CN101718651A - Evaluation method of residual strength and service life under strengthening and damaging combined action - Google Patents
Evaluation method of residual strength and service life under strengthening and damaging combined action Download PDFInfo
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Abstract
The invention relates to an evaluation method of residual strength and service life under strengthening and damaging combined action, which is characterized in that: (A) virtual stress-life curve of a material or a part is obtained by test or technical resources, and the fatigue strength and low amplitude loading range are determined; (B) the low-load strengthening general rule of the material or the part with different strength grades are built according to low-load strengthening performance data of low-strength and high-strength materials or parts by virtue of a numerical method of spline interpolation; (C) the fatigue strength changing rule is built according to the stress-life curve of the material or the part under the strengthening and damaging combined action; in addition, according to the loading grade and frequency of a loading spectrum, the instantaneous fatigue strength value, that is, the residual fatigue strength of the part with given using mileage is calculated; (D) according to the instantaneous stress-life curve, the life evaluation of the material or the part is carried out according to Miner linear damage theory. The invention provides a specific method for the service life of the part and the light-weight design.
Description
Technical field
The present invention relates to a kind of material or evaluation method in mechanical component residual intensity and life-span of being used for, be specifically related to a kind of varying strength rank material or part of being used for, strengthen and damage the evaluation method in low load reinforcing rule, residual intensity and life-span under the acting in conjunction.
Background technology
In the Vehicle Engineering field, traditional linearity and non-linear fatigue damage theory are only based on S-N curve or strain-life curve, consider the strength reinforcing that the low amplitude load in the use causes structure hardly, only consider the damage that big load causes structure, structure residual intensity of carrying out in view of the above and life assessment are partial to safety often, are unfavorable for the lightweight structure design of automobile.
The random load that automobile runs in using, the 80%th, low amplitude load (being that amplitude is lower than fatigue limit), before crack initiation appearred in structure, these load may cause reinforcement to structure.(, behind the structure germinating visible crack, promptly thinking inefficacy) so-called low load reinforcing in the vehicle industry, refer to that low amplitude load acts on repeatedly after, the phenomenon that structural strength (comprising static strength and fatigue strength) increases.Although since over half a century, the researchist constantly finds the existence of low load reinforcing phenomenon both at home and abroad, does not see so far and uses the concrete report that low load reinforcing carries out the vehicle structure light-weight design.
At present, light-weighted effective ways of automobile and approach are all being inquired in the whole world, but do not see yet so far not only lightweight but cheaply automobile appear on the market.In view of the present situation of china natural resources poorness, explore and study not only loss of weight but also vehicle structure design theory and method are significant cheaply; The strength characteristics of further investigation and grasp load character and material or part is a key problem in technology of realizing the vehicle structure light-weight design.
Summary of the invention
The invention discloses the residual intensity and the evaluation method in life-span under a kind of reinforcement and the damage acting in conjunction, based on the research accumulation of a large amount of low amplitude loads in the vehicle maneuvering load spectrum to the invigoration effect (being low load reinforcing) of part strength, proposition is from strengthening and damaging two aspects and handle load, with near the actual physical process, carry out rational loss of weight in the design phase, improve the life prediction precision simultaneously; Overcome the evaluation of prior art to structure residual intensity and life-span, only consider the damage that big load causes structure, and do not consider the strength reinforcing that the low amplitude load in the use causes structure, evaluation precision is low, cause structure relatively safely, be unfavorable for the light-weight design of structure, to such an extent as to waste material, waste energy.
The residual intensity and the evaluation method in life-span under a kind of reinforcement and the damage acting in conjunction, it is characterized in that: the step of described method is as follows:
(A) obtain the virgin stress-life curve of material or part and the fatigue strength of definite part and the low amplitude load scope in the use by test or with reference to the correlation technique data;
(B), set up the universal law of varying strength rank material or part low load reinforcing by the numerical method of spline interpolation by the low load reinforcing property data of existing low-intensity and high-strength material or part;
(C) S-N curve of bond material or part and low load reinforcing rule are set up the fatigue strength Changing Pattern under reinforcement and the damage acting in conjunction; And, calculate the instantaneous fatigue strength values of part under the given use mileage, i.e. residual fatigue intensity according to the load level and the frequency characteristic parameter of loading spectrum;
(D) set up instantaneous stress-life curve according to the residual fatigue intensity that constantly changes, instantaneous stress-life curve carries out material or part life evaluation by Miner linear damage theory in view of the above.
Utilize this method, can be by the strength grade and the low load reinforcing rule realization material or the evaluation in part residual intensity and life-span under reinforcement and damage acting in conjunction of analysis of material or part.
The present invention provides theory and method for residual intensity and the life assessment technology under intensity and the damage acting in conjunction in the correct assessment random load spectrum, overcome the defective that the Miner theory only " is regarded the merit that big load is done as damage; ignore the merit that low amplitude load is done ", solved the drawback of the theoretical unidirectional consideration strength of materials decay of traditional fatigue damage accumulation, make the structural design slip-stick artist can accurately assess the permanance of part in the design phase, guarantee to make the structural design lightweight under the prerequisite in serviceable life satisfying, economize on resources.
Description of drawings
Fig. 1 is the mechanical model of low load reinforcing.
Embodiment
Below in conjunction with drawings and Examples the present invention is done and to describe in further detail.
Semiaxis (middle intensity part with tractor.Material is 40Cr, yield limit 905.5MPa, tensile strength 1000.3MPa) life experiment is an example under the random load spectrum, all tests are all carried out on the electro-hydraulic servo reverse torsion machine.The sample installation way is fixed for the left end spline, and the right-hand member spline imposes pure torsional load, does not limit axial slight the moving of test specimen in the loading procedure, and the axial deformation when guaranteeing that sample is turned round is unrestricted.The residual intensity and the evaluation method in life-span under shaft of tractor reinforcement and the damage acting in conjunction are as follows:
(A) obtain the virgin stress-life curve of material or part and the fatigue strength of definite part and the low amplitude load scope in the use by test or with reference to the correlation technique data;
The 5 grades of stress spectrums of test that obtain after the processing of shaft of tractor raw payload spectrum are seen Table 1.
Table 1 shaft of tractor test (not strengthening) stress spectrum
Strenuous test loading spectrum S-N curve is not as follows:
τ
m＝134.72，lgN
f＝17.3897-5.7244lgτ
a；
τ
m＝121.09，lgN
f＝17.5389-5.7808lgτ
a；
τ
m＝107.32，lgN
f＝17.6884-5.8371lgτ
a；
τ
m＝93.81，lgN
f＝17.8340-5.8918lgτ
a。
To the part torsional fatigue test, be the fatigue limit of life-span radix estimation often with 2,000,000 times.Press Goodman mean stress correction formula, have only the 5th grade to be low amplitude load in the table 1, the equivalent stress amplitude is 81.21MPa, is lower than fatigue limit 90.68MPa.
(B), set up the universal law of varying strength rank material or part low load reinforcing by the numerical method of spline interpolation by the low load reinforcing property data of existing low-intensity and high-strength material or part;
(i) low-intensity part low load reinforcing test
With homemade certain automobile front axle (low-intensity material, fatigue limit is 144.5MPa) be example, carry out the quantitative test of low load reinforcing, promptly under given low amplitude load, adopt different reinforcement number of times to carry out strenuous test, obtain the different average checking life-spans, test findings sees Table 2.The corresponding relation of test load and maximum nominal stress is: 1KN is equivalent to 7.22MPa.
Certain automobile front axle low load reinforcing quantitative test result of table 2
Experimental result shows, strengthens load equivalent amplitude 130.01MPa, strengthens number of times 2.0 * 10
5About inferior, will cause best strengthening effect to structure.185.63MPa the following life-span of load is 6.30 * 10
5, with this life-span substitution front axle S-N curvilinear equation formula, see formula (1):
lgs＝3.3481-0，1948lgN(1)
The stress that obtains on the corresponding S-N curve is: S=165.35MPa.That is to say that load S=165.35MPa has the identical life-span with reinforcement after load S=185.63MPa before strengthening, promptly the ratio that fatigue strength improves after strengthen best strengthening point is:
(185.63-165.35)/165.35×100％＝12.26％(2)
Arrive after the best strengthening point, continue to apply low amplitude load to 3.0 * 10
5, 4.5 * 10
5With 6.0 * 10
5Inferior, verify that on average the life-span reduces with strengthening increased frequency, be respectively 4.45 * 10
5, 3.57 * 10
5, 3.10 * 10
5Inferior.Therefore, suppose that the low amplitude load action effect is a separation with best strengthening point, arrive before the best strengthening point that low amplitude load has invigoration effect; Low amplitude load has damaging action after arriving best strengthening point.
Experiment shows that low-intensity part low load reinforcing serves as the best number of times of strengthening with 200,000 times, and the maximum ratio of strengthening back fatigue strength raising is 12.26%.Continue afterwards to strengthen, fatigue strength is tending towards reducing, and sees Table 3 again.
The best strengthening point of table 3 is strengthened number of times and fatigue strength variation relation later on
Can obtain fatigue strength attenuation law behind the low-intensity part low load reinforcing after further handling, see Table 4.
Table semi-finals number of times and fatigue strength rate of change relation
Annotate: the tired fatigue strength of "+" representative improves in the table, and on behalf of fatigue strength, "-" reduce.
The (ii) low load reinforcing of high strength part test
With certain automobile gearbox reverse gear cylindrical gear (high-strength material, fatigue limit are 359.22MPa) is example, by the quantitative test of low load reinforcing, obtains the different average checking life-spans, and test findings sees Table 5.The corresponding relation of test load and maximum nominal stress is: 1KN is equivalent to 86.35MPa.
Certain automobile gearbox gear low load reinforcing quantitative test result of table 5
Experimental result shows, strengthens load equivalent amplitude 287.55MPa and strengthens number of times 3.0 * 10
5About inferior, will cause best strengthening effect to structure.The 422MPa following life-span of load is 2.25 * 10
5, with this life-span substitution gear S-N curvilinear equation formula, see formula (3):
S＝676.9075-50.4767lgN(3)
The stress that obtains on the corresponding S-N curve is: S=406.75MPa.The load that before strengthening is 406.75MPa has the identical life-span with the reinforcement back for the load of 422MPa, and the maximum ratio of promptly strengthening back fatigue strength raising is:
(422-406.75)/406.75×100％＝3.75％(4)
Arrive after the best strengthening point, continue to apply low amplitude load and affact 4.0 * 10
5Inferior, verify that on average the life-span is reduced to 1.49 * 10 with strengthening increased frequency
5Inferior.Here, reflected the duality of low amplitude load action effect equally, promptly arrived before the best strengthening point that low amplitude load has invigoration effect; Low amplitude load has damaging action after arriving best strengthening point.
Experiment shows that high strength part low load reinforcing serves as the best number of times of strengthening with 300,000 times, and the maximum ratio of strengthening back fatigue strength raising is 3.75%.After the low load reinforcing 300,000 times, fatigue strength is tending towards reducing, and remakes with 100,000 times, and the ratio of strengthening back fatigue strength reduction is 2.22%.After further handling, can obtain fatigue strength attenuation law behind the high strength part low load reinforcing, see Table 6.
Table 6 high strength part is strengthened number of times and fatigue strength rate of change relation
Annotate: the tired fatigue strength of "+" representative improves in the table, and on behalf of fatigue strength, "-" reduce.
(C) S-N curve of bond material or part and low load reinforcing rule are set up the fatigue strength Changing Pattern under reinforcement and the damage acting in conjunction; And, calculate the instantaneous fatigue strength values of part under the given use mileage, i.e. residual fatigue intensity according to the load level and the frequency characteristic parameter of loading spectrum.
According to high strength and the test of low-intensity part low load reinforcing, getting high and low intensity part the best reinforcement number of times and fatigue strength raising ratio intermediate value is best number of times and the fatigue strength raising ratio strengthened of middle intensity part.The intensity part serves as the best number of times of strengthening with 250,000 times in obtaining, and this moment, the fatigue strength improvement value was 8.01%.Be that this is strengthened and the fatigue strength Changing Pattern that damages under the acting in conjunction is listed in table 7 and table 8.
Strengthen number of times and fatigue strength variation relation before the best strengthening point of table 7
According to high strength and the test of low-intensity part low load reinforcing, reinforcement number of times and fatigue strength variation relation are listed in table 8 behind the best strengthening point of different intensity grades part.
The best strengthening point of table 8 is strengthened number of times and fatigue strength variation relation later on
The different intensity grades part be can obtain according to table 3 and number of times and residual intensity variation relation strengthened.Apply the 5th grade of low amplitude load continuously, then strengthen number of times and the residual intensity variation relation is as shown in table 9 below.
Table 9 is strengthened number of times and residual intensity variation relation
Annotate: on behalf of residual intensity, "+" improve in the table, and on behalf of residual intensity, "-" reduce.
(D) according to the corresponding relation between material or part low load reinforcing rule and the life assessment technology, carry out material or part life evaluation.
When being estimated the fatigue lifetime under the strenuous test loading spectrum not, will test approximate hypothesis and be divided into three parts and carry out according to the low load reinforcing theory.
First applies 250,000 times to obtain best strengthening effect with the 5th grade of stress, and the spectrum piece number of corresponding this strengthening effect is:
At this moment, the maximum ratio 8.01% that part fatigue strength improves, its original S-N curve I brings up to new S-N curve II.Move to the D point by the A point, under the promptly identical stress, increase fatigue lifetime, as shown in Figure 1.
The original S-N curve of part is curve I, and the new S-N curve after the reinforcement is curve II.Fig. 1 shows that prolong fatigue lifetime under same load, and whole S-N curve is to the upper right side translation, and the S-N slope of a curve is constant substantially.
Hypothesis test at first applies low amplitude load.When the low amplitude load effect was applied to the best reinforcement number of times, part fatigue strength was brought up to maximum, and this moment, the S-N curve of part was brought up to new curve II by original curve I.At identical stress σ
1Down, fatigue lifetime is by N
1Be increased to N
2, promptly move to the D point by the A point.
The computing method that specifically find D to order are (is example with stress level 1):
(1) on original S-N curve, by the corresponding stress σ of B point
2Carry out the transition to the corresponding stress σ of D point
1, part fatigue strength increases by 8.01%, promptly by formula: (σ
1-σ
2)/σ
2* 100%=a% obtains:
(127.99-σ
2)/σ
2×100％＝8.01％
Known σ
1, can obtain the corresponding stress σ of B point
2=118.50MPa.
(2) τ on original S-N curve
m=134.72 o'clock, lgN
f=17.3897-5.7244lg τ
aThe stress σ that known B point is corresponding
2Can obtain the corresponding life-span N of B point
2=3.31 * 10
5Inferior.And 2 of B, D have identical N fatigue lifetime
2
(3) the corresponding stress σ of known D point
1With N fatigue lifetime
2, know that again new S-N curve II slope is identical with original S-N curve I slope, can obtain the τ of new S-N curve II
m=134.72, lgN
f=17.5821-5.7244lg τ
a
Because the 5th grade of stress is the low amplitude load effect that stress amplitude is lower than fatigue limit, suppose part strengthen afterwards the 5th grade of stress on the S-N curve fatigue lifetime N
iConstant.Other each stress levels are corresponding life-span such as table 10 on new S-N curve II.
Table 10 part is strengthened the pairing life-span of stress at different levels in the after load spectrum
Be also noted that after the first effect, semiaxis reaches new S-N curve, during utilization Miner Theoretical Calculation amount of damage, only need to calculate second and third part damage and be 1.
Second portion only applies the big load that 152.44 stress levels are 1-4.Total damage is:
X is the loop blocks number in the formula.
Third part is the stress of 1-5 according to proof stress spectrum stress application level.Up to the semiaxis fatigue break.The progressive damage of a cyclic test spectrum piece is:
Total damage equals above-mentioned second and third part damage sum:
7.68×10
-2+7.44×10
-4X＝1
Solve the X=1240.86 piece, 1240.86+152.44=1393.3 piece altogether circulates.Test life is under the spectrum loading
N
f=1393.3 * 2298=3.20 * 10
6(inferior)
Consider that low load reinforcing fatigue lifetime is 3.20 * 10 this moment
6Inferior.According to the test findings that document provides, the random load spectrum corresponding with table 1 test life of semiaxis down is 3.05 * 10
6Inferior, both compare, and error is 5%.
Claims (1)
- One kind strengthen and the damage acting in conjunction under the residual intensity and the evaluation method in life-span, it is characterized in that: the step of described method is as follows:(A) obtain the virgin stress-life curve of material or part and the fatigue strength of definite part and the low amplitude load scope in the use by test or technical information;(B), set up the universal law of varying strength rank material or part low load reinforcing by the numerical method of spline interpolation by the low load reinforcing property data of existing low-intensity and high-strength material or part;(C) S-N curve of bond material or part and low load reinforcing rule are set up the fatigue strength Changing Pattern under reinforcement and the damage acting in conjunction; And, calculate the instantaneous fatigue strength values of part under the given use mileage, i.e. residual fatigue intensity according to the load level and the frequency characteristic parameter of loading spectrum;(D) set up instantaneous stress-life curve according to the residual fatigue intensity that constantly changes; Instantaneous stress-life curve carries out material or part life evaluation by Miner linear damage theory in view of the above.
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