatigue limit so as to realize the purpose of quickly predicting the fatigue limit from the fatigue damage entropy production rate, and can solve the problems in the background technology.
In order to achieve the purpose, the invention provides the following technical scheme:
a method for rapidly predicting the fatigue limit of a welding joint based on the entropy production rate of fatigue damage comprises the following steps:
s1: fatigue test is carried out on a PLG-200 high-frequency fatigue testing machine by using a butt joint test piece, and the specified service life is 2 multiplied by 106Secondly;
s2: and (3) measuring the temperature rise of the butt joint in the fatigue test process: uniformly spraying a layer of black matte paint on the surface of the butt joint to improve the surface emissivity of the butt joint, and monitoring the local hot spot temperature of the surface of a welding seam in real time by adopting a Fluke Ti450 thermal infrared imager in the process of carrying out a fatigue test on the butt joint;
s3: temperature rise data processing: outputting and processing temperature data by means of SmartView software, drawing a curve graph of real-time temperature change and cycle frequency under different load levels based on Origin software, and establishing a functional relation between the real-time temperature change and the cycle frequency T-N under different load levels;
s4: establishing an entropy model of accumulated fatigue damage of the butt joint: the method comprises the steps of establishing a fatigue damage entropy mathematical model of an accumulated damage butt joint by integrating obtained material parameters such as T-N function relation, specific heat capacity and material density under different load levels, calculating accumulated fatigue damage entropies of test pieces under different load levels when the test pieces break, and verifying that the accumulated fatigue damage entropies of the test pieces under different load levels are a constant value;
s5: preliminarily establishing a welding joint fatigue limit rapid prediction mathematical model based on the accumulated damage entropy production rate: according to the fact that the accumulated fatigue damage entropy of the butt joint under different load levels is a fixed value, the accumulated fatigue damage entropy production rate under different load levels is obtained;
s6: dividing the cumulative fatigue damage entropy production rate data under different load levels into two groups, and linearly fitting two groups of functional graphs of the cumulative fatigue damage entropy production rate and the load level, wherein the load level is an abscissa, and the cumulative fatigue damage entropy production rate is an ordinate;
s7: acquiring an intersection point between the two linear fitting functions, wherein the intersection point is a predicted value of the fatigue limit of the butt joint;
s8: predicting the welded joint by using a lifting method to obtain a fatigue limit test value;
s9: and comparing the fatigue limit predicted value with the fatigue limit test value to obtain an error value.
Further, the fatigue test in the S2 adopts a temperature measuring system, the temperature measuring system comprises a PLG-200 high-frequency fatigue testing machine, a thermal infrared imager and a computer, the PLG-200 high-frequency fatigue testing machine is connected with the computer through a cable, the computer is connected with the thermal infrared imager through a cable, and a test piece on the PLG-200 high-frequency fatigue testing machine forms a waveform diagram through the thermal infrared imager.
Further, the process of changing the fatigue temperature with the cycle change in S3 may be divided into three stages, i.e., a first stage in which the temperature sharply rises under initial load, a second stage in which the temperature changes steadily around a constant value after load is stably applied, and a third stage in which the temperature sharply rises when the test piece breaks.
Further, in S9, dividing the multiple cumulative fatigue damage entropy production rates according to a specified rule, and obtaining two sets of cumulative fatigue damage entropy production rate data includes:
dividing the entropy rate of the accumulated fatigue damage entropy into a group with the entropy rate less than or equal to a specified threshold;
and dividing the cumulative fatigue damage entropy production rate larger than a specified threshold into a group.
Further, the specified threshold is an empirical value, and in the embodiment of the present invention, the threshold may be set to 0.4092.
Compared with the prior art, the invention has the beneficial effects that:
1. the method is based on a Fluke Ti450 thermal infrared imager to monitor the temperature of a hot spot on the surface of a welding seam in real time, establishes a functional relation between real-time temperature change and cycle T-N under different load levels, and finds that the fatigue temperature rise in the first stage is gradually increased and the fatigue life is gradually increased along with the increase of the load level, which shows that the irreversible entropy increase rate and the fatigue limit have a certain functional relation.
2. The method is based on the second law of thermodynamics, and brings parameters such as the surface temperature, the specific heat capacity and the material density of a welding seam in the fatigue process into the establishment of the entropy production rate model of the cumulative fatigue damage, so that the fatigue process is basically an irreversible entropy increase process, and the method proves that when a test piece is broken, the entropy production rate of the cumulative fatigue damage of the test piece under different load levels is a fixed value, and the corresponding entropy production rate of the cumulative fatigue damage changes along with the increase of the load and exists at a turning point.
3. The functional relation between the cumulative fatigue damage entropy production rate and the load level is established, the turning point of the degree of change of the cumulative fatigue damage entropy production rate along with the load level is used as the predicted value of the fatigue limit, and the rapid prediction of the fatigue limit of the angle welding joint from the fatigue damage entropy production rate is realized.
4. The welding joint is subjected to fatigue test by adopting the traditional lifting method, the error of predicting the fatigue limit by the two methods is 5.557%, the consistency is high, and the fatigue limit of the welding joint can be rapidly predicted.
Drawings
FIG. 1 is a geometric dimension of a Q460 mild steel butt joint provided by an embodiment of the invention;
FIG. 2 is a temperature measurement system for a fatigue test based on a method for rapidly predicting a fatigue limit of a butt joint based on an accumulated fatigue damage entropy production rate, provided by an embodiment of the invention;
FIG. 3 is a schematic flowchart of a method for rapidly predicting fatigue limit of a butt joint based on cumulative fatigue damage entropy production rate according to an embodiment of the present invention;
FIG. 4 is a graph of a typical fatigue temperature rise versus cycle number for a fatigue test specimen provided in accordance with an embodiment of the present invention at a load level of 330 MPa;
FIG. 5 is a graph showing the relationship between fatigue temperature rise and cycle frequency of a fatigue test specimen under different load levels according to an embodiment of the present invention;
FIG. 6 is a comparison graph of cumulative fatigue damage entropies of test pieces provided by the embodiment of the invention under different load levels;
FIG. 7 is a graph showing the relationship between the cumulative fatigue damage entropy production rate and the load level of a test piece under different load levels according to an embodiment of the present invention;
FIG. 8 is a graph illustrating the use of conventional lifting and lowering methods to predict fatigue limit for a butt joint in accordance with the teachings of the present invention.
In the figure: 1. PLG-200 high frequency fatigue testing machine; 2. a thermal infrared imager; 3. a computer; 4. and (5) testing the test piece.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
A method for rapidly predicting the fatigue limit of a welding joint based on the entropy production rate of fatigue damage comprises the following steps:
s1: the fatigue test is carried out on a PLG-200 high-frequency fatigue testing machine 1 by utilizing a butt joint test piece, and the specified service life is 2 multiplied by 106Secondly;
s2: and (3) measuring the temperature rise of the butt joint in the fatigue test process: uniformly spraying a layer of black matte paint on the surface of the butt joint to improve the surface emissivity of the butt joint, and monitoring the local hot spot temperature of the surface of a welding seam in real time by adopting a Fluke Ti450 thermal infrared imager in the process of carrying out a fatigue test on the butt joint;
in the examples, the Q460 butt joint fatigue test piece was subjected to high frequency fatigue tensile test, as shown in FIG. 1, and seven load levels of 330MPa (as shown in FIG. 4), 280MPa, 270MPa, 260MPa, 240MPa, 230MPa and 220MPa of maximum nominal stress were selected for loading. The relation curve of fatigue temperature rise and cycle under different load levels is shown in figure 5, and when the fatigue test is carried out, the temperature of a hot spot on the surface of the welding seam is synchronously measured by means of Fluke Ti450, so that all data of real-time temperature change of the surface of the welding seam in the fatigue test process are obtained.
Referring to fig. 2, the fatigue test adopts a temperature measuring system, the temperature measuring system comprises a PLG-200 high-frequency fatigue testing machine 1, a thermal infrared imager 2 and a computer 3, the PLG-200 high-frequency fatigue testing machine 1 is connected with the computer 3 through a cable, the computer 3 is connected with the thermal infrared imager 2 through a cable, and a test piece 4 on the PLG-200 high-frequency fatigue testing machine 1 forms a waveform diagram through the thermal infrared imager 2.
S3: temperature rise data processing: outputting and processing temperature data by means of SmartView software, drawing a curve graph of real-time temperature change and cycle frequency under different load levels based on Origin software, and establishing a functional relation between the real-time temperature change and the cycle frequency T-N under different load levels;
in the embodiment, the process of the fatigue temperature changing with the cycle frequency change can be divided into three stages, namely a first stage of the temperature sharply rising under the initial loading, a second stage of the temperature changing around a fixed value after the load is stably loaded, and a third stage of the temperature sharply rising when the test piece is broken, and a relation graph of the weld surface hot spot temperature change and the cycle frequency and a corresponding infrared thermal image graph of the process are shown in fig. 3 by taking the weld surface hot spot fatigue temperature change under 330MPa as an example.
S4: establishing an entropy model of accumulated fatigue damage of the butt joint: the obtained material parameters such as T-N function relation, specific heat capacity, material density and the like under different load levels are integrated to establish a fatigue damage entropy mathematical model of the accumulated damage butt joint, the accumulated fatigue damage entropy of the test piece when the test piece is broken under different load levels is calculated, and the accumulated fatigue damage entropy of the test piece under different load levels is verified to be a fixed value, as shown in figure 6;
s5: preliminarily establishing a welding joint fatigue limit rapid prediction mathematical model based on the accumulated damage entropy production rate: according to the fact that the accumulated fatigue damage entropy of the butt joint under different load levels is a fixed value, the accumulated fatigue damage entropy production rate under different load levels is obtained;
s6: dividing the cumulative fatigue damage entropy production rate data under different load levels into two groups, and linearly fitting two groups of functional graphs of the cumulative fatigue damage entropy production rate and the load level, wherein the load level is an abscissa, and the cumulative fatigue damage entropy production rate is an ordinate, as shown in FIG. 7;
s7: acquiring an intersection point between the two linear fitting functions, wherein the intersection point is a predicted value of the fatigue limit of the butt joint;
in which a graph of the relationship between the cumulative fatigue damage entropy production rate and the load level is established, as shown in fig. 6. As can be seen from the figure, the cumulative fatigue damage entropy production rate values corresponding to the load levels of 220MPa, 230MPa and 240MPa are close to 0 and are smaller than the specified cumulative fatigue damage entropy production rate threshold value 0.4092, and the cumulative fatigue damage entropy production rates of the rest five load levels are obviously changed along with the increase of the load levels. In order to determine the load size corresponding to the turn of the cumulative fatigue damage entropy production rate, and using the load size as a fatigue limit predicted value, linear fitting is respectively carried out on two groups of cumulative fatigue damage entropy production rate data, so that the following results are obtained:
y1＝0.05838x-14.0955
y2＝-0.00137x+0.74966
wherein x is the nominal stress and y1、y2The entropy production rate is the cumulative fatigue damage entropy production rate. The two formulas are combined, and the abscissa corresponding to the intersection point of the two straight lines is 248.45 MPa. Therefore, the predicted fatigue limit of the welded joint was 248.45 MPa. In addition, in order to verify the authenticity and reliability of the fatigue limit prediction method of the present invention, a fatigue test by a lifting method is subsequently employed.
S8: predicting the welded joint by using a lifting method to obtain a fatigue limit test value;
s9: and comparing the fatigue limit predicted value with the fatigue limit test value to obtain an error value. Wherein, the dividing the multiple cumulative fatigue damage entropy production rates according to a specified rule to obtain two groups of cumulative fatigue damage entropy production rate data comprises:
and dividing the cumulative fatigue damage entropy production rate into a group with the entropy production rate less than or equal to a specified threshold value.
And dividing the cumulative fatigue damage entropy production rate larger than a specified threshold into a group.
The threshold is designated as an empirical value and may be set to 0.4092 in an embodiment of the present invention.
For the lifting method fatigue verification test of the invention:
the fatigue strength of a group of samples is obtained by using a lifting method, the average fatigue strength of the north side material is necessarily estimated, the estimated average fatigue strength is used as a first-stage stress level to carry out a test, a first sample is randomly selected, and if the first-stage stress level is lower, the sample fails at a given cycle number. A specimen was also randomly selected and the stress level increased by one stress level if the previous specimen did not fail. If the previous specimen failed, one stress level was reduced and the test continued until all specimens were tested in this manner. According to GB/T24176-2009 statistical scheme and analytical method of fatigue test data of metal materials, in order to obtain an S-N curve, fatigue tests are carried out at different stress levels under the failure probability of 50%, and in consideration of the test purpose and the reliability of test materials, at least 8 samples are selected for explanation tests to calculate the average fatigue limit of the materials. The specified cycle of the fatigue test is 2 multiplied by 106According to the standard, the fatigue fracture of the sample in a specified cycle is determined as 'pass', otherwise, the sample is regarded as 'fail', and the test result is shown in figure 8.
Statistical analysis is carried out on the test data, four broken and continuous subsample pairs are selected to be averaged, and the 2 x 10 of Q460 butt joints can be obtained6The fatigue limit predicted value corresponding to the cycle is as follows:
the fatigue limit predicted value obtained by the method of the invention is compared with a fatigue limit test value obtained by a lifting method, and the error is calculated as follows:
therefore, the error between the predicted fatigue limit value and the fatigue limit test value is 5.557%, the consistency is high, and the fatigue limit of the welding joint can be rapidly predicted.
The invention relates to the technical field of fatigue reliability and safe service evaluation of a welding structure. The method comprises the following steps: the method comprises the steps of dividing accumulated fatigue damage entropy production rate data under different load levels into two groups by calculating the accumulated fatigue damage entropy production rate under the load levels; linearly fitting two groups of function graphs of the cumulative fatigue damage entropy production rate and the load level, wherein the load level is an abscissa, and the cumulative fatigue damage entropy production rate is an ordinate; and acquiring an intersection point between the two linear fitting functions, wherein the intersection point is the predicted value of the fatigue limit of the butt joint. The method takes a nominal stress value corresponding to the turn of the entropy production rate of the accumulated fatigue damage as a quick prediction value of the fatigue limit, predicts the fatigue limit by adopting a traditional lifting method, then obtains an error value between the two, and has authenticity and reliability as a result.
The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art should be able to cover the technical solutions and the inventive concepts of the present invention within the technical scope of the present invention.
Claims (5)
1. A method for rapidly predicting the fatigue limit of a welding joint based on the entropy production rate of fatigue damage is characterized by comprising the following steps:
s1: fatigue test is carried out on a PLG-200 high-frequency fatigue testing machine by using a butt joint test piece, and the specified service life is 2 multiplied by 106Secondly;
s2: and (3) measuring the temperature rise of the butt joint in the fatigue test process: uniformly spraying a layer of black matte paint on the surface of the butt joint to improve the surface emissivity of the butt joint, and monitoring the local hot spot temperature of the surface of a welding seam in real time by adopting a Fluke Ti450 thermal infrared imager in the process of carrying out a fatigue test on the butt joint;
s3: temperature rise data processing: outputting and processing temperature data by means of SmartView software, drawing a curve graph of real-time temperature change and cycle frequency under different load levels based on Origin software, and establishing a functional relation between the real-time temperature change and the cycle frequency T-N under different load levels;
s4: establishing an entropy model of accumulated fatigue damage of the butt joint: the method comprises the steps of establishing a fatigue damage entropy mathematical model of an accumulated damage butt joint by integrating obtained material parameters such as T-N function relation, specific heat capacity and material density under different load levels, calculating accumulated fatigue damage entropies of test pieces under different load levels when the test pieces break, and verifying that the accumulated fatigue damage entropies of the test pieces under different load levels are a constant value;
s5: preliminarily establishing a welding joint fatigue limit rapid prediction mathematical model based on the accumulated damage entropy production rate: according to the fact that the accumulated fatigue damage entropy of the butt joint under different load levels is a fixed value, the accumulated fatigue damage entropy production rate under different load levels is obtained;
s6: dividing the cumulative fatigue damage entropy production rate data under different load levels into two groups, and linearly fitting two groups of functional graphs of the cumulative fatigue damage entropy production rate and the load level, wherein the load level is an abscissa, and the cumulative fatigue damage entropy production rate is an ordinate;
s7: acquiring an intersection point between the two linear fitting functions, wherein the intersection point is a predicted value of the fatigue limit of the butt joint;
s8: predicting the welded joint by using a lifting method to obtain a fatigue limit test value;
s9: and comparing the fatigue limit predicted value with the fatigue limit test value to obtain an error value.
2. The method for rapidly predicting the fatigue limit of the welding joint based on the entropy production rate of the fatigue damage as claimed in claim 1, wherein a temperature measuring system is adopted in the fatigue test in S2, the temperature measuring system comprises a PLG-200 high-frequency fatigue testing machine (1), a thermal infrared imager (2) and a computer (3), the PLG-200 high-frequency fatigue testing machine (1) is connected with the computer (3) through a cable, the computer (3) is connected with the thermal infrared imager (2) through a cable, and a test piece (4) on the PLG-200 high-frequency fatigue testing machine (1) forms a waveform diagram through the thermal infrared imager (2).
3. The method for rapidly predicting the fatigue limit of the welding joint based on the entropy production rate of the fatigue damage as claimed in claim 1, wherein the process of changing the fatigue temperature along with the cycle frequency in S3 is divided into three stages, namely a first stage of rapidly increasing the temperature under the initial loading, a second stage of stably changing the temperature around a fixed value after the load is stably loaded, and a third stage of rapidly increasing the temperature when the test piece is broken.
4. The method for rapidly predicting the fatigue limit of the welding joint based on the entropy production rate of the fatigue damage as claimed in claim 1, wherein the step of dividing the entropy production rates of the cumulative fatigue damage according to the specified rule in the step S9 to obtain two groups of entropy production rate data of the cumulative fatigue damage comprises the following steps:
dividing the entropy rate of the accumulated fatigue damage entropy into a group with the entropy rate less than or equal to a specified threshold;
and dividing the cumulative fatigue damage entropy production rate larger than a specified threshold into a group.
5. The entropy production rate welding joint fatigue limit rapid prediction method based on fatigue damage as claimed in claim 4, wherein the specified threshold is an empirical value, and the threshold can be set to 0.4092.
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Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
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| CN112288135A (en) * | 2020-09-29 | 2021-01-29 | 南京航空航天大学 | Intelligent monitoring method for fan blade fatigue limit attenuation based on image recognition |
| CN112284942A (en) * | 2020-10-23 | 2021-01-29 | 哈尔滨工业大学 | Thermal boundary condition control method for fatigue limit evaluation of welding joint |
| CN112924307A (en) * | 2021-01-29 | 2021-06-08 | 有研工程技术研究院有限公司 | Fatigue limit rapid prediction method based on infrared thermal imaging |
| CN114397206A (en) * | 2022-01-07 | 2022-04-26 | 太原理工大学 | Method for predicting fatigue limit of metal material based on entropy production stability value |
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| CN112288135A (en) * | 2020-09-29 | 2021-01-29 | 南京航空航天大学 | Intelligent monitoring method for fan blade fatigue limit attenuation based on image recognition |
| CN112288135B (en) * | 2020-09-29 | 2024-04-16 | 南京航空航天大学 | Image recognition-based intelligent monitoring method for fatigue limit attenuation of fan blade |
| CN112284942A (en) * | 2020-10-23 | 2021-01-29 | 哈尔滨工业大学 | Thermal boundary condition control method for fatigue limit evaluation of welding joint |
| CN112284942B (en) * | 2020-10-23 | 2024-03-26 | 哈尔滨工业大学 | Thermal boundary condition control method for fatigue limit assessment of welded joint |
| CN112924307A (en) * | 2021-01-29 | 2021-06-08 | 有研工程技术研究院有限公司 | Fatigue limit rapid prediction method based on infrared thermal imaging |
| CN112924307B (en) * | 2021-01-29 | 2023-01-20 | 有研工程技术研究院有限公司 | Fatigue limit rapid prediction method based on infrared thermal imaging |
| CN114397206A (en) * | 2022-01-07 | 2022-04-26 | 太原理工大学 | Method for predicting fatigue limit of metal material based on entropy production stability value |
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