Creep-fatigue life prediction method and system for strain-controlled notched parts based on critical distance theory

Abstract

The present invention discloses a method and system for predicting creep-fatigue life of strain-controlled notched parts based on critical distance theory. The method comprises the following steps: S1. Assigning a random fatigue life N of a sample to be predicted. f1 , based on the random fatigue life N f1 Solve the predicted critical distance value l0′; S2. Perform finite element analysis on the predicted sample based on the critical distance value l0′ to obtain the corresponding predicted equivalent plastic strain #imgabs0# S3. Solve the fatigue predicted life N based on the predicted equivalent plastic strain #imgabs1# f2 ; S4. When N f1 =N f2 When the fatigue prediction life N is output f2 , otherwise the fatigue prediction life N f2 Assign to N f1 And repeat S1-S3 until the assigned random fatigue life value is equal to the calculated fatigue prediction life value. The present invention combines the test data under strain control, finite element simulation and critical distance theory, which is simple and intuitive, and the steps are concise; the present invention can be used to predict the fatigue life of notched parts under strain control.

Description

Technical Field

[0001] The invention belongs to the technical field of fatigue life reliability analysis of hydrogenation reactors, and in particular relates to a creep-fatigue life prediction method and system for strain-controlled notched parts based on critical distance theory. Background Art

[0002] Hydrogenation reactors are common equipment in the chemical industry. Since hydrogenation reactors work under high temperature and high pressure conditions for a long time, creep damage is inevitable. In addition, hydrogenation reactors will be subjected to long-term alternating loads in production engineering. The causes include pressure fluctuations and start-up and shutdown fluctuations, which lead to fatigue damage to the reactor. In addition to single damage, there is also a kind of interactive damage during the operation of hydrogenation reactors. In addition, in the manufacturing process of hydrogenation reactors, in order to meet process requirements, it is necessary to open pipes or holes, etc., which will cause discontinuity in structural dimensions. The uneven distribution of stress and strain caused by this will also threaten the safe operation of hydrogenation reactors and greatly affect the efficiency of chemical production. Therefore, studying the low-cycle fatigue life prediction method of hydrogenation reactors with notches under high temperature environments has important research value and engineering significance for ensuring the high-quality and healthy development of my country's petrochemical industry.

[0003] In order to consider the influence of stress state in high stress area near notch on fatigue damage, critical distance theory came into being. However, most of the existing critical distance theories are based on low cycle fatigue tests under stress control, and most of them only study low cycle fatigue tests. Few scholars have studied the effect of critical distance theory on low cycle fatigue and creep-fatigue life prediction under strain control. In addition, since traditional methods require a large amount of experimental data when predicting life, which increases the cost of money and time, there is an urgent need for a notch life prediction model based on critical distance theory under strain control. Summary of the invention

[0004] The present invention aims to solve the deficiencies of the prior art and provides the following solutions:

[0005] The creep-fatigue life prediction method of strain-controlled notched parts based on critical distance theory includes the following steps:

[0006] S1. Assign a random fatigue life N to the sample to be predicted f1 , based on the random fatigue life N f1 Solve the predicted critical distance value l0';

[0007] S2. Based on the critical distance value l0', a finite element analysis is performed on the sample to be predicted to obtain the corresponding predicted equivalent plastic strain

[0008] S3. Predict the equivalent plastic strain according to the Solve the fatigue prediction life N f2 ;

[0009] S4. When N f1 =N f2 When the fatigue prediction life N is output f2 , otherwise the fatigue prediction life N f2 Assign to N f1 And repeat the steps S1 to S3 until the assigned random fatigue life value is equal to the calculated fatigue predicted life value.

[0010] Preferably, the method for solving the predicted critical distance value l0' comprises:

[0011] l0=A×N f B

[0012] Among them, l0 is the critical distance value, A and B are material constants, N f is fatigue life; let N f =N f1 , solve the formula to obtain the predicted critical distance value l0'.

[0013] Preferably, the finite element analysis method comprises:

[0014] Establish a two-dimensional axisymmetric simulation model of the notched part;

[0015] Assigning material properties to the two-dimensional axisymmetric simulation model, meshing the two-dimensional axisymmetric simulation model, and setting load and boundary constraint conditions;

[0016] Perform static analysis on the two-dimensional symmetrical simulation model, take the path from the maximum value point of the notch root to the center of the notch as the fastest attenuation path, find the node on the path where the plastic strain is equal to the plastic strain of the smooth part, then the plastic strain at the node is the equivalent plastic strain required for the critical distance value l0'

[0017] Preferably, solve the fatigue prediction life N f2 The methods include:

[0018]

[0019] ε A =σ A / E

[0020] Among them, N A and k are material constants, σ A is the fatigue limit, E is the elastic modulus, is the equivalent plastic strain; Solve the formula to obtain the fatigue prediction life Nf2 .

[0021] The present invention also provides a creep-fatigue life prediction system for strain-controlled notched parts based on the critical distance theory, wherein the prediction system applies any of the above-mentioned prediction methods and comprises: a critical distance calculation module, a finite element analysis module, a predicted life calculation module and a result judgment unit;

[0022] The critical distance calculation module is used to assign a random fatigue life N to the sample to be predicted. f1 , based on the random fatigue life N f1 Solve the predicted critical distance value l0';

[0023] The finite element analysis module performs finite element analysis on the sample to be predicted based on the critical distance value l0' to obtain the corresponding predicted equivalent plastic strain

[0024] The predicted life calculation module is based on the predicted equivalent plastic strain Solve the fatigue prediction life N f2 ;

[0025] The result judgment unit is used to judge the calculated fatigue prediction life value. f1 =N f2 When the fatigue prediction life N is output f2 , otherwise the fatigue prediction life N f2 Assign to N f1 The fatigue predicted life value is recalculated until the assigned random fatigue life value is equal to the calculated fatigue predicted life value.

[0026] Preferably, the workflow of the critical distance calculation module includes:

[0027] l0=A×N f B

[0028] Among them, l0 is the critical distance value, A and B are material constants, N f is fatigue life; let N f =N f x, solve the formula to obtain the predicted critical distance value l0'.

[0029] Preferably, the finite element analysis module comprises: a model building unit, a condition setting unit and an analysis unit;

[0030] The model building unit is used to establish a two-dimensional axisymmetric simulation model of the notched part;

[0031] The condition setting unit is used to assign material properties to the two-dimensional axisymmetric simulation model, perform meshing on the two-dimensional axisymmetric simulation model, and set load and boundary constraint conditions;

[0032] The analysis unit is used to perform static analysis on the two-dimensional symmetrical simulation model, taking the path from the maximum value point of the notch root to the center of the notch as the fastest attenuation path, and finding the node on the path where the plastic strain is equal to the plastic strain of the smooth part. The plastic strain at the node is the equivalent plastic strain required for the critical distance value l0'.

[0033] Preferably, the workflow of the predicted life calculation module includes:

[0034]

[0035] ε A =σ A / E

[0036] Among them, N A and k are material constants, σ A is the fatigue limit, E is the elastic modulus, is the equivalent plastic strain; Solve the formula to obtain the fatigue prediction life N f2 .

[0037] Compared with the prior art, the present invention has the following beneficial effects:

[0038] (1) Under strain control, the critical distance corresponding to notched parts with different notch forms has a different functional relationship with fatigue life. Therefore, different types of notched parts need to obtain corresponding functional relationships based on the steps of the present invention, or add additional parameter stress concentration factors. In addition, the present invention also extends the critical distance method to the creep-fatigue test life prediction part. Based on the test data of smooth parts under strain control, the present invention fits the ε under strain control smooth parts. p -N f Curve, and combined with finite element simulation to obtain the critical distance l0 and equivalent plastic strain and fatigue life N f The function expression of can be used to predict the creep-fatigue life of notched parts.

[0039] (2) The present invention is suitable for fitting smooth parts ε p -N f The curve and finite element simulation do not require a large amount of test data, which can save a lot of time and capital costs. On the other hand, the steps of the invention are concise, the various functional relationships are simple, it is convenient and fast in actual engineering applications, and has certain engineering application value.

[0040] (3) The present invention combines test data, finite element simulation and critical distance theory under strain control, which is simple and intuitive with concise steps. The critical distance theory life prediction model is extended from low-cycle fatigue to creep-fatigue. The present invention can be used to predict the fatigue life of notched parts under strain control, providing a technical basis for the influence of stress concentration caused by the structural characteristics of the hydrogenation reactor on the fatigue life of the structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] In order to more clearly illustrate the technical solution of the present invention, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0042] Figure 1 The smooth member ε of the embodiment of the present invention p -N f Schematic diagram of the curve;

[0043] Figure 2 Schematic diagram of a two-dimensional axisymmetric simulation model of a notch component according to an embodiment of the present invention;

[0044] Figure 3 The critical distance l0 and fatigue life N in the embodiment of the present invention are f Function relationship diagram of ;

[0045] Figure 4 is the equivalent plastic strain of the embodiment of the present invention and fatigue life N f Function diagram of ;

[0046] Figure 5 A schematic diagram of a method flow chart of an embodiment of the present invention;

[0047] Figure 6 Schematic diagram of prediction results according to an embodiment of the present invention. DETAILED DESCRIPTION

[0048] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0049] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0050] Embodiment 1

[0051] In this embodiment, the formula to be used must first be derived through experiments.

[0052] The plastic strain ε of the smooth part is plotted using the low-cycle fatigue and creep-fatigue test data of the smooth part. p and fatigue life N f ε p -N f Curves, such as Figure 1 As shown:

[0053] ε p ×N f β =C

[0054] Among them, ε p is the plastic strain, N f is the fatigue life, β and C are material constants. According to the notched part test fatigue life N f , from the smooth part ε p -N f The corresponding plastic strain is obtained from the curve

[0055] The equivalent plastic strain on the notch root cross section is obtained by finite element analysis of notched parts under strain-controlled low-cycle fatigue and creep-fatigue loads. distribution; specifically, a two-dimensional axisymmetric simulation model of the notch is established, and the notch is analyzed using finite element software, including a pre-processing stage and a post-processing stage; the pre-processing stage includes: assigning material properties to the two-dimensional axisymmetric simulation model, meshing the two-dimensional axisymmetric simulation model, and setting load and boundary constraints; the post-processing stage includes performing static analysis and checking the analysis results, such as Figure 2 shown.

[0056] Take the path from the maximum value point of the notch root to the center of the notch as the fastest attenuation path, and find the path where the plastic strain is equal to the plastic strain of the smooth part. node, the plastic strain at the node is the equivalent plastic strain required by the critical distance model The distance from this node to the maximum value point of the notch root is the fatigue life N f corresponds to half of the critical distance.

[0057] According to the above steps, several groups of notched parts are selected again to test fatigue life N f , combined with the critical distance theoretical point method, the critical distance value l0 and equivalent plastic strain corresponding to different experimental fatigue lives are obtained respectively.

[0058] Fitting the critical distance value l0 and the fatigue life Nf , the expression of the first nonlinear function is obtained as:

[0059] l0=A×N f B

[0060] Among them, l0 is the critical distance value, A and B are material constants, N f is the fatigue life; then, through the test data of several groups of notched parts, the specific values ​​of A and B are fitted according to the above formula, as shown in Figure 3 shown.

[0061] Fitting the equivalent plastic strain With the fatigue life N f , the expression of the second nonlinear function is obtained as:

[0062]

[0063] ε A =σ A / E

[0064] Among them, N A and k are material constants, σ A is the fatigue limit, E is the elastic modulus, is the equivalent plastic strain; then, through the test data of several groups of notched parts, N is obtained by fitting according to the above formula A and k and specific values, such as Figure 4 shown.

[0065] After deriving the expression, such as Figure 5 As shown, the creep-fatigue life prediction method of strain-controlled notched parts based on critical distance theory includes the following steps:

[0066] S1. Assign a random fatigue life N to the sample to be predicted f1 , based on the random fatigue life N f1 Solve to predict the critical distance value l0'.

[0067] The method for solving the predicted critical distance value l0' includes:

[0068] l0=A×N f B

[0069] Among them, l0 is the critical distance value, A and B are material constants, N f is fatigue life; let N f =N f1 , solve the formula to get the predicted critical distance value l0'.

[0070] S2. Based on the critical distance value l0', finite element analysis is performed on the predicted sample to obtain the corresponding predicted equivalent plastic strain

[0071] The finite element analysis method includes: establishing a two-dimensional axisymmetric simulation model of the notched part; assigning material properties to the two-dimensional axisymmetric simulation model, meshing the two-dimensional axisymmetric simulation model, and setting load and boundary constraints; performing static analysis on the two-dimensional symmetric simulation model, taking the path from the maximum value point of the notch root to the center of the notched part as the fastest attenuation path, and finding the node on the path where the plastic strain is equal to the plastic strain of the smooth part, then the plastic strain at the node is the equivalent plastic strain required for the critical distance value l0'.

[0072] S3. Based on the predicted equivalent plastic strain Solve the fatigue prediction life N f2 .

[0073] Solve the fatigue prediction life N f2 The methods include:

[0074]

[0075] ε A =σ A / E

[0076] Among them, N A and k are material constants, σ A is the fatigue limit, E is the elastic modulus, is the equivalent plastic strain; Solve the formula to get the fatigue prediction life N f2 .

[0077] S4. When N f1 =N f2 When the fatigue prediction life N is output f2 , otherwise the fatigue prediction life N f2 Assign to N f1 Repeat S1-S3 until the assigned random fatigue life value is equal to the calculated fatigue predicted life value. The prediction result is as follows: Figure 6 shown.

[0078] Embodiment 2

[0079] In this embodiment, the creep-fatigue life prediction system of strain-controlled notched parts based on critical distance theory includes: a critical distance calculation module, a finite element analysis module, a predicted life calculation module and a result judgment unit.

[0080] The critical distance calculation module is used to assign the random fatigue life N to the sample to be predicted. f1, based on the random fatigue life N f1 Solve to predict the critical distance value l0'.

[0081] The workflow of the critical distance calculation module includes:

[0082] l0=A×N f B

[0083] Among them, l0 is the critical distance value, A and B are material constants, N f is fatigue life; let N f =N f1 , solve the formula to get the predicted critical distance value l0'.

[0084] The finite element analysis module performs finite element analysis on the predicted sample based on the critical distance value l0' to obtain the corresponding predicted equivalent plastic strain

[0085] The finite element analysis module includes: model building unit, condition setting unit and analysis unit. The model building unit is used to establish a two-dimensional axisymmetric simulation model of the notched part; the condition setting unit is used to assign material properties to the two-dimensional axisymmetric simulation model, mesh the two-dimensional axisymmetric simulation model, and set load and boundary constraints; the analysis unit is used to perform static analysis on the two-dimensional symmetric simulation model, taking the path from the maximum value point of the notch root to the center of the notched part as the fastest attenuation path, and finding the node on the path where the plastic strain is equal to the plastic strain of the smooth part, then the plastic strain at the node is the equivalent plastic strain required for the critical distance value l0'.

[0086] The life prediction module is based on the predicted equivalent plastic strain. Solve the fatigue prediction life N f2 .

[0087] The workflow of the predicted life calculation module includes:

[0088]

[0089] ε A =σ A / E

[0090] Among them, N A and k are material constants, σ A is the fatigue limit, E is the elastic modulus, is the equivalent plastic strain; Solve the formula to get the fatigue prediction life N f2 .

[0091] The result judgment unit is used to judge the calculated fatigue prediction life value. f1=N f2 When the fatigue prediction life N is output f2 , otherwise the fatigue prediction life N f2 Assign to N f1 The fatigue predicted life value is recalculated until the assigned random fatigue life value is equal to the calculated fatigue predicted life value.

[0092] The embodiments described above are only descriptions of the preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A creep-fatigue life prediction method for strain-controlled notched parts based on critical distance theory, characterized in that: The following steps are involved: S1. Assign a random fatigue life N to the sample to be predicted f1 , based on the random fatigue life N f1 Solve the predicted critical distance value l0′; S2. Based on the critical distance value l'0, a finite element analysis is performed on the sample to be predicted to obtain the corresponding predicted equivalent plastic strain. S3. Predict the equivalent plastic strain according to the Solve the fatigue prediction life N f2 ; S4. When N f1 =N f2 When the fatigue prediction life N is output f2 , otherwise the fatigue prediction life N f2 Assign to N f1 and repeating the steps S1 to S3 until the assigned random fatigue life value is equal to the calculated fatigue predicted life value; Method for solving the predicted critical distance value l'0 include: l0=A×N f B Among them, l0 is the critical distance value, A and B are material constants, N f is fatigue life; let N f =N f1 , solve the formula to obtain the predicted critical distance value l0'; The finite element analysis method comprises: Establish a two-dimensional axisymmetric simulation model of the notched part; Assigning material properties to the two-dimensional axisymmetric simulation model, meshing the two-dimensional axisymmetric simulation model, and setting load and boundary constraint conditions; The two-dimensional axisymmetric simulation model is subjected to static analysis, and the path from the maximum value point of the notch root to the center of the notch is taken as the fastest attenuation path. The node on the path where the plastic strain is equal to the plastic strain of the smooth part is found. The plastic strain at the node is the equivalent plastic strain required for the critical distance value l0'. Solve the fatigue prediction life N f2 The methods include: e A =s A / E Among them, N A and k are material constants, σ A is the fatigue limit, E is the elastic modulus, is the equivalent plastic strain; Solve the formula to obtain the fatigue prediction life N f2 .

2. A creep-fatigue life prediction system for strain-controlled notched parts based on critical distance theory, wherein the prediction system applies the prediction method described in claim 1, characterized in that: include: Critical distance calculation module, finite element analysis module, predicted life calculation module and result judgment unit; The critical distance calculation module is used to assign a random fatigue life N to the sample to be predicted. f1 , based on the random fatigue life N f1 Solve the predicted critical distance value l0'; The finite element analysis module performs finite element analysis on the sample to be predicted based on the critical distance value l0' to obtain the corresponding predicted equivalent plastic strain The predicted life calculation module is based on the predicted equivalent plastic strain Solve the fatigue prediction life N f2 ; The result judgment unit is used to judge the calculated fatigue prediction life value. f1 =N f2 When the fatigue prediction life N is output f2 , otherwise the fatigue prediction life N f2 Assign to N f1 The fatigue predicted life value is recalculated until the assigned random fatigue life value is equal to the calculated fatigue predicted life value; The workflow of the critical distance calculation module includes: l0=A×N f B Among them, l0 is the critical distance value, A and B are material constants, N f is fatigue life; let N f =N f1 , solve the formula to obtain the predicted critical distance value l0'; The finite element analysis module includes: a model building unit, a condition setting unit and an analysis unit; The model building unit is used to establish a two-dimensional axisymmetric simulation model of the notched part; The condition setting unit is used to assign material properties to the two-dimensional axisymmetric simulation model, perform meshing on the two-dimensional axisymmetric simulation model, and set load and boundary constraint conditions; The analysis unit is used to perform static analysis on the two-dimensional axisymmetric simulation model, taking the path from the maximum value point of the notch root to the center of the notch as the fastest attenuation path, and finding the node on the path where the plastic strain is equal to the plastic strain of the smooth part. The plastic strain at the node is the equivalent plastic strain required for the critical distance value l0'. The workflow of the predicted life calculation module includes: e A =s A / E Among them, N A and k are material constants, σ A is the fatigue limit, E is the elastic modulus, is the equivalent plastic strain; make Solve the formula to obtain the fatigue prediction life N f2 .

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