JP5267391B2 - Method and apparatus for evaluating fatigue strength of casting material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method and a device for accurately evaluating the fatigue strength of a casting material having internal defects. <P>SOLUTION: An S-N diagram is produced in advance based on results of a fatigue test performed on a plurality of prepared test pieces of a casting material. Each of internal defects of the respective test piece is modeled as a semi-circular or circular crack and a specific crack length a<SB>0</SB>thereof is calculated. An estimated fatigue limit &sigma;<SB>w</SB>of each test piece is calculated based on the radium a<SB>eq</SB>and the specific crack length a<SB>0</SB>of the semi-circular or circular crack by the formula (2) &sigma;<SB>w</SB>=&sigma;<SB>w0</SB>[a<SB>eq</SB>/(a<SB>eq</SB>+a<SB>0</SB>)]<SP>1/2</SP>. A stress amplitude &sigma;<SB>a</SB>of the S-N diagram is divided by the estimated fatigue limit &sigma;<SB>w</SB>to produce an S-N diagram in consideration of internal defects. In the formula, &sigma;<SB>w0</SB>is a fatigue limit of a material with no defect. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to a method and an apparatus for evaluating the fatigue strength of a casting material suitable for designing the fatigue strength of a structure using a material such as a casting material having inherent defects.

  In the case of a member having a complicated shape or a large-sized member, a casting material is generally used instead of machining or welding because of ease of production and manufacture.

  Casting materials have the advantage of being easy to manufacture and manufacture because any shape can be obtained by pouring the metal into the mold, but impurities and air are concentrated during the metal cooling process. There is a problem that an area that cannot be solidified, that is, an intrinsic defect occurs. This is an unavoidable problem in manufacturing. In particular, when the mass of a casting material to be manufactured increases, it is inevitable that an intrinsic defect is included.

  A material such as a casting material in which an inherent defect exists has a lower fatigue strength than a material in which the inherent defect does not exist or is extremely small. Therefore, when evaluating the material strength such as fatigue in the cast material, it is necessary to perform an evaluation in consideration of the inherent defects.

  Conventionally, when evaluating the fatigue strength of a casting material having such an internal defect, a fatigue test was performed by preparing a test piece made of a casting material in the same manner as a normal material and conducting a fatigue test. The fatigue strength of the casting material was evaluated using an evaluation diagram (SN diagram). As an example, FIG. 10 shows an SN diagram when a fatigue test is performed using a round bar test piece made of a cast material.

More specifically, in the conventional method, as shown by a broken line in FIG. 10, an evaluation diagram that envelops all the plots (plots of stress amplitude σ _a and repetition number N _f ) obtained in the fatigue test is shown. The fatigue strength of the casting material was evaluated (or designed) using this evaluation diagram.

  As prior art document information related to the invention of this application, there are Patent Document 1 and Non-Patent Documents 1 to 3.

JP 2005-8913 A

Murakami, Yoshihiro Endo, “Effects of hardness and crack shape on the lower limit stress intensity factor width ΔKth of microcracks”, Japan Society for Materials Science, Materials, Vol. 35, No. 395, p. 911-917 Murakami Y., Aoki S., Hasebe N., Itoh Y., Miyata H., Miyazaki N., Terada H., Tohgo K., Toya M. and Yuuki R. ed., `` STRESS INTENSITY FACTORS HANDBOOK '', Pergamon Press., 1987 Keisuke Tanaka, “Propagation of Micro Fatigue Crack”, Japan Society for Materials Science, Materials, Vol. 33, No. 371, 1984, p. 961-972

  However, in the test piece made of a casting material, there is an internal defect having a different size for each test piece. Therefore, in the SN diagram prepared without considering the internal defect, the internal defect of each test piece is obtained. The fatigue strength variation (variation with respect to the evaluation parameters (stress and strain) obtained by structural analysis or the like) becomes very large depending on the magnitude of.

  Therefore, when an evaluation diagram that envelops all such variations is used, the evaluation becomes excessively safe, and the evaluation criteria (evaluation criteria) cannot be clearly set as absolute values. . In other words, the set evaluation diagram itself uses an unclear safety factor, and the basis for setting the evaluation diagram is unclear, so this evaluation diagram is used to include internal defects such as casting materials. When the fatigue strength of the structure to be evaluated is evaluated, there is a possibility that excessive safety evaluation may be performed particularly when the SN diagram has a large variation.

  In addition, as a method for evaluating the fatigue strength of a structure containing a built-in defect such as a casting material, a method of performing a relative evaluation by comparison with an existing product is also conceivable, but in this method, a conventional machine (existing product) is considered. As a result of the relative comparison, the new machine is developed with the same or higher strength as the conventional machine. Therefore, the work such as structural analysis is more than doubled, and there is a problem that the accuracy is low.

  Therefore, an object of the present invention is to solve the above-mentioned problems, suppress variation in the SN diagram, and accurately evaluate the fatigue strength of a casting material having an inherent defect, To provide an apparatus.

The present invention was devised in order to achieve the above object, and is a method for evaluating the fatigue strength of a casting material having an inherent defect, wherein a plurality of test pieces of the casting material are prepared and subjected to a fatigue test. From the test results, an SN diagram, which is a relationship between the stress amplitude σ _a and the number of repetitions N _f , is prepared. On the other hand, the internal defect of each test piece is a semi-ellipse considering the size of the internal defect. Alternatively, the radius a _eq of the semi-circular or circular crack in which the maximum stress intensity factor is equal to that of the semi-elliptical or elliptical crack is obtained by an elliptical crack, and the inherent defect of each specimen is While modeling as a semi-circle or circular crack, Equation (1) shown in [Equation 1]

Is used to calculate the natural crack length a ₀ , which is the crack length that does not affect the fatigue limit, and the semicircular or circular crack radius a _{eq of} each of the obtained specimens and the specific crack Based on the crack length a ₀ , the formula (2) shown in [Expression 2]

The estimated fatigue limit σ _w of each specimen is calculated by the above, and the stress amplitude σ _a of the SN diagram created in the fatigue test is divided by the estimated fatigue limit σ _w of each specimen obtained. Then, an SN diagram in consideration of the internal defect, which is a relationship between the stress amplitude σ _a / the estimated fatigue limit σ _w and the number of repetitions N _f , is created, and the created SN diagram in consideration of the internal defect is used. This is a method for evaluating the fatigue strength of a casting material for evaluating the fatigue strength of the casting material.

  The semi-elliptical or elliptical crack may have a major axis and a minor axis determined from the size of the inherent defect in the fracture surface of each test piece.

Further, the present invention is an apparatus for evaluating the fatigue strength of a casting material having an inherent defect, and the stress amplitude σ _a and the number of repetitions N _f are determined from the results of a fatigue test performed by producing a plurality of test pieces of the casting material. The S—N diagram creation unit for creating a related S—N diagram, and the internal defects of each test piece are represented by a semi-ellipse or an elliptical crack in consideration of the dimensions of the internal defects, A semicircle or circular crack radius a _{eq in} which the maximum stress intensity factor is equal to an ellipse or an elliptical crack is obtained, and the internal defect of each specimen is modeled as the semicircular or circular crack. Along with Equation (1) shown in [Equation 3]

Is used to calculate the natural crack length a ₀ , which is the crack length that does not affect the fatigue limit, and the semicircular or circular crack radius a _{eq of} each of the obtained specimens and the specific crack Based on the crack length a ₀ , the formula (2) shown in [Expression 4]

The fatigue limit calculation unit for calculating the estimated fatigue limit σ _w of each test piece by the above, and the estimated fatigue limit σ _w of each test piece obtained by the fatigue limit calculation unit, the SN prepared in the fatigue test by dividing the stress amplitude sigma _a of the diagram, modified S-N line to create a S-N diagram in consideration of the inherent defect is the relationship of the stress amplitude sigma _a / estimated fatigue limit sigma _w and the number of repetitions N _f And a fatigue strength evaluation unit that evaluates the fatigue strength of the casting material using an SN diagram that takes into account the inherent defects created by the modified SN diagram creation unit. It is a fatigue strength evaluation apparatus of a casting material.

  ADVANTAGE OF THE INVENTION According to this invention, the fatigue strength evaluation method and apparatus of a casting material which can suppress the dispersion | variation of a SN diagram and can evaluate the fatigue strength of the casting material which has an internal defect accurately can be provided.

It is a functional block diagram of the fatigue strength evaluation apparatus of the casting material used for the fatigue strength evaluation method of the casting material which concerns on one embodiment of this invention. It is a flowchart of the fatigue strength evaluation method of the casting material which concerns on one embodiment of this invention. In this invention, it is explanatory drawing explaining the dimension of an intrinsic defect. In this invention, it is a figure which shows an example of the SN diagram which an SN diagram preparation part produces. (A), (b) is explanatory drawing explaining modeling an intrinsic defect to a semi-ellipse or an elliptical crack in this invention. (A), (b) is explanatory drawing explaining modeling the internal defect modeled by the semi-ellipse or the crack of an ellipse into a semicircle or a circle crack further in this invention. It is a figure which shows the relationship between a crack growth rate and a stress intensity factor range. It is a figure which shows the relationship with crack length a when lower limit stress range (DELTA) (sigma) _th . In this invention, it is a figure which shows an example of the SN diagram and evaluation diagram which considered the intrinsic defect. It is a figure which shows the SN diagram and evaluation diagram which were used for the fatigue strength evaluation method of the conventional casting material.

  Preferred embodiments of the present invention will be described below with reference to the accompanying drawings.

  First, a cast material fatigue strength evaluation apparatus used in the cast material fatigue strength evaluation method according to the present embodiment will be described.

As shown in FIG. 1, the cast material fatigue strength evaluation apparatus 1 is a fatigue test result (stress amplitude σ _a and number of repetitions N _f ) of _a plurality of cast material test pieces (cast steel test pieces). The relationship between the stress amplitude σ _a and the number of repetitions N _f from the input part 2 for inputting the size of the inherent defect of each specimen and the name of the material used for the specimen, and the fatigue test result inputted in the input part 2 The SN diagram creation unit 3 for creating the SN diagram, the material data storage unit 4 for storing the material data of the material used for the test piece, and the test piece input by the input unit 2 A fatigue limit calculation unit 5 for calculating an estimated fatigue limit σ _w of each test piece based on the size of the intrinsic defect and the material data stored in the material data storage unit 4, and each test piece obtained by the fatigue limit calculation unit 5 The stress amplitude σ _a of the SN diagram created by the SN diagram creation unit 3 is divided by the estimated fatigue limit σ _w of Thus, a modified SN diagram is created to create an SN diagram (modified SN diagram) that takes into account the inherent defects that are the relationship between the stress amplitude σ _a / the estimated fatigue limit σ _w and the number of repetitions N _f A modified SN chart storage section 7 for storing an SN chart in consideration of the intrinsic defect created by the section 6, the modified SN chart creation section 6, and a modified SN chart storage section 7. A fatigue strength evaluation unit 8 that evaluates the fatigue strength of the casting material, and a fatigue strength evaluation result performed by the fatigue strength evaluation unit 8 are monitored using an SN diagram that takes into account the stored internal defects. And an output unit 9 for outputting to a display.

  These are: input unit 2, SN diagram creation unit 3, material data storage unit 4, fatigue limit calculation unit 5, modified SN diagram creation unit 6, modified SN diagram storage unit 7, fatigue strength evaluation The unit 8 and the output unit 9 are realized by appropriately combining an interface, a memory, a CPU, software, and the like.

The material data storage unit 4 stores material-specific data such as a lower limit stress intensity factor range ΔK _th and Vickers hardness Hv as material data.

The fatigue limit calculation unit 5 models the internal defect of each specimen as a semi-circle or circular crack to obtain the radius a _eq of the semi-circle or circular crack, and the crack length that does not affect the fatigue limit. calculating a specific crack length a ₀ is a, the radius a _eq semicircular or circular crack of the specimen, based on the specific crack length a _0, the estimated fatigue limit sigma _w of each specimen Is configured to calculate. A detailed procedure for calculating the estimated fatigue limit σ _w will be described later.

  Next, the method for evaluating the fatigue strength of the casting material according to the present embodiment will be described in detail together with the operation of the fatigue strength evaluation apparatus 1 for the casting material.

As shown in FIG. 2, in the method for evaluating fatigue strength of a casting material according to the present embodiment, first, a plurality of test pieces of the casting material are produced and a fatigue test is performed (step S1). In the present embodiment, a plurality of round bar test pieces made of a cast material were produced and subjected to a tensile and compression fatigue test using an axial force, and the stress amplitude σ _a and the number of repetitions N _f were measured for each test piece.

  Moreover, the dimension of the internal defect exposed to the torn surface when each test piece fractures in the fatigue test is measured for each test piece. Although the intrinsic defect actually has a three-dimensional shape, the largest internal defect included in the test piece is exposed on the fracture surface, so in this embodiment, the largest intrinsic defect size on the fracture surface is set. It was decided to measure.

  As shown in FIG. 3, the inherent defect 31 generally has a complicated shape. Therefore, in the present embodiment, the length d in the longest direction (referred to as the length direction) in plan view of the inherent defect 31 is The width w in the direction perpendicular to the vertical direction was measured.

Thereafter, the stress amplitude σ _a and the number of repetitions N _f of each test piece obtained in the fatigue test, the dimensions (length d and width w) of the inherent defect of each test piece, and the material used for the test piece are input to the input unit 2. A name is input (step S2).

When each input value is input to the input unit 2, the SN diagram creation unit 3 generates an SN diagram based on the stress amplitude σ _a and the repetition number N _f of each test piece input to the input unit 2. Create (step S3). An example of the SN diagram created by the SN diagram creation unit 3 is shown in FIG. In the SN diagram shown in FIG. 4, since the inherent defects of each test piece are not taken into consideration, the variation is very large.

  Thereafter, the fatigue limit calculation unit 5 models the internal defect into a semi-elliptical or elliptical crack based on the dimensions (length d and width w) of the internal defect input to the input unit 2 (step S4). Specifically, as shown in FIGS. 5 (a) and 5 (b), when there is an internal defect (defect) on the surface 51 of the test piece, there is a semi-elliptical crack 52, and there is an internal defect inside the test piece. The case is modeled as an elliptical crack 53. At this time, the major axis 2a of the semi-elliptical or elliptical crack is the length d of the intrinsic defect, and the minor axis 2c is the width w of the intrinsic defect. The crack is set (see the broken line in FIG. 3). Thereby, a safe design is possible.

  After modeling the intrinsic defect into a semi-elliptical or elliptical crack, the fatigue limit calculation unit 5 further models the semi-elliptical or elliptical crack into a semi-circular or circular crack (step S5).

  Specifically, first, the fatigue limit calculation unit 5 calculates the stress intensity factor K in the set semi-elliptical or elliptical crack by the equation (3) shown in [Equation 5].

In the equation (3), F ₁ is a coefficient depending on the shape and position of the crack and the target part (here, round bar test piece), and this coefficient F ₁ is determined by the major axis 2a and the minor axis 2c. It is a value considering the ratio of. The value of the coefficient F ₁ is stored in advance in the material data storage unit 4 or input to the input unit 2. Note that since Equation (3) is described in detail in Non-Patent Document 2, description thereof is omitted here.

After calculating the stress intensity factor K, the fatigue limit calculator 5 shows the radius a _eq of the semi-elliptical or elliptical crack and the semi-circular or circular crack where the maximum stress intensity factor is equal to [ _Equation 6]. It calculates | requires by Formula (4).

In Formula (4), F ₂ is a coefficient when the crack shape is a semicircle or a circle, and is different from the coefficient F ₁ when the crack shape is a semi-ellipse or an ellipse. The value of the coefficient F ₂ is stored in advance in the material data storage unit 4 or input to the input unit 2.

As described above, the inherent defect of each test piece is modeled as a semicircle or circular crack with a radius _aeq . As shown in FIGS. 6 (a) and 6 (b), when there is an internal defect (defect) on the surface 61 of the test piece, a semi-circular crack 62, and when there is an internal defect inside the test piece, Modeled into a crack 63.

By modeling the internal defect of each specimen as a semi-circle or circular crack, it is possible to have only one radius a _eq parameter for the size of the internal defect and simplify the model of the internal defect Is possible.

After modeling the internal defects of each specimen as a semi-circle or circular crack, the fatigue limit calculation unit 5 is an inherent crack length that is a crack length that does not affect the fatigue limit even if it exists in a smooth material. a ₀ is calculated (step S6).

Specifically, the fatigue limit calculation unit 5 first determines the lower limit stress intensity factor range ΔK _th and the Vickers hardness Hv from the material data storage unit 4 based on the material name of the test piece input by the input unit 2. To get.

Thereafter, the natural crack length a ₀ is calculated using the equation (1) shown in [Equation 7].

In general, the fatigue limit σ _w0 of a material having no inherent defect (defect-free material) can be expressed as 1.6 times the Vickers hardness Hv (σ _w0 = 1.6 Hv), so the lower limit stress in the equation (1) The range Δσ _w0 is obtained by further doubling the fatigue limit σ _{w0 of a} material having no intrinsic defect.

Expression (1) is a relational expression that is established when there is a marginal dimension that does not affect the fatigue limit even if there is an intrinsic defect, and the crack length a ₀ at that time is defined as the intrinsic crack length.

After that, the fatigue limit calculation unit 5 uses the equation (2) shown in [Equation 8] based on the semicircular or circular crack radius a _eq and the inherent crack length a _{0 of} each obtained test piece.

Thus, the estimated fatigue limit σ _w of each test piece is calculated (step S7). In equation (2), since the fatigue limit σ _w0 and the inherent crack length a ₀ of the material having no inherent defect are values inherent to the material, the estimated fatigue limit σ _w is the radius a _{eq of the} semicircular or circular crack. It turns out that it depends only on.

  Here, the basis of Expression (2) will be described.

FIG. 7 shows the relationship between the crack growth rate da / dn and the stress intensity factor range ΔK. As shown in FIG. 7, when the stress intensity factor range ΔK is smaller than the lower limit stress intensity factor range ΔK _th , the crack does not propagate even if stress is repeatedly applied.

That is, even if stress is repeatedly applied to the intrinsic defect, if the stress intensity factor range ΔK using the stress intensity factor K obtained by the above equation (4) is smaller than the lower limit stress intensity factor range ΔK _th , This means that the crack does not propagate even if there is a defect. Therefore, it can be said that the lower limit stress range Δσ _th corresponding to the lower limit stress intensity factor range ΔK _th corresponds to the fatigue limit of a material having an inherent defect.

As described in Non-Patent Document 3, the relationship between the lower limit stress range Δσ _th and the crack length a can be represented by a curve shown in FIG. 8 (FIG. 8 is cited from Non-Patent Document 3). A curve indicated by a solid line in FIG. 8 is expressed by Expression (5) shown in [Equation 9].

In the present embodiment, the crack length a in the equation (5) corresponds to the radius a _eq of the semicircle or circular crack, so a = a _eq and the lower limit stress range on the left side is the amplitude (fatigue limit). (5) becomes Equation (6) shown in [Equation 10].

When σ _w0 in equation (6) is _shifted to the right side, equation (2), which is an estimation equation of the fatigue limit when any inherent defect exists, is derived.

That is, in the present embodiment, the underlying defect is regarded as a crack, and the lower limit stress at an arbitrary crack length is calculated from the relational expression between the crack size and the lower limit stress. Since this lower limit stress can be considered as the fatigue limit of a defect material (casting material), the estimated fatigue limit σ _w taking into account the dimensional effect of the inherent defect should be estimated using the estimation formula of Equation (2). Is possible.

Returning to FIG. 2, after calculating the estimated fatigue limit σ _w of each test piece, the modified SN diagram creation unit 6 uses the estimated fatigue limit σ _w of each test piece obtained by the fatigue limit calculation unit 5 to calculate S By dividing the stress amplitude σ _a (vertical axis) of the SN diagram created by the −N diagram creation unit 3, the vertical axis is made dimensionless, and the stress amplitude σ _a / the estimated fatigue limit σ _w and the number of repetitions An SN diagram is created in consideration of the intrinsic defect which is the relationship of N _f (step S8). FIG. 9 shows an example of the SN diagram in consideration of the intrinsic defect created by the corrected SN diagram creation unit 6.

  As shown in FIG. 9, the SN diagram in consideration of the intrinsic defects is suppressed in variation compared with the SN diagram (see FIG. 4) in which the intrinsic defects are not considered. The SN diagram in consideration of the intrinsic defect created by the modified SN diagram creation unit 6 is stored in the modified SN diagram storage unit 7.

  Thereafter, the fatigue strength evaluation unit 8 evaluates the fatigue strength of the casting material using the SN diagram in consideration of the inherent defects stored in the corrected SN diagram storage unit 7 (step S9). Specifically, the fatigue strength evaluation unit 8 is an evaluation diagram (a straight line indicated by a broken line in FIG. 9) obtained by multiplying a SN diagram (a straight line indicated by a solid line in FIG. 9) in consideration of an inherent defect by a predetermined safety factor. ) And evaluate (or design) the fatigue strength of the casting material using this evaluation diagram. The evaluation diagram is created so as to envelop all the plots. The fatigue strength evaluation result performed by the fatigue strength evaluation unit 8 is output to a display device such as a monitor via the output unit 9.

In addition, when obtaining stress (for example, 10 ⁶ times fatigue strength, 10 ⁷ times fatigue strength, etc.) at a predetermined number of repetitions N _f of the casting material using the created evaluation diagram, the fatigue strength evaluation unit 8 Then, the value of the stress amplitude σ _a / the estimated fatigue limit σ _w corresponding to the predetermined number of repetitions N _f is obtained, and the stress amplitude σ _a is calculated backward from this value.

In this case, as the radius a _eq used when calculating the estimated fatigue limit σ _w , for example, the radius a _eq is calculated by measuring the dimensions (length d, width w) of inherent defects in a large number of specimens. , and performs a statistical processing of the calculated radius a _eq calculates the probability distribution, it is sufficient to use a radius a _eq for a given probability can be determined that a sufficiently safe side (e.g. the probability of about 99%) . That is, as the radius a _eq used when calculating the estimated fatigue limit σ _w , a value that can be determined to be sufficiently safe may be used.

As described above, in the method for evaluating the fatigue strength of a casting material according to the present embodiment, a plurality of test pieces of the casting material are produced and subjected to a fatigue test. From the fatigue test result, the stress amplitude σ _a and the number of repetitions are determined. An SN diagram, which is the relationship of N _f , is prepared, and on the other hand, the inherent defect of each specimen is modeled as a semicircle or circular crack, and the inherent crack length a ₀ is calculated and obtained. The estimated fatigue limit σ _w of each specimen is calculated according to the above equation (2) based on the semicircular or circular crack radius a _{eq of} each specimen and the inherent crack length a _0. By dividing the stress amplitude σ _a of the SN diagram prepared in the fatigue test by the estimated fatigue limit σ _w of each test piece, the stress amplitude σ _a / the estimated fatigue limit σ _w and the number of repetitions N _f Create an SN diagram that takes into account the inherent defects that are related, and use the created SN diagram to take into account the internal defects. And to carry out the evaluation of the fatigue strength of the wood.

In other words, in the present embodiment, an SN diagram (SN diagram considering an intrinsic defect) taking into account the estimated fatigue limit σ _w is created with respect to the conventional SN diagram, It is used to evaluate the fatigue strength of the casting material.

  Since the SN diagram in consideration of the internal defects is one in which the dimensions of the internal defects of each test piece are taken into account, the variation of the fatigue strength diagram (SN diagram) can be reduced, and excessively It is possible to create a rational evaluation diagram (fatigue design diagram) without using a significant safety factor. Therefore, it becomes possible to accurately evaluate the fatigue strength of the casting material having an inherent defect.

  In the above embodiment, the size of the internal defect on the fracture surface of the test piece is measured. However, the present invention is not limited to this, and the size of the internal defect may be measured by any method, for example, using ultrasonic waves. The size of the inherent defect may be measured by a conventional UT inspection or a PT inspection using colored ink.

  Moreover, in the said embodiment, although the casting material was mentioned as a material containing an intrinsic defect, this invention is applicable not only to a casting material but the other material containing an intrinsic defect. In addition, inclusions and second layers that become the starting point of fatigue (starting point of fracture due to fatigue) can be handled in the same manner as in the present invention. That is, according to the present invention, unified evaluation is possible.

  Furthermore, in the above embodiment, a fatigue test of tensile compression by axial force was performed. However, in the case where various stress components such as multiaxial tension are included, equivalent stresses when the stress components are uniaxial tension (for example, Equivalent stress of Mises or equivalent stress of Tresca) or evaluation using the largest stress acting on the test piece.

31 Internal defects 51, 61 Surface of specimen 52 Semi-elliptical crack 53 Elliptical crack 62 Semi-circular crack 63 Circular crack

Claims (3)

A method for evaluating the fatigue strength of a casting material having inherent defects,
A plurality of test pieces of the casting material were prepared and subjected to a fatigue test. From the result of the fatigue test, an SN diagram that is a relationship between the stress amplitude σ _a and the number of repetitions N _f was prepared,
On the other hand, the internal defect of each test piece is represented by a semi-elliptical or elliptical crack in consideration of the size of the internal defect, and the semi-elliptical or elliptical crack has the same maximum stress intensity factor as that of the semi-elliptical or elliptical crack. A crack radius a _eq is determined, and the inherent defect of each specimen is modeled as the semi-circle or circular crack,
Formula (1) shown in [Formula 1]

Is used to calculate the inherent crack length a ₀ , which is the crack length that does not affect the fatigue limit,
Based on the semicircular or circular crack radius a _{eq of} each of the obtained test pieces and the inherent crack length a ₀ , the equation (2) shown in [Expression 2]

To calculate the estimated fatigue limit σ _w of each test piece,
By dividing the stress amplitude σ _a of the SN diagram created in the fatigue test by the estimated fatigue limit σ _w of each test piece obtained, the stress amplitude σ _a / the estimated fatigue limit σ _w and the number of repetitions Create an SN diagram that takes into account the intrinsic defects that are the relationship of N _f ,
A method for evaluating the fatigue strength of a casting material, wherein the fatigue strength of the casting material is evaluated using an S—N diagram in consideration of the created inherent defect.   2. The method for evaluating the fatigue strength of a casting material according to claim 1, wherein the semi-elliptical or elliptical crack has a major axis and a minor axis determined from the size of an inherent defect in the fracture surface of each test piece. A fatigue strength evaluation device for a casting material having an inherent defect,
From the result of a fatigue test performed by producing a plurality of test pieces of the cast material, an SN diagram creation unit that creates an SN diagram that is a relationship between the stress amplitude σ _a and the number of repetitions N _f ;
The internal defect of each test piece is represented by a semi-elliptical or elliptical crack in consideration of the size of the internal defect, and the semi-elliptical or elliptical crack has the same maximum stress intensity factor as that of the semi-elliptical or elliptical crack. Determining the radius a _eq of the crack and modeling the inherent defect of each specimen as the semi-circle or circular crack;
Formula (1) shown in [Formula 3]

Is used to calculate the natural crack length a ₀ , which is the crack length that does not affect the fatigue limit, and the semicircular or circular crack radius a _{eq of} each of the obtained specimens and the specific crack Based on the crack length a ₀ , the formula (2) shown in [Expression 4]

A fatigue limit calculator for calculating an estimated fatigue limit σ _w of each test piece by
By dividing the stress amplitude σ _a of the SN diagram created in the fatigue test by the estimated fatigue limit σ _w of each test piece obtained by the fatigue limit calculation unit, the stress amplitude σ _a / estimated fatigue A modified SN diagram creating unit for creating an SN diagram in consideration of an inherent defect that is a relationship between the limit σ _w and the number of repetitions N _f , and the intrinsic defect created by the modified SN diagram creating unit A fatigue strength evaluation apparatus for a casting material, comprising: a fatigue strength evaluation unit that evaluates the fatigue strength of the casting material using an SN diagram in consideration of the above.

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