JPH08243778A - Weld zone fatigue strength improvement method - Google Patents

Abstract

PURPOSE: To provide the fatigue strength improvement method of weld zone by reducing stress concentration at welded end part so as to improve fatigue strength of weld joint. CONSTITUTION: On whole surface or partial surface near welded end part of a fillet weld zone 17 of metal and part of a principal plate 11 of metal to weld, further with use of a fiber reinforced plastic layer, in which a plastic constant of a fiber reinforced plastic 13 in the longitudinal direction and the direction along surface is 5-150% of that of the principal plate, the fiber reinforced plastic layer, in which (plastic constant)×(thickness) is 1-50% of that of the principal plate, is adhered. By this method, stress concentration of weld joint is alleviated to increase fatigue design load, it can achieve quality improvement and weight reduction of welded structure.

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 metal material welded portion.

[0002]

2. Description of the Related Art Fillet welding is widely used in the manufacture of structures using metal materials such as steel and aluminum alloys.
The fatigue strength of the fillet weld is much lower than that of the base metal due to stress concentration at the toe and residual stress caused by welding.

Therefore, the fatigue strength of the entire structure is governed by the welded joint, and the characteristics of the base metal cannot be fully utilized. Therefore, methods for improving the fatigue strength of welds have been widely researched, and the methods are roughly classified into two methods: relaxation of stress concentration and control of residual stress.

On the other hand, as the toe treatment for the purpose of controlling the residual stress, an excessive load is applied in advance to generate a tensile stress exceeding the yield stress at the weld toe,
Preloading treatment to give compressive residual stress after unloading, heating and quenching treatment to give compressive residual stress by heating the entire joint and then quenching, and hitting the weld toe with wires, balls, etc. A method such as a peening treatment for giving a compressive residual stress is known.

On the other hand, the toe treatment for the purpose of alleviating stress concentration is made smooth by cutting the welding toe with a grinder or the like, grinding treatment, and remelting the welding toe with TIG. Processing, smoothing by remelting with plasma (Japanese Patent Publication No. 54-30386), and processing for obtaining a smooth toe portion using a decorative welding rod.

[0006] Further, by the addition of the steel plate and the filler,
Japanese Patent Application Laid-Open No. 62-296964 discloses a method of reinforcing the welded portion to prevent fatigue cracks from being generated from the weld toe portion and improve the fatigue strength of the joint.

Further, when the welded joint is subjected to hot dip galvanizing and then rapidly cooled, zinc having a low elastic modulus adheres to the surface, so that stress concentration at the toe portion is relieved, and the surface is rapidly cooled to cause residual stress of compression. It is known that the fatigue strength is improved by the two combined effects of being added to the toe.

[0008]

It is said that stress concentration is the major cause of the decrease in fatigue strength of welded joints, and stress concentration and welding residual stress that have a greater effect. Study [Study on Improvement of Fatigue Strength of Toe Fitted Joint: Summary of Welding Society National Lecture Meeting 49th p. 96 ('91)], it has been clarified that various fatigue strength improving methods by controlling residual stress are less effective than methods of relaxing stress concentration.

Further, the grinding / cutting treatment and the remelting treatment by TIG or plasma, which are the methods for relaxing the above-mentioned stress concentration, are aimed at adjusting the toe shape, and the flow of stress itself is changed. I can't.
Therefore, the effect is limited.

The most effective method is to fix an additive around the weld toe portion and change the flow of stress itself to alleviate the stress concentration at the toe portion.
The method disclosed in Japanese Patent No. 6964 is mainly intended to reduce the bending moment and is not effective when the main plate receives a tensile force.

Further, the quenching method after hot dip galvanizing has the effect of relaxing the stress concentration at the toe portion by adhering zinc having a low elastic modulus to the surface and the residual stress of compression due to the surface quenching. However, since the plating layer is very thin, its effect is not sufficient from the viewpoint of relaxing stress concentration.

The present invention relates to a fiber reinforced plastic (hereinafter referred to as F
Provided is a method for improving the fatigue strength of a welded portion, in which the stress concentration at the weld toe portion is reduced by fixing a layer called RP) and the fatigue strength of the welded joint is improved.

[0013]

SUMMARY OF THE INVENTION The gist of the present invention is that the entire surface of a fillet weld of a metal or a partial surface near the weld toe and a part of a main plate of the metal have a longitudinal direction of a joint. And the elastic modulus of the fiber-reinforced plastic in the direction along the surface is 5% to 150% of the metal main plate, (elastic modulus) ×
A method of improving fatigue strength of a welded part is characterized in that a fiber-reinforced plastic layer having a (thickness) of 1% to 50% of the metal main plate is fixed.

[0014]

The outline of the operation of the present invention is to reinforce the fillet welds with FRP to relieve the stress concentration at the weld toes and improve the fatigue strength of the joint.
The details will be described below.

In order to reduce the stress concentration at the weld toe, the FRP layer provided on the surface of the joint must share the load applied to the joint. At this time, if the elastic modulus of the FRP layer in the load direction is significantly lower than that of the base metal, the load is not shared, and the effect of stress concentration relaxation at the weld toe cannot be obtained.

Therefore, it is necessary that the FRP layer has an elastic modulus of 5% or more of the elastic modulus of the metal base material. On the contrary,
When the elastic modulus of the FRP layer is larger than 150% of that of the base material, the load transmitted by the FRP layer becomes very large. In such a case, stress concentration becomes large at the end portion of the FRP on the main plate side, which causes a fatigue crack to occur.

Further, if the elastic modulus of the FRP layer is larger than 150% of that of the metal base material, shear stress is generated at the interface between the metal base material and the FRP layer, which may cause interfacial peeling when repeated stress is applied. Become. For these two reasons, the elastic modulus of the FRP layer must not exceed 150% of the elastic modulus of the metal base material in order to obtain effective reinforcement.

In order to alleviate the stress concentration at the weld toe, it is necessary to share the load with the FRP layer on the surface.
For this purpose, the (modulus of elasticity) × (thickness) of the FRP layer must be 1% or more of the welded metal main plate.

When (modulus of elasticity) × (thickness) exceeds 50% of the welded metal main plate, the load sharing of this portion becomes too large, and the stress concentration on the metal side at the end of the FRP layer becomes large. As a result, fatigue cracks occur at this part. For this reason,
It is necessary to suppress (elastic modulus) × (thickness) to 50% or less of the welded metal main plate.

Further, the range in which the FRP layer is fixed must be a range sufficient to relieve the stress concentration at the weld toe. In order for the FRP layer to share the stress that should flow into the weld toe, the fillet welded portion and the main plate side must be fixed at least 30% of the weld leg length around the weld toe.

Further, the effect of reducing the stress is increased by increasing the coating portion, but the effect is saturated from the weld toe to the fillet welds and the main plate side at about 150% of the weld leg length, so that the stress that extends beyond this is increased. There is no point in terms of mitigation. However, there is no reason to prevent the extension of the FRP layer for reasons such as ensuring workability during production.

As the type of FRP to be used, any reinforcing fiber may be used, such as carbon fiber, aramid fiber, glass fiber, etc., as long as it satisfies the above requirements. Also, as the resin, any resin such as epoxy, polyester or engineering plastic may be used. However, epoxy is most suitable from the viewpoint of the fixing method described below.

As a method of fixing the FRP layer to the metal joint, there is a method of first bonding the molded FRP to a metal by using an adhesive. In this case, as the adhesive, any adhesive may be used as long as it has sufficient strength for joining the metal and the resin used for the FRP, such as an epoxy adhesive.

However, the most suitable fixing method is a method in which a sheet called prepreg in which fibers are impregnated with a resin is laminated on a metal joint, and the sheet is molded by heating and pressurizing the sheet to directly fix the sheet. In particular, by using a prepreg impregnated with an epoxy resin, it is possible to form a strongly adhered FRP layer on the metal joint.

[0025]

FIG. 1 shows an example of a cross section of a welded portion subjected to the fatigue strength improving method of the present invention, in which a main plate 11 and a rib plate 12 are joined by fillet welding 17 to form a non-load-transmitting rib cross joint. are doing. The FRP layer 13 is fixed to the surface of each fillet weld 17 so as to cover the main plate 11. In addition, 14 is an end of the FRP layer on the main plate side, 15 is a welding toe, and 16 is a welding leg length.

As Example 1, a non-load-transmitting rib cross joint (base material main plate thickness 22 m, using 360 MPa class steel (YP 440 MPa, TS 520 MPa) as a material.
m, rib plate thickness 10 mm, width 40 mm) and 1 mm
A joint reinforced with a thick CFRP layer was manufactured and a fatigue test was conducted.

CFRP is a PAN-based carbon fiber woven fabric (epoxy resin Vf = 50%) having an elastic modulus of 240 GPa, and the elastic modulus as FRP is 60 GPa. This is 28% of the elastic modulus of steel, 214 GPa.

For the production of the CFRP layer for reinforcement, the prepreg of the above carbon fiber woven fabric using a low temperature curing type (80 ° C.) epoxy resin was used. The prepreg was laminated so that one fiber direction of the woven fabric coincided with the longitudinal direction of the main plate and the entire surface of the welded portion of the joint and the weld toe of the main plate were covered by a length equal to twice the weld leg length.

A bag was produced around this laminated prepreg, and was formed by the so-called vacuum bug method in which the inside of the bag was decompressed by a vacuum pump. A light for illumination was used for heating during molding, and the epoxy resin was cured by leaving it for 2 hours while being heated to 80 ° C.

The test was carried out under a tensile repetitive load with a nominal stress range of 200 MPa and a stress ratio of 0. The life of the as-welded joint was 410,000 times, whereas that of the CFRP reinforced joint was 1.
With 030,000 cycles, the fatigue life was extended by a factor of 2.5 due to CFRP reinforcement.

As Example 2, a non-load transmission type T joint (base metal main plate thickness 22 mm, rib plate thickness 10 mm) using 360 MPa class steel (YP 440 MPa, TS 520 MPa) as a material, A joint was produced by reinforcing this with a CFRP layer having a thickness of 1 mm, and a fatigue test was performed.

CFRP is a PAN having an elastic modulus of 240 GPa.
Unidirectional carbon fiber (epoxy resin, Vf = 60%)
The elastic modulus in the direction in which the fibers as FRP are oriented is 144 GPa. This is the elastic modulus of mild steel 214G
It becomes 67% of Pa.

For the production of the CFRP layer for reinforcement, the prepreg of the above carbon fiber unidirectional material using a medium temperature curing type (120 ° C.) epoxy resin was used. The prepreg was laminated so that the fiber direction was aligned with the longitudinal direction of the main plate, and the entire surface of the welded portion of the joint and the weld toe of the main plate were covered by a length three times the weld leg length.

A bag was produced around this laminated prepreg, the inside of the bag was decompressed by a vacuum pump, and the bag was molded by a so-called autoclave method in which heating and heating were performed using an autoclave. 120 in autoclave
The epoxy resin was cured by heating at 0 ° C. and leaving it under a pressure of 2 atm for 2 hours.

The test was carried out in a three-point bending (span 300 mm) with a nominal stress range of 250 MPa and a stress ratio of 0, and the life of the as-welded joint was 250,000 times, whereas that of the CFRP reinforced joint was 530,000 times, and CFRP was 530,000 times. The fatigue life was extended about 2.1 times by the reinforcement.

As a third embodiment, the material is aluminum alloy A50.
Non-load transmission type rib cross joint using 83P-O (base material plate thickness 10 mm, rib plate thickness 5 mm) and ArFRP
A layer reinforced joint was made.

ArFRP is a aramid fiber woven fabric (epoxy resin, Vf = 50%) having an elastic modulus of 130 GPa, and the elastic modulus as FRP is 32 GPa. This is 45% of the elastic modulus of 71 GPa of the aluminum alloy.

For the production of the reinforcing ArFRP layer, the prepreg of the aramid fiber woven fabric using a low temperature curing type (80 ° C.) epoxy resin was used. Using this prepreg, an ArFRP layer was molded in the same manner as in Example 1.

The test was carried out at a nominal stress range of 60 MPa and a stress ratio of 0.
The tensile life was 120,000 times for the as-welded joint, whereas it was 32 for the CFRP reinforced joint.
10,000 times, the fatigue life is about 2.7 due to reinforcement by CFRP.
Doubled.

[0040]

As described above, according to the present invention, by fixing the fiber reinforced plastic to the surface of the fillet welded portion, stress concentration is relieved, the fatigue strength of the welded joint is improved, and the fatigue design is required. It is possible to increase the design load of various structures, improve the quality of welded structures, and reduce the weight of structures.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing an embodiment of the present invention and showing a rib cross joint formed by adding an FRP layer to a fillet weld.

[Explanation of symbols]

 11 Main Plate 12 Rib Plate 13 FRP Layer 14 Main Plate Side End of FRP Layer 15 Weld Toe 16 Weld Leg Length 17 Fillet Weld

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yukio Seiya 20-1 Shintomi, Futtsu City Nippon Steel Corporation Technology Development Division

Claims (1)

[Claims] 1. Elasticity of the fiber-reinforced plastic in the longitudinal direction of the joint and in the direction along the surface of the entire surface of the fillet weld of the metal or a part of the surface near the weld toe and a part of the main plate of the metal. Improved weld fatigue strength, characterized in that a fiber-reinforced plastic layer having a modulus of 5% to 150% of the metal main plate and an (elastic modulus) × (thickness) of 1% to 50% of the metal main plate is fixed. Method.