CN113010973A - Plastic gear accelerated fatigue test method considering temperature effect - Google Patents

Plastic gear accelerated fatigue test method considering temperature effect Download PDF

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CN113010973A
CN113010973A CN202110079348.4A CN202110079348A CN113010973A CN 113010973 A CN113010973 A CN 113010973A CN 202110079348 A CN202110079348 A CN 202110079348A CN 113010973 A CN113010973 A CN 113010973A
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CN113010973B (en
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刘怀举
兰宇轩
宋海蓝
卢泽华
张仁华
魏沛堂
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Chongqing University
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Abstract

The invention discloses a plastic gear accelerated fatigue test method considering temperature effect, which comprises the following steps: 1. obtaining the fatigue life of the plastic gear at the temperature T and the cyclic stress S according to fatigue test data or fatigue simulation analysis at different temperatures; 2. determining equivalent injury factorDAnd temperatureTAnd cyclic stressSThe relationship of (1); 3. obtaining equivalent damage factor by adopting data fitting modeDAnd fatigue lifeNThe relationship between; 4. is found at temperatureTAnd cyclic stressSAn evolution equation of damage of the plastic gear under action; 5. and selecting any one of a constant load fatigue test and a stepped accelerated fatigue test for testing. The invention has the technical effects that:the temperature and load double-factor damage variables are equivalent to a single damage factor, a plastic material damage-life equation is constructed, the fatigue failure of the plastic gear under the joint influence of the operating temperature and the load can be determined, the obtained result is accurate, and the fatigue test cost of the plastic gear is reduced.

Description

Plastic gear accelerated fatigue test method considering temperature effect
Technical Field
The invention belongs to the technical field of mechanical part service life tests, and particularly relates to a temperature effect-considered plastic gear fatigue life acceleration test method.
Background
The plastic gear has the advantages of light weight, corrosion resistance, low cost and the like, and is widely applied to the fields of intelligent home, automobile delivery, medical health and the like. Common materials for plastic gears are Polyoxymethylene (POM), nylon (PA), Polyetheretherketone (PEEK) and their composites, which belong to the category of semi-crystalline thermoplastics. Because the gear operation temperature under load changes violently, and the plastic gear is extremely sensitive to the temperature change, the conventional metal gear accelerated fatigue test method cannot be directly used, and the product delivery detection lacks an effective performance evaluation means.
The fatigue life test of the existing plastic gear products has two types: one is to carry out life test under the condition inferior to the rated service environment according to the experience of manufacturers, for example, the electric door lock plastic gear box usually applies 2-3 times rated load under the dry contact condition, and the normal 6000 hours service life under the grease lubrication condition is replaced by the fault-free operation for 300 hours as a qualified index, but the problems that the load setting is unreasonable, the designed gear performance is far higher than the required level and is excessively conservative and the like exist; the other method is to carry out a full life cycle test according to the actual service environment, such as a sweeping robot driving gear box, and simulate the working environment according to the actual life requirement to carry out a 6000-hour fatigue test, but the method has the problems of long test period, high test cost and the like. The plastic gear industry is developing forward in the directions of high speed, high load and light weight, the current plastic gear fatigue life test method cannot meet the development trends of market diversification, high response and low cost, and a plastic gear material characteristic and service environment accelerated fatigue life test method based on considering the temperature effect is urgently needed to be found.
Disclosure of Invention
Aiming at the problems in the prior art, the invention aims to solve the technical problem of providing a plastic gear accelerated fatigue test method considering the temperature effect, breaking through the limitation that the traditional metal material single-factor fatigue damage accumulation is not suitable for plastic materials, equating the temperature and load double-factor damage variable to be a single damage factor, constructing a plastic material damage-life equation, determining the fatigue failure of the plastic gear under the common influence of the operation temperature and the load, obtaining an accurate result and reducing the fatigue test cost of the plastic gear.
The technical problem to be solved by the invention is realized by the technical scheme, which comprises the following steps:
step 1, obtaining the fatigue life of the plastic gear at temperature T and cyclic stress S according to fatigue test data or fatigue simulation analysis at different temperatures;
step 2, determining the relation between the equivalent damage factor D and the temperature T and the cyclic stress S:
D＝α×T+β×S
in the formula, alpha and beta are respectively the influence coefficients of temperature T and cyclic stress S on damage;
step 3, obtaining a relation between the equivalent damage factor D and the fatigue life N by adopting a data fitting manner, wherein logN is λ (a × D + b), and a and b are fatigue life coefficients; lambda is the lubrication coefficient and takes the value of 1.08
Step 4, according to the linear fitting relation between the equivalent damage factor D and the fatigue life N in the step 3, the plastic gear damage evolution equation under the action of the temperature T and the cyclic stress S is D (logN)/dD ═ C, wherein C is a constant;
step 5, selecting plastic gear fatigue test method
The plastic gear fatigue test method selects any one of a constant load accelerated fatigue test and a stepped accelerated fatigue test:
1) and constant load accelerated fatigue test:
loading by adopting a constant damage factor, carrying out fatigue test on a plurality of plastic gears, and recording the cycle number of each plastic gear when each plastic gear fails;
averaging the test data to obtain the cycle number N of the plastic gear in actual failureiSubscript i denotes the number of test gears; comparing the equivalent damage factor D-fatigue life N curve, if the cycle number NiGreater than the theoretical number of cycles N0The fatigue life of the plastic gear is qualified, otherwise, the plastic gear is unqualified;
2) step-type accelerated fatigue test:
loading by adopting a step-type variable damage factor, continuously carrying out a fatigue life test until the gear fails, wherein the cycle times under each damage factor are the same; carrying out fatigue tests on the plurality of plastic gears, and recording the level of damage factors and the cycle times when each plastic gear fails; averaging the number of cycles of failure;
calculating and summing the ratio of the cycle times under each damage factor value to the theoretical cycle times to obtain an accumulated value of the damage rate; if the accumulated damage rate value is equal to or greater than 1, the fatigue life of the plastic gear is qualified, otherwise, the plastic gear is unqualified.
Preferably, in the constant load accelerated fatigue test, the constant damage factor is 80% of the plastic gear yield limit.
Preferably, in the step-type accelerated fatigue test, the step-type change damage factor is selected from four numerical levels of 20%, 40%, 60% and 80% of the yield limit of the plastic gear.
The invention has the technical effects that:
the method has the advantages that the fatigue failure problem of the plastic gear is analyzed under the conditions of temperature and load effect, the theoretical life value of the plastic gear is determined to be used as reference, the fatigue test of the plastic gear is further developed, the obtained analysis result is well matched with test data, a basis is provided for manufacturing and using the plastic gear, the economic cost and the time cost of the test for detecting the contact fatigue life of the plastic gear in engineering practice are reduced, and meanwhile, accidents and economic losses caused by the contact failure of the plastic gear are reduced.
Drawings
The drawings of the invention are illustrated as follows:
FIG. 1 is a graph of plastic gear fatigue life at different temperatures T and cyclic stresses S;
FIG. 2 is a theoretical fitting relationship between the equivalent damage factor D and the fatigue life N;
FIG. 3 is a test fitting relationship between the equivalent damage factor D and the fatigue life N;
FIG. 4 is a graph of the number of cycles to failure versus the D-N curve for a constant load fatigue test;
FIG. 5 is a graph of the number of cycles of a stepped accelerated fatigue test versus a D-N curve;
FIG. 6 is the constant load accelerated fatigue test data for example 1;
fig. 7 shows the stepwise load spectrum and the cumulative damage rate of example 2.
Detailed Description
The invention is further illustrated by the following examples in conjunction with the accompanying drawings:
the invention comprises the following steps:
step 1, obtaining the fatigue life of the plastic gear under the temperature T and the cyclic stress S
According to fatigue test data or fatigue simulation analysis at different temperatures, the data of the fatigue life N of the plastic gear at different temperatures T and cyclic stress S (contact stress) are obtained, and the data are shown in figure 1. Fatigue life at 50% reliability can be obtained in fig. 1 for different stresses and temperatures.
Since The contact fatigue life curves at different temperatures require a large number of repeated tests, The literature "The simulation and experiment of research on contact failure performance of The access devices", MECMAT, 2021: 154 (simulation and test research on contact fatigue performance of acetal gears, MECAT, 2021: 154), establishing a life simulation analysis model of the plastic gear at different temperatures, performing contact fatigue test on the POM gear at different temperatures, acquiring a contact fatigue SN curve of the POM gear at 30 ℃ and test data of part of the POM gear at high and low temperatures, and verifying the correctness of the simulation analysis model by using the test data. In addition, the life data at different temperatures disclosed in German plastic gear standard VDI 2736-2 are used for verifying the simulation analysis model, and the results of the simulation analysis model are well matched.
In order to verify the correctness of the simulation result, a contact fatigue test is carried out, and the output torques of 40, 60, 80 and 100Nm and the input rotating speed are selected to be kept at 1000r/min at the oil temperature of 30 +/-2 ℃. The test device is a dynamic open type test bed capable of predicting vibration signals in the test process. The gear sample is processed by hobbing, so that the injection molding defect is avoided. The effectiveness of the method was verified by the fact that the actual fatigue fracture occurred in the same area. The microscopic morphology of the gear teeth was observed by an optical microscope and fatigue fractures were found to occur between the pitch line and the tooth spaces. Due to the hobbing process, the original machining traces are still observed around the pitch line region in the lubricated state. Tooth flank contact fatigue life was compared and analyzed by contact fatigue testing, finite element simulation, and VDI2736 prediction. The tests were repeated twice at each load level and the flank contact fatigue failure pattern was the same for all tests. It can be found that the simulation results, the test results and the standard prediction results agree quite well.
Meanwhile, a large number of tests on the tensile property of the plastic material at different temperatures have been developed in the early stage by the conventional material suppliers, relevant data are disclosed, and the fatigue strength limit of the material at different temperatures and cycle times can be deduced according to the elastic modulus, the poisson ratio and the yield strength at different temperatures and by combining the Beeger equation. On the basis of test, the gear fatigue strength under different temperatures and cyclic stresses can be indirectly obtained according to the numerical calculation result, and the workload is greatly saved. As mentioned above, the POM gear contact fatigue test at different temperatures is carried out, and the correctness of the model is verified.
The material used in the test of FIG. 1 was a copolymer POM available from Nippon Baysian corporation, and was designated as material No. M90-44. The simulated fatigue curves are from The literature "The simulation and experiment of The research on contact fatigue performance of acetal gears", MECAT, 2021.154 ("simulation and test study of The contact fatigue performance of acetal gears", MECAT, 2021.154), The material parameters are from The literature "The effect of injection molding of lubricant on The performance of polymer gears", Int J Mec Sci.2020.180:105665 ("impact of injection molding of lubricant on The durability performance of polymer gears", Int J Mec Sci.2020.180:105665), and The test data are from The literature "The research on The bearing of lubricant under dry conditions" of lubricant under lubricating conditions of lubricant on The bearing of lubricant assemblies and lubricant conditions ", chemistry.2020.180: 105665 (" test of lubricant on The bearing of lubricant under lubricating conditions ", 82.180: 105665).
As can be seen from the curves in fig. 1, like many S-N curves, the contact stress is reduced and the fatigue life is improved under the same temperature environment; as the test temperature increases, the contact fatigue life measured by the contact stress of the gears of the POM material at the same level decreases.
Step 2, obtaining the relation between the equivalent damage factor D and the temperature T and the cyclic stress S
Since both temperature and load have an influence on contact fatigue life, the contact pressure S and temperature T in the plurality of SN curves obtained in step 1 are extracted to obtain a new influence factor, i.e., an equivalent damage factor D. The relationship can be expressed as
D＝α×T+β×S
Wherein alpha and beta are the influence coefficients of temperature and stress on damage, which are called temperature damage coefficient and cyclic stress damage coefficient. α and β are empirical coefficients obtained by linear regression from the lifetime under stress at different temperatures, and α and β are different for different materials, and reference values for POM materials are listed in the expressions of the following examples.
Six data points at each test temperature are selected on the basis of the data of fig. 1, and the relationship between the equivalent damage factor D and the contact fatigue life under the theoretical value is fitted according to a linear regression method, as shown in fig. 2.
Step 3, verifying the relation between the equivalent damage factor D obtained in the step 2 and the fatigue life N
When the theoretical DN curve obtained in the step 2 is verified, because the states under the combination of the temperature T and the cyclic stress S are more, in order to simplify the test, a more accurate DN curve is obtained in a short time, the test temperature T and the cyclic stress S under the condition of the step 2 are selected, a contact fatigue test is carried out, linear regression fitting is carried out according to the obtained data points, and the fact that the ideal DN curve has better coincidence with the DN curve in the figure 2 is found. See FIG. 3
The relationship between the equivalent damage factor D and the fatigue life N is obtained by a data fitting (linear regression) method, which can be generally described as logN ═ λ (a × D + b), where a and b are related to the parameters of the material and the lubrication conditions, and are fatigue life coefficients, and a and b are obtained by fitting data points and linear regression as shown in fig. 3; lambda is the lubrication coefficient and takes the value of 1.08.
Step 4, determining a damage evolution equation of the plastic gear
The change rule of the damage inside the plastic gear material along with the action of external factors (such as load, temperature change and the like) is described by using an equation logN ═ lambda (a multiplied by D + b), and the damage evolution equation of the plastic gear is D (logN)/dD ═ C, wherein C is a constant.
Step 5, selecting plastic gear fatigue test method
And (4) designing an accelerated fatigue test method of the plastic gear according to the damage change rate obtained in the step (4) and by combining a Miner linear accumulated damage law. The main assumptions of the Miner's linear cumulative injury law are: under the action of the constant-amplitude fatigue load spectrum, the damage amount caused by each cyclic loading is equal and can be independently added; under the action of a multi-stage amplitude-variable load spectrum, damage caused by cyclic loading at each stage can be independently added; the critical damage corresponding to the fatigue failure of the material is a constant, depends only on the characteristics of the material, and is independent of the load and the loading process. With niRepresenting the number of cycles under a certain level of stress in the load spectrum, NiRepresenting the total number of cycles that will result in failure at this stress level, fatigue failure occurs in the case of:
Figure BDA0002908671980000061
the relation equation of the stress amplitude and the service life of the plastic gear is as follows:
σi mNi＝C
the following formula is combined:
Figure BDA0002908671980000062
in the formula, niNumber of cycles at each stress level, NiAt σiContact fatigue life at stress level, σiStress at each stage, σ1Is the maximum stress value, σkIs the equation constant of the minimum stress value, the index m, the contact fatigue life cycle number and the contact fatigue life cycle number. (i ═ 1,2, …, k)
It can be seen from the above equation that if the magnitude of the applied stress is larger, the number of cycles will be smaller; if the magnitude of the applied stress is smaller, the number of test cycles is longer, and the test period is longer. If a suitable acceleration load is applied and the load level increases regularly with the number of cycles or cycle time, the time required for the test can be reduced considerably without altering the failure mechanism.
Assuming that the part is at a damage factor DiCycle number of fatigue life under NiThe Miner linear cumulative damage law holds that when the part cycle number reaches niWithout reaching the theoretical number of damage NiThe lifetime damage per cycle is ni/NiWhen the cumulative damage value is the sum of the damage rates
Figure BDA0002908671980000063
When (i ═ 1,2, …, k), contact fatigue failure occurred.
In the process of a constant load accelerated fatigue test or a step type accelerated fatigue test, when the sum of damage accumulation values under all damage factor levels reaches 1, the gear generates contact fatigue failure. The actual contact fatigue life of the gear obtained through tests can be compared with the DN curve (figure 2) obtained theoretically, and the qualification condition of the plastic gear can be judged. Therefore, any one of the constant load accelerated fatigue test and the stepped accelerated fatigue test can be selected:
1) constant load accelerated fatigue test
And (3) loading by adopting a constant damage factor, carrying out fatigue test on the plurality of plastic gears, and recording the cycle number of each plastic gear when the plastic gear fails.
In order to obtain a more accurate test result, 5 groups of plastic gears obtained under the same condition are selected for testing, loading is carried out according to 80% of the yield limit of the plastic gears, and the cycle number of the plastic gears when the plastic gears fail is recorded.
Common materials for plastic gears include POM (polyoxymethylene), PA6 (polyamide 6 or nylon 6), PA66 (polyamide 66 or nylon 66), PC (polycarbonate), PPA (polyparaphenyleneterephthalamide), PBT (polyester), PPS (polyphenylene sulfide), PEEK (polyetheretherketone), and composite materials thereof.
And (3) judging the result of the constant load accelerated fatigue test:
according to the test data obtained in the step 1), the fatigue life of 5 groups of plastic gears under the same level of damage factors is averaged to obtain the cycle number N when the plastic gears actually fail. Comparing the D-fatigue life N curve of the equivalent damage factor in the step 3, and if the cycle number N is more than the theoretical cycle number N0And if the fatigue life of the plastic gear meets the requirement of a theoretical value, otherwise, the fatigue life of the plastic gear does not meet the requirement of the theoretical value.
Data processing see FIG. 4, D0The damage factor value is corresponding to the constant load of the fatigue test; n is a radical of0Is D0The corresponding theoretical fatigue life; n is a radical of1And N2Data points for both cases obtained for the experiment, and N1<N0，N2>N0。
When the experimental result is N1In the meantime, the actual fatigue life of the plastic gear is less than the theoretical fatigue life obtained by the DN curve, and if the batch of gears are unqualified in actual production; when the experimental result is N2In time, the actual fatigue life of the plastic gear is greater than the theoretical fatigue life obtained from the DN curve, and if the batch of gears is qualified in actual production.
2) Stepped accelerated fatigue test
In actual production life, besides a constant load fatigue test method, a stepped accelerated fatigue test can be used to obtain the relation between the test fatigue life and the theoretical value of the plastic gear. Therefore, another test method, namely a stepped accelerated fatigue test, is proposed on the basis of the test method. Using stepped stress spectroscopy, test specimens are first tested at a given stress level for a predetermined period of time, and then at a slightly higher stress level for a further period of time. The above process continues with increasing stress levels until either a test specimen fails or the test is terminated when the test is run to a maximum stress level. This method can more quickly disable the product for analysis.
And (3) according to the equivalent damage factor D-fatigue life N curve obtained in the step (3), evaluating the fatigue life of the plastic gear by using a step-type accelerated fatigue test and adopting a step-type variable damage factor loading mode.
Loading by adopting a step-type variable damage factor, continuously carrying out a fatigue life test until the gear fails, wherein the cycle times under each damage factor are the same; carrying out fatigue tests on the plurality of plastic gears, and recording the level of damage factors and the cycle times when each plastic gear fails; the number of cycles to failure is averaged.
Specifically, 20%, 40%, 60% and 80% of the yield limit of the plastic gear is selected for loading, and the cycle times under each damage factor condition are the same. The fatigue life test was continued until failure of the gear occurred. The level of injury factor and cycle number at failure were recorded. Five experiments were repeated with an average.
Judging the stepped accelerated fatigue test result:
according to the test data obtained in the step 2), averaging multiple groups of test data to obtain the failure condition of the plastic gear, and calculating and summing the ratios of the cycle times under different loads and the theoretical cycle times by combining the Miner fatigue life criterion to obtain the accumulated value of the damage rate. If the accumulated damage rate value is equal to or greater than 1, the fatigue life of the plastic gear meets the requirement of a theoretical value, otherwise, the fatigue life is lower than the theoretical value.
Data processing see FIG. 5, D1、D2、D3、D4Stepwise loading of the Damage factor, N, at different levels1、N2、N3、N4Is a corresponding theoretical fatigue life value, N, under different levels of damage factorsaThe number of cycles of the plastic gear under different damage factor levels. Calculating damage rate cumulative value
Figure BDA0002908671980000081
Compared to 1. If the value is less than 1, the actual fatigue life of the plastic gear is less than the theoretical fatigue life obtained by the DN curve, and if the batch of gears is not qualified in actual production; when the value is greater than 1, the actual fatigue life of the plastic gear is greater than the theoretical fatigue life obtained from the DN curve, and if the batch of gears is qualified in actual production.
Example 1 constant load accelerated fatigue test
Step 1, obtaining the contact fatigue life of the plastic gear under the temperature T and the cyclic stress S
According to fatigue test data or fatigue simulation analysis at different temperatures, the data of the fatigue life N of the plastic gear at different temperatures T and cyclic stress S are obtained, and a curve shown in figure 1 is drawn.
Step 2, obtaining the relation between the equivalent damage factor D and the temperature T and the cyclic stress S
Selecting five data points on each curve according to the fatigue life data under different temperatures T and cyclic stresses S obtained in the step 1, calculating an equivalent damage factor D according to a formula D of alpha multiplied by T + beta multiplied by S of 11.34 multiplied by T +7.71 multiplied by S, drawing a scatter diagram and performing linear regression to obtain a DN curve theoretical value curve which is as shown in figure 2, wherein the curve equation is D of (-7.5 multiplied by 10)-7)N+100.56。
TABLE 1 Damage factor D and fatigue Life N finite element simulation data
Figure BDA0002908671980000082
Figure BDA0002908671980000091
And 3, obtaining the relation between the equivalent damage factor D and the fatigue life N.
In order to verify the correctness of the curve obtained in the step 2, the test temperature T and the cyclic stress S corresponding to part of scatter points are selected, the equivalent damage factor D is calculated, a scatter diagram is drawn and linear regression fitting is carried out to obtain a DN test value curve in the form of a graph 3, and the curve equation is that D is (-7 multiplied by 10)-7) N + 102.32. And finding better coincidence with a theoretical value curve.
TABLE 2 Damage factor D and fatigue Life N test data
Figure BDA0002908671980000092
Figure BDA0002908671980000101
Step 4, defining a damage evolution equation of the plastic gear
According to the relationship between the equivalent damage factor D and the fatigue life N in the step 3, the damage change rate D (logN)/dD under the action of the temperature T and the cyclic stress S is constant, namely DN is linear relationship.
Step 5, selecting a constant load accelerated fatigue test
And (4) selecting a constant load accelerated fatigue mode to perform a fatigue test on the POM plastic gear. Selection of injury factor level D0The fatigue life of each gear was recorded and averaged to compare with the theoretical fatigue life, determined from the formula D ═ α × T + β × S ═ 11.34 × T +7.71 × S (50 ℃, 60 MPa).
Table 3 constant load accelerated fatigue test example data recording table
Test gear number i Cycle number logN at failure i
1 9.3958
2 9.59501
3 9.18572
4 9.21591
5 9.14861
Mean value of the number of cycles at failure logN 9.30821
As shown in fig. 6, the theoretical fatigue life value logN of the plastic gear is calculated and obtained from the relationship between the equivalent damage factor D and the fatigue life N, i.e., logN ═ λ (a × D + b) ═ 1.08 × (0.085 × D +0.058), and the theoretical fatigue life value logN of the plastic gear is calculated and obtained09.2965, see logN>logN0And the fatigue life logarithm value obtained by the actual constant-load accelerated fatigue test is larger than the logarithm value of the theoretical fatigue life, namely the fatigue life of the plastic gear of the batch is larger than the theoretical value of the DN curve, and the qualified condition is met.
Example 2 stepwise accelerated fatigue test
Steps 1 to 4 of this example are the same as in example 1.
Step 5, selecting a step type accelerated fatigue test
Loading by adopting a stepwise variable damage factor, selecting 20%, 40%, 60% and 80% of the yield limit 73.9MPa of the plastic gear as a load part, and determining the damage factor at different levels by selecting the temperature of 50 DEG CZi Ji is denoted as D1、D2、D3、D4The number of cycles under each injury factor was 1.5X 104. The fatigue life test was continued until failure of the gear occurred. The level of injury factor and cycle number at failure were recorded. Five experiments were repeated with an average.
TABLE 4 data recording table of stepwise accelerated fatigue test examples
Figure BDA0002908671980000111
The results of the stepped accelerated fatigue test are judged and shown in FIG. 7: the test data shows that the damage rate cumulative value is 0.6019<1, namely the plastic gear subjected to the stepped accelerated fatigue test in the batch has fatigue failure before the damage rate cumulative value reaches 1, namely the plastic gear subjected to the actual fatigue test has a life not reaching the theoretical value, and the plastic gear in the batch is unqualified.

Claims (3)

1. A plastic gear accelerated fatigue test method considering temperature effect is characterized by comprising the following steps:
step 1, obtaining the fatigue life of the plastic gear at temperature T and cyclic stress S according to fatigue test data or fatigue simulation analysis at different temperatures;
step 2, determining the relation between the equivalent damage factor D and the temperature T and the cyclic stress S:
D＝α×T+β×S
in the formula, alpha and beta are respectively the influence coefficients of temperature T and cyclic stress S on damage;
step 3, obtaining a relation between an equivalent damage factor D and a fatigue life N in a data fitting mode, wherein the relation is that log N is lambda (a multiplied by D + b), and a and b are fatigue life coefficients; lambda is the lubrication coefficient and takes the value of 1.08
Step 4, according to the linear fitting relation between the equivalent damage factor D and the fatigue life N in the step 3, the plastic gear damage evolution equation under the action of the temperature T and the cyclic stress S is D (logN)/dD ═ C, wherein C is a constant;
step 5, selecting plastic gear fatigue test method
The plastic gear fatigue test method selects any one of a constant load accelerated fatigue test and a stepped accelerated fatigue test:
1) and constant load accelerated fatigue test:
loading by adopting a constant damage factor, carrying out fatigue test on a plurality of plastic gears, and recording the cycle number of each plastic gear when each plastic gear fails;
averaging the test data to obtain the cycle number N of the plastic gear in actual failureiSubscript i denotes the number of test gears; comparing the equivalent damage factor D-fatigue life N curve, if the cycle number NiGreater than the theoretical number of cycles N0The fatigue life of the plastic gear is qualified, otherwise, the plastic gear is unqualified;
2) step-type accelerated fatigue test:
loading by adopting a step-type variable damage factor, continuously carrying out a fatigue life test until the gear fails, wherein the cycle times under each damage factor are the same; carrying out fatigue tests on the plurality of plastic gears, and recording the level of damage factors and the cycle times when each plastic gear fails; averaging the number of cycles of failure;
calculating and summing the ratio of the cycle times under each damage factor value to the theoretical cycle times to obtain an accumulated value of the damage rate; if the accumulated damage rate value is equal to or greater than 1, the fatigue life of the plastic gear is qualified, otherwise, the plastic gear is unqualified.
2. The accelerated fatigue test method for plastic gears considering temperature effect according to claim 1, wherein: in the constant load accelerated fatigue test, the constant damage factor is 80% of the plastic gear yield limit.
3. The accelerated fatigue test method for plastic gears considering temperature effect according to claim 1, wherein: in the step-type accelerated fatigue test, four numerical levels of 20%, 40%, 60% and 80% of the yield limit of the plastic gear are selected as the step-type variable damage factors.
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