CN113295400A - Rubber sealing life evaluation method - Google Patents
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Abstract
The application provides a rubber sealing life evaluation method, which comprises the following steps: testing the compression set rate before and after the rubber material aging test, and obtaining material aging test data about a stress-strain curve; substituting the material aging test data into a finite element model of the sealing system to obtain the time-varying relation between the contact pressure and the sealing width of the rubber product; obtaining a microscopic morphology image of the surface of the sealing interface; constructing a microscopic leakage channel of the sealing interface according to the time-varying relation between the contact pressure and the sealing width and the surface microscopic topography image of the sealing interface; determining the flow state of the leakage fluid in the microscopic leakage channel according to the geometric dimension of the microscopic leakage channel; and (4) calculating the leakage rate of the rubber product in a fluid state, and predicting the rubber sealing service life. The beneficial effect of this application is: quantitative analysis on the leakage performance of the rubber seal can be realized through the steps, the degradation rule that the leakage rate of the rubber seal increases along with time is obtained, and the service life of the rubber seal is scientifically predicted.
Description
Technical Field
The disclosure relates to the technical field of service life prediction of sealing devices, in particular to a rubber sealing service life evaluation method.
Background
Because the orbit spacecraft including the space station operates and works in the space vacuum, the working cabin of the spacecraft is completely sealed, and thus the air leakage rate is strictly limited. If the spacecraft working cabin not only can influence the normal work of equipment in the cabin, but also can cause the change of the motion orbit and the attitude of the spacecraft due to the aerodynamic force generated by air leakage. For manned space flight, the control of gas leakage rate is more important, and the gas leakage caused by untight sealing has serious influence on the survival of astronauts. The ground test of the spacecraft and the vacuum equipment sealing of the launching equipment are also very important, and the air tightness and the vacuum degree of the spacecraft influence the smooth launching operation of the spaceflight test.
For spacecraft systems that require long-term storage or in-orbit service, the rubber seals therein are weak links that affect their performance. Namely, the quality and reliability of the sealing device can have important influence on the performance and the service life of the spacecraft system. Therefore, it is necessary to develop the research on the degradation rule of the rubber sealing performance under the long-term in-orbit service condition.
At present, for a spacecraft system with long-term on-orbit service requirements, if the sealing structure fails and loses the sealing performance, the sealing device cannot be replaced on the orbit by the prior art, and the aging of a rubber material body is one of the main reasons for the reduction of the sealing performance of rubber. In the prior art, the research on the degradation rule of the rubber sealing performance in a long service life is limited to qualitatively analyzing the contact pressure and the contact area between the rubber sealing contact surfaces, and the quantitative research on the calculation aspect of the leakage rate of the rubber sealing is not involved, which inevitably has great influence on the evaluation precision of the rubber sealing service life.
Disclosure of Invention
The application aims to solve the problems and provides a rubber sealing life evaluation method.
In a first aspect, the present application provides a method for evaluating rubber seal life, comprising the steps of:
testing the compression set rate before and after the rubber material aging test, and obtaining material aging test data about the relationship between the stress-strain curve of the rubber material and the temperature and time;
substituting the material aging test data into a finite element model of the sealing system to obtain the change relation between the contact pressure and the sealing width along with time when the rubber product is in contact with the sealing interface;
acquiring a surface micro-topography image of a sealing interface in contact with a rubber product;
constructing a microscopic leakage channel of the sealing interface according to the change relation between the contact pressure and the sealing width along with time and the surface microscopic topography image of the sealing interface;
determining the flow state of the leakage fluid in the microscopic leakage channel according to the geometric dimension of the microscopic leakage channel;
and (4) calculating the leakage rate of the rubber product in a fluid state, and predicting the rubber sealing service life.
According to the technical scheme provided by the embodiment of the application, the construction of the microscopic leakage channel specifically comprises the following steps:
constructing a primary microscopic leakage channel according to the change relation between the contact pressure and the sealing width along with time and the surface microscopic topography image of the sealing interface;
and developing a rubber product leakage test, and correcting the primary microscopic leakage channel to obtain a corrected microscopic leakage channel.
According to the technical scheme provided by the embodiment of the application, the determining the flow state of the leakage fluid in the microscopic leakage channel according to the geometric dimension of the microscopic leakage channel specifically comprises the following steps: the flow pattern of the leakage fluid in the microscopic leakage path is determined by correcting the geometric dimension of the microscopic leakage path.
According to the technical scheme provided by the embodiment of the application, the method for predicting the rubber sealing life specifically comprises the following steps:
fitting the leakage rate with the service life of the rubber product, and establishing a leakage rate degradation rule calculation formula;
and deducing a calculation formula for calculating the leakage rate degradation rule to obtain the predicted service life of the rubber seal.
According to the technical scheme provided by the embodiment of the application, the developing of the rubber product leakage test and the correction of the primary microscopic leakage channel to obtain the corrected microscopic leakage channel specifically comprise the following steps:
establishing a rubber material thermo-oxidative aging model according to the compression set rate;
predicting the sealing life of a rubber product by using a rubber material thermal oxidation aging model;
carrying out a rubber product leakage rate test on a leakage test stand, and recording the test leakage rate of the rubber product;
and correcting the primary microscopic leakage channel according to the test leakage rate to obtain a corrected microscopic leakage channel.
The invention has the beneficial effects that: the application provides a rubber seal life evaluation method, which comprises the steps of establishing a finite element simulation model according to stress-strain constitutive relation before and after a test rubber material aging test, and analyzing the change rule of rubber seal contact pressure and contact area along with aging time; and constructing a microcosmic leakage channel according to the microcosmic appearance of the surface of the obtained material, and solving the leakage performance of the microcosmic leakage channel to obtain the leakage rate and the performance degradation rule of the rubber seal more accurately, thereby realizing the prediction of the service life of the rubber seal. The method is based on the time-temperature equivalent theory, the influence of the aging effect of the rubber material is taken into account when the rubber sealing performance is analyzed, the change rule of the contact pressure and the contact area of the rubber sealing along with the increase of the aging time is obtained, the microcosmic leakage channel of the rubber sealing is established on the basis, the quantitative analysis of the leakage performance of the rubber sealing is realized, the degradation rule of the leakage rate of the rubber sealing along with the increase of the time is obtained, and the service life of the rubber sealing can be predicted.
Drawings
FIG. 1 is a flow chart of a first embodiment of the present application;
FIG. 2 is a schematic block diagram of a first embodiment of the present application;
FIG. 3 is a diagram showing a prediction of a rubber property change index under a thermal oxidation condition in a first example of the present application;
FIG. 4 is a graph of performance change index versus storage time at-10 ℃ storage temperature for a first example of the present application;
FIG. 5 is a graph showing the relationship between the index of change in properties at a storage temperature of 0 ℃ and the storage time in the first example of the present application;
FIG. 6 is a graph showing the relationship between the index of change in properties at a storage temperature of 10 ℃ and the storage time in the first example of the present application;
FIG. 7 is a graph showing the performance variation index at a storage temperature of 20 ℃ as a function of the storage time in the first example of the present application;
FIG. 8 is a graph showing the relationship between the index of change in properties at a storage temperature of 30 ℃ and the storage time in the first example of the present application;
FIG. 9 is a graph of stress-strain curves of rubber after thermo-oxidative aging in a first embodiment of the present application;
FIG. 10 is a diagram of a finite element model of a rubber seal according to a first embodiment of the present application;
FIG. 11 is a graph comparing the distribution of contact stress in the first embodiment of the present application;
FIG. 12 is a graph of contact width versus performance index for the first embodiment of the present application.
Detailed Description
In order that those skilled in the art will better understand the technical solutions of the present invention, the following detailed description of the present invention is provided in conjunction with the accompanying drawings, and the description of the present section is only exemplary and explanatory, and should not be construed as limiting the scope of the present invention in any way.
Fig. 1 and fig. 2 are schematic diagrams illustrating a first embodiment of the present application, which includes the following steps:
and S1, testing the compression set rate before and after the rubber material aging test, and obtaining material aging test data about the relationship between the stress-strain curve of the rubber material and the temperature and time.
In the step, the main key points of the rubber material aging test are that the temperature range and equivalent parameter indexes of the aging test are determined.
In the step, the key point of the aging test of the rubber material is the determination of the test parameter index. In the aging process of the high polymer material, all performance parameters are changed, and theoretically any one parameter can be used as a performance residual index for establishing a time-temperature equivalent model, but considering that the relationship between the life prediction of the high polymer material and the life of a sealing product (rubber product) needs to be established, and the performance of the material is not equal to the performance of the sealing product, the material performance parameter closest to the life prediction model of the sealing product is searched. However, in the field of sealing research at home and abroad, no mature method or theory for establishing a quantitative relation between the service life of a material and the service life of a sealing product exists at present.
For rubber materials, the aging performance indexes are commonly evaluated as follows: the elongation at break, the compression set, the compression stress relaxation coefficient and the like, and the time-temperature equivalent model established based on rubber compression set data is reported in documents and can be used for predicting the rule of the service life attenuation of a sealing product, so that the aging parameters of the rubber are determined as the compression set.
In the step, the rubber material is subjected to an aging test by using a rubber cylindrical sample or a dumbbell-shaped sample.
In the step, a universal testing machine is adopted to test the stress-strain constitutive relation of the rubber material in the whole aging test period.
And S2, substituting the material aging test data into the finite element model of the sealing system to obtain the time-dependent change relationship between the contact pressure and the sealing width when the rubber product is in contact with the sealing interface.
In the step, the key point is that the relevant test result of the rubber material aging is applied to a finite element model of the rubber product to obtain the change relation between the contact pressure and the sealing width when the rubber product is in contact with the sealing interface, so that the equivalent relation between the material aging and the product aging is realized.
In the step, boundary conditions are set, displacement load is applied to simulate the state of the rubber product with given compression ratio, the contact pressure and the contact width of the rubber product are obtained through solving, and important parameters are provided for subsequently establishing a microcosmic leakage model of the rubber product.
In one embodiment, the rubber product is an O-ring rubber seal.
And S3, acquiring a surface micro-topography image of the sealing interface contacted with the rubber product.
In this step, a ZYGO Nexview 3D optical surface profiler is used to test the surface micro-topography image of the metal material in contact with the rubber product.
And S4, constructing a microscopic leakage channel of the sealing interface according to the change relation of the contact pressure and the sealing width along with the surface microscopic topography image of the sealing interface.
In the step, a microcosmic leakage channel of the sealing interface is constructed according to the change relation between the contact pressure and the sealing width along with time and the surface microcosmic appearance image of the sealing interface based on a statistical theory, and important parameters are provided for quantitatively calculating the leakage rate of the sealing system.
In the present embodiment, regarding the structure of the rubber product, since the deformation of the metal material is only about one ten-thousandth of the deformation of the rubber material under the same stress condition, in the microcontact model, the surface of the rubber product is generally regarded as a smooth plane, and the surface of the metal material in contact therewith is regarded as a rough surface. According to the critical amplification theory of Persson, when the contact area is gradually amplified, the details of the concave-convex body of the smooth plane are gradually shown, the contact area is gradually reduced, and when the critical amplification factor is reached, the fluid leakage channel starts to be shown, so that the amplification factor of the contact area is continuously increased, and finally the microscopic leakage channel can be found.
The surface of the metal material in the rubber seal, which is in contact with the rubber product, is formed by turning, and the rough peak of the surface is a spiral line. Establishing a spiral equation according to the actual feed amount of metal turning, drawing a contact area by the size of a rubber product and the macroscopic contact width, wherein the length of a circumferential leakage channel is the spiral length of the contact area, a radial leakage channel spans a plurality of rough peaks and valleys, the macroscopic contact pressure of each rough peak and valley can be determined in groups according to the contact pressure of finite element analysis of the rubber product, the maximum contact stress of a period can be obtained through the finite element analysis of the rough peak and valley contact of the single period, and the radial leakage channel is calculated according to the maximum contact stress of the period.
The circumferential leakage channels and the radial leakage channels with different diameters, which are caused by different contact pressures, are connected in series (the total flow resistance is the sum of the flow resistances of all parts), so that the inner part and the outer part of the rubber product are communicated, and the circumferential leakage channels and the radial leakage channels which are communicated with the inner part and the outer part are connected in parallel to form the microcosmic leakage channels of the whole rubber product.
In a preferred embodiment, this step comprises:
and S41, constructing a primary microscopic leakage channel according to the change relation of the contact pressure and the sealing width along with the time and the surface microscopic topography image of the sealing interface.
And S42, developing a rubber product leakage test, and correcting the primary microscopic leakage channel to obtain a corrected microscopic leakage channel.
In a preferred embodiment, the step S42 specifically includes:
and S421, establishing a rubber material thermal-oxidative aging model according to the compression set rate.
In the step, a thermal oxidation aging model is established according to a time-temperature equivalent theory, and referring to GJB92.2-1986, the relation between a performance change index P and an aging time tau of vulcanized rubber in an aging process is as follows:
P＝Ae-Kτ (1)
in the formula, P: tensile Property test is the elongation at break L at any timetElongation at break L before aging0The ratio of (A) to (B); the compression stress is relaxed to be a relaxation coefficient; the compression set is 1-P, and P is the compression set at time tau; a is a constant; tau is aging time, [ d](ii) a K is a temperature-dependent rate constant of change of property, [ d ]-1]。
The specific expression of the equation depends on the material aging mechanism, so that the corresponding relation between the microstructure change and the macroscopic property change of the sealing material under the simulation test condition is the basis for establishing a mathematical model.
If LnP is not well linearly related to τ, it can be corrected by:
wherein α is a constant.
After the kinetic expression is clear, the reaction rate constant K is determined by the reaction rate constant K and Arrhenius equation:
K＝Ze-E/RT (3)
wherein T is absolute temperature, [ K ]](ii) a E is apparent activation energy, [ J.mol-1](ii) a Z is a frequency factor, [ d ]-1](ii) a R is a gas constant, [ J.K ]-1.mol-1]。
An expression of P ═ F (τ, T) (where T is the aging temperature) is obtained, and then numerical processing is performed using experimental data, and finally coefficients of the functional formula are fitted.
The data fitting method comprises the following steps: for example, aging test obtains n aging time and performance indexes
When X is τ, Y is LnP, and b is-K, formula (1) can be expressed by Y being a + bX, and coefficients a and b and a correlation coefficient r are obtained by the least square method:
wherein:
looking up a correlation coefficient table in a standard GJB92.2-1986, comparing the r value with the calculated r value according to the degree of confidence of 99% and the degree of freedom of f ═ n-2, and determining the r valueCalculated value|＞rTabular valueThen, a linear relationship between X and Y is established, which can be expressed by Y ═ a + bX, the slope b of the equation is the constant of the rate of change of the performance at the corresponding aging test temperature, and if | rCalculated value|＜rTabular valueThe linear relationship does not hold.
And similarly, calculating the relation between the performance change speed constant K and the temperature T:
order to
Then formula (3) can be represented by Y1＝a1+b1X1Expressing, the coefficient a is obtained by the least square method1、b1And the correlation coefficient r1：
Wherein:
looking up a correlation coefficient table, comparing the r value with the confidence coefficient of 95% and the degree of freedom of f ═ m-2 with the calculated r1 value, and determining if | r1 calculated value|＞rTabular valueThen X1And Y1The linear relationship holds true, available as Y1＝a1+b1X1Denotes if r1 calculated value|＜rTabular valueThe linear relationship does not hold.
Thus, can be according to Y1＝a1+b1X1Calculating the storage temperature Qs[℃]Average value of lower performance change rate constant
For storage temperature
Namely, the method comprises the following steps:
then:
the principle described in GB92.2-1986, these basic theoretical formulas are applicable to high molecular materials.
And S422, predicting the sealing life of the rubber product by using a rubber material thermal oxidation aging model.
Through the aging model established in the step S421, the life of the rubber product under the set use condition can be predicted.
And S423, carrying out the rubber product leakage test on a leakage test bench, and recording the test leakage rate of the rubber product.
Through the prediction of the service life of the rubber product under the set service condition in the step S422, the rubber product is subjected to the aging test under the set condition in the step, and the test time of the aging test is less than the predicted service life.
And S424, correcting the primary micro leakage channel according to the test leakage rate to obtain a corrected micro leakage channel.
It can be known from the above steps S41-S42 and S421-S421 that the preliminary microscopic leakage path established in step S41 is the theoretically established microscopic leakage path, the actual aging test of the rubber product is performed based on the established thermo-oxidative aging model through steps S421-S424, the test leakage rate of the rubber product is obtained through the aging test, the test leakage rate is compared and corrected with the preliminary microscopic leakage path established theoretically, that is, the parameter of the test leakage rate is the accurate parameter, the parameter of the test leakage rate is substituted for the parameter of the leakage rate calculated by the preliminary microscopic leakage path, and the corrected leakage rate is then pushed back to the preliminary microscopic leakage path to obtain the corrected microscopic leakage path.
And S5, determining the flow state of the leakage fluid in the microscopic leakage channel according to the geometric dimension of the microscopic leakage channel.
In a preferred embodiment, the flow pattern of the leakage fluid in the microscopic leakage path is determined by modifying the geometry of the microscopic leakage path, as seen in step S42.
Gas molecular mean free path
The path traveled by one gas molecule in two consecutive collisions with other gas molecules is called the free path. The length of the free path of a single molecule varies widely, but the average of the free paths of a large number of molecules is constant, and is called the mean free path.
The mean free path of a single gas molecule is calculated by the following formula:
from the above formula, the mean free path
Inversely proportional to the gas pressure p, the lower the pressure, the longer the mean free path, and when the temperature T is constant,
the product with p is a constant:
wherein the content of the first and second substances,
is mean free path [ m]N is the number density of gas molecules [ m-3]And σ is the gas molecular diameter [ m ]]T is the thermodynamic temperature [ K ]]K is Boltzmann constant 1.381X 10-23J/K, and p is gas pressure [ Pa ]]。
Flow conditions of gas along the pipe
The flow conditions of the gas along the pipe can be divided into four types, turbulent flow, viscous-molecular flow and molecular flow. Turbulence occurs when gas pressure and flow velocity are high, inertial force plays a major role in gas flow, gas flow lines are not straight nor regular, and this flow state does not occur substantially in static seals. Viscous flow occurs when gas pressure is high and flow velocity is low, inertia force is low, and internal friction of gas plays a main role. When the molecular flow is generated at low pressure of the pipeline, the average free path of gas molecules is larger than the diameter of the pipeline, the collision times among molecules are few, and mainly the gas molecules collide with the pipe wall. And the state between viscous flow and molecular flow is called viscous-molecular flow.
Viscous flow is generally distinguished from molecular flow by the mean free path of the gas molecules and the diameter of the pipe:
wherein d is the diameter of the pipe.
And S6, calculating the leakage rate of the rubber product in the fluid state, and predicting the rubber sealing life.
The leakage amount of viscous flow or molecular flow is calculated according to a Poiseul formula or a Knudsen formula, the flow state of the rubber sealing gas during leakage is the viscous flow, the molecular flow or between the viscous flow and the molecular flow, the section of the microscopic leakage channel can be approximately round or rectangular, and the length of the microscopic leakage channel is far greater than the radius of the section.
The flow conductance calculation formula of different flow states is as follows:
conductance of the pipe in viscous flow:
the pipeline conductance can be solved by a Poiseuille equation during viscous flow, the following four assumptions are required to be met, and the velocity distribution profile is irrelevant to the position; viscous flow; the speed at the wall of the container is zero; the Mach number of the gas is less than 0.3, and when the section of the long pipe is circular (the length is more than 20 times of the diameter of the pipe), the derived flow (conductance) formula is as follows:
wherein Q isnThe flow rate [ Pa.m ] of the long-term pipeline of viscous flow3/s]；UnConductance of long pipe for viscous flow3/s](ii) a d is the diameter of the pipeline, L is the length of the pipeline, eta is the viscosity coefficient of the gas,
is the average pressure [ Pa ] in the pipe]，
p1，p2Respectively the gas pressure [ Pa) at both ends of the pipeline]。
When the long pipe section is rectangular, two sides of the rectangle are a and b (b > a), respectively, the length of the pipeline is L, and the unit of a, b and L is [ m ], then the conductance formula of the rectangular section is:
where ψ is a coefficient relating to a, b,
where th (x) sh (x)/ch (x), sh (x) ex-e-x)/2ch(x)＝(ex+e-x)/2。
Conductance of the pipe during molecular flow:
the conductance of the long pipe with the circular cross section in molecular flow is as follows:
wherein, UfConductance of a long tube with a circular cross-section in molecular flow [ m3/s]R is a molar gas constant 8.3143[ J/(K.mol)]M is the gas molar mass [ kg/mol ]]D is the diameter of the circular section, and in the approximate calculation of the flow conductance of the irregular pipeline, both viscous flow and molecular flow can be calculated by adopting an equivalent hydraulic diameter d of 4A/P, wherein A is the sectional area, and P is the sectional perimeter.
The conductance of the rectangular cross-section long tube during molecular flow is as follows:
wherein a, b and L are respectively two sides of the rectangular section and the length of the pipeline, and the unit is [ m [ ]]，KjIs a form factor.
Molecular conductance for a particular cross-section, such as variable cross-section pipe conductance, can be calculated by the Knudsen (Knudsen) formula:
wherein H is the perimeter [ m ] of the section of the pipeline]L is the length of the pipeline [ m ]]，
Is the average velocity of gas molecule thermal motion [ m/s ]]。
The molecular flow conductance of the straight pipe with uniform section is as follows:
wherein, KxIs the pipe cross-sectional shape factor.
Conductance of the pipe in viscous-molecular flow:
viscous-molecular flow conductance of circular cross-section pipes:
wherein, Un-fConductance of circular cross-section pipeline in viscous-molecular flow3/s]。
The viscous-molecular flow conductance of the rectangular cross-section pipeline is as follows:
wherein, KjIs the shape coefficient of the pipeline with the rectangular cross section, and a and b are respectively the short side and the long side [ m ] of the rectangular cross section]。
The rubber seal leakage performance index is used as a criterion for judging the failure of the rubber seal performance, so that the actual service life of the rubber seal can be predicted.
In this step, the flow regime of the leakage fluid in the microscopic leakage path is determined by the step S5, the conductance in the flow regime is calculated by the conductance formula of the different flow regimes in the step S6, and the leakage rate can be directly converted by the conductance according to the basic knowledge in the art.
In a preferred embodiment of this step, the predicting the rubber sealing life specifically includes:
s61, fitting the leakage rate with the service life of the rubber product, and establishing a leakage rate degradation rule calculation formula;
and S62, deducing a calculation formula for calculating the leakage rate degradation rule to obtain the predicted service life of the rubber seal.
The following are examples of the implementation of steps S1-S6:
(1) developing a rubber material aging test
Taking an accelerated aging test of a rubber material as an example, the rubber material is processed into a cylindrical sample, a clamp with a special compression ratio is prepared, and the degradation rule of the compression set of the rubber material under the environment of different temperatures and hot oxygen media along with time is examined under the set compression ratio.
The test period is determined as 50 days, 5 temperature points are selected, namely T1 … … T5(120 ℃, 130 ℃, 140 ℃, 150 ℃ and 160 ℃), the test time is 8 (3, 9, 15, 20, 25, 30, 40 and 50 days), each test point sample is 5, the average value is taken, and the test results are shown in table 1.
TABLE 1 rubber Material aging test record
(2) Construction of rubber material thermo-oxidative aging model
The performance change index of the rubber under the condition of 120-160 ℃ hot oxygen in the prediction test is shown in figure 3 by using a statistical method of GJB92.2-4.2 'hot air aging method for measuring vulcanized rubber storage performance guide rule'.
Typical storage temperatures of-10 deg.C, 0 deg.C, 10 deg.C, 20 deg.C, and 30 deg.C were selected to predict the rubber property change index P (as shown in FIGS. 4 to 8).
The prediction of the average value of the specific performance change index P is shown in Table 2, and the performance change index P is about 70-50% in 10 years under the normal working condition of the rubber, and a macroscopic contact model is analyzed on the basis of the performance change index P.
TABLE 2 prediction of the mean value of the index of variation of rubber Properties P
(3) Testing the relation between stress and strain before and after aging test of rubber material
In the previous rubber material aging test, the stress-strain curve of the rubber aged in the free state is measured at the same time as shown in fig. 9, and it can be seen from the figure that after the rubber is aged by heat and oxygen, the stress-strain curve is bent to the upper left relative to the initial state, and the rubber becomes 'hard' along with the increase of the aging time of the heat and oxygen, and the mechanical property of the rubber can be represented by using an M-R model.
According to the constructed rubber material thermo-oxidative aging model, the performance change index in the 10-year aging process is about 70% -50% (as shown in table 3), the rubbers with the performance change indexes of 70%, 65%, 60%, 55% and 50% are taken as research objects, and the corresponding relations between the corresponding performance change indexes and the aging time are respectively 140 ℃ thermo-oxidative aging for 20 days, 160 ℃ thermo-oxidative aging for 25 days, 160 ℃ thermo-oxidative aging for 40 days and 160 ℃ thermo-oxidative aging for 50 days.
TABLE 3 correlation between Performance Change index and aging time
| Approximate actual aging time | When aged by thermal oxidationWorkshop | Index of |
| Year | ||
| 0 | |
100％ |
| Storing at 0 deg.C for 5 years | Thermal oxidative aging at 140 deg.C for 20 |
70％ |
| Storing at 10 deg.C for 5 years | Aging at 160 deg.C for 20 |
65％ |
| Storing at 5 deg.C for 10 years | Aging at 160 deg.C for 25 |
60％ |
| Storing at 15 deg.C for 10 years | Aging at 160 deg.C for 40 |
55％ |
| Storing at 25 deg.C for 10 years | Aging at 160 deg.C for 50 |
50％ |
(4) Finite element analysis of macroscopic contact performance of rubber seal
According to the structural characteristic that the rubber sealing ring has circumferential circular symmetry, a 3-degree region of the sealing ring (rubber product) can be selected as a research object to develop related research. The finite element model of the rubber seal is shown in fig. 10, and the boundary conditions for the performance analysis are: applying x and z direction displacement constraints at position 1; circularly symmetrical constraint is applied to the rubber sealing ring, the sealing groove and the pressure plate circumferential truncation part 2; applying y-direction displacement excitation at position 3; applying displacement constraints in x, y and z directions at the position of the sealing groove 4; a gas pressure load is applied at position 5.
As shown in fig. 11, the maximum contact stresses of the 3 contact pairs are all located at the contact center, in order to more intuitively and accurately represent the contact stress distribution, a left-to-right path is established by a node connected with the top of the O-ring, the absolute distance on the path is taken as an X axis, the contact stress is taken as a Y axis, and the contact stress distribution under different working conditions is analyzed.
The contact stress distribution curve comparing the initial state and different performance change indexes is shown in figure 10, the performance change indexes of the rubber are gradually reduced along with the increase of aging time, the contact stress is gradually reduced, the contact stress curves are all in a parabola shape with a downward opening, after the rubber is stored for 10 years at the storage temperature of 25 ℃, the performance change index of the rubber is reduced to 50 percent, namely, the compression permanent deformation is 50 percent, and meanwhile, as the hardness of the rubber is increased, the highest contact stress is still larger than the pressure of a sealing medium under the comprehensive action, and the sealing effect can be achieved. The analysis reason needs to consider that the rubber performance change index is reduced along with the increase of the aging time, and the contact stress is reduced due to the reduction of the rebound rate, but the rubber stress-strain curve is changed, the contact stress is increased due to the fact that the rubber is hardened, after the aging is carried out for 10 years at the temperature of 25 ℃, the contact stress increased due to the hardening of the rubber is smaller than the contact stress reduced due to the reduction of the rebound rate, and finally the contact stress is reduced.
As shown in fig. 12, which is a schematic diagram of the contact width variation, the contact width gradually increases with the performance variation index, and both the contact width and the contact width are smaller than the initial contact width by 2.39 mm.
(5) Construction of rubber-sealed microscopic leakage passages
According to the size of a sealing ring, the size of a contact circular ring of a sealing contact interface is obtained by combining contact width data obtained by a macroscopic finite element contact model, according to a macroscopic millimeter scale three-dimensional white light measurement result of a metal flange, a flange turning spiral line track is superposed on the contact circular ring to obtain a sealing contact area model, according to the sealing state, a sealing medium can be directly leaked along the diameter direction, the leakage is mainly carried out along the turning contour valley bottom with lower contact stress in the circumferential direction, and the leakage path is in a spiral line shape. Since the macroscopic contact stress distribution is in a peak shape with two low sides and a high middle part, the contact stress difference between the contact center and the contact edge is large, the flow state of the leakage fluid may be different, and therefore, the contact area needs to be calculated in a segmented mode along the radius direction.
(6) Calculating the leakage rate of rubber seals
In each section of the contact area, the average value of macroscopic contact stress is taken, the sizes of radial and circumferential leakage channels are calculated according to the result of a microscopic finite element contact model, the conductance of each section is obtained according to different conductance calculation formulas according to the difference of flow states, finally, the section conductances are sequentially connected in series to obtain the total conductance of the radial and circumferential directions, and the leakage rates of the two directions are respectively calculated and summed to obtain the total leakage rate of the sealing ring.
In a thermo-oxidative aging test, the compression permanent deformation and the stress-strain relation of rubber under different temperatures and aging time are obtained through testing, a time-temperature equivalent model is established, the corresponding relation between the thermo-oxidative condition and the actual storage condition is obtained, 6 typical values are calculated according to the rubber initial sample and the thermo-oxidative aging performance indexes (100% -compression permanent deformation rate) of 70%, 65%, 60%, 55% and 50%, respectively, and the leakage rate of the sealing ring is calculated.
Setting the working condition as room temperature (20 ℃), the sealing medium as helium, the internal diameter of the sealing ring as 30mm, the pressure of the sealing medium as 0.6MPa and the external pressure of the sealing cavity as atmospheric pressure, and calculating the leakage rate of the sealing ring under each working condition from the initial 9.2 multiplied by 10-8Pa.m3Increase of/s to 2.03X 10-6Pa.m3And s, the sealing performance is obviously reduced.
The principles and embodiments of the present application are explained herein using specific examples, which are presented only to assist in understanding the method and its core concepts of the present application. The foregoing is only a preferred embodiment of the present application, and it should be noted that there are objectively infinite specific structures due to the limited character expressions, and it will be apparent to those skilled in the art that a plurality of modifications, decorations or changes may be made without departing from the principle of the present application, and the technical features described above may be combined in a suitable manner; such modifications, variations, combinations, or adaptations of the invention using its spirit and scope, as defined by the claims, may be directed to other uses and embodiments, or may be learned by practice of the invention.
Claims (5)
1. A rubber seal life evaluation method is characterized by comprising the following steps:
testing the compression set rate before and after the rubber material aging test, and obtaining material aging test data about the relationship between the stress-strain curve of the rubber material and the temperature and time;
substituting the material aging test data into a finite element model of the sealing system to obtain the change relation between the contact pressure and the sealing width along with time when the rubber product is in contact with the sealing interface;
acquiring a surface micro-topography image of a sealing interface in contact with a rubber product;
constructing a microscopic leakage channel of the sealing interface according to the change relation between the contact pressure and the sealing width along with time and the surface microscopic topography image of the sealing interface;
determining the flow state of the leakage fluid in the microscopic leakage channel according to the geometric dimension of the microscopic leakage channel;
and (4) calculating the leakage rate of the rubber product in a fluid state, and predicting the rubber sealing service life.
2. The rubber seal life evaluation method of claim 1, wherein said constructing a microscopic leakage path, comprises in particular the steps of:
constructing a primary microscopic leakage channel according to the change relation between the contact pressure and the sealing width along with time and the surface microscopic topography image of the sealing interface;
and developing a rubber product leakage test, and correcting the primary microscopic leakage channel to obtain a corrected microscopic leakage channel.
3. The method for evaluating the life of a rubber seal according to claim 2, wherein the determining the flow state of the leakage fluid in the microscopic leakage path according to the geometric dimension of the microscopic leakage path specifically comprises: the flow pattern of the leakage fluid in the microscopic leakage path is determined by correcting the geometric dimension of the microscopic leakage path.
4. The method for evaluating rubber seal life according to claim 3, wherein the predicting rubber seal life specifically comprises the steps of:
fitting the leakage rate with the service life of the rubber product, and establishing a leakage rate degradation rule calculation formula;
and deducing a calculation formula for calculating the leakage rate degradation rule to obtain the predicted service life of the rubber seal.
5. The method for evaluating the service life of a rubber seal according to claim 2, wherein the step of developing a test for testing the leakage rate of the rubber product and correcting the preliminary microscopic leakage path to obtain a corrected microscopic leakage path comprises the following steps:
establishing a rubber material thermo-oxidative aging model according to the compression set rate;
predicting the sealing life of a rubber product by using a rubber material thermal oxidation aging model;
carrying out a rubber product leakage rate test on a leakage test stand, and recording the test leakage rate of the rubber product;
and correcting the primary microscopic leakage channel according to the test leakage rate to obtain a corrected microscopic leakage channel.
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