Method for estimating performance of spot-welded joint

Abstract

To provide a method for estimating performance of a spot-welded joint capable of estimating a limit initial crack depth according to a plate thickness and a nugget diameter.SOLUTION: A method includes: an analysis step (#5); a classification step (#10); first to fourth derivation steps (#15 to #30); a setting step (#35); a calculation step (#40); and an estimation step (#45). From the analysis step (#5) to the fourth derivation step (#30), a function is derived having a coefficient of a nugget diameter as an independent variable and a limit initial crack ratio as a dependent variable. In the calculation step (#40), the coefficient of a nugget diameter of an evaluation target set in the setting step (#35) is substituted into the coefficient of the nugget diameter in the function derived in the fourth derivation step (#30), and the limit initial crack ratio of the evaluation target is calculated. In the estimation step (#45), a limit initial crack depth is defined by multiplying the limit initial crack ratio calculated in the calculation step (#40) by a sheet thickness set in the setting step (#35).SELECTED DRAWING: Figure 5

Description

Technical Field

[0001] The present disclosure relates to a method for estimating the performance of spot weld joints.

Background Art

[0002] In the automotive industry, a large number of steel sheets are used, and resistance spot welding is widely used as a method for joining steel sheets. Conventionally, for example, when resistance spot welding is performed on high-strength steel sheets having zinc-based plating, it is known that microcracks sometimes occur in the welded portion. The occurrence of these cracks is due to Liquid Metal Embrittlement (LME). In spot weld joints formed by resistance spot welding, not limited to cracks caused by such LME, if microcracks exist as initial cracks, there is a concern that the joint strength may decrease depending on the depth of the initial cracks. Therefore, it is desired that the limit depth that does not affect the joint strength can be estimated for the initial cracks.

[0003] Japanese Patent Application Laid-Open No. 2023-093288 (Patent Document 1) describes a method for estimating the performance of spot weld joints. In this estimation method, finite element analysis is performed to derive the relationship between the initial crack depth and the joint strength, and the limit of the initial crack depth that does not affect the joint strength is estimated from this relationship.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In the conventional estimation method described in Patent Document 1, the relationship between the initial crack depth and the joint strength is used to estimate the critical initial crack depth. This relationship is determined based on the results of finite element analysis under the condition that the plate thickness is 1.6 mm at one level. That is, the influence when the plate thickness changes is not considered. Then, in the estimation method of Patent Document 1, when the plate thickness is 1.6 mm, it is possible to estimate the critical initial crack depth according to the nugget diameter. However, when the plate thickness is different from 1.6 mm, it may be difficult to estimate the critical initial crack depth.

[0006] An object of the present disclosure is to provide a method for estimating the performance of a spot weld joint that can estimate the critical initial crack depth that does not affect the joint strength according to the plate thickness and the nugget diameter.

Means for Solving the Problems

[0007] The method for estimating the performance of a spot welding joint according to the present disclosure includes an analysis step, a classification step, a first derivation step, a second derivation step, a third derivation step, a fourth derivation step, a setting step, a calculation step, and an estimation step. The analysis step performs a finite element analysis of the tensile condition of the spot welding joint by changing the plate thickness, the nugget diameter represented by the product of the coefficient and the square root of the plate thickness, and the initial crack depth. The classification step classifies the results of the analysis step by the coefficient of the nugget diameter. The first derivation step derives the relationship between the initial crack rate defined by the ratio of the initial crack depth to the plate thickness and the joint strength for each plate thickness among those classified in the classification step. The second derivation step derives, for each of the relationships between the initial crack rate and the joint strength derived in the first derivation step among those classified in the classification step, the initial crack rate at which the joint strength changes abruptly during a decrease as the abrupt initial crack rate. The third derivation step derives, among those classified in the classification step, the average value of the abrupt initial crack rates derived in the second derivation step as the limit initial crack rate. The fourth derivation step derives a function having the coefficient of the nugget diameter as an independent variable and the limit initial crack rate as a dependent variable based on the limit initial crack rate derived in the third derivation step and the coefficient of the nugget diameter corresponding to the limit initial crack rate. The setting step sets the coefficient of the nugget diameter and the plate thickness in the spot welding joint to be evaluated. The calculation step substitutes the coefficient of the nugget diameter of the object to be evaluated set in the setting step into the coefficient of the nugget diameter, which is the independent variable, in the function derived in the fourth derivation step, and calculates the limit initial crack rate in the spot welding joint to be evaluated. The estimation step estimates, as the limit initial crack depth, the value obtained by multiplying the limit initial crack rate calculated in the calculation step by the plate thickness set in the setting step.

Effect of the Invention

[0008] According to the method for estimating the performance of a spot welding joint according to the present disclosure, it is possible to estimate the limit initial crack depth that does not affect the joint strength according to the plate thickness and the nugget diameter.

Brief Description of the Drawings

[0009]

Figure 1

Figure 2

Figure 3

Figure 4A

Figure 4B

Figure 4C

Figure 5

Figure 6A

Figure 6B

Figure 6C

Figure 7

DETAILED DESCRIPTION OF THE INVENTION

[0010] To achieve the above object, the inventors first considered the utilization of the conventional estimation method. Specifically, an analysis model of the spot weld joint was created and finite element analysis (hereinafter, also referred to as "FE analysis") was performed to investigate the joint strength. Hereinafter, the state of the FE analysis will be described with reference to FIGS. 1 to 3.

[0011] FIG. 1 is a perspective view showing a test piece 10 for investigating the joint strength of a spot weld joint assumed in finite element analysis. The test piece 10 shown in FIG. 1 assumes a tensile shear joint as a spot weld joint. By using this test piece 10, the tensile shear strength (TSS) can be investigated as the joint strength. The test piece 10 is composed of two steel plates 11 and 12. The steel plates 11 and 12 are stacked on top of each other in a mutually displaced state in the longitudinal direction LD of the test piece 10, and a welded portion 13 of resistance spot welding is formed at the center of the overlapping region.

[0012] FIG. 2 is a perspective view showing an analysis model 20 based on the test piece 10 shown in FIG. 1. The analysis model 20 shown in FIG. 2 is composed of two steel plates 21 and 22 that overlap each other, similar to the test piece 10 shown in FIG. 1, and a welded portion 23 is formed in the overlapping region. However, due to the symmetry in the width direction WD of the test piece 10, the analysis model 20 has the shape obtained by cutting the test piece 10 at the center in the width direction WD, that is, half of the shape of the test piece 10. A nugget 24 is formed in the welded portion 23.

[0013] FIG. 3 is an enlarged view of the welded portion 23 of the analysis model 20 shown in FIG. 2 and its vicinity. Referring to FIG. 3, in the analysis model 20, a HAZ (heat affected zone) 25 is formed so as to surround the nugget 24, and a transition layer 26 is formed between the HAZ 25 and the base materials 27 of the respective steel plates 21 and 22. In the analysis model 20, the nugget 24, the HAZ 25, the transition layer 26, and the base material 27 are divided into element sets as different structures from each other, and a deformation resistance curve and a fracture criterion are set for each of them.

[0014] In the analysis model 20, further, an initial crack 28 is formed in the HAZ 25 starting from the contact portion between the steel plate 21 and the steel plate 22. The initial crack 28 extends in a range of 90° in the circumferential direction centered on the nugget 24 among the periphery of the nugget 24. The depth d of the initial crack 28 means the dimension of the initial crack 28 in the plate thickness direction TD.

[0015] Returning to FIG. 2, during the FE analysis, one end of the analysis model 20 (the open end 21a in the longitudinal direction LD of the steel plate 21) is fully constrained, and a tensile load in the longitudinal direction LD (the thick arrow in FIG. 2) is applied to the other end of the analysis model 20 (the open end 22a in the longitudinal direction LD of the steel plate 22). The FE analysis is performed under the condition that the tensile load is applied. As a result of the FE analysis, the joint strength is obtained. Also, the fracture mode can be recognized. The joint strength is the maximum value of the load applied to the analysis model 20 or the test piece 10 during tension. In the FE analysis, elements that reach the fracture criterion are deleted to reduce the rigidity, and thereby, the FE analysis simulates a tensile test using the actual test piece 10.

[0016] Referring to FIG. 3, in the FE analysis, a number of analysis models 20 were created. Specifically, the plate thickness t was variously changed. This plate thickness t is the plate thickness of each of the steel plates 21 and 22. The plate thickness t of the steel plate 21 is the same as the plate thickness t of the steel plate 22. The plate thickness t was set to three levels of 1.2 mm, 1.6 mm, and 2.0 mm. Also, regarding the nugget diameter (k√t) represented by the product of the coefficient k and the square root of the plate thickness t, the coefficient k was variously changed. The coefficient k of the nugget diameter was set to three levels of 3.0, 4.0, and 5.0. In this case, the variations of the analysis model 20 based on the plate thickness t and the coefficient k of the nugget diameter were nine levels. In each variation, the initial crack depth d was variously changed. The initial crack depth d included 0 (zero) and was further increased at a pitch of 0.1 mm from 0.1 mm. As the steel plates 21 and 22, 980 MPa grade high-tensile steel plates were used.

[0017] FIGS. 4A to 4C show the FE analysis results arranged according to the conventional estimation method. FIG. 4A is a diagram showing the relationship between the initial crack depth d and the joint strength TSS when the plate thickness t is 1.2 mm. FIG. 4B is a diagram showing the relationship between the initial crack depth d and the joint strength TSS when the plate thickness t is 1.6 mm. FIG. 4C is a diagram showing the relationship between the initial crack depth d and the joint strength TSS when the plate thickness t is 2.0 mm. Among these figures, the relationship shown in FIG. 4B coincides with the relationship used in the conventional estimation method. The relationships shown in FIGS. 4A and 4C are not considered in the conventional estimation method.

[0018] In each figure, the open marks (◇, 〇, □) indicate that the breakage started from an initial crack. Hereinafter, this breakage form is also referred to as "crack-origin breakage". On the other hand, the solid marks (◆, ●, ■) indicate that the breakage occurred without starting from an initial crack, that is, in the same way as when the initial crack depth is zero. Hereinafter, this breakage form is also referred to as "normal breakage".

[0019] Referring to FIG. 4B, in the conventional estimation method, for any of the three levels of the lug diameters, the joint strength of the open mark (crack-origin breakage) is lower than that of the solid mark (normal breakage). If the initial crack depth is small, the breakage form becomes normal breakage and the joint strength is maintained. On the other hand, when the initial crack depth exceeds a certain point, the breakage form changes to crack-origin breakage and the joint strength decreases. Therefore, the initial crack depth when the joint strength suddenly changes due to the decrease can be said to be the critical initial crack depth that does not affect the joint strength. At each of the three levels of the lug diameters, the critical initial crack depth is the initial crack depth when the joint strength suddenly changes due to the decrease, that is, when it is closest to the solid mark (normal breakage) among the open marks (crack-origin breakage). Therefore, in the conventional estimation method, as described above, under the condition that the plate thickness is set to one level, the critical initial crack depth can be estimated according to the lug diameter.

[0020] On the other hand, the relationships shown in FIGS. 4A and 4C are not considered in the conventional estimation method, but have the same tendency as the relationship shown in FIG. 4B. Then, the conventional estimation method may be applicable to the relationships shown in FIGS. 4A and 4C. However, the joint strength and the critical initial crack depth that can be said from the relationships shown in FIGS. 4A and 4C are clearly different from those shown in FIG. 4B. It can be said that this is affected by the plate thickness. Furthermore, if the plate thickness is different from that in the cases shown in FIGS. 4A to 4C, it can be said that it is difficult to cope with it by the conventional estimation method.

[0021] Therefore, in order to achieve the above object, the present inventors have conducted intensive studies and tried to organize the above FE analysis results from a perspective different from the conventional estimation method. As a result, the following findings were obtained.

[0022] First, the above FE analysis results were classified by the coefficient k of the nugget diameter. Next, as a new index, an initial crack ratio R defined by the ratio of the initial crack depth d to the plate thickness t was introduced. Next, among the classified results, the relationship between the initial crack ratio R and the joint strength was derived for each plate thickness. Then, it was found that the initial crack ratio R when the joint strength suddenly decreases is equivalent among the plate thicknesses in the classification. In this specification, these equivalent initial crack ratios R are each referred to as the sudden-change initial crack ratio Rc. Next, among the classified results, the average value of these sudden-change initial crack ratios Rc was defined as the limit initial crack ratio Cr. As a result, the limit initial crack ratio Cr is determined for each coefficient k of the nugget diameter in the classification. And based on the limit initial crack ratio Cr and the coefficient k of the nugget diameter corresponding to the limit initial crack ratio Cr, a function with the coefficient k of the nugget diameter as the independent variable and the limit initial crack ratio Cr as the dependent variable could be derived.

[0023] After deriving such a function, if the coefficient ke of the nugget diameter and the plate thickness te in the spot weld joint to be evaluated are set, the limit initial crack depth dc can be estimated using that function. That is, the function is a function with the coefficient k of the nugget diameter as the independent variable and the limit initial crack ratio Cr as the dependent variable, and the limit initial crack ratio Cr is the ratio of the limit initial crack depth dc to the plate thickness t. Therefore, in that function, if the coefficient ke of the evaluation target is substituted for the coefficient k of the nugget diameter and the plate thickness te of the evaluation target is substituted for the plate thickness t, the limit initial crack depth dc can be uniquely obtained.

[0024] The method for estimating the performance of the spot weld joint according to this embodiment was completed based on the above findings.

[0025] The performance estimation method of the spot welding joint according to this embodiment includes an analysis step, a classification step, a first derivation step, a second derivation step, a third derivation step, a fourth derivation step, a setting step, a calculation step, and an estimation step. The analysis step performs a finite element analysis of the tensile condition of the spot welding joint by changing the plate thickness, the nugget diameter represented by the product of the coefficient and the square root of the plate thickness, and the initial crack depth. The classification step classifies the results of the analysis step by the coefficient of the nugget diameter. The first derivation step derives the relationship between the initial crack rate defined by the ratio of the initial crack depth to the plate thickness and the joint strength for each plate thickness among those classified in the classification step. The second derivation step derives, for each of the relationships between the initial crack rate and the joint strength derived in the first derivation step among those classified in the classification step, the initial crack rate at the time when the joint strength suddenly changes as the sudden change initial crack rate. The third derivation step derives, among those classified in the classification step, the average value of the sudden change initial crack rates derived in the second derivation step as the limit initial crack rate. The fourth derivation step derives a function having the coefficient of the nugget diameter as an independent variable and the limit initial crack rate as a dependent variable based on the limit initial crack rate derived in the third derivation step and the coefficient of the nugget diameter corresponding to the limit initial crack rate. The setting step sets the coefficient of the nugget diameter and the plate thickness in the spot welding joint to be evaluated. The calculation step substitutes the coefficient of the nugget diameter of the evaluation target set in the setting step into the coefficient of the nugget diameter, which is the independent variable, in the function derived in the fourth derivation step, and calculates the limit initial crack rate in the spot welding joint to be evaluated. The estimation step estimates, as the limit initial crack depth, the value obtained by multiplying the limit initial crack rate calculated in the calculation step by the plate thickness set in the setting step (the first configuration).

[0026] According to the performance estimation method of the first configuration, in order to follow the steps along the above findings, it is possible to estimate the limit initial crack depth that does not affect the joint strength according to the plate thickness and the nugget diameter.

[0027] In the performance estimation method according to the first configuration, for example, the function derived in the fourth derivation step is a linear function (the second configuration).

[0028] In the performance estimation method according to the second configuration, preferably, the linear function is represented by Expression (1). Cr = a×k + b (1) In Expression (1), the meanings of the symbols are as follows; Cr: Critical initial crack ratio, k: Coefficient of the nugget diameter (k√t), a: Coefficient represented by a numerical value of -10 or more and 10 or less, and b: Coefficient represented by a numerical value of -10 or more and 10 or less (third configuration).

[0029] In the performance estimation method according to the third configuration, preferably, the critical initial crack ratio Cr in Expression (1) is in the range of 0.1 or more and 0.9 or less (fourth configuration).

[0030] In the performance estimation method according to the third configuration or the fourth configuration, for example, the spot weld joint is a tensile shear joint. In this case, in Expression (1), it is preferable that the coefficient a is a negative numerical value and the coefficient b is a positive numerical value (fifth configuration).

[0031] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In each figure, the same or corresponding components are denoted by the same reference numerals, and redundant explanations will not be repeated.

[0032] [Performance Estimation Method for Spot Weld Joints] With reference to FIGS. 5 to 7, the performance estimation method for the spot weld joint according to the present embodiment will be described. FIG. 5 is a flowchart showing the performance estimation method for the spot weld joint according to the present embodiment. With reference to FIG. 5, the performance estimation method includes an analysis step #5, a classification step #10, a first derivation step #15, a second derivation step #20, a third derivation step #25, a fourth derivation step #30, a setting step #35, a calculation step #40, and an estimation step #45. Hereinafter, each of steps #5 to #45 will be described in detail.

[0033] [Analysis Step #5] Analysis step #5 performs a FE analysis of the tensile conditions of the spot weld joint. In this FE analysis, a number of analysis models 20 described with reference to FIGS. 2 and 3 are used. That is, the analysis model 20 is one in which the plate thickness t, the nugget diameter (k√t) represented by the product of the coefficient k and the square root of the plate thickness t, and the initial crack depth d are variously changed.

[0034] In this embodiment, as described above with reference to FIG. 3, the plate thickness t has three levels of 1.2 mm, 1.6 mm, and 2.0 mm. The coefficient k of the nugget diameter has three levels of 3.0, 4.0, and 5.0. In this case, the variation of the analysis model 20 based on the plate thickness t and the coefficient k of the nugget diameter is nine levels. In each variation, the initial crack depth d is increased at a pitch of 0.1 mm from 0.1 mm. Furthermore, there is also one in which the initial crack depth d is 0 (zero), that is, there is no initial crack.

[0035] Regarding the plate thickness t, the number to be changed (the number of levels) may be a plurality and is not particularly limited. However, in the case of a steel plate used for an automobile, the plate thickness t is, for example, in the range of 0.6 mm or more and 2.4 mm or less. Regarding the coefficient k of the nugget diameter, the number to be changed (the number of levels) may be a plurality and is not particularly limited. However, in practical terms, the coefficient k is, for example, in the range of 3.0 or more and 5.5 or less. Regarding the initial crack depth d, the more the number of changes, the better, and the smaller the pitch of the change, the better. This is because the accuracy is improved.

[0036] In this embodiment, the steel plate is a 980 MPa grade high-tensile steel plate. However, the steel plate may be a high-tensile steel plate of 980 MPa grade or more, for example, a 1180 MPa grade high-tensile steel plate.

[0037] For such a number of analysis models 20, FE analysis is performed as described with reference to FIG. 2. As a result of the FE analysis, the joint strength is obtained. Also, the fracture mode can be recognized.

[0038] [Classification step #10] The classification step #10 classifies the results of the analysis step #5 by the coefficient k of the spatter diameter. In the present embodiment, the analysis results are classified into those with the coefficient k of 3.0, those with the coefficient k of 4.0, and those with the coefficient k of 5.0.

[0039] [First Derivation Step #15] The first derivation step #15 derives the relationship between the initial cracking ratio R and the joint strength for each plate thickness t among those classified in the classification step #10. In the present embodiment, among those with the coefficient k of the spatter diameter being 3.0, the relationship between the initial cracking ratio R and the joint strength is derived for each plate thickness t. Similarly, among those with the coefficient k of the spatter diameter being 4.0, the relationship between the initial cracking ratio R and the joint strength is derived for each plate thickness t. Similarly, among those with the coefficient k of the spatter diameter being 5.0, the relationship between the initial cracking ratio R and the joint strength is derived for each plate thickness t. In this specification, the initial cracking ratio R is a newly introduced index and is defined as the ratio of the initial crack depth d to the plate thickness t. In the present embodiment, as described above, the plate thickness t is 1.2 mm, 1.6 mm, and 2.0 mm.

[0040] Figures 6A to 6C show the classified FE analysis results. Figure 6A is a diagram showing the relationship between the initial cracking ratio R and the joint strength TSS when the coefficient k of the spatter diameter is 3.0. Figure 6B is a diagram showing the relationship between the initial cracking ratio R and the joint strength TSS when the coefficient k of the spatter diameter is 4.0. Figure 6C is a diagram showing the relationship between the initial cracking ratio R and the joint strength TSS when the coefficient k of the spatter diameter is 5.0.

[0041] In each figure, the open marks (◇, 〇, □) indicate that the crack initiation has broken. On the other hand, the solid marks (◆, ●, ■) indicate that it has broken normally. Referring to Figures 6A to 6C, among the classified coefficients k, for any of the three levels of the plate thickness t, the joint strength of the open marks (crack initiation break) is lower than the joint strength of the solid marks (normal break).

[0042] [Second Derivation Step #20] The second derivation step #20 derives, for each of the relationships between the initial cracking ratio R derived in the first derivation step #15 and the joint strength TSS among those classified in the classification step #10, the initial cracking ratio R at the time when the joint strength TSS changes abruptly during decrease as the abrupt change initial cracking ratio Rc.

[0043] Referring to FIGS. 6A to 6C, if the initial cracking ratio R is small, the fracture mode is normal fracture and the joint strength is maintained. On the other hand, when the initial cracking ratio R exceeds a certain point, the fracture mode changes to crack initiation fracture and the joint strength decreases. Therefore, it can be said that the initial cracking ratio R at the time when the joint strength changes abruptly during decrease is the abrupt change initial cracking ratio Rc at which the joint strength begins to be affected. As indicated by the arrows in each figure, for each of the three levels of plate thickness t, the abrupt change initial cracking ratio Rc is the initial cracking ratio R when the joint strength changes abruptly during decrease, that is, at the time of the open mark (crack initiation fracture) closest to the solid mark (normal fracture). Although the open mark closest to the solid mark is defined as the abrupt change initial cracking ratio Rc, alternatively, the average value of the solid mark closest to the open mark and the open mark closest to the solid mark may be defined as the abrupt change initial cracking ratio Rc. Among the classified coefficients k, the three abrupt change initial cracking ratios Rc are equivalent among the plate thicknesses t.

[0044] [Third Derivation Step #25] The third derivation step #25 derives, among those classified in the classification step #10, the average value of the abrupt change initial cracking ratios Rc derived in the second derivation step #20 as the limit initial cracking ratio Cr. In the present embodiment, among the classified coefficients k, the average value of the three abrupt change initial cracking ratios Rc is taken as the limit initial cracking ratio Cr.

[0045] Referring to FIG. 6A, when the coefficient k is 3.0, the limit initial cracking ratio Cr is 0.75. Referring to FIG. 6B, when the coefficient k is 4.0, the limit initial cracking ratio Cr is 0.47. Referring to FIG. 6C, when the coefficient k is 5.0, the limit initial cracking ratio Cr is 0.15.

[0046] [Fourth Derivation Step #30] The fourth derivation step #30 derives a function with the coefficient k of the nozzle diameter as the independent variable and the critical initial cracking ratio Cr as the dependent variable, based on the critical initial cracking ratio Cr derived in the third derivation step #25 and the coefficient k of the nozzle diameter corresponding to the critical initial cracking ratio Cr.

[0047] Fig. 7 shows the function derived in the fourth derivation step #30. Fig. 7 is a diagram showing the relationship between the coefficient k of the nozzle diameter and the critical initial cracking ratio Cr. In Fig. 7, the horizontal axis represents the coefficient k of the nozzle diameter, and the vertical axis represents the critical initial cracking ratio Cr. In this embodiment, the function with the coefficient k of the nozzle diameter as the independent variable and the critical initial cracking ratio Cr as the dependent variable is a linear function.

[0048] Specifically, the linear function is represented by Equation (1). Cr = a×k + b (1) In Equation (1), the meanings of the symbols are as follows: Cr: Critical initial cracking ratio, k: Coefficient of the nozzle diameter (k√t), a: Coefficient represented by a numerical value of -10 or more and 10 or less, and b: Coefficient represented by a numerical value of -10 or more and 10 or less.

[0049] Referring to Fig. 7, in the linear function represented by Equation (1), the coefficient a is -0.3014 and the coefficient b is 1.662. Then, Equation (1) is rewritten as the following Equation (1A). Cr = -0.3014×k + 1.662 (1A)

[0050] In the case of Equation (1A), considering the practical coefficient k of the nozzle diameter, the critical initial cracking ratio Cr may be in the range of 0.1 or more and 0.9 or less.

[0051] Also, when the spot weld joint is a tensile shear joint as in this embodiment, in Equation (1), it is preferable that the coefficient a is a negative numerical value and the coefficient b is a positive numerical value.

[0052] [Setting step #35] Setting step #35 sets the coefficient ke of the nugget diameter and the plate thickness te in the spot weld joint to be evaluated. In this embodiment, for example, 4.5 is set as the coefficient ke, and 1.4 mm is set as the plate thickness te.

[0053] [Calculation step #40] In calculation step #40, in the function derived in the fourth derivation step #30, the coefficient ke of the nugget diameter of the evaluation target set in setting step #35 is substituted for the coefficient k, which is the independent variable, to calculate the critical initial crack ratio Cre in the spot weld joint to be evaluated. In this embodiment, in Equation (1A), 4.5, which is the coefficient ke of the nugget diameter of the evaluation target, is substituted for the coefficient k. As a result, 0.31 is obtained as the critical initial crack ratio Cre in the spot weld joint to be evaluated.

[0054] [Estimation step #45] As described above, the initial crack ratio R is the ratio of the initial crack depth d to the plate thickness t. Therefore, the critical initial crack ratio Cr is the ratio of the critical initial crack depth dc to the plate thickness te. Therefore, in estimation step #45, the value obtained by multiplying the critical initial crack ratio Cre calculated in calculation step #40 by the plate thickness te set in setting step #35 is estimated as the critical initial crack depth dc.

[0055] In this embodiment, 0.31, which is the critical initial crack ratio Cre, is multiplied by 1.4 mm, which is the plate thickness te of the evaluation target, and the resulting value is 0.434 mm. This 0.434 mm can be estimated as the critical initial crack depth dc when the plate thickness te is 1.4 mm and the coefficient ke of the nugget diameter is 4.5. Therefore, when the plate thickness te is 1.4 mm and the coefficient ke of the nugget diameter is 4.5, it can be said that there is no influence on the joint strength as long as the initial crack depth d does not exceed 0.434 mm.

[0056] The processing of each of the above steps #5 to #45 is executed by a computer in which the program is installed.

[0057] [Effect] According to the method for estimating the performance of the spot welding joint according to this embodiment, by sequentially performing the above-described steps #5 to #45, the critical initial crack depth dc that does not affect the joint strength can be estimated according to the plate thickness te and the nugget diameter (specifically, the coefficient ke of the nugget diameter (ke√te)). If the initial crack depth d does not exceed the critical initial crack depth dc, there is no effect on the joint strength. Conversely, if the initial crack depth d appears to exceed the critical initial crack depth dc, since the joint strength decreases, it is possible to prompt attention in the design.

[0058] As described above, the embodiments according to the present disclosure have been described. However, the above embodiments are merely examples. Therefore, the present disclosure is not limited to the above embodiments, and the above embodiments can be appropriately modified and implemented without departing from the spirit thereof.

[0059] In the above embodiment, a tensile shear joint is assumed as the spot welding joint to be evaluated. However, the spot welding joint to be evaluated is not limited to a tensile shear joint, and may be, for example, a cross tensile joint or an L-shaped tensile joint. In the case of a cross tensile joint, the cross tensile strength (CTS) is used as the joint strength. In the case of an L-shaped tensile joint, the L-shaped tensile strength (LTS) is used as the joint strength.

[0060] In the above embodiment, in the analysis model 20, the initial crack 28 extends in a range of 90° in the circumferential direction centered on the nugget 24. However, the range in the circumferential direction in which the initial crack 28 extends may be, for example, 45°, 60°, or 180°.

Explanation of Reference Numerals

[0061] 20: Analysis model 21, 22: Steel plates 23: Welded part 24: Nugget 25: HAZ 28: Initial crack t: Plate thickness d: Initial crack depth k: Coefficient of nugget diameter

Claims

1. A method for estimating the performance of a spot welding joint, comprising: an analysis step of performing a finite element analysis of the tensile conditions of the spot welding joint by changing the plate thickness, the nugget diameter represented by the product of the coefficient and the square root of the plate thickness, and the initial crack depth; a classification step of classifying the results of the analysis step by the coefficient of the nugget diameter; a first derivation step of deriving, for each plate thickness among those classified in the classification step, the relationship between the initial crack ratio defined by the ratio of the initial crack depth to the plate thickness and the joint strength; a second derivation step of deriving, for each of the relationships between the initial crack ratio and the joint strength derived in the first derivation step among those classified in the classification step, the initial crack ratio at the time when the joint strength suddenly changes as the sudden change initial crack ratio; a third derivation step of deriving, among those classified in the classification step, the average value of the sudden change initial crack ratios derived in the second derivation step as the limit initial crack ratio; a fourth derivation step of deriving a function having the coefficient of the nugget diameter as an independent variable and the limit initial crack ratio as a dependent variable based on the limit initial crack ratio derived in the third derivation step and the coefficient of the nugget diameter corresponding to the limit initial crack ratio; a setting step of setting the coefficient of the nugget diameter and the plate thickness in the spot welding joint to be evaluated; a calculation step of substituting the coefficient of the nugget diameter of the spot welding joint to be evaluated set in the setting step into the coefficient of the nugget diameter as the independent variable in the function derived in the fourth derivation step to calculate the limit initial crack ratio in the spot welding joint to be evaluated; an estimation step of estimating, as the limit initial crack depth, a value obtained by multiplying the limit initial crack ratio calculated in the calculation step by the plate thickness set in the setting step. A method for estimating the performance of a spot welding joint.

2. The method for estimating the performance of a spot welding joint according to claim 1, wherein the function derived in the fourth derivation step is a linear function. A method for estimating the performance of a spot welding joint.

3. The method for estimating the performance of a spot welding joint according to claim 2, wherein the linear function is represented by formula (1). A method for estimating the performance of a spot welding joint. Cr = a×k + b (1) In formula (1), the meanings of the symbols are as follows: Cr: limit initial crack ratio, k: coefficient of the nugget diameter (k√t), a: a coefficient represented by a numerical value of -10 or more and 10 or less, and b: a coefficient represented by a numerical value of -10 or more and 10 or less. **Claim 4** A method for estimating the performance of a spot welding joint according to claim 3, wherein the limit initial crack ratio Cr in the formula (1) is within a range of 0.1 or more and 0.9 or less, the method for estimating the performance of a spot welding joint. **Claim 5** A method for estimating the performance of a spot welding joint according to claim 3 or 4, wherein the spot welding joint is a tensile shear joint, and in the formula (1), the coefficient a is a negative numerical value and the coefficient b is a positive numerical value, the method for estimating the performance of a spot welding joint.

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