Method for estimating spot-welded joint performance

Abstract

To accurately estimate the relation between initial crack depth and joint strength in a spot welding joint.SOLUTION: A method for estimating spot welding joint performance comprises: a process of deriving the relation between an initial crack depth or a nugget diameter and joint strength, which derives a joint strength limit line and a joint strength estimation line without initial cracks, by the finite element method analysis for a tensile condition of the spot welding joint; a joint condition input process of inputting a welding nugget diameter and / or the initial crack depth of a spot welding joint to be evaluated; and a joint performance estimation process of applying the welding nugget diameter and / or the initial crack depth input in the joint condition input process to the joint strength limit line and the joint strength estimation line without initial cracks, and of estimating the limit of the initial crack depth that does not affect the joint strength and / or the limit of the welding nugget diameter that does not affect the joint strength.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to predicting spot weld joint performance. [Background technology]

[0002] In recent years, liquid metal embrittlement (LME) cracking has been reported in spot welds of ultra-high-tensile steels with zinc-based plating, and cracking can occur in pressure welds between steel sheets, especially under disturbance conditions such as the presence of a clearance between the electrode and the steel sheet. Initial cracks that occur during such welding processes can affect the strength of the welded joint depending on their depth, so it is important to clarify the relationship between the depth of the initial crack and the joint strength.

[0003] As a technique related to FEM analysis for elucidating the fracture mechanism of spot welds, Non-Patent Document 1 discloses a technique related to a fracture prediction method for spot welds targeted at steel sheets for automobiles. The technique disclosed in Non-Patent Document 1 makes it possible to accurately predict the joint strength and fracture mode for any sheet combination with different load modes on the weld.

[0004] Patent Document 1 describes a spot weld fracture analysis method using a finite element model, Patent Document 2 describes a method for collision analysis that takes into account spot weld fracture using finite element analysis, and Patent Document 3 describes a method for determining spot weld fracture using a finite element analysis model. However, these methods do not take into consideration initial cracks in the weld, and no clarification of the relationship between the depth of the initial crack and the strength of the joint has been made.

[0005] Non-Patent Document 2 describes the relationship between initial cracking of welded parts and joint strength, taking into account the initial cracking of welded parts using finite element analysis. However, this method is limited to fracture morphology originating from the heat affected zone (hereinafter referred to as HAZ) around the nugget, and does not address changes in fracture morphology due to initial cracking. Furthermore, there is no mention of the changes in fracture morphology and joint strength due to differences in weld nugget diameter.Since fracture morphology and joint strength can change depending on the weld nugget diameter, it is important to clarify the influence of weld nugget diameter and fracture morphology on joint strength. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] Hideki Ueda and three others, "Study on Spot Weld Fracture Prediction Technology Considering Stress Triaxiality (1st Report)," Transactions of the Society of Automotive Engineers of Japan, Vol. 44, No. 2, p. 727 (2013) [Non-patent document 2] Fracture modeling of resistance spot welded ultra-high-strength steel considering the effect of liquid metal embrittlement crack, Materials&Design, 210(2021)110075 [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-090366 [Patent Document 2] Japanese Patent Application Laid-Open No. 2007-263830 [Patent Document 3] Japanese Patent Application Laid-Open No. 2010-127933 Summary of the Invention [Problem to be solved by the invention]

[0008] Therefore, an object of the present invention is to accurately estimate the relationship between the initial crack depth and joint strength. [Means for solving the problem]

[0009] Non-Patent Document 1 discloses a technique for predicting fracture of spot welds in automotive steel sheets. By applying this technique, it is possible to construct a sophisticated analytical model for evaluating the strength of spot welds, in which an appropriate deformation resistance curve and fracture criteria are set according to the microstructure around the weld. The inventors believed that by further developing this analytical model, it would be possible to accurately predict the fracture mode and joint strength, and that it would be possible to accurately predict the fracture mode and joint strength of spot-welded joints under conditions that take initial cracking into account. The present invention was completed by obtaining such an idea and realizing it. The present invention will be described below.

[0010] One aspect of the present invention is a method for estimating the performance of a spot-welded joint, the method including: a step of deriving the relationship between the initial crack depth or nugget diameter and the joint strength by finite element analysis of the tensile conditions of the spot-welded joint, which derives a joint strength limit line and an initial crack-free joint strength estimation line; a joint condition input step of inputting the weld nugget diameter and / or the initial crack depth of the spot-welded joint to be evaluated; and a joint performance estimation step of applying the weld nugget diameter and / or the initial crack depth input in the joint condition input step to the joint strength limit line and the initial crack-free joint strength estimation line, and estimating the limit of the initial crack depth that does not affect the joint strength and / or the limit of the weld nugget diameter that does not affect the joint strength.

[0011] The joint strength obtained in the process of deriving the relationship between the initial crack depth or nugget diameter and the joint strength may be at least one of tensile shear strength, cross tensile strength, and L-shaped tensile strength.

[0012] The joint strength limit line may be a joint strength expressed by a linear function with the initial crack depth as a variable, and the crack-free joint strength estimation line may be a joint strength expressed by a linear function with the weld nugget diameter as a variable. [Effects of the Invention]

[0013] According to the present invention, it is possible to accurately estimate the relationship between initial crack depth and joint strength without performing a tensile test on the target joint each time, and members equipped with welded parts can be manufactured using the method for estimating joint performance. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a flowchart showing the outline of the procedure of a method S10 for estimating the performance of a spot-welded joint according to one embodiment. [Figure 2] FIG. 2 shows an example of an analytical model of a spot-welded joint for tensile shear strength (TSS) analysis. [Figure 3] FIG. 3 shows the relationship between the initial crack depth and TSS. [Figure 4] Figure 4 shows the relationship between the weld nugget diameter and the TSS of joints without initial cracks. [Figure 5] FIG. 5 is a diagram showing an example of an analytical model of a spot-welded joint for cross tensile strength (CTS) analysis. [Figure 6] FIG. 6 is a graph showing the relationship between the initial crack depth and CTS. [Figure 7] FIG. 7 shows the relationship between the weld nugget diameter and the CTS of joints without initial cracks. [Figure 8] FIG. 8 is a diagram showing an example of an analytical model of a spot-welded joint for L-tensile strength (LTS) analysis. [Figure 9] FIG. 9 is a graph showing the relationship between the initial crack depth and LTS. [Figure 10] Figure 10 shows the relationship between the weld nugget diameter and the LTS of joints without initial cracks. [Figure 11] FIG. 11 is a diagram illustrating the configuration of a spot-welded joint performance estimation device 70. DETAILED DESCRIPTION OF THE INVENTION

[0015] 1. Spot-welded joint performance estimation method S10 1 shows the flow of a method S10 for estimating spot-welded joint performance according to one embodiment. The method S10 for estimating spot-welded joint performance includes a step S11 for deriving the relationship between the initial crack depth or nugget diameter and the joint strength, a step S12 for inputting joint conditions, and a step S13 for estimating joint performance. Each step is described below.

[0016] 1.1. Process for deriving the relationship between initial crack depth or nugget diameter and joint strength S11 In step S11 of deriving the relationship between the initial crack depth or nugget diameter and the joint strength, at least one of a joint strength limit line for tensile shear strength (TSS), a joint strength limit line for cross tensile strength (CTS), and a joint strength limit line for long-length tensile strength (LTS) is derived by finite element analysis (FEM analysis) of the tensile conditions of the spot-welded joint, and at least one of a joint strength estimate line for TSS for a joint without an initial crack, a joint strength estimate line for CTS for a joint without an initial crack, and a joint strength estimate line for LTS for a joint without an initial crack. This will be explained in more detail below.

[0017] 1.1.1. Tensile shear strength (TSS) Figure 2 shows an example of an analytical model of a joint for achieving TSS based on spot welding, as disclosed in Non-Patent Document 1. Figure 2(a) shows the shape of a test piece for a joint for TSS analysis. Steel plates 11 and 12 are partially overlapped, and the center of the overlap is spot-welded to form spot weld 13. Figure 2(b) shows an analytical model 20 of a joint for TSS analysis. Steel plates 21 and 22 are partially overlapped, and the test piece model is modeled as half-symmetric in the plate width direction, with spot weld 23 at the center of the overlap. Figure 2(c) is an enlarged view of spot weld 23 in analytical model 20. In spot weld 23, element sets are divided into weld nugget 24, HAZ 25, transition layer 26 from HAZ 25 to base metal 27, and the structure of base metal 27, and deformation resistance curves and fracture criteria are set for each.

[0018] Furthermore, an initial crack 28 is also set, originating from the pressure-welded portion between the steel plates 21 and 22. FEM analysis is performed under the conditions that one end 21a of the steel plate 21 is fully restrained and one end 22a of the steel plate 22 is subjected to a tensile load in the direction of the arrow, and the fracture mode and joint strength are determined.

[0019] Here, the joint strength is the maximum load applied to the test piece during tension. In this FEM analysis, elements that have reached the fracture criterion are deleted to reduce rigidity, thereby simulating a tensile test using an actual test piece. In this embodiment, the weld nugget diameter (the size of the weld nugget in the direction along the interface of the welded steel plates) is set to 5 × t (mm) with respect to the thickness t of the steel plate. 0.5 (mm), 4×t 0.5 (mm), and 3×t 0.5 For example, the thickness t can be set to 1.6 mm. In addition, in this embodiment, a similar FEM analysis is performed using an analysis model in which the depth of the initial crack 28 is set in 0.1 mm increments from 0.1 mm to 1.4 mm, and an analysis model without cracks (depth of the initial crack 28 is 0 mm).

[0020] As a result of the above FEM analysis, Fig. 3 shows the relationship between the depth of the initial crack (mm) and the tensile shear strength (TSS) (kN) of the joint. 0.5 (mm) are examples of ◆ and ◇, and the weld nugget diameter is 4×t 0.5 (mm) are examples of weld nugget diameter 3×t 0.5 Examples where the fracture diameter is (mm) are indicated by ■ and □. Among them, solid marks (◆, ●, ■) indicate the same fracture mode as joints without initial cracks (hereinafter referred to as normal fracture), while open marks (◇, ◯, □) indicate fracture modes that originate from an initial crack (hereinafter referred to as crack-initiated fracture).

[0021] As can be seen from Figure 3, the TSS for open marks (crack-initiated fracture) is lower than that for solid marks (normal fracture) at all weld nugget diameters, and the TSS decreases as the fracture mode changes from normal fracture to crack-initiated fracture at all weld nugget diameters.

[0022] Under the FEM analysis conditions, an initial crack is set in the pressure welded portion of the HAZ structure (initial crack 28 in Figure 2), and the fracture rapidly propagates in the plate thickness direction depending on the depth of the initial crack, leading to crack-initiated fracture. In other words, there are TSS limit points (A, B, and C in Figure 3) where the TSS of the joint decreases under conditions where the fracture mode changes from normal fracture to rapid crack-initiated fracture. These TSS limit points (A, B, and C) are represented by the boundaries where the fracture mode changes for each weld nugget diameter (the boundaries where it changes from normal fracture to crack-initiated fracture). In Figure 3, they are the open mark (crack-initiated fracture) closest to the solid mark (normal fracture) for each weld nugget diameter.

[0023] From this TSS limit point, the TSS limit line (joint strength limit line) can be approximated by a linear function, as shown by the straight line in Figure 3. Specifically, it can be expressed as a linear function with the initial crack depth as a variable, as shown in the following equation (1). F1 T =a T CD+b T …(1) Here, F1 Tis TSS (kN), CD is the initial crack depth (mm), a T and b T indicates a constant in the range of -100 to 100. Specific values ​​of these constants can be obtained from multiple FEM analysis data in which the depth of the initial crack and the diameter of the weld nugget are changed.

[0024] Meanwhile, the relationship between weld nugget diameter and TSS of joints without initial cracks is shown in Figure 4. The weld nugget diameter and TSS of joints without initial cracks is a linear function with the weld nugget diameter as a variable, as shown in equation (2), and can be approximated as an estimation line for joint strength without initial cracks. F2 T =c T ND+d T …(2) Here, F2 T is TSS (kN), ND is weld nugget diameter (mm), c T and d T indicates a constant in the range of -50 to 50. Specific values ​​of these constants can be obtained from multiple FEM analysis data in which the diameter of the weld nugget without initial cracks is changed.

[0025] 1.1.2. Cross tensile strength (CTS) In this embodiment, in addition to or in addition to the joint strength limit line and the strength estimate line without initial cracks for the tensile shear strength (TSS) described above, a joint strength limit line and a joint strength estimate line without initial cracks for the cross tensile strength (CTS) can also be considered. The idea behind obtaining these joint strength limit lines and joint strength estimate lines without initial cracks for the CTS is the same as the idea behind the joint strength limit line and joint strength estimate line without initial cracks for the TSS described above, but is specifically as follows.

[0026] Figure 5 shows an example of an analytical model of a joint for cross-tensile strength analysis of spot welds. Figure 5(a) shows a test specimen 30 of the joint. Steel plates 31 and 32 are overlapped in a cross shape and joined by a spot weld at the center, forming a spot weld 33. Figure 5(b) shows an analytical model 40 of the joint for cross-tensile strength analysis. The steel plates 41 and 42 of the test specimen are overlapped in a cross shape and have a central weld 43. The width direction of the steel plates 41 and 42 is halved and modeled as a quarter symmetrical shape. Figure 5(c) shows an enlarged view of the spot weld 43 in the analytical model 40. The spot weld 43 is divided into element sets for the weld nugget 44, HAZ 45, transition layer 46 from HAZ 45 to base metal 47, and the microstructure of the base metal 47. Deformation resistance curves and fracture criteria are set for each. An initial crack 48 originating from the pressure weld between the steel plates is also set. FEM analysis is performed under the condition that the gripping portion 42a of one steel plate 42 is completely restrained and a tensile load is applied to the gripping portion 41a of the other steel plate 41 in the direction of the arrow, and the fracture mode and joint strength are determined.

[0027] Figure 6 shows the relationship between the initial crack depth and the cross tensile strength (CTS) of the joint. Figure 6 is plotted in the same way as Figure 3, and by approximating the joint strength limit line from the CTS limit points (D, E) of the joint, it can be expressed as a linear function with the initial crack depth as a variable, as in equation (3). Note that in this example, 3 × t 0.5 There are also conditions under which fracture does not occur at the crack initiation point, such as (mm). F1 C =a C CD+b C …(3) Here, F1 C is CTS (kN), CD is initial crack depth (mm), a C and b C indicates a constant in the range of -100 to 100. Specific values ​​of these constants can be obtained from multiple FEM analysis data in which the initial crack depth and weld nugget diameter are changed.

[0028] The relationship between the weld nugget diameter and the CTS of joints without initial cracks is shown in Figure 7. The weld nugget diameter and the CTS of joints without initial cracks is a linear function with the weld nugget diameter as a variable, as shown in equation (4), and can be approximated as an estimation line for joint strength without initial cracks. F2 C =c C ND+d C …(4) Here, F2 C is CTS (kN), ND is weld nugget diameter (mm), c C and d C indicates a constant in the range of -50 to 50. Specific values ​​of these constants can be obtained from multiple FEM analysis data in which the diameter of the weld nugget without initial cracks is changed.

[0029] 1.1.3.L-shaped tensile strength (LTS) Similarly, for L-shaped tensile strength (LTS), a joint strength limit line and an estimated joint strength line for a joint without initial cracks can also be considered. The concept for obtaining these joint strength limit lines and estimated joint strength lines for a joint without initial cracks for LTS is similar to that for the joint strength limit line and estimated joint strength line for a joint without initial cracks for TSS, but is specifically as follows. In the present invention, only LTS may be considered, or it may be considered together with at least one of the TSS and CTS.

[0030] Figure 8 shows an example of an analytical model of a joint for analyzing the L-shaped tensile strength of spot welds. Figure 8(a) shows a test specimen 50 of the joint. L-shaped steel plates 51 and 52 are overlapped as shown in Figure 8(a), and spot weld 53 is formed at the center of the flange. Figure 8(b) shows an analytical model 60 of a joint for analyzing L-shaped tensile strength. The steel plates 61 and 62 of the L-shaped test specimen are overlapped and have a weld 63 at the center of the flange, and the plate width direction of each is modeled as half symmetrical. Figure 8(c) shows an enlarged view of the spot weld 63 and its surroundings in the analytical model 60. The spot weld 63 is divided into element sets for the weld nugget 64, HAZ 65, transition layer 66 from HAZ 65 to base metal 67, and the microstructure of the base metal 67. Deformation resistance curves and fracture criteria are set for each. An initial crack 68 originating from the pressure weld between the steel plates is also set. FEM analysis is performed under the condition that the gripping portion 62a of one steel plate 62 is completely restrained and a tensile load is applied to the gripping portion 61a of the other steel plate 61 in the direction of the arrow, and the fracture mode and joint strength are determined.

[0031] The relationship between the initial crack depth and the L-shaped tensile strength (LTS) of the joint is shown in Figure 9. Figure 9 is plotted in the same way as Figure 3, and by approximating the joint strength limit line from the LTS limit points (F, G, H) of the joint, it can be expressed as a linear function with the initial crack depth as a variable, as shown in equation (5). F1 L =a L CD+b L …(5) Here, F1 L is LTS (kN), CD is initial crack depth (mm), a L and b L indicates a constant in the range of -100 to 100. Specific values ​​of these constants can be obtained from multiple FEM analysis data in which the initial crack depth and weld nugget diameter are changed.

[0032] The relationship between the weld nugget diameter and the LTS of joints without initial cracks is shown in Figure 10. The weld nugget diameter and the LTS of joints without initial cracks is a linear function with the weld nugget diameter as a variable, as shown in equation (6), and can be approximated as an estimation line for joint strength without initial cracks. F2 L =c L ND+d L …(6) Here, F2 L is LTS (kN), ND is weld nugget diameter (mm), c L and d L indicates a constant in the range of -50 to 50. Specific values ​​of these constants can be obtained from multiple FEM analysis data in which the diameter of the weld nugget without initial cracks is changed.

[0033] Other In the above, an analytical model was created and each joint strength limit line and each estimated strength line without initial cracking were obtained by FEM analysis. However, this is not limiting and actual test pieces may be prepared and tested, and the above-mentioned each joint strength limit line and each estimated strength line without initial cracking may be obtained using the actual measurement data.

[0034] In addition, when determining the joint strength limit line for tensile shear strength (TSS) in equation (1), the joint strength limit line for cross tensile strength (CTS) in equation (3), and the joint strength limit line for long-length tensile strength (LTS) in equation (5), the point used to obtain the limit lines (approximate equations) was the limit point (the boundary where the fracture mode changes for each weld nugget diameter, i.e., the boundary where it changes from normal fracture to crack-initiated fracture). In Figures 3, 6, and 9, only the open mark (crack-initiated fracture) closest to the solid mark (normal fracture) for each weld nugget diameter was used. Alternatively, all points at which crack-initiated fracture occurred (i.e., all open marks in Figures 3, 6, and 9) may be used to obtain the joint strength limit line. The inventors' investigations have confirmed that roughly the same approximate formula can be obtained in either case.

[0035] 1.2. Joint condition input process S12 In the joint condition input step S12, the weld nugget diameter (mm) and / or initial crack depth (mm) of the spot-welded joint to be evaluated are input. That is, in this step, information necessary to obtain values ​​for each joint strength limit line (Equation (1), Equation (3), Equation (5)) and each crack-free joint strength estimation line (Equation (2), Equation (4), Equation (6)) is input. Therefore, the initial crack depth (mm) is input for Equation (1), Equation (3), and Equation (5), and the weld nugget diameter (mm) is input for Equation (2), Equation (4), and Equation (6).

[0036] 1.3. Joint performance estimation process S13 In the joint performance estimation step S13, an equation among Equations (1) to (6) is applied as needed to calculate the limit of the initial crack depth that does not affect the joint strength and / or the limit of the weld nugget diameter that does not affect the joint strength, and these limits are estimated. More details are as follows.

[0037] 1.3.1. Estimation of the limit of initial crack depth that does not affect joint strength First, the nugget diameter of the spot welded joint to be evaluated, which was entered in the joint condition input process S12, is set to ND. e Then, ND in equation (2) is e By substituting this, the TSS of the joint without initial cracks is calculated. e F2 is the TSS of the joint without initial cracking. Te get.

[0038] Next, calculate the initial crack depth using formula (1). Te F1 in equation (1) T Substitute into and solve for the initial crack depth CD, and find the initial crack depth CD e get.

[0039] According to this, the diameter is ND e In the weld nugget, the initial crack depth is CD e The fracture mode changes (from normal fracture to crack initiation fracture), that is, the joint strength decreases, so e In the weld nugget ofe It can be assumed that there is no effect on the joint strength up to this point.

[0040] 1.3.2. Estimation of the Limit of Weld Nugget Diameter That Does Not Affect Joint Strength First, the initial crack depth of the spot welded joint to be evaluated, which was entered in the joint condition input process S12, is calculated as CD f Then, add CD to CD in equation (1). f This calculation calculates the TSS by substituting the initial crack depth CD. f F1, the TSS of Tf get.

[0041] Next, the weld nugget diameter is calculated using equation (2). Tf F2 in equation (2) T and solve for the weld nugget diameter, and the weld nugget diameter ND f get.

[0042] According to this, the initial crack depth CD f Then, the weld nugget diameter ND f The fracture mode changes (from normal fracture to crack initiation fracture), that is, the joint strength decreases, and the initial crack depth increases to CD. f Then, the weld nugget diameter ND f It can be assumed that there is no effect on the joint strength up to this point.

[0043] Estimation was performed using equations (1) and (2) above. That is, the process of estimation using TSS was explained, but in addition to or instead of this, estimation using CTS can be performed using equations (3) and (4), and estimation using LTS can be performed using equations (5) and (6). In this case, simply use equations (3) and (5) instead of equation (1), and equations (4) and (6) instead of equation (2), and replace TSS with CTS and LTS.

[0044] 2. Effects etc. According to the present invention, it is possible to estimate the limit of the depth of an initial crack that does not affect joint strength. This makes it possible to estimate, from the weld nugget diameter of the spot-welded joint being evaluated, the depth up to which the initial crack can occur without affecting joint strength, and to set the allowable range of the initial crack. Furthermore, the present invention can estimate the size of the weld nugget diameter that does not affect the joint strength, thereby making it possible to set the size of the weld nugget diameter that allows for an initial crack to be detected based on the initial crack depth of the spot-welded joint being evaluated. As described above, according to the present invention, it is possible to accurately estimate the relationship between the initial crack depth and the joint strength without performing a tensile test on the joint to be evaluated each time, and it is possible to manufacture components equipped with welded parts using the method for estimating joint performance.

[0045] 3. Spot-welded joint performance estimation device 11 is a conceptual diagram showing the configuration of a spot-welded joint performance estimation device 70 according to one embodiment that specifically performs calculations in accordance with the above-described spot-welded joint performance estimation method S10. The spot-welded joint performance estimation device 70 has input means 71, a calculation device 72, and a display means 78. The calculation device 72 is equipped with calculation means 73, RAM 74, storage means 75, receiving means 76, and output means 77. The input means 71 also includes a keyboard 71a, a mouse 71b, and an external storage device 71c that functions as one of the storage media.

[0046] The calculation means 73 is configured by a so-called CPU (central processing unit), and is connected to and capable of controlling the above-mentioned components. The calculation means 73 also executes various programs stored in storage means 75, which functions as a storage medium, and performs calculations based on these programs as a means for generating data for each process of the above-mentioned method for estimating spot-welded joint performance S10 and selecting data from a database.

[0047] The RAM 74 is a component that functions as a working area for the calculation means 73 and as temporary data storage means. The RAM 74 can be configured with an SRAM, a DRAM, a flash memory, or the like, and is similar to known RAMs.

[0048] The storage means 75 is a component that functions as a storage medium for storing programs and data that are the basis for various calculations. The storage means 75 may also be capable of storing various intermediate and final results obtained by executing the programs. More specifically, the storage means 75 stores (stores) programs. Other information may also be stored therein.

[0049] The stored programs include programs that are the basis for calculating each step of the above-described method S10 for estimating spot-welded joint performance. That is, the program includes a step of storing in storage means 75 equations (1) to (6) obtained in step S11 of deriving the relationship between the initial crack depth or nugget diameter and the joint strength, and performing the calculations described in step S13 of estimating joint performance based on the results of input to input means 71 in step S12 of inputting joint conditions, so as to correspond to each step of method S10 for estimating spot-welded joint performance shown in Fig. 1 .

[0050] The receiving means 76 is a component having a function for appropriately receiving information from the outside into the arithmetic unit 72, and is connected to the input means 71. This also includes so-called input ports, input connectors, and the like.

[0051] The output means 77 is a component that has the function of appropriately outputting information that should be output from among the obtained results to the outside, and is connected to a display means 78 such as a monitor and various devices. This also includes so-called output ports, output connectors, etc.

[0052] The input device 71 includes, for example, a keyboard 71a, a mouse 71b, an external storage device 71c, etc. The keyboard 71a and the mouse 71b may be well-known devices, and a description thereof will be omitted. The external storage device 71c is a known externally connectable storage means, and also functions as a storage medium. It is not particularly limited, and various necessary programs and data can be stored therein. For example, the same programs and data as those in the storage means 75 may be stored here. The external storage device 71c may be a known device, such as a CD-ROM and a CD-ROM drive, a DVD and a DVD drive, a hard disk, or various types of memory.

[0053] Additionally, information may be provided to the computing device via a network or communication through receiving means 76. Similarly, information may be transmitted to an external device via output means 77 through a network or communication.

[0054] The spot-welded joint performance estimation method S10 described above can be performed efficiently and accurately using this spot-welded joint performance estimation device 70. A computer, for example, can be used as this joint performance estimation device 70, and the program can be a computer program. [Example]

[0055] The method for estimating the performance of a spot-welded joint according to the present invention will be described in more detail below with reference to examples, although the present invention is not limited to these examples.

[0056] The object of evaluation was a spot-welded joint for tensile shear analysis using 980 MPa-class steel plate with a thickness of 1.6 mm.

[0057] In the process S11 of deriving the relationship between the initial crack depth or nugget diameter and the joint strength, the size of the weld nugget diameter is 3 × t 0.5 (mm), 4×t 0.5 (mm), 5×t 0.5For each of the initial crack depths (mm), analytical models were created with initial crack depths ranging from 0.0 mm (no cracks) to 1.4 mm in 0.1 mm increments, as shown in Figure 2. Then, for each analytical model, FEM analysis of the tensile conditions was performed, as explained above, to obtain equations (1)' and (2)' corresponding to equations (1) and (2). Specifically, the approximate equations were obtained as follows: the approximate equations were determined by the least squares method using spreadsheet software (Microsoft Excel). Note that this example is for a joint for tensile shear analysis, but for reference, a joint for cross tensile strength analysis was also analyzed to obtain equations (3)' and (4') corresponding to equations (3) and (4). F1 T =-17.1·CD+32.0 …(1)' F2 T =6.7·ND-13.2 …(2)' F1 C =-3.5·CD+10.3 …(3)' F2 C =1.5·ND-0.3 …(4)'

[0058] Next, in the joint condition input process S12, the weld nugget diameter is 4.5 × t 0.5 The target joints were those with welds of approximately 5.7 mm. Then, in step S13 of estimating joint performance, it is estimated from equations (1)' and (2)' that the fracture mode changes at an initial crack depth of 0.4 mm, i.e., the joint strength decreases, and therefore that an initial crack depth of up to 0.4 mm does not affect the joint strength.

[0059] In addition, in the joint condition input step S12, joints having welds with a crack depth of 0.3 mm were targeted. Then, in the joint performance estimation process S13, it is estimated from equations (1)' and (2') that the fracture mode changes at a weld nugget diameter of 5.96 mm, i.e., the joint strength decreases, and therefore that there is no effect on the joint strength up to a weld nugget diameter of 5.96 mm. [Explanation of symbols]

[0060] 20 Analytical model of joints for obtaining TSS by spot welding 40 Analytical model of spot-welded joints for achieving CTS 60 Analytical model of spot-welded joints for obtaining LTS 70 Spot-welded joint performance estimation device S10 Estimation method for spot welded joint performance S11 Derivation process of the relationship between initial crack depth or nugget diameter and joint strength S12 Joint condition input process S13 Joint performance estimation process

Claims

1. 1. A method for estimating performance of a spot welded joint, comprising: A process of deriving a joint strength limit line and an estimated joint strength line without initial cracks by finite element analysis of the tensile conditions of the spot-welded joint, and deriving the relationship between the initial crack depth and the joint strength and the relationship between the nugget diameter and the joint strength; A process of inputting joint conditions, which involves inputting the weld nugget diameter and initial crack depth of the spot-welded joint to be evaluated; a joint performance estimation process of applying the weld nugget diameter and the initial crack depth input in the joint condition input process to the joint strength limit line and the initial crack-free joint strength estimation line, and estimating a limit of the initial crack depth that does not affect the joint strength and a limit of the weld nugget diameter that does not affect the joint strength, A method for estimating spot welded joint performance.

2. 2. The method for estimating spot-welded joint performance according to claim 1, wherein the joint strength obtained in the process of deriving the relationship between the initial crack depth and the joint strength and the relationship between the nugget diameter and the joint strength is at least one of tensile shear strength, cross tensile strength, and L-tensile strength.

3. The joint strength limit line is a joint strength expressed by a linear function with the initial crack depth as a variable, The crack-free joint strength estimation line is a joint strength expressed by a linear function with the weld nugget diameter as a variable. The method for estimating spot-welded joint performance according to claim 1 or 2.

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