CN114065556A - Amplitude-variable fatigue life prediction method based on dissipation energy - Google Patents
Amplitude-variable fatigue life prediction method based on dissipation energy Download PDFInfo
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Abstract
The invention discloses a variable amplitude fatigue life prediction method based on dissipation energy, which comprises the following steps: determining all levels of loads of the variable amplitude fatigue load spectrum and the cycle times under all levels of loads; carrying out fatigue test with load amplitude gradually increased on the material sample until the material sample is fatigue failure, carrying out temperature collection on the surface of the material sample in each step of load constant amplitude loading fatigue test process until the material sample stops loading and the surface temperature is reduced to room temperature, and stopping temperature collection; determining the dissipation energy and energy tolerance of each stage of load in the variable amplitude fatigue load spectrum in each cycle based on the acquired temperature data; aiming at a given variable amplitude fatigue load, establishing a nonlinear damage accumulation model based on dissipated energy and energy tolerance; based on the damage accumulation model, the amplitude-variable fatigue life of the material can be finally calculated; the inherent dissipation energy is used as the damage parameter of the variable amplitude fatigue model, and the method has the advantages of considering the damage mechanism, being high in service life prediction precision and easy to obtain the dissipation energy.
Description
Technical Field
The invention belongs to the technical field of variable amplitude fatigue life prediction, and particularly relates to a variable amplitude fatigue life prediction method based on dissipation energy.
Background
In engineering practice, the mechanical structural members in service are mostly subjected to complex load spectrums, and one of the important reasons for the complexity is that the amplitudes of the reciprocating loading loads constituting the load spectrums may be different. Therefore, in order to be closer to the real service condition, the fatigue problem needs to be considered in the research of the fatigue problem, and the core aim of the research is to establish a reliable variable-amplitude service life prediction model.
Aiming at the problem of variable amplitude fatigue, a large number of fatigue models are available at present, and the mainstream method is to adopt a rain flow counting method to disassemble a variable amplitude load spectrum into a combination of constant amplitude loads and then evaluate the fatigue failure life of a material or a structure under a certain damage accumulation criterion. In the method, the influence of the load loading history is not considered, and the load loading history such as the accumulative damage of the preamble has a great influence on the subsequent fatigue damage evolution. In addition, in the existing variable amplitude fatigue life prediction model, the fatigue damage parameters are mostly in an energy form based on stress, strain or a combination of stress and strain.
Disclosure of Invention
The invention aims to overcome the defects of the prior art, and provides a prediction method for estimating the fatigue life based on the inherent dissipation energy calculated based on temperature data as a fatigue damage parameter and the damage accumulation of load loading history, in particular to a variable amplitude fatigue life prediction method based on the dissipation energy.
The invention provides a variable amplitude fatigue life prediction method based on dissipation energy, which comprises the following steps:
step 1: for a variable amplitude fatigue load spectrum experienced by a given material sample, defining alternating loads with the same amplitude and continuous time as the same level of load, and dividing each level of load sequence according to load loading history; the load spectrum is divided into a stress spectrum or a strain spectrum;
step 2: according to the stress spectrum or the strain spectrum, carrying out a fatigue test with the stress or strain amplitude gradually increased on the material sample until the sample fails due to fatigue, and ending the fatigue test with the stress or strain amplitude gradually increased; in the process of the fatigue test with the stress or strain amplitude gradually increased, the temperature collection of the surface of the sample is required in each step of the stress or strain constant amplitude loading test until the sample stops loading and the surface temperature is reduced to the room temperature, and the temperature collection is stopped; wherein, each step of stress loading needs to be circulated for many times;
and step 3: calculating the energy dissipated per cycle and the energy tolerance; performing heat source analysis based on temperature data acquired in the fatigue test process with the stress or strain amplitude gradually increased, and calculating the inherent dissipation energy of the material sample under each constant amplitude load so as to obtain the corresponding dissipation energy per cycle and energy tolerance under each level of load;
and 4, step 4: according to a given variable amplitude fatigue load spectrum and the obtained dissipation energy and energy tolerance of each cycle, the damage accumulated by cyclic loading in the fatigue test process is gradually increased for the stress or strain amplitude, and a nonlinear damage accumulation model based on the dissipation energy and the energy tolerance is established;
and 5: calculating the amplitude-variable fatigue life of the material; and (3) calculating the total accumulated damage after loading of each stage of load according to the load loading sequence based on a nonlinear damage accumulation model of the dissipated energy and the energy tolerance, judging that the sample has fatigue failure until the accumulated total damage is 1, obtaining the cycle number under the loading of the last stage of load before the fatigue failure, then calculating the total cycle number of the variable amplitude load, wherein the total cycle number experienced by the sample is the variable amplitude fatigue life of the sample.
Preferably, the specific content of step 2 is: carrying out a stress or strain amplitude gradual increase fatigue test on the material sample until the sample fails due to fatigue, ending the stress or strain amplitude gradual increase fatigue test, and recording the number of different amplitudes experienced in the process asC 1(ii) a The number of cycles required for each step of constant stress loading was recorded asC 2Wherein inC 1Step stress amplitude down cycleC 3Fatigue failure occurs next time, andC 3 < C 2and in the process, collecting the temperature of the surface of the sample until the temperature of the surface of the sample is reduced to room temperature, stopping temperature collection, and carrying out the next stress or strain loading fatigue test.
Preferably, the process of step 3 is: based on the temperature data acquired in the test process and a heat conduction equation, calculating the inherent dissipation energy under each constant amplitude fatigue load according to heat source analysis, wherein the heat conduction equation is as follows:
wherein,ρandCdividing the density and specific heat capacity of the sample material;θthe average temperature rise of a target area on the surface of the material sample is obtained, namely the real-time temperature minus the initial temperature;d 1is inherent dissipated energy;tis time;τ eq is a time constant;
for calculated inherent dissipation energyd 1Integrating in a cycle, and calculating to obtain the energy dissipated per cycle under each constant amplitude fatigue loadE d The calculation formula is as follows:
whereint 0In order to stabilize the start time of the cycle,Tconstant amplitude fatigue alternating load cycle period;
integrating the obtained energy dissipated per cycle under each constant amplitude load within the fatigue life range of the fatigue test with the stress or strain amplitude increasing gradually to obtain the fatigue energy tolerance of the materialE C The calculation formula can be simplified as follows:
wherein,E d n()gradual loading of stress or strain amplitude in fatigue testnThe energy dissipated per cycle under step stress constant amplitude loading,
gradual loading fatigue test for stress or strain amplitudeC 1Energy dissipated per cycle at step stress amplitude.
Preferably, step 3 further comprises: determining the relationship between the constant amplitude stress or strain amplitude and the energy dissipated per cycle, and recording the relationship asε a -E d The relational formula is as follows:
whereina、bAre all the fitting coefficients of the two-dimensional image,ε a representing the a-th order constant amplitude stress or strain amplitude.
Preferably, the process of step 4 is:
according to the dissipation energy per cycle under each stage of load in the variable amplitude fatigue load spectrum, the stress or strain amplitude is gradually increased to damage accumulated by cyclic loading in the fatigue test process, and a nonlinear damage accumulation model based on the dissipation energy and the energy tolerance is established; in the first placeiThe cyclic loading times under the stage load is deltan i Of 1 atiUnder a stage load, the energy dissipated per cycle isE d,i Then cycle of deltan i Total injury accumulated after a whileD i The calculation formula of (2) is as follows:
wherein,n i e-1,before showingiTotal damage accumulated after class 1 stress loading, on the secondiEquivalent cycle times required for generating the same accumulated damage under the loading of the grade stress;
δ i the definition is as follows:
μ i represents the firstiClass 1 load to class 2iA factor influencing the evolution of fatigue damage under a grade load,μ i the definition is as follows:
whereinE d,i-1Is shown asi-dissipated energy per cycle under class 1 stress loading;
will totally damageD i As a nonlinear damage accumulation model based on dissipated energy and energy tolerance.
Preferably, step 4 comprises:
step 4.1: calculating first order stressε 1 Loaded down cycle Δn 1Post-cumulative damageD 1The calculation formula is as follows:
wherein,E d,1representing first order stressε 1 Dissipation energy per cycle under loading;
index of refractionδ 1The definition is as follows:
step 4.2: stress of the first orderε 1 Accumulated damage under loadD 1As stress of the second orderε 2 The starting point for the accumulation of cycles under load is then:
wherein,n e1,the equivalent cycle number required for the sample to generate the accumulated damage which is the same as that generated after the first-stage stress loading under the second-stage stress loading is shown;n e1,the starting point of damage evolution under the second-stage stress loading is obtained;E d,2dissipating energy per cycle for a second level stress level;
index of refractionδ 2The definition is as follows:
μ 2representing the influence factor of the fatigue damage evolution under the first-stage stress loading and the second-stage stress loading, then
μ 2The definition is as follows:
number of equivalent cyclesn e1,Is recorded as:
step 4.3: calculating second order stressε 2 Load cycle Δn 2Injury accumulated after timesD 2The calculation formula is as follows:
deducing the second step according to the steps 4.1, 4.2 and 4.3iCumulative total damage after stage stress loading.
Preferably, the number of cycles is equivalentn i e-1,The calculation formula of (2) is as follows:
whereinq i Expressed as:
preferably, step 5 comprises:
step 5.1: determining the condition of fatigue failure, i.e. total damage, of the specimenD i When the value is 1, the fatigue failure of the sample is judged, namely:
D i =1
number of cycles under the last stage of load before fatigue failure of the specimen, i.e. the first stageiNumber of cycles Δ under a level stress loadingn iCalculated by the following calculation formula:
wherein,E C indicating the energy tolerance for fatigue failure of the material,E d,i is shown asiThe energy dissipated per cycle under a stress loading of order,n i e-1,showing that the sample is atiUnder the action of a secondary stress, the primary stress is generatedi-the same accumulated damage after level 1 stress loading, required equivalent cycle number;
the total fatigue life under variable amplitude fatigue loading can therefore be expressed as:
preferably, the time constantτ eq The calculation formula of (a) is as follows:
whereinR(τ eq ) Representing time constantsτ eq A function of the integration over the integration time r,τ opt representing the optimal time constant calculated by the least square methodτ eq ，d 1(t,τ eq )2Expressing heat transfer equation
，tIs time.
Has the advantages that:
1. the material can be accompanied with the generation of inherent dissipation energy under the fatigue alternating load, so the inherent dissipation energy is taken as a fatigue damage parameter, and the fatigue damage state and the evolution process can be represented from the point of damage mechanism.
2. The fatigue life prediction method based on the dissipation energy belongs to an energy method, and the life prediction precision of the method is generally superior to that of a life prediction method based on stress and strain states.
3. The acquisition process of the inherent dissipation energy is based on temperature acquisition and temperature data calculation, and compared with stress and strain states, the acquisition process is easier in engineering practice by virtue of the advantages of non-contact, full-field, nondestructive and real-time measurement of a new infrared thermography.
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In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings needed to be used in the description of the embodiments will be briefly introduced below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on these drawings without creative efforts.
Fig. 1 is a flowchart of a method for predicting a dissipation energy-based variable amplitude fatigue life in the practice of the present invention.
Fig. 2 is a schematic diagram of a stress spectrum of a two-stage variable amplitude test from high stress loading to low stress loading in the variable amplitude fatigue life prediction method based on dissipated energy in the implementation of the invention.
Fig. 3 is a graph of the relationship between stress and dissipation energy per cycle of a dissipation energy-based variable amplitude fatigue life prediction method in the implementation of the invention.
Fig. 4 is a graph of the relationship between the predicted amplitude fatigue life and the experimental amplitude fatigue life of the amplitude-variable fatigue life prediction method based on the dissipated energy in the implementation of the present invention.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
The sample material adopted by the embodiment is 316L stainless steel, the thermal infrared imager is erected, the position of the thermal infrared imager is adjusted, so that the parallel section of the material sample is just imaged in the visual field of the thermal imager, the thermal imager is subjected to non-uniform correction, and the temperature acquisition range and the sampling frequency are set; the method is characterized in that black matte paint is sprayed on the surface of a material sample, and aims to improve the thermal radiance of the surface of the material sample so as to ensure the accuracy of temperature acquisition of a thermal infrared imager.
The embodiment provides a variable amplitude fatigue life prediction method, and firstly, a fatigue damage parameter representing the damage size of a material in a model is inherent dissipation energy, which is different from damage parameters based on stress and strain, the inherent dissipation energy is expressed as dissipated heat energy in the material fatigue process, and the inherent dissipation energy can be obtained by obtaining the surface temperature of the material and then reversely calculating a heat source. Since the dissipated energy is directly related to the plastic strain energy that causes fatigue failure of the material, it can be used to characterize the fatigue damage parameter. In addition, the model proposed by the present embodiment can thus take into account the influence of the load loading history by introducing load order influence factors.
As shown in fig. 1, the method for predicting the fatigue life of variable amplitude provided by this embodiment includes the following steps:
for a variable amplitude fatigue load spectrum experienced by a given material, defining alternating loads with the same amplitude and continuous time as the same-level load, and dividing the order of each level of load according to the load loading history, wherein the load spectrum is a strain spectrum in the embodiment.
Step 1: carrying out fatigue test with gradually increased strain amplitude and temperature collection on the material sample: selecting a proper strain loading range, and performing a group of gradual increase fatigue tests containing 5 different strain amplitudes, wherein the strain ratio is-1; ending the fatigue test with the strain amplitude gradually increased until the material sample is subjected to fatigue failure; the number of different amplitudes experienced in this process is recorded asC 1(ii) a The number of cycles required for each step of constant strain loading was recorded asC 2(inC 1Step stress amplitude down cycleC 3The fatigue failure occurs the next time,C 3 < C 2) And in the process, collecting the temperature of the surface of the sample until the temperature of the surface of the sample is reduced to room temperature, stopping temperature collection, and performing a next strain control constant amplitude loading fatigue test.
Step 2: calculating the energy dissipated per cycle; performing heat source analysis based on temperature data acquired in the process of increasing the fatigue test step by the strain amplitude, and calculating the inherent dissipation energy of the material under each constant amplitude strain so as to obtain the dissipation energy per cycle under each level of load;
based on the temperature data acquired in the test process and a heat conduction equation, calculating the inherent dissipation energy under each constant amplitude load according to heat source analysis, wherein the heat conduction equation is as follows:
wherein,ρandCis divided into materialsDensity and specific heat capacity;θthe average temperature rise of a target area on the surface of the material sample is obtained, namely the real-time temperature minus the initial temperature;d 1is inherent dissipated energy;tis time;τ eq is a time constant;
time constantτ eq The calculation formula of (a) is as follows:
whereinR(τ eq ) Representing time constantsτ eq A function of the integration over the integration time r,τ opt representing the optimal time constant calculated by the least square methodτ eq ，d 1(t,τ eq ) 2Expressing heat transfer equation
，tIs time;
according to the constant amplitude uniaxial tension-compression fatigue test on 316L stainless steel, as shown in FIG. 3, the strain amplitude isε a Determiningε a -E d The relationship is represented by the following formula:
wherein,a、bare all the fitting coefficients of the two-dimensional image,ε a representing the a-th level constant amplitude stress or strain amplitude; fitting by the formulaε a AndE d to obtain a relationship ofa = 1.26，b= 5.86; by establishing the relation, the calculated inherent dissipation energy is further processedd 1Integrating in a cycle, and calculating to obtain the energy dissipated per cycle under each constant amplitude strain loadE d The calculation formula is as follows:
whereint 0In order to stabilize the start time of the cycle,Tconstant amplitude fatigue alternating load cycle period;
integrating the obtained energy dissipated per cycle under each constant amplitude load within the fatigue life range of the strain amplitude increasing step by step to obtain the fatigue energy tolerance of the materialE C The calculation formula can be simplified as follows:
wherein,E d n()gradual loading of strain amplitude in fatigue testnThe energy dissipated per cycle under step strain constant amplitude loading,
gradual loading fatigue test for stress or strain amplitudeC 1Energy dissipated per cycle at step stress amplitude.
And step 3: according to the dissipation energy of each cycle under each stage of strain load, the damage accumulated by the cyclic loading in the fatigue test process of gradually increasing the stress or strain amplitude is established, and a nonlinear damage accumulation model based on the dissipation energy and the energy tolerance is established;
step 3.1: calculating first order strainε 1Loaded down cycle Δn 1Post-cumulative damageD 1The calculation formula is as follows:
wherein,E d,1representing first order strainε 1Dissipation energy per cycle under loading;
index of refractionδ 1The definition is as follows:
step 3.2: straining the first orderε 1Accumulated damage under loadD 1As second order strainε 2The starting point for the accumulation of cycles under load is then:
wherein,n e1,indicating that the material is subjected to the same accumulated damage as the material subjected to the first-order strain loading under the second-order strain loadingD 1The required equivalent cycle number;n e1,namely the starting point of the damage evolution under the second-stage strain loading;E d,2dissipated energy per cycle for the second level strain level;
index of refractionδ 2The definition is as follows:
μ 2representing the influence factor of the first-stage strain level on the fatigue damage evolution under the second-stage strain loading, then
μ 2The definition is as follows:
number of equivalent cyclesn e1,Is recorded as:
step 3.3: calculating second order strainε 2Cycle after load Δn 2Post-cumulative damageD 2The calculation formula is as follows:
deducing the second from step 3.1, step 3.2 and step 3.3iCumulative total damage after secondary strain loading;
in the first placeiThe cyclic loading times under the stage load is deltan i Of 1 atiThe energy dissipated per cycle under class strain isE d,i Then cycle of deltan i Total injury accumulated after a whileD i The calculation formula of (2) is as follows:
wherein,n i e-1,is frontiTotal damage accumulated after 1-order strain loading, on the firstiEquivalent cycle times required for generating the same accumulated damage under the secondary strain loading;E d,i is shown asiEnergy dissipated per cycle under a secondary strain loading;
δ i the definition is as follows:
μ i is shown asi-level of strain of order 1 vsiThe fatigue damage evolution influence factor under the grade strain loadingμ i The definition is as follows:
whereinE d,i-1Is shown asi-energy dissipated per cycle under strain loading of order 1;
will totally damageD i Is calculated byThe formula acts as a nonlinear damage accumulation model based on dissipated energy and energy tolerance.
Furthermore, since the fatigue failure mechanism is different between high cycle fatigue (low strain range region) and low cycle fatigue (high strain range region), the energy dissipation energy accumulated per cycle during fatigue failure of the material is also different, and thus the energy tolerance is here madeE C The evaluation was carried out in the high strain range and in the low strain range, respectively, according to the following formula;
E
C
= E
d
×N
f
whereinE d For the energy dissipated per cycle at this strain amplitude,N f represents the fatigue life under the constant amplitude strain load; tolerance of energyE C According to its correspondingE d,i Fall into the low strain range or the high strain rangeE C,LCF AndE C,HCF (ii) a The energy tolerance of the material in a high strain range and a low strain range is respectively obtainedE C,LCF = 2.66×1010 W/m3，E C,HCF = 17.2×1010 W/m3；
And 4, step 4: calculating the amplitude-variable fatigue life of the material; calculating the total accumulated damage after strain loading at each stage according to a load loading sequence based on a nonlinear damage accumulation model of dissipated energy and energy tolerance, judging that the material has fatigue failure until the accumulated total damage is 1, obtaining the cycle number under the last stage of strain loading before the fatigue failure, and then calculating the total cycle number of the material, wherein the total cycle number of the material is the amplitude-variable fatigue life of the material;
wherein the equivalent number of cyclesn i e-1,The calculation formula of (2) is as follows:
whereinq i Expressed as:
total injuryD i For determining a condition for fatigue failure of the material;
carrying out two-stage strain loading amplitude-variable symmetrical tension-compression (strain ratio is-1) fatigue test, wherein the material is subjected to strain loadingε a1Lower cycle deltan 1Wherein isn 1The number of times of constant strain loadingε a1Lower test amplitude fatigue lifeN f1Then at the strain amplitudeε a2（ε a2≠ε a1) Continuing to perform the fatigue test until the material fails due to fatigue, and recording the fatigue failure inε a2Down cycle loading to failure cycle number Δn 2The fatigue test under two stages of different strain loading comprises from high strain loading to low strain loading (ε a1>ε a2) And low strain loading to high strain loading (ε a1<ε a2) In both cases, as shown in FIG. 2, the abscissa of the graphtRepresenting time, ordinateεRepresenting strain amplitude or stress loading, H-L representing the loading sequence from high strain loading to low strain loading, L-H representing the loading sequence from low strain loading to high strain loading, the total number of cycles of the two phases, i.e. Δn 1+Δn 2As a prediction of the fatigue life of the amplitude variation,
step 4.1: determining the condition of fatigue failure, i.e. total damage, of a materialD i And 1, judging that the material has fatigue failure, namely:
D i =1；
number of cycles under load of last stage before fatigue failure of material, i.e. firstiNumber of cycles under secondary strain loadingNumber deltan i Calculated by the following calculation formula:
wherein,E C indicating the energy tolerance for fatigue failure of the material,E d,i is shown asiThe energy dissipated per cycle under a secondary strain loading,n i e-1,the material is shown iniUnder a secondary strain loading, the primary strain is generatediEquivalent number of cycles required for the same accumulated damage after 1-order strain loading.
Testing and predicting the amplitude-variable fatigue life: according to the graph shown in FIG. 4, the straight line in the graph is an ideal line for predicting the equivalent variable amplitude fatigue life to the experimental variable amplitude fatigue life, and the two dotted lines represent the 2-fold factor life discrete range, and it can be known from the graph that all the other points except 1H-L test data point are within the 2-fold factor range, wherein all the 4L-H test data points fall within the range of the 2-fold factorN fp = N f Near the straight line of (A), (B)N fp In order to predict the fatigue life of the amplitude variation,N f for testing the amplitude-variable fatigue life), the test data points of H-L are uniformly located on both sides of the straight line, so that the conclusion that the prediction result for predicting the amplitude-variable fatigue life is approximately similar to the test amplitude-variable fatigue life can be drawn, namely the prediction precision for predicting the amplitude-variable fatigue life is high.
The variable amplitude fatigue life prediction method provided by the embodiment has the following beneficial effects:
1. the material can be accompanied with the generation of inherent dissipation energy under the fatigue alternating load, so the inherent dissipation energy is taken as a fatigue damage parameter, and the fatigue damage state and the evolution process can be represented from the point of damage mechanism.
2. The fatigue life prediction method based on the dissipation energy belongs to an energy method, and the life prediction precision of the method is generally superior to that of a life prediction method based on stress and strain states.
3. The acquisition process of the inherent dissipation energy is based on temperature acquisition and temperature data calculation, and compared with stress and strain states, the acquisition process is easier in engineering practice by virtue of the advantages of non-contact, full-field, nondestructive and real-time measurement of a new infrared thermography.
Therefore, the use of the inherent dissipation energy as the fatigue damage parameter has the advantages of high accuracy of life prediction and easy acquisition from the viewpoint of mechanism.
The model provided by the present embodiment is constructed based on inherent dissipated energy, with energy tolerance compared to conventional modelsE C Replaces the S-N curve under constant amplitude fatigue, so one of the advantages isE C Compared with an S-N curve, the method is easier to obtain; in this study, energy tolerance is defined in the fields of low cycle fatigue (low stress/strain range region) and high cycle fatigue (low stress/strain range region)E C,LCF AndE C,HCF the two values can be obtained by carrying out a calibration test in two fields respectively or obtaining the two values respectively through a variable amplitude fatigue test from two times of gradual loading until failureE C,LCFAndE C,HCF . In fact, if the load range involved in the variable-amplitude fatigue load spectrum is small and is in the low-cycle fatigue field or the high-cycle fatigue field, only calibration is neededE C,LCF OrE C,HCF And (4) finishing. Accordingly, the determination of the S-N curves for the low cycle fatigue domain and the cycle fatigue domain often means that a large number of fatigue tests need to be performed.
The model of the embodiment is based on damage accumulation in the variable amplitude fatigue strain loading process, and the influence of the strain loading history on the variable amplitude fatigue life is further considered in the model by introducing and considering the load loading sequence influence factor in the damage accumulation process.
The present invention is not limited to the above preferred embodiments, and any modification, equivalent replacement or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (9)
1. A variable amplitude fatigue life prediction method based on dissipation energy is characterized by comprising the following steps:
step 1: for a variable amplitude fatigue load spectrum experienced by a given material sample, defining alternating loads with the same amplitude and continuous time as the same level of load, and dividing each level of load sequence according to load loading history; the load spectrum is divided into a stress spectrum or a strain spectrum;
step 2: according to the stress spectrum or the strain spectrum, carrying out a fatigue test with the stress or strain amplitude gradually increased on the material sample until the sample fails due to fatigue, and ending the fatigue test with the stress or strain amplitude gradually increased; in the process of the fatigue test with the stress or strain amplitude gradually increased, the temperature collection of the surface of the sample is required in each step of the stress or strain constant amplitude loading test until the sample stops loading and the surface temperature is reduced to the room temperature, and the temperature collection is stopped; wherein, each step of stress loading needs to be circulated for many times;
and step 3: calculating the energy dissipated per cycle and the energy tolerance; performing heat source analysis based on temperature data acquired in the process of increasing the fatigue test step by stress or strain amplitude, and calculating the inherent dissipation energy of the material sample under each constant amplitude load so as to obtain the corresponding dissipation energy per cycle and energy tolerance under each level of load;
and 4, step 4: according to a given variable amplitude fatigue load spectrum and the obtained dissipation energy and energy tolerance of each cycle, the damage accumulated by cyclic loading in the fatigue test process is gradually increased for the stress or strain amplitude, and a nonlinear damage accumulation model based on the dissipation energy and the energy tolerance is established;
and 5: calculating the amplitude-variable fatigue life of the material sample; and (3) calculating the total accumulated damage after loading of each stage of load according to the load loading sequence based on a nonlinear damage accumulation model of the dissipated energy and the energy tolerance, judging that the sample has fatigue failure until the accumulated total damage is 1, obtaining the cycle number under the loading of the last stage of load before the fatigue failure, then calculating the total cycle number of the variable amplitude load, wherein the total cycle number experienced by the sample is the variable amplitude fatigue life of the sample.
2. A method according to claim 1 based on dissipated energyThe variable amplitude fatigue life prediction method is characterized in that the specific content of the step 2 is as follows: carrying out a stress or strain amplitude gradual increase fatigue test on the material sample until the sample fails due to fatigue, ending the stress or strain amplitude gradual increase fatigue test, and recording the number of different amplitudes experienced in the process asC 1(ii) a The number of cycles required for each step of constant stress loading was recorded asC 2Wherein inC 1Step stress amplitude down cycleC 3Fatigue failure occurs next time, andC 3 < C 2and in the process, collecting the temperature of the surface of the sample until the temperature of the surface of the sample is reduced to room temperature, stopping temperature collection, and carrying out the next stress or strain loading fatigue test.
3. The amplitude-variation fatigue life prediction method based on dissipated energy as claimed in claim 2, wherein the process of step 3 is as follows: based on the temperature data acquired in the test process and a heat conduction equation, calculating the inherent dissipation energy under each constant amplitude fatigue load according to heat source analysis, wherein the heat conduction equation is as follows:
wherein,ρandCrespectively dividing the material density and the specific heat capacity;θthe average temperature rise of a target area on the surface of the material sample is obtained, namely the real-time temperature minus the initial temperature;d 1is inherent dissipated energy;tis time;τ eq is a time constant;
for calculated inherent dissipation energyd 1Integrating in a cycle, and calculating to obtain the energy dissipated per cycle under each constant amplitude fatigue loadE d The calculation formula is as follows:
whereint 0In order to stabilize the start time of the cycle,Ta constant amplitude fatigue load cycle period;
integrating the obtained energy dissipated per cycle under each constant amplitude fatigue load within the fatigue life range of the fatigue test with the stress or strain amplitude increasing gradually to obtain the fatigue energy tolerance of the materialE C The calculation formula can be simplified as follows:
wherein,E d n()gradual loading of stress or strain amplitude in fatigue testnThe energy dissipated per cycle under step stress constant amplitude loading,
gradual loading fatigue test for stress or strain amplitudeC 1Energy dissipated per cycle at step stress amplitude.
4. The amplitude-variation fatigue life prediction method based on dissipated energy as claimed in claim 3, wherein the step 3 further comprises: determining the relationship between the constant amplitude stress or strain amplitude and the energy dissipated per cycle, and recording the relationship asε a -E d The relational formula is as follows:
whereina、bAre all the fitting coefficients of the two-dimensional image,ε a representing the a-th order constant amplitude stress or strain amplitude.
5. The amplitude-variation fatigue life prediction method based on dissipated energy as claimed in claim 4, wherein the process of step 4 is as follows:
according to the dissipation energy per cycle under each stage of load in the variable amplitude fatigue load spectrum, the stress or strain amplitude is gradually increased to damage accumulated by cyclic loading in the fatigue test process, and a nonlinear damage accumulation model based on the dissipation energy and the energy tolerance is established; in the first placeiThe cyclic loading times under the stage load is deltan i Of 1 atiUnder a stage load, the energy dissipated per cycle isE d,i Then cycle of deltan i Total injury accumulated after a whileD i The calculation formula of (2) is as follows:
wherein,n i e-1,before showingiTotal damage accumulated after class 1 stress loading, on the secondiEquivalent cycle times required for generating the same accumulated damage under the loading of the grade stress;
δ i the definition is as follows:
μ i represents the firstiClass 1 load to class 2iA factor influencing the evolution of fatigue damage under a grade load,μ i the definition is as follows:
whereinE d,i-1Is shown asi-dissipated energy per cycle under class 1 stress loading;
will totally damageD i As a nonlinear damage accumulation model based on dissipated energy and energy tolerance.
6. The amplitude-variation fatigue life prediction method based on dissipated energy as claimed in claim 5, wherein the step 4 comprises:
step 4.1: calculating first order stressε 1 Loaded down cycle Δn 1Post-cumulative damageD 1The calculation formula is as follows:
wherein,E d,1representing first order stressε 1 Dissipation energy per cycle under loading;
index of refractionδ 1The definition is as follows:
step 4.2: stress of the first orderε 1 Accumulated damage under loadD 1As stress of the second orderε 2 The starting point for the accumulation of cycles under load is then:
wherein,n e1,the equivalent cycle number required for the sample to generate the accumulated damage which is the same as that generated after the first-stage stress loading under the second-stage stress loading is shown;n e1,the starting point of damage evolution under the second-stage stress loading is obtained;E d,2dissipating energy per cycle for a second level stress level;
index of refractionδ 2The definition is as follows:
μ 2representing the influence factor of the fatigue damage evolution under the first-stage stress loading and the second-stage stress loading, then
μ 2The definition is as follows:
number of equivalent cyclesn e1,Is recorded as:
step 4.3: calculating second order stressε 2 Load cycle Δn 2Post-cumulative damageD 2The calculation formula is as follows:
deducing the second step according to the step 4.1, the step 4.2 and the step 4.3iCumulative total damage after stage stress loading.
7. The method for predicting the amplitude-variation fatigue life based on dissipated energy as claimed in claim 5, wherein the equivalent cycle numbern i e-1,The calculation formula of (2) is as follows:
whereinq i Expressed as:
8. the amplitude-variation fatigue life prediction method based on dissipated energy as claimed in claim 7, wherein the step 5 comprises:
step 5.1: determining the condition of fatigue failure, i.e. total damage, of the specimenD i When the value is 1, the fatigue failure of the sample is judged, namely:
D i =1；
number of cycles under the last stage of load before fatigue failure of the specimen, i.e. the first stageiNumber of cycles Δ under a level stress loadingn i Calculated by the following calculation formula:
wherein,E C indicating the energy tolerance for fatigue failure of the specimen material,E d,i is shown asiThe energy dissipated per cycle under a stress loading of order,n i e-1,showing that the sample is atiUnder the action of a secondary stress, the primary stress is generatedi-the same accumulated damage after level 1 stress loading, required equivalent cycle number;
the total fatigue life under variable amplitude fatigue loading can therefore be expressed as:
9. the method for predicting the amplitude-variation fatigue life based on dissipated energy as claimed in claim 3, wherein the time constant isτ eq The calculation formula of (a) is as follows:
whereinR(τ eq ) Representing time constantsτ eq A function of the integration over the integration time r,τ opt representing the optimal time constant calculated by the least square methodτ eq ，d 1(t,τ eq )2Expressing the equation of thermal conductivity
，tIs time.
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