JP2008107322A - Fracture strain calculation method for spot welded zone, and standard fracture strain calculation method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fracture strain calculation method capable of calculating the fracture strain regardless of the size of the element, in the analysis of a spot-welded zone by finite element method. <P>SOLUTION: A plurality of analysis models with respective elements sizes are created regarding the identical spot-welded joint, and fracture strain values ε<SB>CR</SB>of a base material and/or a thermally affected portion of the spot-welded zone in the analysis models are calculated, respectively, by using finite element method; and the relation between the fracture strain values ε<SB>CR</SB>and element size parameters Rb, Rh and Div that define the element sizes is obtained; and then the fracture strain value ε<SB>CR</SB>of the base material and/or the thermally affected portion is calculated from the values of the element size parameters Rb, Rh and Div by using this relation. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a fracture strain calculation method for a spot welded portion using a finite element method analysis (hereinafter referred to as “FEM (Finite Element Method) analysis”), a standard fracture strain calculation method, and the like.

  Spot welding is widely used as a method for joining steel plates in an automobile assembly process. The automobile absorbs the collision energy by breaking the spot welded part of the automobile member at the time of the collision, and aims at the safety of the occupant. Therefore, in order to grasp the collision energy absorption performance of the automobile, it is important to accurately predict the breakage of the spot weld.

  As a method for predicting the fracture, there is a method using FEM analysis. FEM analysis is a kind of numerical analysis method, and mainly involves modeling an object with a computer, dividing the model into fine mesh elements, and approximating numerical values for each element. The FEM analysis can use a subdivided finite element analysis model (hereinafter, simply referred to as “analysis model”) in a short time if the shape is simple. However, since the calculation time is enormous for a complicated shape such as an automobile member, an analysis model simplified with a shell element or the like is used, and the element size is increased to reduce the calculation time. However, since the accuracy of FEM analysis is determined by the element size, it is difficult to achieve both analysis time and analysis accuracy.

  In Patent Document 1, a fracture strength parameter is calculated based on a cross-type tensile test and / or a shear-type tensile test, and this fracture strength parameter is introduced into a fracture prediction formula obtained by a finite element method, so that a spot welded portion is obtained. A method for predicting the fracture of a spot weld and a spot welded portion fracture prediction apparatus having means for executing these are disclosed. According to such a technique, it is said that, for example, it is possible to accurately perform fracture prediction at a portion where spot welding of an automobile member is modeled by finite element method analysis on a computer.

  In Non-Patent Document 1, as a model structure of a spot welded portion used in the dynamic explicit method, an element for fracture determination is provided around a welding surface (hereinafter referred to as “nugget”). According to this, both the displacement state and displacement until the crack occurrence in the spot welded part are simulated well by CAE, the plate thickness difference, all load modes (shearing, peeling, cross tension, U-shaped tension, etc.) It is said that it is possible to simulate a plug fracture.

In Non-Patent Document 2, the stress-strain relationship of each part of each part of spot welded base material, heat-affected zone (hereinafter sometimes referred to as “HAZ (Heat Affected Zone)”), and nugget using ultra-small test pieces. Individual and quantitative measurement is performed, and the fracture strain of each part is obtained by FEM analysis of the ultra-small test piece from the stress-strain relationship. And the fracture | rupture position and fracture | rupture strength of a welded joint are estimated by determining that the site | part which reached this fracture | rupture distortion | strain previously in an analysis model is fracture | rupture, and calculating | requiring the intensity | strength. According to this, it is said that the fracture position and fracture strength of the welded joint can be predicted with high accuracy.
JP 2005-326401 A Toyota Motor Co., Ltd. “Development of analysis method for seat belt anchor strength by dynamic explicit method” Automobile Society of Japan Vol.32, No.2, April 2001 Research Paper (18) Sumitomo Metal Industries, Ltd. “Measuring mechanical properties of spot welds and predicting joint tensile strength” Automobile Engineering Society Proceedings Vol.36, No.1, January 2005

  However, in the technique described in Patent Document 1, since the fracture strength parameter cannot be calculated unless the joint width and rotation angle are input, spot welding is irregularly performed on a complex spot welded portion, for example, a curved steel plate. There was a problem that it was difficult to predict the fracture in the formed member. In addition, there is no description for setting the material characteristics of the nugget, HAZ, and base material separately in the CAE model, and the failure prediction cannot be performed accurately because the material characteristics are not set separately for the nugget, HAZ, and base material. was there.

  In the method described in Non-Patent Document 1, an appropriate limit plastic strain that becomes a fracture strain must be set by trial and error, and there is no description about the element size of the analysis model that affects the strain. There was a problem that trial and error for determining the breaking strain had to be made each time. In addition, there is no description that sets the material characteristics of the nugget, HAZ, and base material separately in the analysis model, and the failure prediction cannot be performed accurately because the material characteristics are not set separately for the nugget, HAZ, and base material. was there.

  In the method described in Non-Patent Document 2, it is necessary to divide the element size of the analysis model into solid elements of several tens of μm. Therefore, a complicated and large analysis model such as an automobile member has a problem that the calculation time is increased, and furthermore, the creation of the analysis mesh requires a great amount of time and human labor.

  Therefore, in order to solve the above problems, the present invention can calculate the fracture strain regardless of the shape of the analysis model, the nugget diameter, the element size, and the fracture strain calculation method for a spot weld with good analysis accuracy, and It is an object of the present invention to provide a reference fracture strain calculation method and the like.

  The present invention will be described below. In order to facilitate understanding of the present invention, reference numerals in the accompanying drawings are appended in parentheses, but the present invention is not limited to the illustrated embodiment.

  The invention according to claim 1 is a method for calculating a fracture strain of a spot welded part using a finite element method, wherein the same spot welded joint (1-3) having a spot welded part is analyzed with a plurality of element sizes. (4-6), a step of calculating the fracture strain of the base material (33) and / or the heat-affected zone (32) of the spot welded portion in each analysis model by the finite element method, and the element size A spot characterized by having a step of determining a relationship between an element size parameter to be determined and a breaking strain, and a step of calculating a breaking strain of a base material and / or a heat affected zone from a value of a predetermined element size parameter based on the relationship. The said subject is solved by providing the fracture | rupture distortion calculation method of a welding part.

  The “element size parameter” is a variable (parameter) representing the size of each element in the analysis model, and may be the element size itself or a value defined from the element size. As specific examples of the element size parameter, as the element size itself, a radial length Rb of a base material element to be described later, a radial length Rh of a HAZ element, a circumferential length Cb of the base material element, a circumferential direction of the HAZ element Length Ch, circumferential element division number Div when considering a circle at the center of the nugget, diagonal length RbDia of the base material element, diagonal length RhDia of the HAZ element, radial length RbMax of the base material element, Examples include the radial length RhMax of the HAZ element. Further, examples of the value defined from the element size include Eb, Eh, and Ebh described later. However, the element size parameter is not limited to these variables, and any other variable can be used as long as it represents a size of the element.

  The “predetermined” element size parameter value is an element size parameter value obtained from an element size of an element for which fracture strain is to be obtained in an analysis model of a workpiece to be analyzed for fracture of a spot weld. .

  The invention according to claim 2 is characterized in that the element size parameter of 1 or 2 is used in the fracture strain calculation method of the spot welded portion according to claim 1.

  The invention according to claim 3 is the spot strain weld fracture strain calculation method according to claim 1 or 2, wherein each analysis model (4 to 6) includes a spot weld base material (33), a heat affected zone. (32) and the material properties of the nugget (31) are set.

  “Material properties” means properties of the material itself such as the Young's modulus, Poisson's ratio, stress σ-strain ε relationship of the base material, the heat affected zone, and the nugget, which are the parts of spot welding.

  The invention according to claim 4 is a method for calculating a reference fracture strain of a spot welded portion using a finite element method, and has a spot welded portion and a plurality of spot welds having different load modes and / or nugget diameters (ND). Element size parameters for determining the spot weld joint tensile test for joints (1-3) and the element size of the base metal (33) and / or heat affected zone (32) of the spot welds of all spot weld joints A process of creating an analysis model (4-6) so that the values of the same value are the same, a process of calculating the fracture strain of the base material and / or the heat-affected zone in each analysis model by the finite element method, and a tensile test And a step of calculating a reference fracture strain of the base metal and / or the heat-affected zone using the fracture strain. To solve the above problems by.

  The “reference breaking strain” is a breaking strain that can be used for each part of the base material and / or the heat affected zone regardless of the load mode and / or the nugget diameter. The load mode is a type of load applied to the spot weld, and generally includes tensile shear, L-shaped joint tension, U-shaped joint tension, etc., but is not particularly limited thereto. Other loads can also be used.

  According to a fifth aspect of the present invention, in the reference fracture strain calculation method of the spot welded portion according to the fourth aspect, the base material (33) of the spot welded portion, the heat affected zone ( 32) and material properties of the nugget (31) are set.

  The invention according to claim 6 is the base material (33) and / or the heat affected zone (32) for the values of a plurality of element size parameters using the method for calculating the reference fracture strain of the spot welded portion according to claim 4 or 5. And a step of obtaining a fracture criterion which is a relationship between the element size parameter and the criterion fracture strain of the base material and / or the heat-affected zone. The problem is solved by providing a reference creation method.

  The invention according to claim 7 is a method for predicting a fracture of a spot welded portion by a finite element method, the step of creating an analytical model (60) of a workpiece having a spot welded portion, and an analytical model of the workpiece The element size parameter value for determining the element size of the base material (33) and the base material (33) from the element size parameter value of the workpiece to be welded using the fracture criterion created by the fracture criterion creation method according to claim 6 And / or the step of obtaining the reference fracture strain of the heat affected zone (32), the step of analyzing the analysis model of the work piece by the finite element method to obtain the strain of the element, and the element when the strain exceeds the reference fracture strain The above-mentioned problem is solved by providing a method for predicting the fracture of a spot welded portion, which includes the step of determining that the fracture is a fracture.

  The fracture prediction is to predict a spot welding fracture site, a fracture position, a fracture strain at the time of fracture, the presence or absence of fracture at a predetermined load, and the like.

  According to an eighth aspect of the present invention, in the spot welded portion fracture prediction method according to the seventh aspect of the present invention, the spot welded base material (33) and the heat affected zone (32) are included in the analysis model (60) of the work piece. And material properties of the nugget (31) are set.

  The invention according to claim 9 is an analysis model creation means (52) for creating an analysis model (60) divided into elements, and the relationship between an element size parameter for determining an element size of the analysis model and an element fracture strain. A storage means (51) for storing, a fracture strain setting processing means (56) for obtaining an element size parameter value from the element size, and obtaining a fracture strain from the element size parameter value using the storage means, and an analysis model as a finite element By providing an analysis means (54) for analyzing by the method and comparing the strain and fracture strain of the element obtained by the analysis, the above-mentioned problem is provided by providing a fracture prediction device (50) for a spot weld To solve.

  According to the first aspect of the present invention, the base material and / or the heat-affected zone is obtained by determining the value of the element size parameter for any element size from the relationship between the element size parameter and the fracture strain. Can be obtained.

  According to the second aspect of the present invention, since the relationship between the element size parameter and the breaking strain is simplified by using the element size parameter of 1 or 2, the breaking strain can be easily obtained from the element size parameter. Can do.

  According to the invention described in claim 3, the accuracy of the FEM analysis can be improved by separately setting the base material, the heat-affected zone, and the nugget material characteristics in the analysis model. Thereby, an accurate fracture strain can be obtained.

  According to the fourth aspect of the present invention, the reference breaking strain that is the breaking strain not related to the load mode and / or the nugget diameter is obtained. As a result, regardless of the shape of the analysis model and / or the nugget diameter of the spot welded portion, the reference fracture strain can be used as a fracture criterion for strain for each part of the base material and / or the heat affected zone.

  According to the invention described in claim 5, the accuracy of the FEM analysis can be improved by separately setting the base material, the heat-affected zone, and the nugget material characteristics in the analysis model. Thereby, an accurate reference fracture strain can be obtained.

  According to the sixth aspect of the present invention, the fracture criterion that is the relationship between the element size parameter and the standard fracture strain of the base material and / or the heat affected zone is obtained. Thereby, the standard breaking strain can be obtained by obtaining the value of the element size parameter from the element size of the analysis model. Therefore, it is possible to obtain a criterion for fracture regarding the strain of an element regardless of the shape of the analysis model and / or the nugget diameter of the spot weld and the element size of the analysis model.

  According to the invention described in claim 7, by using the fracture criterion created by the fracture criterion creating method according to claim 6, the shape of the workpiece and / or the nugget diameter of the spot weld, the analysis model Regardless of the element size, it is possible to obtain a criterion for fracture regarding the strain of the element. Therefore, it is possible to predict the fracture of the spot welded portion regardless of what is to be welded.

  According to the invention described in claim 8, by setting the material properties of the base material, the heat-affected zone and the nugget separately in the analysis model of the workpiece, the accuracy of the FEM analysis in the analysis model of the workpiece is increased. Can be improved. Thereby, the precision of fracture prediction of the spot welded portion can be improved.

  According to the ninth aspect of the present invention, the spot welded portion rupture predicting apparatus includes a storage unit that stores a relationship between an element size parameter and an element rupture strain, and a rupture strain setting processing unit. The fracture strain can be obtained regardless of the element size of the analysis model. Therefore, regardless of the element size of the analysis model, it is possible to predict the breakage of the spot welded part of the workpiece.

  In addition, by using the standard breaking strain as the breaking strain, it is possible to obtain a criterion for breaking regarding the strain of the element regardless of the shape of the workpiece and / or the nugget diameter of the spot weld and the element size of the analysis model. . Therefore, it is possible to predict the fracture of the spot welded portion for any workpiece.

  The present invention will be described below based on the embodiments shown in the drawings. However, what is described below is an example of the embodiments of the present invention, and the present invention is not limited to the following descriptions unless it exceeds the gist. is not.

<Test model and analysis model>
FIGS. 1A to 1A are diagrams showing test pieces 1 to 3 of the spot welded joint, and FIG. 1B is a view showing finite element analysis models 4 to 6 of the test pieces 1 to 3. FIG. 1 shows a specimen for a tensile shear test (hereinafter referred to as “TS”), FIG. 2 shows a specimen for an L-shaped joint tensile test (hereinafter referred to as “LT”), and FIG. 3 shows a test for a U-shaped joint tensile test. It is a piece (hereinafter referred to as “UT”), and the state of the tensile test is shown. In (a) of FIGS. 1 to 3, the test pieces 1 to 3 spot-welded with the nugget 10 are fixed to the chucks 12 and 12 or the jig 13. With this configuration, the tensile load 11 is applied to the test pieces 1 to 3 until the spot welded portion breaks. Thereby, the relationship between the tensile load 11 and the strain ε by the tensile test can be obtained.

  The analysis models 4 to 6 in FIGS. 1 to 3 are divided into meshes by shell elements for the purpose of shortening the calculation time, TS is a 1/2 symmetric model in the width direction, and LT is the width direction. The UT is a 1/4 symmetric model on the welding surface, and the UT is a 1/8 symmetric model on the length direction, the width direction, and the welding surface. Each analysis model 4-6 is spot-welded with a nugget 20 in the same manner as the test pieces 1-3. Further, a tensile load 23 is generated in the nugget 20 by applying a tensile load 21 to the load portion 22. Thereby, the relationship of the tensile load 21, strain, and epsilon by the analysis models 4-6 can be calculated | required.

<Spot welded part>
FIG. 4 is a diagram showing a spot weld. 4A is an external perspective view, and FIG. 4B is a longitudinal sectional view taken along a broken line 30 in FIG. 4A. FIG. 4 shows TS, but the same applies to other test pieces. As shown in FIG. 4 (a), the spot welding sites are the nugget 31 that is the welded surface after melting, the HAZ 32 that is the area affected by the welding heat around the nugget 31, and the base material that is not affected by the welding heat. 33. As shown in FIG. 4B, the nugget 31 has a nugget diameter ND and the HAZ has a HAZ width 32a. Here, the HAZ width 32a is the length of the HAZ 32 in the radial direction. Each dimension of spot welding, nugget diameter ND, and HAZ width 32a can be measured from a cross section. Moreover, you may use the calculation formula which calculates each part dimension of spot welding. For example, the nugget diameter ND can be obtained by the following equation (1) known as a general equation.

ここで、ｔは被溶接物の板厚である。また、ｋは係数であり、断面の計測で蓄積した寸法データから求めることができる。なお、各部の寸法算出の計算式は、上述した（１）式の他にも、スポット溶接部の断面を計測し、蓄積した寸法のデータから作成することが可能である。このようにして測定又は算出した寸法から、解析モデルにスポット溶接部の各部位であるナゲット３１、ＨＡＺ３２、および母材３３を作成する。

Here, t is the thickness of the workpiece. Further, k is a coefficient and can be obtained from the dimension data accumulated by the measurement of the cross section. In addition to the above-described formula (1), the calculation formula for calculating the dimensions of each part can be created from data of dimensions accumulated by measuring the cross section of the spot welded part. From the dimensions measured or calculated in this way, the nugget 31, HAZ 32, and base material 33, which are each part of the spot welded portion, are created in the analysis model.

FIG. 5 is a diagram showing the relationship of stress σ-strain ε of the nugget 31, HAZ 32, and base material 33 (see FIG. 4). The stresses σ ₃₁ , σ ₃₂ , and σ ₃₃ of the nugget 31, the HAZ ₃₂ , and the base material 33 are data obtained by collecting a test piece from each part and testing it. With regard to HAZ32, the tensile strength varies depending on the location where the specimen is collected. Therefore, the stress σ ₃₂ ′ of the HAZ ₃₂ ′ can be obtained by calculation according to the following equation (2) that represents the generally established relationship of stress σ−strain ε.
σ ₃₂ ′ = a · σ ₃₃ + (1−a) · σ ₃₁ (2)
(However, 0 <a <1)
Here, a is a constant. FIG. 5 shows an example of σ ₃₂ ′. As shown in FIG. 5, the tensile strengths of the portions 31 to 33 are in the relationship of the base material 33 <HAZ32 <nugget 31. Therefore, in order to ensure the prediction accuracy of the fracture of the spot welded part, it is necessary to set the material characteristics of the nugget 31 and the HAZ 32 in the analysis model in the FEM analysis. Material properties include Young's modulus, Poisson's ratio, stress σ-strain ε relationship, and the like.

<Calculation of breaking strain>
FIG. 6 is a calculation example of the breaking strain ε _CR in the analysis model of the test piece. Here, the test piece is LT. In FIG. 6, the horizontal axis represents the displacement amount λ of the entire test piece, and the vertical axis represents the tensile load P or the element strain ε in the analytical model. 6 shows a load change 40 in the test, a load change 41 in the FEM analysis, a maximum equivalent plastic strain change 42 in the FEM analysis, a joint maximum load 43, a fracture displacement 44 in the FEM analysis, and a fracture in the FEM analysis. The strain ε _CR is shown. Hereinafter, the breaking strain of the base material 33 is ε _{(Base) CR} and the breaking strain of the HAZ32 is ε _{(HAZ) CR} .

A method for obtaining the fracture strain ε _{(Base) CR} of the base material 33 (see FIG. 4) element of the spot welded portion of LT will be described. First, the LT is subjected to a tensile test as shown in FIG. Next, the load change 41 in the FEM analysis is obtained by performing FEM analysis with the analysis model as shown in FIG. At this time, a change 42 in the maximum equivalent plastic strain of the base material is obtained by FEM analysis. The peak load of the load change 40 in the test is the joint maximum load 43, and the spot welded portion of the test piece is broken at the joint maximum load 43. Therefore, the fracture displacement 44 in the FEM analysis is obtained from the intersection of the joint maximum load 43 and the load change 41 in the FEM analysis. Then, the fracture strain ε _{(Base) CR} of the _base material can be obtained from the intersection of the fracture displacement 44 in the FEM analysis and the change 42 in the maximum equivalent plastic strain of the base material.

  In the above, the joint maximum load 43 is used as a load when the spot welded portion breaks. The joint maximum load 43 is preferably averaged by performing a plurality of tensile tests. Thereby, since a suitable load can be used, it can prevent that the precision of FEM analysis falls. Moreover, since the load when the spot welded portion is broken differs depending on the material of the test piece, a load other than the joint maximum load 43 can be used in accordance with the test piece.

In order to obtain the breaking strain ε _{(HAZ) CR} of the HAZ32 (see FIG. 4) element, the change in the maximum equivalent plastic strain of the HAZ32 is obtained by FEM analysis and used in FIG. Similarly, for test pieces having different load modes such as TS (see FIG. 1), UT (see FIG. 3), and / or nugget diameter ND (see FIG. 4 (b)), the base material 33 and / or FIG. Alternatively, the breaking strains ε _{(Base) CR} and ε _{(HAZ) CR} of the _HAZ 32 can be obtained.

  Table 1 shows an example of a result of a tensile test performed on four test pieces having different load modes and / or nugget diameters ND.

試験片には、荷重モードＴＳ、ＬＴ、及びＵＴを用い、ＴＳはナゲット径ＮＤが異なる２つの物を用いている。それぞれの試験片を、ＴＳ−１、ＴＳ−２、ＬＴ−１、及びＵＴ−１とする。引張試験は試験１〜試験３まで各試験片について３回実施し、結果としてスポット溶接部の破断箇所が示されている。試験片により破断箇所が異なり、ＴＳ−１、ＴＳ−２は母材３３（図４参照）、ＬＴ−１はＨＡＺ３２（図４参照）、ＵＴ−１は母材３３又はＨＡＺ３２のいずれかで破断している。

As the test piece, load modes TS, LT, and UT are used, and two TSs having different nugget diameters ND are used as TS. Let each test piece be TS-1, TS-2, LT-1, and UT-1. The tensile test was carried out three times for each test piece from Test 1 to Test 3, and as a result, the fractured portion of the spot welded portion is shown. The fracture location differs depending on the test piece. TS-1 and TS-2 are fractured by the base material 33 (see FIG. 4), LT-1 is fractured by either HAZ32 (see FIG. 4), and UT-1 is fractured by either the base material 33 or HAZ32. is doing.

FIG. 7 is a diagram showing the breaking strain ε _CR of the test piece of Table 1 obtained from FEM analysis. The breaking strain ε _CR is obtained by the method described in FIG. Since each test piece has been subjected to three tensile tests as shown in Table 1, the results are averaged to obtain a load change 40 in the test (see FIG. 6). Then, by performing the FEM analysis on each test piece, the breaking strains ε _{(Base) CR 1} and ε _{(HAZ) CR} are obtained for the _HAZ 32 and the base material 33 of each test piece as shown in FIG. it can.

At this time, the accuracy of the FEM analysis is improved by setting the material properties of the nugget 31, the HAZ 32, and the base material 33 (see FIG. 4) as the elements of the analysis model of each test piece. As a result, as shown in FIG. 7, the difference in fracture strain ε _{(Base) CR} of the _base materials of TS-1, TS-2, and UT-1 that cause the base material fracture, and LT-1 and UT− that cause the HAZ fracture The difference in breaking strain ε _{(HAZ) CR} of 1 HAZ becomes smaller.

<Calculation of standard breaking strain>
Furthermore, the strain at break epsilon _CR of each specimen as shown in Figure 7, determine the reference strain at break Esuipushiron _CR for each preform 33 and HAZ32. It uses the broken portion by a tensile test of a test sample can be determined, such as by averaging the rupture strain epsilon _CR. For example, as shown in Table 1, the test pieces TS-1, TS-2, and UT-1 break at the base material 33. Therefore, the breaking strain ε _{(Base) CR} of the _base material 33 of these test pieces is averaged to obtain the reference breaking strain Sε _{(Base) CR} of the base material 33. Moreover, the fracture | rupture distortion | strain ₍ epsilon _{) (HAZ) CR} of HAZ32 of the test piece LT-1 and UT-1 which fracture | rupture with HAZ32 is averaged, and it is set as the reference | standard fracture | rupture distortion | strain S ₍ epsilon _{) (HAZ) CR of} HAZ32. Thereby, a standard breaking strain which is a breaking strain not related to the load mode and / or the nugget diameter is obtained. Therefore, regardless of the shape of the analysis model and / or the nugget diameter of the spot welded portion, the reference breaking strain can be used as a judgment criterion for breaking regarding the strain.

Further, breaking stress multi Jikudo (hereinafter, breaking stress multi Jikudo) can be used as a parameter for the reference strain at break Esuipushiron _CR calculation. Here, the “stress multiaxiality” is expressed as a ratio of the average stress in the vertical direction and the equivalent stress. When the stress multiaxiality is high, ductile fracture occurs even if the equivalent plastic strain is low. Although TS is pulled in a direction parallel to the spot weld surface, LT and UT are pulled in a direction perpendicular to the spot weld surface, so that the effect of bending around the spot weld surface is increased. When the bending effect is large, the stress in the vertical direction is maintained at a longer balance between tension and compression on the upper and lower surfaces of the plate, and the stress multiaxiality increases. Therefore, if the fracture location is the same, LT and UT are better than TS. The breaking stress multiaxiality tends to be high.

  Table 2 shows a test example of a fractured portion when the load mode is changed with the same nugget diameter (TS, UT).

As shown in Table 2, the test pieces TS-1a and UT-1a are broken at the base material 33. FIG. 8 is a distribution of calculation results of the breaking strain and the breaking stress multiaxiality according to the analysis model of the test piece shown in Table 2. Here, “Base”, which is a broken portion, is omitted. P1 is the result of TS-1a and P2 is the result of UT-1a. Breaking stress multi Jikudo T _CR, like the calculation of the strain at break epsilon _CR shown in FIG. 6, it is calculated by replacing the strain epsilon stress multi Jikudo T. As fracture limit line line connecting P1 and P2, it is possible to determine the reference strain at break Esuipushiron _CR a linear approximation from the stress at break multiaxiality. This is effective when there is a large difference in fracture strain between test pieces and averaging is not suitable. The range of P1 and P2, and Esuipushiron _CR strain break-away break strain epsilon _CR P1 and P2. To summarize the calculation equation of the reference strain at break Esuipushiron _CR by breaking stress multiaxiality (3), (4), the equation (5). When there are three or more test pieces, the number of fracture limit lines may be increased and a plurality of equations (4) may be used, or one fracture limit line may be obtained by the error minimization method. Alternatively, it may be obtained by approximating with a polynomial of breaking stress multiaxiality.
_{Sε CR = ε CR 1: (} T CR <T CR 1) ··· (3)
_{Sε CR = T CR · a1 +} b1: (T CR 1 ≦ T CR ≦ T CR 2) ··· (4)
_{Sε CR = ε CR 2: (} T CR 2 <T CR) ··· (5)
Here, a1 and b1 in the formula are coefficients.

Incidentally, how to obtain the reference strain at break Esuipushiron _CR is not limited to the above method, generally a method of determining the one representative value from a plurality of values used can be applied. In particular, when the load mode and / or nugget diameter ND of the analysis model is clear, the fracture strain ε _CR of a specific test piece that matches or approximates the load mode and / or the nugget diameter ND is determined as the fracture reference strain Sε _CR. Is possible. For the same reason, with a breaking strain epsilon _CR of some specimens, by and weighted average according to the degree of the average or approximate, it is possible to determine the reference strain at break Sε _CR.

<Fracture determination and fracture prediction by FEM analysis>
The fracture determination in the FEM analysis can be performed under the condition of the expression (6) or (6) ′.
ε _{(Base) CR} ≦ ε _(Base) or ε _{(HAZ) CR} ≦ ε _(HAZ) (6)
Sε _{(Base) CR} ≦ ε _(Base) or Sε _{(HAZ) CR} ≦ ε _(HAZ) (6) ′
Here, ε _(Base) is the maximum equivalent plastic strain of the base material, and ε _(HAZ) is the maximum equivalent plastic strain of the HAZ. As shown in equation (6), the maximum equivalent plastic strain ε _(Base) of the base material reaches the fracture strain ε _{(Base) CR} of the _base material, or the maximum equivalent plastic strain ε _(HAZ) of the HAZ is rupture strain ε of the HAZ. _{When (HAZ) CR} is reached, it is determined that the fracture has occurred at the part that has reached first. The same applies to the case where the fracture reference strains Sε _{(Base) CR 1} and Sε _{(HAZ) CR} are used instead of the fracture strains ε _{(Base) CR 1} and ε 2 _{(HAZ) CR as shown} in the equation (6) ′.

  If the base material 33 (see FIG. 4) that is about 1 mm away from the nugget 31 (see FIG. 4) becomes a fractured portion by the above determination, it may be identified as a button fracture that breaks at the base material 33 and the welded portion comes out. it can. The button breakage is characterized by not being brittle breakage because it sufficiently stretches and breaks at the base material 33, and it can be said to be a preferred breakage mode with a large amount of energy absorption. On the other hand, when HAZ32 (see FIG. 4) becomes a fractured portion, it is considered to include a risk of causing interface fracture because it is adjacent to the nugget. Interfacial rupture that breaks at the interface of the nugget 31 is considered to be dangerous because it is brittle and breaks without sufficiently absorbing energy. As a result of the tensile test being carried out 3 times each for 40 types of specimens with different steel types (590-980 MPa class), load mode, nugget diameter, and plate thickness, interfacial fracture occurred in 16 types of specimens, of which the base metal There were 0 test pieces that could break at 33, but there were 4 test pieces that could break at HAZ32. Therefore, predicting the fracture location of the base material 33 and the HAZ 32 is also effective in predicting a spot weld that includes the risk of interface fracture.

  FIG. 9 is an enlarged view of the vicinity of the spot welded portion of the analysis model. FIG. 9A shows a quarter of the spot weld. FIG. 9B is an enlarged view of the HAZ element 41 and the base material element 42 of FIG. With the center O of the nugget 31 as the center of the circle, the elements are divided circumferentially and circumferentially with respect to the HAZ 32 and the base material 33. The number of element divisions in the circumferential direction is assumed to be Div (hereinafter simply referred to as “Div”). As shown in FIG. 4B, the width 32a of the HAZ can be measured from the cross section of the spot weld. Moreover, it can obtain | require using the calculation formula which calculates the dimension of each part of the spot welding part produced from the data of the dimension accumulate | stored by the measurement of the general or cross section. Thus, the element size Rh in the radial direction of the HAZ element 41 is determined by the number of HAZ elements 41 set in the radial direction. Here, the HAZ elements 41 are set in one row in the radial direction. Further, the element size Ch in the circumferential direction of the HAZ element 41 is obtained by determining the element division number Div in the circumferential direction so as to be the size at the intermediate position of the size in the radial direction of the HAZ element 41. This Div can be arbitrarily determined. In this way, the size of the HAZ element 41 can be determined by Rh and Ch, or Rh and Div. On the other hand, the base material element 42 can arbitrarily determine the element size Rb in the radial direction. In addition, the element size Cb in the circumferential direction of the base material element 42 is obtained by determining the element size Rb and the number of element divisions Div in the circumferential direction so as to be the size at the intermediate position in the radial direction of the base material element 42. Div is the same as Div of the HAZ element 41. Thereby, the size of the base material element 42 can be determined by Rb and Cb or Rb and Div.

FIG. 10A is a diagram showing the relationship between the number of element divisions Div in the circumferential direction of the base material element 42 (see FIG. 9) and the breaking strain ε _{(Base) CR} of the base material 33 (see FIG. 9). A plurality of analysis models that are the same except for Div are created, and the fracture strain ε _{(Base) CR} of the _base material 33 is determined for each analysis model with reference to FIG. Since the size of the base material element 42 increases as the Div becomes smaller, the breaking strain ε _{(Base) CR} tends to become smaller. As described above, since the size Cb (see FIG. 9) of the base material element 42 can be obtained from Div, FIG. 10 (a) shows the relationship between the element size Cb and the breaking strain ε _{(Base) CR} of the _base material 33. It can also be a relationship. FIG. 10B is a diagram showing the relationship between the element size Rb (see FIG. 9) of the base material element 42 and the breaking strain ε _{(Base) CR} of the _base material 33. A plurality of analysis models that are the same except for the element size Rb are created, and the fracture strain ε _{(Base) CR} of the _base material 33 is determined for each analysis model with reference to FIG. The smaller the Rb, the larger the breaking strain ε _{(Base) CR} .

(Element size parameter)
As described above, the value of the breaking strain ε _{(Base) CR} of the _base material 33 changes with the change of the element sizes Rb and Cb or Div. Therefore, by determining this relationship, the fracture strain ε _{(Base) CR} of the _base material 33 can be obtained with an arbitrary element size Rb and Cb or Div. Therefore, the element size parameter Eb is set from the element sizes Rb and Cb. The element size parameter Eb is defined by equation (7).
Eb = Rb · s1 + Cb · s2 (7)
(0 ≦ s1, 0 ≦ s2)
Here, s1 and s2 are weighting factors of the element sizes Rb and Cb related to the breaking strain ε _{(Base) CR,} and can be obtained from the ratio of the plastic strain components at the time of breaking as follows.

FIG. 11 is a diagram showing plastic strain at the time of fracture of an element. Fig.11 (a) is a figure which shows the analysis output of the plastic strain at the time of the fracture | rupture in 1 element. FIG. 11B is a diagram showing an analysis output of plastic strain at the time of fracture in the HAZ element 41 and the base material element 42. As shown in FIG. 11A, plastic strains PEx and PEy are output in the directions of the two axes X and Y orthogonal to each element. Therefore, as shown in FIG. 11B, the HAZ element 41 outputs plastic strains PEx _(HAZ) and PEy _(HAZ) . In the base material element 42, plastic strains PEx _(Base) and PEy _(Base) are output. Here, in the base material element 42, the directions of the element sizes Rb and Cb are not orthogonal as shown in FIG. Therefore, the element size Rb and the direction of the plastic strain PEx _(Base) coincide, but the element size Cb and the direction of the plastic strain PEy _(Base) do not coincide. Therefore, the element size Cb and the direction of the plastic strain PEy _(Base) are approximated by increasing the element division number Div in the circumferential direction, for example, 128. Thereby, the weighting factors s1 and s2 can be obtained by the following equations (8) and (9).
s1 = PEx _(Base) / (PEx _(Base) + PEy _(Base) ) (8)
s2 = 1-s1 (9)
The above formulas (8) and (9) are examples of methods for calculating the weighting factors s1 and s2, and the method for calculating the weighting factors s1 and s2 is not limited to this, and other calculation methods may be used. Is possible.

FIG. 12 is a diagram showing the relationship between the element size parameter Eb and the breaking strain ε _{(Base) CR} of the base material 33 (see FIG. 9). A plurality of analysis models are created by changing the element size parameter Eb, and the fracture strain ε _{(Base) CR} of the _base material of each analysis model is obtained from FIG. The distribution of the breaking strain ε _{(Base) CR} can be approximated by a master curve Cm such as a hyperbola. As a result, the breaking strain ε _{(Base) CR} can be obtained for an arbitrary element size parameter Eb.

FIG. 13A is a diagram showing the relationship between the size Rh (see FIG. 9) of the HAZ element 41 and the breaking strain ε _{(HAZ) CR} of the _HAZ 32 (see FIG. 9). A plurality of analysis models that are the same except for the element size Rh are created, and the fracture strain ε _{(HAZ) CR} of the _HAZ 32 is obtained for each analysis model with reference to FIG. As Rh decreases, the breaking strain ε _{(HAZ) CR} of HAZ32 tends to increase. The same applies to the relationship between the size Ch of the HAZ element 41 and the breaking strain ε _{(HAZ) CR} of the _HAZ 32. FIG. 13B is a diagram showing the relationship between the size Rb of the base material element 42 (see FIG. 9) and the breaking strain ε _{(HAZ) CR} of the _HAZ 32 (see FIG. 9). A plurality of analysis models that are the same except for the element size Rb are created, and the fracture strain ε _{(HAZ) CR} of the _HAZ 32 is obtained for each analysis model with reference to FIG. Unlike the element size Rh, the breaking strain ε _{(HAZ) CR} of the _HAZ 32 tends to decrease as the element size Rb decreases. This is because the element size Rb is the element size of the base material 33, and the base material 33 is greatly extended by reducing the element size Rb. However, since the overall extension does not change, the extension of the HAZ element 41 is increased. This is because it becomes smaller. The same applies to the relationship between the element size Cb and the breaking strain ε _{(HAZ) CR of} HAZ32. Thus, the value of the breaking strain ε _{(HAZ) CR} of the _HAZ 32 changes with the change of the element sizes Rh, Ch, Rb, and Cb. Therefore, by determining this relationship, the breaking strain ε _{(HAZ) CR} of the _HAZ 32 can be obtained with an arbitrary element size Rh, Ch, Rb and Cb. For this reason, the element size parameter Eb set from the element sizes Rb and Cb is used as shown in the above equation (7). Also, an element size parameter Eh is set from the element sizes Rh and Ch. The element size parameter Eh is defined by equation (10).
Eh = Rh · t1 + Ch · t2 (10)
(0 ≦ t1, 0 ≦ t2)
Here, t1 and t2 are weighting factors of the element sizes Rh and Ch related to the breaking strain ε _{(HAZ) CR,} and can be obtained from the ratio of the plastic strain components at the time of breaking as follows.

Using the plastic strains PEx _(HAZ) and PEy _(HAZ) at the time of breakage in the HAZ element 41 in FIG. (11) and (12) can be obtained.
t1 = PEx _(HAZ) / (PEx _(HAZ) + PEy _(HAZ) ) (11)
t2 = 1−t1 (12)
The above formulas (11) and (12) are examples of methods for calculating the weighting factors t1 and t2. The method for calculating the weighting factors t1 and t2 is not limited to this, and other calculation methods may be used. Is possible.

FIG. 14 is a diagram showing the relationship between the element size parameters Eb and Eh and the breaking strain ε _{(HAZ) CR} of the _HAZ 32 (see FIG. 4). A plurality of analysis models are created by changing the element size parameters Eb and Eh, and the fracture strain ε _{(HAZ) CR} of the _HAZ 32 of each analysis model is obtained from FIG. The distribution of the breaking strain ε _{(HAZ) CR} can be approximated by a master curved surface Sm such as an orthogonal product curved surface. An orthogonal product curved surface is a curved surface in which control points are arranged in a grid pattern. Here, the control points are replaced with a fracture strain ε _{(HAZ) CR} calculated in advance by FEM analysis, and the two axes are in a grid pattern with Eb and Eh. To place. Since the height direction is set to the value of breaking strain ε _{(HAZ) CR} , it is distributed on the curved surface. As a result, the fracture strain ε _{(HAZ) CR} of the _HAZ 32 can be obtained for arbitrary element size parameters Eb and Eh.

The element size parameter used for calculating the breaking strain ε _{(HAZ) CR} is not limited to Eb and Eh, and other element size parameters can be used. For example, an element size parameter Ebh in which the element sizes Rb, Cb, Rh, and Ch of the base material 33 and the HAZ 32 are combined may be used. The element size parameter Ebh is defined by equation (13).
Ebh = Rb− ^m · r1 + Cb− ⁿ · r2 + Rh · r3 + Ch · r4 (13)
(However, 0 ≦ m, 0 ≦ n, 0 ≦ r1, 0 ≦ r2, 0 ≦ r3, 0 ≦ r4)

Here, m and n are influence coefficients of the element sizes Rb and Cb. R1, r2, r3, and r4 are weighting factors of element sizes Rb, Cb, Rh, and Ch related to the breaking strain ε _{(HAZ) CR,} and can be obtained from the ratio of the plastic strain components at the time of breaking. As described above, when the sizes Rh and Ch of the HAZ element 41 (see FIG. 9) are reduced, the breaking strain ε _{(HAZ) CR} is increased, but the sizes Rb and Cb of the base material element 42 (see FIG. 9) are reduced. Then, the breaking strain ε _{(HAZ) CR} becomes small. Therefore, the base material element 42 (see FIG. 9) sizes Rb and Cb are given Rb ^−m and Cb ⁻ⁿ and negative indices. Thereby, when the element sizes Rb and Cb are reduced, the element size parameter Ebh is increased, so that the breaking strain ε _{(HAZ) CR} is decreased.

The element size parameter Ebh can be determined by the following equation (14) using, for example, the above-described weighting factors s1, s2, t1, and t2.
Ebh = (Rb.s1) ^-m + (Cb.s2) ^-n + Rh.t1 + Ch.t2...
(14)
The above formula (14) is an example of the element size parameter Ebh, and the formula of the element size parameter Ebh or the weighting factors r1, r2, r3, r4 is not limited to this, and other formulas or values are used. It is also possible.

Further, as the element size parameters of the HAZ element 41 and the base material element 42, the diagonal lengths RbDia and RhDia of the element shown in FIG. 15 (a), or the radial maximum length RbMax of the element shown in FIG. 15 (b), RhMax may be used. R1 in FIG. 15B is the inclination of the element with respect to the radial direction, and varies depending on the number of element divisions Div in the circumferential direction. As shown in FIG. 10A, when Div is decreased, the breaking strain ε _{(Base) CR} of the base material 33 (see FIG. 4) greatly decreases. However, even if Div is increased, the breaking strain ε of the base material 33 is increased. It can be seen that the change to the increase trend of _{(Base) CR} is small. This is because when Div is increased, the change in R1 is reduced, and the changes in the radial lengths RbMax and RhMax of the element are reduced. Therefore, it is considered that the breaking strain ε _CR is greatly affected by the radial maximum lengths RbMax and RhMax, which are the maximum regions occupied by the elements in the radial direction.

FIG. 16 is a diagram showing the relationship between the element size parameter RbMax and the breaking strain ε _{(Base) CR} of the base material 33 (see FIG. 4). When RbMax is on the horizontal axis, the change in fracture strain of the base material 33 is linear, and therefore linear approximation can be performed. The same applies to the relationship between RhMax and the breaking strain ε _{(HAZ) CR of} HAZ32 (see FIG. 4). Therefore, the fracture strains of the base material 33 and the HAZ 32 can be easily obtained for arbitrary element size parameters RbMax and RhMax.

  With respect to the master curved surface Sm shown in FIG. 14, when the combination of Eb and Eh is irregular and the orthogonal product curved surface cannot be used, approximation may be performed with a lofted curved surface that interpolates an arbitrary curve group. Tables 3 and 4 show examples.

Table 3 shows the FEM analysis using 12 kinds of analysis models created by combining element sizes of Rb = 0.5 mm, 1 mm, 2 mm, 4 mm, Rh = 0.25 mm, 0.5 mm, and 1 mm, and the fracture strain ε _{CR ( i, j)} . In this example, since the breaking strains εCR _{(i, j)} can be arranged in a lattice pattern with Rb and Rh, orthogonal product curved surfaces can be used. However, in Table 4, the fracture strain ε _{CR (i, j)} is not calculated for some of the combinations of the same element sizes as in Table 3, and the orthogonal product curved surface cannot be used because it cannot be arranged in a lattice pattern. In this case, it may be approximated by a lofted curved surface or the like.

FIG. 17 is a diagram showing a method for obtaining the breaking strain ε _CR from the element size parameter. FIG. 17A is a diagram showing the relationship between the element sizes Rb, Rh, and Div and the breaking strain εCR. In FIG. 17A, master curved surfaces Sm1, Sm2, and Sm3 are created with three types of Div = 8, 16, and 32 as an example. Then, for each of the curved surfaces Sm1, Sm2, Sm3, element size Rb elements to be determined breaking strain epsilon _CR, from Rh, strain at break _{ε CR 1, ε CR 2,} ε CR 3 is calculated.

FIG. 17B is a diagram showing a method for obtaining the breaking strain for an arbitrary element size Cb from the breaking strains ε _CR 1, ε _CR 2, and ε _CR 3. In FIG. 17 (b), the horizontal axis element size Cb, vertical axis represents the strain at break epsilon _CR. Figure 17 (a) ε _CR 1 calculated in, ε _CR 2, Div of epsilon _CR 3 is converted into Cb, are marked as Figure 17 (b). Then, a line approximating the breaking strain ε _CR is drawn by the slope between ε _CR 1 and ε _CR 2 and the slope between ε _CR 2 and ε _CR 3. Thereby, the fracture | rupture distortion | strain (epsilon) _CR can be calculated | required from arbitrary Cb43. Therefore, it is possible to calculate the breaking strain ε _CR from any Rb, Rh, Div.

The degree of influence from the element sizes of the base material 33 and the HAZ 32 (see FIG. 4) varies depending on the balance of the tensile strengths of the base material 33 and the HAZ 32. Therefore, the master curve Cm (see Fig. 12) or the master curved Sm to be used for breaking strain epsilon _CR calculation varies depending on the material of the object to be welded. The relationship quantification techniques and fracture strain epsilon _CR and element sizes described above are not intended to have only the target fracture strain epsilon _CR, it can be applied even in the case of the other analysis results in fracture criterion. FIG. 18A shows the relationship between Div and the stress of the base material element 42 (see FIG. 9). The smaller the Div, the larger the element size, so the stress tends to decrease. FIG. 18B is a diagram showing the relationship between the element size Rb (see FIG. 9) and the stress of the base material element 42. The stress tends to increase as the element size Rb decreases. Thus, since the analysis results other than the breaking strain ε _CR are also affected by the element size, the present invention can be applied to the analysis results other than the breaking strain ε _CR .

(Breaking prediction)
By the above method, the fracture prediction of the spot welded portion is performed. A tensile test is performed using four types of test pieces shown in Table 1. The test pieces are as shown in FIGS. 1 (a) to 3 (a), and only the TS of FIG. 1 (a) changes the nugget diameter ND into two types of test pieces. Three tensile tests are performed on each type of test piece. The break points are as shown in Table 5.

  Next, an analysis model is created for the above four types of test pieces. In the analysis model, the plate thickness is 1.2 mm, the plate width is 40 mm, the tensile strength of the steel type is 780 MPa class, and material characteristics are set for each of the base material 33, HAZ32, and nugget 31 (see FIG. 4). The material properties are Young's modulus 205000 MPa, Poisson's ratio 0.3, and the stress σ-strain ε relationship is set for the base material 33, HAZ 32, and nugget 31 shown in FIG. FIG. 19 shows the relationship of stress σ-strain ε after plasticity in the FEM analysis. Element sizes Rb, Rh, and Div are used as the element size parameters. Rb is 1 mm, 2 mm, 4 mm, Rh is 0.1625 mm, 0.325 mm, 0.65 mm, and Div is 8, 16, 32. With these combinations, FEM analysis is performed with 27 analysis models for each test piece.

A case where Rb is 1 mm, Rh is 0.65 mm, and Div is 32 from the combination of the element size parameters described above will be described. The analytical model of each test piece is in this element size parameter performed FEM analysis, calculates the strain at break epsilon _CR. The calculation method is as described in FIG. Table 5 also shows the breaking strain ε _CR by FEM analysis. Now, the base material 33 and HAZ32, determine the reference strain at break Sε _CR. The base breaking strain Sε _{(Base) CR} of the base material 33 is 0 by averaging the breaking strain ε _{(Base) CR} of the _base material 33 of the test pieces TS-1, TS-2, and UT-1 that are broken by the base material 33. .258. The reference breaking strain Sε _{(HAZ) CR} of HAZ32 is 0.152 by averaging the breaking strain ε _{(HAZ) CR} of HAZ32 of the test piece LT-1 and UT-1 that were broken by HAZ32.

Similarly, we performed FEM analysis on 27 as the combination of all the element size parameter, determine the reference strain at break Sε _CR. Thereby, the fracture | rupture reference | standard is created as shown in Table 6 about each of the base material 33 and HAZ32.

なお、破断基準は表６に限定されず、例えば要素サイズパラメータＲｂ、Ｒｈ、Ｄｉｖのサイズを変更することはもちろん、要素サイズパラメータとして、上述したＣｂ、Ｃｈ、Ｅｂ、Ｅｈ、Ｅｂｈ、ＲｈＤｉａ、ＲｂＤｉａ、ＲｈＭａｘ、ＲｂＭａｘなどを用いることも可能である。

Note that the fracture criterion is not limited to Table 6. For example, the size of the element size parameters Rb, Rh, Div is changed, and as the element size parameter, the above-described Cb, Ch, Eb, Eh, Ebh, RhDia, RbDia are used. , RhMax, RbMax, etc. can also be used.

Then, an analysis model is created for the workpiece to be predicted for breakage of the spot weld. The element size at this time is not particularly limited, and can be created with an arbitrary size using an automatic mesh function or the like. The elements of the analysis model, determine the size of each element size parameter, it calculates a reference strain at break Esuipushiron _CR than breaking criteria in Table 6. Here, the element size parameters are Rb, Rh, Div, and in the method described with reference to FIGS. 17A and 17B, the vertical rupture is set as the reference break strain Sε _CR , so that the reference break strain Sε _{CR is obtained.} Can be requested. Then, a FEM analysis of the analytical model of the object to be welded, when the strain ε of as elements of the above-mentioned (6) 'equation becomes a reference strain at break Esuipushiron _CR, determines that the fracture. As a result, it is possible to predict ruptures such as a rupture site as to whether the rupture occurs in the base material 33 or HAZ32, a rupture position from the ruptured element, a rupture stress at that time, and whether or not there is a rupture at a predetermined load. .

FIG. 20 is a diagram showing a fracture prediction device 50 for spot welds. The rupture prediction device 50 includes a rupture strain database 51 that is a storage unit that stores a relationship between an element size parameter and an element rupture strain, a preprocessor 52 that is an analysis model creation unit, an analysis model data 53 that is created by a preprocessor 52, and the like. It has a fracture strain setting processing means 56 connected to a solver 54 as FEM analysis means, a post processor 55 as analysis result display means, analysis model data 53 and a fracture strain database 51. The breaking strain setting processing means 56 has an element size calculating means 57 and a breaking strain approximation means 58. Incidentally, the strain at break database 51, as shown in Table 5 described above, element size parameter - the motor and the reference strain at break Esuipushiron _CR may store the fracture criterion was quantified relationship of. Further, the preprocessor 52, the solver 54, and the postprocessor 55 are not particularly limited, and generally used analysis model creation, FEM analysis, analysis result display software, and the like can be used.

  With this configuration, the analysis model 60 is created for the workpiece to be predicted for breakage of the spot weld by the preprocessor 52. Fig.21 (a) is a figure which shows the analysis model around the spot weld part of a to-be-welded object. The workpieces are spot-welded with overlapping plate materials. Here, for simplification of the analysis model, half of the spot welds are divided into elements by shell elements. FIG. 21B is an enlarged view of the spot welded part in FIG. Material characteristics are set for each part of the nuggets 61 and 61, the HAZs 62 and 62, and the base materials 63 and 63, respectively. Each data of the analysis model 60 is output as analysis model data 53. The analysis model data 53 may be not only the data output to the preprocessor 52 using the analysis model creation software, but also the data created by hand calculation. In addition, the shape data created by a modeling tool such as CAD is used as mesh data. It may be converted.

FIG. 22 is a diagram showing the element size calculation means 57. Analytical model data 53 such as element node information, element information, node set information, and element set information is read and the HAZ element 64 has a radial length Rh ₆₀ , a circumferential length Ch ₆₀ , and a base material adjacent to the HAZ element A radial length Rb ₆₀ and a circumferential length Cb ₆₀ of the element 65 are obtained.

FIG. 23 is a diagram showing the breaking strain approximation means 58. Here, as an example, a method of creating a master curved surface of the base material 63 (see FIG. 21) from the rupture strain database 51 (see FIG. 20) and obtaining the rupture strain ε _{(Base) CR} of the _base material 63 will be described. From the fracture strain database 51, as shown in FIG. 23A, the fracture strain ε _{(Base) CR} of the _base material 63 corresponds to Div = 8, 16, and 32, with Rb as the first axis and Rh as the second axis, respectively. Master curved surfaces Sm1, Sm2, and Sm3 are created.

As shown in FIG. 23B, in Sm1, a bilinear patch is applied to the calculation of the breaking strain ε _{(Base) CR} 1 at Rb ₆₀ and Rh ₆₀ (see FIG. 22) calculated by the element size calculation means 56. . In the bilinear patch, the value at an arbitrary position can be approximated by performing linear interpolation from the values of four control points surrounding the arbitrary position on the orthogonal product curved surface. With the bilinear patch, ε _{(Base) CR} 1 can be obtained by the following equation (15).
ε _{(Base) CR} 1 = (1-v) {(1-u) · E _0,0 + u · E _0,1 }
+ V · {(1-u) · E _1,0 + u · E _1,1 } (15)
_{However, u = (Rb 60 -Rb 0,0} ) / (Rb 0,1 -Rb 0,0)
v = (Rh ₆₀ −Rh _0,0 ) / (Rh _1,0 −Rh _0,0 )
Here, E _0,0 , E _0,1 , E _1,0 , E _1,1 are the fracture strain ε _{(Base) CR} of the _base material 63 at the control point. Rb _0,0 is the Rb coordinate of the control point E _0,0 , Rh _0,0 is the Rh coordinate of the control point E _0,0 , Rb _0,1 is the Rb coordinate of the control point E _0,1 , Rh _{1, 0} is the Rh coordinate of the control point _E1,0 . The other master curved surfaces Sm2 and Sm3 can also obtain the breaking strains ε _{(Base) CR} 2 and ε _{(Base) CR} 3 by the same method.

Since Div of the fracture strain database 51 can be converted to Cb, Div = 8 to Cb1, Div = 16 to Cb2, and Div = 32 to Cb3 are calculated. The breaking strain ε _{(Base) CR} of the _base material 63 at the element size Cb ₆₀ calculated by the element size calculation means 56 (see FIG. 20) is the value of Cb1, Cb2, Cb3 and Cb _{60 as} shown in FIG. Can be obtained by linear interpolation from the slope SL1 between ε _{(Base) CR} 1 and ε _{(Base) CR} 2 or the slope SL2 between ε _{(Base) CR} 2 and ε _{(Base) CR} 3. . Similarly, the breaking strain ε _{(HAZ) CR} of HAZ62 (see FIG. 21) can be obtained. As a result, the fracture strains ε _{(Base) CR 1} and ε _{(HAZ) CR} of the base material 63 and the HAZ 62 are set in the analysis model data 53 (see FIG. 20).

FIG. 24 is a diagram illustrating a state of FEM analysis of the analysis model 60 (see FIG. 21). For the lower plate material of the analysis model 60, the elements are grayed out for ease of illustration. Using the analysis model data 53, the FEM analysis is performed by the solver 54 and displayed by the post processor 55 (see FIG. 20). By using commercially available software ABAQUS / Explicate for the solver 54, the breaking strains ε _{(Base) CR 1} and ε _{(HAZ) CR} obtained in FIG. 23 are set for each part of the base material, HAZ, and nugget. It is possible to forcibly set the stress of the element reaching ε _{(Base) CR 1} and ε _{(HAZ) CR} to 0, and proceed with the analysis with the element removed. FIG. 24A shows an initial state. FIG. 24B shows a state in which the base material 63 (see FIG. 21) is broken. The equivalent plastic strain of some of the base material elements 65 reaches the set breaking strain ε _{(Base) CR} , and the elements are removed. FIG. 24C shows a state where the fracture has further progressed. In this way, by using the fracture prediction device 50 of the present invention (see FIG. 20), it is predicted strain at break is also a member there are a plurality of spot welds epsilon _CR and breaking process. The solver 54 is not limited to the above software, and other commercially available software may be used.

Next, an analysis model 70 is created by the preprocessor 52 in order to predict fracture of the spot welded part of the actual member. FIG. 25A is a diagram showing an analysis model having a hat shape. FIG. 25 (b) is an enlarged view of the spot welded portion of FIG. 25 (a). The hat-type member 74 and the plate 75 are joined by a spot weld 76. The plates 77 and 78 are joined at both ends. The plate 78 is constrained and an impact load 79 is applied to the plate 77 to crush the hat-shaped member 74 and the plate 75 that are to-be-welded. Material characteristics are set for each part of the nugget 71, the HAZ 72, and the base material 73. In the analysis model 70, since the tensile strength of the steel type was set to 440 MPa class, material characteristics of 440 MPa class and breaking strains ε _{(Base) CR} and ε _{(HAZ) CR} were set.

  FIG. 26 is a diagram illustrating a state of FEM analysis of the analysis model 70. The plate 77 and the plate 78 are omitted from the viewability of the figure. Analysis was performed using ABAQUS / Explicit as in the analysis model 60. FIG. 26A shows an initial state. FIG. 26B shows a collapsed state with a displacement amount of 60 mm, and FIG. 26C shows a displacement amount of 150 mm. Similar to the analysis model 60 shown in FIG. 24, the analysis can proceed with the elements that have reached the fracture strain removed, so the process in which the spot weld 76 breaks and the hat-shaped member 74 and the plate 75 separate is simulated. can do. FIG. 27 is a comparison of the absorption energy by the experiment and the FEM analysis for the analysis model 70. It can be seen that the FEM analysis results almost coincide with the experimental results.

(Comparative Example 1)
In Comparative Example 1, the breaking strain ε _CR of each test piece was obtained without setting the material properties of HAZ32 (see FIG. 4). FIG. 28 is a diagram showing the results of Comparative Example 1. In Comparative Example 1, for the test piece of Table 1, the stress σ-strain ε relationship between the HAZ 32 and the base material 33 in FIG. 5 is not distinguished, and the base material 33 other than the nugget 31 (see FIG. 4) is used. A stress σ-strain ε relationship was set. As shown in Table 1, in the tensile test, although broken by TS-1, TS-2, UT-1 is the base material 33, a large difference in FIG. 28 strain of breaking epsilon _CR. Therefore, when the strength of the spot welded portion is calculated based on the standard breaking strain of the base material 33 obtained by averaging these, an error of a maximum of 16% occurs as compared with the actual strength. The tensile in the test, but the test piece LT-1, UT-1 is broken at HAZ32, a large difference in FIG. 28 strain of breaking epsilon _CR. Therefore, when the strength of the spot welded portion was calculated with the HAZ32 standard breaking strain obtained by averaging these, an error of 30% at maximum was generated as compared with the actual strength. Since generally the prediction error of the intensity of spot welds is that it is desirable to around 10%, she is impossible to obtain a strain at break in the epsilon _CR accurate reference strain at break shown in FIG. 28. If the material properties of the HAZ 32 are not set, the material around the constrained nugget 31 is stretched the most, and the fracture location is also the position of the HAZ 32 adjacent to the nugget 31. However, since the deformation resistance of the HAZ 32 is larger than that of the base material 33, the base material 33 that is about 1 mm away from the HAZ 32 may be stretched most than the HAZ 32 adjacent to the nugget 31 in the actual phenomenon. In order to perform fracture prediction independent of the load mode, the strain must be calculated by properly calculating the elongation of each part in any load mode. Therefore, it was confirmed that the effect of setting the material properties of HAZ32 was great.

(Comparative Example 2)
In Comparative Example 2, FEM analysis was performed by changing the material property setting, and the relationship between the displacement λ and strain ε of the entire test piece in the HAZ element 41 and the base material element 42 (see FIG. 9) was obtained. 29A to 29C are diagrams showing the results of Comparative Example 2. FIG. FIG. 29A is a diagram showing an analysis result for the test piece TS-1 in Table 1. FIG. The material pattern 1 is a case where material characteristics are set for the base material 33, the HAZ 32, and the nugget 31 (see FIG. 4), respectively. The material pattern 2 is a case where the material characteristics of the base material 33 are set except for the nugget 31 and the material characteristics of the HAZ 32 are not set. In the material pattern 1, the base material element 42 is extended more than the HAZ element 41 adjacent to the constrained nugget 31, but in the material pattern 2, the opposite result is obtained.

  FIG. 29B is a diagram showing the analysis results for the test piece LT-1 in Table 1. FIG. In the material pattern 1, the base material element 42 is stretched more than the HAZ element 41, but in the material pattern 2, the elongation of the HAZ element 41 and the base material element 42 is substantially the same.

  FIG. 29 (c) is a diagram showing an analysis result for the test piece UT- 1 in Table 1. In the material pattern 1, the base material element 42 is extended more than the HAZ element 41, but in the material pattern 2, the opposite result is obtained. Thus, each load mode is affected by the material properties of the HAZ 32 (see FIG. 4). In order to perform fracture prediction independent of the load mode, the strain must be calculated by properly calculating the elongation of each part in any load mode. Therefore, it was confirmed that the effect of setting the material properties of HAZ32 was great.

  In the above embodiment, the present invention is applied to the base material and the HAZ. However, the present invention can also be applied to only one of the base material and the HAZ. The effect of the present invention can also be obtained by using other values such as stress instead of the strain of the above embodiment.

  While the present invention has been described in connection with embodiments that are presently the most practical and preferred, the present invention is not limited to the embodiments disclosed herein. However, the present invention can be changed as appropriate without departing from the spirit or concept of the invention that can be read from the claims and the entire specification, and the fracture strain calculation method, the standard fracture strain calculation method, and the like accompanying such changes are also included in the present invention. It should be understood as encompassed within the technical scope.

It is a figure which shows the test piece and analysis model of the tensile shear of a spot welded joint. It is a figure which shows the test piece and analysis model of L-shaped joint tension | pulling of a spot welded joint. It is a figure which shows the test piece and analysis model of U-shaped joint tension | tensile_strength of a spot welded joint. It is a figure which shows a spot weld part. It is a figure which shows the stress (sigma) -strain (epsilon) relationship of each site | part of spot welding. It is a figure which shows the example of calculation of the fracture strain in the analysis model of a test piece. It is a figure which shows the fracture | rupture distortion of the test piece calculated | required from the FEM analysis. It is a figure which shows the relationship between breaking strain and breaking stress multiaxiality. It is an enlarged view near the spot weld part of an analysis model. (A) is a figure which shows the relationship between the element division number Div of the circumferential direction, and the fracture | rupture distortion of a base material. (B) is a figure which shows the relationship between element size Rb of a base material element, and the fracture | rupture distortion of a base material. It is a figure which shows the plastic strain at the time of the fracture | rupture of an element. It is a figure which shows the relationship between the element size parameter Eb and the fracture | rupture distortion of a base material. (A) is a figure which shows the relationship between the size Rh of a HAZ element, and the fracture | rupture distortion | strain of HAZ. (B) is a figure which shows the relationship between the size Rb of a base material element, and the fracture | rupture distortion of HAZ. It is a figure which shows the relationship between the element size parameters Eb and Eh and the fracture strain of HAZ. (A) is a figure which shows element size parameter RbDia and RhDia. (B) is a diagram showing element size parameters RbMax, RhMax. It is a figure which shows the relationship between element size parameter RbMax and the fracture | rupture distortion of a base material. It is a figure which shows the method of calculating | requiring a fracture | rupture distortion from an element size parameter. (A) is a figure which shows the relationship between Div and the stress of an element. (B) is a figure which shows the relationship between element size Rb and the stress of an element. It is a figure which shows the stress (sigma) -strain (epsilon) relationship of each site | part of spot welding. It is a figure which shows the fracture prediction apparatus of a spot weld part. It is a figure which shows the analytical model of the to-be-welded object which performs the fracture | rupture prediction of a spot weld part. It is a figure which shows the element size calculation means in the fracture prediction apparatus of a spot weld part. It is a figure which shows the fracture | rupture distortion approximation means in the fracture prediction apparatus of a spot weld part. It is a figure which shows the condition of the FEM analysis of an analysis model. It is a figure which shows the analysis model of a hat-shaped shape which performs the fracture | rupture prediction of a spot weld part. It is a figure which shows the condition of the FEM analysis of a hat-shaped analysis model. It is a figure which shows the comparison with the absorption energy by the experiment and FEM analysis which made the hat-shaped analysis model object. It is a figure which shows the fracture | rupture distortion of a base material when not setting the material characteristic of HAZ. In FEM analysis, it is a figure which shows the relationship between a material characteristic and distortion.

Explanation of symbols

ε _CR rupture strain ε _{(Base) CR} base material rupture strain ε _{(HAZ) CR} heat affected zone (HAZ) rupture strain Sε _CR standard rupture strain Sε _{(Base) CR} base material rupture strain Sε _{(HAZ) CR} heat Basic fracture strain of affected zone (HAZ) T Stress multiaxiality T _CR Multiaxiality of fracture stress 1 Tensile shear specimen 2 L-shaped joint tensile specimen 3 U-shaped joint tensile specimen 31 Nugget 32 Thermal effect Department (HAZ)
33 Base material 41 Heat-affected zone (HAZ) element 42 Base material element 50 Spot weld fracture rupture prediction device 51 Break strain database (storage means)
56 Breaking strain setting processing means 57 Element size calculating means 58 Breaking strain approximation means

Claims (9)

A fracture strain calculation method for spot welds using a finite element method,
Creating an analysis model with a plurality of element sizes for the same spot weld joint having the spot weld,
Calculating the fracture strain of the base material of the spot weld and / or the heat affected zone in each of the analysis models by the finite element method;
Determining a relationship between an element size parameter defining the element size and the breaking strain;
And a step of calculating the fracture strain of the base material and / or the heat-affected zone from the value of the predetermined element size parameter according to the relationship.   The fracture strain calculation method for a spot weld according to claim 1, wherein the element size parameter of 1 or 2 is used.   3. The spot welded portion fracture strain calculation method according to claim 1 or 2, wherein material characteristics of the base metal, the heat affected zone, and the nugget of the spot welded portion are set in each of the analysis models. A method for calculating a reference fracture strain of a spot weld using a finite element method,
A step of performing a tensile test of the spot welded portion with respect to a plurality of spot welded joints having the spot welded portion and different load modes and / or nugget diameters;
Creating an analysis model so that the value of the element size parameter that determines the element size of the base material adjacent to the heat affected zone and / or the heat affected zone of the spot welded portion of all the spot welded joints is the same; ,
Calculating a fracture strain of the base material and / or the heat-affected zone in each analysis model by the finite element method;
A method for calculating a reference fracture strain of a spot welded part, comprising: calculating a reference fracture strain of the base material and / or the heat-affected zone using the fracture strain from the result of the tensile test.   5. The method for calculating the reference fracture strain of a spot weld according to claim 4, wherein material characteristics of the base material, the heat affected zone and the nugget of the spot weld are set in each of the analysis models. Calculating the reference fracture strain of the base material and / or the heat-affected zone for a plurality of values of the element size parameter using the method of calculating the reference fracture strain of the spot weld according to claim 4 or 5;
A method for preparing a fracture criterion for a spot welded portion, comprising a step of obtaining a fracture criterion that is a relationship between the element size parameter and the reference fracture strain of the base material and / or the heat affected zone. A method for predicting fracture of a spot weld by a finite element method,
Creating an analysis model of an object to be welded having the spot welds;
Obtaining a value of an element size parameter for determining an element size of the analysis model of the work piece;
A step of obtaining the reference fracture strain of the base material and / or the heat-affected zone from the value of the element size parameter of the workpiece using the fracture criterion created by the fracture criterion creation method according to claim 6; ,
Analyzing the analysis model of the workpiece to be welded by the finite element method, and obtaining a strain of the element;
And a step of determining the fracture of the element when the strain becomes equal to or greater than the reference fracture strain.   The spot welded portion fracture prediction method according to claim 7, wherein material characteristics of the base material, the heat affected zone, and the nugget of the spot welded portion are set in the analysis model of the workpiece. An analysis model creating means for creating an analysis model of the work piece divided into elements;
Storage means for storing a relationship between an element size parameter for determining an element size of the analysis model and a breaking strain of the element;
The value of the element size parameter is determined from the element size, and the breaking strain setting processing means for determining the breaking strain from the value of the element size parameter using the storage means;
Analyzing means for analyzing the analysis model by a finite element method, and comparing the strain of the element obtained by analysis with the breaking strain;
An apparatus for predicting breakage of a spot welded portion.

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