JP2002035986A - Estimation method of dangerous cracking part on spot- welded structure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To acquire a method to distinguish a dangerous part which has a high risk for crack and requires a detail model and a less dangerous part which has a low risk for crack and requires only simple model. SOLUTION: A finite element method is applied on a sport welded part using a model which is described in simple model. After performing the finite element method, a value, corresponding to a peel distance in the direction of perpendicular to a surface, is calculated and compared with the prescribed value. Even using a simple model which has low analysis accuracy, a large value of the peel distance is calculated on the spot welded part which has the high risk of crack, and a small value of the peel distance is calculated on the spot welded part which has less risk of crack. Therefore, it is exactly judged whether the details model is required or not by the value of peel distance.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a computer having a structure in which a plurality of panels are spot-welded at a plurality of locations, for example, a body of an automobile, in which a crack is likely to be generated. It relates to technology to predict using.

[0002]

2. Description of the Related Art In order to respond to a reduction in the development period and a reduction in development cost of a structure, a technique of analyzing the strength of the structure using an electronic computer has begun to be frequently used. It has been verified that the finite element method is useful for structural strength analysis. Some structures, such as automobile bodies, have a plurality of panels formed by spot welding at a plurality of locations. When analyzing the strength of this type of structure using an electronic computer, a spot weld is modeled and the finite element method is applied to the model. Several methods of modeling a spot weld so that it can be calculated by the finite element method are known, and a typical example is shown in FIG.

FIG. 2 shows a shared node model. The shared node model includes nodes 21-1, 21-2,..., 22-1, 2 provided on each of the plurality of panels 21 and 22.
2-2,... Describes spot welding by adding a constraint that one node 21-n and 22-n share the same position for each panel corresponding to the spot welding point. You can create a model in a short time. On the other hand, the phenomenon that a solidified portion formed over a plurality of panels by welding transfers a bending moment from panel to panel cannot be accurately described, and is so fine that a locally generated high stress state can be reproduced. It is difficult to provide nodes. (If a model is arranged with nodes densely around spot welds, the advantage of a shared node model that can be built in a short time is lost. The nodes are densely arranged throughout the structure. In the model, the calculation time becomes too long.
Japanese Unexamined Patent Application Publication No. 153130 discloses a technique for locally arranging nodes locally and densely. For that purpose, it is necessary to obtain an error. To obtain the error, a true value is required. In the shared node model, there is a problem that the shared nodes 21-n and 22-n
Tends to be calculated lower than the actual stress generated in the plate element 23 adjacent to. It can be said that the analysis accuracy is not high. FIG. 3 shows a spider web model. The spider web model describes spot welding with a beam element 30, an adjacent plate element 32 arranged at a fine density around the beam 30, and a rigid element 31 between the beam 30 and the adjacent plate element 32 and connecting the two. . According to this model, it is possible to describe an event in which a solidified portion transmits a bending moment from panel to panel, and to describe a locally generated high stress state, so that the analysis accuracy is high. However, the model is dense and takes a long time to create.

Each of the modeling methods shown in FIG. 1 has advantages and disadvantages. A model that can be created with a small number of man-hours has low analysis accuracy, and a model with a high analysis accuracy requires many man-hours to create. At present, a modeling method that requires only a small number of man-hours and yet has high analysis accuracy has not been developed.

[0005] When a structure is manufactured as a trial and a durability test is performed by repeatedly applying an external force to the manufactured structure, a crack is usually formed in a spot welded portion. When a structure formed by spot welding a plurality of panels at a plurality of locations is used in a vibration environment, the service life is determined by cracks in the spot welds.

When the finite element method is performed using a model in which the number of man-hours is small but the analysis accuracy is low among the modeling methods for the finite element method, cracks that determine the life of the structure are generated. Cannot be calculated, and the life cannot be calculated to a reliable degree. When the finite element method is performed using a model with high analysis accuracy, high stress is calculated around the spot weld where a crack occurs, and the area where the calculated stress is high is extracted to predict the area where the crack is likely to occur be able to. FIG. 4A shows the result of performing the finite element method using a model with high analysis accuracy (hereinafter referred to as a detailed model). FIG. 4A shows an analysis result when the spider web model is used, and indicates that the darker the color, the higher the calculated stress. A high stress is calculated at the spot welding point 41 where a crack has occurred in the durability test of the prototype structure, and it is shown that a crack occurrence risk site can be predicted by using the calculated stress as an index. FIG. 4B shows the result of performing the finite element method using a model with low analysis accuracy (hereinafter, referred to as a simple model). FIG. 4 (B) shows an analysis result when the shared node model is used. High stress is not calculated even at the spot welding point 41 where a crack has occurred in the durability test of the prototype structure, and the calculated stress is used as an index. This indicates that it is not possible to predict the crack initiation risk site from the above.

It is apparent from the above that when a finite element method is used to analyze the life of a structure formed by spot welding a plurality of panels at a plurality of locations, a reliable analysis result is obtained. However, a simple modeling method that can be easily modeled but has low analysis accuracy cannot be adopted, and a detailed modeling method that requires many man-hours for modeling must be adopted.

In the case of an automobile body, the number of spot welds is extremely large, and it is not practical because modeling all spot welds with a detailed model requires a very long operation. Therefore, at present, about 200 spot welding points, which are empirically known to be easily cracked, are described with a detailed model, and other spot welding points are described with a common node model.

[0009]

[Problems to be Solved by the Invention] It takes one month or more to prepare a model that describes a spot welding point reaching about 200 places by a detailed model. This is too long,
In many cases, analysis results cannot be fed back to the structure development process. In addition, since spot welding points for creating detailed models and spot welding points for simple models are classified according to the experience of workers, cracks occur at spot welding points for which detailed models were not created during actual durability tests. This phenomenon also occurs. Therefore, in the present invention,
An object of the present invention is to solve the above-described problem by introducing a finite element method into a process of distinguishing a spot welding point that requires creation of a detailed model from a spot welding point that does not require it.

[0010]

Means for Solving the Problems, Functions and Effects In the present invention, the finite element method is executed using a model in which spot welding points are described by a simple model. As described above, in the model describing the spot welding points by the simple model, an analysis result sufficient to evaluate the product life cannot be obtained. However, by devising an index for evaluating the analysis results, it is possible to objectively determine whether a spot welding point requires the creation of a detailed model or a spot welding point sufficient to create a simple model. It has been found that the number of spot welds that must be reduced significantly. That is,
Even if the number of spot welds for which a detailed model is created is reduced, a model is obtained in which a detailed model is created for spot welds for which a detailed model must be created, and a model that can provide reliable analysis results is obtained. This enables creation in a relatively short time.

According to the present invention, a step of modeling a spot welded portion with a shared node model in predicting a crack generation danger site in a structure formed by spot welding a plurality of panels at a plurality of locations; Applying the finite element method to the model to calculate a value corresponding to the separation distance in the direction perpendicular to the surface at a node adjacent to the shared node, and performing a step of comparing the calculated value with a predetermined value to perform cracking Predict the danger site. The value corresponding to the peeling distance is the peeling distance itself or a tilt angle indirectly indicating the peeling distance.

When comparing many calculation results with many endurance test results, at a spot welding point where a crack has occurred,
It was confirmed that there was a regularity that the calculated peel distance was large. In the model where the spot welding point is described by the simple model, the calculated peel distance responds well to the risk of cracking, even though the analysis accuracy is low, and the crack occurs at the spot welding point where the calculated peeling distance is short Without
It was confirmed that cracks occurred only in the spot welding point group where the calculated peel distance was long.

Using the present invention, it is possible to extract spot welding points that require the creation of a truly detailed model. For example, empirically, about 200 places require a detailed model, but by using this invention,
It can be narrowed down to about 50 places. As a result, the creation of the detailed model can be shortened to about one week. In this case, the analysis result can be fed back to the structure development process.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The lower right part of FIG. 5 shows a joint node model near a spot welding point of a structure before the start of a durability test. In this case, two panels extending substantially horizontally are overlapped, and the nodes 1-0 and 2-0 share a position corresponding to the fact that the nodes 1-0 and 2-0 are spot-welded. . There are eight nodes adjacent to the shared node for each panel. Each node is distributed so as to form a plate element having a side length of about 25 mm, and the panels are almost in close contact with each other, so that eight adjacent nodes almost overlap with each other on two panels. I do. At the lower right of FIG. 5, the corresponding nodes of the two panels are displayed overlapping.

[0015] The upper left part of FIG. 5 illustrates an analysis result when the external force is applied by applying the external force to the structure model and applying the external force. In this case, the spot welding points 1 of the panels 1 and 2 are shown. A force that separates the panel 1 from the panel 2 acts on the right side of −0 and 2-0, and the right side of the panel 1 is deformed so as to be separated from the panel 2. FIG. 5 illustrates that after the deformation, the largest peeling occurred between the nodes 1-1 and 2-1.

In this technique, the Z direction between the nodes after deformation (here, the panels 1 and 2 extend substantially in the XY plane,
The Z-axis direction is a direction perpendicular to the panel surface.) The distance is used as an index. Specifically, the difference between the Z-axis coordinates of the corresponding two nodes is 8
All of the adjacent nodes are determined, and the largest one is determined as the separation distance of the spot welding point.

FIG. 6 shows a first structure (body of vehicle A).
FIG. 6 shows the peel distance calculated at four spot welds on the radiator support lean hose and the peel distance calculated at six spot welds on the apron upper member. In this case, referring to the peel distance calculated using the peel distance of 1.0E-04mm as the reference value, it can be seen that the total of four points during the spot welding of 10 points is more than the reference value, and the details are shown at four points. It turns out that a model needs to be created. On the other hand, in actual durability tests, it was known that cracks occurred at two places, and the two spot welds were included in the spot weld group that was determined to require the creation of a detailed model. It is confirmed.

FIG. 8 shows a second structure (body of vehicle B).
Distance calculated at five spot welds on the front side member of the vehicle and 5 mm on the rear upper support member.
5 shows the peel distance calculated at two spot welds. Also in this case, referring to the peel distance calculated using the peel distance of 1.0E-04mm as the reference value, it can be seen that the total of six points during the spot welding of 10 points is more than the reference value.
It turns out that a detailed model needs to be created in this respect. On the other hand, in an actual endurance test, it was known that cracks occurred at three places, and the three spot welds were included in the spot weld group determined to require the creation of a detailed model. It is confirmed.

FIG. 7 shows the result of calculating a bending moment applied to a spot weld by applying a finite element method by creating a detailed model for the structure analyzed in FIG. FIG.
Shows a view similar to FIG. 7 for the structure of FIG. FIG.
From FIG. 9 and FIG. 9, it is confirmed that a crack is generated at a high point by the bending moment. When spot welded joints of various shapes are prepared and tested, it has been found that fatigue occurs quickly when a bending moment acts on the welded portion, and hardly occurs when tensile or shear force acts on the welded portion. This is consistent with the results of research on linear fracture mechanics. FIG. 7 and FIG. 9 confirm this fact, and support that cracks may easily occur at points where the bending moment is high.

From the comparison of FIGS. 6 and 7 and FIGS. 8 and 9,
The peel distance calculated by the simplified model relatively approximates the bending moment calculated by the detailed model. It has been confirmed that the bending moment is closely related to crack initiation. Since the stress calculated by the simple model is far from the true value, it is not an effective index. On the other hand, a relatively accurate value can be calculated for the amount of deformation in the direction perpendicular to the surface even with a simple model.

As is apparent from the above description, in the simplified model in which all the spot welding points are described by the common node model, the stress cannot be calculated and the accuracy of predicting the occurrence of the crack cannot be obtained, but the separation distance is focused on. As a result, it was confirmed that a spot welding spot having a high risk of crack occurrence and a spot welding spot having a low risk of cracking can be objectively determined.

In the above description, the difference between the Z-axis coordinates of two corresponding nodes in the adjacent node group is used as the separation distance. Instead, a peeling angle α shown in FIG. 11 may be used. This is because the peel angle α substantially gives a value corresponding to the peel distance. Also, instead of using the difference between the Z-axis coordinates of the corresponding two nodes after deformation as the peel distance, the difference between the distance in the Z-axis direction of the corresponding two nodes after deformation and the distance in the Z-axis direction before deformation is used. You may. Similarly, a difference between the angle after the deformation and the angle after the deformation may be used as the value corresponding to the peel distance. Since the panels do not adhere to each other before deformation and there is a distance in the Z-axis direction between the two nodes, the latter index may be useful.

It is desirable that the reference value used as a comparison of the peel distance be changed depending on the material and thickness of the panel, whether the spot welding point is at the flange, at the corner, or at the end, and the like. In this case, it has been found that the peeling distance is significantly different between a welding point having a high risk of generating a crack and a welding point having a low risk, and it is easy to determine a reference value.

FIG. 11 is a drawing for comparing the period required for strength analysis before and after the present invention, which took one month or more before the present invention. Shortened. With the current development speed of the vehicle body, it is difficult to feed back the analysis results to the development process if the analysis takes more than one month, but if it takes about 10 days, it will be more reasonable to refer to the analysis results Can develop a simple structure. According to the present invention, not only can the analysis be performed in a short time, but also the reliability of the analysis result is improved, which is extremely effective.

[Brief description of the drawings]

FIG. 1 shows types of modeling methods for spot welds.

FIG. 2 shows a shared node model.

FIG. 3 shows a spider web model.

FIG. 4 shows the stress analyzed by the finite element method, and (A)
Shows a case using a detailed model, and (B) shows a case using a simple model.

FIG. 5 shows a state before deformation calculated by the shared node model and a state after deformation in comparison.

FIG. 6 shows a peel distance calculated by a shared node model.

FIG. 7 shows a bending moment calculated by a detailed model.

FIG. 8 shows a peel distance calculated by a shared node model.

FIG. 9 shows the bending moment calculated by the detailed model.

FIG. 10 shows a period required to obtain an analysis result by executing a finite element method by modeling and comparing before and after the present invention.

FIG. 11 shows angles that can be used instead of peel distances.

[Explanation of symbols]

 1, 2: Panel 1-1, 1-2, ..., 2-1, 2-2 ... Nodes 1-0, 2-0: Shared nodes

Claims (1)

[Claims] 1. A method for predicting a crack generation danger site of a structure formed by spot welding a plurality of panels at a plurality of locations, comprising: modeling a spot weld with a shared node model; Applying the finite element method to the model to calculate a value corresponding to the separation distance in the direction perpendicular to the surface at a node adjacent to the common node, and comparing the calculated value with a predetermined value to crack How to predict the danger site.