Ratchet wheel evaluation method based on accumulative damage

Abstract

The invention relates to the technical field of ratchet evaluation methods. Specifically, the invention relates to a ratchet wheel evaluation method based on accumulative damage. The method comprises the following steps: S1, calculating the allowable life Nk of the structure under the kth cyclic load; s2, calculating ratchet wheel damage Dk caused by the cyclic load; s3, calculating a damage superposition coefficient D caused by various cyclic loads borne by the structure; and S4, judging whether D is not greater than 1 or not, if D is not greater than 1, passing ratchet wheel evaluation, and if D is greater than 1, not passing ratchet wheel evaluation. Due to the adoption of the technical scheme, compared with the prior art, the method has the advantages that the method is simple, economical and accurate, and can be used for the condition that equipment bears the combined action of various cyclic loads.

Description

Technical Field

[0001] The invention relates to the technical field of ratchet evaluation methods.

[0002] Specifically, it relates to a ratchet assessment method based on cumulative damage. Background Art

[0003] When pressure-bearing components are subjected to alternating thermal mechanical loads, they may experience incremental plastic damage or inelastic deformation, i.e., ratchet failure. Ratchet failure is one of the main failure modes of pressure-bearing components under cyclic loads, and preventing ratchet failure has always been an important research topic for researchers around the world.

[0004] In 1959, Miller conducted an ideal elastic-plastic analysis on a simplified thin-walled cylinder model and established the critical conditions for thermal stress ratcheting. In the 1960s, Bree studied the elastic-plastic behavior of thin-walled cylinders under mechanical film stress and thermal bending stress. According to different stress states of the structure, he divided the two-dimensional coordinate diagram composed of mechanical film stress and thermal bending stress into six regions, namely the famous Bree diagram. Later, Nadarajah et al. confirmed the validity of the Bree diagram based on ideal elastic-plastic finite element analysis. Bree theory has always been the basis for thermal stress ratchet assessment in European and American standards. In 2002, Kalnins summarized the methods used for shakedown analysis and ratchet assessment in the ASME pressure vessel and piping specifications. By comparing two elastic-plastic analysis examples of cylinders subjected to radial temperature gradient and pressure and cylinders subjected to axial temperature gradient and pressure, it was found that when there is thermal film stress or even when the thermal film stress is dominant, although the Bree method is verified, it cannot pass the evaluation of elastic-plastic ratchet analysis. At this time, the Bree method is unconservative and dangerous. In 2008, the ratchet boundary was derived when mechanical film stress and thermal film plus bending stress exist at the same time, pointing out that alternating thermal film stress is sometimes more dangerous than bending stress. Therefore, the study of ratchet effect was extended to three stress parameters, but the stable mechanical bending stress was not considered. In some cases, the mechanical bending stress in the structure can also have a significant impact on the safety of the structure. On this basis, in 2018, based on the non-cyclic method, the ratchet boundary considering constant mechanical film stress and mechanical bending stress, cyclic thermal film stress and thermal bending stress was derived in detail.

[0005] The study of ratchet effect has expanded from considering two stress parameters to three stress parameters and then to four stress parameters. Great progress has been made from the theoretical level, and the numerical methods used to solve structural stability and ratchet analysis have gradually matured. In the past two decades, pressure vessel specifications in various countries have begun to fully introduce elastic-plastic analysis based on numerical calculations for various failure modes. Japan's C-TDF method gives two plastic criteria based on finite element analysis to evaluate the ratchet effect: evaluating the change in plastic strain increment and evaluating the change in elastic core width. In 2002, Appendix B of EN13445-3 proposed a new method for checking the ratchet of pressure vessels, and gave two application criteria to check the stability behavior of the structure. Application criterion 1 means that the maximum absolute value of the main structure strain is limited to 5% under the load condition and the specified number of cycles. Starting from the 2007 edition, ASME VIII-2 also gives the elastic-plastic analysis method of ratchet and three evaluation criteria:

[0006] The plastic strain is zero.

[0007] There is an elastic core at the load-bearing boundary of the element.

[0008] There is no permanent change in the overall dimensions of the component.

[0009] The elastic-plastic stress analysis method based on numerical analysis is a cycle-by-cycle incremental analysis by finite element. Although it is simple and easy to use, it is very time-consuming and computationally intensive for working conditions with a large number of cycles. In addition, most methods are based on research under a single working condition and do not allow the pressure-bearing components to enter the ratchet state, which is relatively conservative. In actual engineering applications, various structures and load conditions are often complex. Therefore, it is increasingly important to find a simple and economical assessment method to prevent ratchet failure of pressure-bearing components.

[0010] The AMCD method proposed in this application is a method for ratcheting assessment based on the results of a smaller number of cyclic calculations. The essential difference from the existing assessment methods is that it allows the structure to enter the ratchet state, but it needs to be limited. This method is not only simple and economical, but also relatively accurate and can be used when the equipment is subjected to the combined action of multiple cyclic loads. Summary of the invention

[0011] The purpose of the present invention is to overcome the shortcomings of the above-mentioned traditional technology and provide a ratchet assessment method based on cumulative damage, which is simple, economical, relatively accurate and can be used in situations where the equipment is subjected to the combined action of multiple cyclic loads.

[0012] The object of the present invention is achieved by the following technical measures: A ratchet assessment method based on cumulative damage comprises the following steps:

[0013] Step S1, calculate the allowable life N of the structure under the kth cyclic load k;

[0014] Step S2, calculating the ratchet damage D caused by the cyclic load k , D k The calculation formula is:

[0015]

[0016] Among them, n k is the actual number of cycles of the cyclic load;

[0017] Step S3, calculating the damage superposition coefficient D caused by various cyclic loads borne by the structure, the calculation formula of D is:

[0018]

[0019] Step S4, judging whether D is not greater than 1, if D is not greater than 1, the ratchet assessment is passed, and if D is greater than 1, the ratchet assessment is failed.

[0020] As an improvement: In step S1, N k The calculation formula is:

[0021]

[0022] Among them, |ε R | is the allowable total strain limit, ε0 is the equivalent ratchet strain accumulated in the unstable stage, is the average value of the equivalent ratcheting strain increment in the stable cycle, and n0 is the number of cycles in the unstable stage.

[0023] As an improvement: in step S1, an elastic-plastic finite element analysis is performed on the kth cyclic load, the equivalent ratchet strain is calculated using the peak-to-valley average value, and a curve of the change of the equivalent ratchet strain increment with the number of cycles is plotted. The initial n0 cycles are an unstable stage, and the equivalent ratchet strain increment decays rapidly. Then the curve enters a stable stage, and the calculation ends at the preset number A cycles after the stable stage.

[0024] As an improvement: the preset number of times A is 6-10.

[0025] Due to the adoption of the above technical solution, compared with the prior art, the advantages of the present invention are: simple and economical, relatively accurate and applicable to the situation where the equipment is subjected to the combined action of multiple cyclic loads.

[0026] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Attached Figure 1 is a schematic diagram of the biplane model used for validation.

[0028] Attached Figure 2 Schematic diagram of the meshing of the dual-plane model used for verification.

[0029] Attached Figure 3 It is a schematic diagram of the operating condition numbering based on the Bree diagram.

[0030] Attached Figure 4 It is a schematic diagram of the position of each working condition on the Bree diagram.

[0031] Attached Figure 5 It is a schematic diagram of the location and combination of working conditions.

[0032] Attached Figure 6 It is a schematic diagram of the allowable number of cycles for each operating condition based on the Bree ratchet area.

[0033] Attached Figure 7 It is a schematic diagram of ratchet damage under various working conditions in the Bree ratchet area in a single cycle.

[0034] Attached Figure 8 It is a projection diagram of the ratchet damage contour lines on the X and Y planes based on various working conditions in the Bree ratchet area under a single cycle.

[0035] Attached Fig. 9 It is a schematic diagram of a geometric model of a calculation example of a ratchet assessment method based on cumulative damage of the present invention.

[0036] Attached Fig.10 It is a schematic diagram of the mesh division of a geometric model in a calculation example of a ratchet assessment method based on cumulative damage of the present invention.

[0037] Attached Fig.11 It is a schematic diagram of the temperature cycle history on the inner and outer surfaces of working condition 1 in a calculation example of a ratchet assessment method based on cumulative damage of the present invention.

[0038] Attached Fig.12 It is a schematic diagram of working condition 1 - temperature distribution along the thickness direction in a calculation example of a ratchet assessment method based on cumulative damage of the present invention.

[0039] Attached Fig.13 The present invention is a ratchet assessment method based on cumulative damage, in which a curve showing the variation of equivalent ratchet strain with the number of cycles in working condition 1 is shown in a calculation example.

[0040] Attached Fig.14 The present invention is a ratchet assessment method based on cumulative damage, and the equivalent ratchet strain of working condition 2 in a calculation example varies with the number of cycles.

[0041] Attached Fig.15 The present invention is a ratchet evaluation method based on cumulative damage, and the equivalent ratchet strain of working condition 3 in a calculation example varies with the number of cycles.

[0042] Attached Fig.16 It is a schematic flow chart of a ratchet assessment method based on cumulative damage of the present invention. DETAILED DESCRIPTION

[0043] Embodiment 1: As attached Fig.16 As shown, a ratchet assessment method based on cumulative damage is characterized by comprising the following steps:

[0044] Step S1, calculate the allowable life n of the structure under the kth cyclic load k ;

[0045] Step S2, calculating the ratchet damage D caused by the cyclic load k , D k The calculation formula is:

[0046]

[0047] Among them, n k is the actual number of cycles of the cyclic load;

[0048] Step S3, calculating the damage superposition coefficient D caused by various cyclic loads borne by the structure, the calculation formula of D is:

[0049]

[0050] Step S4, judging whether D is not greater than 1, if D is not greater than 1, the ratchet assessment is passed, and if D is greater than 1, the ratchet assessment is failed.

[0051] In step S1, N k The calculation formula is:

[0052]

[0053] Among them, |ε R | is the allowable total strain limit, ε0 is the equivalent ratchet strain accumulated in the unstable stage, is the average value of the equivalent ratcheting strain increment in the stable cycle, and n0 is the number of cycles in the unstable stage.

[0054] In step S1, an elastic-plastic finite element analysis is performed on the kth cyclic load, the equivalent ratchet strain is calculated using the peak-to-valley average value, and a curve of the change of the equivalent ratchet strain increment with the number of cycles is plotted. The initial n0 cycles are an unstable stage, and the equivalent ratchet strain increment decays rapidly. Then the curve enters a stable stage, and the calculation ends at the preset number A of cycles after the stable stage.

[0055] Specifically, the preset number of times A is 6-10. Specifically, it can be 6, 8 or 10.

[0056] Example 2: Numerical calculation and verification.

[0057] 1. Geometric model.

[0058] In order to verify the correctness of the above ratchet evaluation method and to facilitate comparison with existing methods, a typical Bree case is taken as an example to carry out numerical research based on a single-plane model. Figure 1 As shown in the figure, a constant pressure P is applied on the boundary line of the plane to obtain uniformly distributed mechanical film stress, and a cyclic temperature gradient linearly distributed along the X direction is applied to the plane to generate linearly distributed cyclic thermal bending stress in the X direction. It can consider any combination of constant mechanical film stress and thermal bending stress. On this basis, various working conditions are combined and loaded in different ways, and finite element software is used to analyze and verify the Bree problem based on cumulative damage.

[0059] The material properties are shown in Table 1. The elastic-perfectly plastic material model is used in the analysis. The Von-Mises yield function and the related flow rules are used. The Von-Mises yield condition is the same as the Tresca yield condition under uniaxial stress state.

[0060] Table1 Material properties

[0061]

[0062] Grid division as Figure 2 As shown in the figure, the unit used in plane stress analysis is Plane183 unit. The mesh independence has been verified.

[0063] 2. Analysis and verification.

[0064] In order to record the calculation results of different working conditions, the Bree diagram is evenly divided and the working conditions are numbered. When Y is 0, X is 0.1, and the corresponding number is 01; when Y is 0.5, X is 0.1, and the corresponding number is 11. Similarly, each row is numbered from left to right. 42 working conditions in the right ratchet area are selected for loading, such as Figure 3As shown. In addition to the working conditions 19 and 73 near the boundary, three other working conditions were taken. The coordinates in the Bree diagram are (0.23, 4.5), (0.42, 2.49), and (0.8, 0.9), which are evenly distributed near the upper, middle, and lower parts of the boundary. The numbers are approximately 92, 54, and 18, respectively. Each working condition is set to cycle 50 times, and 6s is one cycle. Three points on the inner wall, middle, and outer wall of the single-plane model were selected for verification, which include the points with the largest plastic strain, that is, the most demanding points. The equivalent ratchet strain and main ratchet strain of these three points under these 45 working conditions were calculated using the peak-to-valley average method and the load average method, and the curves of the change of the equivalent ratchet strain and the main ratchet strain with the number of cycles were plotted to obtain the allowable number of cycles. Finally, the calculation results of the most demanding point among the three points were compared with the allowable number of cycles calculated by the finite element method.

[0065] In order to verify the correctness of the new method, two working conditions were randomly selected for loading, with a total of 10 groups of working conditions, as shown in Table 2. Figure 4 . Figure 5 The location and combination of 10 working conditions are displayed more intuitively.

[0066] Table 2 10 selected working conditions

[0067]

[0068]

[0069] These 10 groups of working conditions cover the following three situations: different mechanical film stresses and the same thermal bending stresses; the same thermal bending stresses and different mechanical film stresses; different mechanical film stresses and thermal bending stresses. The cumulative damage method simply linearly superimposes the ratchet damage of each working condition without considering the loading sequence and method, so it is necessary to further consider whether loading methods in different sequences will have a greater impact on plastic strain. Assuming there are two working conditions AB, there are a total of four accumulation methods: ABAB..., BABA..., AA...BB..., BB...AA..., and the specific meanings are as follows:

[0070] ABAB: Start from A, and cycle A and B alternately.

[0071] BABA: Start from B, and then alternate between B and A.

[0072] AABB: A cycles first, then B cycles.

[0073] BBAA: B cycles first, then A cycles.

[0074] The two loading methods ABAB... and BABA... are essentially the same, because as long as the number of cycles is large enough, the strain generated by the first cycle A condition of ABAB loading can be ignored, which is actually BABA... type loading.

[0075] Therefore, it is only necessary to analyze the three types of loading methods: ABAB..., AA...BB..., BB...AA... The method in Example 1 can be used to obtain the allowable number of cycles for the 45 ratchet working conditions selected on the Bree diagram, and then a suitable number of cycles is set for each group of working conditions so that the cumulative damage is approximately equal to 1. The equivalent plastic strain obtained by finite element calculation with the same number of cycles of ABAB..., AABB..., BBAA... loading methods is compared with the degree of agreement of the allowable total strain limit of 0.07. 3. Calculation and results.

[0076] Under a single working condition, the premise for the new criterion to be established is that the ratchet strain of the initial few cycles is stable, and when the difference between the ratchet strain of the next cycle and the previous cycle is less than 10 -4 , it can be considered to have reached stability. Table 3 records the number of cycles when 42 ratchet working conditions reach stability in the form of a Bree diagram, where X represents the ratio of mechanical film stress to yield strength, and Y represents the ratio of thermal bending stress to yield strength. Working conditions 92, 54, and 18 are approximate numbering values, which are not convenient to list in Table 3. They are explained separately here, and the number of cycles when the increments of the three working conditions are stable is 3. Table 4 records 45 working conditions located in the ratchet area, using the four calculation methods mentioned in Example 1 and the finite element calculation of the allowable number of cycles for each working condition when the total strain limit is 7%, and taking the number of cycles at the most demanding point. It can be clearly seen that the four calculation methods and the allowable number of cycles calculated by the finite element are basically consistent. It shows that the calculation results of the four calculation methods are basically consistent, and consistent with the numerical solution. The consistency of the results is also the premise for the establishment of the new method.

[0077] Table 3 Cycle times when increment is stable

[0078]

[0079]

[0080] Table 4 Allowable cycles for each working condition under different calculation methods

[0081]

[0082]

[0083] Table 5 shows the allowable cycles of the most demanding point under 42 working conditions calculated by the equivalent ratchet strain-peak-valley average method, which is also presented in the form of a Bree diagram. Working conditions 92, 54, and 18 are listed in Table 4. Figure 6 The allowable number of cycles for different working conditions on the Bree diagram is reflected in the form of a three-dimensional graph. The closer to the junction of the ratchet and the stable, the greater the allowable number of cycles. Figure 7 The figure shows the ratchet damage diagram based on the Bree diagram under a single cycle. The farther away from the boundary, the greater the ratchet damage in a single cycle and the smaller the allowable number of cycles. Figure 8 The projection diagram of the ratchet damage contour line of a single cycle on the XY plane shows that the ratchet damage on each contour line is the same. The allowable cycle number diagram can intuitively calculate the allowable cycle number of the ratchet area working condition on the Bree diagram. When multiple cyclic working conditions are loaded, the damage of a single cycle can be obtained according to the damage diagram, and multiplied by the actual number of cycles to obtain the ratchet damage. When the damage of each working condition is linearly superimposed and does not exceed the allowable damage factor of 1, the ratchet assessment passes.

[0084] Table 5 Permissible number of cycles

[0085]

[0086] Under mixed working conditions, Table 6 shows the comparison between the cumulative damage coefficient and the plastic strain results calculated by finite element calculation after the selected 10 groups of working conditions are combined. When the cumulative damage is close to the allowable damage 1, the equivalent plastic strain calculated by loading in AA...BB... or changing the order of working conditions AB is around 0.07, which is consistent with the numerical solution. However, it is also found that when loading in ABAB... mode, the maximum error between the equivalent plastic strain calculated by finite element calculation and the total strain limit of 0.07 reaches 50.7%. This shows that the AMCD method is fully applicable to the loading method of AA...BB..., while for the loading method of ABAB, the calculation results will be biased. Therefore, another calculation scheme is proposed for the loading method of ABAB and verified.

[0087] Table 6 Comparison results of cumulative damage and equivalent plastic strain calculated by finite element method under different working condition combinations

[0088]

[0089] For the loading mode of ABAB, the two working conditions AB can be regarded as a load unit, that is, AB can be regarded as an overall cycle, and the equivalent plastic strain at the peak-to-valley average or load average within 12s of this overall cycle is taken to calculate the allowable number of cycles. Finally, it is compared and verified with the results of finite element cycle-by-cycle calculation, see Table 7.

[0090] Table 7 Comparison of allowable cycles calculated by different methods under mixed working conditions

[0091]

[0092]

[0093] Embodiment 3: Calculation example.

[0094] For a cylinder plus flat head model, by adjusting the inner diameter and thickness, the maximum stress range is in the place where the structure is discontinuous at the bend of the cylinder, that is, the ratchet assessment is performed on the most demanding place. Two working conditions are set, and elastic-plastic finite element analysis is performed under these two working conditions, and the application steps of the AMCD method are explained in detail. Finally, the assessment results are verified by finite element numerical calculation.

[0095] 1. Geometric model.

[0096] The geometric model size is as follows Fig. 9 shown. Fig.10 It is a finite element mesh model. According to the symmetry of the structure and load, a 2D axisymmetric model can be used for analysis. The unit type used is Plane183 unit. After the mesh division is completed, the number of nodes is 4407 and the number of units is 1394.

[0097] The ideal elastic-plastic material model is adopted, and the Von-Mises yield criterion and the associated flow criterion are selected. The specific material parameters are consistent with Table 1. This model adds thermal analysis, so the thermal conductivity also needs to be added, and the specific value is 0.0148W / (mm·℃).

[0098] 2. Loading conditions.

[0099] Two different working conditions are set, each of which is given a constant internal pressure, the initial temperature is set to 300°C, and different temperature gradients are set along the thickness direction:

[0100] Condition 1: P1 = 40 MPa, T max =450℃, T min =150℃

[0101] Condition 2: P2 = 30 MPa, T max =487.5℃, T min =112.5℃

[0102] One cycle is set to 6s, and each working condition is cycled 48 times. Taking working condition 1 as an example, the alternating temperature loads applied to the inner and outer walls are as follows: Fig.11 As shown (only 5 cycles are shown). Fig.12 The temperature gradient along the thickness direction obtained for the cycle holding time. A constant internal pressure of 40 MPa is applied to the inner wall of the container. The axial displacement constraint of the bottom of the container is set to 0.

[0103] 3. Assessment process

[0104] The elastic-plastic finite element analysis is performed on the first cyclic load, and the curve of the equivalent ratchet strain changing with the number of cycles is plotted, as shown in Fig.13 As shown, the first few weeks are unstable, Δε R,k The curve decays rapidly until the 18th cycle, and then enters a stable stage. The average value of the equivalent ratchet strain increment of the 6 even cycles after the 18th cycle is calculated. is 0.00048, the number of cycles in the unstable stage n0 is 18, and the accumulated ratchet strain ε0 in the unstable stage is 0.02768. Substituting the above values ​​into formula (2), the allowable number of cycles N for working condition 1 can be obtained. k is 108.

[0105] The allowable number of cycles N of working condition 1 is known k = 108, assuming that the actual number of cycles of this load during the service life of the equipment is N k is 49 times. Substituting the above value into formula (3), we can obtain that the ratchet damage D1 caused by this load is 0.4537.

[0106] Fig.14 The curve of the equivalent ratchet strain changing with the number of cycles for working condition 2. Similarly, the allowable number of cycles N for working condition 2 can be obtained. k is 91, and the actual number of cycles of this load during the service life of the equipment is N k Assume it is 48 times, and the ratchet damage D2 is obtained as 0.5274.

[0107] The sum of the ratchet damage under the two working conditions is 0.9811, the cumulative damage coefficient D is less than 1, and the ratchet assessment passes.

[0108] If loading is performed in the form of ABAB…, it is necessary to regard working conditions 1 and 2 as one load unit, which is recorded as working condition 3 here. Fig.15 This is the variation curve of equivalent ratchet strain with the number of cycles under the loading condition 3. Similarly, the allowable number of cycles N under the loading condition 3 can be obtained. k It is 58. Assuming that the equipment actually cycles 57 times in total under working conditions 1 and 2 during its service life, the ratchet damage caused by working condition 3 is 0.9827, that is, the cumulative damage coefficient D is 0.9827, which is less than 1, and the ratchet assessment is passed.

[0109] 4. Result verification.

[0110] In order to further verify the correctness of the new method, the evaluation results of the actual number of cycles during the service life of the two assumed equipment were verified by finite element cycle-by-cycle calculation. The first assumption is that the actual number of cycles of working condition 1 is 49 times, and the actual number of cycles of working condition 2 is 48 times, that is, 97 times of loading in the form of AA...BB..., and the equivalent plastic strain obtained is 0.069718, which is less than the total strain limit of 0.07. The ratchet evaluation passed, which is consistent with the results obtained by the new method. The second assumption is that the actual number of cycles of the two working conditions is 57 times in total, 29 times for A and 28 times for B, that is, 57 times of loading in the form of ABAB..., and the equivalent plastic strain obtained is 0.066524, which is less than 0.07, and the ratchet evaluation passed, which is consistent with the evaluation results of the new method.

[0111] In order to prevent ratchet failure, the current specification actually puts the structure in a stable or fully elastic state, that is, the structure is not allowed to enter the ratchet. This application proposes a new method that allows the structure to enter the ratchet. Referring to the EU's plastic strain control method, considering that the risk of ratchet failure is less than the risk of plastic collapse failure, the allowable limit of the equivalent ratchet total strain is set to 7%. The new method expands the scope of application of ratchet assessment. As long as the total ratchet strain does not exceed 7%, the structure can pass even if ratcheting occurs.

[0112] When the elastoplastic analysis method in the ASME specification is used for ratchet verification, a detailed elastoplastic analysis is required. When the number of cycles is large, the amount of calculation is huge. However, when the new method is used, except for the unstable stage in the initial few weeks, it is only necessary to calculate until the end of the sixth cycle of the stable stage, and then calculate the allowable number of cycles according to formula (2). The amount of calculation is much less than the detailed elastoplastic analysis method.

[0113] In order to verify the correctness of the AMCD method, the allowable number of cycles under a single working condition was analyzed. It can be seen from Table 4 that the results of the four calculation methods for the allowable number of cycles are basically consistent and have a good degree of agreement with the results of the finite element cycle-by-cycle calculation. The maximum error between the four calculation methods is within 2.6%. Compared with the results of the finite element cycle-by-cycle calculation, the maximum errors of the peak-valley average value-equivalent ratchet strain and the peak-valley average value-main ratchet strain are both 3.51%; the maximum errors of the load average value-equivalent ratchet strain and the load average value-main ratchet strain are both 4.68%. Therefore, the relatively more accurate peak-valley average method is selected to calculate the equivalent ratchet strain and then calculate the allowable number of cycles.

[0114] The AMCD method adopts the linear damage accumulation theory. For mixed working conditions, it is necessary to consider whether the AMCD method is also applicable for different loading methods and sequences. It was analyzed and verified in Example 2 of the present application. Table 6 compares the equivalent plastic strains obtained under mixed working conditions with different loading methods and sequences, indicating that for AA...BB... type loading methods, the theoretical solution and the numerical solution are in good agreement. However, for ABAB... type loading forms, there will be deviations between the theoretical solution and the numerical solution. Therefore, for the ABAB type loading method, the present application proposes another calculation scheme, which regards the two working conditions AB as a load unit, that is, the allowable number of cycles is calculated according to the method of a single working condition. It can be seen from Table 7 that the results of the AMCD method and the numerical calculation are in good consistency, indicating that the AMCD method is a simple and relatively accurate ratchet evaluation method.

[0115] This application proposes a new ratchet assessment method based on cumulative damage, referred to as the AMCD method. The correctness of this method is studied based on a plane model. The differences between the four allowable cycle calculation methods are analyzed, and the effects of loading in different ways and sequences on the equivalent plastic strain of two working conditions are compared. Finally, through a calculation example, the assessment process of the AMCD method is elaborated in detail, and the consistency of its assessment results is verified. The following conclusions can be drawn:

[0116] The new method is a method of ratchet assessment based on the calculation results of a smaller number of cycles. It only needs to calculate 6-10 cycles after stabilization to obtain the allowable life of the structure under the working condition. Compared with the finite element cycle-by-cycle calculation, it greatly reduces the amount of calculation and shortens the calculation time. Compared with the existing ratchet assessment method, its essential difference is that it allows the pressure-bearing components to enter the ratchet state, which can fully utilize the bearing capacity of the structure. And the AMCD method can also be used for ratchet assessment under a variety of working conditions.

[0117] The calculation errors of the four allowable cycles are within 2.6%. It is determined that the relatively accurate peak-to-valley average method is used to calculate the equivalent ratchet strain and then the allowable cycles.

[0118] Different loading methods have an impact on the equivalent plastic strain. Under mixed working conditions, for the case of successive loading, the AMCD method can be directly used to calculate the ratchet damage of each working condition and then linearly superimpose it. For the case of alternating loading, it is necessary to regard these two working conditions as one load unit and then use the AMCD method for calculation.

[0119] In summary, the AMCD method is a simple, economical and relatively accurate ratchet evaluation method suitable for use in engineering.

[0120] The above is a detailed description of an embodiment of the present invention, but the content is only a preferred embodiment of the present invention and cannot be considered to limit the scope of implementation of the present invention. All equivalent changes and improvements made within the scope of the present invention should fall within the scope of the patent coverage of the present invention.

Claims

1. A ratchet assessment method based on cumulative damage, characterized in that: The following steps are involved: Step S1, calculate the allowable life N of the structure under the kth cyclic load k ; Step S2, calculating the ratchet damage D caused by the cyclic load k , D k The calculation formula is: Among them, n k is the actual number of cycles of the cyclic load; Step S3, calculating the damage superposition coefficient D caused by various cyclic loads borne by the structure, the calculation formula of D is: Step S4, judging whether D is not greater than 1, if D is not greater than 1, the ratchet assessment is passed, and if D is greater than 1, the ratchet assessment is failed.

2. The ratchet evaluation method according to claim 1, characterized in that: In step S1, N k The calculation formula is: Among them, |ε R | is the allowable total strain limit, ε0 is the equivalent ratchet strain accumulated in the unstable stage, is the average value of the equivalent ratcheting strain increment in the stable cycle, and n0 is the number of cycles in the unstable stage.

3. The ratchet evaluation method according to claim 2, characterized in that: In step S1, an elastic-plastic finite element analysis is performed on the kth cyclic load, the equivalent ratchet strain is calculated using the peak-to-valley average value, and a curve of the change of the equivalent ratchet strain increment with the number of cycles is plotted. The initial n0 cycles are an unstable stage, and the equivalent ratchet strain increment decays rapidly. Then the curve enters a stable stage, and the calculation ends at the preset number A of cycles after the stable stage.

4. The ratchet evaluation method according to claim 3, characterized in that: The preset number of times A is 6-10.

Images