JP5834436B2 - Strength evaluation method of laser lap weld joint - Google Patents

Description

  The present invention relates to a method for evaluating the strength of a laser lap weld joint using a finite element method.

  Laser welding is applied as an assembly welding method to parts of automobile bodies mainly because of the merit of production cost.

  Non-Patent Document 1 points out the necessity of elucidating the strength characteristics of the laser welded part and predicts the joint strength of the laser lap joint when the application range of laser welding is further expanded as an assembly welding method for an automobile body. The method describes the relationship between weld bead dimensions and joint strength (tensile shear strength) using a mechanical model.

  In general, when strength evaluation is performed, a finite element method is often used as an effective means. Patent Document 1 describes a fatigue life prediction method for spot welds of an automobile body. A shell model for finite element analysis (hereinafter referred to as FEM model) of spot welds is formed, and finite element method linear elasticity analysis is performed by giving load data to find the 6 component forces and nodal displacement of spot welds. .

  Further, Patent Document 2 relates to a method for estimating the fatigue life of a front structure or the like of a construction machine, and when the stress value obtained by the finite element method is obtained by substituting it into a member SN diagram, it changes depending on the welding shape. It is stated that the stress value is corrected by a notch coefficient.

  The stress concentration rate set according to the local shape in the stress concentration part of the structure and the fatigue life evaluation diagram of the structure are stored in advance, and this stress concentration rate and fatigue life evaluation diagram, and stress analysis by the finite element method The fatigue life corresponding to each local shape is evaluated based on the value. For example, the stress analysis value by the finite element method is the stress of the shell element at the position corresponding to the weld toe of fillet welding or butt welding.

Japanese Patent No. 3842621 Japanese Patent No. 3842621

Miyazaki et al. "Tensile shear strength of laser lap joints" Nippon Steel Technical Report No. 385 2006

  As described above, the joint strength of laser welded lap joints has been elucidated, but in order to expand the scope of application, it is also important to evaluate the strength of fatigue characteristics, etc. It is requested.

  Since the stress calculated by the finite element method is the stress at the center of gravity of the element, the distance from the weld line (toe) to the center of gravity of the element varies depending on the mesh size of the shell element of the FEM model. The stress values differed greatly and stable and accurate strength evaluation could not be performed.

  In addition, the method described in Patent Document 1 uses a welded toe of a T-shape, cross, and butt joint as the fatigue crack occurrence position, and cracks occur from the plate surface where the upper and lower plates are in contact with each other. It cannot be applied as it is to a laser lap welded joint.

  Therefore, the present invention has an object to provide a strength evaluation method for a laser lap weld joint that accurately obtains a stress distribution of a laser lap weld joint using a finite element method and evaluates strength such as tensile shear strength and fatigue characteristics. And

The object of the present invention can be achieved by the following means.
1. A method for evaluating the strength of a laser lap weld joint by a finite element method, comprising the following procedure.
Procedure 1: Create FEM models of upper and lower metal plates and laser welds that constitute a laser lap weld joint.
Procedure 2: The structural analysis is performed with the model, and the stress is calculated from the shell element of the metal plate at the position of the common node of the shell element that models the metal plate and the shell element that models the laser weld.
Procedure 3: An evaluation stress is calculated from the calculated stress, and an evaluation stress distribution of the laser welded portion excluding the end is obtained.
Procedure 4: The evaluation stress at the end of the laser weld is obtained by extrapolation from the evaluation stress distribution, an evaluation stress distribution for the entire length of the weld is created, and the strength is evaluated.
2. 2. The laser lap weld joint according to 1, wherein the evaluation stress of each of the joint nodes is a maximum value of a principal stress obtained from a shell element obtained by modeling the metal plate connected to the joint node. Strength evaluation method.

  According to the present invention, since the evaluation stress of the welded portion of the laser lap weld joint can be obtained accurately regardless of the mesh size of the shell element of the FEM model, the accuracy of strength evaluation using the finite element method can be improved. It is extremely useful in industry.

The figure explaining a laser lap welding joint. The figure which shows the welding part vicinity expanded by an example of the FEM model of a laser welding joint. The figure which shows distribution of the principal stress whose absolute value is the maximum among the principal stress of the shared node position obtained by structural analysis. The figure which shows the evaluation stress distribution of the welding part full length created by calculating | requiring the evaluation stress in the shared node used as the edge part of a welding part by extrapolation from the evaluation stress distribution of FIG. Example (FEM model). Example (a figure showing an evaluation stress distribution of a shared node position obtained by an FEM model of 4 cases).

  The present invention is characterized in that an evaluation stress at a welding start / end portion of a laser lap weld joint obtained by elasto-plastic analysis of an FEM model is obtained by extrapolating from an evaluation stress distribution before and after that. Hereinafter, it will be described in detail with appropriate reference to the drawings.

  In the present invention, first, an FEM model of upper and lower metal plates and a laser welded part constituting a laser lap weld joint is created (procedure 1). FIG. 1 is a diagram for explaining a laser lap weld joint, in which an overlap portion of an upper plate (thin plate A) and a lower plate (thin plate B) is laser-welded. Laser welding is through welding, and weld beads (hereinafter referred to as welds) are formed on the outer surfaces of the upper plate (thin plate A) and the lower plate (thin plate B).

The FEM model is created using a shell element used for a finite element analysis of a normal weld.
FIG. 2 shows an example of an FEM model in which the vicinity of the welded portion of the laser welded joint is enlarged, and shows an upper plate (thin plate A) or lower plate (thin plate B) and a laser welded portion modeled by shell elements.
The laser welded portion is plate-shaped and is upright with respect to the upper plate (thin plate A) or lower plate (thin plate B), and the lower part has a common node with the upper plate (thin plate A) or lower plate (thin plate B). Then.

  Next, using the FEM model, structural analysis is performed when a load is applied, and principal stresses are obtained from the shell elements of the thin plate for other shared nodes excluding the shared nodes that are the ends of laser welding. A plurality of shell elements that model the thin plate B (A) are connected to one shared node, and the principal stress at the shared node position can be obtained from each shell element, but the one with the largest absolute value is shared The evaluation stress at the nodal point. And the evaluation stress distribution of the welding part except a welding part edge part is calculated | required (procedures 2 and 3).

  FIG. 3 shows an evaluation stress distribution when the evaluation stress is a main stress having the maximum absolute value of the shared node position. The principal stress in the figure is a value on the surface of the thin plate B (A) where the shell element of the laser weld exists.

  Finally, as shown in FIG. 4, the evaluation stress at the common node that becomes the end of the weld is extrapolated from the evaluation stress distribution, and the evaluation stress distribution of the entire length of the weld is created. Strength evaluation is performed using the obtained evaluation stress distribution (procedure 4). The strength evaluation is an evaluation of tensile shear strength, fatigue strength, and the like.

  A laser welded lap joint created by overlapping a portion of a thin plate (width 40 mm, total length 145 mm) having a thickness of 0.7 mm was converted into an FEM model, and stress analysis was performed when a tensile load was applied.

  The laser welded portion was welded to have a welding length of 10 mm and through-welded so as to be perpendicular to the tensile load direction.

  FEM modeling was performed for four cases with shell element dimensions of 0.5 mm, 1 mm, 1.25 mm, and 2.5 mm.

  FIG. 5 shows an example of the FEM model, and FIG. 6 shows the evaluation stress distribution at the shared node position for four cases. Even if the shell element dimensions are different, the stress distributions are well matched except for the position corresponding to the end of the laser weld. It can be seen that if the evaluation stress at the end of the laser weld is extrapolated from the obtained evaluation stress distribution, the values match regardless of the shell element dimensions.

  That is, according to the present invention, the same evaluation stress distribution can be obtained regardless of the shell element dimensions of the FEM model. Therefore, stable strength evaluation can be performed by using the evaluation stress distribution.

Claims (2)

A method for evaluating the strength of a laser lap weld joint by a finite element method, comprising the following procedure.
Procedure 1: Model the upper and lower metal plates and laser welds constituting the laser lap weld joint with shell elements.
Step 2: Structural analysis is performed with the model, and the shell element that models the metal plate and the shell element that models the laser weld have a common node, and stress is applied from the shell element of the metal plate at this position. Is calculated.
Step 3: calculating an evaluation stress from the calculated said stress, obtains the evaluation stress distribution of the laser welds for other of the shared node points excluding the ends.
Step 4: The evaluation stress in shared node points from the evaluation stress distribution serving as a laser weld end and extrapolated to create an evaluation stress distribution of the weld total length, performs strength evaluation.   2. The laser lap welding according to claim 1, wherein an absolute value of a principal stress obtained from a shell element obtained by modeling the metal plate connected to the shared node is a maximum value for the evaluation stress of each of the shared nodes. Joint strength evaluation method.