JPH08206831A - Fatigue strength improvement method of structural stress concentration part - Google Patents

Abstract

(57) [Abstract] [Purpose] To provide a method for improving the fatigue strength of a structural stress concentrating portion, which can sufficiently improve the fatigue strength even when high stress is repeatedly applied and whose quality control is easy. [Structure] Stress concentration part A at a specific location due to repeated tensile force
A welding material having a higher strength than that of the structure is used for the structure 1 in which the weld is formed, and a welding bead 7 is formed on the surface of the structure at a distance h from the stress concentrated portion and in parallel with the tensile stress generated in the stress concentrated portion. To form residual compressive stress in the stress concentration portion.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for improving the fatigue strength of a structural stress concentration portion where fatigue cracks are likely to occur.

[0002]

2. Description of the Related Art For example, a ship or the like has a structural stress concentration portion as shown in FIG. 6 in which a fatigue crack (fatigue fracture) is likely to occur. In this figure, (A) is a plate-framed structure in which a reinforcing plate 2 is welded to a metal plate 1, (B) is a metal plate 1 having a notch portion 3, and (C) is a metal plate having a through hole 4. 1 and each of which receives a tensile force indicated by an arrow 5,
A stress-concentrated portion is generated in the region A, and fatigue fracture (cracks or the like) may occur in the region A due to fatigue due to repeated tensile force.

[0003]

In order to prevent such fatigue fracture and improve the fatigue strength of the stress concentrated portion, conventionally, a residual compressive stress is formed in the stress concentrated portion by peening, TIG treatment, water jet and the like. Means were taken. However, these means have a problem that the treatment is complicated and quality control is difficult because there is no trace of the treatment being performed, and there is a possibility that the treatment may be left untreated.

In order to solve these problems, the same applicant as the present invention proposes that "the structural stress concentrating portion has a compressive stress field by heating and cooling the side portions of the stress concentrating portion. "Fatigue strength improvement method" was invented and filed first (Japanese Patent Application No. 2-1
38236). However, this method has a problem in that the degree of improvement in fatigue strength is small when high stress is repeatedly applied. That is, when heating or cooling a part of the structure by gas or high frequency heating, the maximum value of the tensile residual stress generated in the structure of the heating / cooling section is limited by the yield stress of the structural material, and the residual stress The range of occurrence is also narrow. Therefore, when tensile force acts on the structure in this state,
Where the sum of the tensile residual stress and the tensile stress generated by the acting force exceeds the yield stress of the structural material, the residual stress is released due to plastic deformation, and therefore the compressive residual stress generated in the stress concentration part is also released. There is a problem that the effect of improving fatigue strength / life is reduced. In addition, even with this method, quality control is difficult because there is no trace of processing being performed,
There is a problem that it may be used as it is untreated.

The present invention was devised to solve such problems. That is, an object of the present invention is to provide a method for improving the fatigue strength of a structural stress concentrating portion, which can sufficiently improve the fatigue strength even when high stress is repeatedly applied and whose quality control is easy.

[0006]

According to the present invention, in a structure in which a stress concentration portion is formed at a specific location by repeated tensile force, a welding material having a higher strength than that of the structure is used, and the stress concentration is increased. Structural stress concentration characterized by forming a weld bead on the surface of the structure in parallel with the tensile stress generated in the stress concentration part at a distance from the part, thereby generating residual compressive stress in the stress concentration part. A method of improving fatigue strength of a part is provided.

According to a preferred embodiment of the present invention, the weld bead is straight.

[0008]

According to the method of the present invention, the residual compressive stress can be generated in the stress concentrated portion only by forming the weld bead on the surface of the structure. Therefore, the fatigue characteristics of the stress concentrated portion can be reduced by a simple method. (Strength / lifetime) can be improved. Further, since a welding material having a higher strength than that of the structure is used, even when high stress is repeatedly applied, the maximum value of the tensile residual stress generated in the structure can be increased to the yield stress of the high-strength welding material, Moreover, the range in which the residual stress is generated can be freely adjusted by the number and width of the weld beads. Therefore, even if the tensile force acts on the structure in this state and the sum of the tensile residual stress and the tensile stress generated by the acting force exceeds the yield stress of the structural material, as long as it does not exceed the yield stress of the high-strength welding material. , Plastic deformation does not occur and residual stress is not released. Therefore, even when high stress is repeatedly applied, the rate of release of the compressive residual stress generated in the stress concentrated portion is small, and the effect of improving fatigue strength / life can be maintained.

Furthermore, when the method of the present invention is carried out, since weld beads remain on the surface of the structure, it is possible to easily visually confirm that the measures for improving fatigue characteristics have been taken easily, and quality control is also easy. Is.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will be described below with reference to the drawings. In the drawings, common parts are designated by the same reference numerals and used. FIG. 1 is an example of a structure similar to that of FIG. 6 in which the method of the present invention is carried out. (A) is a plate-frame structure part in which a reinforcing plate 2 is welded to a metal plate 1, and (B) is a notch. When the metal plate 1 having the portion 3 and (C) receives the tensile force indicated by the arrow 5 in the metal plate 1 having the through hole 4, the stress concentration portion is generated in the region A.

According to the method of the present invention, the weld bead 7 is formed on the surface of the structure 1 (metal plate) as shown in FIG. It consists of generating residual compressive stress in the concentrated part. The welding bead 7 is formed by using a welding material having a higher strength than the structural material of the structure 1 and using a normal welding means such as arc welding, TIG welding, or the like. As the high-strength welding material, high-strength steel (HT50, HT80, etc.) is preferably used when the material of the structure 1 is mild steel (SS). The weld bead 7 is formed at a distance h from the stress concentrating portion A and in parallel with the tensile stress 6 generated in the stress concentrating portion A. The interval h is set to an appropriate interval so that the residual compressive stress is formed in the stress concentration portion A. The interval h is preferably about 1 to 3 times the width of the welding bead 7 or about 1 to 3 times the plate thickness of the structure 1. Further, as shown in FIG. 1, the weld bead 7 is preferably linear in parallel with the tensile stress 6 generated in the stress concentration portion A. The size of the welding bead 7 is
It is preferable to determine so that a sufficient amount of residual compressive stress is formed in the stress concentration portion A, and if necessary, a plurality of weld beads may be formed in parallel.

FIG. 2 is a residual stress distribution map of a welded portion according to the present invention, where (A) is a surface view of a structure 1 having a weld bead 7, and (B) is a residual stress distribution map along the line AA. Is. As shown in this figure, at a certain portion of the welding bead 7, since the high-strength welding material that thermally expanded during welding is cooled after being integrated with the structure 1, tensile stress (+) is generated at that portion. A compressive stress (indicated by-) that is generated and is balanced with this tensile stress is generated on both sides of the weld bead 7.
The portion where the maximum compressive stress is generated (indicated by B in the figure) is separated from the portion of the welding bead 7 where the maximum tensile stress is generated, and this distance is made equal to the above-mentioned distance h to maximize the stress concentration portion. It is preferable to match the portion (B portion) where the compressive stress occurs.

FIG. 3 shows B of the structure 1 in the case of FIG. 1 (C).
It is a residual stress distribution map in a -B section. As shown in this figure, a tensile stress (indicated by +) is generated in a portion where the weld bead 7 is present, and a compressive stress (indicated by −) that balances this tensile stress is generated on both sides of the weld bead 7. Maximum compressive stress occurs in the concentrated area.

FIG. 4 is a diagram showing a change in residual stress when an external force is applied. (A) shows the case where the same material as the structure (for example, mild steel) is used for the welding bead 7, and (B) shows The case of the present invention in which a high-strength welding material is used for the welding bead 7 is shown. In FIG. 4 (A), the initial residual stress distribution in which the weld bead 7 is formed is as shown in FIG.
The maximum value of the tensile residual stress generated in the structure ₁ is limited by the yield stress σ ₁ of the structural material. When the tensile force acts on the structure in this state, the tensile residual stress and the acting force are as shown in FIG. Yield stress σ of structural material is the sum of tensile stress σ
A portion exceeding ₁ is plastically deformed to release the residual stress, so that the compressive residual stress B generated in the stress concentrated portion is also released, and the effect of improving fatigue strength / life is reduced.

On the other hand, in FIG. 4B according to the present invention, since the welding material having higher strength than the structural material of the structure 1 is used, the maximum value of the tensile residual stress generated in the structure 1 is set to the high strength. The yield stress of the welding material can be increased to σ ₂ (> σ ₁ ), and the tensile force acts on the structure in this state, and the sum of the tensile residual stress and the tensile stress σ generated by the acting force is the yield stress of the structural material. Even if it exceeds σ ₁ , plastic deformation does not occur and residual stress is not released unless the yield stress σ ₂ (> σ ₁ ) of the high-strength welding material is exceeded. Therefore, even when high stress is repeatedly applied, the rate of release of the compressive residual stress generated in the stress concentrated portion is small, and the effect of improving fatigue strength / life can be maintained.

FIG. 5 is a diagram comparing the effect of the present invention with a conventional example, in which the vertical axis represents the magnitude of the repeated load (tensile force) acting on the structure 1, and the horizontal axis represents fatigue due to this tensile force. The number of repetitions that causes destruction is shown. In this figure,
A is the fatigue life of the member without any measures, B is the fatigue life of the member with local heating measures, and C is the fatigue life of the members with the measures of the present invention (addition of high-strength weld beads). Is schematically shown. As shown in this figure, according to the method of the present invention, even when low stress is repeatedly applied, B
It has the same effect as the fatigue life of the member that takes measures against local heating, and even when high stress is repeatedly applied, the release of the compressive residual stress generated in the stress concentration part is small and the fatigue strength / life improvement effect. Can be maintained.

Further, according to the above-mentioned method, the residual compressive stress can be generated in the stress concentrated portion only by forming the weld bead on the surface of the structure. Therefore, the fatigue from the stress concentrated portion can be reduced by a simple method. The characteristics (strength / life) can be improved, and the weld beads remain on the surface of the structure, so it is possible to easily visually confirm that the measures for improving fatigue characteristics have been taken easily, and quality control is also possible. It's easy.

The present invention is not limited to the above-mentioned embodiments, and it goes without saying that various modifications can be made without departing from the gist of the present invention.

[0019]

As described above, according to the method of the present invention, the fatigue characteristics (strength / life) from the stress concentrated portion can be improved by a simple method (application of welding beads), and high stress is repeated. In this case, the fatigue characteristics can be improved, and since the welding bead remains, it is possible to confirm that the measures for improving the fatigue characteristics can be surely taken.

Therefore, the method for improving the fatigue strength of the structural stress concentration portion of the present invention is excellent in that it is possible to sufficiently improve the fatigue strength even when high stress is repeatedly applied and the quality control is easy. Have an effect.

[Brief description of drawings]

FIG. 1 is a schematic view of a structure in which the method of the present invention has been carried out.

FIG. 2 is a residual stress distribution map of a welded portion according to the present invention.

FIG. 3 is a residual stress distribution diagram in the case of FIG. 1 (C).

FIG. 4 is a diagram showing a change in residual stress when an external force is applied.

FIG. 5 is a diagram comparing the effect of the present invention with a conventional example.

FIG. 6 is a diagram showing an example of a structure having a stress concentration portion.

[Explanation of symbols] 1 Metal plate (structure) 2 Reinforcement plate 3 Notch part 4 Through hole 5 Tensile force 6 Tensile stress 7 Weld bead A Stress concentration part B Maximum compression stress part

Claims (2)

[Claims] 1. A structure in which a stress concentration portion is formed at a specific location by repeated tensile force, a welding material having a higher strength than that of the structure is used, and the stress concentration portion is spaced apart from the stress concentration portion. A method for improving fatigue strength of a structural stress concentrating portion, characterized in that a weld bead is formed on the surface of the structure in parallel with the generated tensile stress to generate residual compressive stress in the stress concentrating portion. 2. The method for improving fatigue strength of a structural stress concentrating portion according to claim 1, wherein the weld bead is linear.