JP4418287B2 - Optimization method for increasing the fracture strength of spot welds - Google Patents

Description

  The present invention is suitable for spot welding of structural members for automobiles. More specifically, the member shape, plate thickness, material strength, and spot welding for increasing the strength at which a spot welded portion of a member breaks at the time of collision deformation are described. The present invention relates to an optimization method for increasing the breaking strength of a spot weld for obtaining a nugget diameter.

  In recent years, in the automobile industry, the development of a vehicle body structure that can reduce injury to passengers during a collision has become an urgent issue. Such a vehicle body structure excellent in collision safety can be realized by absorbing the impact energy at the time of collision by a structural member other than the passenger compartment to ensure the living space by minimizing the deformation of the passenger compartment. That is, it is important to absorb impact energy by the structural member.

  The main structural member that absorbs impact energy in automobile full-wrap collision and offset collision is the front side member. The front side member has a closed cross-section by spot welding after forming the member by press molding or the like. Usually, this front side member is buckled to absorb impact energy. In order to improve the absorption of impact energy, it is important to stabilize the buckling form and not to bend or break along the way.

  With regard to spot welding of the above-mentioned members, if the spot welding interval, nugget diameter and welding conditions are not optimized in order to stabilize the buckling, fracture from the weld point will occur at the time of buckling, and a stable buckling form There is a problem that the absorption of impact energy is not reduced.

Explanation paper No. 9705JSAE SYMPOSIUM “New Body Structure Molding Technology” JIS Z3136 JIS Z3137 JP-A-6-182561 JP 2002-31627 A

  Conventionally, in order to solve this problem, for example, as described in Non-Patent Document 1, a sample is made by changing the spot welding interval in various ways, a buckling test is performed, and conditions for stable buckling without breaking at the welding point are set. I was investigating. However, this method has a problem that trial and error such as making a prototype for each automobile and each member and performing a test is necessary, which requires a manufacturing cost and takes time for designing.

  Further, Patent Document 1 proposes a structure for preventing the peeling of the welded portion where the load is applied to the floor panel, but it is a structure only for the floor panel and prevents the peeling of the welding point with all the impact absorbing members. The spot welding method that absorbs impact energy by stable buckling has been trial and error by trial production.

  Further, Patent Document 2 proposes optimization of the spot welding interval, but the individual spot welding strength is only a simple index and is not an accurate prediction of the fracture itself, so it has high accuracy. There was a problem that the design based on the prediction of spot weld fracture could not be made.

  Typical strength indicators for spot welds are the shear tensile test and the cross-shaped tensile test defined in Non-Patent Documents 2 and 3. There are other examples of reports in various test forms that assume various load conditions. Generally, the two types of tests specified by JIS are used to determine the shear tensile test value as the shear strength of the welded part. The cross-shaped tensile test value is treated as the peel strength of the weld.

  However, since the shear strength and peel strength of spot welding obtained by the test are affected by the structure such as the width, the actual member must be corrected and estimated from various viewpoints. In systems that perform optimal design by simulation of automobile collisions on computers that have made great strides in recent years, this estimation accuracy is not sufficient, reducing the reliability of optimal design for collision safety.

  The present invention is a specimen-level spot welding strength data table in which the shape, width, plate thickness, material strength, load loading method, and spot welding nugget diameter are changed, regardless of the trial production / impact test of the member, or Based on the prediction formula of spot welding strength created based on the data table, width, plate thickness, material strength, spot welded nugget diameter to increase the fracture strength at the spot welded portion of any member during impact deformation One or more of them are calculated, and the purpose is to prevent welding part breakage at the time of impact of the member, to optimize the deformation buckling mode, and to improve the absorption of impact energy.

The method for optimizing the spot welded portion of the present invention is the material strength TS (MPa), plate thickness t (mm), spot weld nugget diameter d (mm), and joint width of the joint in the cross-type tensile test. W (mm), maximum load at break Fcts (N), rotation angle θ of the joint of the cross-type tensile test, and / or material strength TS (MPa) of various specimens in the shear-type tensile test, sheet thickness t (Mm), spot welding nugget diameter d (mm), joint plate width W (mm), and maximum load Ftss (N) at the time of breakage are measured, based on the formula (1) or (4), Alternatively, a database is created by calculating the stress concentration factor α based on the relational expression of the stress concentration factor α defined by the equation (1) or (4) and the ratio d / W of the nugget diameter d and the width W. , Any part based on the database Of the breaking limit load so as to maximize the spot weld has a width of a spot weld, the plate thickness, material strength, characterized in that determining the one or more of the nugget diameter.
α = TS · W · t / Ftss (1)
α = 2 · TS · W · t · sinθ / Fcts (4)
Where α is the stress concentration factor at the end of the nugget of the spot weld and the base metal in cross-type tension and / or shear-type tension

  According to the present invention, it is possible to accurately predict the breaking strength of a spot welded portion in an arbitrary member.For example, verification of spot welded portion breakage during a collision test on an actual automobile member may be omitted, The number of verification tests can be greatly reduced. In addition, the design of members that prevent spot welding breakage by large-scale experiments in trial production and collision tests with different spot welding conditions for automobile members is omitted, so that the fracture strength when the load method of the member is decided is maximized Since the shape, width, plate thickness, material strength, and spot welding nugget diameter can be easily determined, it is possible to greatly reduce the cost and shorten the design development period.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing an outline of a shear-type tensile test. As shown in the drawing, the test piece is formed by spot welding two steel plates as the base material 2 to form the nugget 1. A tensile test is performed on the test piece in the direction indicated by the arrow 3 until the test piece breaks. At this time, the displacement and load of the test piece in the tensile direction 3 are measured. A rupture occurs around the nugget 1, and at this time, the maximum load is obtained, which is defined as a rupture limit load Ftss (N). When this limit load Ftss is reached, the average stress σo (MPa) in the base material is Ftss / W · t from the width W (mm) of the base material 2 and the plate thickness t (mm).

Assuming that the maximum stress has reached the tensile strength TS (MPa) around the nugget 1 that is the starting point of the fracture, the stress concentration coefficient α at the end of the nugget 1 and the base material 2 is determined as the tensile strength of the base material. The ratio of the strength TS and the average tensile stress σo of the base material can be defined as shown in equation (1).
α = TS / σo = TS · W · t / Ftss (1)

By measuring the fracture limit load Ftss with various specimen widths W, sheet thickness t, nugget diameter d (mm) with various tensile strength TS materials, this stress concentration factor α is expressed as (1) Calculate from the formula and create a table as a database. From this, the fracture limit load Ftss at an arbitrary tensile strength TS, plate thickness t, width W, and nugget diameter d can be predicted by equation (2) using the stress concentration factor α in the table.
Ftss = TS · W · t / α (2)

Moreover, since the stress concentration factor α becomes a single curve when arranged by the ratio d / W of the nugget diameter d and the width W, Ftss is also calculated from the equation (2) using α calculated from the equation (3). You may predict.
α = k / (p · d / W−q) ⁿ + r (3)
Here, k, p, q, n, and r are parameters for fitting the relationship between the curve of α and d / W by equation (3). The equation for fitting the curve does not necessarily have the form of equation (3), and any equation that can fit the curve relationship may be used. Further, α may be read directly from the curve graph without using the equation (3).

  From the formulas (2) and (3), an arbitrary nugget diameter d, width W, plate thickness t, and fracture limit load Ftss at the material strength TS can be obtained. Conversely, for example, if the thickness t and the material strength TS of the material used for the member are determined, a combination of the nugget diameter d and the width W that maximizes the fracture limit load can be obtained. That is, a combination of nugget diameter, width, plate thickness, and material strength at which the fracture limit load Ftss of the curved surface generated from the equations (2) and (3) is maximized is obtained, and the optimum is selected according to the purpose. Choose the right combination.

  Moreover, even if it is not the maximum value of the curved surface, the combination of the change rate of the nugget diameter, width, plate thickness, and material strength at which the change rate of Ftss including the maximum value is maximum and minimum is obtained, and Ftss is stable within the range. Then, it may be used as a range for maintaining strength. Here, in addition to the test piece width W, for example, as shown in FIG. 2, the width W includes a hat-shaped section flange width 4, a section length 5, a buckling bead interval 6, and a spot welding pitch 7. It corresponds to either. As described above, since the actual members have various shapes, the dimensions around the spot welding and the dimensions that determine the buckling form may be evaluated as the dimensions corresponding to the width W each time.

  FIG. 3 is a diagram showing an outline of the cross-shaped tensile test method. As shown in the drawing, the test piece is formed by spot welding two steel plates as the base material 2 to form the nugget 1. A tensile test is performed on the test piece in the direction indicated by the arrow 3 until the test piece breaks. At this time, the displacement and load of the test piece in the tensile direction 3 are measured. A fracture occurs around the nugget 1, and at this time, the maximum load is obtained, and this is defined as a fracture limit load Fcts (N). When the limit load Fcts is reached, the average stress σo in the plate surface of the base material 2 is calculated from the width W (mm) of the base material 2 and the thickness t (mm) using the angle θ shown in FIG. / (2 W · t · sin θ).

Assuming that the maximum stress has reached the tensile strength TS (MPa) around the nugget 1 that is the starting point of the fracture, the stress concentration coefficient α at the end of the nugget 1 and the base material 2 is determined as the tensile strength of the base material. The ratio of the strength TS (MPa) to the average tensile stress σo (MPa) of the base material can be defined as in the formula (4).
α = TS / σo = 2 · TS · W · t · sinθ / Fcts (4)

This is exactly the same as the equation (1) obtained in the shear type tensile test, and the angle direction θ is entered because the tensile direction is different. Therefore, the fracture limit load Fcts can be calculated from the equation (5) based on an arbitrary width, plate thickness, material strength, and nugget diameter in the same manner as the shear type tension.
Fcts = 2 ・ TS ・ W ・ t ・ sinθ / α (5)

  From the formulas (4) and (5), the fracture limit load Fcts can be obtained in the same way as the shear type tensile test. The smaller one of the fracture limit loads Ftss and Fcts may be set as the design fracture limit load, and if the deformation mode of the spot weld is known from the member shape and load conditions, The corresponding breaking limit load may be used.

  This method can be applied to any material, not just steel materials. In addition to spot welding, all welding such as laser welding, arc welding, seam welding, mash seam welding, etc., and all mechanical bonding such as TOX bonding, rivet bonding, friction bonding, diffusion bonding, friction diffusion bonding, adhesives It can be applied to all joints.

  The calculation method of the stress concentration coefficient α by experiment is not limited to the above-described shear type tensile test and cross-type tensile test, and can be calculated by any test piece shape and load application method.

  Needless to say, the above-described prediction of fracture determination can be applied not only to the collision analysis of the entire automobile and members, but also to parts other than automobiles, and can also be applied to analysis using quasi-static deformation other than collision.

  FIG. 7 is a block diagram showing an example of a computer system capable of executing the optimization method of the present invention. In the figure, reference numeral 1200 denotes a computer PC. The PC 1200 includes a CPU 1201, executes device control software stored in the ROM 1202 or the hard disk (HD) 1211, or supplied from the flexible disk drive (FD) 1212, and collects all devices connected to the system bus 1204. To control.

  Each function unit of the present embodiment is configured by a program stored in the CPU 1201, ROM 1202, or hard disk (HD) 1211 of the PC 1200.

  A RAM 1203 functions as a main memory, work area, and the like for the CPU 1201. Reference numeral 1205 denotes a keyboard controller (KBC), which controls to input a signal input from the keyboard (KB) 1209 into the system main body. Reference numeral 1206 denotes a display controller (CRTC), which performs display control on the display device (CRT) 1210. Reference numeral 1207 denotes a disk controller (DKC), which is a hard disk (boot program (startup program: a program for starting execution (operation) of personal computer hardware and software)), a plurality of applications, editing files, user files, a network management program, and the like. HD) 1211 and flexible disk (FD) 1212 are controlled.

  Reference numeral 1208 denotes a network interface card (NIC) that exchanges data bidirectionally with a network printer, another network device, or another PC via the LAN 1220.

  The functions of the above-described embodiments can also be realized by a computer executing a computer program. Further, means for supplying a computer program to a computer, for example, a computer-readable recording medium such as a CD-ROM in which such a program is recorded, or a transmission medium such as the Internet for transmitting such a program is also applied as an embodiment of the present invention. be able to. A computer program product such as a computer-readable recording medium in which the program is recorded can also be applied as an embodiment of the present invention. The computer program, recording medium, transmission medium, and computer program product are included in the scope of the present invention. As the recording medium, for example, a flexible disk, a hard disk, an optical disk, a magneto-optical disk, a CD-ROM, a magnetic tape, a nonvolatile memory, a ROM, or the like can be used.

(Example)
Using the rupture prediction model, the maximum region of the rupture limit load Ftss in the shear type tensile test was examined. A spot welding nugget diameter d and width W were varied in order to obtain a rupture limit load Ftss that is a maximum in a shear type tensile test piece with a steel plate of 980 MPa thickness t = 1.4 mm. The other parameters used in the equations (2) and (3) are parameters obtained simply by limiting to this specific steel plate of 980 MPa class, TS = 1050 (MPa), t = 1.4. (Mm), p = 0.8013, q = 0.06712, r = 1.0614, k = 0.02695, n = 2.3759. These change if the material changes, but may be obtained each time or a relational expression between the material and the parameter may be used separately.

  FIG. 5 is an example of verification of this prediction model. When the nugget diameter d is fixed to 7.1 mm, the breaking limit load is maximum in the vicinity of the width of 40 mm. If the nugget diameter is determined, the maximum breaking limit load is obtained. It can be seen that the width at that time can be obtained.

In addition, the range between the point where the breaking limit load includes the maximum value and the rate of change is the maximum and the minimum is 20 mm or more and 60 mm or less. Can be used as a high area.

  FIG. 6 is a curved surface showing changes in the breaking limit load when not only the width of FIG. 5 but also the nugget diameter is changed. It can be seen that the width W at which the breaking limit load has a maximum value shifts to a larger side when the nugget diameter is changed, and an optimum combination of the width and the nugget diameter can be selected according to the purpose. That is, if it is desired to increase the breaking limit load, the width can be about 70 mm, the nugget diameter can be about 13 mm, and if the width is determined to be 50 mm from another design requirement of the member, the maximum breaking The nugget diameter at the limit load can be determined to be about 11 mm.

  As described above, the maximum fracture limit load in spot welding was predicted in the basic test, and the optimum conditions for the material, the plate thickness, the nugget diameter, and the width could be determined. In addition, the prediction of spot weld rupture at the time of collision deformation at the part level is verified from an experiment / prediction model, and it is confirmed that the rupture in the prediction model matches the experiment. From the above, it was confirmed that the deformation mode of the member and the absorbed energy can be controlled and designed by obtaining the maximum value of the fracture of the spot weld.

It is a figure which shows the outline | summary of a shear type tensile test. It is a figure which shows the flange width in an actual member, cross-sectional length, the space | interval of a buckling bead, and a spot welding pitch. It is a figure which shows the outline | summary of a cross-shaped tension test method. It is a side view at the time of the test of a cross-type tension test. It is a figure which shows the change with respect to the width | variety W of the breaking limit load of a shear type tensile test. It is a figure which shows the change with respect to the width W and the nugget diameter d of the breaking limit load in a shear type | mold tensile test. It is a block diagram which shows an example of the computer system which can perform the optimization method.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Nugget 2 Base material 3 Tensile direction of both ends of test piece 4 Flange width of hat-shaped section 5 Section length 6 Spacing between buckling beads 7 Spot welding pitch

Claims (1)

Material strength TS (MPa), plate thickness t (mm), spot welding nugget diameter d (mm), joint plate width W (mm) of joints, maximum load Fcts at break ( N), and the rotation angle θ of the joint in the cross-type tensile test, and / or the material strength TS (MPa), plate thickness t (mm), spot weld nugget diameter d (mm) of various test pieces in the shear-type tensile test ), The plate width W (mm) of the joint, and the maximum load Ftss (N) at the time of breaking, and defined based on the formula (1) or (4) or by the formula (1) or (4) Based on the relational expression of the stress concentration factor α and the ratio d / W between the nugget diameter d and the width W, a database is created by calculating the stress concentration factor α, and the spot of an arbitrary member based on the database Fracture limit load at weld zone So as to maximize the width of the spot weld, the plate thickness, material strength, optimization method for increasing the breaking strength of the spot weld, characterized by determining one or more of the nugget diameter.
α = TS · W · t / Ftss (1)
α = 2 · TS · W · t · sinθ / Fcts (4)
Where α is the stress concentration factor at the end of the nugget of the spot weld and the base metal in cross-type tension and / or shear-type tension

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