JP2012108102A - Method for evaluating fatigue fracture of cylindrical metal material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for evaluating the fatigue fracture of a cylindrical metal material that is capable of not only surely reproducing a fatigue fracture occurring on the inner surface of a cylindrical metal material but simply evaluating fatigue characteristics of the inner surface of the cylindrical metal material with high accuracy.SOLUTION: Characteristics of the fatigue fracture of a cylindrical metal material are evaluated in the following manner. First, a cylindrical test piece 1 is prepared in which the outer diameter of the center part 1b is set equal to or less than 90% of that of a restraint part 1a; the ratio of the outer to inner diameters of the center part 1b satisfies the relation of (outer diameter/inner diameter)≤2; and the thickness of the center part 1b is set equal to or greater than 1 mm. Second, a compressive residual stress equal to or greater than 25% of the tensile strength of the metal material is applied on the outer surface of the center part 1b and a compressive residual stress equal to or less than 2.5% of the tensile strength thereof is applied to a point 0.8 mm deep from the outer surface. Third, a fatigue fracture is caused on the inner surface of the cylindrical test piece 1 by applying a repeated bending or torsional load to the cylindrical test piece 1, and then the characteristics of the fatigue fracture of the cylindrical metal material are evaluated based on conditions of the repeated bending or torsional load under which the fatigue fracture occurs.

Description

  The present invention constrains both ends of a cylindrical test material made of a metal material, and generates a fatigue failure on the inner surface of the cylindrical test material by applying a bending load or a torsion load to the cylindrical test material. The present invention relates to a fatigue fracture evaluation method for a cylindrical metal material that reproduces the fatigue fracture occurring on the inner surface of the metal material.

  Conventionally, many cylindrical metal materials have been used for mechanical structures, but quality control of the surface properties of the outer surface of the cylindrical metal material can be easily performed by visual inspection, but the inner surface of the cylindrical metal material There was a problem that the quality control of the surface texture was very difficult to perform because it could not be confirmed visually. As a result, if the surface roughness of the inner surface of the cylindrical metal material (difference in surface irregularities) is too large, etc., the fracture starting from the inner surface of the cylindrical metal material depends on the use conditions and environment of the machine structure. May occur. In particular, in a bent portion of a cylindrical metal material, a large bending stress or torsional stress is applied to the inner surface, and as shown in FIG. 4, the possibility of occurrence of fatigue failure 2 starting from the inner surface becomes higher.

  Therefore, it is important for grasping the fatigue characteristics starting from the inner surface of the cylindrical metal material in advance to enhance the safety of the mechanical structure. However, as shown in FIG. 3, both ends of a cylindrical test material 1 simulating a cylindrical metal material are restrained by a testing machine jig 3, and a bending load or a torsion load is applied to the cylindrical test material 1. If only the addition is made, fatigue fracture 2 will occur from the outer surface before the inner surface of the cylindrical test material 1, and the fatigue fracture occurring on the inner surface of the cylindrical metal material of the machine structure cannot be reproduced. It was.

  From the situation as described above, it has been a conventional problem to develop a test method capable of evaluating the fatigue characteristics of the inner surface of a cylindrical metal material simply and accurately.

  As a test method for evaluating the fatigue characteristics of a cylindrical metal material, a bending fatigue test method for a pipe or the like described in Patent Document 1 has been proposed. This test method is used for a bar material such as a pipe as a test material. A short column-shaped bending jig with a taper on the outer periphery is provided at both ends of the bar so that the outer part has a part that fits outward and the thickness of the outer part becomes thinner toward the center of the bar. This is a test method in which a bending moment is repeatedly applied to portions other than the outer fitting portion of the bending jig.

  However, this proposal is intended to prevent the occurrence of breakage in the restraint part etc. first when the end part of a bar material such as a pipe as a test material is restrained with a bending jig and a bending fatigue test is performed. This is not a proposal intended to reproduce the fatigue fracture that occurs on the inner surface of the cylindrical metal material of the mechanical structure.

  In addition, Patent Document 2 proposes a device for mounting an actual tube test piece of an actual tube rotating bending fatigue tester. This proposal, too, is similar to Patent Document 1, in the case of a rotating bending fatigue test. This is just a proposal to prevent the restraint from being broken.

Japanese Patent Laid-Open No. 62-174630 Japanese Utility Model Publication No.2-135846

  The present invention has been made in view of the above-described conventional situation, and can reliably reproduce the fatigue failure occurring on the inner surface of the cylindrical metal material of the mechanical structure, and can easily and accurately perform the inner surface of the cylindrical metal material. It is an object of the present invention to provide a method for evaluating fatigue fracture of a cylindrical metal material capable of evaluating the fatigue characteristics of steel.

  According to the first aspect of the present invention, both ends of a cylindrical test material made of a metal material are restrained, and repeated bending load or torsion load is applied to the cylindrical test material to cause fatigue fracture on the inner surface of the cylindrical test material. A fatigue fracture evaluation method for a cylindrical metal material that reproduces the fatigue fracture that occurs on the inner surface of the cylindrical metal material, wherein the outer diameter of the central portion of the cylindrical test material is equal to the outer diameter of the restraint portions at both ends. 90% or less, the ratio of the outer diameter to the inner diameter of the central portion is (outer diameter / inner diameter) ≦ 2, the material thickness of the central portion is 1 mm or more, and the central portion of the cylindrical test material The outer surface of the metal material is loaded with a compressive residual stress of 25% or more of the tensile strength of the metal material and at a depth of 0.8 mm from the outer surface of 2.5% or less of the tensile strength of the metal material, Cylindrical test material, repeated bending load or torsion load In addition, fatigue failure occurs on the inner surface of the cylindrical test material, and the fatigue failure characteristics of the cylindrical metal material are evaluated based on the repeated bending load condition or torsional load condition that caused the fatigue failure. This is a fatigue fracture evaluation method for a cylindrical metal material.

  According to the second aspect of the present invention, when a fatigue failure is generated on the inner surface of the cylindrical test material by repeatedly applying a bending load to the cylindrical test material, the outer surface bending of the constraining portions at both ends of the cylindrical test material is performed. When the stress amplitude is 25% or more of the tensile strength of the metal material and the material thickness of the restraint portion is 2.5 mm or less, a deformation suppression jig is fitted inside the cavity of the restraint portion. 2. The fatigue failure evaluation method for a cylindrical metal material according to claim 1, wherein a bending load is repeatedly applied to the cylindrical test material to cause fatigue failure on the inner surface of the cylindrical test material.

  According to a third aspect of the present invention, when fatigue damage is generated on the inner surface of the cylindrical test material by repeatedly applying a torsional load to the cylindrical test material, the outer surface shearing of the restraining portions at both ends of the cylindrical test material is performed. When the stress amplitude is 50% or more of the tensile strength of the metal material and the material thickness of the restraint portion is 2.5 mm or less, a deformation suppression jig is fitted inside the cavity of the restraint portion. 2. The fatigue failure evaluation method for a cylindrical metal material according to claim 1, wherein a torsional load is repeatedly applied to the cylindrical test material to cause fatigue failure on the inner surface of the cylindrical test material.

  According to the fatigue fracture evaluation method for a cylindrical metal material of the present invention, the fatigue fracture occurring on the inner surface of the cylindrical metal material of the machine structure is reliably reproduced without causing the fatigue fracture from the outer surface first. It is possible to evaluate the fatigue characteristics of the inner surface of the cylindrical metal material easily and accurately.

The state which is implementing the rotation bending fatigue test by the fatigue fracture evaluation method of the cylindrical metal raw material of this invention is shown, (a) is a front view, (b) is a side view which shows the end surface of a cylindrical test material. The state which is performing the rotation bending fatigue test by the fatigue fracture evaluation method of the cylindrical metal raw material of Claim 2 or 3 is shown, (a) is a front view which shows a deformation | transformation suppression jig | tool through, (b) is shown. It is XX sectional drawing of (a). It is a front view which shows the state which is implementing the rotation bending fatigue test by the fatigue fracture evaluation method of the conventional cylindrical metal raw material. It is sectional drawing of the cylindrical metal raw material for showing the position where the fatigue fracture which starts the inner surface in the bending shape part of a cylindrical metal raw material generate | occur | produces.

  Hereinafter, the present invention will be described in more detail based on embodiments shown in the accompanying drawings.

  For example, as shown in FIG. 1, the fatigue fracture evaluation method for a cylindrical metal material according to the present invention uses a testing machine jig 3 to attach both ends of a cylindrical test material 1 made of a metal material such as iron, copper, or aluminum. By restraining and repeatedly applying a bending load or a torsion load to the cylindrical test material 1, a fatigue failure 2 starting from the position shown in FIG. 4 is generated on the inner surface of the cylindrical test material 1. This is a method for reproducing the fatigue fracture 2 occurring on the inner surface of the cylindrical metal material 1A of the structure.

  A cylindrical test material 1 used for this fatigue fracture evaluation method is composed of constraining portions 1a and 1a at both ends and a central portion 1b located in the middle thereof. In the embodiment shown in FIG. 1, the lengths of the restraining portions 1 a and 1 a and the central portion 1 b at both ends are approximately 1: 1: 1, and the length varies depending on the overall length and diameter of the cylindrical test material 1. The length of the central portion 1b is preferably 20 to 30 mm.

  In addition, the outer diameter of the central portion 1b of the cylindrical test material 1 is 90% or less of the outer diameter of the restraining portions 1a and 1a at both ends. Furthermore, the ratio of the outer diameter to the inner diameter of the central portion 1b is set to (outer diameter / inner diameter) ≦ 2, and the material thickness of the central portion 1b is set to 1 mm or more.

  In the method for evaluating fatigue fracture of a cylindrical metal material according to the present invention, the cylindrical test material 1 having such a shape is previously subjected to 25% or more of the tensile strength of the metal material on the outer surface of the central portion in advance from its outer surface. After applying a compressive residual stress of 2.5% or less of the tensile strength of the metal material at a depth of 8 mm, a cylindrical shape is obtained by repeatedly applying a bending load or a torsion load to the cylindrical test material 1. A fatigue failure 2 is generated on the inner surface of the test material 1 to reproduce the fatigue failure 2 generated on the inner surface of the cylindrical metal material of the mechanical structure.

  Next, the reason for limiting the numerical values such as the outer diameter of the central portion 1b of the cylindrical test material 1 will be described.

(The outer diameter of the central part of the cylindrical test material is 90% or less of the outer diameter of the restraining part at both ends)
Since stress concentration occurs in the restraining portions 1a and 1a at both ends of the cylindrical test material 1, in the conventional fatigue fracture evaluation method for a cylindrical metal material as shown in FIG. 3, if a bending load or a torsion load is applied, The fatigue fracture 2 starting from the position restrained by the testing machine jig 3 occurs. As described above, when the fatigue failure 2 starting from the position constrained by the testing machine jig 3 occurs, the fatigue failure 2 generated on the inner surface of the cylindrical metal material 1A of the mechanical structure cannot be reproduced.

  Therefore, by making the outer diameter of the central portion 1b of the cylindrical test material 1 smaller than the outer diameters of the restraining portions 1a and 1a at both ends, the generated stress of the central portion 1b of the cylindrical test material 1 is generated in the restraining portion 1a. The stress can be made larger than the stress, and the fatigue failure 2 can be generated in the central portion 1b of the cylindrical test material 1.

  In order to make the generated stress of the central portion 1b of the cylindrical test material 1 larger than the generated stress of the constraining portions 1a and 1a at both ends, the outer diameter of the central portion 1b of the cylindrical test material 1 is set to the constraining portions 1a and 1a at both ends. In order to make the generated stress of the central portion 1b of the cylindrical test material 1 larger than the generated stress of the restraining portions 1a and 1a at both ends, it is necessary to make it sufficiently smaller than the outer diameter of the cylindrical test material 1 It has been found that the outer diameter of the central portion 1b of 1 needs to be 90% or less of the outer diameter of the restraining portion 1a. By setting the outer diameter of the central portion 1b of the cylindrical test material 1 to 90% or less of the outer diameter of the restraining portion 1a, the fatigue failure 2 can be generated in the central portion 1b of the cylindrical test material 1.

(The ratio of the outer diameter to the inner diameter of the central part of the cylindrical test material is (outer diameter / inner diameter) ≦ 2, and the outer surface of the central part of the cylindrical test material has a compressive residual stress of 25% or more of the tensile strength of the metal material. load)
In general, when bending deformation or torsional deformation is applied to the cylindrical test material 1, the stress on the outer surface becomes larger than the stress on the inner surface of the central portion 1 b of the cylindrical test material 1. It is conceivable that the fatigue fracture 2 starting from the outer surface of 1b occurs.

  Thus, as a measure for preventing the occurrence of fatigue failure 2 starting from the outer surface of the central portion 1b of the cylindrical test material 1, a crack starting from the outer surface of the central portion 1b of the cylindrical test material 1 is used. It is conceivable to employ a technique for increasing resistance to generation and a technique for reducing the difference between the outer surface stress and the inner surface stress of the central portion 1 b of the cylindrical test material 1. When the previous method is adopted, it is effective to apply compressive stress to the outer surface of the cylindrical test material 1, and when the later method is adopted, the difference between the outer diameter and the inner diameter of the cylindrical test material 1 is reduced. It is effective to make it.

  Based on the above view, as a result of conducting an experimental study on the conditions under which fatigue failure 2 occurs starting from the inner surface of the central portion 1b of the cylindrical test material 1, the central portion of the cylindrical test material 1 is obtained. The outer surface of 1b was loaded with a compressive residual stress of 25% or more of the tensile strength of the metal material. In addition, the ratio of the outer diameter to the inner diameter of the central portion 1b of the cylindrical test material 1 was expressed as (outer diameter / It was found that the fatigue fracture 2 starting from the inner surface of the central portion 1b of the cylindrical test material 1 occurs stably by setting the inner diameter) ≦ 2.

(A compressive residual stress of 2.5% or less of the tensile strength of the metal material is loaded at a depth of 0.8 mm from the outer surface of the cylindrical test material, and the thickness of the central part of the cylindrical test material is 1 mm or more)
As a technique for applying compressive residual stress to the outer surface of the central part 1 b of the cylindrical test material 1, it is conceivable to employ a technique such as shot peening, but the compressive residual stress is applied to the outer surface of the central part 1 b of the cylindrical test specimen 1. , The compressive residual stress affects not only the outer surface but also a deeper position of the central portion 1b of the cylindrical test material 1. However, when this compressive residual stress affects more than a certain depth of the central portion 1b of the cylindrical test material 1, the fatigue fracture 2 starting from the inner surface of the central portion 1b of the cylindrical test material 1 is caused. The situation is unlikely to occur. Moreover, when the plate | board thickness of the center part 1b of the cylindrical test material 1 is too thin, it will have an influence of a compressive residual stress to the inner surface of the center part 1b of the cylindrical test material 1, In such a case, The fatigue characteristics of the material cannot be evaluated.

  Here, the distribution of compressive residual stress by shot peening is investigated in the thickness direction, the thickness of the central portion 1b of the cylindrical test material 1 is set to 1 mm or more, and the outer surface of the central portion 1b of the cylindrical test material 1 is measured. By setting the compressive residual stress at a depth of 0.8 mm to 2.5% or less of the tensile strength of the metal material, the effect of the compressive residual stress applied to the outer surface of the central portion 1b of the cylindrical test material 1 can be reduced. I found that it can be avoided.

  In addition, as a result of experimental investigation, the outer surface of the central portion 1b of the cylindrical test material 1 has a tensile strength of 2.5% or more at a position of 25% or more of the tensile strength of the metallic material and a depth of 0.8 mm from the outer surface. When we examined the conditions of shot peening to achieve a compressive residual stress of less than 10%, shot peening was divided into the first and second stages, and the first and second stages of shot peening were It was confirmed that it should be carried out under the conditions exemplified in.

  For example, when performing fatigue evaluation of a cylindrical test material using a steel material having a tensile strength of 1000 MPa or more and 2000 MPa or less, as shown in the examples described later, the shot peening condition is the first stage shot peening. , Shot particle size: φ0.5-1 mm, projection speed: 75-100 m / sec, projection time: 40-60 sec, second stage shot peening, shot particle size: φ0.3-1 mm, projection speed: 30 ˜75 m / sec, projection time: 40 to 60 sec.

  By the fatigue fracture evaluation method for cylindrical metal material 1A described above, the fatigue fracture that occurs on the inner surface of cylindrical metal material 1A of the machine structure can be reliably confirmed by the test without causing fatigue fracture from the outer surface first. The fatigue characteristics of the inner surface of the cylindrical metal material 1A can be evaluated easily and accurately.

  However, when the experiment is performed with an increased bending load or torsion load applied to the cylindrical test material 1, in order to prevent the restraint portion 1 a of the cylindrical test material 1 from slipping, the restraint portion 1 a is applied by applying a larger pressure. You will be restrained. When the experiment is performed by such a method, depending on the conditions, the constraining portion 1a may be deformed by the large pressure, and a crack is generated from the inner surface of the constraining portion 1a before the inner surface of the central portion 1b. It cannot be denied that it may occur.

  As a result of examination by experiment, it was found that the conditions differ between when the bending load is repeatedly applied to the cylindrical test material 1 and when the torsional load is repeatedly applied to the cylindrical test material 1.

  The condition when the bending load is applied to the cylindrical test material 1 is that the outer surface bending stress amplitude of the restraining portion 1a at both ends of the cylindrical test material 1 is 25% or more of the tensile strength of the metal material, and the restraint thereof. This is a case where the material thickness of the portion 1a is 2.5 mm or less.

  On the other hand, the condition when the torsional load is applied to the cylindrical test material 1 is that the outer surface shear stress amplitude of the restraining portion 1a at both ends of the cylindrical test material 1 is 50% or more of the tensile strength of the metal material, and This is a case where the material thickness of the restraining portion 1a is 2.5 mm or less.

  When each of the above conditions is satisfied, in order to prevent the restraint portion 1a of the cylindrical test material 1 from slipping, if the restraint portion 1a is restrained with a larger pressure, the restraint portion 1a is deformed by the large pressure. There is a possibility that a crack may occur from the inner surface of the restraining portion 1a before the inner surface of the central portion 1b. However, as shown in FIG. 2, if the deformation suppressing jig 4 is fitted inside the cavity 1c of the restricting portion 1a, deformation of the restricting portion 1a due to a large external pressure can be suppressed. As a result, it is possible to prevent a crack from being generated from the inner surface of the restraining portion 1a, and to reliably generate a crack from the inner surface of the central portion 1b. That is, by inserting the deformation suppressing jig 4 into the cavity 1c of the restraining portion 1a, it is possible to reliably reproduce the fatigue failure that occurs on the inner surface of the cylindrical metal material 1A.

  The deformation suppressing jig 4 is formed of a cylindrical shaft portion 4a and a head portion 4b. The diameter of the shaft portion 4a is only about 0.01 mm smaller than the inner diameter of the cavity 1c of the restraint portion 1a of the cylindrical test material 1, and the shaft portion 4a of the deformation suppressing jig 4 is placed inside the cavity 1c of the restraint portion 1a. The deformation of the restraining portion 1a due to a large external pressure can be suppressed. Further, the tip of the shaft portion 4a is hemispherical, and makes it difficult for shape discontinuity to occur at the boundary between the deformation suppressing jig 4 and the inner surface of the cavity 1c. On the other hand, the head 4b serves as a stopper for preventing the deformation suppressing jig 4 from entering the inside of the cavity 1c of the restraining portion 1a more than necessary, and has a condition that it has a larger diameter than the shaft portion 4a. However, in the embodiment shown in FIG. 2, the circular cross-sectional shape has the same size as the restraining portion 1 a of the cylindrical test material 1.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, and the present invention is implemented with appropriate modifications within a range that can meet the gist of the present invention. These are all included in the technical scope of the present invention.

  Cylindrical test materials under various conditions of A to L and a to f shown in Table 1 and Table 2 were prepared, and a rotating bending fatigue test and a torsional fatigue test shown in JIS Z 2274 were performed. The length of the central part of the cylindrical test material was all 25 mm.

  After producing the cylindrical test material, shot peening was performed under the following conditions, and a compressive residual stress was applied to the cylindrical test material. A steel ball was used for shot peening.

Shot peening conditions (A, B, C, D, F, G, H, I, J, L, a, b, c, d, e, f)
First stage: shot particle diameter φ0.8mm, projection speed 88m / sec, projection time: 50sec
Second stage: Shot particle diameter φ0.3mm, Projection speed 50m / sec, Projection time: 50sec
Shot peening conditions (E, K)
First stage: shot particle diameter φ0.8mm, projection speed 88m / sec, projection time: 10sec
Second stage: Shot particle diameter φ0.3mm, Projection speed 50m / sec, Projection time: 10sec

  Table 1 shows the results of the rotating bending fatigue test.

  A to C are examples of the invention, and in any case, fatigue failure occurred on the inner surface of the cylindrical test material. Among these invention examples, Invention Example A having the largest surface roughness (difference in surface irregularities) of the cylindrical test material has the shortest rupture life, and the surface roughness (surface irregularities) of the cylindrical test material. Inventive example C having the smallest difference) has the longest fracture life. This test result is a result showing that the surface roughness (difference in surface irregularities) of the inner surface of the cylindrical metal material has an influence on the fracture life.

  D is a comparative example in which the ratio of the outer diameter to the inner diameter (outer diameter / inner diameter) of the central part of the cylindrical test material is 2.5, which is too large. E is a compressive residual stress on the outer surface of the central part of the cylindrical test material. Comparative example 21 which is too small, 21% of the tensile strength of the material, F is a comparative example where the plate thickness (material thickness) of the central part of the cylindrical test material is too thin, 0.5 mm.

  As a result, in Comparative Example D, the stress at the central portion of the cylindrical test material was not sufficiently increased and fractured at the restraint portion. In Comparative Example E, the fracture resistance of the outer surface of the central portion of the cylindrical test material was not sufficiently increased, and the fracture occurred on the outer surface of the central portion.

  In Comparative Example F, since the plate thickness (material thickness) of the central portion of the cylindrical test material is too thin, the effect of compressive residual stress is exerted on the inner surface of the central portion. Although the surface roughness (difference in surface irregularities) of the inner surface of the material was large, a contradictory result was obtained that the fatigue life was longer than that of Invention Example B. In other words, it can be seen that an accurate test result cannot be obtained in the test using the cylindrical test material as in Comparative Example F.

  Although ac is an example of an invention, the test result of the rotation bending fatigue test for confirming the effect acquired when the deformation | transformation suppression jig | tool of Claim 2 is used is shown. In invention example a, the outer surface bending stress amplitude of the restraint portion is 25% or more of the tensile strength of the metal material, but the plate thickness (material thickness) of the restraint portion exceeds 2.5 mm. Destruction has occurred from the inside.

  On the other hand, in invention examples b and c, the outer surface bending stress amplitude of the restraint portion is 25% or more of the tensile strength of the metal material, and the plate thickness (material thickness) of the restraint portion is 2.5 mm. Is a case where the deformation suppression jig was not used, and Invention Example c was a case where the deformation suppression jig was inserted into the cavity of the restraint portion and tested. As a result, the result was obtained that the invention example c using the deformation suppressing jig had a longer fracture life than the invention example b not using the deformation suppressing jig. Fatigue fracture basically occurs from the inner surface of the central portion. However, as a result of repeated tests, in Invention Example b in which the deformation suppression jig was not used, fracture occurred from the inner surface of the restraining portion before the inner surface of the central portion. I was able to get results that

  From the above test results, the outer surface bending stress amplitude of the restraint portion at both ends of the cylindrical test material is 25% or more of the tensile strength of the metal material, and the plate thickness (material thickness) of the restraint portion is 2.5 mm or less. In some cases, fatigue failure may occur from the inner surface of the constraining portion before the inner surface of the central portion, but by using a deformation suppression jig, the fatigue failure is reliably generated from the inner surface of the central portion. I was able to confirm that it was possible.

  Table 2 shows the test results of the torsional fatigue test.

  GI are invention examples, and in any case, fatigue fracture occurred on the inner surface of the cylindrical test material. Among these invention examples, Invention Example G, which has the largest surface roughness (difference in surface irregularities) of the cylindrical test material, has the shortest fracture life, and the surface roughness (surface irregularities) of the cylindrical test material. Inventive Example I having the smallest difference) has the longest fracture life. This test result is a result showing that the surface roughness (difference in surface irregularities) of the inner surface of the cylindrical metal material has an influence on the fracture life.

  J is a comparative example in which the ratio of the outer diameter to the inner diameter (outer diameter / inner diameter) of the central portion of the cylindrical test material is too large as 2.5, and K is a compressive residual stress of the outer surface of the central portion of the cylindrical test material is a metal. A comparative example in which the tensile strength of the raw material is 20% which is too small, and L is a comparative example in which the thickness of the central portion of the cylindrical test material is too thin at 0.5 mm.

  As a result, in Comparative Example J, the stress at the central portion of the cylindrical test material was not sufficiently increased, and fractured at the restraint portion. In Comparative Example K, the fracture resistance of the outer surface of the central portion of the cylindrical test material was not sufficiently increased, and the fracture occurred on the outer surface of the central portion.

  In Comparative Example L, since the plate thickness of the central portion of the cylindrical test material is too thin, the inner surface of the central portion is affected by the compressive residual stress, and the surface of the inner surface of the cylindrical metal material is more than that of Invention Example H. Although the roughness (difference in surface irregularities) was large, a contradictory result was obtained in which the fatigue life was longer than that of Invention Example H. In other words, it can be seen that an accurate test result cannot be obtained in a test using a cylindrical test material as in Comparative Example L.

  df is an example of an invention, The test result of the rotation bending fatigue test for confirming the effect acquired when the deformation | transformation suppression jig | tool of Claim 3 is used is shown. In invention example d, the outer surface shear stress amplitude of the restraint portion is 50% or more of the tensile strength of the metal material, but the plate thickness (material thickness) of the restraint portion exceeds 2.5 mm. Destruction has occurred from the inside.

  On the other hand, invention examples e and f have an outer surface shear stress amplitude of the restraint portion of 50% or more of the tensile strength of the metal material, and the plate thickness (material thickness) of the restraint portion is 2.5 mm. Is an example in which the deformation suppression jig was not used, and Invention Example f was an example in which the test was performed by inserting the deformation suppression jig into the cavity of the restraint portion. As a result, the result that the fracture life of the invention example f using the deformation suppression jig was longer than that of the invention example e not using the deformation suppression jig was obtained. Fatigue fracture basically occurs from the inner surface of the central portion, but as a result of repeated tests, in Invention Example e in which the deformation suppression jig was not used, fracture occurred from the inner surface of the restraining portion before the inner surface of the central portion. I was able to get results that

  From the above test results, the outer surface shear stress amplitude of the restraint portion at both ends of the cylindrical test material is 50% or more of the tensile strength of the metal material, and the plate thickness (material thickness) of the restraint portion is 2.5 mm or less. In some cases, fatigue failure may occur from the inner surface of the constraining portion before the inner surface of the central portion, but by using a deformation suppression jig, the fatigue failure is reliably generated from the inner surface of the central portion. I was able to confirm that it was possible.

DESCRIPTION OF SYMBOLS 1 ... Cylindrical test material 1a ... Restraint part 1b ... Center part 1c ... Cavity 1A ... Cylindrical metal material 2 ... Fatigue failure 3 ... Test machine jig 4 ... Deformation suppression jig 4a ... Shaft part 4b ... Head

Claims (3)

By constraining both ends of a cylindrical test material made of a metal material and repeatedly applying a bending load or a torsion load to the cylindrical test material, fatigue failure occurs on the inner surface of the cylindrical test material, and the cylindrical metal material A method for evaluating fatigue fracture of a cylindrical metal material that reproduces fatigue fracture occurring on the inner surface,
The outer diameter of the central portion of the cylindrical test material is 90% or less of the outer diameter of the restraining portions at both ends, and the ratio of the outer diameter to the inner diameter of the central portion is (outer diameter / inner diameter) ≦ 2, and The material thickness of the central part is 1 mm or more,
Compressive residue of 25% or more of the tensile strength of the metal material on the outer surface of the central portion of the cylindrical test material and 2.5% or less of the tensile strength of the metal material at a depth of 0.8 mm from the outer surface. With stress applied,
By applying repeated bending load or torsional load to the cylindrical test material, fatigue failure occurs on the inner surface of the cylindrical test material, and based on the repeated bending load condition or torsional load condition where the fatigue failure occurred. A method for evaluating fatigue fracture of a cylindrical metal material characterized by evaluating fatigue fracture characteristics of the cylindrical metal material. When generating fatigue failure on the inner surface of the cylindrical test material by repeatedly applying a bending load to the cylindrical test material,
When the outer surface bending stress amplitude of the constraining portion at both ends of the cylindrical test material is 25% or more of the tensile strength of the metal material, and the material thickness of the constraining portion is 2.5 mm or less,
The fatigue fracture is generated on the inner surface of the cylindrical test material by repeatedly applying a bending load to the cylindrical test material in a state where a deformation suppressing jig is fitted inside the cavity of the restraint portion. The fatigue fracture evaluation method for the cylindrical metal material described. When generating fatigue fracture on the inner surface of the cylindrical test material by repeatedly applying a torsional load to the cylindrical test material,
When the outer surface shear stress amplitude of the restraint portion at both ends of the cylindrical test material is 50% or more of the tensile strength of the metal material, and the material thickness of the restraint portion is 2.5 mm or less,
The fatigue fracture is generated on the inner surface of the cylindrical test material by repeatedly applying a torsional load to the cylindrical test material in a state in which a deformation suppressing jig is fitted inside the cavity of the restraint portion. The fatigue fracture evaluation method for the cylindrical metal material described.

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