JP2015001409A - Fatigue life evaluation method of structure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fatigue life evaluation method of a structure capable of evaluating a fatigue life with high reliability.SOLUTION: A fatigue life evaluation method of a structure is adapted, which includes: a first step S1 of detecting a defect in a structure; a second step S2 of measuring a dimension of the defect; a third step S4 of measuring a residual stress in a defect part having a defect; a fourth step S5 of acquiring a stress intensity factor by the residual stress of the defect part on the basis of a measurement result of the residual stress and the measurement result of the dimension; a fifth step S7 of adding the stress intensity factor to a stress intensity factor range by an external force applied to the defect part and acquiring the stress intensity factor range of the defect part; and a sixth step S8 of executing a fatigue test under a test piece loading condition set based on the stress intensity factor range of the defect part and evaluating the fatigue life of the structure.

Description

  The present invention relates to a method for evaluating fatigue life of a structure.

  Patent Document 1 below discloses a defect evaluation apparatus for appropriately evaluating the progress life of defects generated in a member. The defect evaluation apparatus includes a first storage unit that stores correspondence data representing the shape of the member of the welded part and a temporal change in the operation cycle, and a second storage that stores defect condition data representing a defect generated in the member. A crack growth amount due to fatigue and creep is calculated based on the correspondence data and the defect condition data, and the defect life of the weld is evaluated from the crack growth amount.

JP 2011-232206 A

By the way, as in the above prior art, a stress intensity factor is often used for the life evaluation of a structure having a crack defect. Generally, this stress intensity factor is calculated from a handbook of a predetermined standard, numerical analysis, or the like.
However, the stress intensity factor calculated in this way is often different from the actual stress intensity coefficient. For this reason, the conventional fatigue life evaluation has low reliability. As a result, for example, an unfavorable situation may occur in terms of maintenance of the structure, such as a short maintenance period of the structure and unnecessary maintenance work.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a fatigue life evaluation method for a structure capable of performing highly reliable fatigue life evaluation.

  In order to solve the above-described problems, the present invention provides a first step for flaw detection in a structure, a second step for measuring the size of the defect, and a second step for measuring residual stress in the defect portion having the defect. 3 steps, a fourth step of obtaining a stress intensity factor by the residual stress of the defect portion based on the measurement result of the residual stress and the measurement result of the dimension, and a stress due to an external force applied to the defect portion Fifth step of adding to the expansion factor range, obtaining the stress intensity factor range of the defect portion, and performing a fatigue test under the test piece load condition set based on the stress intensity factor range of the defect portion, And a sixth step of evaluating the fatigue life of the structure.

  In the present invention, in the sixth step, a technique is adopted in which a fatigue test piece is formed from the defect portion collected from the structure.

  In the present invention, a technique is adopted in which the residual stress is welding residual stress.

  In the present invention, a technique is adopted in which the structure is a spherical gas holder.

  In the present invention, a technique of using an ultrasonic flaw detection technique is employed in the first step.

  In the present invention, the fourth step employs a technique of obtaining a stress intensity factor range due to an external force applied to the defective portion by FEM analysis.

In this method, a defect in a structure is detected, and a stress intensity factor due to the residual stress in the defective portion is obtained based on the measurement result of the residual stress in the defective portion and the size of the defect. The stress intensity factor due to the residual stress of the defective part depending on the actual machine condition is likely to deviate from the one calculated from the pamphlet etc. The stress intensity factor due to the residual stress in the defective portion can be obtained.
Moreover, in this method, the fatigue life of the final structure is evaluated by actually performing a fatigue test without using simulation. In this method, first, the calculated stress intensity factor due to the residual stress of the defect portion is added to the stress intensity factor range due to the external force applied to the defect portion to obtain the stress intensity factor range of the defect portion in the actual machine. Then, the test piece load condition is set so that the stress intensity factor at the crack end of the fatigue test piece coincides with the calculated stress intensity factor range of the defect part in the actual machine. Thus, by matching the stress intensity factor ranges between the actual machine and the test piece, the fatigue life of the structure can be evaluated from the test result of the fatigue test.
Therefore, according to the present invention, a highly reliable fatigue life evaluation can be performed. For this reason, it is possible to set an appropriate maintenance period for the structure, and it is possible to reduce unnecessary maintenance work and reduce the maintenance cost of the structure.

It is a flowchart which shows the fatigue life evaluation method of the structure in embodiment of this invention. It is a figure which shows the structure in embodiment of this invention. It is a graph which shows the measurement result of the welding residual stress of the welding defective part in the embodiment of the present invention. It is a figure which shows the positional relationship of the measurement point of the welding residual stress of the weld defect part in embodiment of this invention, and a defect. It is a graph which shows the comparison with the measurement result of the welding residual stress of the weld defect part in embodiment of this invention, and the calculation result by a specification. It is a figure for demonstrating the calculation method of the stress intensity factor by the welding residual stress in embodiment of this invention. It is a figure which shows the collection piece and fatigue test piece in embodiment of this invention. It is a graph which shows the load conditions in embodiment of this invention. It is a graph which shows the load conditions in embodiment of this invention. It is a graph which shows the test result of the fatigue test in embodiment of this invention. It is a figure for demonstrating the calculation method of the stress intensity factor by the welding residual stress in another embodiment of this invention. It is a graph which shows the test result of the fatigue test in another embodiment of this invention.

  Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a flowchart showing a fatigue life evaluation method for a structure according to an embodiment of the present invention. FIG. 2 is a diagram showing a structure in the embodiment of the present invention.
In this method, the weld defect structure A shown in FIG. The weld defect structure A of the present embodiment is a spherical gas holder in which a plurality of weld lines 2 are formed vertically and horizontally, and is an existing one (hereinafter may be referred to as an actual machine).

As is well known, the spherical gas holder repeatedly applies stress (external force) due to internal pressure at a predetermined interval (for example, one day interval) by storing and venting gas. Due to this repeated stress, the weld line 2 or its vicinity that becomes a welded joint is fatigued, and defects (cracks) are generated and propagated.
This method is intended to evaluate the fatigue life by carrying out a fatigue crack growth confirmation test on the weld defect structure A. As shown in FIG. 1, the present method has a flow F1 on the actual machine side and a flow F2 on the fatigue test side.

In this method, first, in step S1, a defect inspection (first process) of the weld defect structure A is performed.
In this step S1, defects are detected along the weld line 2 of the weld defect structure A shown in FIG. 2 by the TOFD (Time Of Flight Diffraction) method which is a kind of ultrasonic flaw detection technique. In the present embodiment, rails are laid along the weld line 2 and a known TOFD type ultrasonic flaw detector is automatically run to detect defects along the weld line 2.

In this method, the defect size measurement (second step) of the weld defect structure A is performed in the next step S2.
The TOFD method is a method of transmitting and receiving ultrasonic waves so that a pair of transmitting and receiving two probe elements are arranged to face each other and transmit a cross section to be inspected. The TOFD method uses the difference in propagation time between the surface transmitted wave (lateral wave) and the bottom reflected wave directly below the surface and the diffracted wave generated at the upper and lower ends of the “defect” to geometrically determine the dimension of the “defect” (defect height). The length of the defect) can be measured. In particular, the TOFD method can measure the dimension in the thickness direction with high accuracy.

In this method, in the next step S3, a defective weld defect portion (denoted by reference numeral 3 in FIG. 2) is selected.
In step S3, when defects are detected at a plurality of locations as a result of ultrasonic flaw detection in step S1, one of the plurality of welding defect portions 3 is selected based on the measurement result of the dimensions. Is to select. In this embodiment, the weld defect part 3 in which the largest defect is measured is selected based on the measurement result of the defect size.

In this method, in the next step S4, the welding residual stress of the selected welding defect portion 3 is measured (third step).
FIG. 3 is a graph showing a measurement result of the welding residual stress of the weld defect portion 3 in the embodiment of the present invention. In FIG. 3, the vertical axis represents the distance z (mm) in the welding direction (plate thickness direction) with the thickness center of the weld defect 3 as the reference “zero”, and the horizontal axis represents the welding residual stress σ _Y (MPa). . FIG. 4 is a diagram showing a positional relationship between the measurement point (a, b) of the welding residual stress of the weld defect portion 3 and the defect 4 in the embodiment of the present invention. Reference numeral 5 denotes an outer surface of the weld defect structure A, and reference numeral 6 denotes an inner surface of the weld defect structure A.

  In this step S4, as shown in FIG. 3, the welding residual stress is measured at a plurality of points in the welding direction, and the distribution of the welding residual stress in the welding direction of the weld defect portion 3 is measured. FIG. 3 shows the distribution of welding residual stress at the measurement point a and the distribution of welding residual stress at the measurement point b. As shown in FIG. 4, the measurement point a is a point offset from the center line of the weld line 2 to the base material side, and is a point on a straight line passing through the formation position of the defect 4. The measurement point b is a point on the center line of the weld line 2.

  Welding stress can be measured using various methods such as destructive and non-destructive methods. In this method, since ultrasonic flaw detection is performed, the sound velocity of ultrasonic waves propagating in the medium is the stress state of the medium. Measurement is performed by the ultrasonic acoustoelastic residual stress measurement method using a phenomenon that slightly changes depending on In step S4, the measurement result at the measurement point a is adopted as the measurement result of the welding residual stress of the weld defect portion 3. As shown in FIG. 4, the measurement point a is closer to the defect 4 than the measurement point b, and the measurement result at the measurement point a matches the actual crack growth evaluation of the defect 4.

FIG. 5 is a graph showing a comparison between the measurement result of the welding residual stress of the weld defect portion 3 and the calculation result according to the standard in the embodiment of the present invention. In FIG. 5, the vertical axis represents welding residual stress σ (MPa), and the horizontal axis represents z / t (t: total thickness) obtained by making the thickness dimensionless.
FIG. 5 shows a calculation result (BS7910) calculated according to a predetermined procedure and calculation formula defined in the pamphlet of the international standard (BS7910: 2005). Thus, it can be seen that the result calculated from the pamphlet of the international standard deviates from the measurement result (actual machine) of the welding residual stress in the actual machine.

Returning to FIG. 1, in the next step S <b> 5, the present technique calculates a stress intensity factor (fourth step) based on the welding residual stress of the weld defect portion 3.
In step S5, the stress intensity factor due to the welding residual stress of the weld defect portion 3 is obtained based on the measurement result of the welding residual stress in step S4 and the measurement result of the dimension of the defect 4 in step S2. In this embodiment, according to the Japan Welding Association standard (WES2805: 2007), the stress intensity factor due to welding residual stress is calculated as follows.

FIG. 6 is a diagram for explaining a method of calculating a stress intensity factor by welding residual stress in the embodiment of the present invention. In FIG. 6, the vertical axis represents the welding residual stress σ (MPa), and the horizontal axis represents the thickness z (mm).
The defect 4 of this embodiment is an embedded crack (see FIG. 4). In the present embodiment, as shown in FIG. 6, the stress intensity factor due to the welding residual stress is obtained by scaling the welding residual stress distribution of the weld defect portion 3 with respect to the defect size.

Specifically, the intersection of both ends of the defect 4 (for example, dimension 2.4 mm) in the welding direction (plate thickness direction) is taken (intersection σ (a1) on the outer surface side, intersection σ (a2) on the inner surface side), and Then, an approximate straight line (approximate straight line of BS standard (1), approximate straight line of actual machine (2)) passing through intersection σ1 on the outer surface 6 side and intersection σ (a2) on the inner surface 5 side is obtained, and the outer surface of the actual machine The stress σ _{1 at} 6 and the stress σ _{2 at} the inner surface 5 of the actual machine are obtained.
The characteristics of the welding residual stress (tensile stress component σ _t , bending stress component σ _b ) can be obtained from the following equations (1) and (2).

And the stress intensity _{| strength} coefficient ( _Kres ) by welding residual stress can be _{calculated |} required from the following Formula (3). The shape correction factor for F _t is the tensile stress, F _b is the shape correction factor for the bending stress, l denotes the defect size. As the shape correction coefficients of F _t and F _b , appropriate coefficients defined in WES2805 are used.

  Table 1 below shows an example of welding residual stress calculated from the approximate straight line of the BS standard (1) (case (1)) and an example of welding residual stress calculated from the approximate straight line of the actual machine (2) (case (2)). It shows. As shown in Table 1 below, bending (+) welding residual stress is generated in the case (1), whereas compression (−) welding residual stress is generated in the case (2). Thus, it can be seen that the case (1) is a condition where the crack of the defect 4 is more likely to progress.

Returning to FIG. 1, in this method, in the next step S <b> 6, a stress intensity factor range is calculated by an external force.
In step S6, the stress intensity factor range due to the external force of the weld defect structure A is obtained by FEM analysis. In this embodiment, the FEM analysis model of the spherical gas holder shown in FIG. 2 is generated, and the stress intensity factor range is obtained from the stress (σ _max , σ _min ) due to the gas internal pressure. In the case of a spherical gas holder, since the gas internal pressure acts uniformly, the stress intensity factor range due to the external force is constant regardless of the position of the weld defect portion 3.

In this method, in the next step S7, the stress intensity factor range of the weld defect portion 3 is calculated (fifth step).
In step S7, the stress intensity factor (in this embodiment, a negative value) due to the welding residual stress in step S5 is added to the stress intensity factor range due to the external force in step S6 to obtain the stress intensity factor range of the weld defect portion 3. is there. Specifically, the stress intensity factor range (ΔK) of the weld defect portion 3 can be expressed as a load case (0) shown in FIG.

The maximum value and the minimum value (K _max , K _min ) of the stress intensity factor of the weld defect 3 are expressed by the following expressions (4) and (5). K _σmax is the maximum value of the stress intensity factor due to external force, and K _σmin is the minimum value of the stress intensity factor due to external force.
K _max = K _σmax + K _res (4)
K _min = K _{σ min} + K _res (5)

In this method, a fatigue test is performed and the fatigue life of the weld defect structure A is evaluated (sixth step: steps S8 and S11 to S15).
For this reason, as shown in FIG. 1, in this method, the process proceeds from step S3 to step S11 of the flow F2 on the fatigue test side.

In this method, the welding defect portion 3 is sampled in step S11 of the flow F2 on the fatigue test side.
In this step S11, the welding defect portion 3 selected in step S3 is cut out by trepanning, and the defect 4 and its peripheral portion are collected. In addition, the actual machine sampling points will be repaired afterwards.

FIG. 7 is a view showing a collected piece and a fatigue test piece in the embodiment of the present invention.
In this method, the crack size of the sample piece 7 is measured in the next step S12.
In this step S12, the collected weld defect portion 3 is sliced to form a sample piece 7 having a predetermined thickness where the defect 4 is exposed on the slice surface as shown in FIG. 7 (a). It measures with a scale.

In this method, the fatigue test piece 8 is produced from the collected piece 7 in the next step S13.
In this step S13, the hatched portion shown in FIG. 7 (b) is polished from the sample piece 7 into a rectangular chip of a predetermined size, and the grip portion 9 is laser welded as shown in FIG. 7 (c). By doing so, the fatigue test piece 8 is formed.

Returning to FIG. 1, in the next step S14, the test piece load condition is set based on the stress intensity factor range of the weld defect portion 3 obtained in step S7.
This step S14 sets the test piece load conditions so that the stress intensity factor range of the crack end of the defect 4 of the prepared fatigue test piece 8 matches the calculated stress intensity factor range of the weld defect portion 3. It is.

Specifically, the stress intensity factor (K) at the crack tip of the defect 4 (eccentric crack) of the fatigue test piece 8 can be obtained from the following equation (6) according to WES2805. σ _t is the stress, c is the half length of the crack, F _t2 (ξ) and F _t3 (ξ, λ) are the shape correction factors, e is the distance from the center line to the crack center, and W is the specimen H represents the distance from the center of the crack to the end of the specimen.

From the above equation (6), the specimen load conditions (σ _max , σ _min ) can be set by the following equations (7) and (8).

In this method, in the next step S13, a fatigue test is performed under the set test piece load conditions.
8 and 9 are graphs showing load conditions in the embodiment of the present invention. 8 and 9, the vertical axis represents the stress intensity factor K, and the horizontal axis represents the time T. 8 and 9 show load cases (0) to (4) of the fatigue test of the present embodiment.

As shown in FIG. 8A, the load case (0) is obtained by adding the stress intensity factor (K _res ) due to welding residual stress to the stress intensity factor range due to external force (obtained in step S7). .
As shown in FIG. 8 (a), the load case (1) is obtained by adjusting the negative value K _min to zero while maintaining K _max . The load case (1) has a stronger load than the load case (0). belongs to.
As shown in FIG. 8B, the load case (2) is obtained by offsetting the load case (0) so that the negative _Kmin is zero, and the load case (2) is tighter than the load case (1). It is the safety side.

As shown in FIG. 9A, the load case (3) does not add the stress intensity factor (K _res ) due to welding residual stress, and is only in the stress intensity factor range due to external force. The load is tighter than that on).
As shown in FIG. 9B, the load case (4) is obtained by _{adding the} stress intensity factor (K _BSres ) due to welding residual stress calculated according to the BS standard to the range of stress intensity factors due to external force (of the conventional method). The load is tighter than the load case (3) and is on the safe side.

  As shown in Table 2 below, the fatigue test was performed by selecting and setting the load cases (0) to (4) for each number of repetitions (150,000 cycles to) (step 1 to step 4). As shown in Table 2 below, the fatigue test was performed by forming a plurality of fatigue test pieces (A-1 to A-4) and selecting different load cases for each fatigue test piece. The fatigue test pieces (A-1 to A-4) were produced from the sampled pieces 7 sliced in step S12 and formed from the same weld defect portion 3.

In this method, in the final step S8, the fatigue life of the weld defect structure A is evaluated from the fatigue test.
FIG. 10 is a graph showing a test result of a fatigue test in the embodiment of the present invention. In FIG. 10, the vertical axis indicates the dimensionless crack length a / W (a is the crack length), and the horizontal axis indicates the total number of repetitions N (cycle). When the dimensionless crack length is 1, it indicates that the test piece has been broken.

As shown in Table 2, the fatigue test piece (A-1) is obtained by repeatedly applying a load in the load case (2) → the load case (3), and the fatigue test piece (A-2) (1) → Load case (2) was repeatedly applied, but as shown in FIG. Further, the fatigue test piece (A-3) is obtained by repeatedly applying a load in the load case (1) → the load case (2) → the load case (3). As shown in FIG. The fracture occurred when A-1) and (A-2) were slightly exceeded.
On the other hand, in the fatigue test piece (A-4), when the load is repeatedly applied in the load case (1) → the load case (0) (this method: load condition close to the actual machine), as shown in FIG. When the specimen (A-3) was greatly exceeded, it broke.

  As shown in FIG. 10, for example, when the load case (4) on the safer side than the fatigue test piece (A-1) is selected, the fatigue life may be shorter than that of the fatigue test piece (A-1). is expected. Thus, it can be seen that the fatigue life evaluation by the conventional method tends to be short-lived. On the other hand, for example, when the load case (0) is selected, the fatigue life is predicted to be longer than that of the fatigue test piece (A-4). Thus, the fatigue life evaluation by this method is close to the conditions of the actual machine, and it can be seen that the actual fatigue life is long.

  Thus, in this method, the defect 4 of the weld defect structure A is detected, and based on the measurement result of the weld residual stress in the weld defect portion 3 and the dimension of the defect 4, the welding residual stress of the weld defect portion 3 is determined. A stress intensity factor is obtained (steps S1, S2, S4, S5). The stress intensity factor due to the welding residual stress of the weld defect portion 3 is likely to depend on the state of the actual machine, and is likely to deviate from that calculated from the pamphlet or the like. In this method, since the said value is calculated | required based on the measured welding residual stress and the dimension of the defect 4, the stress intensity factor by the welding residual stress of the welding defect part 3 nearer to an actual machine can be calculated | required.

  Moreover, in this method, the fatigue life evaluation of the final weld defect structure A is performed by actually performing a fatigue test without using simulation (step S8). In this method, first, the calculated stress intensity factor due to the welding residual stress of the weld defect portion 3 is added to the stress intensity factor range due to the external force applied to the weld defect portion 3, and the stress intensity factor of the weld defect portion 3 in the actual machine is added. A range is obtained (step S7). Then, the test piece load condition is set so that the stress intensity factor at the crack end of the fatigue test piece matches the calculated stress intensity factor range of the weld defect part 3 in the actual machine (step S14). Thus, by matching the stress intensity factor range between the actual machine and the test piece, the fatigue life of the weld defect structure A can be evaluated from the test result of the fatigue test (see the fatigue test piece (A-4) shown in FIG. 10). ).

  As described above, according to this method, it is possible to perform a highly reliable fatigue life evaluation that is close to an actual machine. For this reason, an appropriate maintenance period of the weld defect structure A can be set, unnecessary maintenance work can be reduced, and the maintenance cost of the weld defect structure A can be reduced.

[Another embodiment]
The following shows results when this method is applied to another actual machine (spherical gas holder B (the same type as the above-described spherical gas holder A)).
FIG. 11 is a diagram for explaining a method of calculating a stress intensity factor by welding residual stress in another embodiment of the present invention.
The defect size of the actual machine B is 3.2 mm, and by scaling the welding residual stress distribution of the weld defect portion 3 as shown in FIG. Ask.

  Table 3 below shows an example of welding residual stress calculated from the approximate straight line of BS standard (1) (case (1)) and an example of welding residual stress calculated from the approximate straight line of actual machine (2) (case (2)). It shows. As described above, it can be seen that the case (1) is a condition in which the crack of the defect 4 is easily propagated as in the above embodiment.

  As shown in Table 4 below, the fatigue test was performed by selecting and setting the load cases (0) to (4) for each number of repetitions (150,000 cycles to) (step 1 to step 4). As shown in Table 4 below, the fatigue test was performed by forming a plurality of fatigue test pieces (B-1 to B-4) and selecting different load cases for each fatigue test piece.

FIG. 12 is a graph showing a test result of a fatigue test in another embodiment of the present invention.
The fatigue test piece (B-4) is a load case (1) → load case (0) with repeated load applied (this method: load condition close to the actual machine). As shown in FIG. Was the longest.
On the other hand, in the case where the load case (4) on the safer side than the fatigue test piece (B-1) is selected (conventional method), the fatigue life is predicted to be shorter than that of the fatigue test piece (B-1). Thus, it can be seen that the fatigue life evaluation by the conventional method tends to be short-lived.

  Therefore, according to this method, when a welding defect is found by inspection, it is possible to set an appropriate repair period by experimentally evaluating the life of the actual machine, thereby reducing the maintenance cost of the actual machine. Can be reduced.

  As mentioned above, although preferred embodiment of this invention was described referring drawings, this invention is not limited to the said embodiment. Various shapes, combinations, and the like of the constituent members shown in the above-described embodiments are examples, and various modifications can be made based on design requirements and the like without departing from the gist of the present invention.

  For example, in the above embodiment, since the weld defect part having the longest defect size is collected and the part is repaired, the actual fatigue life is slightly longer than the evaluated fatigue life. For this reason, for example, a weld defect with the longest defect size and a weld defect with the third longest defect size are sampled and subjected to a fatigue test. By estimating and evaluating the fatigue life of a long weld defect, the fatigue life of the actual machine can be made closer.

  For example, in the above embodiment, the spherical gas holder is exemplified as the structure, but the present invention is not limited to this. For example, when a residual stress is generated in a member by processing such as driving a rivet, this method can be applied to a fatigue life evaluation method for the opening.

  A ... Weld defect structure (structure), 3 ... Weld defect (defect), 4 ... Defect, 8 ... Fatigue specimen

Claims (6)

A first step for flaw detection in a structure;
A second step of measuring the size of the defect;
A third step of measuring the residual stress in the defective part having the defect;
Based on the measurement result of the residual stress and the measurement result of the dimension, a fourth step of obtaining a stress intensity factor due to the residual stress of the defect portion;
A fifth step of adding the stress intensity factor to a stress intensity factor range due to an external force applied to the defect portion to obtain a stress intensity factor range of the defect portion;
A sixth step of performing a fatigue test under a test piece load condition set based on a stress intensity factor range of the defect and evaluating a fatigue life of the structure;
A method for evaluating fatigue life of a structure.   The fatigue life evaluation method for a structure according to claim 1, wherein, in the sixth step, a fatigue test piece is formed from the defect portion collected from the structure.   The fatigue life evaluation method for a structure according to claim 1, wherein the residual stress is welding residual stress.   The said structure is a spherical gas holder, The fatigue life evaluation method of the structure as described in any one of Claims 1-3 characterized by the above-mentioned.   The fatigue life evaluation method for a structure according to any one of claims 1 to 4, wherein an ultrasonic flaw detection method is used in the first step.   The fatigue life evaluation method for a structure according to any one of claims 1 to 5, wherein, in the fourth step, a stress intensity factor range due to an external force applied to the defect portion is obtained by FEM analysis.

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