A method for evaluating ultra-low cycle fatigue life

Abstract

The application relates to a kind of super low cycle fatigue life evaluation methods, which belongs to the technical field of steel structure fatigue prediction. Super low cycle fatigue is large-scale reciprocating plastic deformation of material, and the cycle frequency is not a control factor. The super low cycle fatigue fracture of steel is a failure form between low cycle fatigue fracture and ductile fracture, with high ductile deformation component before breaking, and the critical failure mechanical parameter is related to stress state. The method considers the influence of stress state on super low cycle fatigue life, the model is widely applicable to fatigue load spectrum, and can reflect the influence of stress triaxiality and Lode angle parameter on super low cycle fatigue. Existing experimental data show that the average error of the new super low cycle fatigue model is 25%.

Description

Technical Field

[0001] This invention relates to a method for assessing ultra-low cycle fatigue life, which belongs to the technical field of fatigue prediction for steel structures. Background Technology

[0002] Steel structures undergo fatigue failure under seismic loads, with fatigue lives typically around twenty cycles, a condition known as ultra-low cycle fatigue. Ultra-low cycle fatigue is characterized by large-scale cyclic plastic deformation of the material, while the cycle frequency is not the controlling factor. To investigate the ultra-low cycle fatigue fracture behavior and mechanism of steel under cyclic high-strain loads, many scholars have conducted related research. Studies have shown that ultra-low cycle fatigue fracture of steel is a failure mode intermediate between low-cycle fatigue fracture and ductile fracture, characterized by a high ductile deformation component before fracture, and the critical failure mechanical parameters are related to the stress state.

[0003] The Manson-Coffin (CM) model is widely used for low-cycle and ultra-low-cycle fatigue life assessment. However, for alloy steel materials, the CM model overestimates their ultra-low-cycle fatigue life. More critically, the CM model cannot consider the influence of stress state on ultra-low-cycle fatigue life. Summary of the Invention

[0004] To address the problems existing in the prior art, this invention provides a novel prediction method for ultra-low cycle fatigue model. This model is not only widely applicable to fatigue load spectra, but also reflects the influence of stress triaxiality and Rhodes angle parameters on ultra-low cycle fatigue.

[0005] The technical solution adopted in this invention is: a method for evaluating ultra-low cycle fatigue life, characterized in that the method can reflect the influence of stress triaxiality and Rhodes angle parameters on ultra-low cycle fatigue, and the specific method is as follows:

[0006]

[0007] In the formula, N f For fatigue cycles, Δε p The strain range is 2 times the strain amplitude, and A, B, and k are material parameters;

[0008] In the formula ε f The fracture strain is defined as the equivalent plastic deformation at the fracture moment, and has the following form:

[0009]

[0010]

[0011] In the formula, n and H are the strengthening exponents in the power hardening function, H is the strengthening coefficient; c is the intensity coefficient; m1 and m2 are the positive and negative hydrostatic pressure participation coefficients, respectively.

[0012] The Rhodes angle function is defined as follows:

[0013]

[0014]

[0015]

[0016] In the formula, c t c is the generalized tensile coefficient. c c is the generalized compression factor. s γ is the generalized shear coefficient; γ is an intermediate variable. Calibration parameter c t c c c s When the stress state to which the three parameters belong is defined, it needs to be defined as the reference stress state, and its corresponding parameter is also set to a unit quantity. Generally, generalized tension is defined as the reference stress state, and parameter c... t It is specified as a unit quantity, and the other two parameters are in c. t The calibration is based on the data to reflect the relativity of material ductility under three stress states.

[0017]

[0018] In the formula, Δp ii and Δp ij The difference between the plastic strain tensor components at adjacent load inflection points is given by i and j, where i and j take values ​​of 1, 2, or 3, respectively.

[0019] For a given alloy material, parameters A, B, k, n, H, c, and c are calibrated experimentally. t c c c s m1 and m2, and then calculate the pipeline ultra-low cycle fatigue cycle N using formula (1). f .

[0020] Beneficial effects: This model not only inherits the advantage of the CM model in its wide applicability to fatigue load spectrum, but also reflects the influence of stress triaxiality and Rhodes angle parameters on ultra-low cycle fatigue. Existing experimental data show that the average prediction error of the new ultra-low cycle fatigue model is 25%. Attached Figure Description

[0021] Figure 1 It is an API 5L X65 steel tensile specimen.

[0022] Figure 2 It is a square cross-section specimen (OH) with an elliptical hole in the center.

[0023] Figure 3 This is a graph showing the experimental data of ultra-low cycle fatigue of API 5L X65 steel.

[0024] Figure 4 This is a comparison chart of ultra-low cycle fatigue experiments and theoretical predictions from the new model for API 5L X65 steel. Detailed Implementation

[0025] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0026] This study obtained ultra-low cycle fatigue experimental data for API 5L X65 steel from the literature. The ultra-low cycle fatigue experimental data came from three types of test specimens: SP, OH, and CH. SP specimens were smooth square cross-section specimens, with dimensions shown in [reference needed]. Figure 1 And Table 1; OH is a square cross-section specimen with an elliptical hole at its center, and the geometric dimensions of the specimen are shown in Table 1. Figure 2 And Table 1; CH is a square cross-section specimen with a circular hole in the center, and the geometric dimensions of the specimen are shown in Table 1. Figure 1 .

[0027] Table 1 Geometric parameters of API 5L X65 steel tensile specimens

[0028]

[0029] Figure 3 This figure reflects the relationship between ultra-low cycle (low cycle) fatigue life and strain stroke of API 5L X65 steel in a double logarithmic coordinate system. It shows that fatigue life decreases with increasing strain stroke. More importantly, the correlation between fatigue life and strain stroke is difficult to express directly using the traditional Mansion-Coffin formula, depending on the specimen type.

[0030] An ultra-low cycle fatigue life assessment method was adopted, which can reflect the influence of stress triaxiality and Rhodes angle parameters on ultra-low cycle fatigue. The specific method is as follows:

[0031]

[0032] In the formula, N f For fatigue cycles, Δε p The strain range is 2 times the strain amplitude, and A, B, and k are material parameters;

[0033] In the formula ε fThe fracture strain is defined as the equivalent plastic deformation at the fracture moment, and has the following form:

[0034]

[0035]

[0036] In the formula, n and H are the strengthening exponents in the power hardening function, H is the strengthening coefficient; c is the intensity coefficient; m1 and m2 are the positive and negative hydrostatic pressure participation coefficients, respectively.

[0037] The Rhodes angle function is defined as follows:

[0038]

[0039]

[0040]

[0041] In the formula, c t c is the generalized tensile coefficient. c c is the generalized compression factor. s γ is the generalized shear coefficient; γ is an intermediate variable. Calibration parameter c t c c c s When the stress state to which the three parameters belong is defined, it needs to be defined as the reference stress state, and its corresponding parameter is also set to a unit quantity. Generally, generalized tension is defined as the reference stress state, and parameter c... t It is specified as a unit quantity, and the other two parameters are in c. t The calibration is based on the data to reflect the relativity of material ductility under three stress states.

[0042]

[0043] In the formula, Δp ii and Δp ij The difference between the plastic strain tensor components at adjacent load inflection points is given by i and j, where i and j take values ​​of 1, 2, or 3, respectively.

[0044] For a given alloy material, parameters A, B, k, n, H, c, and c are calibrated experimentally. t c c c s m1 and m2, and then the ultra-low cycle fatigue life of the pipeline is calculated using formula (1).

[0045] Using equation (1), the ultra-low cycle life was calibrated and predicted. The ultra-low cycle fatigue model parameters of API 5L X65 steel are shown in Table 2, and the theoretical predicted life is listed in Table 3.

[0046] Table 2 Parameters of the Ultra-Low Cycle Fatigue Model

[0047]

[0048] Table 3. Predictions of different specimens in API 5L X65 steel under the new ultra-low cycle fatigue model.

[0049]

[0050] Figure 4 The comparison between the experimental data of ultra-low cycle fatigue of API 5L X65 steel and the theoretical prediction of the new model shows that the average prediction error of the present invention is 25%, which has good prediction accuracy.

[0051] The above description is merely a preferred embodiment of the present invention; however, the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and its improved concepts, should be covered within the scope of protection of the present invention.

Claims

1. A method for assessing ultra-low cycle fatigue life, characterized in that, This method can reflect the influence of stress triaxiality and Rhodes angle parameters on ultra-low cycle fatigue, with fatigue cycles N. f With strain range Δε p The relationship is as follows: In the formula, N f For fatigue cycles, Δε p Where A, B, and k represent the strain range, and A, B, and k are material parameters. In equation (1), ε f The fracture strain is defined as the equivalent plastic deformation at the fracture moment, and has the following form: In the formula, n and H are the strengthening exponents in the power hardening function, H is the strengthening coefficient; c is the intensity coefficient; m1 and m2 are the positive and negative hydrostatic pressure participation coefficients, respectively. The Rhodes angle function is defined as follows: In the formula, c t c is the generalized tensile coefficient. c c is the generalized compression factor. s γ is the generalized shear coefficient; γ is an intermediate variable. For Rhodes angle parameters; Strain range Δε p Defined as: In the formula, Δp ii and Δp ij The difference between the plastic strain tensor components at adjacent load inflection points is given by i and j, where i and j take values ​​of 1, 2, or 3, respectively. For a given alloy material, parameters A, B, k, n, H, c, and c are calibrated experimentally. t c c c s m1 and m2, and then the ultra-low cycle fatigue cycle N of the pipeline is calculated using formula (1). f .

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