CN114065556A - Amplitude-variable fatigue life prediction method based on dissipation energy - Google Patents

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

A variable amplitude fatigue life prediction method based on dissipated energy

technical field

The invention belongs to the technical field of variable amplitude fatigue life prediction, in particular to a variable amplitude fatigue life prediction method based on dissipated energy.

Background technique

In engineering practice, most of the mechanical structural parts in service are subjected to complex load spectra. One of the important reasons for the complexity is that the amplitudes of the reciprocating loading loads that constitute the load spectrum may be different. Therefore, in order to be closer to the real service conditions, the research on fatigue problems needs to consider this variable amplitude fatigue problem, and the core goal of the research is to establish a reliable variable amplitude life prediction model.

For the variable amplitude fatigue problem, there are a large number of fatigue models. The mainstream method is to use the rainflow counting method to decompose the variable amplitude load spectrum into a combination of constant amplitude loads, and then evaluate the fatigue failure of materials or structures under a certain damage accumulation criterion. life. In this kind of method, the influence of the load history is not considered, and the load history such as pre-order cumulative damage will have a greater impact on the subsequent fatigue damage evolution. In addition, in the existing variable-amplitude fatigue life prediction models, the fatigue damage parameters are mostly in the form of energy based on stress, strain or stress-strain combination.

SUMMARY OF THE INVENTION

The purpose of the present invention is to overcome the above-mentioned deficiencies of the prior art, and to provide a method for predicting fatigue life based on the inherent dissipative energy calculated based on temperature data as a fatigue damage parameter, and based on the accumulated damage of the load loading history, specifically a method based on A variable amplitude fatigue life prediction method for dissipated energy.

The invention provides a variable amplitude fatigue life prediction method based on dissipated energy, comprising the following steps:

Step 1: For the variable-amplitude fatigue load spectrum experienced by a given material sample, define alternating loads with the same amplitude and continuous time as the same level of load, and divide the load order of each level according to the load loading history; the load spectrum is divided into stress spectrum or strain spectrum;

Step 2: According to the stress spectrum or strain spectrum, carry out the fatigue test with the gradually increasing stress or strain amplitude on the material sample, until the fatigue failure of the sample occurs, then the fatigue test with the gradually increasing stress or strain amplitude ends; The amplitude is gradually increased. During the fatigue test, each step of the stress or strain constant amplitude loading test needs to collect the temperature of the sample surface until the sample stops loading and the surface temperature drops to room temperature, and the temperature collection is stopped; Need to cycle multiple times;

Step 3: Calculate the dissipated energy and energy tolerance per cycle; perform heat source analysis based on the temperature data collected during the fatigue test with the stress or strain amplitude gradually increasing, and calculate the inherent dissipation of the material sample under each constant amplitude load energy, and then obtain the corresponding dissipated energy per cycle and energy tolerance under each level of load;

Step 4: According to the given variable-amplitude fatigue load spectrum and the obtained dissipated energy per cycle and energy tolerance, gradually increase the stress or strain amplitude for the accumulated damage of cyclic loading during the fatigue test. A nonlinear damage accumulation model for energy tolerance;

Step 5: Calculate the variable-amplitude fatigue life of the material; based on the nonlinear damage accumulation model of dissipated energy and energy tolerance, calculate the accumulated total damage after loading at all levels according to the load loading sequence, until the accumulated total damage is 1. Fatigue failure of the sample occurs, the number of cycles under the last load before fatigue failure is obtained, and then the total number of cycles of the variable amplitude load is calculated. The total number of cycles experienced by the sample is the variable amplitude fatigue life of the sample.

Preferably, the specific content of step 2 is: perform a fatigue test on the material sample with a gradually increasing stress or strain amplitude until the sample fails fatigue, then the stress or strain amplitude is gradually increased and the fatigue test ends. The number of different amplitudes of C 1 is recorded as C ₁ ; the number of cycles required for each step of stress constant amplitude loading is recorded as C ₂ , and fatigue failure occurs when C ₃ cycles are cycled under the stress amplitude of C ₁ step, and there is C ₃ < C ₂ , in this process, the temperature of the sample surface is collected until the temperature of the sample surface drops to room temperature, the temperature collection is stopped, and the next stress or strain loading fatigue test is carried out.

Preferably, the process of step 3 is: based on the temperature data collected in the test process and the heat conduction equation, according to the heat source analysis, calculate the inherent dissipated energy under each constant amplitude fatigue load, wherein the heat conduction equation is:

Among them, ρ and C are the density and specific heat capacity of the sample material, respectively; θ is the average temperature rise of the target area on the surface of the material sample, that is, the real-time temperature minus the initial temperature; d ₁ is the inherent dissipative energy; t is the time; τ _eq is time constant;

Integrate the calculated inherent dissipative energy d ₁ in one cycle, and calculate the dissipated energy E _d per cycle under each constant amplitude fatigue load. The calculation formula is:

Among them, t ₀ is the starting time of the stable cycle, and T is the cycle period of constant amplitude fatigue alternating load;

The obtained dissipated energy per cycle under each constant amplitude load is integrated within the fatigue life range of the fatigue test with the stress or strain amplitude gradually increasing, and the fatigue energy tolerance E _C of the material can be obtained. The calculation formula can be simplified as follows :

为应力或应变幅值逐步加载疲劳试验中C ₁步应力幅值下的每循环耗散能。where E _{d ( n )} is the dissipated energy per cycle under the constant stress amplitude load at the nth step in the stress or strain amplitude stepwise loading fatigue test,

is the energy dissipated per cycle at C ₁ step stress amplitude in a stepwise loading fatigue test for stress or strain amplitude.

Preferably, step 3 further includes: determining the relationship between each constant amplitude stress or strain amplitude and the dissipated energy per cycle, denoted as ε _a - E _d , and the relationship formula is:

Among them, a and b are fitting coefficients, and ε _a represents the constant amplitude stress or strain amplitude of the a-th grade.

Preferably, the process of step 4 is:

According to the dissipated energy per cycle under each level of load in the variable-amplitude fatigue load spectrum, the damage accumulated by the cyclic loading during the fatigue test is gradually increased to the stress or strain amplitude, and a nonlinear model based on dissipated energy and energy tolerance is established. Damage accumulation model; under the i -th level load, the number of cyclic loadings is Δ n _i , and under the i -th level load its dissipated energy per cycle is E _{d,i ,} then the accumulated total damage Di after cycling Δ n _i times is The calculation formula is:

Among them, n _{i -1, e} represents the total damage accumulated after the first i -1 level of stress loading, and the equivalent number of cycles required to produce the same accumulated damage under the i -th level of stress loading;

δi is _defined as follows:

μ _i represents the influence factor of the i -1st level load on the fatigue damage evolution under the i -th level load, and μ _i is defined as follows:

where E _{d,i -1} represents the dissipated energy per cycle under the i -1st stress loading;

The _formula for calculating the total damage Di is used as a nonlinear damage accumulation model based on dissipated energy and energy tolerance.

Preferably, step 4 includes:

Step 4.1: Calculate the accumulated damage D ₁ after the cycle Δ n ₁ under the loading of the first-level stress ε ₁ , the calculation formula is:

Among them, E _{d ,1} represents the dissipated energy per cycle under the loading of the first-order stress ε ₁ ;

The exponent δ ₁ is defined as follows:

；

;

Step 4.2: Taking the accumulated damage D ₁ under the loading of the first-level stress ε ₁ as the starting point of the cyclic accumulation under the loading of the second-level stress ε ₂ , there are:

Among them, n _{1, e} represents the equivalent number of cycles required to generate the same cumulative damage as the first-level stress loading of the specimen under the second-level stress loading; n _{1, e} is the second-level stress loading The starting point of damage evolution under stress loading; E _{d ,2} is the dissipated energy per cycle corresponding to the second stress level;

The exponent δ ₂ is defined as follows:

μ ₂ represents the influence factor of the first-level stress loading on the fatigue damage evolution under the second-level stress loading, then

μ2 is defined as follows _:

The equivalent number of cycles n _{1, e} is recorded as:

Step 4.3: Calculate the accumulated damage D ₂ after the second-level stress ε ₂ loading cycle Δ n ₂ times, the calculation formula is:

Derive the accumulated total damage after the i -th level of stress loading according to steps 4.1, 4.2, and 4.3.

Preferably, the calculation formula of the equivalent number of cycles n _{i -1, e} is:

where q _i is expressed as:

。

.

Preferably, step 5 includes:

Step 5.1: Determine the conditions for fatigue failure of the sample, that is, when the total _{damage Di} is 1, determine the fatigue failure of the sample, that is:

D _i =1

The number of cycles obtained under the last stage of load before the fatigue failure of the sample, that is, the number of cycles under the i -th stage of stress loading, Δn _i , is calculated by the following formula:

Among them, E _C represents the energy tolerance of the material for fatigue failure, _Ed,i represents the dissipated energy per cycle under the i -th level of stress loading, n _{i -1, e} represents the sample under the i -th level of stress loading. The equivalent number of cycles required to produce the same cumulative damage as after the i -1st stress loading;

Therefore, the total fatigue life under variable amplitude fatigue load can be expressed as:

。

.

Preferably, the calculation formula of the time constant τ _eq is as follows:

，t为时间。where R ( τ _eq ) represents the function of integrating the time constant τ _eq over the integration time г, τ _opt represents the optimal time constant τ _eq calculated by the least squares method, and d ₁ ( t , τ _eq ) ² represents the heat conduction equation

, t is time.

Beneficial effects:

1. The material will be accompanied by the generation of inherent dissipative energy under the fatigue alternating load. Therefore, using the inherent dissipative energy as the fatigue damage parameter can characterize the fatigue damage state and evolution process from the perspective of damage mechanism.

2. The fatigue life prediction method based on dissipated energy belongs to an energy method, and its life prediction accuracy is generally better than the life prediction method based solely on stress and strain state.

3. The acquisition process of inherent dissipative energy is based on temperature acquisition and calculation of temperature data. With the help of the advantages of non-contact, full-field, non-destructive and real-time measurement of the emerging infrared thermal imaging method, in engineering practice, compared with stress, Strain status is easier to obtain.

Description of drawings

In order to illustrate the technical solutions in the embodiments of the present invention more clearly, the following briefly introduces the accompanying drawings used in the description of the embodiments. Obviously, the accompanying drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can also be obtained from these drawings without creative effort.

FIG. 1 is a flow chart of a method for predicting variable amplitude fatigue life based on dissipated energy in the implementation of the present invention.

FIG. 2 is a schematic diagram of a two-stage variable amplitude test stress spectrum from high stress loading to low stress loading of a variable amplitude fatigue life prediction method based on dissipated energy in the implementation of the present invention.

FIG. 3 is a graph showing the relationship between stress and dissipated energy per cycle of a variable amplitude fatigue life prediction method based on dissipated energy in the implementation of the present invention.

FIG. 4 is a diagram showing the relationship between the predicted variable amplitude fatigue life and the experimental variable amplitude fatigue life of a method for predicting variable amplitude fatigue life based on dissipated energy in the implementation of the present invention.

Detailed ways

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the implementations. example. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

The sample material used in this example is 316L stainless steel, an infrared thermal imager is set up, and the position of the infrared thermal imager is adjusted so that the parallel section of the material sample is just in the field of view of the thermal imager, and the thermal imager is non-uniformly Correct and set the temperature collection range and sampling frequency; spray black matte paint on the surface of the material sample to improve the thermal emissivity of the material sample surface to ensure the accuracy of the temperature collection of the infrared thermal imager.

This embodiment proposes a variable-amplitude fatigue life prediction method. First, the fatigue damage parameter that characterizes the material damage in the model is the inherent dissipative energy, which is different from the damage parameters based on stress and strain. It is expressed as the dissipated heat energy, which can be obtained by obtaining the surface temperature of the material and then inverting the heat source. Because dissipated energy is directly related to the plastic strain energy that causes fatigue failure of the material, it can be used to characterize fatigue damage parameters. In addition, the model proposed in this embodiment can take into account the influence of the load loading history by introducing the load order influencing factor.

As shown in FIG. 1 , the variable amplitude fatigue life prediction method provided in this embodiment includes the following steps:

For the variable-amplitude fatigue load spectrum experienced by a given material, the alternating load with the same amplitude and continuous time is defined as the same-level load, and the load sequence of each level is divided according to the load loading history. In this embodiment, the load spectrum is the strain spectrum .

Step 1: Carry out fatigue test and temperature collection of material samples with gradually increasing strain amplitude: select an appropriate strain loading range, and conduct a group of fatigue tests with 5 different strain amplitudes gradually increasing, with a strain ratio of -1; until The fatigue failure of the material sample occurs, and the fatigue test with the strain amplitude gradually increasing ends; the number of different amplitudes experienced in this process is recorded as C ₁ ; the number of cycles required for constant strain amplitude loading at each step is recorded as C ₂ (fatigue failure occurs after C ₃ cycles under the stress amplitude of C ₁ step, C ₃ < C ₂ ), during this process, the temperature of the sample surface is collected until the temperature of the sample surface drops to room temperature, and the temperature is stopped. Acquisition, and the next step is the strain-controlled constant amplitude loading fatigue test.

Step 2: Calculate the dissipated energy per cycle; gradually increase the temperature data collected during the fatigue test based on the strain amplitude, conduct heat source analysis, calculate the inherent dissipative energy of the material under each constant amplitude strain, and then obtain the of energy dissipated per cycle;

Based on the temperature data collected during the test and the heat conduction equation, according to the heat source analysis, the inherent dissipated energy under each constant amplitude load is calculated, where the heat conduction equation is:

Among them, ρ and C are the material density and specific heat capacity, respectively; θ is the average temperature rise of the target area on the surface of the material sample, that is, the real-time temperature minus the initial temperature; d ₁ is the inherent dissipated energy; t is the time; τ _eq is the time constant ;

The calculation formula of the time constant τ _eq is as follows:

，t为时间；where R ( τ _eq ) represents the function of integrating the time constant τ _eq over the integration time г, τ _opt represents the optimal time constant τ _eq calculated by the least squares method, and d ₁ ( t , τ _eq ) ² represents the heat conduction equation

, t is time;

According to the constant amplitude uniaxial tensile-compression fatigue test of 316L stainless steel, as shown in Figure 3, the strain amplitude in the figure is ε _a , and the relationship between ε _a - E _d is determined. The relationship formula is:

Among them, a and b are both fitting coefficients, ε _a represents the constant amplitude stress or strain amplitude of the a-th grade; using this formula to fit the relationship between ε _a and E _d , a = 1.26, b = 5.86; by establishing This relationship is then used to integrate the calculated inherent dissipative energy d ₁ within one cycle to obtain the per-cycle dissipative energy E _d under each constant amplitude strain load. The calculation formula is:

Among them, t ₀ is the starting time of the stable cycle, and T is the cycle period of constant amplitude fatigue alternating load;

The obtained dissipated energy per cycle under each constant amplitude load is integrated within the fatigue life range with the strain amplitude gradually increasing, and the fatigue energy tolerance E _C of the material can be obtained. The calculation formula can be simplified as follows:

为应力或应变幅值逐步加载疲劳试验中C ₁步应力幅值下的每循环耗散能。where E _{d ( n )} is the dissipated energy per cycle under the constant-amplitude strain load at the n -th step in the strain-amplitude step-by-step loading fatigue test,

is the energy dissipated per cycle at C ₁ step stress amplitude in a stepwise loading fatigue test for stress or strain amplitude.

Step 3: According to the dissipated energy per cycle under each level of strain load, the damage accumulated by the cyclic loading during the fatigue test is gradually increased to the stress or strain amplitude, and the nonlinear damage based on the dissipated energy and energy tolerance is established cumulative model;

Step 3.1: Calculate the accumulated damage D ₁ after the cycle Δ n ₁ under the first-order strain ε ₁ loading, the calculation formula is:

where E _{d ,1} represents the dissipated energy per cycle under the loading of the first-order strain ε ₁ ;

The exponent δ ₁ is defined as follows:

Step 3.2: Taking the accumulated damage D ₁ under the loading of the first-order strain ε ₁ as the starting point of the cyclic accumulation under the loading of the second-order strain ε ₂ , we have:

Among them, n _{1, e} represents the equivalent number of cycles required for the material to generate the same accumulated damage D ₁ as that after the first-level strain loading under the second-level strain loading; n ₁ , e is the second-level strain loading E _{d ,2} is the dissipated energy per cycle corresponding to the second strain level;

The exponent δ ₂ is defined as follows:

μ ₂ represents the influence factor of the first-order strain level on the fatigue damage evolution under the second-order strain loading, then

μ2 is defined as follows _:

The equivalent number of cycles n _{1, e} is recorded as:

Step 3.3: Calculate the accumulated damage D ₂ after the second-order strain ε ₂ is loaded after the cycle Δ n ₂ , the calculation formula is:

Derive the accumulated total damage after the i -th strain loading according to steps 3.1, 3.2 and 3.3;

Under the i -th level of load, the number of cyclic loadings is Δ n _i , and under the i -th level of strain, the dissipated energy per cycle is E _{d,i ,} then the calculation formula of the accumulated total damage Di after cycling Δ n _i times is:

Among them, n _{i -1, e} is the total accumulated damage after the first i -1 level of strain loading, and the equivalent number of cycles required to generate the same accumulated damage under the i- th level of strain loading; Ed _,i represents the i -th level Dissipated energy per cycle under strain loading;

δi is _defined as follows:

μ _i represents the influence factor of the i -1st strain level on the fatigue damage evolution under the i -th strain loading, then μ _i is defined as follows:

where E _{d,i -1} represents the dissipated energy per cycle under the i -1 strain loading;

The _formula for calculating the total damage Di is used as a nonlinear damage accumulation model based on dissipated energy and energy tolerance.

In addition, because the fatigue failure mechanisms of high-cycle fatigue (low strain range) and low-cycle fatigue (high strain range) are different, the accumulated dissipated energy per cycle during the fatigue failure of the material is also different, so here The energy tolerance E _C is evaluated in the high strain range and the low strain range according to the following formula;

E _C = E _d ×N _f

Among them, Ed is the dissipated energy per cycle under the strain amplitude, N f _{is the} fatigue life under the constant amplitude strain load; the energy tolerance E _C is based on the low strain range or high strain range to which the corresponding Ed _,i belongs. It is divided into E _{C, LCF} and E _{C, HCF} ; the energy tolerances of the material in the high strain range and low strain range are obtained as E _{C, LCF} = 2.66×10 ¹⁰ W/m ³ , E _{C, HCF} = 17.2×10 ¹⁰ W/m ³ ;

Step 4: Calculate the variable-amplitude fatigue life of the material; based on the nonlinear damage accumulation model of dissipated energy and energy tolerance, calculate the accumulated total damage after all levels of strain are loaded according to the load loading sequence, until the accumulated total damage is 1. When the material has fatigue failure, the number of cycles under the last strain loading before fatigue failure is obtained, and then the total number of cycles of the material is calculated, and the total number of cycles of the material is the variable amplitude fatigue life of the material;

The calculation formula of the equivalent number of cycles n _{i -1, e} is:

where q _i is expressed as:

The total damage D _i is used to determine the conditions under which the material will experience fatigue failure;

Two-stage strain loading and variable amplitude symmetrical tension-compression (strain ratio of -1) fatigue test was carried out. The material was cycled Δn ₁ times under strain loading ε _{a 1} , and the number of Δ n 1 _was the test under constant strain loading ε _{a 1} half of the variable amplitude fatigue life N _{f 1} , then continue the fatigue test at the strain amplitude ε _{a 2} ( ε _{a 2} ≠ ε _{a 1} ) until the material fails fatigue, record the cycles of cyclic loading to failure at ε _{a 2} The number of times Δn ₂ , the two-stage fatigue test under different strain loading includes from high-strain to low-strain loading ( ε _{a 1} > ε _{a 2} ) and low-strain loading to high-strain loading ( ε _{a 1} < ε _{a 2} ) In both cases, as shown in Fig. 2, the abscissa t in the figure represents time, the ordinate ε represents the strain amplitude or stress loading, HL represents the loading sequence from high strain loading to low strain loading, and LH represents from low strain loading to high strain loading. The loading order of the strain loading, the total number of cycles of the two stages, i.e. Δn ₁ + Δn ₂ as the predicted variable-amplitude fatigue life,

Step 4.1: Determine the conditions for the fatigue failure of the material, that is, when the total _{damage Di} is 1, determine the fatigue failure of the material, that is:

D _i = 1;

The number of cycles under the last level of load before the material fatigue failure, that is, the number of cycles under the i -th level of strain loading Δ n _i is calculated by the following formula:

Among them, E _C represents the energy tolerance of the material for fatigue failure, E _d,i represents the dissipated energy per cycle under the i -th strain loading, n _{i -1, e} represents the material under the i -th strain loading, to produce The equivalent number of cycles required for the same cumulative damage as after the i -1 strain loading.

Test and predict the variable amplitude fatigue life: as shown in Figure 4, the straight line in the figure is an ideal line that is equal to the predicted variable amplitude fatigue life and the test variable amplitude fatigue life, and the two dashed lines represent the 2-fold factor life dispersion range. HL test data points, all other points are within a factor of 2, of which all 4 LH test data points, all fall near the line of N _fp = N _f ( N _fp is the predicted variable amplitude fatigue life, N _f is The experimental variable amplitude fatigue life), the test data points for HL are also evenly located on both sides of the straight line, so it can be concluded that the prediction results of the predicted variable amplitude fatigue life are generally similar to the experimental variable amplitude fatigue life, that is, the predicted variable amplitude fatigue life The lifetime prediction accuracy is high.

The variable amplitude fatigue life prediction method provided in this embodiment has the following beneficial effects:

1. The material will be accompanied by the generation of inherent dissipative energy under the fatigue alternating load. Therefore, using the inherent dissipative energy as the fatigue damage parameter can characterize the fatigue damage state and evolution process from the perspective of damage mechanism.

2. The fatigue life prediction method based on dissipated energy belongs to an energy method, and its life prediction accuracy is generally better than the life prediction method based solely on stress and strain state.

3. The acquisition process of inherent dissipative energy is based on temperature acquisition and calculation of temperature data. With the help of the advantages of non-contact, full-field, non-destructive and real-time measurement of the emerging infrared thermal imaging method, in engineering practice, compared with stress, Strain status is easier to obtain.

Therefore, using the inherent dissipative energy as the fatigue damage parameter has the advantages of starting from the mechanism, high accuracy of life prediction and easy acquisition.

The model provided in this embodiment is constructed based on the inherent dissipative energy. Compared with the conventional model, the energy tolerance EC is used to replace the SN curve under constant amplitude fatigue _. Therefore, one of its advantages is that the _EC is easier to obtain than the SN curve. ; The energy tolerances E _C,LCF and E _C,HCF are defined in the field of low cycle fatigue (low stress/strain range region) and high cycle fatigue (low stress/strain range region) fields respectively in this study, which can be defined in These two values are obtained by performing a single calibration test in both fields, or by two variable amplitude fatigue tests with stepwise loading until failure, respectively, for EC , _LCF and EC _,HCF . In fact, if the load range involved in the variable amplitude fatigue load spectrum is small, all in the low cycle fatigue field or the high cycle fatigue field, it is only necessary to calibrate E _{C, LCF} or E _{C, HCF} . Correspondingly, the determination of the SN curves in the low-cycle fatigue domain and the cycle fatigue domain often means that a large number of fatigue tests need to be carried out.

The model of this embodiment is based on the damage accumulation in the variable amplitude fatigue strain loading process. In the damage accumulation process, the influence factor of the load loading order is introduced to realize the consideration of the influence of the strain loading history on the variable amplitude fatigue life in the model.

The above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modification, equivalent replacement or improvement made within the spirit and principle of the present invention shall be included in the protection scope of the present invention. Inside.

Claims (9)

1. a variable amplitude fatigue life prediction method based on dissipated energy, is characterized in that, comprises the steps: Step 1: For the variable-amplitude fatigue load spectrum experienced by a given material sample, define alternating loads with the same amplitude and continuous time as the same level of load, and divide the load order of each level according to the load loading history; the load spectrum is divided into stress spectrum or strain spectrum; Step 2: According to the stress spectrum or strain spectrum, carry out the fatigue test with the gradually increasing stress or strain amplitude on the material sample, until the fatigue failure of the sample occurs, then the fatigue test with the gradually increasing stress or strain amplitude ends; The amplitude is gradually increased. During the fatigue test, each step of the stress or strain constant amplitude loading test needs to collect the temperature of the sample surface until the sample stops loading and the surface temperature drops to room temperature, and the temperature collection is stopped; Need to cycle multiple times; Step 3: Calculate the dissipated energy and energy tolerance per cycle; gradually increase the temperature data collected during the fatigue test based on the stress or strain amplitude, conduct heat source analysis, and calculate the inherent dissipated energy of the material sample under each constant amplitude load , and then obtain the corresponding dissipated energy per cycle and energy tolerance under each level of load; Step 4: According to the given variable-amplitude fatigue load spectrum and the obtained dissipated energy per cycle and energy tolerance, gradually increase the stress or strain amplitude for the accumulated damage of cyclic loading during the fatigue test. A nonlinear damage accumulation model for energy tolerance; Step 5: Calculate the variable-amplitude fatigue life of the material sample; based on the nonlinear damage accumulation model of dissipated energy and energy tolerance, calculate the accumulated total damage after loading at all levels according to the loading sequence, until the accumulated total damage is 1 When the fatigue failure of the sample is determined, the number of cycles under the last stage load before fatigue failure is obtained, and then the total number of cycles of the variable amplitude load is calculated, and the total number of cycles experienced by the sample is the variable amplitude fatigue life of the sample. . 2. A variable amplitude fatigue life prediction method based on dissipated energy according to claim 1, wherein the specific content of the step 2 is: performing a stress or strain amplitude gradually increasing fatigue test on the material sample , until the fatigue failure of the sample occurs, the stress or strain amplitude gradually increases and the fatigue test ends, and the number of different amplitudes experienced in this process is recorded as C ₁ ; the number of cycles required for constant stress loading at each step It is denoted as C ₂ , in which the fatigue failure occurs after C ₃ cycles under the stress amplitude of C ₁ step, and C ₃ < C ₂ . During this process, the temperature of the sample surface is collected until the surface temperature of the sample drops to Room temperature, stop temperature acquisition, and proceed to the next stress or strain loading fatigue test. 3. A variable amplitude fatigue life prediction method based on dissipated energy according to claim 2, wherein the process of step 3 is: based on the temperature data collected in the test process, and the heat conduction equation, according to the heat source Analyze and calculate the inherent dissipated energy under each constant amplitude fatigue load, where the heat conduction equation is:

Among them, ρ and C are the material density and specific heat capacity, respectively; θ is the average temperature rise of the target area on the surface of the material sample, that is, the real-time temperature minus the initial temperature; d ₁ is the inherent dissipated energy; t is the time; τ _eq is the time constant ; Integrate the calculated inherent dissipative energy d ₁ in one cycle, and calculate the dissipated energy E _d per cycle under each constant amplitude fatigue load. The calculation formula is:

where t ₀ is the starting time of the stable cycle, and T is the cycle period of constant amplitude fatigue load; The obtained dissipated energy per cycle under each constant amplitude fatigue load is integrated within the fatigue life range of the fatigue test with the stress or strain amplitude gradually increasing, and the fatigue energy tolerance E _C of the material can be obtained. The calculation formula can be simplified as follows:

where E _{d ( n )} is the dissipated energy per cycle under the constant stress amplitude load at the nth step in the stress or strain amplitude stepwise loading fatigue test,

is the energy dissipated per cycle at C ₁ step stress amplitude in a stepwise loading fatigue test for stress or strain amplitude. 4 . The variable amplitude fatigue life prediction method based on dissipated energy according to claim 3 , wherein the step 3 further comprises: determining the difference between each constant amplitude stress or strain amplitude and the dissipated energy per cycle. 5 . The relationship between , denoted as ε _a - E _d , the relationship formula is:

Among them, a and b are fitting coefficients, and ε _a represents the constant amplitude stress or strain amplitude of the a-th grade. 5. A variable amplitude fatigue life prediction method based on dissipated energy according to claim 4, wherein the process of the step 4 is: According to the dissipated energy per cycle under each level of load in the variable-amplitude fatigue load spectrum, the damage accumulated by the cyclic loading during the fatigue test is gradually increased to the stress or strain amplitude, and a nonlinear model based on dissipated energy and energy tolerance is established. Damage accumulation model; under the i -th level load, the number of cyclic loadings is Δ n _i , and under the i -th level load its dissipated energy per cycle is E _{d,i ,} then the accumulated total damage Di after cycling Δ n _i times is The calculation formula is:

Among them, n _{i -1, e} represents the total damage accumulated after the first i -1 level of stress loading, and the equivalent number of cycles required to produce the same accumulated damage under the i -th level of stress loading; δi is _defined as follows:

μ _i represents the influence factor of the i -1st level load on the fatigue damage evolution under the i -th level load, and μ _i is defined as follows:

where E _{d,i -1} represents the dissipated energy per cycle under the i -1st stress loading; The _formula for calculating the total damage Di is used as a nonlinear damage accumulation model based on dissipated energy and energy tolerance. 6 . The variable amplitude fatigue life prediction method based on dissipated energy according to claim 5 , wherein the step 4 comprises: Step 4.1: Calculate the accumulated damage D ₁ after the cycle Δ n ₁ under the loading of the first-level stress ε ₁ , the calculation formula is:

Among them, E _{d ,1} represents the dissipated energy per cycle under the loading of the first-order stress ε ₁ ; The exponent δ ₁ is defined as follows:

; Step 4.2: Taking the accumulated damage D ₁ under the loading of the first-level stress ε ₁ as the starting point of the cyclic accumulation under the loading of the second-level stress ε ₂ , there are:

Among them, n _{1, e} represents the equivalent number of cycles required to generate the same cumulative damage as the first-level stress loading of the specimen under the second-level stress loading; n _{1, e} is the second-level stress loading The starting point of damage evolution under stress loading; E _{d ,2} is the dissipated energy per cycle corresponding to the second stress level; The exponent δ ₂ is defined as follows:

μ ₂ represents the influence factor of the first-level stress loading on the fatigue damage evolution under the second-level stress loading, then μ2 is defined as follows _:

The equivalent number of cycles n _{1, e} is recorded as:

Step 4.3: Calculate the accumulated damage D ₂ after the second-level stress ε ₂ loading cycle Δ n ₂ , the calculation formula is:

The accumulated total damage after stress loading of the i -th level is derived according to the described steps 4.1, 4.2 and 4.3. 7. The variable amplitude fatigue life prediction method based on dissipated energy according to claim 5, wherein the calculation formula of the equivalent cycle number n _{i -1, e} is:

where q _i is expressed as:

. 8 . The variable amplitude fatigue life prediction method based on dissipated energy according to claim 7 , wherein the step 5 comprises: Step 5.1: Determine the conditions for fatigue failure of the sample, that is, when the total _{damage Di} is 1, determine the fatigue failure of the sample, that is: D _i = 1; The number of cycles obtained under the last stage of loading before the fatigue failure of the sample, that is, the number of cycles Δ n _i under the i -th stage of stress loading, is calculated by the following formula:

Among them, E _C represents the energy tolerance of the specimen material for fatigue failure, _Ed,i represents the dissipated energy per cycle under the i -th level of stress loading, n _{i -1, e} represents the sample under the i -th level of stress loading , the equivalent number of cycles required to produce the same cumulative damage as after the i -1st stress loading; Therefore, the total fatigue life under variable amplitude fatigue load can be expressed as:

. 9. A kind of variable amplitude fatigue life prediction method based on dissipated energy according to claim 3, is characterized in that, the calculation formula of described time constant τ _eq is as follows:

where R ( τ _eq ) represents the function of integrating the time constant τ _eq over the integration time г, τ _opt represents the optimal time constant τ _eq calculated by the least squares method, and d ₁ ( t , τ _eq ) ² represents the heat conduction equation

, t is time.

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