JP4774515B2 - Prediction method of stress corrosion cracking behavior and remaining life of actual structure - Google Patents

Description

The present invention relates to a method for predicting the stress corrosion cracking behavior of a real structure that predicts the occurrence, coalescence, and propagation behavior of micro surface cracks due to stress corrosion cracking in structural materials such as power plants, and the life of the actual structure or from the present time. The remaining life expectancy (hereinafter referred to as “remaining life”) is predicted.

Damage and destruction of equipment and structures such as power plants (hereinafter also referred to as “real structures”) are life threatening and cause great economic loss. Therefore, ensuring the soundness and reliability of these real structures. Is an important issue. In addition, in order to make more effective use of structures that have already been used for many years, it is desired to develop a technology that can be used safely for a long period of time.

From the viewpoint of aging degradation, countermeasures against stress corrosion cracking (hereinafter also referred to as “SCC”) that gradually proceeds in a low stress, corrosive environment are particularly important. SCC is an extremely complicated fracture phenomenon caused by a combination of materials, environment and stress. Detailed research for scientifically elucidating its behavior and predicting SCC behavior and remaining life of structural materials such as actual power plants A method is being developed. Research to elucidate the SCC behavior is conducted at the laboratory level in an accelerated environment, and a progressive surface is generated by the generation of corrosion pits, the occurrence of minute surface cracks from the pits, and the coalescence of minute surface cracks Behaviors such as the formation of cracks, the growth of surface cracks and the formation of large cracks by coalescence have been clarified.

On the other hand, SCC behavior and remaining life evaluation in structures such as power plants are performed as described in Patent Document 1, by placing loaded metal test pieces in equipment and actual structures, and SCC behavior in actual structure environments. In addition, the laboratory tests the acceleration test at the laboratory level, predicts the future SCC behavior in the actual structure environment from the SCC data and the acceleration test data in the actual structure environment, and evaluates the remaining life. Proposed. Further, as disclosed in Patent Document 2, a method has been proposed in which a material is cut out from an actual structure, tested for SCC sensitivity, and the remaining life is evaluated. As disclosed in these, the SCC behavior of an actual structure, the method already proposed as a remaining life evaluation method, captures a relatively large detectable crack (several mm to several tens mm), The remaining life is to be predicted by the subsequent crack propagation behavior, and the process up to the formation of a minute surface crack and the formation of a detectable surface crack by coalescence is not considered.

As seen in Non-Patent Document 1, there has been proposed a simulation method that takes into account a stochastic process from the generation of a small crack in SCC to coalescence and progress to a large crack. However, this assumes a uniform stress field penetration crack, and the coalescence and propagation conditions are also based on specific experimental results. SCC behavior has not been predicted.

As described above, in the conventional methods for predicting the SCC behavior and remaining life in actual equipment and structures, the occurrence of small surface cracks until the formation of detectable surface cracks in SCC, coalescence and The process of progress was not taken into account, and the rationality and reliability were not sufficient. For this reason, the evaluation was excessively safe. In addition, a simulation method that considers the generation process, coalescence, and propagation of a microcrack has not yet been applicable to actual equipment and structures.
JP 05-297181 A JP 11-173970 A (Y.-Z.Wang et al., The behavior of multiple stress corrosion cracks in a Mn-Cr and Ni-Cr-M0-V steel: III Monte Carlo simulation, Corrosion Science, Vol. 37, No. 11 (1995 ), pp. 1705-1720)

The present invention provides a surface of a size that can be detected by the occurrence, coalescence, and propagation of microscopic surface cracks in the contact surface of a structural material with a corrosive environment under any stress distribution as found in actual equipment and structures. Let's provide a method for predicting the process until a crack is formed and the process until the formation of a large crack by surface crack growth and coalescence by computer simulation and a method for predicting the remaining life based on the prediction result It is what.

Stress corrosion cracking behavior of an actual structure predicting the stress corrosion cracking behavior of an actual structure by an algorithm that assumes the occurrence of surface cracks in a real structure environment as a stochastic process and the coalescence and propagation of surface cracks as a definite process In the prediction method, the algorithm divides the analysis area into a plurality of stress areas in order to consider the stress distribution in the analysis area of the actual structure, and sets the tensile stress for each of the stress areas. The generation time of the surface crack in the crack generation process is assigned by a random number based on the number of possible surface cracks and the cumulative probability distribution, the number of possible surface cracks is determined according to the area for each stress region, the cumulative probability distribution, the determined in accordance with the tensile stress of each stress region, in the coalescence and development processes of the surface crack, a crack prudent裂深Satono ratio of surface crack a Those comprising as a feature to predict the evolution of have cleft with and aspect ratio.

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The stress corrosion cracking behavior prediction method for an actual structure according to claim 2 is based on the data obtained from the acceleration test and the test piece under the actual structure environment in addition to the requirement of claim 1. It is characterized by obtaining.

In addition to the above requirements, the method for predicting stress corrosion cracking behavior of an actual structure according to claim 3 is characterized in that, in the algorithm, the occurrence position of the surface crack is a random number with a uniform distribution, and the surface crack length is a random number with a normal distribution. It is characterized by the fact that

In addition to the above requirements, the method for predicting stress corrosion cracking behavior of an actual structure according to claim 4 is characterized in that, in the algorithm, the surface cracks whose distance from the tip to a neighboring surface crack is within the critical distance are combined. It is characterized by the fact that

The stress corrosion cracking behavior prediction method according to claim 5 actual structure described, in addition to the requirements of claim 4, wherein the critical distance, the coalescence region radius r _c,
(Where K _IA is the stress intensity factor at the surface of each crack, σ _ys is the yield stress of the material, and k is a factor depending on the material and the environment.)
It is characterized by being given by.

Further, the stress corrosion cracking behavior prediction method for a real structure according to claim 6 includes, in addition to the requirements of claim 4 , the crack length of the surface crack after coalescence includes two surface cracks before coalescence. The length dimension is used, and the crack depth is characterized by the deeper one of the two surface cracks.

In addition to the requirements of claims 4 to 6 , the method for predicting stress corrosion cracking behavior of a real structure according to claim 7 includes a plurality of surface cracks between several surface cracks when they are within a critical distance. It is characterized by the fact that the surface cracks considering the interaction are combined.

The stress corrosion cracking behavior prediction method for an actual structure according to claim 8 is characterized in that, in addition to the requirements of claim 7 , the surface cracks considering the interaction between the multiple surface cracks are the surface cracks. There calculates the tip distance d for a set of all surface cracks satisfies coalescing,-out coalescence region radius r _c divided by the d / r _c is the smallest surface to make the surface-out裂同worker to the appropriate this It is characterized by the fact that it is a fissure.

In addition to the requirements of claims 3 to 8 , the method for predicting stress corrosion cracking behavior of a real structure according to claim 9 includes the following in the stress release region where the stress is released by the generation, coalescence, and propagation of surface cracks: It is characterized by the fact that the generation, coalescence, or propagation of the surface cracks of the film is impossible.

In addition to the requirements of claim 9 , the stress corrosion cracking behavior prediction method for an actual structure according to claim 10 is a circle whose diameter is the crack length of the surface crack. Is a feature.

Further, the stress corrosion cracking behavior prediction method for an actual structure according to claim 11 is characterized in that, in addition to the requirements of claims 3 to 10 , in the algorithm, the coalescence and progress of the surface crack are the time from the occurrence of the immediately preceding surface crack. This is characterized in that the time Δt is subdivided and sequentially advanced during Δt.

In addition to the requirement of claim 1, the algorithm for predicting stress corrosion cracking behavior of an actual structure according to claim 12 includes the step of setting an analysis region of the actual structure and a stress distribution in the analysis region; Step 2 to obtain the number of surface cracks that can be generated in each stress region and all regions, and assign the generation time for all surface cracks that can be generated in each stress region by random numbers based on the cumulative probability distribution of each stress region. Step 3 for determining the number of generation times of all surface cracks that can occur in the region, and the generation of surface cracks can occur in the corresponding stress region in order from the shortest surface crack generation time. The crack initiation position is assigned by a uniform random number within the stress region, and the crack length is assigned by a normal random number. Step 4 If the crack initiation location is within the stress release region, In step 5 that returns to step 4 for the next occurrence of surface cracks, and where the crack generation position is outside the stress release zone, the surface crack is generated and the immediately preceding surface is generated. A step 6 for calculating the time Δt from crack initiation, and a step 7 for merging with a neighboring surface crack when the tip-to-tip distance with the neighboring surface crack is within a critical distance outside the stress release zone; And a step 8 of causing the surface crack that has progressed to coalescence to reach the length of the progressing surface crack to propagate by the time Δt based on the surface crack propagation characteristics. .

Further, the stress corrosion cracking behavior prediction method for a real structure according to claim 13 is not limited to the requirements of claim 12 , and in the coalescence of step 7, the crack length of the surface crack after coalescence is two before the coalescence. The length dimension includes the surface crack, and the crack depth is the deeper dimension of the two surface cracks.

The remaining life prediction method according to claim 14 repeats the stress corrosion cracking behavior prediction method according to claims 1 to 13 many times, and estimates the remaining life until the critical crack length is reached from its statistical characteristics. It consists of features.

According to the present invention, it is possible to accurately predict the stress corrosion cracking behavior and remaining life of an actual structure in a corrosive environment such as a power plant. In particular, according to the first aspect of the present invention, the behavior from the generation stage of a minute surface crack can be well predicted.

Further , according to the invention described in claim 1, it is possible to predict the behavior reflecting the actual stress distribution of the actual structure.

Further, according to the first aspect of the invention, it can be predicted as behavior actually along the generation time in the developmental stage of-out fine surface cracks.

According to invention of Claim 2, it can estimate as a behavior reflecting the structural material of an actual structure, stress distribution, and the substance of an environment.

According to the third aspect of the present invention, it is possible to predict the crack generation shape at the generation stage of the minute surface crack as a behavior along the actual condition.

According to invention of Claim 4, it can estimate as a behavior in consideration of the coalescence conditions of the surface crack.

According to the fifth aspect of the present invention, coalescence at the coalescence stage of surface cracks can be predicted as a behavior reflecting the structural material, stress distribution, and environmental substance of the actual structure.

According to the sixth aspect of the present invention, the coalescence at the coalescence stage of the surface crack can be predicted as a more realistic behavior. In particular, according to the present invention, it is possible to predict a behavior that is significantly different from the case where a through crack is assumed.

According to the seventh aspect of the present invention, coalescence in the coalescence stage of surface cracks can be predicted as a behavior when a large number of cracks are close to each other.

According to the eighth aspect of the present invention, coalescence in the coalescence stage of surface cracks can be better predicted as a behavior when a large number of cracks are close to each other.

According to the invention described in claim 9, it can be predicted as a behavior considering that stress is released in the vicinity of the crack due to the generation, coalescence, and propagation of the surface crack.

According to the invention described in claim 10 , since the stress release region is simply expressed by the occurrence, coalescence and progress of the surface crack, the behavior considering the stress release is predicted in a reasonable time. be able to.

According to the eleventh aspect of the invention, it is possible to predict the coalescence and propagation of surface cracks as a more accurate behavior.

According to the twelfth aspect of the present invention, the stress distribution in the actual structure can be reflected and the behavior can be predicted in consideration of each process of generation, coalescence, and propagation of the surface crack.

According to the invention of claim 13 , it reflects the stress distribution in the actual structure and predicts the behavior in consideration of each process of generation, coalescence, and propagation of the surface crack. It can be predicted that the coalescence is a behavior that actually follows the behavior, and that the behavior is significantly different from the case where the penetration crack is assumed.

According to the invention of claim 14, as a result of reflecting the stress distribution in the actual structure and predicting the behavior in consideration of each process of generation, coalescence and progress of the surface crack, and as a statistical value thereof, It is possible to predict the remaining life according to the actual situation.

BEST MODE FOR CARRYING OUT THE INVENTION The best mode for carrying out the present invention includes one described in the following examples, and further includes various methods that can be improved within the technical idea.

Therefore, FIG. 1 shows a schematic flow chart of the method for predicting the remaining life of an actual structure for the SCC of the present invention. In the remaining life prediction method of the present invention, first, data on the accelerated test in the laboratory of the structural material and the behavior of the occurrence, coalescence, and propagation of the minute surface crack in the small test piece in the actual structure environment are acquired. Then, using these data as input data, computer simulation is performed in consideration of the stochastic process and confirmation process of SCC surface crack generation, coalescence, and propagation in actual structures. In this computer simulation, the stress corrosion cracking behavior prediction method of the actual structure of the present invention is used. In the present residual life prediction method, the SCC life until the critical surface crack is reached by repeating this computer simulation several times. Is obtained as a statistic, and the remaining life of the actual structure can be predicted.

Here, in a structural material in an SCC environment, first, a large number of non-progressive micro surface cracks are generated on a smooth surface and coalesced with other micro surface cracks generated in close proximity. Then, when this united surface crack has a certain critical length, thereafter, steady crack propagation starts. The generation process of this small surface crack is a stochastic process with respect to generation time, generation position, and crack length, and the coalescence and propagation of cracks are definite processes, so a stochastic approach is used for quantitative evaluation of SCC behavior. The introduction of a deterministic approach is essential. Therefore, in the method for predicting stress corrosion cracking behavior of the actual structure of the present invention, the occurrence of minute surface cracks occurs randomly based on the stochastic processes of occurrence time, occurrence position, and crack length, and The progress is characterized by computer simulations based on Monte Carlo methods that are deterministic based on the concept of fracture mechanics. Next, the computer simulation algorithm will be described with reference to the flowchart shown in FIG.

Step 1: First, an area to be analyzed and a stress distribution in the analysis area are set. The stress distribution received by the member (structural material) in the actual structure changes smoothly including the residual stress, but here, for the sake of simplicity, for example, a stepped shape as shown in FIG. Consider the stress distribution that changes to. That is, the area of each stress regions A _1, · · ·, and A _n.

Step 2: The generated surface crack is a semi-elliptical surface crack with surface length 2a, depth b, and aspect ratio b / a = α. The surface dimensions of the crack are on the order of crystal grains, and the crack is in the direction of tensile stress. I think that it occurs almost perpendicular to the. Therefore, as shown in FIG. 4, since one minute surface crack can be generated in one crystal region, the number of surface cracks that can be generated in each stress region and all regions (analysis region) is expressed by the following formula 2. And given by Equation 3.

Step 3: Surface crack initiation is described by the cumulative probability distribution F _i (t) of surface crack initiation at time t. Since F _i (t) is a function depending on stress, the cumulative probability distribution of each stress region in FIG. 3 is described by a different function. The generation time for N _kmax surface cracks in each stress region is assigned by a random number based on the cumulative probability distribution of each stress region, and the generation times for N _max surface cracks are respectively determined.

Step 4: Surface cracks are generated in order from the surface crack with the shortest occurrence time assigned in Step 3, and the crack generation position in the corresponding stress region (x, y coordinates of the surface crack center) Is determined by a uniform random number in the stress region, and the crack length is determined by a normal random number.

Step 5: Since the top and bottom of the existing surface cracks are places where surface cracks are unlikely to occur due to stress release, the surface crack generation position is in the place shown in Fig. 5 (a), that is, within the stress release area. In this case, assuming that a new surface crack cannot be generated, the process returns to Step 4 to assign a new crack number to the crack generation position and crack length based on a new random number for the next generation crack. The stress release area can be set, for example, in a circle having a crack length as a diameter.

Step 6: When a surface crack can be generated, the interval time Δt from the previous crack generation time is calculated, and the coalescence and propagation of the surface crack at Δt time are calculated in the next Step 7 and Step 8. It becomes.

Step 7: For the coalescence of cracks, consider that stress concentration occurs at both ends of the surface crack. In other words, considering the activation region described by the stress intensity factor at the tip of the surface crack, the surface of the surface crack becomes closer to the right (left) tip of one surface crack and the left (right) tip of another surface crack. Assume that coalescence of cracks occurs. Here, as shown in FIG. 6, the stress intensity factor of the surface crack having a semi-elliptical cross section is evaluated by the following Equation 4 at the surface portion and deepest portion of the crack.
However, F _A and F _B are correction coefficients and functions of the aspect ratio, and are given, for example, in FIG. The critical distance between the surface crack tips for coalescence of both surface cracks is given by the stress intensity factor of both surface cracks. For example, as shown in FIG. 8, considering the circular activation region at the tip of the surface crack, coalescence occurs when the left tip of Crack 2 enters the circular region at the right tip of Crack 1, and the critical radius of the coalescence region is It is expressed by the following formula 1.
Here, K _IA is the stress intensity factor at the surface of each surface crack, σ _ys is the yield stress of the material, k is the coefficient depending on the material and the environment, and can be obtained by considering experiments or conventional literature .

When several surface cracks satisfy the above-mentioned coalescence condition, as shown in FIG. 9, the distance d between the tips is calculated for all the conditions that satisfy the condition, and these are formed by the corresponding surface cracks. the value d / r _c obtained by dividing the combined region radius r _c is the possible to combine the smallest surface-out裂同workers. As a result, the distance between the tips is shorter and the larger the surface cracks, the easier the coalescence.

FIG. 10 shows a new surface crack formed by coalescence of surface cracks. The surface dimension of the surface crack formed by coalescence was the length including the two surface cracks before coalescence, and the depth dimension was the depth dimension deeper than the two surface cracks. Therefore, a surface crack with a small aspect ratio b / a is formed by coalescence of the surface cracks. In fact, this point is significantly different from the case of a through crack. That is, the smaller aspect ratio means that the F _A, which is the correction factor of the stress intensity factor K _IA at the crack surface, becomes smaller, as can be seen from the curve in FIG. As in the case of a simulation in the case of a through-crack, the longer the crack is, the easier it is to coalesce and the crack progresses in an instant, avoiding predicting behavior far from reality, This leads to the ability to predict behavior that more closely matches reality.

Step8: Then, a surface crack reaching the development surface crack length by coalescence, as has been introduced in various documents such as the steady progress based on crack growth properties (da / dt-K _I relationship) As it begins, the amount of surface crack propagation is calculated for all surface cracks that have reached the length of the progressive surface crack. At this time, the surface crack growth amount in the surface direction of the surface crack is the stress intensity factor K _IA in the surface portion, and the surface crack growth amount in the depth direction is the stress intensity factor K _IB in the deepest portion. Calculated using

In Step 7 and Step 8, since the coalescence and progress of the surface crack are affected by the stress release by the existing surface crack, as shown in FIGS. 5 (b) and 5 (c), the surface within the stress release region It was assumed that it did not coalesce with the crack and would not progress into the stress relief zone. In addition, it is preferable from the viewpoint of improving prediction accuracy that the coalescence and progress of cracks in Δt time are calculated so as to proceed sequentially by subdividing Δt into a certain time interval and repeating Step 7 and Step 8. Since this requires extra calculation time, it is better to subdivide it into reasonable time intervals.

Step 9: In the flowchart shown in FIG. 2, Step 4 to Step 8 are repeated for the generation of a new surface crack until the maximum surface crack length reaches the set limit surface crack length. However, in the method for predicting stress corrosion cracking of the actual structure of the present invention, it is not always necessary to repeat until the maximum surface crack length reaches the set limit surface crack length, as long as the desired behavior can be grasped. In addition, the occurrence of cracks, coalescence, and progress may be repeated up to the point of deadlock where neither progresses.

Step 10: When the maximum surface crack length reaches the set critical length, the distribution of the surface cracks over time, the surface dimensions and depth dimensions of each surface crack are output, and the simulation is terminated. Of course, other behavior information may be output in the same manner.

It is sufficient if the behavior to be obtained can be grasped by one simulation, but by repeating this simulation with a new random number, the statistical characteristics of the SCC behavior under the set SCC conditions can be predicted. For example, when fifteen simulations are performed and a certain limit surface crack length is set, as shown in FIG. 11, the cumulative probability of an event (SCC life) reaching the limit surface crack length with respect to time is plotted on probability paper. By doing so, the remaining life until the limit surface crack length is reached can be estimated as a statistic.

As described above, the stress corrosion cracking behavior prediction method of the actual structure of the present invention sets the possible number of minute surface cracks and random numbers based on the cumulative probability distribution of surface cracks with respect to the surface crack generation time. It is characterized by considering the stress distribution in the simulation region, and considering the occurrence, coalescence, and propagation of surface cracks. It enables the simulation of SCC behavior for real time of structural materials.

The setting conditions that affect the results of stress corrosion cracking behavior prediction of actual structures include the cumulative probability distribution of surface crack initiation, evaluation of stress intensity factors, surface crack coalescence conditions, and propagating surface cracks. The critical length and surface crack propagation characteristics can be considered. Among these, the evaluation of stress intensity factor, surface crack coalescence conditions, critical length of progressive surface cracks, and surface crack propagation characteristics have been published. Available from many references. However, although data on detectable surface cracks have been published, there is no data available on the cumulative probability distribution of the occurrence of micro surface cracks on the order of grains.

Therefore, in order to determine the cumulative probability distribution of the occurrence of micro surface cracks, the effects of the environment and stress on the cumulative probability distribution are clarified by laboratory-level accelerated tests, corresponding to the conditions that actual structural materials receive. A method for estimating the cumulative probability distribution is considered. Furthermore, a large number of small multi-stage stress test pieces 1 that can change the stress distribution as shown in FIG. 12 in a load device as shown in FIG. 13 are installed in the actual structure. This is taken out after a certain period of time, and the number of minute surface cracks generated in the multistage stress test piece 1 is measured, and plotted on a probability paper as shown in FIG. The cumulative probability distribution under each stress in can be determined. Of course, when such data is already held, it may be used. In the load device, 2 is a load jig pedestal (lower) that fixes the lower end of the multistage stress test piece 1 and 3 constitutes the jig body, 3 is a load jig pedestal (upper) that constitutes the jig body, and 4 is an upper and lower pedestal. Are connected to a load bolt 9 that fixes the upper end of the multi-stage stress test piece 1 and applies a load thereto, and 6 is connected via the load bolt 9 and the connection tool 5. A load spring for applying a load to the multistage stress test piece 1, 7 is a spring seat, and 8 is a load nut capable of adjusting the load applied to the multistage stress test piece 1 by the load spring 6 by changing the tightening length of the load bolt 9. .

Hereinafter, embodiments of the present invention performed with specific numerical values will be described with reference to the drawings. The structural material is SUS304 with an average crystal grain size of 0.1 mm, and 10 mm x 10 mm under uniform tensile stress (σ = 437 MPa) assuming an accelerated SCC test with a constant strain bending beam (CBB) at the laboratory level. A simulation based on the above algorithm was performed for a square region.

FIG. 15 is a cumulative probability distribution of microcrack generation, and assumes an exponential distribution according to the following Equation 5.
In this example, since the analysis area is 10 mm × 10 mm and the average crystal area is 0.01 mm ² , the number of possible minute surface cracks is 10,000. Therefore, each crack initiation time was assigned to 10,000 fine surface cracks by a random number according to this exponential distribution, and was generated from a surface crack with a short time. The crack initiation position (x, y) is a uniform random number between 0 and 10 mm for each x and y, and the surface crack length 2a is a regular random number with an average length of 0.1 mm and a standard deviation of 0.02 mm. Were determined. Of course, random numbers based on probability distributions based on actual phenomena may be used instead of uniform random numbers and normal random numbers. The coalescence condition of the surface crack was set to k = 0.5 in Equation 1. For the critical length of a progressive surface crack, the lower limit value of SCC surface crack growth was set to K _ISCC = 5 MPa√m, and it started to grow when this value was exceeded. Further, the surface crack growth rate follows the surface crack growth characteristics shown in FIG. 16, and 15 simulations were performed under such conditions.

FIG. 17 shows the surface crack distribution on the surface at each elapsed time. When 100 hours have passed, there is almost no coalescence of surface cracks, and only minute surface cracks are uniformly distributed. After 300 hours, the number of surface cracks increases, and several large cracks can be observed due to coalescence of surface cracks and crack growth. After 500 hours, the number of surface cracks that have grown further increases, and it can be seen that some of the surface cracks are larger than 5 mm.

FIG. 18 shows the relationship between the number of surface cracks and the elapsed time. The figure also shows the results calculated based on the exponential distribution. The number of surface cracks is equal to the number of surface cracks that occurred at the initial stage of the surface crack, and agrees with the result of the exponential distribution. It can be seen that a region that cannot be generated tends to be saturated because it is formed.

FIG. 19 shows the relationship between the maximum surface crack length and the elapsed time. After the incubation period, there is a period of formation of a progressive surface crack due to coalescence of small surface cracks. After reaching _KISCC , the surface crack grows gradually due to propagation and coalescence. Since growth and coalescence contribute to the growth of the surface crack, it can be seen that the single surface crack grows at a faster rate than the result predicted from the growth rate.

FIG. 20 shows the relationship between the maximum surface crack depth and the elapsed time. Since crack coalescence does not occur in the crack depth direction, it is evaluated from the stress intensity factor and surface crack growth characteristics at the deepest part of the surface crack. It depends on the variation of the surface crack formation period.

FIG. 21 shows the relationship between the aspect ratio of the surface crack having the maximum depth and the elapsed time. The aspect ratio of the surface crack decreases from 1 at the generation stage of the minute surface crack to 0.2 to 0.4 as the surface crack increases with time. This well predicts the characteristics of the actual SCC surface cracks.

FIG. 22 is a plot of the cumulative probability of SCC life on an exponential distribution probability paper when the SCC life is set to a maximum surface crack length of 5 mm and the maximum surface crack depth is 1 mm. Since these plots show a linear relationship, it can be seen that the SCC life F (t) until reaching the limit surface crack length is also represented by an exponential distribution.
When the maximum crack length is 5 mm
When the limit crack depth is 1 mm
The above simulation results were performed for a region under uniform stress after Step 3 in the flow shown in FIG. 2, but the characteristics of stress corrosion cracking behavior observed in experiments or actual structures so far Is reproduced well. Therefore, when performing on a region having a stress distribution in an actual structure, performing Step 1 and Step 2 in the same flow and then performing the above will reproduce the characteristics of stress corrosion cracking behavior considering the stress distribution. Become. In other words, according to this simulation algorithm, if conditions such as stress distribution applied to the structural material in the region to be analyzed in the actual structure are used as input data, prediction of stress corrosion cracking behavior in the actual structure, It is clear that the life from the start of use and the remaining life from the present time can be predicted if it is already in use.

It is a schematic flowchart of the remaining life prediction method of the actual structure with respect to SCC of this invention. It is a flowchart of the computer simulation at the time of implementing the stress corrosion cracking behavior prediction method of the actual structure of the present invention. It is explanatory drawing which shows typically the analysis area | region which has stress distribution. It is explanatory drawing typically shown about the possible number of micro surface cracks. With regard to the stress release area, which is the effect of the existing surface crack, (a) the effect on surface crack initiation, (b) the effect on surface crack coalescence, and (c) the effect on surface crack growth, FIG. It is explanatory drawing which shows typically a surface crack and its stress intensity factor. It is a characteristic view which shows the relationship between the correction factor of the stress intensity factor in a surface crack, and an aspect ratio. It is explanatory drawing shown typically about the area | region where two surface cracks unite | combine. It is explanatory drawing which shows typically about a mutual relationship when several surface cracks satisfy | fill coalescence conditions. It is explanatory drawing which shows typically the state (a) of two surface cracks before coalescence, and the state (b) of surface crack after coalescence. It is explanatory drawing which shows typically that a remaining life can be estimated based on a simulation result. It is a front view of the multistage stress test piece for a test for obtaining the cumulative probability distribution of a minute surface crack generation. It is a front view of the constant load tensile load jig for a test used in order to obtain the cumulative probability distribution of a minute surface crack generation. It is explanatory drawing which shows typically determining the cumulative probability distribution of surface crack generation | occurrence | production by the SCC test of a multistage stress test piece. It is a characteristic view which shows the cumulative probability distribution of the surface crack generation | occurrence | production which becomes an exponential distribution. It is a characteristic view which shows the growth characteristic of the surface crack of stress corrosion cracking. FIG. 6 is a plan view of an analysis region showing distribution examples of surface cracks obtained as simulation results after (a) 100 hours, (b) 300 hours, and (c) 500 hours. It is a characteristic view which shows the relationship between the number of surface cracks, and elapsed time. It is a characteristic view which shows the relationship between the maximum surface crack length and elapsed time. It is a characteristic view which shows the relationship between the maximum surface crack depth and elapsed time. It is a characteristic view showing the relationship between the aspect ratio of the surface crack having the maximum depth and the elapsed time. FIG. 6 is a characteristic diagram showing a cumulative probability distribution of stress corrosion cracking life when the limit surface crack length is 5 mm and the maximum depth is 1 mm.

Explanation of symbols

1 Multistage Stress Test Specimen 2 Load Jig Pedestal (Lower) 3 Load Jig Pedestal (Upper) 4 Post 5 Connector 6 Load Spring 7 Spring Receiving Seat 8 Load Nut 9 Load Bolt

Claims (14)

Stress corrosion cracking behavior of an actual structure predicting the stress corrosion cracking behavior of an actual structure by an algorithm that assumes the occurrence of surface cracks in a real structure environment as a stochastic process and the coalescence and propagation of surface cracks as a definite process A prediction method,
In the algorithm,
In order to consider the stress distribution in the analysis region of the actual structure, the analysis region is divided into a plurality of stress regions and a tensile stress is set for each stress region.
The surface crack initiation time in the surface crack initiation process is assigned by a random number based on the number of possible surface cracks and the cumulative probability distribution,
The number of possible surface cracks is determined according to the area of each stress region,
The cumulative probability distribution is determined according to the tensile stress for each stress region,
In the process of coalescence and propagation of the surface crack, stress corrosion cracking of an actual structure characterized by predicting crack propagation using an aspect ratio which is a ratio of the crack length and crack depth of the surface crack. Behavior prediction method. The cumulative probability distribution, the stress corrosion cracking behavior prediction method according to claim 1 real structure, wherein the be based on data obtained by the accelerated test and actual structures environment of the specimen. In the algorithm, the random number generation position is a uniform distribution of surface crack, a surface crack length stress corrosion cracking according to claim 1 or 2 real structure wherein it has to be due to random numbers normally distributed Behavior prediction method. In the algorithm, the stress corrosion cracking behavior of actual structures of claims 1 to 3, wherein the tip distance between-out neighboring surface cleft surface-out裂同workers became the critical distance is characterized in that the uniting Prediction method. The critical distance, as coalescence region radius r _c,
(Where K _IA is the stress intensity factor at the surface of each crack, σ _ys is the yield stress of the material, and k is a factor depending on the material and the environment.)
5. The method for predicting stress corrosion cracking behavior of an actual structure according to claim 4, wherein: The crack length of the surface crack after coalescence is the length dimension including the two surface cracks before coalescence, and the crack depth is the deeper dimension of the two surface cracks. The method for predicting stress corrosion cracking behavior of an actual structure according to claim 4 . When several surface-out裂同workers is within the critical distance, actual structures of claim 4 to 6, wherein the surface-out裂同worker in consideration of the interaction of a number of surface-out裂間is characterized in that the coalescing For predicting stress corrosion cracking behavior of steel. The surface cracks taking into account the interaction between the multiple surface cracks are calculated by calculating the inter-tip distance d for all sets of surface cracks that satisfy the condition that the surface cracks coalesce with each other. stress corrosion cracking behavior prediction method according to claim 7 real structure, wherein the value d / r _c obtained by dividing the combined region radius r _c to make the surface-out裂同worker has to be the smallest surface-out裂同mechanic . In the algorithm, the generation of surface cracks, coalescence, and the stress relief region that is stress released by the development, the occurrence of-out next surface cracks, coalescence, or progress 3 through claim, characterized in that the can not 8. A method for predicting stress corrosion cracking behavior of an actual structure according to 8 . 10. The method for predicting stress corrosion cracking behavior of an actual structure according to claim 9 , wherein the inside of the stress release region is a circle whose diameter is the crack length of the surface crack. In the algorithm, coalescence and development of surface cracks, between the time Δt from the surface crack initiation immediately preceding, claims 3 to 10 the time Δt is characterized in that the advancing finely divided and sequentially A method for predicting stress corrosion cracking behavior of the described actual structure. The algorithm includes step 1 for setting an analysis region of an actual structure and a stress distribution in the analysis region, step 2 for obtaining the number of surface cracks that can be generated in each stress region and all regions, and generation in each stress region. Step 3 of determining the generation time of all surface cracks that can be generated in all areas by assigning generation times for all surface cracks by random numbers based on the cumulative probability distribution of each stress area, and generation of surface cracks Step 4 in which the crack generation position is assigned by a uniform random number in the stress region and the crack length is assigned by a normal random number. If the crack generation position is within the stress release region, it is determined that the surface crack could not be generated, and the process returns to step 4 for the next surface crack generation. In step 5 where the crack generation position is outside the stress release region, the surface crack is generated and step 6 for calculating the time Δt from the immediately preceding surface crack generation is detected. When the tip-to-tip distance with the surface crack is within the critical distance, step 7 is performed for coalescence with the adjacent surface crack, and the surface crack that has progressed to coalescence and has reached the progressing surface crack length is surface cracked. 2. The method for predicting stress corrosion cracking behavior of an actual structure according to claim 1, further comprising a step 8 of making progress by an amount corresponding to time Δt based on the progress characteristics. In the coalescence of the step 7, the crack length of the surface crack after the coalescence is a length dimension including the two surface cracks before the coalescence, and the crack depth is the length of the two surface cracks. 13. The method for predicting stress corrosion cracking behavior of an actual structure according to claim 12 , wherein the depth dimension is set to a deeper depth. Claims 1 to stress corrosion cracking behavior prediction method according 13 is repeated a number of times, residual life prediction method of estimating the remaining life from the statistical properties until the limit crack length.