Vehicle front structure, method for designing vehicle front structure, and method for manufacturing vehicle front structure

Abstract

To provide a vehicle body front structure characterized in that front side members are bent and deformed after crash boxes are fully compressed and deformed in a case of a vehicle collision, a design method of the vehicle body front structure, and a manufacturing method of the vehicle body front structure.SOLUTION: A vehicle body front structure 1 in accordance with the present invention includes a pair of left and right front side members 11 that extend in a fore-and-aft direction of a vehicle body and each have a closed cross section, and crash boxes 21 that extend from the distal ends of the front side members 11 toward a front part of the vehicle body and each have a closed cross section. The thicknesses and tension strengths of metal plates to be made into the front side members 11 and crash boxes 21 respectively are designated so that a ratio Fx1 / Fx2 of an axial force Fx1 of a cross section minimizing the cross-sectional line length of the closed cross section of the crash box 21 to an axial force Fx2 of a cross section minimizing the cross-sectional line length of the closed cross section of the front side member 11 becomes equal to or smaller than a predefined critical axial force ratio.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a vehicle front structure including a pair of left and right front side members extending in the vehicle front-rear direction and a crash box extending from the tip of each front side member to the vehicle front side, a design method for the vehicle front structure, and a manufacturing method for the vehicle front structure.

Background Art

[0002] In recent years, as a measure for environmental protection, electric vehicles that can reduce the CO 2 emission during vehicle travel compared to gasoline vehicles have begun to spread. To increase the cruising range of electric vehicles, it is effective to increase the battery load, but this increases the vehicle body weight and becomes a factor in increasing the collision energy during a vehicle collision. Therefore, in electric vehicles and the like, a vehicle body structure that can sufficiently absorb the collision energy is required. Conventionally, in a vehicle front collision (front impact), a vehicle front structure that absorbs collision energy by axially compressing and deforming (axial crushing) a front side member in the longitudinal direction is known. Such a front side member bends and deforms (bending deformation) so that the front side compresses and deforms in the vehicle front-rear direction and the rear side protrudes outward of the vehicle body in the vehicle width direction or downward in the vehicle vertical direction during a vehicle front impact. Thereby, while absorbing the collision energy input to the vehicle front structure, it prevents the deterioration of the occupant protection performance (occupant injury value) due to the intrusion of vehicle body parts such as a dash panel into the passenger compartment by on-vehicle equipment such as an engine and a motor.

[0003] Generally, in the compression deformation and bending deformation of vehicle body parts, even with the same stroke, the compression deformation can absorb more collision energy. Therefore, for example, in Patent Document 1, a front body structure is disclosed in which a rigidity difference is provided between the front side and the rear side of a front side frame (corresponding to the front side member of the present application), and a rigidity difference is alternately provided in the vehicle longitudinal direction for each region. In this technology, during a vehicle frontal collision, compression deformation starts from the front side of the front side frame with relatively low rigidity, and then the rear side of the front side frame with relatively high rigidity is compressed and deformed. As a result, it is possible to compress and deform the front side member smoothly in a bellows shape over the entire longitudinal direction, so it is said that collision energy can be sufficiently absorbed.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In the front body structure, there is one in which a crush box is provided in front of the front side member. During a vehicle frontal collision, it is possible to further absorb collision energy by compressing and deforming the crush box in the vehicle longitudinal direction. In such a front body structure including the front side member and the crush box, a steel plate with a relatively low strength of about 440 MPa in tensile strength is used for the crush box in order to be sufficiently compressed and deformed. Furthermore, a steel plate with a relatively high strength of 780 MPa or more in tensile strength is often used for the front side member. In response to the demand for increasing the amount of collision energy absorption during a frontal collision of an electric vehicle, using a steel plate (ultra-high tensile material) with a tensile strength of 980 MPa or more for the crush box has been under consideration.

[0006] In such a case, it is important to cause compressive deformation on the front side of the crash box and the front side member to obtain a necessary amount of absorbed collision energy. However, even if the strength of the steel plate used for the crash box is simply increased, there has been a case where the front side member is bent and deformed before sufficient compressive deformation occurs in the crash box. In such a case, there has been a problem that the collision energy cannot be sufficiently absorbed by the crash box and a predetermined collision performance such as the amount of vehicle body deformation cannot be satisfied. Therefore, in order to increase the strength of the crash box, it is important that the front side member is bent and deformed after the crash box is sufficiently compressed and deformed. And for that purpose, it is considered that the plate thickness and material strength of the metal plate such as the steel plate used for the crash box and the front side member may be appropriately determined.

[0007] However, the technique disclosed in Patent Document 1 sufficiently absorbs collision energy by causing the front side member to be compressed and deformed in a smooth bellows shape over the entire longitudinal direction. Therefore, Patent Document 1 does not disclose or suggest anything regarding determining an appropriate plate thickness and material strength of the crash box in relation to the front side member.

[0008] In addition, an electric vehicle loaded with a battery is required to make the vehicle body front structure compact in order to secure a battery loading space in addition to ensuring a predetermined collision energy absorption performance. And in order to make the vehicle body front structure compact, it is required that the crash box and the front side member can sufficiently absorb collision energy even with a small amount of compressive deformation during a frontal collision of the vehicle.

[0009] However, in the technology of Patent Document 1, the load input during a vehicle frontal collision decreased significantly due to the reduction in the rigidity of the front side of the front side member. Therefore, in order to ensure the amount of collision energy absorbed by the compression deformation during a frontal collision, it was necessary to lengthen the front side member so that the amount of compression deformation increased. Thus, with the technology of Patent Document 1, it was impossible to achieve both ensuring the collision energy absorption performance and compactness in the front body structure of an electric vehicle.

[0010] The present invention has been made to solve the above problems, and an object thereof is to provide a front body structure, a design method of the front body structure, and a manufacturing method of the front body structure that can achieve both compactness while satisfying predetermined collision performance during a vehicle frontal collision.

Means for Solving the Problems

[0011] (1) The front body structure according to the present invention includes a pair of left and right front side members extending in the vehicle front-rear direction and having a closed cross-section structure, and a crush box extending from the tip of each front side member to the front side of the vehicle body and having a closed cross-section structure, and the axial force F of the cross-section where the cross-sectional line length of the closed cross-section structure in the crush box is minimized x1 and the axial force F of the cross-section where the cross-sectional line length of the closed cross-section structure in the front side member is minimized x2 and the ratio F x1 / F x2 is set to be equal to or less than a predetermined critical axial force ratio, and the plate thickness and tensile strength of the metal plate used for each of the front side member and the crush box are set so that the crush box is sufficiently compressed and deformed during a vehicle frontal collision and then the front side member is bent and deformed.

[0012] (2) The design method of the vehicle front body structure according to the present invention is for a vehicle front body structure including a pair of left and right front side members extending in the vehicle longitudinal direction and having a closed cross-section structure, and a crash box extending from the tip of each front side member toward the front side of the vehicle body and having a closed cross-section structure. The design is such that, when the vehicle undergoes a frontal collision, the front side members are bent and deformed after the crash box is sufficiently compressed and deformed. A vehicle model acquisition step of acquiring a vehicle model having the vehicle front body structure and setting an intrusion amount evaluation part for evaluating the intrusion amount of vehicle body parts toward the passenger compartment side during a frontal collision of the vehicle model. A cross-sectional line length minimum cross-section determination step of determining, for each of the crash box and the front side member in the acquired vehicle model, a cross-sectional line length minimum cross-section where the cross-sectional line length is minimized. An axial force ratio calculation step of calculating, for each of the crash box and the front side member in the vehicle model, the axial force at the cross-sectional line length minimum cross-section when various plate thicknesses and tensile strengths are set, and calculating the ratio of the axial force calculated for the crash box to the axial force calculated for the front side member. A vehicle body part passenger compartment side intrusion amount calculation step of performing a collision analysis regarding the frontal collision of the vehicle model with various plate thicknesses and tensile strengths set, and calculating the intrusion amount of the vehicle body parts toward the passenger compartment side at the intrusion amount evaluation part set in the vehicle model during the frontal collision of the vehicle model. A critical axial force ratio determination step of determining a critical axial force ratio at which the intrusion amount saturates to a substantially constant value from the relationship between the axial force ratio calculated by setting various plate thicknesses and tensile strengths in the axial force ratio calculation step and the intrusion amount of the vehicle body parts toward the passenger compartment side at the intrusion amount evaluation part calculated by the collision analysis of the vehicle model with various plate thicknesses and tensile strengths set in the vehicle body part passenger compartment side intrusion amount calculation step. It is characterized by including a plate thickness and tensile strength determination step of determining a combination of plate thickness and tensile strength that is equal to or less than the critical axial force ratio determined in the critical axial force ratio determination step among the combinations of various plate thicknesses and tensile strengths for which the axial force ratio has been calculated.

[0013] (3) In the case of the above (2), In the axial force ratio calculation step, the weights of the crash box and the front side member in the combinations of the various plate thicknesses and tensile strengths are calculated, In the plate thickness and tensile strength determination step, among the combinations of plate thickness and tensile strength that result in an axial force ratio equal to or less than the critical axial force ratio, the combination of plate thickness and tensile strength that minimizes the total weight of the crash box and the front side member calculated in the axial force ratio calculation step is determined. This is the gist of the invention.

[0014] (4) In the case of the above (2), In the plate thickness and tensile strength determination step, among the combinations of plate thickness and tensile strength that result in an axial force ratio equal to or less than the critical axial force ratio, the combination of plate thickness and tensile strength that maximizes the axial force calculated for the crash box is determined. This is the gist of the invention.

[0015] (5) The method for manufacturing a vehicle body front structure according to the present invention By the design method of the vehicle body front structure according to any one of the above (2) to (4), for each of the front side member and the crash box of the vehicle body front structure, a combination of plate thickness and tensile strength is determined. Using the metal plate with the determined combination of plate thickness and tensile strength, the axial force F of the cross-section where the cross-sectional line length of the closed cross-section structure in the crash box is minimized x1 and the axial force F of the cross-section where the cross-sectional line length of the closed cross-section structure in the front side member is minimized x2 and the ratio F x1 / F x2 is equal to or less than the critical axial force ratio, and a vehicle body front structure including the front side member and the crash box is manufactured. This is the gist of the invention.

Advantages of the Invention

[0016] According to the present invention, when the vehicle collides head-on, after the crash box is sufficiently axially crushed, the front side member can be bent and deformed, the intrusion amount of the vehicle body parts into the passenger compartment side can be suppressed below a predetermined level, and the collision energy can be sufficiently absorbed. Furthermore, according to the present invention, the vehicle body can be lightened and made more compact without degrading collision performance such as the absorption of collision energy and the intrusion amount of vehicle body parts into the passenger compartment side during a head-on collision of the vehicle, and it can contribute to the improvement of the marketability of automobiles.

Brief Description of the Drawings

[0017]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Mode for Carrying Out the Invention

[0018] Prior to explaining the vehicle front structure and the design method of the vehicle front structure according to the embodiment of the present invention, the background leading to the present invention will be explained. In the drawings of the present application, the X direction, the Y direction, and the Z direction indicate the vehicle front-rear direction, the vehicle width direction, and the vehicle vertical direction, respectively.

[0019] <Background Leading to the Present Invention> Fig. 2 shows a figure schematically showing a vehicle front structure 1 including a front side member 11 and a crash box 21 targeted by the present invention.

[0020] As shown in FIG. 1, the front side member 11 extends in the longitudinal direction of the vehicle body, and the end portion on the rear side of the vehicle body is connected to the dash lower panel 31 that partitions the cabin (passenger compartment) of the vehicle and the front body structure 1. When the vehicle undergoes a frontal collision, the front side member 11 is compressed and deformed (axial crushing) in the longitudinal direction (the longitudinal direction of the vehicle body), and is also bent and deformed (bending crushing) in a direction orthogonal to the longitudinal direction (the outer side in the left-right direction of the vehicle body or the lower side in the up-down direction of the vehicle body). Further, as shown in FIG. 2, since the front side member 11 is provided with the fold bead 13, bending deformation is induced starting from the fold bead 13 when the vehicle undergoes a frontal collision. Also, as shown in FIG. 7(b), the front side member 11 has a closed cross-sectional structure perpendicular to the longitudinal direction.

[0021] The crash box 21 extends from the tip of the front side member 11 toward the front side of the vehicle body, and is axially crushed and deformed in a bellows shape in the longitudinal direction when the vehicle undergoes a frontal collision. And, in order to induce bellows-shaped compression deformation in the longitudinal direction (the longitudinal direction of the vehicle body) during a frontal collision, the crash box 21 is provided with a plurality of beads 23 in the longitudinal direction, for example, as shown in FIG. 2. Also, as shown in FIG. 7(c), the crash box 21 has a closed cross-sectional structure perpendicular to the longitudinal direction.

[0022] FIG. 3 shows a graph schematically representing the load-stroke curve of the front body structure 1 during a frontal collision of the vehicle. In FIG. 3, the value obtained by integrating the load with respect to the distance (stroke) (the area of the range surrounded by the curve) represents the amount of collision energy absorption.

[0023] In the front body structure 1 in which the fold bead 13 is provided on the front side member 11 as in Patent Document 1, since bending deformation occurs due to the fold bead 13 and the load is greatly reduced, smooth compression deformation in the front side member 11 can be induced. However, in order to obtain a collision energy absorption amount equivalent to that of the vehicle body front structure 1 (conventional vehicle body front structure) without providing the folding bead 13 on the front side member 11, it is necessary to increase the total length of the front side member 11 and the crush box 21, and increase the stroke (deformation amount) until the front side member 11 undergoes folding deformation (x in FIG. 3 A →x A ’).

[0024] On the other hand, from the perspective of weight reduction, by using a high-strength metal plate (for example, a high-tensile steel plate) for the crush box 21, the yield strength during a frontal collision is improved (F → F’ in FIG. 3), and the subsequent load is also improved. Thus, it is considered that the stroke (deformation amount) required to obtain a collision energy absorption amount equivalent to that of the conventional vehicle body front structure can be suppressed (x in FIG. 3 A →x B ).

[0025] However, even if only the metal plate used for the crush box 21 is strengthened, if the front side member 11 undergoes folding deformation during the compression deformation of the crush box 21, the compression deformation of the crush box 21 will be inhibited, and the collision energy absorption amount will decrease.

[0026] Therefore, the inventors have intensively studied this problem. As a result, they conceived of optimizing the balance of the axial forces of the crush box 21 and the front side member 11 so that the front side member 11 is strengthened in accordance with the strengthening of the crush box 21. The axial force means the design allowable load (load-bearing capacity) in the longitudinal direction of a member having a closed cross-section structure such as the crush box 21 or the front side member 11, and is represented by Equation (1) described later.

[0027] By optimizing the balance of the axial forces between the crash box 21 and the front side member 11 and then increasing the strength of the crash box 21, it has been found that smooth bellows-like compressive deformation can be promoted after the start of buckling deformation of the crash box 21. As a result, it has been found that the crash box 21 can sufficiently absorb the collision energy, and further, the amount of intrusion of the vehicle body parts into the passenger compartment side can be reduced to a predetermined level or less, thereby improving the safety of the cabin. Hereinafter, the optimization of the balance of the axial forces between the crash box 21 and the front side member 11 will be described.

[0028] Figures 4 and 5 show the load-stroke curves when the front body structure 1 including the front side member 11 and the crash box 21 as shown in Figure 2 is deformed under a collision load applied from the front side to the rear side of the vehicle during a frontal collision of the vehicle. The load-stroke curves shown in Figures 4 and 5 are graphs schematically representing the change in the load received by the front side member 11 with respect to the deformation amount (stroke) of the front body structure 1 in the vehicle front-rear direction.

[0029] As shown by the solid line in Figure 4, during a frontal collision of the vehicle, when a collision load is applied to the front body structure 1, first, the crash box 21 first elastically (compresses) deforms slightly in the vehicle front-rear direction. Thereafter, the crash box 21 buckles under the load F1 and starts compressive deformation (axial crushing) (deformation amount d1), and is crushed completely and the crushing ends (deformation amount d2). Subsequently, the front side member 11 undergoes transient elastic deformation and then buckles and starts bending deformation (deformation amount d3) when it receives the transmitted load F3, and reaches the yield strength (maximum load) and completes the bending deformation (deformation amount d4).

[0030] Here, if only the high-tensile steel plate is applied to the crash box 21 to increase its strength, as shown by the dashed line in Fig. 4, the buckling load of the crash box 21 increases as the yield stress increases (F1 → F1'). Therefore, at the stage of the deformation amount d3' (<d2) before the crash box 21 is completely crushed, the load transmitted to the front side member 11 reaches F3, and the front side member 11 buckles and starts to bend. In this way, the compressive deformation of the crash box 21 is inhibited, and the intrusion amount of the vehicle body parts toward the passenger compartment increases by the amount (= d2 - d3') by which the deformation amount of the crash box 21 due to the compressive deformation becomes smaller. As a result, the crash box 21 cannot sufficiently absorb the collision energy.

[0031] On the other hand, when the high-tensile steel plate is also applied to the front side member 11 to increase its strength in accordance with the increase in the strength of the crash box 21, and the load at which the front side member 11 buckles and starts to bend is set to F3' (F3' > F3), it is shown by the dashed line in Fig. 5. In this case, the crash box 21 can be sufficiently compressed and deformed until it is completely crushed, and the buckling deformation of the front side member 11 can be started (deformation amount d3 > d2) after the crushing is completed (deformation amount d2). Therefore, the compressive deformation of the crash box 21 is not inhibited.

[0032] Also, even if the thickness of the closed cross-sectional structure of the crash box 21 is increased or the sectional line length of the closed cross-sectional structure is increased so that the crash box 21 becomes less likely to be compressed and deformed without increasing the strength of the crash box 21, the load at which the crash box 21 buckles and starts to be compressed and deformed increases. Therefore, similar to the case where the strength of the crash box 21 is increased, the compressive deformation of the crash box 21 is inhibited.

[0033] In this way, the inventors focused on the balance of the yield strength between the crash box 21 and the front side member 11 so that the crash box 21 can be sufficiently compressed and deformed before the front side member 11 starts to bend, and conceived to appropriately adjust the plate thickness and tensile strength used for the crash box 21 and the front side member 11.

[0034] Next, the inventors considered how to optimize the balance of the strength between the crash box 21 and the front side member 11. Therefore, first, the inventors decided to use the axial force represented by the following formula (1) as an index corresponding to the strength of the crash box 21 and the front side member 11. The axial force is an index corresponding to the allowable load (load-bearing capacity) in the longitudinal direction of the member in design. When the allowable load in design is the buckling load, the yield strength may be used instead of the tensile strength in formula (1). Axial force (N) = sectional line length (mm) × plate thickness (mm) × tensile strength (MPa) ···(1)

[0035] Regarding the balance of the axial forces in the crash box 21 and the front side member 11, the axial force ratio (F x1 of the front side member 11 to the axial force F x2 of the crash box 21, that is, the ratio (= F x1 / F x2 ), is represented by the axial force ratio (F X ).

[0036] Then, based on the axial force ratio defined in this way, the inventors examined whether the balance of the axial forces in the crash box 21 and the front side member 11 can be appropriately adjusted. Therefore, the plate thickness and tensile strength of the crash box 21 and the front side member 11 were changed to various combinations, and the relationship between the axial force ratio of the crash box 21 and the front side member 11 and the amount of intrusion of the vehicle body parts into the passenger compartment side during a frontal collision of the vehicle was investigated. As a result, as shown in FIG. 6, it was obtained that when the axial force ratio is below a predetermined value, the amount of intrusion of the vehicle body parts saturates at a certain level.

[0037] In the present application, the axial force ratio between the crash box 21 and the front side member 11 at the point when the intrusion amount of the vehicle body part into the passenger compartment side reaches saturation is referred to as the critical axial force ratio. Further, FIG. 6 shows the axial force ratio F x which is a graph plotting the dash panel intrusion amount, which is the intrusion amount of the vehicle body part into the passenger compartment side, against the axial force ratio.

[0038] Thus, it has been found that regarding the balance of the axial forces in the crash box 21 and the front side member 11, it is sufficient to adjust the axial force ratio between the crash box 21 and the front side member 11 to be equal to or less than the critical axial force ratio.

[0039] That is, by determining the plate thickness and tensile strength (or yield strength) of the crash box 21 and the front side member 11 such that the axial force ratio between the crash box 21 and the front side member 11 becomes an appropriate value equal to or less than the critical axial force ratio, it has been found that the balance of the bearing strengths in the crash box 21 and the front side member 11 can be optimized. And in the vehicle body front structure 1 in which the axial force ratio between the crash box 21 and the front side member 11 is optimized, at the time of a frontal collision of the vehicle, after the crash box 21 is sufficiently compressed and deformed, the front side member 11 can be bent and deformed. As a result, it has been found that the intrusion amount of the vehicle body part into the passenger compartment side can be suppressed to a predetermined level or less, and the collision energy can be sufficiently absorbed by the compression deformation of the crash box 21. The present invention has been made based on the above-described findings, and its specific configuration will be described below.

[0040] <Vehicle body front structure> FIG. 1 shows, as an example, a vehicle body front structure 1 according to an embodiment of the present invention. The front body structure 1 includes a pair of left and right front side members 11 extending in the longitudinal direction of the vehicle body and having a closed cross-sectional structure, and a crush box 21 extending forward from the tip of each front side member 11 and having a closed cross-sectional structure.

[0041] As shown in FIG. 7 for example, the front side member 11 is formed by joining a front side member inner 11a having a hat cross-sectional shape and a front side member outer 11b having a panel shape to form a closed cross-sectional structure.

[0042] As shown in FIG. 7 for example, the crush box 21 is formed by joining a crush box inner 21a having a U-shaped cross-sectional shape and a crush box outer 21b having a U-shaped cross-sectional shape to form a closed cross-sectional structure. Further, in the front body structure 1, as shown in FIG. 1, a bumper beam 33 extending in the vehicle width direction is connected to the tip of each crush box 21.

[0043] And in the front body structure 1, the axial force F of the crush box 21 x1 and the axial force F of the front side member 11 x2 and the ratio F of them x1 / F x2 are set such that the thickness and tensile strength of the metal plates used for each of the front side member 11 and the crush box 21 are below a predetermined critical axial force ratio. Here, the axial force F of the crush box 21 x1 is assumed to be applied at the cross section where the cross-sectional line length of the closed cross-sectional structure in the crush box 21 is minimized. Also, the axial force F of the front side member 11 x2 is assumed to be applied at the cross section where the cross-sectional line length of the closed cross-sectional structure in the front side member 11 is minimized.

[0044] Thereby, the front body structure 1 is configured such that the front side member 11 is bent and deformed after the crush box 21 is sufficiently compressed and deformed during a frontal collision of the vehicle.

[0045] Thus, according to the vehicle front structure 1 according to this embodiment, at the time of a frontal collision of the vehicle, the front side member 11 can be bent and deformed after the crush box 21 is sufficiently compressed and deformed. As a result, without increasing the length of the front side member 11 in the vehicle front-rear direction, the amount of intrusion of the vehicle body parts into the passenger compartment side can be made equal to or less than a predetermined level, and the crush box 21 can sufficiently absorb the collision energy.

[0046] Furthermore, in the present invention, if the axial force ratio between the crush box 21 and the front side member 11 is equal to or less than a predetermined critical axial force ratio, a metal plate having a high tensile strength and a thin plate thickness can be used for the crush box 21 and the front side member 11. Thereby, it is possible to reduce the weight and size of the vehicle body without degrading the collision performance such as absorption of collision energy and intrusion of vehicle body parts into the passenger compartment side at the time of a frontal collision of the vehicle, and it is possible to contribute to an improvement in the marketability of the automobile.

[0047] Note that, as shown in FIG. 6 described above, the critical axial force ratio may be a value determined based on the amount of intrusion into the passenger compartment side, which is the amount of deformation of the vehicle body parts (for example, the dash lower panel 31) into the passenger compartment side at the time of a frontal collision of the vehicle.

[0048] Also, regarding the lower limit value of the axial force ratio, as long as it is within the range of the plate thickness and the tensile strength (or yield strength) of a metal plate (for example, a steel plate) practically used for the front side member 11 and the crush box 21, it does not cause a significant difference in the amount of intrusion into the passenger compartment side.

[0049] For example, in the vehicle front structure 1 shown in FIG. 1, the plate thickness of the steel plate practically used as the material of the front side member 11 and the crush box 21 is 1.2 mm to 2.3 mm, and the tensile strength is in the range of 270 MPa class to 1470 MPa class. In this case, among the steel plates applicable to the crush box 21, the steel plate with the minimum axial force F x1 is the one with a plate thickness of 1.8 mm and a tensile strength of 270 MPa class. Also, among the steel plates applicable to the front side member 11, the axial force Fx2 The steel plate with the maximum [value] has a plate thickness of 1.0 mm (outer) and 1.6 mm (inner), and a tensile strength of 1470 MPa grade. Therefore, the axial force ratio of 0.15 given by the combination of the plate thickness and the tensile strength of these steel plates becomes the lower limit value. And even in the vehicle body front structure 1 with an axial force ratio of 0.15, the crash box 21 was sufficiently axially crushed and then the front side member 11 was bent and deformed, and no increase was seen in the intrusion amount of the vehicle body parts into the passenger compartment side.

[0050] <Design method of vehicle body front structure> The design method of the vehicle body front structure according to the embodiment of the present invention is to design a vehicle body front structure 1 including a pair of left and right front side members 11 and a crash box 21 as shown in FIG. 1. And the design method of the vehicle body front structure according to the present embodiment is designed such that the front side member 11 is bent and deformed after the crash box 21 is sufficiently compressed and deformed during a frontal collision of the vehicle.

[0051] The design method of the vehicle body front structure according to the present embodiment includes, as shown in FIG. 8, a vehicle model acquisition step S1, a cross-sectional line length minimum cross-section determination step S3, an axial force ratio calculation step S5, and a vehicle body part passenger compartment side intrusion amount calculation step S7. Further, the design method of the vehicle body front structure according to the present embodiment includes, as shown in FIG. 8, a critical axial force ratio determination step S9 and a plate thickness and tensile strength determination step S11. Hereinafter, for the case of designing the vehicle body front structure 1 shown in FIG. 1 described above, each of the above steps will be described with reference to FIGS. 8 and 9.

[0052] ≪Vehicle model acquisition step≫ In the vehicle model acquisition step S1, a vehicle model having the vehicle body front structure 1 is acquired, and a part for evaluating the intrusion amount of the vehicle body parts into the passenger compartment side during a frontal collision of the vehicle model 101 shown in FIG. 9 is set. In the present embodiment, as the frontal collision of the vehicle model 101, as shown in FIG. 9, an offset collision against the collision body 103 is targeted. In the present embodiment, the dash lower panel 31 is used as the vehicle body part for evaluating the amount of intrusion into the passenger compartment in a frontal collision of the vehicle model 101 (FIG. 1). Then, as shown in FIG. 10, an intrusion amount evaluation site 31a is set on the dash lower panel 31, and the amount of intrusion into the passenger compartment at each intrusion amount evaluation site 31a is calculated.

[0053] ≪Minimum cross-sectional line length cross-section determination step≫ In the minimum cross-sectional line length cross-section determination step S3, for each of the crash box 21 and the front side member 11 in the front body structure 1 of the acquired vehicle model 101, a cross-section with the minimum cross-sectional line length is determined. In the present embodiment, for each of the crash box 21 and the front side member 11 of the front body structure 1 shown in FIG. 7(a), as the cross-section with the minimum cross-sectional line length in the cross-section perpendicular to the longitudinal direction, an A-A cross-section (FIG. 7(c)) and a B-B cross-section (FIG. 7(b)) provided with beads 23 and folded beads 13 are determined, respectively.

[0054] ≪Axial force ratio calculation step≫ In the axial force ratio calculation step S5, first, the axial forces F x1 and F x2 in the cross-sections with the minimum cross-sectional line lengths determined for the crash box 21 and the front side member 11 in the vehicle model 101 are calculated by Equation (1). Then, in the axial force ratio calculation step S5, the axial force F x1 calculated for the crash box 21 and the axial force F x2 calculated for the front side member 11 are used to calculate the ratio F x1 / F x2 (=axial force ratio F x ). Here, in the axial force ratio calculation step S5, for various combinations within the range of the plate thickness and tensile strength (or yield strength) of the metal plates (for example, steel plates) practically used for the front side member 11 and the crash box 21, the axial forces F x1 , F x2 and the axial force ratio F x are calculated.

[0055] <<Calculation Steps for Intrusion Amount of Passenger Compartment Side of Body Parts>> In the calculation step S7 for the intrusion amount of the passenger compartment side of the body parts, first, a collision analysis regarding the frontal collision of the vehicle model 101 is performed, in which various combinations of the plate thickness and tensile strength of the metal plates used for the front side member 11 and the crash box 21 are set. Then, the intrusion amount of the body part (dash lower panel 31) into the passenger compartment side at the intrusion amount evaluation site 31a set in the vehicle model 101 during the frontal collision of the vehicle model 101 is calculated.

[0056] Note that the collision analysis of the vehicle model 101 in the calculation step S7 for the intrusion amount of the passenger compartment side of the body parts may be performed by executing collision analysis software.

[0057] <<Critical Axial Force Ratio Determination Step>> In the critical axial force ratio determination step S9, the critical axial force ratio is determined from the relationship between the axial force ratio calculated in the axial force ratio calculation step S5 and the intrusion amount calculated in the calculation step S7 for the intrusion amount of the passenger compartment side of the body parts. As the axial force ratio, the axial force ratios calculated for various combinations of the plate thickness and tensile strength of the metal plates used for the front side member 11 and the crash box 21 in the axial force ratio calculation step S5 are used. On the other hand, as the intrusion amount of the body part, the values calculated by the collision analysis of the vehicle model 101 in which various combinations are set as the plate thickness and tensile strength of the metal plates used for the front side member 11 and the crash box 21 in the calculation step S7 for the intrusion amount of the passenger compartment side of the body parts are used.

[0058] And in the critical axial force ratio determination step S9, the relationship between the axial force ratio and the intrusion amount of the body part can be expressed as exemplified in FIG. 6. As shown in FIG. 6, as the axial force ratio decreases, the intrusion amount of the body part also decreases, but when the axial force ratio is 0.69 or less, the intrusion amount saturates to values within a substantially constant range (within ±20%). From this, it can be determined that the critical axial force ratio is 0.69.

[0059] <<Determination Step for Plate Thickness and Tensile Strength>> In the plate thickness and tensile strength determination step S11, a combination of the plate thickness and tensile strength of the metal plates used for the front side member 11 and the crash box 21 that is equal to or less than the critical axial force ratio determined in the critical axial force ratio determination step S9 is determined. In determining the combination of the plate thickness and tensile strength of the metal plates used for the front side member 11 and the crash box 21, it is preferable to select from among various combinations of the plate thickness and tensile strength for which the axial force ratio was calculated in the axial force ratio calculation step S5.

[0060] As described above, according to the method for designing the front body structure according to the embodiment of the present invention, the front body structure 1 can be designed so that the front side member is bent and deformed after the crash box 21 is sufficiently compressed and deformed during a frontal collision of the vehicle. Then, during a frontal collision of a vehicle equipped with the front body structure 1 designed in this way, without increasing the length of the front side member 11 in the vehicle front-rear direction, the amount of intrusion of the vehicle body parts into the passenger compartment side can be made equal to or less than a predetermined level, and the crash box 21 can sufficiently absorb the collision energy.

[0061] In the plate thickness and tensile strength determination step, any combination of the plate thickness and tensile strength that results in an axial force ratio equal to or less than the critical axial force ratio can be appropriately determined. However, in the plate thickness and tensile strength determination step, for example, as described below, it is preferable to determine the plate thickness and tensile strength using the weight as an index.

[0062] In this case, first, in the axial force ratio calculation step S5, for various combinations of the plate thickness and tensile strength of the metal plates used for the front side member 11 and the crash box 21, in addition to the axial force ratio, the weights of the front side member 11 and the crash box 21 are calculated.

[0063] In the plate thickness and tensile strength determination step S11, among the combinations of plate thickness and tensile strength that result in an axial force ratio equal to or less than the critical axial force ratio, the combination of plate thickness and tensile strength that minimizes the total weight of the front side member 11 and the crash box 21 calculated in the axial force ratio calculation step is determined. Thereby, it is possible to design the vehicle front structure 1 that achieves weight reduction of the vehicle body while ensuring collision performance.

[0064] Note that the above description was for the case of determining the optimal plate thickness and tensile strength using weight as an index, but the rigidity of the vehicle front structure or the collision energy absorption amount may also be used as an index. When determining the combination of plate thickness and tensile strength of the metal plates used for the front side member 11 and the crash box 21 using the collision energy absorption amount of the crash box 21 as an index, in the plate thickness and tensile strength determination step, the axial force ratio F that is equal to or less than the critical axial force ratio x Among the combinations of plate thickness and tensile strength that result in, the combination of plate thickness and tensile strength that maximizes the axial force F x1 calculated for the crash box 21 is determined. Thereby, while promoting sufficient compressive deformation of the crash box 21, the yield strength of the crash box 21 is maximized, and the collision energy absorption amount (the value obtained by integrating the load in the load-stroke curve with respect to the distance (stroke)) can be maximized.

[0065] Also, the vehicle body part passenger compartment side intrusion amount calculation step S7 according to the present embodiment uses a full vehicle model in which the entire vehicle is modeled as the vehicle model 101. However, a partial vehicle model in which the vehicle front structure and its surrounding vehicle body parts of the vehicle are modeled may also be used.

[0066] Also, the design method of the vehicle front structure according to the present embodiment may be implemented using a design device for the vehicle front structure configured by a computer. In the design device for the vehicle front structure, the computer may execute a program for implementing each step of the design method of the vehicle front structure.

[0067] <Method for manufacturing the front body structure> The method for manufacturing the front body structure according to the embodiment of the present invention determines the combination of the plate thickness and the tensile strength for each of the front side member 11 and the crash box 21 of the front body structure 1 by the design method of the front body structure according to the embodiment of the present invention described above. Then, using the metal plate having the determined combination of the plate thickness and the tensile strength, the axial force F of the cross section where the sectional line length of the closed cross section structure in the crash box 21 is minimized x1 and the axial force F of the cross section where the sectional line length of the closed cross section structure in the front side member 11 is minimized x2 and the ratio F x1 / F x2 manufactures the front body structure 1 including the front side member 11 and the crash box 21 so as to be equal to or less than the critical axial force ratio determined in the critical axial force ratio determination step S9.

[0068] According to the method for manufacturing the front body structure according to the embodiment of the present invention, the front body structure 1 can be manufactured so that the front side member 11 is bent and deformed after the crash box 21 is sufficiently compressed and deformed during a frontal collision of the vehicle. Then, during a frontal collision of a vehicle equipped with the front body structure 1 manufactured in this way, the amount of intrusion of the vehicle body parts into the passenger compartment side can be made equal to or less than a predetermined level. Furthermore, the crash box 21 can sufficiently absorb the collision energy without increasing the length of the front side member 11 in the vehicle body front-rear direction.

Example

[0069] An analysis for verifying the operational effects of the present invention was performed, and this will be described below. In the analysis, as shown in FIG. 9, an offset collision was targeted in which a collision body 103 was made to collide with the front surface of a vehicle model 101 having a vehicle body front structure 1 (FIG. 1) in a state shifted from the center in the vehicle body width direction of the vehicle model 101. Then, based on the analysis of the offset collision, the optimum plate thickness and tensile strength were determined so that the front side member 11 would be bent and deformed after the crush box 21 in the vehicle body front structure 1 was sufficiently compressed and deformed.

[0070] First, a vehicle model 101 having the vehicle body front structure 1 shown in FIG. 1 was obtained, and an intrusion amount evaluation site for evaluating the intrusion amount of vehicle body parts toward the passenger compartment side during a frontal collision of the vehicle model 101 was set. In the embodiment, the dash lower panel 31 and the foot brake (not shown) were used as vehicle body parts for evaluating the intrusion amount toward the passenger compartment side. For the dash lower panel 31, as shown in FIG. 10, six intrusion amount evaluation sites 31a were set. Also, for the foot brake, it was set as an intrusion amount evaluation site itself.

[0071] Next, for each of the crush box 21 and the front side member 11 in the vehicle body front structure 1 of the vehicle model 101 shown in FIG. 7(a), the cross-sectional line length minimum cross-section where the cross-sectional line length is minimum was determined as the A-A cross-section (FIG. 7(c)) and the B-B cross-section (FIG. 7(b)), respectively.

[0072] Next, for each of the crush box 21 and the front side member 11, the axial force F x1 and F x2 at the cross-sectional line length minimum cross-section were calculated, and the axial force ratio F x (=F x1 / F x2 ) was calculated. Here, the axial force ratio F x was calculated for various combinations of the plate thickness and tensile strength of the metal plates used for the front side member 11 and the crush box 21. Note that for calculating the axial force of each of the crush box 21 and the front side member 11, PRIMER 1.80 of ARUP software manufactured by JSOL Corporation was used. Also, the weights of the crash box 21 and the front side member 11 were calculated for various combinations of the plate thickness and tensile strength of the metal plates used for the front side member 11 and the crash box 21.

[0073] Table 1 shows the combinations of the plate thickness and tensile strength set for the crash box 21 and the front side member 11, and the axial forces and axial force ratios F of the crash box 21 and the front side member 11 calculated for each combination. x are shown.

Table 1

[0074] Subsequently, an offset collision analysis of the vehicle model 101 with various combinations of plate thickness and tensile strength was performed, and the intrusion amount of the vehicle body parts into the passenger compartment side at the intrusion amount evaluation site set for the vehicle model 101 was calculated. In the collision analysis, the initial velocity of the vehicle model 101 was set to 64 km / h.

[0075] Next, the axial force ratio F of the crash box 21 and the front side member 11 obtained for each combination of the plate thickness and tensile strength shown in No. 1 to No. 10 of Table 1 x and the intrusion amount at the intrusion amount evaluation site obtained by the collision analysis of the vehicle model 101 were plotted. Then, the critical axial force ratio was determined from the plotted relationship between the axial force ratio and the intrusion amount. Fig. 6 shows a graph plotting the relationship between the axial force ratio and the dash panel intrusion amount. As shown in Table 1 and Fig. 6, in the region where the axial force ratio is greater than 0.69 (No. 2 and No. 4 in Table 1), the dash panel intrusion amount tends to decrease as the axial force ratio decreases. And in the region where the axial force ratio is 0.69 or less (No. 1, No. 3, No. 5 to No. 10 in Table 1), the dash panel intrusion amount is within the range of ±20% regardless of the axial force ratio and is almost constant. From this, the critical axial force ratio for the dash panel intrusion amount was determined to be 0.69.

[0076] In Table 1, the crash box 21 and the front side member 11 with the combinations of plate thickness and tensile strength shown in No. 1, No. 3, and No. 5 to No. 10 have an axial force ratio F x that is a value below the critical axial force ratio determined by FIG. 6. Therefore, the vehicle front structure 1 having the crash box 21 and the front side member 11 with these combinations of plate thickness and tensile strength was within the scope of the present invention (invention example).

[0077] On the other hand, in Table 1, the crash box 21 and the front side member 11 with the combinations of plate thickness and tensile strength shown in No. 2 and No. 4 have an axial force ratio F x that is a value greater than the critical axial force ratio determined by FIG. 6. Therefore, the vehicle front structure 1 having the crash box 21 and the front side member 11 with these combinations of plate thickness and tensile strength was outside the scope of the present invention (comparative example).

[0078] Table 1 also shows the results of obtaining the weight differences between the crash box 21 and the front side member 11 in each combination of plate thickness and tensile strength according to No. 2 to No. 10, based on the weight when the plate thickness of No. 1 was set for the crash box 21 and the front side member 11.

[0079] In the invention examples shown in No. 3 and No. 6 to No. 9, by increasing the tensile strength of the crash box 21 and the front side member 11 and reducing the plate thickness compared to No. 1, the crash box 21 and the front side member 11 were lightened without reducing the dash panel intrusion amount.

[0080] FIG. 11 shows, as an example, the load-stroke curves obtained by the offset collision analysis of the vehicle model 101 with the combinations of the plate thickness and tensile strength of the crash box 21 and the front side member 11 being No. 3 (invention example 2) and No. 4 (comparative example 2) shown in Table 1.

[0081] Furthermore, FIG. 12 shows a graph summarizing the results of the dashboard intrusion amount, foot brake intrusion amount, and collision energy absorption amount by the crash box (crash box EA amount) in the inventive examples (No. 1, No. 3, and No. 10) and the comparative example (No. 4). In FIGS. 11 and 12, the crash box EA amount is the amount of collision energy absorbed by the compressive deformation of the crash box 21 up to the distance and time at which buckling (bending deformation) of the front side member 11 starts.

[0082] When taking No. 1 (Inventive Example 1) shown in Table 1 for the combination of the plate thickness and tensile strength of the crash box 21 and the front side member 11, as shown in FIGS. 11(a) and 12, the crash box EA amount up to the distance of 668.8 mm and time of 0.041 s at which buckling of the front side member 11 starts was 58.5 kN·m. Also, the dashboard intrusion amount was 22.5 mm and the foot brake intrusion amount was 15.0 mm.

[0083] The combination of the plate thickness and tensile strength of the crash box 21 and the front side member 11 in No. 3 (Inventive Example 2) of Table 1 has a larger axial force ratio F x than that of No. 1 (Inventive Example 1) (No. 1: 0.50, No. 3: 0.69), and the axial force ratio F x of No. 3 (Inventive Example 2) is equal to the critical axial force ratio (= 0.69). And as shown in FIG. 11(a), smooth bellows-like compressive deformation of the crash box 21 could be promoted until the start of buckling (bending deformation) of the front side member 11. As a result, the crash box EA amount improved by 6.6% (No. 3: 62.4 kN·m, No. 1: 58.5 kN·m), the dashboard intrusion amount decreased by 9% (No. 3: 20.4 mm, No. 1: 22.5 mm), and the foot brake intrusion amount decreased by 20% (No. 3: 12.0 mm, No. 1: 15.0 mm) compared to No. 1 (Inventive Example 1). Furthermore, the axial force F of the crash box 21 in No. 3 (Inventive Example 2) is larger than that in No. 1 (Inventive Example 1). x1is 53% larger (No.1: 304.9 kN, No.3: 466.1 kN). Due to sufficient compressive deformation of the crash box 21, the EA amount of the crash box of No.3 (invention example) is 6.6% higher than that of No.1 (invention example 1) (No.3: 62.4 kN·m, No.1: 58.5 kN·m).

[0084] The combination of the plate thickness and tensile strength of the crash box 21 and the front side member 11 in No.4 (comparative example 2) in Table 1 has an axial force ratio F x that is larger than the critical axial force ratio of 0.69 (=0.84). As shown in Fig. 11(b), the distance until buckling (bending deformation) of the front side member 11 starts is 18% shorter than that of No.1 (invention example 1) (No.4: 550.6 mm, No.1: 668.8 mm), and sufficient compressive deformation of the crash box 21 could not progress. Therefore, in No.4 (comparative example 2), even though the axial force was increased by 87% compared to No.1 (invention example 1) (No.4: 568.8 kN, No.1: 304.9 kN) with the tensile strength of the steel plate used for the crash box 21 being 1180 MPa, the EA amount of the crash box decreased by 12.8% compared to No.1 (invention example 1) (No.4: 51.0 kN·m, No.1: 58.5 kN·m). Furthermore, in No.4 (comparative example 1), due to the small deformation amount of the compressive deformation of the crash box 21 until the front side member 11 starts to bend, the dash panel intrusion amount increased by 73% (No.4: 38.9 mm, No.1: 22.5 mm), and the foot brake intrusion amount increased by 58% (No.4: 23.7 mm, No.1: 15.0 mm).

[0085] The combination of the plate thickness and tensile strength of the crash box 21 and the front side member 11 shown in No.10 (invention example 8) in Table 1 has an axial force ratio F higher than that of No.1 (invention example 1). xIt was made smaller (No.1: 0.50, No.10: 0.19), and the smooth bellows-like axial crushing of the crash box could be promoted until the buckling (bending deformation) of the front side member 11 started. As a result, compared with No.1 (Inventive Example 1), No.10 (Inventive Example 8) had a 11% reduction in the dash panel intrusion amount (No.10: 20.0 mm, No.1: 22.5 mm) and an 18% reduction in the foot brake intrusion amount (No.10: 12.3 mm, No.1: 15.0 mm). Note that since No.10 (Inventive Example 8) used a steel plate with a lower tensile strength than No.1 (Inventive Example 1) for the crash box 21 (No.1: 440 MP grade, No.10: 270 MPa grade), the EA amount of the crash box decreased by 57% (No.10: 25.3 kN·m, No.1: 58.5 kN·m).

[0086] In this embodiment, further, when changing the combination of the plate thickness and the tensile strength, the axial force ratio and the weights of the front side member 11 and the crash box 21 were calculated, and the optimal combination of the plate thickness and the tensile strength was determined from the viewpoint of weight reduction of the vehicle body front structure 1.

[0087] Table 2 shows the results of obtaining the axial force ratio for various combinations of the plate thickness and the tensile strength. Also, Table 3 shows the weight difference of each of the front side member 11 and the crash box 21 and the total weight difference of them, based on the crash box 21 and the front side member 11 according to No.1 (Inventive Example 1) of Table 1 described above, for various combinations of the plate thickness and the tensile strength.

[0088]

Table 2

[0089]

Table 3

[0090] In the axial force ratio in Table 2 and the total weight difference between the front side member 11 and the crash box 21 in Table 3, the columns filled in gray indicate combinations of plate thickness and tensile strength where the axial force ratio is less than or equal to the critical axial force ratio shown in Fig. 6 (≤0.69).

[0091] From the results shown in Table 2 and Table 3, when the plate thickness of the front side member 11 is 1.0 mm for the front side member inner 11a (inner) and 1.6 mm for the front side member outer 11b (outer), the tensile strength is of the 1470 MPa class, the plate thickness of the crash box 21 is 1.2 mm, and the tensile strength is of the 1470 MPa class, the absolute value of the total weight difference between the front side member 11 and the crash box 21 is the largest at -2151.2 g, indicating that it is the most optimal among the combinations of plate thickness and tensile strength shown in Table 2 and Table 3.

[0092] Also, the combination of plate thickness and tensile strength that maximizes the collision energy absorption amount by the crash box 21 (crash box EA amount) is the combination among the combinations with a critical axial force ratio less than or equal to (≤0.69) that fills the columns of Table 2 in gray, for which the axial force F x1 calculated for the crash box 21 is the maximum. In this embodiment, it can be determined that the plate thickness of the front side member 11 is 1.2 mm for the front side member inner 11a (inner) and 1.8 mm for the front side member outer 11b (outer), the tensile strength is of the 1470 MPa class, and the plate thickness of the crash box 21 is 1.4 mm and the tensile strength is of the 1470 MPa class.

[0093] As described above, according to the present invention, at the time of a frontal collision of a vehicle, after sufficiently compressing and deforming the crash box, the front side member can be bent and deformed, the intrusion amount of the vehicle body parts into the passenger compartment side can be suppressed below a predetermined level, and it has been shown that the collision energy absorption amount can be ensured. Furthermore, it has been demonstrated that the vehicle body can be lightened without degrading the collision performance (collision energy absorption amount and intrusion amount of vehicle body parts into the passenger compartment).

Description of Reference Numerals

[0094] 1 Front body structure 11 Front side member 11a Inner front side member 11b Outer front side member 13 Folding bead 21 Crash box 21a Inner crash box 21b Outer crash box 23 Bead 31 Dash lower panel 31a Intrusion amount evaluation site 33 Bumper beam 101 Vehicle model 103 Collision object

Claims

1. A vehicle front structure comprising a pair of left and right front side members extending in the longitudinal direction of the vehicle body and having a closed cross-sectional structure, and a crash box extending from the tip of each front side member toward the front side of the vehicle body and having a closed cross-sectional structure, wherein The axial force F defined by the following formula (1) in the cross-section where the cross-sectional line length of the closed cross-sectional structure in the crash box is minimized x1 and the axial force F defined by the following formula (1) in the cross-section where the cross-sectional line length of the closed cross-sectional structure in the front side member is minimized x2 and the ratio F x1 / F x2 is 0.69 or less, which is a predetermined critical axial force ratio. The plate thickness and tensile strength of the steel plates used for the front side member and the crash box are set within the ranges of a plate thickness of 1.2 mm to 2.3 mm and a tensile strength of 270 MPa class to 1470 MPa class, so that the front side member is bent and deformed after the crash box is sufficiently compressed and deformed during a frontal collision of the vehicle. A front body structure characterized by this is provided. Axial force (N) = sectional line length (mm) × plate thickness (mm) × tensile strength (MPa) ··· (1)

2. In a vehicle front structure comprising a pair of left and right front side members extending in the longitudinal direction of the vehicle and having a closed cross-sectional structure, and a crash box extending from the tip of each front side member toward the front side of the vehicle body and having a closed cross-sectional structure, a design method for a vehicle front structure designed to cause the front side member to bend and deform after sufficiently compressing and deforming the crash box during a frontal collision of the vehicle, comprising a vehicle model acquisition step of acquiring a vehicle model having the vehicle front structure and setting an intrusion amount evaluation site for evaluating the intrusion amount of vehicle body parts toward the passenger compartment side during a frontal collision of the vehicle model; a sectional line length minimum cross-section determination step of determining, for each of the crash box and the front side member in the acquired vehicle model, a sectional line length minimum cross-section where the sectional line length is minimized; an axial force ratio calculation step of calculating the axial force at the sectional line length minimum cross-section when various plate thicknesses and tensile strengths are set for each of the crash box and the front side member in the vehicle model, and calculating the ratio of the axial force calculated for the crash box to the axial force calculated for the front side member; a vehicle body part passenger compartment side intrusion amount calculation step of performing a collision analysis regarding the frontal collision of the vehicle model with various plate thicknesses and tensile strengths set, and calculating the intrusion amount of the vehicle body parts toward the passenger compartment side at the intrusion amount evaluation site set in the vehicle model during the frontal collision of the vehicle model; a critical axial force ratio determination step of determining a critical axial force ratio at which the intrusion amount saturates to a substantially constant value from the relationship between the axial force ratio calculated by setting various plate thicknesses and tensile strengths in the axial force ratio calculation step and the intrusion amount of the vehicle body parts toward the passenger compartment side at the intrusion amount evaluation site calculated by the collision analysis of the vehicle model with various plate thicknesses and tensile strengths set in the vehicle body part passenger compartment side intrusion amount calculation step Among the axial force ratios calculated for various combinations of plate thickness and tensile strength, a plate thickness and tensile strength determination step of determining a combination of plate thickness and tensile strength that is equal to or less than the critical axial force ratio determined in the critical axial force ratio determination step, and a method for designing a front body structure, characterized by including this.

3. In the axial force ratio calculation step, the weights of the crash box and the front side member in the various combinations of plate thickness and tensile strength are calculated. In the plate thickness and tensile strength determination step, among the combinations of plate thickness and tensile strength that result in an axial force ratio equal to or less than the critical axial force ratio, the combination of plate thickness and tensile strength that minimizes the total weight of the crash box and the front side member calculated in the axial force ratio calculation step is determined. The method for designing a front body structure according to claim 2, characterized by this.

4. In the plate thickness and tensile strength determination step, among the combinations of plate thickness and tensile strength that result in an axial force ratio equal to or less than the critical axial force ratio, the combination of plate thickness and tensile strength that maximizes the axial force calculated for the crash box is determined. The method for designing a front body structure according to claim 2, characterized by this.

5. By the method for designing a front body structure according to any one of claims 2 to 4, for each of the front side member and the crash box of the front body structure, a combination of plate thickness and tensile strength is determined. Using a metal plate with the determined combination of plate thickness and tensile strength, the axial force F of the cross-section where the cross-sectional line length of the closed cross-sectional structure in the crash box is minimized x1 and the axial force F of the cross-section where the cross-sectional line length of the closed cross-sectional structure in the front side member is minimized x2 and the ratio F x1 / F x2 Manufacturing a front body structure including the front side member and the crash box such that the ratio becomes equal to or less than the critical axial force ratio. A method for manufacturing a front body structure, characterized by this.

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