JP6432267B2 - Welded structural members that reduce stress concentration - Google Patents

Description

  The present invention relates to a welded structural member that reduces stress concentration at a contact point between a weld bead and a bracket, and more particularly to a welded structural member to which a repeated load is applied, such as an underbody member of an automobile.

  In the automobile field, in order to preserve the environment, there is a demand for improved collision safety as well as improved fuel economy by reducing the weight of the vehicle body. For this reason, various efforts have been made so far in order to reduce the thickness by using a high-strength steel sheet and optimize the vehicle body structure to reduce the weight of the vehicle body and improve the collision safety.

  Fatigue strength is also required for high-strength steel sheets that reduce the weight of the vehicle body. Usually, the fatigue strength of the base material used for the welded member increases in proportion to the steel plate strength, but it is known that the fatigue strength of the welded joint hardly increases even when the steel plate strength increases. This hinders the weight reduction of the vehicle body due to the use of high-strength steel plates.

  In particular, the fatigue strength of the welded portion is more important in automobile underbody members such as suspension arms and subframes. Fillet arc welding is often used for welding these suspension members, and the welding structure member for the suspension is only on one side of the joint (overlapping portion) between the steel member and the steel member due to the structure of the member. There are welds where fillets are welded, and fatigue cracks are likely to occur in such places. Therefore, in order to achieve weight reduction of the suspension member, it is a problem to increase the fatigue strength of the fillet welded joint that is fillet welded only on one side of the joint.

  Conventionally, as means for improving the fatigue strength of fillet welded joints, finish processing of bead toes by means of grinder processing or decorative fill welding, and welding of reinforcing members such as ribs to appropriate shapes and positions have been performed. It has been broken. Finishing the toe portion is time consuming, and there is a problem that welding another member leads to an increase in cost, and it is not a technology that can be applied to the manufacture of mass-produced products such as automobile parts.

  A technique for reducing stress concentration from the structural surface of the member to be welded to the toe is disclosed in Patent Document 1. Patent Document 1 discloses that an arc-shaped notch is provided on the side surface of the bracket to reduce the stress at the toe at the start and end of the welded portion between the structural member and the bracket.

  However, in the technique described in Patent Document 1, in fillet welding of a welded structural member to which repeated loads are applied, such as an automobile underbody member, stress reduction at the toe portion is not sufficient, and stress is further reduced. It was necessary to let them.

  On the other hand, Patent Document 2 discloses a technique for reducing stress concentration from the surface of the welding method to the toe portion. Patent Document 2 discloses a weld bead structure in which the end face of the other steel plate is butted against one surface of the steel plate, fillet beads are formed on both sides of the butted portion, and this weld bead is extended. The weld bead structure disclosed in Patent Document 2 relaxes the stress concentration on the toe portion by moving the toe portion away from the end portions of other steel plates. However, there is no effect of reducing stress concentration on the root portion, and the occurrence of fatigue failure cannot be effectively suppressed.

JP 2004-169864 A JP 09-253843 A

  In the present invention, in view of such a situation, only one side of the joint portion is fillet welded, and a welded structure member to which a repeated load such as an undercarriage member of an automobile is applied has a simple structure, a toe portion and a route. It is an object of the present invention to provide a welded structure member that can prevent the occurrence of cracks due to fatigue at the joint.

  The present inventors diligently studied on means for solving the above problems. As a result, an extension bead is formed in the welded structural member that is fillet welded only on one side of the joint portion between the bracket and the structural member provided with the arc-shaped notch on the side surface. At that time, the notch and the weld bead are formed. It was found that the stress concentration at the toe portion of the weld bead is further relaxed and the stress concentration at the root portion is also mitigated by limiting the position with the upper end portion.

The present invention has been made based on such knowledge, and the gist thereof is as follows.
(1) A welded structural member in which a bracket formed from a steel plate and a structural member are fillet welded, the bracket having one or two flanges, and a circle on at least one side of the open side surface of the bracket An arc-shaped notch is formed, and the fillet welding is performed only on one side of the bracket with respect to the overlapping portion of the bracket and the structural member, and the weld bead by the fillet welding is at least the cut. It is formed so as to extend from the end of the bracket on the side surface where the notch is formed, and the upper end of the weld bead is formed so as to come into contact with the notch portion below the center height position of the notch. A welded structure member characterized by comprising:
(2) When the bracket width is W and the distance from the side surface to the notch at the center height of the notch is d, d / W is 0.05 to 0.15. The welded structure member according to (1).

  According to the present invention, in a welded structural member fixed to a structural member by fillet welding the bracket, even if the member is repeatedly exposed to vibration stress, the bracket notch and the weld bead are in contact with each other. Thus, it is possible to provide a welded structure member that significantly suppresses the occurrence of fatigue failure and has excellent fatigue characteristics.

It is a figure which shows the bracket which has an arc-shaped notch. (A) is the bracket formed from the steel plate, (b) shows a flange. It is a figure which shows the welded structural member which a notch and the weld bead are not contacting. (A) shows a welded structural member and (b) shows a flange. It is a figure which shows the welding structural member which the notch and the weld bead are contacting. (A) shows a welded structural member and (b) shows a flange. It is a figure which shows the welding structure member comprised from the bracket which has an arc-shaped notch. (A) shows the welding structural member comprised from the bracket which made the U-shaped opening the lower side, (b) shows the welding structural member comprised from a plate-shaped bracket. It is a figure which shows the relationship of the stress concentration factor Kt with respect to the ratio (d / W) of the bracket width W and the notch depth d. It is a figure which shows the external appearance of the bracket for a test.

  First, an example of a bracket having a notch according to the present invention will be described with reference to the drawings.

  FIG. 1 shows a bracket having an arcuate notch. (A) is the bracket formed from the steel plate, (b) shows a flange. The bracket 1 includes a flange 2 that is a constituent element, various connecting members, and the like (not shown) (see Patent Document 1). The bracket 1 is formed of a steel plate having a thickness of 5 mm or less, and the steel type and component composition of the steel plate are not particularly limited. The bracket 1 is U-shaped and has a notch 4 on the open side surface 3.

  When the direction from the welded end of the flange 2 on the side close to the notch 4 to the other end is defined as the height direction, the bracket 1 has a bracket height H and the direction perpendicular thereto is the width. When the direction is taken, the bracket 1 has a bracket width W. The notch 4 has an arc shape with the center of the circle indicated by the dotted line as the center 5 of the notch, and has a notch radius r. Further, the distance d between the notch portion at the height position of the notch center 5 and the side surface 3 is shown smaller than the notch radius r. Hereinafter, the distance d may be referred to as “notch depth”. The height H1 from the end of the bracket 1 to the position of the notch at the lower end is formed to be ½ or more of the weld bead height, for example, 3 mm or more so that the fatigue strength does not decrease. Yes.

  In order to make such a welded structural member welded with a bracket and a structural member a welded structural member to which a repeated load is applied, such as an automobile underbody member, stress at the toe portion and the root portion is further reduced. Therefore, it is necessary to improve the fatigue characteristics.

  First, the present inventors examined extending the weld bead in order to further alleviate the stress concentration at the toe portion of the weld bead.

  FIG. 2 is a view showing a welded structural member in which the notch and the weld bead are not in contact with each other. (A) shows a welded structural member and (b) shows a flange. This welded structural member is formed by fillet welding with the structural member 6 only from one side of the flange 2 of the bracket 1 having an arc-shaped notch so that the weld bead extends from the side surface 3. In addition, the dotted line of (a) has shown the weld bead of the hidden part.

  With respect to the bracket 1 of this welded structural member, stress distribution analysis was performed by a finite element method (FEM). As a result, stress was concentrated on the peripheral portion (hereinafter referred to as “contact portion”) of the notch 4 where the bracket 1 and the weld bead 7 are in contact.

  Thus, the stress is concentrated on the contact portion 8 because the stress is not efficiently dispersed in the notch 4. Therefore, the present inventors anticipate that it is necessary to ensure that the notch 4 and the weld bead have an appropriate positional relationship in order to efficiently distribute the stress to the notch 4, and the notch 4 and the weld bead. The positional relationship was examined.

  FIG. 3 is a view showing a welded structural member in which a notch and a weld bead are in contact with each other. (A) shows a welded structural member and (b) shows a flange. When this welded structural member is welded to the structural member 6 only from one side of the flange 2 of the bracket 1 having an arc-shaped cutout, the lower end of the cutout 4 and the upper end of the weld bead 7 are brought into contact with each other. Further, the weld bead extends from the side surface 3. In addition, the dotted line of (a) has shown the weld bead of the hidden part.

  In this welded structural member, the stress distribution analysis by the finite element method (FEM) was performed on the bracket 1 in the same manner as described above. As a result, the stress at the contact portion 8 was reduced. It is considered that when the notch 4 and the weld bead 7 are brought into contact with each other, the stress is not concentrated on the contact portion 8 and the stress is dispersed by the notch.

  Further, in the welded structure member in which the notch portion at a position exceeding the height of the center 5 of the notch and the weld bead 7 are in contact with each other, the stress is concentrated on the contact portion 8. This is because the weld bead 7 is formed so as to cover the notch, so that the stress dispersion effect by the notch 4 is reduced.

The examination results are summarized as follows.
By configuring the welded structure member so that the upper end portion of the weld bead 7 is in contact with the notch portion at a position equal to or lower than the height of the center 5 of the notch, stress concentration at the contact portion 8 is reduced, and fatigue characteristics are reduced. It can be set as the welded structure member excellent in. The range from the notch center 5 to the notch lower end is referred to as “stress reduction range a”.

  The present invention has reached the invention described in the above (1) through the examination process as described above, and the necessary and preferred requirements will be further described in order.

(Bracket shape)
The bracket of the present invention is not limited to that shown in FIG. 1, but has one or two flanges and an arc-shaped cutout on at least one side of the opened side surface. Next, another example of the bracket is shown.

  FIG. 4 is a view showing a welded structure member constituted by a bracket having an arcuate notch. (A) is a welding structure member comprised from the bracket which made the U-shaped opening the lower side, (b) is a welding structure member comprised from a plate-shaped bracket. Each of the brackets 1 in FIGS. 1 and 4A is a U-shaped bracket 1 in which the direction of the U-shaped opening is changed.

  The bracket 1 shown in FIG. 1 has notches on both sides of the two side surfaces 3 where the bracket 1 is opened, but the bracket 1 shown in FIG. The notch 4 is provided in two places in the side surface 3 of this. In the bracket 1, it is necessary to provide at least one notch on the surface of the flange 2 to be welded, and the notch may be provided on all of the open side surfaces 3.

  FIG. 4B shows a plate-like bracket, and notches 4 are provided on both sides of the open side surface 3 of the bracket 1. Further, a notch may be provided on one side of the open side surface 3 of the bracket 1.

  The shape of the bracket shown in FIGS. 1 and 4 is an example, and the effects of the present invention can be obtained with other shapes.

(Notch shape)
Although FIG. 1 shows the notch 4 formed from a circular curved arc, it may be a circular arc formed by an elliptical curve or a part of a parabolic curve. When the notch 4 is a circular arc and an elliptical curved arc, the notch center 5 is the center of the circular and elliptical curve, and is an annular shape formed by a meandering (waving) curve. In the case of an arc, the notch center 5 is the circle equivalent center. When the notch 4 is a part of a parabolic curve, the parabola has no center, and the position of the curve that is most concave in the bracket width W direction from the side surface 3 is defined as the height position of the notch center.

  The notch depth d is preferably equal to or smaller than the notch radius r. When the notch depth d is larger than the notch radius r, the strength of the bracket 1 is lowered. When the notch 4 is a circular curved arc, the notch radius r is a circular radius.

When the notch 4 is an elliptical curved arc, the notch radius r is set as follows.
・ If the direction of the major axis of the elliptical curve matches the bracket width W direction, the major radius ・ If the direction of the minor axis of the elliptical curve matches the bracket width W direction, the minor radius ・ Other In this case, the distance from the position of the arc that is most recessed in the bracket width W direction from the side surface 3 to the position where the line in the bracket width direction that passes through the position intersects the line in the height direction that passes through the notch center.

  In the case of an annular arc formed by a meandering (waving) curve, the radius r of the notch is a circle equivalent radius. In the case of a parabolic curved arc, the position where the line in the bracket width direction passing through the position intersects the line in the height direction along the side surface 3 from the position most recessed in the bracket width W direction from the side surface 3. The distance to is the radius.

(Weld bead)
The weld bead is formed so as to be welded to the structural member 6 only from one side of the flange 2 of the bracket 1 and extend from the side surface 3, and the upper end portion of the weld bead 7 is in a stress reduction range a. It is formed so as to be in contact with the notch 4.

  The upper end portion of the weld bead 7 is in contact with the notch 4 outside the range of the stress reduction range a, that is, when the upper part of the weld bead 7 is not above the stress reduction range a, or in contact with the portion without the notch 4 below the stress reduction range a. If so, the stress dispersion is reduced. In FIG. 3, the lower end portion of the notch 4 and the upper end portion of the weld bead 7 are in contact with each other, but the weld bead 7 is notched so as to cover the notch 4 within the range of the stress reduction range a. You may make it contact 4 part.

  Further, the weld bead 7 forms an extended bead beyond the contact portion of the flange 4 at each of the start and end points in order to reduce stress concentration at the toe portion of the weld and improve fatigue strength. The length of the extended bead is not particularly limited as long as the stress at the toe portion can be reduced, but is preferably 10 to 40 mm. In FIGS. 2 and 3, the weld bead 7 is formed in a U-shape in accordance with the U-shaped bracket 1, but the lower end of the two flanges 2 having the notches 4. Only the welding bead 7 may be welded to the structural member 6 and two welding beads 7 may be formed linearly one by one.

(Relationship between notch depth and bracket width)
The ratio (d / W) of the bracket width W to the notch depth d is preferably 0.05 to 0.15. By setting it as this range, the stress concentration in the periphery of the contact portion 8 between the notch 4 of the bracket 1 and the weld bead 7 is further reduced.

  FIG. 5 shows the relationship of the stress concentration factor Kt to the ratio (d / W) between the bracket width W and the notch depth d. The right figure shows the direction of stress applied to the bracket. Thus, compared with the case where there is no notch, when the notch is provided and the contact with the weld bead is adjusted, the stress concentration coefficient Kt is lowered. When the ratio (d / W) is in the range of 0.05 to 0.15, the stress concentration coefficient Kt is preferably 3 or less.

(Other requirements)
The welded structural member is preferably assembled by arc welding. The arc welding conditions at the time of welding bead construction and the composition of the welding wire to be used may be in accordance with ordinary methods and are not limited to specific ones. In addition to 100% CO ₂ gas, a mixed gas of Ar gas and 3 to 20% CO ₂ gas can be used as the shielding gas.

  Next, examples of the present invention will be described. The conditions in the examples are one example of conditions used for confirming the feasibility and effects of the present invention, and the present invention is based on this one example of conditions. It is not limited. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

  FIG. 6 shows the appearance of the test bracket. The test bracket is a U-shaped steel plate having a component composition shown in Table 1 having a thickness of 2.6 mm and a height of 50 mm, and a notch radius r, a notch depth d, and a bracket width W shown in Table 2. It processed so that it might become.

  These test brackets were arranged on a plate-like structural member as shown in FIG. 3, and a welded structural member was prepared by welding the joints of the two by arc welding from the outside of the bracket. Arc welding was performed using the welding wires shown in Table 3 under the welding conditions shown below. However, each test bracket was welded so as to have a distance Lb between the center notch position and the upper end of the weld bead shown in Table 2. The center notch position is the position of the notch at the height of the notch center, and the lower end notch position indicates the position of the lowest notch.

Welding method Consumable electrode welding Shielding gas Ar + 20% CO ₂
Shielding gas flow rate 20L / min
Welding speed 80cm / min
Welding current and voltage Set values that are not undercut, 230A, 25V

  Comparative Example 1 is a test bracket that does not have a notch. In Comparative Example 2, the distance Lb between the center notch position and the upper end of the weld bead is larger than the distance Lc (stress reduction range a) between the center notch position and the lower end notch position, that is, the weld bead is notched. Welded to avoid contact. In Invention Examples 1 to 7, welding was performed such that the distance Lb between the center notch position and the upper end of the weld bead was smaller than the distance Lc between the center notch position and the lower end notch position. Further, an extension bead La was formed so that the start and end of the weld bead extended 20 mm each from the flange width of the bracket.

  The manufactured welded structural member for test is gripped by an electrohydraulic fatigue test apparatus so that the load direction P is the direction indicated by the arrow in FIG. 6, and the load range is constant (stress range is constant) and the load ratio is -1. A fatigue test was conducted at a repetition frequency of 5 Hz. Table 4 shows the results of the fatigue test.

  In Comparative Example 1, since notches are not formed in the bracket, stress concentration occurs at the contact point between the weld bead and the bracket, and the fatigue life is low. In Comparative Example 2, a notch was formed in the bracket, but since the weld bead was not in contact with the notch, stress concentration occurred at the contact portion between the weld bead and the bracket. Although the fatigue life of Comparative Example 2 is greater than that of Comparative Example 1, sufficient fatigue strength is still not obtained. Accordingly, sufficient fatigue strength cannot be obtained simply by forming a notch in the bracket, and therefore the positional relationship between the notch and the weld bead needs to be appropriate.

  In Invention Examples 1 and 2, the upper end portion of the weld bead is formed between the center notch position and the lower end notch position, and the weld bead has a larger portion in contact with the notch. In Invention Examples 3 to 7, the upper end portion of the weld bead is formed so as to contact the lower end notch. Inventive Examples 1 to 5 are circular arc cutouts, Inventive Example 6 is an elliptical arc cutout, and Inventive Example 7 is a partial cutout of a parabolic curve. The number of these fatigue lives exceeds 70,000, and the upper end of the weld bead and the notch are in contact with each other within the range of the stress reduction range a, so that the stress concentration can be reduced. Recognize.

  Inventive Examples 1 to 3 and 5 to 7 have a ratio d / W of the bracket width W to the notch depth d of 0.05 to 0.15, whereas Inventive Example 4 has a ratio d / W. 0.20. The number of times of life fatigue of Invention Example 4 is smaller than that of other Invention Examples. Thus, by optimizing the ratio of the notch depth d to the bracket width W, the stress at the contact portion between the weld bead and the bracket can be further reduced, and the fatigue life can be increased.

  As described above, according to the present invention, in the welded structural member fixed by welding the bracket to the structural member, the weld bead and the notch of the bracket come into contact even when the member is repeatedly subjected to vibration stress. By adopting a structure, it is possible to provide a welded structural member having excellent fatigue characteristics that can significantly suppress the occurrence of fatigue failure. The present invention has high industrial applicability.

DESCRIPTION OF SYMBOLS 1 Bracket 2 Flange 3 Side surface 4 Notch 5 Center of notch 6 Structural member 7 Weld bead 8 Contact part a Stress reduction range d Distance between side surface and notch (notch depth)
r Notch radius H Bracket height H1 Height from bracket end to bottom notch position W Bracket width P Load direction

Claims (2)

A welded structural member in which a bracket formed from a steel plate and a structural member are fillet welded, and the bracket has one or two flanges, and an arcuate cut is formed on at least one side of the open side surface of the bracket. A notch is formed, and the fillet welding is performed only from one side surface of the bracket to the overlapping portion of the bracket and the structural member, and at least the notch is formed in the weld bead by the fillet welding. The side surface is formed to extend from the end portion of the bracket, and the upper end portion of the weld bead is formed so as to be in contact with the notch portion below the center height position of the notch. A welded structural member.   The d / W is 0.05 to 0.15, where d is the bracket width W and d is the distance from the side surface to the notch portion at the center height of the notch. The welded structure member according to 1.

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