FIELD

The present invention relates to a lap joint structure comprised of a plurality of overlapping sheet members cut out from a sheet or sheet members formed from a sheet and with overlapping parts joined by a plurality of non-melting joined parts.

BACKGROUND

In the past, in the automotive field, spot welding has been made much use of for assembling car bodies, attaching parts, etc. Pluralities of steel sheet members including high strength steel sheets are also being joined by spot welding. However, in a spot welded joint including steel sheets with a tensile strength of 780 MPa or more, the toughness of the nuggets falls and stress concentrates at the nugget ends in the peeling direction, so there is the problem that even if the steel sheets increase in tensile strength, the cross tensile strength (CTS) will not increase or will decrease. As one art for solving this problem, there is the art of mechanically joining a plurality of metal sheets using rivets, screws, or other mechanical joining means without causing the matrix material to melt. By using this art, there is a possibility that auto parts can be produced higher in reliability of strength than the past.

Further, in the body of a car etc., for the purpose of lightening the weight, sometimes combinations of different materials such as steel sheets and aluminum sheets or steel sheets and carbon fiber reinforced plastic (CFRP) sheets are joined. If in this way the materials combined are materials with different melting points, linear expansion coefficients, and other physical properties, the materials are fastened and joined by mechanical joining means such as described in for example PTLs 1 and 2. Further, in aluminum sheets with their low electrical resistances, sometimes friction stir spot joining is used instead of resistance spot welding.

CITATION LIST

Patent Literature

[PTL 1] Japanese Unexamined Patent Publication No. 2000-272541[PTL 2] Japanese Unexamined Patent Publication No. 2005-119577

SUMMARY

Technical Problem

If joining overlapping parts of a plurality of overlapping sheet members by blind rivets or other mechanical joining means at joined parts, the joined parts of the members are formed with holes through which the rivets are passed. Further, if spot joining overlapping parts by friction joining means, the joined parts of the sheet members at the rotary tool side are left with holes formed by the indentation marks of the probe at the tip of the rotary tool. In the studies of the inventors, in a lap joint member joining overlapping parts by a mechanical joining means or friction joining means, if the lap joint member as a whole receives tensile deformation, strain concentrates at the holes formed at the joined parts and, as shown in Test No. 1 ofFIG. 19showing the results of a tensile test in the later explained examples, there is the problem of the sheet members breaking starting from the holes due to small deformation. Note that, in the present invention, sometimes the mechanical joining means and friction stir spot joining will be referred to together as “non-melting joining means”.

Therefore, the present invention has as its object to provide a lap joint structure of a lap joint member able to keep sheet members from breaking starting from holes formed at the time of joining the members when joining overlapping parts formed by a plurality of overlapping sheet members by non-melting joining means.

Solution to Problem

In the present invention, to deal with such a problem, the inventors studied means for dispersing strain so that strain does not concentrate at the end parts of holes formed at the joined parts. As a result, they obtained the discovery that by forming cutaway recessed parts at the overlapping parts of sheet members between adjacent joined parts in the direction from the end parts of the overlapping parts to the joined part sides, the structure becomes one dispersing strain due a tensile load and fracture of the lap joint member starting from the holes is suppressed. The present invention was made based on such a discovery and has as its gist the following:

(1) A lap joint structure comprised of a plurality of sheet members with an overlapping part joined at a plurality of joined parts in a point manner by mechanical joining means or friction stir spot joining means and with the joined parts formed with, at least at one of the sheet members, holes through which the mechanical joining means are inserted or holes formed at a time of spot joining by the friction stir spot joining means, in which lap joint structure,the overlapping part of at least one of the sheet members among the above sheet members is formed with a cutaway recessed part between adjacent joined parts in a direction from an end part of the overlapping part to the joined parts, andan inside bottom part of the cutaway recessed part is formed at a position of K or more apart from the end part of the overlapping part, where K is an inside diameter of the hole.

(2) The lap joint structure according to the above (1) wherein the hole is positioned so that a distance L between an end part of the hole and an end part of a sheet member at which the cutaway recessed part is formed satisfies relationship of L≥0.8K with respect to the inside diameter K of the hole.

(3) The lap joint structure according to the above (1) or (2) wherein the hole is a through hole passing through the at least one of the sheet members.

(4) The lap joint structure according to any one of the above (1) to (3) wherein a shortest distance between an end part of the hole and an end part of the cutaway recessed part is 0.8K or more.

(5) The lap joint structure according to any one of the above (1) to (4) wherein the cutaway recessed part is formed in at least a sheet member where the hole is formed.

(6) The lap joint structure according to any one of the above (1) to (5) wherein the inside bottom part of the cutaway recessed part is formed sunken in from the end part of the overlapping part over a range of up to 1.5K or more.

(7) The lap joint structure according to any one of the above (1) to (6) wherein the inside bottom part of the cutaway recessed part has a parallel part parallel with the end part of the overlapping part.

(8) The lap joint structure according to the above (7) wherein a length of the parallel part is 0.5K or more.

(9) The lap joint structure according to any one of the above (1) to (8) wherein the sheet member formed with the cutaway recessed part is a hat-shaped member with bent parts formed at an opposite side to the end part of the overlapping part across the joined parts and with a hat-shaped cross-section and the cutaway recessed part is formed over a range from the bent parts of the hat-shaped member to an end part side.

(10) The lap joint structure according to any one of the above (1) to (9) wherein the sheet members are steel sheet members and the cutaway recessed part is formed in a steel sheet member with a tensile strength of 590 MPa or more.

(11) The lap joint structure according to any one of the above (1) to (10) wherein the mechanical joining means are any of blind rivets, self piercing rivets, drill screws, bolts, and resistance element welding.

(12) The lap joint structure according to any one of the above (1) to (11) wherein joining by a resin is jointly used in addition to joining by the mechanical joining means or friction stir spot joining means.

Advantageous Effects of Invention

According to the present invention, when joining a plurality of metal sheet members or CFRP sheet members by non-melting joining means, it is possible to keep the joined parts from breaking starting from the holes through which mechanical joining means are passed or holes formed at the time of spot joining by friction stir spot joining means. As a result, for example, it is possible to produce high strength auto parts excellent in performance in protecting passengers at the time of collision.

