A method for predicting the life of crack defects in pressure vessels based on the safety attenuation rate

Abstract

The present invention belongs to the field of safety assessment of pressure vessels, and particularly relates to a method for predicting the remaining life of crack defects in pressure vessels based on the safety attenuation rate. It mainly aims at the characterization of the safety attenuation process of crack defects in pressure vessels and the prediction of the remaining life. Based on the safety attenuation time-varying curve, the present invention constructs a mathematical model of the safety attenuation rate of crack defects. Through the transient mathematical relationship between the safety attenuation rate and the safety attenuation path, a prediction model for the remaining life of crack defects in pressure vessels based on the safety attenuation rate and a safety margin characterization model are obtained. The beneficial effects of the present invention are that the safety attenuation rate model can more intuitively reflect the safety attenuation process of crack defects in pressure vessels; the remaining life prediction model can effectively predict the remaining life of crack defects; and the attenuation characteristic of the safety margin characterization model, which is fast at the front and slow at the back, is more conducive to the identification and detection of crack defects in pressure vessels in the later stage.

Description

Technical Field

[0001] The present invention belongs to the technical field of safety assessment of pressure vessels, and particularly relates to a method for predicting the life of crack defects in pressure vessels based on the safety attenuation rate. Background Art

[0002] As a basic equipment in industrial production, metal pressure vessels are widely used in various fields such as chemical industry, aerospace, and nuclear power. Since they often need to store dangerous media such as flammable, explosive, highly toxic, and corrosive substances inside, once a metal pressure vessel explodes or leaks, serious safety accidents will occur. Therefore, the prediction of the service life of pressure vessels with crack defects has always been one of the key issues concerned in this field.

[0003] Currently, the main methods for safety assessment of pressure vessels are: ray method, parallel line method, etc. The ray method determines the safety degree of the current crack defect by the distance between the assessment point and the threshold curve and the distance between the threshold curve and the origin in the general assessment diagram. However, this method fails to consider the trajectory change of the safety attenuation path of the crack defect, and only uses a straight-line trajectory to represent the safety attenuation process of the crack defect, resulting in a large error in its calculation results. The parallel line method divides the safety level with parallel curves in the safety area of the general assessment diagram, and determines the safety degree of the defect by observing the safety level area where the assessment point falls. Although this method can determine the safety degree of the current defect of the pressure vessel, due to the lack of a numerical quantification index for the safety margin of the crack defect, the gradient parallel line method cannot predict the remaining life of the crack defect. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to propose a method for predicting the remaining life of a pressure vessel considering the time-varying nature of the safety attenuation rate of crack defects.

[0005] The technical solution adopted by the present invention to solve its technical problems is: obtaining the time-varying curve of the safety attenuation of the crack defect according to the expansion data of the crack defect along the depth dimension direction and combining with the safety assessment standard of the in-service pressure vessel with defects. The moving rate of the safety assessment point of the crack defect along the time-varying curve of the safety attenuation during the failure process is defined as the safety attenuation rate of the crack defect and is solved by the change curve of the crack defect along the depth dimension direction. According to the transient mathematical relationship between the safety attenuation rate and the time-varying curve of the safety attenuation, a prediction model for the remaining life of the crack defect in the pressure vessel in the form of a line integral is obtained. Brief Description of the Drawings

[0006] Figure 1 It is a comparison diagram of the iterative dimension data of the Paris formula and the actual crack data.

[0007] Figure 2 It is a schematic diagram of the time-varying curve of the safety attenuation of the crack defect.

[0008] Figure 3 It is a schematic diagram for calculating the safe attenuation rate P(n) of a crack defect at any point n at any time.

[0009] Figure 4 It is the curve form of P(n) of the safe attenuation rate of the crack defect.

[0010] Figure 5 It is for the safe attenuation rate P(L r , K r ) form of the schematic diagram.

[0011] Figure 6 It is a schematic diagram for the derivation of the remaining life calculation formula.

[0012] Figure 7 It is a schematic diagram of the remaining life prediction curve of the crack defect.

[0013] Figure 8 It is a schematic diagram of the safety margin calculation curve at any point before the failure of the crack defect.

[0014] Figure 9 It is a curve graph of the relationship between the safety margin and the remaining life of the crack defect. Specific implementation manner

[0015] The safety attenuation rate and the remaining life calculation method of this article are described in detail with reference to the accompanying drawings.

[0016] 1) Through Abaqus crack fatigue simulation, Paris fatigue propagation formula iteration, or taking metal specimens from the pressure vessel to be tested for fatigue crack tensile experiments, the depth and length dimension data of the crack defect to be detected after each load application during the safe failure process are obtained. The Paris formula iteration calculation form of the crack depth and length dimensions is as follows:

[0017]

[0018] In the formula, and are respectively the stress intensity factor amplitude at the depth end point and the length end point of the crack defect during the i-th fatigue load application; a0 and c0 are respectively the initial depth dimension and the initial length dimension of the crack defect; a n and c n are the depth and length dimensions of the crack after n fatigue load applications; C and m are Paris formula constants, which are only related to the material type and stress environment of the crack defect. Figure 1 It is a comparison diagram of the crack depth and length dimension data generated by Paris formula iteration and the actual crack data.

[0019] 2) According to Figure 1The depth and length dimension data of the crack defect and the mechanical environment parameters are used to calculate the fracture ratio K n , c n ) and the load ratio L r (a n , c n ) for each size sample point (a r (a n , c n ) according to the standard evaluation process of the failure assessment diagram method, and the safety attenuation time-varying curve G(L r , K r ) of the crack is plotted on the failure assessment diagram. Figure 2 It is a schematic diagram of the safety attenuation time-varying curve of the crack defect.

[0020] 3) Calculate the safety attenuation rate of the current crack defect according to the moving distance of the crack defect along the safety attenuation time-varying curve under the action of a single fatigue load. Thus, the calculation formula for the safety attenuation rate P(n) at any size sample point n of the pressure vessel crack defect is:

[0021]

[0022] In the formula, K r (a n , c n ) and L r (a n , c n ) represent the fracture ratio and the load ratio of the crack defect when it is subjected to the nth fatigue load, respectively. Figure 3 It is a schematic diagram for calculating the safety attenuation rate at any point n of the crack defect, Figure 4 It is the P(n) curve form of the safety attenuation rate.

[0023] 4) Obtain the form of the safety attenuation rate P(L r (a n , c n ) and K r (a n , c n ) of the crack defect through the numerical correspondence between the safety attenuation rate P(n) in Equation 2 and its parameters L r , K r ). Figure 5 It is a schematic diagram of the form of the safety attenuation rate P(L r , K r ) of the crack defect.

[0024] 5) Through the transient mathematical relationship between the safety attenuation time-varying curve and the safety attenuation rate, the calculation formula for the remaining life at any point on the failure assessment diagram of the crack defect can be obtained as:

[0025]

[0026] Wherein, R(L r ,K r ) is the remaining number of cycles that the crack defect can withstand the current fatigue load before failure when it is at the position of (L r ,K r ), that is, the remaining life of the crack defect; G t～A (L r ,K r ) is the remaining safety attenuation path from the current safety assessment point t of the crack defect to the critical failure point A, Figure 6 is the schematic diagram of the mathematical derivation of Equation 3, Figure 7 is the schematic diagram of the remaining life curve of the crack defect.

[0027] 6) According to the curve characteristics of the safety attenuation rate P(L r ,K r ) and the safety attenuation time-varying curve G(L r ,K r ), the safety margin of the crack defect is defined as:

[0028]

[0029] Wherein, M(L r ,K r ) represents the safety margin of the crack defect at any point before failure. Figure 8 is an example diagram of the safety margin calculation curve of the crack defect at any point before failure, Figure 9 is an example diagram of the relationship curve between the safety margin and the remaining life.

Claims

1. A method for predicting the life of crack defects in a pressure vessel based on the safe decay rate, The features include the following steps: 1) Obtain the depth dimension and length dimension data of the crack defect to be detected after each load application during the safe failure process by means of Abaqus crack fatigue simulation, fatigue propagation formula iteration, or taking metal specimens from the pressure vessel to be tested for fatigue crack tensile experiments; 2) According to the depth and length dimension data of the crack defect and the mechanical environment parameters, and in accordance with the standard assessment process of the failure assessment diagram method, calculate the fracture ratio K n , c n ) and the load ratio L r (a n , c n ) for each size sample point (a r , c n , c n ), and plot them on the failure assessment diagram to obtain the safety attenuation time-varying curve G(L r , K r ) of the crack; 3) Calculate the safe attenuation rate of the current crack defect according to the moving distance of the crack defect along the safe attenuation time-varying curve under a single fatigue load. Thus, the calculation formula for the safe attenuation rate P(n) at any size sample point n of the pressure vessel crack defect is: where K r (a n , c n ) and L r (a n , c n ) represent the fracture ratio and the load ratio of the crack defect under the action of the nth fatigue load, respectively; 4) Through the safety attenuation rate P(n) in formula (1) and its parameters L r (a n ,c n ) and K r (a n ,c n ), the P(L r ,K r ) form of the safety attenuation rate of the crack defect is obtained; 5) Through the transient mathematical relationship between the safe attenuation time-varying curve and the safe attenuation rate, the calculation formula for the remaining life at any point on the failure assessment diagram of the crack defect can be obtained as: Wherein, R(L r ,K r ) is the remaining number of cycles that the crack defect can withstand the current fatigue load before failure when it is at the position of (L r ,K r ), that is, the remaining life of the crack defect; G t～A (L r ,K r ) is the remaining safety attenuation path from the current safety assessment point position t of the crack defect to the critical failure point A; 6) According to the safety attenuation rate P(L r , K r ) and the curve characteristics of the safety attenuation time-varying curve G(L r , K r ), the safety margin of the crack defect is defined as: where M(L r , K r ) represents the safety margin of the crack defect at any point before failure.

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