Limit load evaluation method and limit load evaluation device

Abstract

To provide a limit load evaluation method capable of swiftly providing a maintenance plan by quickly performing a soundness assessment on cracks detected during inspection.SOLUTION: The limit load evaluation method is a limit load evaluation method of the ductile failure limit in a structure with n adjacent multiple cracks. The strain distribution of a structure with n adjacent multiple cracks is predicted by correcting the distortion εpre at any position based on an equation (1) that is expressed as the product of the strain distribution εi resulting from elastic-plastic analysis for a single crack condition and the ratio εi / ε0 of the strain ε0 in a sound state. The critical load (plastic collapse load) is estimated and evaluated from the obtained strain distribution. Here, 2≤n.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a limit load evaluation method and apparatus for evaluating the soundness of parts of piping used in, for example, a power plant, particularly parts where aging deterioration or fatigue damage is expected. [Background technology]

[0002] In power plants, for example, where aging is progressing, improving the accuracy of health assessment techniques for aging equipment is important for improving safety and extending the plant life.

[0003] With conventional integrity assessment technology, if a crack is detected by non-destructive testing during a regular inspection, a crack progression assessment and a critical crack size assessment are carried out, taking into account the expected aging deterioration phenomena based on the operating conditions of the equipment, and it is confirmed that the equipment's integrity can be maintained until the next inspection.

[0004] Furthermore, in general, the derivation of the critical crack size in integrity assessment is performed using a simplified evaluation formula or the finite element method. In integrity assessment using a simplified evaluation formula, as shown in Patent Document 1, small cracks of a predetermined size or less are evaluated using a nonlinear fracture mechanics evaluation method, while large cracks are evaluated using a simplified evaluation formula based on the limit load evaluation method. Therefore, when there are multiple cracks close to each other, the cracks are projected onto the same plane for evaluation, which may result in an overly conservative evaluation.

[0005] As shown in Patent Document 2, limit evaluation using the finite element method requires a process of creating an analytical model that matches the geometric shape of the stress concentration area and the crack shape detected during inspection, and performing calculations based on that model, which can require extremely high calculation costs depending on the conditions. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-14683 [Patent Document 2] Japanese Patent Application Laid-Open No. 2014-149246 Summary of the Invention [Problem to be solved by the invention]

[0007] As a result, limit load evaluations that reflect the crack shapes obtained through non-destructive testing could not be carried out promptly, and the time required to formulate maintenance plans often led to extended shutdown periods for power plants.

[0008] Furthermore, because there is no means to quickly and efficiently incorporate the crack shapes obtained through non-destructive testing into an analysis model and provide crack growth evaluation results in a short period of time, conservative life and remaining life assessments with a large safety factor are being carried out. As a result, the remaining life of power plant equipment is estimated to be short, leading to a decrease in the efficiency of the entire power plant.

[0009] The present invention is an invention for solving the above-mentioned problems, and aims to provide a limit load evaluation method and limit load evaluation device that can quickly perform a soundness evaluation of cracks detected during inspection, thereby providing a rapid maintenance plan. [Means for solving the problem]

[0010] In order to solve the above problems, the limit load evaluation method of the present invention is a limit load evaluation method for the ductile fracture limit of a structure having n adjacent multiple cracks, and is a method for evaluating the strain distribution ε i The ratio ε to the strain in the healthy state ε0 i The strain ε at any position is calculated based on the formula (1) expressed as the product of / ε0. pre The method is characterized by predicting the strain distribution of a structure with n adjacent cracks by correcting the strain distribution, and estimating and evaluating the limit load (plastic collapse load) from the obtained strain distribution.

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[0011] The limit load evaluation device of the present invention is a limit load evaluation device for a structure having a plurality of adjacent cracks, and includes a memory unit that stores elastic-plastic analysis results for a single crack under a plurality of shape conditions, and a data storage unit that stores information acquired by the memory unit and information acquired by an inspection device. Re Based on the crack condition to be evaluated, the strain distribution ε i The ratio ε to the strain in the healthy state ε0 i The strain ε at any position is calculated based on the formula (1) expressed as the product of / ε0. pre The present invention is characterized by having a calculation unit that predicts the strain distribution of a structure having n adjacent cracks by correcting the above, and estimates and evaluates the fracture limit (plastic collapse load) from the obtained strain distribution. Other aspects of the present invention will be described in the following embodiments. [Effects of the Invention]

[0012] According to the present invention, it is possible to quickly perform a limit load assessment for a structure having multiple cracks that are close together and that are detected during an inspection, thereby enabling the provision of a prompt maintenance plan. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 2 is a flowchart showing an evaluation procedure in the limit load evaluation system according to the embodiment. [Figure 2] FIG. 10 is a diagram showing an example of crack size ratio conditions used in analysis for DB generation of strain distribution for a structure having a single through crack. [Figure 3] FIG. 10 is a diagram showing an example of crack size ratio conditions used in analysis for DB analysis of strain distribution for a structure having a single surface semi-elliptical crack. [Figure 4] FIG. 1 shows a schematic diagram of a structure having multiple closely spaced through cracks. [Figure 5] FIG. 1 shows a schematic diagram of a structure with a single through crack. [Figure 6] FIG. 10 is a diagram showing the strain distribution in the results of an elastic-plastic analysis of a flat plate having multiple adjacent through cracks. [Figure 7]FIG. 10 is a diagram showing the results of predicting strain distribution in a flat plate having multiple closely spaced through cracks based on an embodiment. [Figure 8] FIG. 8 is a diagram showing the results of comparing the strain distribution in the horizontal direction from the end of the through-cracks in the flat plates having multiple adjacent through-cracks shown in FIGS. 6 and 7. [Figure 9] 1 is a block diagram showing a configuration of an evaluation device according to an embodiment. [Figure 10] FIG. 1 shows a schematic diagram of a structure with multiple closely spaced surface semi-elliptical cracks. DETAILED DESCRIPTION OF THE INVENTION

[0014] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment for carrying out the present invention will be described in detail with reference to the accompanying drawings. Fig. 1 is a flow diagram showing an evaluation procedure in a limit load evaluation system 1 according to an embodiment. The limit load evaluation system 1 shown in Fig. 1 evaluates, in particular, the limit load (plastic collapse load) of equipment in, for example, a power plant facility. The processing steps in the limit load evaluation system 1 are outlined below: a preparatory step 11 in which elastic-plastic analysis is performed in advance on a single crack model assuming multiple crack shapes and the results of an elastic-plastic analysis of the part to be evaluated in a sound state, and the results are stored in a database; an on-site measurement step 12 in which inspection by nondestructive testing is performed during a periodic inspection to measure the crack shapes of the part to be evaluated; and an evaluation step 13 in which strain distribution is corrected and estimated in accordance with the measured crack shape and arrangement, and the limit load is calculated based on the strain distribution to evaluate the soundness.

[0015] In this embodiment, it is assumed that the advance preparation process 11 and the on-site measurement process 12 are performed mainly manually in advance, and the evaluation process 13 is performed by a computer, etc. Here, the evaluation of a flat plate having multiple closely spaced through cracks will be described.

[0016] <Advance preparation process 11> First, in the preliminary preparation process 11 (first stage), specifically in processing step S10, an elastic-plastic analysis is performed using a single crack model for multiple crack shapes assumed in the evaluation target. Next, in processing step S11, the crack shape, location, and strain distribution data obtained from the elastic-plastic analysis are stored in a database. Here, it is recommended to use the constitutive equations defined by Ramberg-Ossgood and power-law plasticity in equations (2) and (3) for the elastic-plastic analysis.

[0017]

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[0018] The DB to be prepared in advance is the crack size (surface crack width L i , depth a i ) and plate thickness t as a standard.

[0019] Figure 2 shows examples of crack size ratio conditions used in analysis for DB analysis of strain distribution in a structure with a single through crack. Figure 3 shows examples of crack size ratio conditions used in analysis for DB analysis of strain distribution in a structure with a single surface semi-elliptical crack.

[0020] For example, Figure 2 shows the case of evaluating a single through crack, where the ratio L / t of plate thickness t to crack width L is calculated under four conditions. When modeling a surface semi-elliptical crack, the crack depth a must also be considered as a parameter, so as shown in Figure 3, calculations are performed under four conditions for each of the crack width L and depth a (a total of 16 conditions), and the desired strain distribution ε i The number of conditions increases.

[0021] Although an example of DB construction with four conditions is shown here, the strain distribution ε can be calculated more accurately by constructing a DB with more detailed settings. i It goes without saying that it is possible to estimate the strain distribution εi can be approximately calculated by interpolating or extrapolating based on the crack size.

[0022] <On-site measurement process 12> Next, in the on-site measurement process 12 (second stage), in which non-destructive testing is conducted during regular inspections to measure the crack shape of the area to be evaluated, specifically in processing step S12, the crack shape and crack arrangement are measured using UT testing (ultrasonic flaw detection testing), which is generally used to detect cracks.

[0023] <Evaluation step 13> In the evaluation process 13 (third stage), which evaluates the limit load of the structure based on the obtained crack shape and position information, the crack distribution data of the evaluation target evaluated in the inspection of the second stage is used, and the strain distribution is estimated using the elasto-plastic analysis results that were compiled into a database in the first stage, and the limit load is calculated to provide the soundness evaluation results. Specifically, in the processing step S13, the strain distribution ε i The ratio ε to the strain in the healthy state ε0 i The strain ε at any position is calculated based on the formula (1) expressed as the product of / ε0. pre For a structure with multiple adjacent cracks, the strain distribution is finally estimated by multiplying the cyclic strain ratio according to the number of cracks, n.

[0024]

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[0025] The limit load is calculated in process step S14 based on the obtained strain distribution, and the limit load result is provided by comparing it with the load applied in the actual device in process step S15.

[0026] Fig. 4 is a schematic diagram of a structure having multiple closely spaced through cracks. Fig. 5 is a schematic diagram of a structure having a single through crack. Fig. 4 is a schematic diagram showing a situation in which a tensile load or displacement 23 is applied to a flat plate 20 having two through cracks 21 (surface crack width L1) and a through crack 22 (surface crack width L2). In this embodiment, the limit load in a structure having multiple cracks such as that shown in Fig. 4 is estimated using the strain distribution obtained in advance by analyzing the single crack condition of the through crack 28 (surface crack width L3) shown in Fig. 5.

[0027] Here, when estimating the limit load for a flat plate with two through cracks as shown in Figure 4, first, for through crack 21, the coordinates of the strain distribution obtained in an analysis of a single crack with an arbitrary surface crack width L3 are corrected by the surface crack length ratio L3 / L1 and mapped. The strain distribution obtained at this time is designated ε1. Next, for the other through crack 22, the coordinates of the strain distribution obtained in an analysis of a single crack with a surface crack width L3 are similarly corrected by the surface crack length ratio L3 / L2 and mapped. The strain distribution obtained at this time is designated ε2.

[0028] By using the obtained strain distributions ε1 and ε2 and correcting the strain distribution at any position according to equation (1), the strain distribution in the plate with two through cracks in Figure 4 can be obtained. The obtained strain distribution ε pre The limit load can be estimated by using the strain distribution around the crack as a criterion.

[0029] Fig. 6 shows strain distributions in elastic-plastic analysis results for a plate with multiple closely spaced through-cracks. Fig. 7 shows predicted strain distributions for a plate with multiple closely spaced through-cracks based on an embodiment.

[0030] That is, FIG. 6 is a diagram showing the strain distribution around the cracks in the elastic-plastic analysis results for the flat plate having two adjacent cracks as described above, and FIG. 7 is a diagram showing the strain distribution ε obtained as a result of evaluation under the same crack conditions as FIG. 6 based on the embodiment. pre is.

[0031] FIG. 8 shows the results of comparing the strain distribution in the horizontal direction from the end of the through crack 21 of the flat plate having multiple adjacent through cracks shown in FIGS. 6 and 7. In FIG. 8, reference numeral 44 indicates the distance x from the crack end, and reference numeral 43 indicates the strain distribution output position. From the comparison results, it can be seen that the strain distribution (ε pre ) is the strain distribution (ε FEA ) can be accurately reproduced, demonstrating the effectiveness of this method.

[0032] 9 is a block diagram showing the configuration of the evaluation device 31 according to this embodiment. The evaluation device 31 according to this embodiment is configured to include a communication unit 33, a calculation unit 34, a memory unit 35, an output unit 41, and an input unit 42. The memory unit 35 is configured to include a database unit 39 that stores the results of the elastic-plastic analysis and a crack information unit 40 that complements the crack shape and position information obtained by the inspection. Data can be stored directly in the database unit 39 and the crack information unit 40, or it can be downloaded via the communication unit 33 and the network NW.

[0033] The calculation unit 34 is composed of a crack position evaluation unit 36 ​​that evaluates the relative coordinates of the crack, a strain distribution estimation unit 37 that estimates the strain distribution according to the crack position, and a limit load evaluation unit 38 that calculates the limit load based on the estimated strain distribution. Here, the physical positional relationship of the evaluation device 31 with the inspection device 30 is not particularly limited, and the evaluation device 31 may be located in a different location. The strain distribution estimation unit 37 estimates the strain distribution and limit load based on the formula (1) shown in the processing step S13 described above.

[0034] The input unit 42 is a device such as a keyboard or a mouse for inputting instructions to the computer, and inputs instructions such as starting a program. The output unit 41 is a display or the like, and displays the execution status and execution results of the processing by the evaluation device 31. The communication unit 33 exchanges various data and commands with other devices.

[0035] The calculation unit 34 is a central processing unit (CPU) that executes various programs stored in memory. The storage unit 35 is an external storage device that stores various data for the evaluation device 31 to execute processing.

[0036] <Modification> FIG. 4 illustrates the flat plate 20 having two through cracks 21 (surface crack width L1) and 22 (surface crack width L2), but the present invention is not limited to this.

[0037] Figure 10 is a schematic diagram showing a situation in which a tensile load or displacement 23 is applied to a plate with two surface semi-elliptical cracks. In the case of surface semi-elliptical cracks, crack depths a1 and a2 are added as parameters to the conditions for a through crack, but the evaluation procedure is the same.

[0038] Surface crack width L when targeting a non-penetrating crack of a surface elliptical crack j The surface crack width L to be evaluated is based on i Then, the crack depth a j and plate thickness t j Aspect ratio a j / L j , and L j / L i The surface crack width L i The coordinates in the strain distribution are expressed as L j / L i Correction is made using the ratio of

[0039] The limit load evaluation method and limit load evaluation device of the present embodiment described above have the following features. (1) A method for evaluating the limit load of the ductile fracture limit of a structure with n adjacent cracks, in which the strain distribution ε resulting from an elastic-plastic analysis for a single crack condition is i The ratio ε to the strain in the healthy state ε0 i The strain ε at any position is calculated based on the formula (1) expressed as the product of / ε0. preThe method is characterized by predicting the strain distribution of a structure with n adjacent cracks by correcting the strain distribution, and evaluating the limit load (plastic collapse load) from the obtained strain distribution.

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[0040] (2) In (1), the surface crack width L in the elastic-plastic analysis with a single crack j The surface crack width L to be evaluated is based on i The position (coordinate) in the strain distribution is expressed as the surface crack width ratio L j / L i The correction is characterized by:

[0041] (3) In (1), the surface crack width L when the surface elliptical crack is a non-penetrating crack j Based on the crack depth a j and plate thickness t j Aspect ratio a j / L j , and L j / L i The surface crack width L i The position (coordinate) in the strain distribution is L j / L i The correction is performed at the ratio of (see FIG. 10).

[0042] (4) The limit load evaluation method is based on elastic-plastic analysis for multiple crack conditions, including surface elliptical cracks and through cracks, and the aspect ratio a j / L j and L j / L i Based on each crack shape, including the strain distribution, stress distribution, load and surface crack length ratio l j / l i The method includes a database that stores coordinates standardized by the above, a process of measuring cracks in a pipe by non-destructive testing and obtaining the dimensions and arrangement of the cracks, and a process of estimating the fracture limit under multiple crack conditions from a single crack based on the limit load evaluation method described in any one of (1) to (3).

[0043] (5) In (1), the elastic-plastic analysis is characterized by using the characteristics defined by Ramberg-Ossgood and power-law plasticity as shown in equations (2) and (3).

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[0044] (6) A limit load evaluation device (e.g., evaluation device 31) for a structure having multiple adjacent cracks, which includes a memory unit 35 that stores elastic-plastic analysis results for a single crack under multiple shape conditions, and a strain distribution ε as a result of elastic-plastic analysis for a single crack condition based on information acquired from the memory unit 35 and the crack conditions of the evaluation target obtained by the inspection device 30. i The ratio ε to the strain in the healthy state ε0 i The strain ε at any position is calculated based on the formula (1) expressed as the product of / ε0. pre and a calculation unit 34 that predicts the strain distribution of a structure having n adjacent cracks by correcting the strain distribution, and estimates and evaluates the limit load (plastic collapse load) from the obtained strain distribution.

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[0045] 1. Limit Load Evaluation System 11 Preparation process 12 On-site measurement process 13 Evaluation process 20 flat plate 21, 22, 28 Through cracks 23 Displacement 30 Inspection equipment 31 Evaluation device (limit load evaluation device) 33 Communications Department 34 Arithmetic section 35 Storage section 36 Crack position evaluation section 37 Strain distribution estimation section 38 Limit Load Evaluation Section 39 Database Department 40 Crack Information Department 41 Output section 42 Input section 43 Sign (strain distribution output position) 44 Sign (distance from crack tip x) a1, a2 crack depth L1, L2, L3 surface crack width t Plate thickness

Claims

1. A method for evaluating the limit load of a ductile fracture limit of a structure having n adjacent multiple cracks, comprising: Strain distribution ε resulting from elastic-plastic analysis for a single crack condition i and the healthy state strain ε 0 The ratio ε i / ε 0 The strain ε at any position is calculated based on the product of equation (1). pre By correcting the strain distribution of a structure with n adjacent cracks, the strain distribution is predicted and the limit load is evaluated from the obtained strain distribution. A limit load evaluation method characterized by: [Equation 1] Here, 2≦n.

2. Surface crack width L in elastic-plastic analysis with a single crack ｊ The surface crack width L to be evaluated is i The position in the strain distribution is determined by the surface crack width ratio L j / L i Correct with 2. The limit load evaluation method according to claim 1.

3. Surface crack width L when targeting a non-penetrating crack of a surface elliptical crack j Based on the crack depth a j Aspect ratio a j / L j , and the surface crack width ratio L j / L i The surface crack width L to be evaluated is calculated using the analysis results in which i The coordinates in the strain distribution are L j / L i Correct by the ratio of 2. The limit load evaluation method according to claim 1.

4. Based on elastic-plastic analysis for multiple crack conditions including surface elliptical crack and through crack, aspect ratio a j / L j and the surface crack width ratio L j / L i a database that stores coordinates normalized by strain distribution, stress distribution, load, and surface crack width ratio L j / L i based on each crack shape including measuring cracks in the pipe by non-destructive testing and obtaining the dimensions and locations of the cracks; and a step of estimating a fracture limit under a multiple crack condition from a single crack based on the limit load evaluation method according to any one of claims 1 to 3. A limit load evaluation method characterized by:

5. The elastic-plastic analysis uses the characteristics defined by Ramberg-Ossgood and power-law plasticity in equations (2) and (3).

2. The limit load evaluation method according to claim 1. [Equation 2] where ε is the total strain, σ is the stress, E is Young's modulus, and a, b, and m are arbitrary material constants for the plasticity characteristic terms.

6. 1. An apparatus for assessing limit loads of a structure having multiple closely spaced cracks, comprising: a storage unit that stores elastic-plastic analysis results for a single crack under a plurality of geometric conditions; Based on the information acquired from the storage unit and the crack conditions of the evaluation target acquired by the inspection device, Strain distribution ε resulting from elastic-plastic analysis for a single crack condition i and the healthy state strain ε 0 The ratio ε i / ε 0 The strain ε at any position is calculated based on the product of equation (1). pre and a calculation unit that predicts the strain distribution of a structure having n adjacent cracks by correcting the above, and estimates and evaluates the limit load (plastic collapse load) from the obtained strain distribution. A limit load evaluation device characterized by: [Equation 1] Here, 2≦n.

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