CN113312755B - Multi-parameter related accelerated degradation test method for spring for bullet - Google Patents
Multi-parameter related accelerated degradation test method for spring for bullet Download PDFInfo
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Abstract
The invention discloses a multi-parameter related accelerated degradation test method for an elastic spring. The method comprises the following steps (1): determining the degradation performance parameters and the sensitive environmental stress of the spring; (2): making an accelerated degradation test scheme; (3): designing a test fixture, simulating the installation state of the spring, carrying out an accelerated degradation test on the spring, and recording test data; (4): analyzing and processing the test data, and establishing a degradation track model; (5): the performance degradation condition of the spring at the storage temperature is calculated by combining the degradation track model with the acceleration model; (6): and establishing a storage joint reliability model of the spring according to the failure threshold value of the degradation performance parameter and the Copula function, and evaluating the storage life of the spring. The invention comprehensively considers a plurality of performance parameters of the spring, adopts a constant stress accelerated degradation test to quickly obtain the degradation data of the spring, establishes a spring multi-performance parameter reliability model based on a Copula function, and can more accurately estimate the reliability change during the spring storage period.
Description
Technical Field
The invention belongs to the field of reliability tests, and particularly relates to a multi-parameter related accelerated degradation test method for an elastic spring.
Background
Ammunition belongs to a product of 'long-term storage and one-time use', the storage period refers to the storage time meeting the requirement of specified storage reliability under specified storage conditions, and the ammunition is one of important technical indexes of ammunition. At present, china requires the reliable storage life of ammunition to be more than 10 years, and the lower limit of the action reliability of the ammunition is not less than 0.85.
The spring is a common energy storage element, mainly plays a role in limiting the movement of the explosion-proof part or providing energy for the movement of the firing pin in ammunition, and is an important part influencing the reliability of the ammunition. If the storage life is evaluated by using a conventional test method, the time cost and the test cost are too large.
Accelerated degradation tests are currently widely used as important test methods for evaluating the storage life of products under normal stress. The accelerated degradation test of the current spring only considers the stress loss rate and lacks the consideration of other performance parameters of the spring. The spring is used as a product with high reliability and long service life, if only single performance parameter is considered, the obtained degradation data is too little during the test, so that the reliability evaluation result is inaccurate; the free length is an important parameter of the spring, the value of the free length can influence the performance of an ammunition system, for example, the length of the spring in a missile wing unfolding mechanism can influence the motion state change of missile wing unfolding, and thus the overall performance of the ammunition is influenced. The free length of the spring is therefore also an important performance parameter.
Disclosure of Invention
The invention aims to provide a multi-parameter related accelerated degradation test method for an elastic spring.
The technical solution for realizing the purpose of the invention is as follows: a multi-parameter related accelerated degradation test method for an elastic spring comprises the following steps:
step (1): performing storage failure mechanism analysis and reliability test on the spring, and determining the degradation performance parameters of the spring as stress relaxation rate and permanent deformation rate and the stress of a sensitive environment as constant high temperature;
step (2): making an accelerated degradation test scheme according to the degradation performance parameters and the sensitive environmental stress determined in the step (1);
and (3): designing a test fixture, simulating the mounting state of the spring along with the storage of the spring, carrying out an accelerated degradation test on the spring, and recording test data including spring stress and spring length;
and (4): analyzing and processing the test data obtained in the step (3) and establishing a degeneration track model;
and (5): calculating the performance degradation condition of the spring at the storage temperature according to the degradation track model in the step (4) and by combining an acceleration model;
and (6): and establishing a storage joint reliability model of the spring according to the failure threshold value of the degradation performance parameter and the Copula function, and evaluating the storage life of the spring.
Further, the step (1) specifically comprises the following steps:
step (11): analyzing a storage failure mechanism by combining the material, the manufacturing process, the storage environment and the test data of a touch-up test of the spring, and selecting the main degradation performance parameters of the spring as a stress relaxation rate and a permanent deformation rate;
step (12): determining the highest acceleration stress through a groping test according to the extreme use condition of the spring;
step (13): and selecting the constant high temperature as the sensitive environmental stress of the environmental influence factor promoting the plastic deformation of the spring by combining the storage environment of the spring and the test data.
Further, the step (3) specifically includes the following steps:
step (31): designing a test fixture, and simulating the installation state of the spring along with the storage of the spring;
step (32): according to different characteristics of two degradation performance parameters of the stress relaxation rate and the permanent deformation rate of the spring, a measuring scheme of the stress of the spring and the length of the spring in a test is made.
Further, the test fixture adopted in the step (31) comprises a sleeve and a cover plate which are connected through a bolt and a nut;
the sleeve and the cover plate are made of LC4 hard aluminum materials, the cover plate is provided with a through hole, the bottom surface of the sleeve is provided with a through hole, and the inner diameter of the sleeve is 5% larger than the outer diameter of the spring; a circular groove with the same outer diameter as the spring is arranged on the lower side of the cover plate, and the height inside the clamp is equal to the height of a real installation environment in which the spring is stored along with the spring;
the bolts sequentially penetrate through the cover plate, the springs are connected with the through holes in the bottom surface of the sleeve through the bolts, and the cover plate and the sleeve are fastened.
Further, the measuring scheme of the spring stress in the step (32) is as follows:
a step (321): placing a test fixture in a pressure instrument, rotating an instrument rocker arm to enable a probe to move downwards to slowly contact with the top end of the test fixture, and pressing a reset key when the probe just contacts the fixture, wherein the point is a compression zero point;
step (322): after the zero point is determined, the probe is moved upwards, then the spring is placed in a test fixture, the rocker arm of the instrument is rotated to enable the probe to move downwards to be in contact with the top end of the spring, and the spring starts to be compressed;
step (323): after the spring is compressed to a predetermined amount, that is, when the distance data is reduced to 0, a pressure reading stably displayed is read, and the data is recorded.
Further, the measurement scheme of the spring length in the step (32) is as follows:
a step (324): turning on a video microscope and a display, plugging a U disk, and turning on an illumination light source;
a step (325): the spring is placed in the center of the display, and the magnification and the focal length of the objective lens are adjusted to enable the spring to be displayed clearly;
step (326): sequentially shooting, pulling out the USB flash disk after the shooting is finished, inserting the USB flash disk into a desktop computer, storing the shot images in the computer, and renaming the images;
a step (327): the length measurement software was opened for spring length measurement and recorded.
Further, the step (4) specifically includes the following steps:
step (41): processing the test data to obtain a performance parameter degradation trajectory equation of the spring: the data obtained by the test is original data which needs to be processed into performance degradation data, and the calculation formula of the stress relaxation rate is as follows:
wherein R is stress relaxation rate/%, P t The elastic pressure/N of the test piece after recovery; p 0 Is the initial elastic pressure/N of the test piece;
the permanent set is calculated as:
wherein D is the permanent deformation rate/%, L t The length/mm of the recovered test piece; l is 0 Is the initial length/mm of the test piece;
estimating parameters of a degradation track model of the degradation performance parameters of each spring sample by using a least square method according to the processed performance degradation data to obtain a performance parameter degradation track equation of the spring sample;
step (42): fitting the common degradation track model with the degradation track equation of the spring performance parameters obtained in the step (41), and selecting the degradation track model with the minimum sum of squared residuals of the degradation performance parameters as an actual degradation model of the spring.
Further, the step (5) specifically includes the following steps:
step (51): adopting an Arrhenius model as an acceleration model; the specific form of the arrhenius acceleration model is as follows:
wherein α is the reaction rate; a is an Arrhenius constant; e a Is activation energy; t is the absolute temperature; r is Boltzmann constant (8.617X 10) -5 eV/K)；
Taking logarithm of two sides of the Arrhenius equation, and linearizing the logarithm:
ln(α)＝a+b/T
wherein a = ln (a); b = ln (-E) a /R)
Setting the degradation rate alpha 1 ，α 2 ，α 3 Corresponding temperature stress is T 1 ，T 2 ，T 3 By (T) 1 ,α 1 )，(T 2 ,α 2 )，(T 3 ,α 3 ) Fitting the linearized arrhenius equation to the three points to obtain acceleration model parameters a and b;
step (52): the storage temperature was set at 25 ℃.
Further, the step (6) specifically includes the following steps:
step (61): performing goodness-of-fit inspection on the degradation quantity distribution of the spring performance parameters by using an Anderson-Darling inspection method;
step (62): respectively establishing reliability models of the performance degradation parameters of the springs according to the test results;
and (63): estimating parameters by adopting a maximum likelihood method, and selecting a proper Copula function by utilizing an AIC (automatic aided learning) criterion;
step (64): establishing a joint reliability model of the spring according to the failure threshold and the Copula function;
step (65): the storage life of the spring was evaluated at a reliability of 0.95.
Further, the storage life when the spring reliability is 0.95 is:
t 0.95 ＝R -1 (0.95)
wherein R (t) is a joint reliability function, R (t) = C (F) 1 (x 1 ),F 2 (x 2 ) F (x) is a cumulative distribution function of the amount of degradation of the performance parameter.
Compared with the prior art, the invention has the remarkable advantages that:
(1) The test method comprehensively considers a plurality of performance parameters of the spring, adopts a constant stress accelerated degradation test to quickly obtain the degradation data of the spring, establishes a Copula function-based reliability model of the spring with the plurality of performance parameters, and can more accurately estimate the reliability change of the spring during the storage period;
(2) The invention specially designs a spring test fixture to accurately obtain the performance degradation data of the storage period of the spring; the spring is not exposed in the air when stored along with the cartridge, but is wrapped by a cartridge case; therefore, in order to simulate the environment condition of the spring along with the storage of the spring to the maximum extent, the designed clamp is of a sleeve and cover plate structure and completely wraps the spring; the use of the clamp is also convenient for developing an accelerated test, and the spring can be placed under high-temperature stress to accelerate the degradation of the spring;
(3) The invention adopts a computer vision method to measure the free length of the spring, and the precision can reach 0.01mm; at present, most of spring length detection methods adopt a steel plate ruler or a tape measure for manual measurement, and the method has the defects of high labor cost, low efficiency, poor precision and the like.
Drawings
FIG. 1 is a flow chart of the accelerated degradation test method for storage of a ballistic spring of the present invention.
FIG. 2 is a view showing the structure of the spring test jig of the present invention.
Fig. 3 is a flow chart of the spring stress measurement for the cartridge of the present invention.
Fig. 4 is a flow chart of the length measurement of the sprung spring of the present invention.
Fig. 5 is a sectional view of an accelerated degradation test of the present invention.
FIG. 6 is a flow chart of a method of establishing a degradation model based on the test data according to the present invention.
FIG. 7 is a flow chart of a method of the present invention for establishing the spring storage reliability model.
Description of reference numerals:
1-sleeve, 2-cover plate.
Detailed Description
In order to facilitate understanding of the present invention, the technical solutions of the embodiments of the present invention will be described clearly and completely with reference to the accompanying drawings in the embodiments, 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.
Fig. 1 is a flowchart of a spring storage accelerated degradation test method for a bullet according to the present invention, which includes steps S100 to S600.
S100: and (3) carrying out storage failure mechanism analysis and reliability test on the spring to determine the degradation performance parameters and the sensitive environmental stress of the spring.
S200: and making a corresponding accelerated degradation test scheme according to the determined degradation performance parameters and the sensitive environmental stress.
S300: and carrying out an accelerated degradation test on the spring, and recording test data.
S400: and analyzing and processing the test data, and establishing a corresponding degradation track model according to the test data.
S500: and calculating the performance degradation condition of the storage temperature according to the degradation track model and by combining an acceleration model.
S600: and establishing a storage reliability model of the spring according to the threshold value of the degradation performance parameter, and evaluating the storage life of the spring.
In one embodiment, the storage failure mechanism analysis and reliability test of the spring to determine the degradation performance parameters and the sensitive environmental stress of the spring includes steps S110 to S120.
S110: the storage failure mechanism analysis is carried out by combining the material, the manufacturing process, the storage environment and the groping test (brief introduction and available for purpose) data of the spring, and main degradation performance parameters of the spring are selected.
S120: according to the extreme use condition of the spring, the highest acceleration stress is determined through a groping test, and the stress needs to meet the principle of consistency of failure mechanisms, namely, the failure mode generated under the acceleration stress is the same as the failure mode under the normal stress, and the failure mechanism of the spring is not changed.
S130: and selecting the environmental influence factors promoting the plastic deformation of the spring as sensitive environmental stress by combining the storage environment of the spring and the test data of the touch-up test.
Stress relaxation and fatigue fracture are the two major failure modes of a spring. For shelf-life sprung springs, the failure mode is primarily stress relaxation, since the spring is in a constant strain state. Stress relaxation refers to the phenomenon in which the stress of a metallic material or element continues to decrease with time under constant strain. The spring includes, but is not limited to, the following failure mechanisms:
1) Because of different smelting methods of steel, inclusions which cause early failure of the spring in different degrees exist in the steel, the mechanical property of the material is influenced by excessive inclusions or overlarge size and poor uniformity, and early stress relaxation failure is easy to occur;
2) During heat treatment, the uneven surface and center temperature distribution of the spring during heating or cooling can cause thermal stress, the phase change process can cause structural stress, and the spring is easy to relax under the action of internal and external stress;
3) At elevated temperatures, the metal expands with heat and the corresponding change in dimensions changes the various properties of the spring. Furthermore, the elastic modulus E and the shear modulus G of the spring decrease, so the amount of deformation of the spring increases even under a constant load. And under the combined action of stress, temperature and time, deformation and stress relaxation will be an important mode of spring failure.
The spring stress relaxation process is actually a process of converting elastic strain into plastic strain from a macroscopic point of view. Elastic strain epsilon during stress relaxation e And plastic strain epsilon p Is a constant c, i.e.:
ε e +ε p ＝c
in one embodiment, the degradation performance parameters include a stress relaxation rate and a set rate. For shelf-life sprung springs, the failure mode is primarily stress relaxation, since the spring is in a constant strain state.
In one embodiment, the sensitive environmental stress is a constant high temperature. According to the failure mechanism analysis result of the spring, the elastic modulus E and the shear modulus G of the spring are reduced by high temperature, so that stress relaxation is promoted, and the failure is caused by time accumulation. Therefore, the sensitive environmental stress is determined as a constant high temperature.
In one embodiment, the step of making a corresponding accelerated degradation test scheme according to the determined degradation performance parameters and the sensitive environmental stress includes steps S210 to S220:
s210: and designing a special test fixture to maximally simulate the mounting state of the spring along with the storage of the spring.
S220: and making a measurement scheme in the test according to different characteristics of two degradation performance parameters of the stress relaxation rate and the permanent deformation rate of the spring.
FIG. 2 is a structural diagram of the spring test fixture for springs, which is designed to be a special test fixture, and comprises a test fixture material of an LC4 duralumin material; the test fixture is a structure of a sleeve and a cover plate, the inner diameter of the sleeve is 5% larger than the outer diameter of the spring, a circular groove is formed in the lower side of the cover plate, and the diameter of the circular groove is equal to the outer diameter of the spring; the sleeve is connected with the cover plate through bolts and nuts, the height of the inside of the whole clamp is 34% of the height of the spring, and a real installation environment for storing the spring along with the spring is simulated.
The measurement scheme in the formulated test comprises that the spring stress is measured by a digital display pressure instrument; the spring length is measured by computer vision.
Fig. 3 is a flowchart of the spring stress measurement for the bullet of the present invention, which includes steps S211 to S215:
s211: and powering on and starting up the digital display type pressure gauge, confirming that each state parameter of the digital display type pressure gauge is normal, ensuring that a probe of the pressure gauge is not contacted with any object, and leading the pressure reading to flutter around 0.0N.
S212: the tested springs and corresponding test fixtures are prepared, and a data record table is prepared (note the sequence of each tested spring sample, and strictly forbid the confusion of the sample sequence, so that the data is disordered).
S213: and (3) placing the test fixture in a pressure instrument, rotating an instrument rocker arm to enable the probe to move downwards to slowly contact with the top end of the test fixture, and pressing a zero clearing key when the probe just contacts the fixture, wherein the point is a compression zero point.
S214: after the zero point is determined, the probe is moved upwards to a certain height, then the spring is correctly placed in the test fixture, the rocker arm of the instrument is rotated to enable the probe to move downwards to slowly contact with the top end of the spring, and the spring begins to be compressed.
S215: after the spring is compressed to a specified amount of compression (i.e., when the distance data is reduced to 0), a stably displayed pressure reading is taken and the data is recorded.
Fig. 4 is a flowchart of the length measurement of the springing spring of the present invention, which includes steps S216 to S219:
s216: and (4) turning on the video microscope and the display, plugging the USB flash disk in the air, and turning on the LED illumination light source.
S217: the spring sample is placed in the center of the display, and the magnification and the focal length of the objective lens are adjusted, so that the spring sample is displayed clearly.
S218: and photographing in sequence, pulling down the USB flash disk after the photographing is finished, inserting the USB flash disk into a desktop computer, storing the photographed images in the computer, and renaming the images.
S219: the length measurement software was opened for spring length measurement and recorded.
FIG. 5 is a cross-sectional view of an accelerated degradation test of the present invention with test time on the abscissa and temperature stress level on the ordinate. The higher the temperature, the faster the performance parameter of the spring degrades and the shorter the time to reach the threshold. Thus, the higher the temperature, the shorter the test time.
The method for carrying out the accelerated degradation test on the spring and recording test data comprises the following steps of S310-S320:
s310: the detection frequency in the early stage of the test is higher, and the detection frequency is reduced along with the slow change of the degradation performance parameters of the spring.
S320: the test was stopped when the degraded performance parameter of the spring degraded to a specified failure threshold.
It should be noted that the failure thresholds of the performance parameters of the spring are different, and when the performance parameters of the spring reach the failure thresholds, the accelerated degradation test can be continued to obtain complete data.
Fig. 6 is a flowchart of a method for establishing a degradation model according to the test data, where the analyzing of the test data and the establishment of a corresponding degradation trajectory model according to the test data include steps S410 to S420:
s410: and processing the test data to respectively obtain performance parameter degradation curves of the spring.
S420: fitting a common degradation track model with the degradation track of the spring performance parameters, and selecting the degradation track model with the minimum sum of squares of residuals of the degradation performance parameters as an actual degradation model of the spring.
The data obtained by the test is original data which needs to be processed into performance degradation data, and the calculation formula of the stress relaxation rate is as follows:
wherein R is stress relaxation rate/%, P t The elastic pressure/N of the test piece after recovery; p is 0 Is the initial elastic pressure/N of the test piece.
The permanent set is calculated by the formula:
wherein D is the permanent deformation rate/%, L t The length/mm of the recovered test piece; l is 0 Is the initial length/mm of the test piece.
And estimating parameters of a degradation track model of the degradation performance parameters of each spring sample by using a least square method according to the processed performance degradation data of the degradation performance parameters to obtain a degradation track equation of the spring sample. Fitting a common degradation track model with the degradation track of the spring performance parameters, and selecting the degradation track model with the minimum sum of squares of residuals of the degradation performance parameters as an actual degradation model of the spring. The common degradation track models are respectively:
1) Linear model: y is i ＝α i ·t+β i
2) An index model: y is i ＝β i ·exp(α i ·t)
3) The power law model:
4) Logarithmic model: y is i ＝α i ·ln(t)+β
Wherein, y i Performance degradation data for the ith spring sample; t is the test time; alpha is alpha i And beta i Are model parameters.
Performance degradation data obtained from three temperature stress levels in an accelerated degradation test can be obtained by fitting a degradation trajectory equation to obtain the degradation rates of the springs corresponding to the three stress levels, and the degradation rates are set as alpha 1 ,α 2 ,α 3 。
In one embodiment, the step of calculating the performance degradation at the storage temperature according to the degradation track model and the acceleration model includes steps S510 to S520:
s510: an arrhenius model was used as the acceleration model.
S520: the storage temperature was set at 25 ℃.
According to relevant standards and engineering experience, when the acceleration stress is temperature, an arrhenius equation is generally selected as an acceleration model, and the specific form is as follows:
wherein α is the reaction rate; a is an Allen-baus constant; e a Is activation energy; t is the absolute temperature; r is Boltzmann constant (8.617X 10) -5 eV/K)
Taking the logarithm of two sides of the arrhenius equation, it can be linearized:
ln(α)＝a+b/T
wherein a = ln (a); b = ln (-E) a /R)。
Setting the degradation rate alpha 1 ,α 2 ,α 3 Corresponding temperature stress is T 1 ,T 2 ,T 3 By (T) 1 ,α 1 ),(T 2 ,α 2 ),(T 3 ,α 3 ) And fitting the linearized Allen-baus equation to the three points to obtain the acceleration model parameters a and b.
According to the requirements in GJB 2515-1995, the annual average temperature of ammunition storage environment should be 20 ℃, and the monthly average temperature of the warehouse should be in the range of-20 ℃ to 30 ℃. The spring storage environment temperature was set to 25 ℃ in consideration of safety and the like. The degradation rate of the spring at the storage temperature can be obtained by substituting the storage temperature into the calculated acceleration model, and the solution method of the other parameter is different according to different degradation track equations.
Fig. 7 is a flowchart of a method for establishing the storage reliability model of the spring according to one embodiment of the present invention, in which the storage reliability model of the spring is established according to the threshold of the degradation performance parameter to evaluate the storage life of the spring, and the method includes steps S610 to S650:
s610: carrying out goodness-of-fit inspection on the degradation quantity distribution of the spring performance parameters by using an Anderson-Darling inspection method;
s620: respectively establishing reliability models of the spring performance degradation parameters according to the test results;
s630: estimating parameters by adopting a maximum likelihood method, and selecting a proper Copula function by utilizing an AIC (automatic aided learning) criterion;
s640: establishing a joint reliability model of the spring according to a failure threshold and a Copula function;
s650: the storage life of the spring was evaluated at a reliability of 0.95.
Anderson-Darling goodness of fit test is a method for testing whether collected data obeys a certain distribution, and is a nonparametric test method, and the statistic A of the method is 2 Is of the discrete form:
wherein F (-) is a cumulative distribution function of the alternative distributions; y is i Performance degradation data for the ith spring sample; n is the number of samples;
generally, A is 2 The smaller the value, the better the fitting effect of the alternative distribution type is, and the distribution type hypothesis can also be tested by using the p value of the AD test.
Since the Weibull distribution, the normal distribution, the log-normal distribution and the Gamma distribution contain most of the types of the degradation amount distribution, the degradation amount distribution of the performance parameters is preferentially selected from the above distribution types.
The Copula function can be used for researching the edge distribution of random variables and the related structure between the variables separately, the edge distribution form is not limited, and a proper distribution form can be selected according to actual conditions. The AIC criterion is suitable for a Copula function obtained by a maximum likelihood estimation method, and the smaller the calculated AIC value is, the higher the fitting degree of the model is, and the specific form is as follows:
AIC＝-2lnMLE+2k
wherein, lnMALE is the whole log-likelihood function value, and k is the number of parameters in the alternative Copula function.
The joint reliability function of each performance degradation parameter of the spring can be easily obtained by using the Copula function:
R(t)＝C(F 1 (x 1 ),F 2 (x 2 ))
where R (t) is a joint reliability function and F (x) is a cumulative distribution function of the amount of degradation of the performance parameter.
From the joint reliability function, the storage life of the spring can be derived at a reliability of 0.95:
t 0.95 ＝R -1 (0.95)。
Claims (9)
1. a multi-parameter related accelerated degradation test method for an elastic spring is characterized by comprising the following steps:
step (1): performing storage failure mechanism analysis and reliability end-touch test on the spring, and determining the degradation performance parameters of the spring as stress relaxation rate and permanent deformation rate, and the stress of a sensitive environment is constant high temperature;
step (2): making an accelerated degradation test scheme according to the degradation performance parameters and the sensitive environmental stress determined in the step (1);
and (3): designing a test fixture, simulating the mounting state of the spring stored along with the spring, carrying out an accelerated degradation test on the spring, and recording test data including spring stress and spring length;
and (4): analyzing and processing the test data obtained in the step (3) and establishing a degradation track model; the step (4) specifically comprises the following steps:
step (41): processing the test data to obtain a performance parameter degradation trajectory equation of the spring: the data obtained by the test is original data which needs to be processed into performance degradation data, and the calculation formula of the stress relaxation rate is as follows:
wherein R is stress relaxation rate/%, P t The elastic pressure/N of the test piece after recovery; p 0 Is the initial elastic pressure/N of the test piece;
the permanent set is calculated by the formula:
wherein D is the permanent deformation rate/%, L t The length/mm of the recovered test piece; l is 0 Is the initial length/mm of the test piece;
estimating parameters of a degradation track model of the degradation performance parameters of each spring sample by using a least square method according to the processed performance degradation data to obtain a performance parameter degradation track equation of the spring sample;
step (42): fitting a common degradation track model with the degradation track equation of the spring performance parameters obtained in the step (41), and selecting the degradation track model with the minimum sum of squares of residuals of the degradation performance parameters as an actual degradation model of the spring;
and (5): calculating the performance degradation condition of the spring at the storage temperature according to the degradation track model in the step (4) and by combining an acceleration model;
and (6): and establishing a storage joint reliability model of the spring according to the failure threshold value of the degradation performance parameter and the Copula function, and evaluating the storage life of the spring.
2. The method according to claim 1, wherein the step (1) comprises in particular the steps of:
step (11): analyzing a storage failure mechanism by combining the material, the manufacturing process, the storage environment and the touch test data of the spring, and selecting the main degradation performance parameters of the spring as a stress relaxation rate and a permanent deformation rate;
step (12): determining the highest acceleration stress through a groping test according to the extreme use condition of the spring;
step (13): and selecting the constant high temperature as the sensitive environmental stress of the environmental influence factor promoting the plastic deformation of the spring by combining the storage environment of the spring and the test data.
3. The method according to claim 2, wherein the step (3) comprises in particular the steps of:
step (31): designing a test fixture, and simulating the installation state of the spring along with the storage of the spring;
step (32): according to different characteristics of two degradation performance parameters of the stress relaxation rate and the permanent deformation rate of the spring, a measuring scheme of the stress of the spring and the length of the spring in a test is made.
4. The method of claim 3, wherein the test fixture employed in step (31) comprises a sleeve and a cover plate connected by bolts and nuts;
the sleeve and the cover plate are made of LC4 hard aluminum materials, the cover plate is provided with a through hole, the bottom surface of the sleeve is provided with a through hole, and the inner diameter of the sleeve is 5% larger than the outer diameter of the spring; a circular groove with the same outer diameter as the spring is arranged on the lower side of the cover plate, and the height inside the clamp is equal to the height of a real installation environment for storing the spring along with the bullet;
the bolts sequentially penetrate through the cover plate, the springs are connected with the through holes in the bottom surface of the sleeve through the bolts, and the cover plate and the sleeve are fastened.
5. The method according to claim 4, characterized in that the measurement scheme of the spring stress in step (32) is as follows:
step (321): placing a test fixture in a pressure instrument, rotating an instrument rocker arm to enable a probe to move downwards to slowly contact with the top end of the test fixture, and pressing a reset key when the probe just contacts the fixture to determine a zero point of compression;
step (322): after the zero point is determined, the probe moves upwards, then the spring is placed in a test fixture, the rocker arm of the instrument is rotated to enable the probe to move downwards to be in contact with the top end of the spring, and the spring begins to be compressed;
step (323): after the spring is compressed to a predetermined amount, that is, when the distance data is reduced to 0, a pressure reading stably displayed is read, and the data is recorded.
6. The method of claim 5, wherein the spring length measurement protocol in step (32) is as follows:
a step (324): turning on a video microscope and a display, plugging a USB flash disk, and turning on an illumination light source;
a step (325): the spring is placed in the center of the display, and the magnification and the focal length of the objective lens are adjusted to enable the spring to be displayed clearly;
step (326): sequentially photographing, pulling out the USB flash disk after the photographing is finished, inserting the USB flash disk into a desktop computer, storing the photographed images in the computer, and renaming the images;
a step (327): the length measurement software was opened for spring length measurement and recorded.
7. The method according to claim 6, characterized in that said step (5) comprises in particular the steps of:
step (51): adopting an Arrhenius model as an acceleration model; the specific form of the arrhenius acceleration model is as follows:
wherein α is the reaction rate; a is an Arrhenius constant; e a Is activation energy; t is the absolute temperature; r is Boltzmann constant, wherein R =8.617 × 10 -5 eV/K；
Taking logarithm of two sides of the Arrhenius equation, and linearizing the logarithm:
ln(α)＝a+b/T
wherein a = ln (a); b = ln (-E) a /R)
Setting the degradation rate alpha 1 ，α 2 ，α 3 Corresponding temperature stress is T 1 ，T 2 ，T 3 By (T) 1 ,α 1 )，(T 2 ,α 2 )，(T 3 ,α 3 ) Fitting the linearized Allen-ius equation to the three points to obtain acceleration model parameters a and b;
step (52): the storage temperature was set at 25 ℃.
8. The method according to claim 7, characterized in that said step (6) comprises in particular the steps of:
step (61): carrying out goodness-of-fit inspection on the degradation quantity distribution of the spring performance parameters by using an Anderson-Darling inspection method;
step (62): respectively establishing reliability models of the spring performance degradation parameters according to the test results;
and (63): estimating parameters by adopting a maximum likelihood method, and selecting a proper Copula function by utilizing an AIC (automatic aided learning) criterion;
step (64): establishing a joint reliability model of the spring according to the failure threshold and the Copula function;
step (65): the storage life of the spring was evaluated at a reliability of 0.95.
9. The method of claim 8, wherein the shelf life at a spring reliability of 0.95 is:
t 0.95 ＝R -1 (0.95)
wherein R (t) is a joint reliability function, R (t) = C (F) 1 (x 1 ),F 2 (x 2 ) F (x) is a cumulative distribution function of the amount of degradation of the performance parameter.
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