WO2006006412A1 - 疲労き裂成長曲線の推定法、推定プログラムおよび推定装置 - Google Patents
疲労き裂成長曲線の推定法、推定プログラムおよび推定装置 Download PDFInfo
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- WO2006006412A1 WO2006006412A1 PCT/JP2005/012046 JP2005012046W WO2006006412A1 WO 2006006412 A1 WO2006006412 A1 WO 2006006412A1 JP 2005012046 W JP2005012046 W JP 2005012046W WO 2006006412 A1 WO2006006412 A1 WO 2006006412A1
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
- G01N3/32—Investigating strength properties of solid materials by application of mechanical stress by applying repeated or pulsating forces
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
- G01N2203/0058—Kind of property studied
- G01N2203/006—Crack, flaws, fracture or rupture
- G01N2203/0062—Crack or flaws
- G01N2203/0064—Initiation of crack
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
- G01N2203/0058—Kind of property studied
- G01N2203/006—Crack, flaws, fracture or rupture
- G01N2203/0062—Crack or flaws
- G01N2203/0066—Propagation of crack
Definitions
- the present invention relates to a fatigue crack initiation 'growth curve estimation method for quantitatively estimating a fatigue crack initiation' growth curve at a stress concentration site in a polycrystalline steel structure.
- the present invention relates to an estimation program and an estimation device.
- Fatigue design takes into account actual physical phenomena based on the S—N curve, where the fracture (cracking) life of a specimen under a constant load amplitude is simply expressed as a function of the stress amplitude. Instead, the focus is on reducing the cumulative stress frequency distribution received by the structure below a certain level of fatigue damage. Fatigue design using SN curves is effective to some extent based on the empirical rule of feedback of actual equipment, but information on the size of cracks cannot be obtained, and in many cases it becomes ineffective in new structural modes. Arise.
- the present inventor has said that a tensile plastic region has been generated in the loading process and a compression plastic region has been generated in the unloading process, and the overlapping region size of both has governed the crack propagation speed. Based on a simple assumption, assuming the case where a constant amplitude load is applied repeatedly, as described in Non-Patent Document 1, the fatigue life of a crack propagating in the first crystal grain is predicted.
- Non-Patent Document 1 Masahiro Toyosada and Toshio Niwa, “Fatigue Life Prediction of Steel Structures”, Kyoritsu Shuppan, 20 December 25, 2001, p. 182- 186
- Non-Patent Document 1 enables the present inventor to estimate a crack force continuous crack growth curve having a magnitude of zero.
- the method described in Non-Patent Document 1 has the following problems in calculating the initial position of the fatigue damage accumulation area.
- the tip position of fatigue damage accumulation area is calculated from RP.
- the initial stage of the crack is a shear-type crack, and the tensile stress acts on the crack surface, whereas it is long! In the crack, the tensile stress does not act on the crack surface! /
- the assumptions described above do not necessarily hold.
- the initial stage of the crack is a shear crack, and the tensile stress is handled by the crack surface, but this phenomenon is completely ignored.
- the estimation of the fatigue crack growth curve assumes a closed-type shear crack, but at the parameter calculation stage necessary for the estimation of the fatigue crack curve, an open-type crack is assumed. Think of a crack It is fixed.
- Non-Patent Document 1 makes it possible to estimate a fatigue crack growth curve including the initial crack growth in the crystal grains. Therefore, the crack in the first crystal grain is an open crack from the beginning from the viewpoint of safety, and the crack that proceeds in the first crystal grain is changed to a shear-type crack force open crack. The detailed state of the transition phenomenon is a component.
- a crack of zero size that is, a state force in which no crack exists at all, is continuously grown in a healthy part.
- the purpose is to provide a fatigue crack growth curve estimation method, estimation program, and estimation device that can accurately estimate the detailed state of fatigue life and crack initiation * growth of metals according to realistic phenomena. To do.
- the fatigue crack growth curve estimation method is an equivalent method for reproducing the relationship between the crack length in an actual structure and the stress intensity factors due to internal and external forces due to residual stress in a one-dimensional crack.
- Fatigue crack growth curve estimation method to estimate fatigue crack growth curve of healthy part force using distributed stress, where tip position of tensile plastic area at maximum load when cyclic load force S is applied to stress concentrated part. The first step of calculating the tensile residual deformation layer thickness, the second step of calculating the tensile residual deformation layer thickness at the minimum load of the repeated load, and calculating the tip position of the compression plastic region from the tensile residual deformation layer thickness Calculate the fatigue damage accumulation area formed in front of the crack, and the fatigue damage accumulation area force by calculating the crack increment.
- the maximum load and the minimum load when a repeated load is applied within the crystal grain.
- the tensile plastic region at the maximum load is larger than the compression plastic region at the minimum load, it is formed only in the compression plastic region, and the tensile plastic region at the maximum load is the one at the minimum load.
- the plastic strain increment formed only in the tensile plastic zone is set to '0'. The crack length from the healthy part is at the notch bottom.
- the plastic strain increment calculates the cumulative plastic strain from the plastic strain increment, and within the crystal grain, the cumulative plastic strain is specific to the material constituting the stress concentration part. If the ductility limit of the crack is reached, it is judged that the crack has transitioned to the open type, and the residual tensile layer thickness incorporated into the crack increment region is calculated outside the grain using the cumulative plastic strain. Includes the fourth step to calculate, the fifth step to calculate the yield point under the next repeated load at the next maximum load and return to the first step.
- the fatigue damage accumulation region formed in front of the crack indicates a region where the tensile plastic region determined from the tip position of the tensile plastic region overlaps with the compression plastic region determined from the tip position of the compression plastic region.
- the crack increment region indicates the region where the crack propagates by the crack increment.
- the fatigue damage accumulation region size which is a cyclic plastic region, governs the growth rate of cracks, there is no crack at all, and the healthy part force is also reduced. According to the realistic phenomenon of continuous crack growth, it is possible to accurately estimate the fatigue life of metal and the details of crack initiation and growth.
- the th threshold is not guaranteed to be kept constant under random loads, but there is no other way, so handling with the desire to remain constant under any load fluctuation is common.
- the fatigue damage accumulation area size controls the crack growth rate, and the fatigue damage accumulation area must be generated by using the fatigue damage accumulation area size. In other words, if the plastic deformation does not proceed, the crack will not progress.
- the tensile residual deformation layer thickness generated in front of the crack differs depending on the load level, and the tensile residual deformation layer thickness incorporated at the load level at which the crack enters is also different, so the crack opening displacement at the minimum load Multiplying the difference from the crack opening displacement when no contact stress is applied due to the absence of a crack closure zone by a ratio determined as a function of cumulative plastic strain is smaller or larger than the crack opening displacement at the minimum load.
- the fatigue crack growth curve estimation method of the present invention is based on the equivalent of reproducing the relationship between the crack length in an actual structure and the stress intensity factor due to internal and external forces due to residual stress in a one-dimensional crack.
- This is a fatigue crack growth curve estimation method that estimates the fatigue crack growth curve of a healthy part force using distributed stress.
- the initial value of the crack length is '0', and shear cracks and open cracks are used.
- any X-axis when the maximum load is applied Normal equivalent distributed stress acting on the X-axis and vertical equivalent distributed stress on the X-axis and the equivalent distributed stress acting on the X-axis, yield point and plastic constraint coefficient under cyclic loading
- the first step of calculating the tensile residual deformation layer thickness at the maximum load from the tip position of the tensile plastic area calculated from the above, on the X-axis when the minimum load of repeated load and the off-unit load are applied The minimum equivalent load from the normal equivalent distributed stress acting on the X axis and the equivalent distributed stress on the X axis and the equivalent distributed stress on the X axis and the tensile residual deformation layer thickness at the maximum load.
- the tensile residual deformation layer thickness at the time is calculated, and the tip position of the compression plastic zone at the minimum load is calculated from the tensile residual deformation layer thickness at the maximum load and the tensile residual deformation layer thickness distribution at the minimum load.
- 2nd step of calculating the layer thickness, the tip position of the tensile plastic zone and the compression plastic zone from the yield point and the plastic constraint coefficient under the repeated tensile load From tip position The third step of calculating the fatigue damage accumulation area formed in front of the crack, calculating the fatigue damage accumulation area force crack increment, and adding this crack increment to the crack length is the crack length from the healthy part.
- the tensile plastic area at the maximum load is the smallest in the grain due to the pair of maximum load and minimum load when the repeated load is applied.
- a load cycle that is larger than the compression plastic zone at the time of loading, it is formed only in the compression plastic zone, and in the case of a load cycle in which the tensile plastic zone at the maximum load is smaller than the compression plastic zone at the minimum load, the tensile plastic zone
- the plastic strain increment that is formed only on the surface is assumed to be '0', and outside the crystal grain, the plastic strain increment is calculated from the change in the tensile residual deformation layer thickness, the cumulative plastic strain is calculated from the plastic strain increment, and Crack length at the bottom of the notch If the grain size is larger than the grain size of the first crystal grain, the plastic strain increment is calculated from the change in tensile residual deformation layer thickness, the cumulative plastic strain is calculated from the plastic strain increment, and the cumulative plastic strain is calculated in the crystal grain.
- the fatigue crack growth can be calculated based on the fatigue damage accumulation region dimension which is a repeated plastic region generated by a pair of maximum load and minimum load, and there is no crack at all. State force that does not continue In accordance with the realistic phenomenon of continuous crack growth, considering the load history applied to the crack, the fatigue life of the metal and the details of crack initiation and growth are accurately estimated be able to.
- the fatigue crack growth curve estimation program of the present invention expresses the initial value of the crack length as '0' and the shear type crack and the open type crack as '1' or '0'.
- a computer that sets the initial value of the crack determination coefficient to '1' and stores it in the storage means.
- the vertical equivalent distributed stress on the X-axis caused by the load and the equivalent distributed stress for the residual stress acting on the X-axis and the memory means force
- the minimum residual load layer thickness at the maximum load read out at the minimum load Calculates the tensile residual deformation layer thickness at, and memorizes the memory means.Calculates the tip position of the compression plastic zone at the minimum load from the tensile residual deformation layer thickness at the maximum load and the tensile residual deformation layer thickness distribution at the minimum load.
- the tensile plastic region tip position read from the storage means and the compression plastic region tip position force, and the fatigue damage accumulation region formed in front of the crack are calculated, Damage accumulation area force Calculates the crack increment, adds this crack increment to the crack length, and stores it in the memory means.
- the crack length of the sound part force is the grain size of the first crystal grain at the notch bottom.
- the tensile plastic zone at the maximum load is larger than the compression plastic zone at the minimum load due to the pair of the maximum load and the minimum load when repeated load force is applied in the grain. Is formed only in the compression plastic zone, and is tensioned at maximum load.
- the plastic strain increment formed only in the tensile plastic region is set to '0', and the plastic strain increment is stored in the memory means outside the crystal grain. Calculate from the read change in tensile residual deformation layer thickness, calculate the accumulated plastic strain from the increment of plastic strain, and store it in the storage means, and the crack length of the healthy part will be the first crystal grain in the notch bottom.
- the plastic strain increment is calculated from the change in the tensile residual deformation layer thickness read from the storage means, the cumulative plastic strain is calculated from the plastic strain increment, and stored in the storage means. If the cumulative plastic strain has reached the inherent ductility limit of the material constituting the stress concentration part, it is determined that it has transitioned to an open crack, and the crack determination coefficient is set to '0' and stored in the storage means. Cumulative plasticity outside the grain. The fourth step of calculating the residual tensile deformation layer thickness taken into the crack increment region from the yield point and plastic constraint coefficient under strain and cyclic loading, and storing it in the memory means, the next maximum load was applied.
- the fatigue crack growth curve estimation apparatus uses a crack judgment in which the initial value of the crack length is expressed as '0', and a shear-type crack and an open-type crack are expressed as '1' or '0'.
- Storage means that stores the initial value of the coefficient as', and when the load force S is repeatedly applied to the stress concentration part, the vertical equivalent distributed stress acting on any X axis when the maximum load is applied and static From the tip position of the tensile plastic zone calculated from the vertical equivalent distributed stress on the X-axis caused by the load, the equivalent distributed stress acting on the X-axis, the yield point under repeated loads, and the plastic constraint coefficient
- the first calculation means that calculates the tensile residual deformation layer thickness at the maximum load and stores the tip position of the tensile plastic zone and the tensile residual deformation layer thickness in the storage means, and the minimum load and non-unit load of the repeated load are X-axis generated by vertical equivalent distributed stress acting on the X-axis and static load when applied Vertical equivalent distributed stress and equivalent distributed stress acting on X-axis and residual stress Memory means force Calculates the tensile residual deformation layer thickness at the minimum load from the read tensile residual deformation layer thickness at the maximum load and stores the memory means
- the tip position of the compressive plastic zone at the minimum load was calculated from the tensile residual deformation layer thickness distribution of the sample, and the yield was compressed from the tensile residual deformation layer thickness at the minimum load, the yield point under repeated loading, and the plastic constraint coefficient.
- a second residual calculation unit that calculates the tensile residual deformation layer thickness of the region and stores the tensile residual deformation layer thickness and the compression plastic region tip position in the storage unit; and the tensile plastic region tip position and the compression plastic value read from the storage unit. Calculate the fatigue damage accumulation area formed in front of the crack tip position force force crack, calculate the crack increment from the fatigue damage accumulation area, add this crack increment to the crack length, and store it in the memory means.
- the fatigue crack growth curve estimation method of the present invention can be implemented.
- the fatigue crack growth curve estimation method of the present invention is such that when a constant amplitude load is repeatedly applied to the stress concentration portion, the third step is to calculate the fatigue damage accumulation region size.
- the number of cycles required for crack increment is calculated from the crack propagation equation, and the crack length from the healthy part is notched. If the grain size is smaller than the first grain size at the bottom, the plastic strain increment is set to '0', and outside the crystal grain, the plastic strain increment is calculated from the change in the tensile residual deformation layer thickness, and the plastic strain increment is cycled.
- the cumulative plastic strain is calculated by adding the number multiplied by the number. If the crack length from the healthy part is equal to or larger than the grain size of the first crystal grain at the notch bottom, the plastic strain increment is calculated as the residual residual deformation layer thickness. Calculated from the change, the plastic strain increment is multiplied by the cycle number In addition, the cumulative plastic strain is calculated, and if the cumulative plastic strain reaches the inherent ductility limit of the material constituting the stress concentration part in the crystal grain, it is determined that the transition has been made to an open crack. With the crack judgment coefficient set to '0', the tensile residual deformation layer thickness incorporated into the crack increment region is calculated from the cumulative plastic strain, the yield point under cyclic loading, and the plastic constraint coefficient outside the crystal grain. Desire ⁇ .
- the upper limit of the crack increment that can be advanced at once in the calculation is 5% of the fatigue damage accumulation region size. Can be grown.
- the method for estimating a fatigue crack growth curve of the present invention includes a load extraction step of extracting only a load pair of a maximum load and a minimum load contributing to crack growth.
- the load extraction step allows the calculation of crack growth to be omitted for load pairs that do not contribute to crack growth.
- the load pair of the maximum load and the minimum load that contribute to the crack growth is determined by repeating the repeated load at the stress concentration part.
- the thickness distribution of the tensile residual deformation layer at the maximum load, the vertical equivalent distributed stress acting on the X axis when a unit load is applied, on the X axis caused by static load The vertical equivalent distributed stress, the equivalent distributed stress with respect to the residual stress acting on the X-axis, the yield point under cyclic loading, and the recompressed plastic zone formation load calculated from the plastic constraint coefficient, and the cyclic load on the stress concentration part
- the tensile residual deformation layer thickness distribution at the minimum load in the load process when continuously applied, vertical equivalent distributed stress acting on the X axis when an off-unit load is applied, on the X axis caused by static load It is desirable to extract the normal equivalent distributed stress, the equivalent distributed stress for the residual stress acting
- the fatigue damage accumulation region size which is a cyclic plastic region, governs the growth rate of the crack, and therefore there is no crack at all. According to a realistic phenomenon in which a crack grows continuously from a healthy part, it is possible to strictly estimate the detailed state of crack initiation and growth when the fatigue life of a metal occurs.
- a shear type crack that initially bears not only a compressive stress but also a tensile stress is formed. After crossing the first grain boundary, dislocations move in a direction inclined with respect to the slip line in the first grain, and in a direction perpendicular to the slip in the first grain due to repeated loading. Plastic strain accumulates, and this accumulated cumulative plastic strain reaches the inherent ductility limit of the material. The actual behavior of gradually changing to a shear-type crack force and an open-type crack can be reproduced by treating the position where it has changed to an open-type crack that has no tensile stress.
- the tensile residual deformation layer thickness generated in front of the crack differs depending on the load level, and the tensile residual deformation layer thickness incorporated at the load level at which the crack enters also differs.
- the difference between the crack opening displacement when the contact stress is not applied and the ratio determined as a function of the cumulative plastic strain is multiplied by the ratio at the minimum load when there is no contact stress because the crack opening area does not occur when the crack opening displacement occurs under load.
- FIG. 1 is a diagram showing an apparatus for estimating a fatigue crack growth curve in Embodiment 1 of the present invention.
- FIG. 2 is a functional block diagram of a fatigue crack growth curve estimation apparatus in Embodiment 1 of the present invention.
- FIG. 3 is a diagram showing details of processing of the fatigue crack growth curve estimation apparatus in Embodiment 1 of the present invention.
- FIG. 4 is a diagram in which the notch bottom force and the tip of the tensile plastic region are divided into n parts.
- FIG. 5 is a diagram showing a method for determining the tip position of a compression plastic region.
- FIG. 6A is a diagram showing a part of a load cycle.
- FIG. 6B is a diagram showing a stress-strain history in the cyclic plastic region in a load site where the tensile plastic region at the maximum load is larger than the compressive plastic region at the minimum load.
- FIG. 6C is a diagram for explaining the plastic strain increment occurring at the notch or crack tip position.
- FIG. 7 is a diagram showing a stress-strain history in a cyclic plastic region in a load cycle in which the tensile plastic region at the maximum load is smaller than the compressive plastic region at the minimum load.
- FIG. 8 Handling of accumulated plastic strain when a crack is propagated at a time under a constant amplitude load.
- FIG. 9A is a diagram showing a linear assumption of a dimensional change in compression plastic zone between ⁇ c.
- FIG. 9B is a diagram showing a linear assumption of a change in compressive plastic strain between ⁇ c.
- FIG. 10 is a diagram showing plastic shrinkage released during crack growth.
- FIG. 11 is a diagram showing a configuration for extracting only load pairs that contribute to crack growth.
- FIG. 1 is a fatigue crack growth curve estimation apparatus according to Embodiment 1 of the present invention.
- the fatigue crack growth curve estimation device 1 includes an input means 2, a storage means 3, an output means 4, and a medium. And a central processing means 10.
- the input means 2 is used to input initial values of parameters necessary for calculation using, for example, a keyboard or a pointing device. Further, the input means 2 may be configured to input by reading a file in which initial values of parameters necessary for calculation are input.
- the central processing calculation means 10 performs calculation by a central processing unit of a computer.
- the storage means 3 temporarily stores parameter values calculated by the central processing calculation means 10 on, for example, a node disk or a computer memory.
- the output means 4 outputs the parameter values calculated by the central processing means 10 to the recording medium.
- the output means 4 records electronic data on a recording medium such as a flexible disk, a node disk, or a CD-ROM.
- the output means 4 can also be configured to output to a recording medium such as paper or sheet by an image forming apparatus or the like.
- FIG. 2 is a functional block diagram of the central processing calculation means 10 of FIG.
- the central processing calculation means 10 includes a first calculation means 11, a second calculation means 12, a third calculation means 13, a fourth calculation means 14, and a fifth calculation. Means 15 are provided.
- the first calculation means 11 calculates the tip position of the tensile plastic zone at the maximum load, calculates the tensile residual deformation layer thickness at the maximum load from the tip position of the tensile plastic zone, The tensile residual deformation layer thickness at the time of loading is stored in the storage means 3.
- the tensile residual deformation layer thickness at the minimum load is calculated from the tensile residual deformation layer thickness at the maximum load read from the storage means 3, and the maximum load and the minimum load are calculated.
- the tip position of the compressive plastic zone at the minimum load is calculated from the tensile residual deformation layer thickness distribution of the material, the yield point force at that time is calculated, and the tensile residual deformation layer thickness in the compression yielding region is calculated.
- the tip position of the compression plastic zone is stored in the storage means 3.
- the third calculation means 13 calculates the fatigue damage accumulation region formed in front of the crack from the tensile plastic region tip position and the compression plastic region tip position read from the storage means 3, and the fatigue damage accumulation region force is also calculated.
- the crack increment is calculated, this crack increment is added to the crack length, and the crack length is stored in the memory means 3.
- the crack length from the healthy part read from the storage means 3 is the notch bottom. If it is smaller than the grain size of the first crystal grain, the plastic strain increment is set to '0' within this grain. Further, outside the crystal grains, the plastic strain increment is calculated from the change in the tensile residual deformation layer thickness read from the storage means 3, and the cumulative plastic strain is calculated from the plastic strain increment and stored in the storage means 3. Furthermore, if the crack length from the sound part read from the storage means 3 is equal to or larger than the grain size of the first crystal grain at the notch bottom, the plastic strain is determined by the change in the tensile residual deformation layer thickness read from the storage means 3.
- the increment is calculated, the cumulative plastic strain is calculated from the plastic strain increment, and stored in the storage means 3. If the cumulative plastic strain read from the storage means 3 reaches the inherent ductility limit in the first crystal grain from the sound part, the crack judgment coefficient at that position is set to '0' and stored in the storage means 3. To do. Further, the tensile residual deformation layer thickness taken into the region of the crack increment from the accumulated plastic strain outside the crystal grain is calculated and stored in the storage means 3.
- the fifth computing means 15 computes the yield point under the next repeated load at the next maximum load from the crack judgment coefficient and the equivalent stress distribution, stores the yield point in the memory means 3, and then stores the first Return to computing means 11.
- FIG. 3 is a flowchart showing details of processing of the central processing calculation means 10 of FIG. A method for estimating a fatigue crack growth curve according to Embodiment 1 of the present invention will be described with reference to FIG.
- step S101 the tip position of the tensile plastic region at the maximum load and the tensile residual deformation layer thickness are calculated (step S101).
- step S102 the tensile residual deformation layer thickness at the minimum load, the tip position of the compression plastic region, and the tensile residual deformation layer thickness in the compression yield region are calculated (step S102).
- step S103 it is determined whether or not the crack length obtained in step S103 is greater than or equal to the initial crystal grain size (step S104). If it is larger than the initial crystal grain size, the process proceeds to step S107, and if not, the process proceeds to step S105 and step S106.
- step S104 when determining whether or not the crack length is equal to or larger than the initial crystal grain size, 1Z2 of the grain size of the first crystal grain may be used as a threshold value.
- step S105 the plastic strain increment is set to '0' in the first crystal grain.
- step S106 the plastic strain increment is calculated outside the first crystal grain, and the cumulative plastic strain is calculated from the plastic strain increment. After step S105 and step S106 are completed, the process proceeds to step S111.
- step S107 the plastic strain increment is calculated, and the cumulative plastic strain is calculated from the plastic strain increment.
- step S111 the yield point under the repeated load at the next maximum load is calculated. Then, the process proceeds to step S101.
- the first calculating means 11 calculates the tensile plastic region tip position at the maximum load and the tensile residual deformation layer thickness (step S101).
- X is the distance from the notch bottom where the shear crack is projected onto the main crack surface. Since the equivalent distributed stress is used, the bullet in front of the crack at the time of the crack of each size is used. The sexual stress distribution is guaranteed to be the same as that in front of the crack being evaluated (eg the deepest part of the surface crack).
- Figure 4 shows the n-division of the notch bottom force and the tip of the tensile plastic zone.
- n between 0 and a, and the value of S (x) at branch point X is S, i i m R expressed as the sum of s (x) and s (x)
- the second computing means 12 computes the tensile residual deformation layer thickness at the minimum load, the compression plastic region tip position, and the tensile residual deformation layer thickness that has undergone compression yielding (step S102).
- Equation (7) is the value of the minimum load
- L (Xj) in Equation (8) is the effective tensile residual deformation layer thickness at the maximum load immediately before. Therefore, the equations (7) and (8) are equalized and arranged into an equation for ⁇ and convergent calculation is performed using the Gauss-Siel method to obtain the distribution of working stress at the minimum load. In this convergence process,
- FIG. 5 shows a method for determining the tip position of the compression plastic region.
- the intersection point between the curve of the elastic residual shrinkage corresponding to the compressive yield and the crack opening displacement V (x) at the minimum load is the tensile residual deformation layer thickness L) at the maximum load.
- the tip position of the compression plastic zone at the minimum load is ⁇ -.
- the third calculation means 13 calculates a fatigue damage accumulation area formed in front of the crack, calculates a crack increment from the fatigue damage accumulation area size, and adds it to the crack length (step S103).
- the crack increment ⁇ c is calculated when the load amplitude changes every cycle.
- the upper limit of the crack increment that can be advanced at once in the calculation is 5% of the fatigue damage accumulation region.
- the fourth computing means 14 first determines whether or not the crack length obtained in step S103 is larger than 1Z2 of the first crystal grain diameter (step S104).
- Figure 6A shows a portion of the load cycle.
- Figure 6B shows that the tensile plastic area at maximum load is the best. It shows the stress-strain history in the repetitive plastic region during the load cycle, which is larger than the compressive plastic region at the time of small load.
- Fig. 6C shows a diagram explaining the increment of plastic strain occurring at the notch or crack tip position.
- Fig. 7 shows the stress-strain history in the cyclic plastic zone in a load cycle in which the tensile plastic zone under load is smaller than the compression plastic zone at the minimum load.
- a compression plastic zone is formed at P, and the positions of X and X are mini 1 2
- the cyclic plastic energy received in one cycle is given by the product of the plastic strain increment and the swing elastic stress amplitude in a completely plastic body.
- Figure 6B shows tension at maximum load.
- the force shown for the stress-strain history in the repetitive plastic zone in a load cycle in which the plastic zone is larger than the compression plastic zone at the minimum load In this case, as shown in Fig. Entering force
- the strain increment to the minimum load corresponds.
- the tensile plastic zone at the maximum load is smaller than the compressive plastic zone at the minimum load V, and in the load cycle, as shown in Fig. 7, the tensile yield yields and the strain increase corresponds to the maximum load.
- the strain difference between '14' and '15' can be defined as the plastic strain increment in one cycle.
- this plastic strain increment distribution is formed only in the compression plastic zone at the minimum load, becomes "0" at the tip, and becomes larger as it approaches the crack tip.
- V (x) at the minimum load based on the crack opening displacement V (x) at the minimum load,
- N is the number of cycles ("1" under variable load).
- the cumulative plastic strain is obtained by the following method.
- the upper limit of the crack increment that can be advanced at once in the calculation is fatigue damage
- the number of cycles can be obtained by the equation, and the cumulative plastic strain can also be obtained by the equation (16).
- FIG. Figure 8 shows the handling of cumulative plastic strain when a crack propagates at a time under a constant amplitude load.
- Figures 9A and 9B show linear assumptions of the fatigue damage accumulation area dimensional change and plastic strain change between Ac.
- the upper limit of the crack increment that can be advanced at once in the calculation is considered to be 5% of the fatigue damage accumulation region.
- the plastic strain increment in one cycle in the previous fatigue damage accumulation region is expressed as ⁇ ⁇ pi
- ⁇ 2 is ⁇ ⁇ 2 ( ⁇ ) (2 1)
- the fourth calculation means 14 determines that the crack length is smaller than 1Z2 of the first crystal grain size in step S104, the plastic strain increment is set to "0" in the first crystal grain. (Step S105). Further, outside the first crystal grain, the cumulative plastic strain is calculated from the plastic strain increment by the method described above (step S106).
- the plastic strain incremental force cumulative plastic strain is calculated by the method described above. Calculate (step S107).
- the thickness of the tensile residual deformation layer taken in the crack increment region is determined from the cumulative plastic strain outside the first crystal grain (step S110).
- the load level at which the crack closes depends on whether the moment when the crack enters the repeated plastic zone is near the minimum load or near the maximum load.
- Figure 10 shows the crack opening displacement V 'when the crack reaches the minimum load without propagating and the crack at the minimum load when the crack propagates A c without forming the crack closure zone.
- the opening displacement V " is shown schematically.
- i I is the thickness of the tensile residual deformation layer formed at the previous maximum load
- ⁇ V'-V ".
- the tensile residual deformation layer thickness actually taken into the actual crack is considered to be between V ' ⁇ and V ⁇ , and the ratio is It is considered to be proportional to the cumulative plastic strain ⁇ (X) received until the occurrence.
- ⁇ is a plastic shrinkage coefficient and a material constant.
- the fifth calculating means 15 calculates the yield point under the repeated load at the next maximum load (step S111). [0095] When the transition from the mixed displacement type, in which the shear type crack and the open type crack are mixed, to the open type in the crack part in the first crystal grain, the crack part does not receive the tensile stress. When the tensile plastic zone tip position a grows beyond the past tensile plastic zone tip position a 'at the maximum load,
- ⁇ is the yield point at the previous minimum load. In this case, the yield point rises by work hardening due to the proportional limit, and the yield point ⁇ is the past unless the yield point ⁇ exceeds the static yield point ⁇ .
- step S111 After obtaining the yield point under the next cyclic load in step S111, the process returns to step S101, but when the yield point ⁇ obtained in equation (28) is greater than the static yield point ⁇
- the crack opening displacement V is obtained from Equation (29).
- the tensile residual deformation layer thicknesses at other positions are maintained at the previous ones.
- the tip position ⁇ + of the tensile plastic zone is a.
- the region tip position a is located within the past tensile plastic region tip position a '. In this case as well, when returning to step S 101, the distribution of working stress at maximum load is calculated using Equation (22). It is necessary to obtain the crack opening displacement.
- Tensile plastic zone tip position a is located in the past tensile plastic zone tip position a '
- FIG. Figure 11 shows a configuration that extracts only load pairs that contribute to crack growth.
- step S101 a recompressed plastic zone forming load P is obtained in step S201.
- CPG is obtained by the following method.
- V (c c ) (l + ⁇ -) L (c c ) (3 3)
- the obtained P is the recompressed plastic zone forming load P
- This load pair is called an invalid load pair, and the other load pairs are called effective load pairs. If it is the maximum load that deviates between these loads first, if the load is greater than the past maximum load P, the process returns to step S101 and the virtual crack is
- step S101 If it is (A in Fig. 11), the process returns to step S101, and the virtual crack tip position a and the tensile plastic zone tip ⁇ + are found again. In addition, when the minimum load is the first to deviate between these loads (the process proceeds in the middle of FIG. 11).
- step S102 a re-tensile plastic zone forming load ⁇ ⁇ ⁇ is obtained in step S202.
- PG is obtained by the following method.
- V (c c ) (l- ⁇ -) L (c c ) (3 4)
- the obtained P is the re-tension plastic zone forming load P
- the state force without any cracks can follow the realistic phenomenon that the crack grows continuously. It is possible to estimate the shape of the crack and the state of growth for each cycle, and it is possible to precisely estimate the fatigue life of metal and the detailed state of crack growth.
- the re-tension plastic zone formation load and the recompression plastic zone formation load are obtained, and these values are used as threshold values to extract only load pairs that contribute to crack initiation and growth. If no tension plastic zone or compression plastic zone occurs, the load does not grow a crack, and the calculation for crack growth can be omitted as a load.
- the fatigue crack growth curve is estimated by quadratic approximation.
- V (xj) P ( ⁇ ,. "+ b t x + c ( . Xj, ⁇ , ⁇ ) dx
- a fatigue life of a metal is determined according to a realistic phenomenon in which a crack of zero size, that is, a state force without any crack, continuously grows in a healthy part. Occurrence of cracks * It is possible to estimate precisely the details of growth. Therefore, the fatigue life of a new structure can be predicted quantitatively at the design stage, contributing to the prevention of fatigue accidents of the structure, and the remaining life of existing structures can be diagnosed accurately and quantitatively. It is possible to formulate a maintenance inspection plan and drastically reduce excessive maintenance inspection expenses so far, so that all equipment and structures (e.g. highway structures, ships, power generation equipment, bridges, towers, It can be used to estimate the life of automobiles, aircraft, civil engineering machinery, steelmaking machinery, etc.).
- equipment and structures e.g. highway structures, ships, power generation equipment, bridges, towers, It can be used to estimate the life of automobiles, aircraft, civil engineering machinery, steelmaking machinery, etc.
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EP05765202A EP1785716A4 (en) | 2004-07-09 | 2005-06-30 | METHOD OF ESTIMATING THE GROWTH CURVE OF A MALICIBILITY CRACK, ESTIMATED PROGRAM AND ESTIMATED APPROACH |
US11/631,863 US7480573B2 (en) | 2004-07-09 | 2005-06-30 | Fatigue crack growth curve estimation method, estimation program, and estimation device |
JP2006528791A JP4378506B2 (ja) | 2004-07-09 | 2005-06-30 | 疲労き裂成長曲線の推定法、推定プログラムおよび推定装置 |
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US7480573B2 (en) | 2009-01-20 |
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