CN108052717B - Fatigue life calibration method based on local stress-strain method - Google Patents
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Abstract
The invention discloses a fatigue life calibration method based on a local stress-strain method, and belongs to the technical field of airplane strength design. The method comprises the following steps: step one, determining the most sensitive parameter K in the local stress-strain methodf(ii) a Step two, to KfCorrecting the fatigue performance curve parameters; determining the corresponding median life t according to the preassigned load stress spectrum50Optimizing for the target; setting an initial value and continuously adjusting KfRepeatedly calculating the median life by using a local stress-strain method until the calculated median life is equal to the determined median life t50At this time, corresponding KfThe value is K in the modified S-N curve three-parameter formulafA value; step three, determining the local stress-strain method and corresponding KfVerifying the value; and predicting the service life under the load spectrum by using the optimized parameters and a local stress-strain method, and comparing the predicted and calculated median service life t' under the load spectrum with the actual measured median service life under the load spectrum to obtain errors.
Description
Technical Field
The invention belongs to the technical field of airplane strength design, and particularly relates to a fatigue life calibration method based on a local stress-strain method.
Background
At present, when the life analysis of the fatigue key part of the airplane under the random spectrum is carried out, because the random spectrum has complex load interaction, no generally applicable high-precision damage accumulation model can better predict the fatigue life at present.
However, researches show that the serialized aircrafts often have great inheritance on structures and load spectrums, so that the fatigue test results of the existing models can be adopted to predict the fatigue life of the subsequent models.
Disclosure of Invention
The purpose of the invention is as follows: in order to solve the problems, the invention provides a fatigue life calibration method based on a local stress-strain method, which can still ensure that the calculated fatigue life has higher precision under the condition of transforming a random load spectrum.
The technical scheme of the invention is as follows: fatigue life calibration method based on local stress-strain method, wherein epsilon of local stress-strain methoda-2NfThe expression of the curve:
in the formula, epsilonaIs the strain amplitude, σ'fIs fatigue strength coefficient, b is fatigue strength index, epsilon'fIs fatigue ductility coefficient, c is fatigue ductility index, 2NfTo the number of reversals of destruction;
the method comprises the following steps:
determining the most sensitive parameters in a local stress-strain method;
the epsilona-2NfThe curve includes 5 parameters: epsilona、σ'f、b、ε'f、c；
a) For epsilonaOptimized, corresponding to the condition of epsilona-2NfMultiplying the left side of the curve expression by a coefficient K, wherein the related sensitive parameter has Kf、σ'f、b、ε'f、c；
b) Since the local stress-strain method is suitable for the conditions that the stress concentration is large and the stress concentration area enters the yielding range in a large range, the performance parameters of the elastic section are not taken as main consideration factors, and therefore K is taken as the parameterfIs the most sensitive parameter;
step two, to KfCorrecting the fatigue performance curve parameters;
c) according to a pre-specified load stress spectrumDetermining the corresponding median lifetime t50Optimizing for the target;
d) setting an initial value and continuously adjusting KfRepeatedly calculating the median life by using a local stress-strain method until the calculated median life is equal to the determined median life t50At this time, corresponding KfThe value is K in the modified S-N curve three-parameter formulafA value;
step three, determining the local stress-strain method and corresponding KfVerifying the value;
predicting the service life under a load spectrum by using the optimized parameters and a local stress-strain method, and comparing the predicted and calculated median service life t' under the load spectrum with the actual measured median service life under the load spectrum;
and if the error is less than 20%, determining the calibration method.
The technical scheme of the invention has the technical effects that: the invention relates to a fatigue life calibration method of a local stress-strain method, which is used for predicting the life under other load spectrums by using parameters obtained by optimization of the calibration method and a nominal stress method, so that the calculation results of the life under other load spectrums still have higher precision.
Drawings
Fig. 1 is a schematic flow chart of a lifetime calibration method by a local stress-strain method according to a preferred embodiment of the present invention.
Detailed Description
In order to make the implementation objects, technical solutions and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be described in more detail below with reference to the accompanying drawings in the embodiments of the present invention. In the drawings, the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The described embodiments are only some, but not all embodiments of the invention. The embodiments described below with reference to the drawings are illustrative and intended to be illustrative of the invention and are not to be construed as limiting the invention. 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. Embodiments of the present invention will be described in detail below with reference to the accompanying drawings.
The core of the fatigue life calibration method is that based on the existing test result, a reasonable fatigue analysis method is adopted to determine the sensitive parameters of fatigue analysis, the calculated life is the same as the test life by adjusting the parameters, and then the life prediction under other load spectrums is carried out by adopting the parameters and the same life analysis method, so that the life calculation results under other load spectrums have higher precision.
The method comprises the steps of converting a nominal stress spectrum acting on structural details into a local stress-strain history of a dangerous point of the structural details through notch elastoplasticity analysis by a local stress-strain method, then equating a cycle obtained by counting the local spectrum by a rain flow to a symmetrical cycle of a smooth test piece by an equivalent cycle method, and finally estimating the damage of the dangerous point of the structure by a strain-life curve of the smooth test piece so as to predict the fatigue life of the structure. The fatigue accumulated damage theory generally adopts the Miner linear accumulated damage theory which is most commonly accepted by the theoretical and engineering circles, so the core for improving the accuracy of the fatigue life prediction result lies in a detail fatigue strain-life curve (epsilon)a-2NfCurve) is determined. Epsilona-2NfThe expression of the curve is as follows:
in the formula, epsilonaIs the strain amplitude; sigma'fThe fatigue strength coefficient; b is fatigue strength index; epsilon'fThe fatigue ductility coefficient; c is fatigue ductility index; 2Nf is the inverse number to the destruction. The above formula knows ∈a-2NfThe curve includes 5 parameters: epsilona、σ′f、b、ε′f、c。
Wherein epsilonaCorresponding to fatigue strength, for εaOptimization is performed as if at εa-2NfThe left side of the curve expression is multiplied by a coefficient k, where k is equivalent to the sum of the pairWhen the partial stress strain is solved, a coefficient K is directly multiplied on the obtained strain value, so that the fatigue notch coefficient K is equivalent tofAnd (6) optimizing. In that
Is, the relevant sensitive parameter has Kf、σ′f、b、ε′f、c。
Epsilon used in local stress-strain methoda-2NfThe curve is a material characteristic parameter curve, and contains more parameters, and when the parameters are optimized, the more the parameters are, the more the number of samples is required, and the worse the robustness of the optimization result is. Considering that the local stress-strain method is suitable for the situation that the stress concentration is large and the stress concentration area enters the yielding state in a large range, the performance parameters of the elastic section should not be taken as the main considered factors. Therefore, taking K out of the above parametersfThe most sensitive parameter is the influence of factors such as detail geometry, surface processing quality and the like on K in the process of obtaining the local stress-strain fieldfAnd correcting the fatigue performance curve parameters.
In optimizing KfWhen the value is obtained, the service life of the test under a certain spectrum can be determined by back-stepping. The reverse method comprises the following steps: according to the specified load (stress) spectrum, with its corresponding median lifetime t50To target, assume KfAnd continuously adjusting K by a certain step lengthfRepeatedly calculating the median life by the local stress-strain method until the calculated median life is equal to t50Calculating to obtain KfThe value is obtained.
And predicting the service life under the load spectrum by using the optimized parameters and a local stress-strain method, comparing the predicted and calculated median service life t' under the load spectrum with the actual measured median service life under the load spectrum, and determining the calibration method if the error is less than 20%.
The service life prediction under other load spectrums is carried out by using the parameters obtained by optimizing the calibration method and the nominal stress method through verification, so that the service life calculation results under other load spectrums still have higher precision.
As shown in fig. 1: the specific implementation is as follows:
1) preparing original data, including a load spectrum, a cyclic stress-strain curve of a material, a strain-fatigue curve and a fatigue notch coefficient of a dangerous part;
2) converting a nominal stress spectrum acting on the structural details into a local stress-strain history through the notch elastoplasticity analysis;
3) counting and counting rain flow to obtain all full cycles and half cycles of the local strain spectrum;
4) calculating equivalent strain, and converting all real strain ranges of full cycle or half cycle into equivalent strain ranges of symmetrical cycle of the smooth test piece;
5) median life was calculated by cumulative damage theory.
To make epsilona-2NfThe curve reflects the original fatigue quality of actual details, and the epsilon of the material which is the same as the key part of the structure is searched from a related material manual based on the grouped fatigue test result of a typical detail simulation test piece under the actual load spectruma-2NfThe curve parameters are taken as initial values, a local stress-strain fatigue analysis method is adopted to analyze the fatigue life under a test spectrum, the calculated life is the same as the test life, and epsilon is continuously optimizeda-2NfA curve parameter;
epsilon used in local stress-strain methoda-2NfThe curve includes 5 parameters: epsilona、σ'f、b、ε'fAnd c, the step (a) is carried out. Wherein epsilonaCorresponding to fatigue strength, for εaOptimization is performed as if at εa-2NfThe left side of the curve expression is multiplied by a coefficient K, and K is equivalent to directly multiplying the obtained strain value by a coefficient K when solving the local stress strain, thereby being equivalent to the fatigue notch coefficient KfAnd (6) optimizing. The relevant sensitive parameter is then Kf、σ'f、b、ε'f、c。
Considering that the local stress-strain method is suitable for the situation that the stress concentration is large and the stress concentration area enters the yielding state in a large range, the performance parameters of the elastic section should not be taken as the main considered factors. Therefore, taking K out of the above parametersfIs the most sensitive parameter.
In optimizing KfWhen the value is obtained, the service life of the test under a certain spectrum can be determined by back-stepping. The reverse method comprises the following steps: according to the specified load (stress) spectrum, with its corresponding median lifetime t50To target, assume KfAnd continuously adjusting K by a certain step lengthfRepeatedly calculating the median life by the local stress-strain method until the calculated median life is equal to t50Calculating to obtain KfThe value is obtained.
And predicting the service life under the load spectrum by using the optimized parameters and a local stress-strain method, comparing the predicted and calculated median service life t' under the load spectrum with the actual measured median service life under the load spectrum, and determining the calibration method if the error is less than 20%.
The service life prediction under other load spectrums is carried out by using the parameters obtained by optimizing the calibration method and the nominal stress method through verification, so that the service life calculation results under other load spectrums still have higher precision.
Finally, it should be pointed out that: the above examples are only for illustrating the technical solutions of the present invention, and are not limited thereto. Although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.
Claims (1)
1. Fatigue life calibration method based on local stress-strain method, wherein epsilon of local stress-strain methoda-2NfThe expression of the curve:
in the formula, epsilonaIs the strain amplitude, σ'fIs fatigue strength coefficient, b is fatigue strength index, epsilon'fIs fatigue ductility coefficient, c is fatigue ductility index, 2NfTo the number of reversals of destruction;
the method is characterized by comprising the following steps:
determining the most sensitive parameters in a local stress-strain method;
the epsilona-2NfThe curve includes 5 parameters: epsilona、σ'f、b、ε'f、c；
a) For epsilonaOptimized, corresponding to the condition of epsilona-2NfMultiplying the left side of the curve expression by a coefficient K, wherein the related sensitive parameter has Kf、σ'f、b、ε'fC, c; wherein, KfIs the fatigue notch coefficient;
b) since the local stress-strain method is suitable for the conditions that the stress concentration is large and the stress concentration area enters the yielding range in a large range, the performance parameters of the elastic section are not taken as main consideration factors, and therefore K is taken as the parameterfIs the most sensitive parameter;
step two, to KfCorrecting the fatigue performance curve parameters;
c) determining the corresponding median life t according to the preassigned load stress spectrum50Optimizing for the target;
d) setting an initial value and continuously adjusting KfRepeatedly calculating the median life by using a local stress-strain method until the calculated median life is equal to the determined median life t50At this time, corresponding KfThe value is K in the modified S-N curve three-parameter formulafA value;
step three, determining the local stress-strain method and corresponding KfVerifying the value;
predicting the service life under a load spectrum by using the optimized parameters and a local stress-strain method, and comparing the predicted and calculated median service life t' under the load spectrum with the actual measured median service life under the load spectrum;
and if the error is less than 20%, determining the calibration method.
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| CN109948216B (en) * | 2019-03-12 | 2023-01-03 | 华东理工大学 | Total strain energy density corrected notched part low-cycle fatigue prediction method |
| CN110489914B (en) * | 2019-08-27 | 2023-01-17 | 中国航空工业集团公司沈阳飞机设计研究所 | Durability calculation method based on stress damage equivalence |
| CN111950163B (en) * | 2020-08-20 | 2023-05-09 | 上海电气风电集团股份有限公司 | Wind blade fatigue life monitoring method |
| CN113109177B (en) * | 2021-03-26 | 2023-01-03 | 北京工业大学 | Based on K f Method for predicting multi-axis constant-amplitude thermal mechanical fatigue life of notch part |
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