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

Abstract

A vehicle body front part structure 1 according to the present invention is provided with: a pair of right and left front side members 11 extending in a front-rear direction of a vehicle body and having a closed cross section structure; and a crash box 21 extending from a tip of each front side member 11 to a vehicle body front side and having a closed cross section structure. A plate thickness and a tensile strength of a metal plate used for each of the front side member 11 and the crash box 21 are set so that a ratio Fx1 / Fx2 of an axial force Fx1 of a cross section in which a cross section line length of the closed cross section structure in the crash box 21 becomes minimum and an axial force Fx2 of a cross section in which a cross section line length of the closed cross section structure in the front side member 11 becomes minimum is equal to or less than a predetermined critical axial force ratio.

Description

Vehicle front body structure, design method for vehicle front body structure, and manufacturing method for vehicle front body structure

[0001] The present invention relates to a vehicle body structure including a pair of left and right front side members extending in the fore-and-aft direction of the vehicle body and crash boxes extending from the tips of the front side members toward the front of the vehicle body, a method for designing the vehicle body structure, and a method for manufacturing the vehicle body structure.

[0002] In recent years, electric vehicles (EVs), which emit less CO2 than gasoline-powered vehicles, have become increasingly popular as an environmental measure. Increasing the battery capacity is an effective way to extend the driving range of EVs. However, this increases the vehicle weight, which in turn increases the collision energy during a vehicle collision. Therefore, EVs and other vehicles require a vehicle body structure that can adequately absorb collision energy. Conventionally, vehicle body structures that absorb collision energy during a front collision (front crash) by compressively deforming (axially crushing) the front side members in the longitudinal direction have been known. During a frontal collision, such front side members undergo compressive deformation in the fore-and-aft direction of the vehicle body, while the rear side undergoes folding deformation, bending so that they protrude outward in the width direction of the vehicle body and downward in the vertical direction of the vehicle body. This not only absorbs the collision energy input to the front body structure, but also prevents deterioration of occupant protection performance (injury value of an occupant) caused by onboard components such as the engine or motor intruding into the passenger compartment.

[0003] Generally, when comparing compressive deformation and bending deformation in vehicle body parts, compressive deformation can absorb more collision energy even with the same stroke. For example, Patent Document 1 discloses a front vehicle body structure in which a front side frame (corresponding to the front side member in the present application) has a difference in rigidity between the front and rear sides, with each region having an alternating difference in rigidity in the longitudinal direction of the vehicle. In this technology, during a frontal collision, compressive deformation begins at the front side of the front side frame, which has a relatively low rigidity, and then compresses at the rear side of the front side frame, which has a relatively high rigidity. This allows the front side member to compress in a smooth bellows-shaped manner throughout the entire longitudinal direction, thereby enabling sufficient absorption of collision energy.

[0004] Japanese Patent Application Laid-Open No. 2021-66202

[0005] Some vehicle front body structures include crash boxes provided in front of the front side members. In the event of a frontal collision, the crash boxes can be compressed and deformed in the longitudinal direction of the vehicle, thereby further absorbing collision energy. In such vehicle front body structures equipped with front side members and crash boxes, the crash boxes have traditionally been made of relatively low-strength steel plates with a tensile strength of approximately 440 MPa to allow for sufficient compressive deformation. Furthermore, the front side members have often been made of relatively high-strength steel plates with a tensile strength of 780 MPa or higher. In response to demands for increased collision energy absorption in the event of a frontal collision of an electric vehicle, the use of steel plates with a tensile strength of 980 MPa or higher (ultra-high strength steel) for crash boxes has been considered.

[0006] In such cases, it is important to compressively deform the front sides of the crash boxes and front side members to obtain the required amount of collision energy absorption. However, simply increasing the strength of the steel plates used in the crash boxes can sometimes result in fracture deformation in the front side members before sufficient compressive deformation occurs in the crash boxes. In such cases, the crash boxes are unable to fully absorb the collision energy, resulting in a problem in which the required collision performance, such as the amount of vehicle body deformation, cannot be met. Therefore, to increase the strength of the crash boxes, it is important that the front side members fracture after sufficient compressive deformation of the crash boxes. To achieve this, it is thought that the thickness and material strength of the metal plates, such as steel plates, used in the crash boxes and front side members can be appropriately determined.

[0007] However, the technology disclosed in Patent Document 1 only sufficiently absorbs collision energy by compressing and deforming the front side member in a smooth bellows shape over the entire longitudinal direction, and therefore does not disclose or suggest anything about determining the appropriate thickness or material strength of the crash boxes in relation to the front side members.

[0008] Furthermore, electric vehicles equipped with batteries are required to have a compact front body structure in order to ensure a certain level of crash energy absorption performance and to secure space for the battery. To achieve a compact front body structure, the crash boxes and front side members must be able to sufficiently absorb the crash energy even if they experience only a small amount of compressive deformation during a frontal collision.

[0009] However, the technology of Patent Document 1 significantly reduces the load input during a frontal collision of the vehicle by reducing the rigidity of the front side member. Therefore, in order to ensure the amount of collision energy absorption by compressive deformation during a frontal collision, it was necessary to lengthen the front side member to increase the amount of compressive deformation. Therefore, the technology of Patent Document 1 was unable 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-mentioned problems, and its purpose is to provide a vehicle front body structure, a design method for a vehicle front body structure, and a manufacturing method for a vehicle front body structure that can achieve compactness while satisfying predetermined collision performance in the event of a frontal collision of the vehicle.

[0011] The vehicle front structure according to the present invention includes a pair of left and right front side members each having a closed cross-section structure and extending in the longitudinal direction of the vehicle body, and crash boxes each having a closed cross-section structure and extending from a tip of each front side member toward the front side of the vehicle body, and the crash boxes each having a closed cross-section structure, and the crash boxes each having a closed cross-section structure and the axial force F x1 and the axial force F of the cross section where the cross-sectional line length of the closed cross-sectional structure of the front side member is the smallest. x2 and the ratio F x1 / F x2 The thicknesses and tensile strengths of the metal plates used for the front side members and the crash boxes are set so that the axial force ratio is equal to or less than a predetermined critical ratio of axial force, so that in the event of a frontal collision of the vehicle, the crash boxes are sufficiently compressed and deformed, and then the front side members are bent and deformed.

[0012] A vehicle front structure design method according to the present invention is for a vehicle front structure including a pair of left and right front side members each having a closed cross-sectional structure and extending in a vehicle longitudinal direction, and crash boxes each having a closed cross-sectional structure and extending forward from a tip of each front side member toward the vehicle body, and for designing the front side members so that, in the event of a frontal collision of the vehicle, the crash boxes are sufficiently compressed and deformed, and then the front side members are folded and deformed. The method includes the steps of: acquiring a vehicle model having the vehicle front structure; and setting an intrusion amount evaluation portion for evaluating the intrusion amount of a vehicle body part into a passenger compartment in the event of a frontal collision of the vehicle model; and determining, for each of the crash boxes and the front side members in the acquired vehicle model, a cross-section where the line length in the cross-sectional direction is the smallest. a cross-sectional line length minimum cross-section determination step of determining a cross-sectional line length minimum cross-section (minimum); an axial force ratio calculation step of calculating an axial force at the cross-sectional line length minimum cross-section when various plate thicknesses and tensile strengths are set for each of the crash boxes and the front side members in the vehicle model, and calculating a ratio between the axial force calculated for the crash boxes and the axial force calculated for the front side members; and a body part passenger compartment calculation step of performing a collision analysis regarding a frontal collision of the vehicle model in which the various plate thicknesses and tensile strengths are set, and calculating an intrusion amount of the body part into the passenger compartment side at the intrusion amount evaluation portion set in the vehicle model when the vehicle model hits the frontal collision. a side intrusion amount calculation step, a critical axial force ratio determination step of determining a critical axial force ratio at which the intrusion amount saturates at 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 body part into the passenger compartment side at the intrusion amount evaluation portion calculated by the collision analysis of the vehicle model in which various plate thicknesses and tensile strengths are set in the body part passenger compartment side intrusion amount calculation step, and 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, from among the axial force ratios calculated for combinations of various plate thicknesses and tensile strengths.Including.

[0013] In the axial force ratio calculation step, the weights of the crash boxes and the front side members for the various combinations of plate thickness and tensile strength are calculated, and in the plate thickness and tensile strength determination step, the combination of plate thickness and tensile strength that results in an axial force ratio that is equal to or less than the critical axial force ratio is determined to be the combination of plate thickness and tensile strength that results in the smallest total weight of the crash boxes and the front side members calculated in the axial force ratio calculation step.

[0014] In the plate thickness and tensile strength determination step, it is preferable to determine the combination of plate thickness and tensile strength that maximizes the axial force calculated for the crash box from among the combinations of plate thickness and tensile strength that result in an axial force ratio that is equal to or less than the critical axial force ratio.

[0015] The manufacturing method of the vehicle body front structure according to the present invention includes determining a combination of plate thickness and tensile strength for each of the front side members and the crash boxes of the vehicle body front structure by the design method of the vehicle body front structure according to the present invention, and using metal plates having the determined combination of plate thickness and tensile strength, determining an axial force F of a cross section that minimizes the cross-sectional line length of the closed cross-sectional structure in the crash box. x1 and the axial force F of the cross section where the cross-sectional line length of the closed cross-sectional structure of the front side member is the smallest. x2 and the ratio F x1 / F x2 a front body structure including the front side members and the crash boxes, such that the axial force ratio is equal to or less than the critical axial force ratio;

[0016] According to the present invention, in the event of a frontal collision of the vehicle, the front side members can bend and deform after the crash boxes have been sufficiently axially crushed, the amount of intrusion of the vehicle body parts into the passenger compartment can be kept below a predetermined level, and the collision energy can be sufficiently absorbed. Furthermore, according to the present invention, the vehicle body can be made lighter and more compact without compromising collision performance, such as absorption of collision energy and the amount of intrusion of the vehicle body parts into the passenger compartment, in the event of a frontal collision of the vehicle, which can contribute to improving the marketability of the automobile.

[0017] FIG. 1 is a diagram illustrating an example of a configuration of a vehicle front body structure according to an embodiment of the present invention. FIG. 2 is a diagram illustrating compressive deformation of a front side member of a vehicle front body structure and bending deformation of a crash box during a vehicle front collision. FIG. 3 is a graph schematically illustrating a load-stroke curve of a vehicle front body structure during a vehicle front collision. FIG. 4 is a graph schematically illustrating the time progression of loads generated in each of the crash boxes and the front side members during a front collision of a vehicle having a vehicle front body structure in which only the crash boxes have been reinforced. FIG. 5 is a graph schematically illustrating the time progression of loads generated in each of the crash boxes and the front side members during a front collision of a vehicle having a vehicle front body structure in which the front side members have been reinforced in accordance with the reinforcement of the crash boxes. FIG. 6 is a plot of the relationship between the axial force ratio, which is the ratio of the axial force of the crash box to the axial force of the front side member, and the dash panel intrusion amount, which is the deformation of the dash lower panel toward the passenger compartment during a vehicle frontal collision, and a diagram illustrating the critical axial force ratio, which is the axial force ratio at which the dash panel intrusion amount saturates at a substantially constant value in the plot. FIG. 7 is a diagram illustrating an example of a cross-sectional shape of the front side member and crash box of a vehicle front body structure according to an embodiment of the present invention, perpendicular to the vehicle front-to-rear direction. FIG. 8 is a flowchart illustrating the processing flow of a design method for a vehicle front body structure according to an embodiment of the present invention. FIG. 9 is a diagram illustrating crashworthiness analysis of an offset collision as a vehicle frontal collision in the examples. FIG. 10 is a diagram illustrating the positions where the dash panel intrusion amount, which is the deformation of the dash lower panel toward the passenger compartment due to a vehicle frontal collision, was measured in the examples. FIG. 11 is a load-stroke curve of a vehicle front body structure obtained by crash analysis during a vehicle frontal collision in the examples ((a) Example (No. 3) and (b) Comparative Example (No. 4)).FIG. 12 is a graph showing the dash panel intrusion amount, foot brake intrusion amount, and crash energy absorption amount by the crash box (crash box EA amount) obtained for the invention examples (Nos. 1, 3, and 10) and the comparative example (No. 4) through a crash analysis of a frontal vehicle collision in the examples.

[0018] Before describing a vehicle body front structure and a design method thereof according to an embodiment of the present invention, the background to the invention will be described. In the drawings of the present application, the X direction, the Y direction, and the Z direction respectively indicate the fore-and-aft direction of the vehicle body, the width direction of the vehicle body, and the up-and-down direction of the vehicle body.

[0019] <Background to the Invention> FIG. 2 is a diagram showing a schematic representation of a vehicle body front structure 1 including a front side member 11 and a crash box 21, which are the subject of the present invention.

[0020] As shown in FIG. 1 , the front side member 11 extends in the fore-and-aft direction of the vehicle body, and its rear end is connected to a dash lower panel 31 that separates the vehicle cabin from the front vehicle body structure 1. In a frontal collision of the vehicle, the front side member 11 undergoes compressive deformation (axial crushing) in the longitudinal direction (the fore-and-aft direction of the vehicle body) and bending deformation (bending crushing) in a direction perpendicular to the longitudinal direction (outside in the left-right direction of the vehicle body or downward in the up-and-down direction of the vehicle body). Furthermore, as shown in FIG. 2 , the front side member 11 is provided with beading beads 13, which induce bending deformation starting from the beading beads 13 during a frontal collision of the vehicle. Furthermore, as shown in FIG. 7B , the front side member 11 has a closed cross-sectional structure perpendicular to the longitudinal direction.

[0021] The crash boxes 21 extend from the ends of the front side members 11 toward the front of the vehicle body and undergo bellows-like compressive deformation (axial crushing) in the longitudinal direction during a frontal collision. To induce bellows-like compressive deformation in the longitudinal direction of the vehicle body during a frontal collision, the crash boxes 21 are provided with a plurality of beads 23 in the longitudinal direction, as shown in Fig. 2 . Furthermore, as shown in Fig. 7(c), the crash boxes 21 have a closed cross-sectional structure perpendicular to the longitudinal direction.

[0022] Fig. 3 shows a graph that schematically illustrates the load-stroke curve of the vehicle front body structure 1 during a frontal collision. In Fig. 3, the value obtained by integrating the load over the distance (stroke) (the area enclosed by the curve) represents the amount of collision energy absorption.

[0023] In a vehicle front body structure 1 in which folding beads 13 are provided to the front side members 11, as in the technology disclosed in Patent Document 1, folding deformation occurs due to the folding beads 13, which significantly reduces the load, and therefore smooth compressive deformation can be induced in the front side members 11. However, in order to obtain the same amount of collision energy absorption as a vehicle front body structure 1 in which folding beads 13 are not provided to the front side members 11 (a conventional vehicle front body structure), it is necessary to increase the overall length of the front side members 11 and crash boxes 21 and increase the stroke (amount of deformation) until folding deformation occurs in the front side members 11 (x in FIG. 3 ). A →x A ').

[0024] In contrast, from the viewpoint of weight reduction, by using a high-strength metal plate (for example, a high-tensile steel plate) for the crash box 21, the yield strength at the time of a frontal collision (F → F' in FIG. 3) and the subsequent load are improved. This is thought to reduce the stroke (amount of deformation) required to obtain the same amount of collision energy absorption as a conventional vehicle front body structure (x in FIG. 3). A →x B ).

[0025] However, even if the metal plate used for the crash box 21 is simply made stronger, if the front side member 11 bends and deforms during the compressive deformation of the crash box 21, the compressive deformation of the crash box 21 will be hindered and the amount of collision energy absorption will be reduced.

[0026] The inventors therefore conducted extensive research into this issue and came up with the idea of ​​optimizing the balance of axial forces between the crash boxes 21 and the front side members 11 so as to increase the strength of the front side members 11 in line with the increase in strength of the crash boxes 21. The axial force refers to the design allowable load (load resistance) in the longitudinal direction of members having a closed cross-sectional structure such as the crash boxes 21 and the front side members 11, and is expressed by equation (1) described below.

[0027] They then found that by optimizing the balance of axial forces between the crash boxes 21 and the front side members 11 and then increasing the strength of the crash boxes 21, it is possible to promote smooth bellows-like compressive deformation of the crash boxes 21 after the start of buckling deformation. As a result, it was found that the crash boxes 21 can sufficiently absorb collision energy, and furthermore, the amount of intrusion of vehicle body parts into the passenger compartment can be kept below a predetermined level, improving cabin safety. Optimizing the balance of axial forces between the crash boxes 21 and the front side members 11 will be described below.

[0028] Figures 4 and 5 show load-stroke curves when the vehicle front body structure 1 including the front side members 11 and crash boxes 21 as shown in Figure 2 is deformed by 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 that schematically show changes in the load received by the front side members 11 relative to the amount of deformation (stroke) of the vehicle front body structure 1 in the fore-and-aft direction of the vehicle.

[0029] As shown by the solid line in Figure 4, when a collision load is applied to the front body structure 1 during a frontal collision, the crash boxes 21 first undergo slight elastic (compressive) deformation in the longitudinal direction of the vehicle. The crash boxes 21 then buckle under load F1 and begin compressive deformation (axial crushing) (deformation amount d1), and are completely crushed, completing the crushing (deformation amount d2). The front side members 11 then undergo transient elastic deformation, and upon receiving the transmitted load F3, buckle and begin bending deformation (deformation amount d3), eventually reaching their yield strength (maximum load) and completing the bending deformation (deformation amount d4).

[0030] If high-tensile steel plates were used only for the crash boxes 21 to increase their strength, the buckling load of the crash boxes 21 would increase as the yield stress increased (F1 → F1'), as shown by the dashed line in Figure 4 . Therefore, the load transmitted to the front side member 11 reaches F3 at deformation d3' (< d2), before the crash boxes 21 are completely crushed. The front side member 11 then begins to buckle and break. Because the compressive deformation of the crash boxes 21 is thus hindered, the amount of intrusion of the vehicle body parts into the passenger compartment increases by the amount of the reduced compressive deformation of the crash boxes 21 (= d2 - d3'). As a result, the crash boxes 21 are unable to adequately absorb the collision energy.

[0031] In contrast to this, the broken line in Figure 5 shows a case where high-tensile steel plate is also used to increase the strength of the crash boxes 21, and the load at which the front side members 11 buckle and begin to buckle is set to F3' (F3' > F3). In this case, the crash boxes 21 are sufficiently compressively deformed and completely crushed, and the folding deformation of the front side members 11 can begin (deformation amount d3 > d2) only after the crush boxes 21 have fully compressed and become completely crushed and the crushing has ended (deformation amount d2), so the compressive deformation of the crash boxes 21 is not hindered.

[0032] Furthermore, even if the plate thickness of the closed cross-sectional structure of the crash box 21 is increased or the cross-sectional line length of the closed cross-sectional structure is increased so that the crash box 21 is less susceptible to compressive deformation without increasing the strength of the crash box 21, the load at which the crash box 21 buckles and begins to undergo compressive deformation will be higher, and thus compressive deformation of the crash box 21 will be hindered, just as in the case where the crash box 21 is increased in strength.

[0033] In this way, the inventors focused on the balance of strength between the crash boxes 21 and the front side members 11, and came up with the idea of ​​appropriately adjusting the plate thickness and tensile strength used for the crash boxes 21 and the front side members 11, so that the crash boxes 21 undergo sufficient compressive deformation before the front side members 11 begin to fold.

[0034] Next, the inventors considered how to optimize the balance between the yield strength of the crash boxes 21 and the front side members 11. Therefore, the inventors first decided to use the axial force expressed by the following formula (1) as an index corresponding to the yield strength of the crash boxes 21 and the front side members 11. The axial force is an index equivalent to the design allowable load (load bearing capacity) in the longitudinal direction of the member. When the design allowable load is the buckling load, the yield strength may be used instead of the tensile strength in formula (1).

[0035] Axial force (N) = cross-sectional wire length (mm) × plate thickness (mm) × tensile strength (MPa) ... (1)

[0036] Regarding the balance of the axial forces in the crash box 21 and the front side member 11, the axial force F x1 The axial force F of the front side member 11 x2 Ratio to (=F x1 / F x2 ) is the axial force ratio (F X ) was used.

[0037] Based on the axial force ratio thus specified, the researchers investigated whether it was possible to appropriately adjust the balance of axial forces in the crash boxes 21 and the front side members 11. Various combinations of plate thickness and tensile strength were then used to investigate the relationship between the axial force ratio between the crash boxes 21 and the front side members 11 and the amount of intrusion of the body parts into the passenger compartment during a frontal collision. As a result, as shown in Figure 6, it was found that when the axial force ratio is equal to or less than a predetermined value, the amount of intrusion of the body parts saturates at a certain level.

[0038] In this 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 is saturated is referred to as the critical axial force ratio. Also, Fig. 6 shows the axial force ratio F when various combinations of plate thickness and material strength shown in Table 1 below are set for the front side member 11 and the crash box 21. x 10 is a graph plotting the dash panel intrusion amount, which is the amount of intrusion of a vehicle body part into the passenger compartment, against the intrusion amount of the dash panel.

[0039] In this way, it has been discovered that the balance of axial forces in the crash boxes 21 and the front side members 11 can be achieved by adjusting the axial force ratio between the crash boxes 21 and the front side members 11 so that it is equal to or less than the critical axial force ratio.

[0040] That is, it was found that the balance of the yield strength between the crash boxes 21 and the front side members 11 can be optimized by determining the plate thicknesses and tensile strengths (or yield strengths) of the crash boxes 21 and the front side members 11 so that the axial force ratio between the crash boxes 21 and the front side members 11 is an appropriate value below the critical axial force ratio. Furthermore, in a vehicle front body structure 1 in which the axial force ratio between the crash boxes 21 and the front side members 11 is optimized, the front side members 11 can be broken and deformed after the crash boxes 21 have undergone sufficient compressive deformation during a frontal collision of the vehicle. This finding indicates that the amount of intrusion of the vehicle body parts into the passenger compartment can be kept below a predetermined level, and that the compressive deformation of the crash boxes 21 can sufficiently absorb the collision energy. The present invention was made based on the above-mentioned study results, and a specific configuration thereof will be described below.

[0041] 1 shows an example of a vehicle front structure 1 according to an embodiment of the present invention. The vehicle front structure 1 includes a pair of left and right front side members 11 that extend in the fore-and-aft direction of the vehicle body and have a closed cross-sectional structure, and crash boxes 21 that extend forward from the tip of each front side member 11 toward the vehicle body front side and also have a closed cross-sectional structure.

[0042] As shown in FIG. 7, the front side member 11 has a closed cross-sectional structure formed by joining a hat-shaped front side member inner 11a and a panel-shaped front side member outer 11b.

[0043] 7, the crash boxes 21 have a closed cross-sectional structure formed by joining a crash box inner 21a having a U-shaped cross section with a crash box outer 21b also having a U-shaped cross section. In the vehicle body front structure 1, a bumper beam 33 extending in the vehicle width direction is connected to the tip of each crash box 21, as shown in FIG.

[0044] In the vehicle body front structure 1, the axial force F of the crash box 21x1 and the axial force F of the front side member 11 x2 and the ratio F x1 / F x2 The thickness and tensile strength of the metal plates used for the front side members 11 and the crash boxes 21 are set so that the axial force F of the crash boxes 21 is equal to or less than a predetermined critical axial force ratio. x1 is given at the cross section where the cross-sectional line length of the closed cross-sectional structure of the crash box 21 is the shortest. x2 is given at the cross section where the cross-sectional line length of the closed cross-sectional structure of the front side member 11 is the smallest.

[0045] As a result, in the vehicle front body structure 1, the crash boxes 21 are sufficiently compressed and deformed during a frontal collision of the vehicle, and then the front side members 11 are bent and deformed.

[0046] As described above, in the event of a frontal collision of the vehicle, the vehicle front body structure 1 according to this embodiment can cause sufficient compressive deformation of the crash boxes 21 and then cause bending deformation of the front side members 11. This makes it possible to keep the amount of intrusion of the vehicle body parts into the passenger compartment at or below a predetermined level without increasing the length of the front side members 11 in the vehicle front-to-rear direction, and enables the crash boxes 21 to sufficiently absorb the collision energy.

[0047] Furthermore, in the present invention, as long as the axial force ratio between the crash boxes 21 and the front side members 11 is equal to or less than a predetermined critical axial force ratio, metal plates with high tensile strength and thin plate thickness can be used for the crash boxes 21 and the front side members 11. This allows the vehicle body to be made lighter and more compact without compromising collision performance, such as absorption of collision energy in the event of a frontal collision of the vehicle and intrusion of vehicle body parts into the passenger compartment, and contributes to improving the marketability of the automobile.

[0048] As shown in FIG. 6 described above, the critical axial force ratio may be a value determined based on the passenger compartment intrusion amount, which is the amount of deformation of a vehicle body part (e.g., dash lower panel 31) toward the passenger compartment during a frontal collision of the vehicle.

[0049] Furthermore, as for the lower limit of the axial force ratio, as long as it is within the range of the plate thickness and tensile strength (or yield strength) of the metal plate (e.g., steel plate) that is practically used for the front side member 11 and the crash box 21, it will not cause a large difference in the amount of intrusion into the passenger compartment.

[0050] For example, in the vehicle front body structure 1 shown in Fig. 1, the thickness of the steel plate practically used as the material for the front side member 11 and the crash box 21 is 1.2 mm to 2.3 mm, and the tensile strength is 270 MPa to 1470 MPa. x1 The steel plate that has the smallest axial force F is 1.8 mm thick and has a tensile strength of 270 MPa. x2 The steel plates with the largest axial force ratio have a thickness of 1.0 mm (outer) and 1.6 mm (inner) and a tensile strength of 1470 MPa. Therefore, the axial force ratio of 0.15, which is given by the combination of the thickness and tensile strength of these steel plates, is the lower limit. Even in the front body structure 1 with an axial force ratio of 0.15, the front side members 11 bent and deformed after the crash boxes 21 had sufficiently collapsed axially, and no increase in the amount of intrusion of the body parts into the passenger compartment was observed.

[0051] <Method for designing a vehicle body front structure> A method for designing a vehicle body front structure according to an embodiment of the present invention is for designing a vehicle body front structure 1 including a pair of left and right front side members 11 and crash boxes 21, as shown in Fig. 1. The method for designing a vehicle body front structure according to this embodiment is designed so that, in the event of a frontal collision of the vehicle, the crash boxes 21 are sufficiently compressively deformed, and then the front side members 11 are folded.

[0052] As shown in Figure 8, the design method for the vehicle body front structure according to this embodiment includes 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 calculation step S7. Furthermore, as shown in Figure 8, the design method for the vehicle body front structure according to this embodiment also includes a critical axial force ratio determination step S9 and a plate thickness and tensile strength determination step S11. Hereinafter, the above steps will be described with reference to Figures 8 and 9 for the case of designing the vehicle body front structure 1 shown in Figure 1 described above.

[0053] <Vehicle Model Acquisition Step> In vehicle model acquisition step S1, a vehicle model having a vehicle body front structure 1 is acquired, and regions for evaluating the amount of intrusion of vehicle body parts into the passenger compartment in a frontal collision of vehicle model 101 shown in FIG. 9 are set. In this embodiment, the target frontal collision of vehicle model 101 is an offset collision with a collision object 103, as shown in FIG. 9. In this embodiment, the vehicle body part for which the amount of intrusion into the passenger compartment in a frontal collision of vehicle model 101 is evaluated is dash lower panel 31 (FIG. 1). Then, as shown in FIG. 10, intrusion amount evaluation regions 31a are set in dash lower panel 31, and the amount of intrusion into the passenger compartment at each intrusion amount evaluation region 31a is calculated.

[0054] <Step for Determining Cross Section with Minimum Cross Section Line Length> In step S3 for determining cross section with minimum cross section line length, a cross section with the minimum cross section line length is determined for each of the crash boxes 21 and the front side members 11 in the vehicle body front structure 1 of the acquired vehicle model 101. In the present embodiment, for each of the crash boxes 21 and the front side members 11 of the vehicle body front structure 1 shown in Figure 7(a), the A-A cross section (Figure 7(c)) and the B-B cross section (Figure 7(b)), where the bead 23 and the folded bead 13 are provided, respectively, are determined as the cross sections with the minimum cross section line length in a cross section perpendicular to the longitudinal direction.

[0055] In the axial force ratio calculation step S5, first, the axial force F at the cross section with the minimum cross-sectional line length determined for the crash box 21 and the front side member 11 in the vehicle model 101 is calculated.x1 and F x2 is calculated by the formula (1). Then, in the axial force ratio calculation step S5, the axial force F calculated for the crash box 21 is x1 and the axial force F calculated for the front side member 11 x2 Ratio to F x1 / F x2 (=axial force ratio F x In the axial force ratio calculation step S5, the axial force F is calculated for various combinations within the range of the plate thickness and tensile strength (or yield strength) of the metal plate (for example, steel plate) that is practically used for the front side member 11 and the crash box 21. x1 , F x2 and axial force ratio F x Calculate.

[0056] <<Body Component Intrusion Amount Into Compartment Passenger Cabin>> In body component intrusion amount to compartment side calculation step S7, first, a collision analysis is performed regarding a frontal collision of vehicle model 101, which is set with various combinations of plate thickness and tensile strength of metal plates used in front side members 11 and crash boxes 21. Then, the amount of intrusion of the body component (dash lower panel 31) into the passenger compartment at intrusion amount evaluation portion 31 a set in vehicle model 101 during a frontal collision of vehicle model 101 is calculated.

[0057] The collision analysis of the vehicle model 101 in the body part passenger compartment side intrusion amount calculation step S7 may be performed by executing collision analysis software.

[0058] <Critical Axial Force Ratio Determination Step> In critical axial force ratio determination step S9, a critical axial force ratio is determined from the relationship between the axial force ratio calculated in axial force ratio calculation step S5 and the intrusion amount calculated in body component passenger compartment side intrusion amount calculation step S7. The axial force ratio used is the axial force ratio calculated in 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 boxes 21. On the other hand, the intrusion amount of the body component is a value calculated in body component passenger compartment side intrusion amount calculation step S7 by crash analysis of the vehicle model 101 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 boxes 21 are set.

[0059] In step S9 for determining the critical axial force ratio, the relationship between the axial force ratio and the penetration amount of the vehicle-body component can be expressed as shown in Fig. 6. As shown in Fig. 6, the penetration amount of the vehicle-body component decreases as the axial force ratio decreases, but when the axial force ratio is 0.69 or less, the penetration amount saturates at a value within a substantially constant range (within ±20%). From this, the critical axial force ratio can be determined to be 0.69.

[0060] <<Plate Thickness and Tensile Strength Determination Step>> In plate thickness and tensile strength determination step S11, a combination of plate thickness and tensile strength of the metal plate to be used for the front side member 11 and the crash box 21 is determined, which results in an axial force ratio equal to or less than the critical axial force ratio determined in critical axial force ratio determination step S9. In determining the combination of plate thickness and tensile strength of the metal plate to be used for the front side member 11 and the crash box 21, it is preferable to select from various combinations of plate thickness and tensile strength for which the axial force ratio has been calculated in axial force ratio calculation step S5.

[0061] As described above, according to the design method for a vehicle front structure according to an embodiment of the present invention, the vehicle front structure 1 can be designed so that, in the event of a frontal collision of the vehicle, the front side members are bent and deformed after the crash boxes 21 are sufficiently compressed. In the event of a frontal collision of a vehicle equipped with the vehicle front structure 1 designed in this manner, the amount of intrusion of the vehicle body parts into the passenger compartment can be kept below a predetermined level without increasing the length of the front side members 11 in the vehicle fore-and-aft direction, and the crash boxes 21 can sufficiently absorb the collision energy.

[0062] In the plate thickness and tensile strength determination step, a combination of 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 weight as an index.

[0063] In this case, first, in the axial force ratio calculation step S5, the weights of the front side member 11 and the crash boxes 21 are calculated in addition to the axial force ratio for various combinations of plate thickness and tensile strength of the metal plates used for the front side member 11 and the crash boxes 21.

[0064] Then, in the plate thickness and tensile strength determination step S11, from 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 members 11 and the crash boxes 21 calculated in the axial force ratio calculation step is determined. This makes it possible to design a vehicle front structure 1 that achieves a reduction in vehicle weight while ensuring collision performance.

[0065] The above explanation has been made on the case where the optimum plate thickness and tensile strength are determined using weight as an index, but the rigidity of the vehicle front structure or the collision energy absorption capacity may also be used as an index. When determining the combination of plate thickness and tensile strength of the metal plate used for the front side member 11 and the crash box 21 using the collision energy absorption capacity of the crash box 21 as an index, in the plate thickness and tensile strength determination step, an axial force ratio F that is equal to or less than the critical axial force ratio is set. xAmong the combinations of plate thickness and tensile strength, the axial force F calculated for the crash box 21 is x1 This maximizes the yield strength of the crash boxes 21 while promoting sufficient compressive deformation of the crash boxes 21, thereby maximizing the amount of collision energy absorption (the value obtained by integrating the load over the distance (stroke) in the load-stroke curve).

[0066] Furthermore, in the body part passenger compartment side intrusion amount calculation step S7 according to this embodiment, a full vehicle model that models the entire vehicle is used as the vehicle model 101. However, a partial vehicle model that models the front body structure of the vehicle and its surrounding body parts may also be used.

[0067] The method for designing a vehicle front structure according to the present embodiment may be implemented using a design device for a vehicle front structure configured by a computer, and the computer in the design device for a vehicle front structure may execute a program for implementing each step of the method for designing a vehicle front structure.

[0068] <Method of manufacturing a vehicle body front structure> A method of manufacturing a vehicle body front structure according to an embodiment of the present invention uses the above-described method of designing a vehicle body front structure according to an embodiment of the present invention to determine a combination of plate thickness and tensile strength for each of the front side members 11 and the crash boxes 21 of the vehicle body front structure 1. Then, using metal plates with the determined combination of plate thickness and tensile strength, a cross-sectional axial force F of the crash box 21 that minimizes the cross-sectional line length of the closed cross-sectional structure is determined. x1 and the axial force F of the cross section where the cross-sectional line length of the closed cross-sectional structure of the front side member 11 is the minimum. x2 and the ratio F x1 / F x2 is manufactured so that the critical axial force ratio is equal to or less than the critical axial force ratio determined in the critical axial force ratio determining step S9.

[0069] According to the manufacturing method of the vehicle front structure of the embodiment of the present invention, the vehicle front structure 1 can be manufactured so that, in the event of a frontal collision of the vehicle, the crash boxes 21 are sufficiently compressively deformed, followed by the front side members 11 being bent and deformed. Furthermore, in the event of a frontal collision of a vehicle equipped with the vehicle front structure 1 manufactured in this manner, the amount of intrusion of the vehicle body parts into the passenger compartment can be kept below a predetermined level. Furthermore, the crash boxes 21 can sufficiently absorb collision energy without increasing the length of the front side members 11 in the vehicle front-to-rear direction.

[0070] An analysis was conducted to verify the effects of the present invention, which will be described below. The analysis used a vehicle model 101 having a vehicle body front structure 1 (FIG. 1), as shown in Figure 9, and targeted an offset collision in which a collision body 103 collided with the front of the vehicle model 101 while being offset from the center of the vehicle body in the width direction. Based on the analysis of the offset collision, the optimal plate thickness and tensile strength were determined so that the crash boxes 21 in the vehicle body front structure 1 were sufficiently compressively deformed before the front side members 11 were bent.

[0071] First, a vehicle model 101 having the vehicle body front structure 1 shown in FIG. 1 was obtained, and intrusion amount evaluation parts were set to evaluate the intrusion amount of vehicle body parts into the passenger compartment during a frontal collision of the vehicle model 101. In this example, the dash lower panel 31 and the foot brake (not shown) were set as the vehicle body parts for which the intrusion amount into the passenger compartment was evaluated. Then, six intrusion amount evaluation parts 31a were set for the dash lower panel 31, as shown in FIG. 10. Furthermore, the foot brake itself was set as an intrusion amount evaluation part.

[0072] Next, for each of the crash boxes 21 and the front side members 11 in the front body structure 1 of the vehicle model 101 shown in Figure 7(a), the cross sections with the smallest cross-sectional line lengths were determined to be the A-A cross section (Figure 7(c)) and the B-B cross section (Figure 7(b)), respectively.

[0073] Next, for each of the crash box 21 and the front side member 11, the axial force F x1 and Fx2 Calculate the axial force ratio F x (=F x1 / F x2 ) Calculate the axial force ratio F x was calculated for various combinations of the thickness and tensile strength of the metal plates used in the front side member 11 and the crash box 21. The axial force of each of the crash box 21 and the front side member 11 was calculated using PRIMER 1.80 (Oasys Primer ver.1.80 for LS-DYNA) from ARUP software manufactured by JSOL Corporation. In addition, the weight of each of the crash box 21 and the front side member 11 was calculated for various combinations of the thickness and tensile strength of the metal plates used in the front side member 11 and the crash box 21.

[0074] Table 1 shows the combinations of plate thickness and tensile strength set for the crash box 21 and the front side member 11, and the axial force and axial force ratio F of each of the crash box 21 and the front side member 11 calculated for each combination. x Shows.

[0075]

[0076] Next, a collision analysis of an offset collision was performed on vehicle model 101 with various combinations of plate thickness and tensile strength set, and the amount of intrusion of the body part into the passenger compartment was calculated at the intrusion amount evaluation portion set on vehicle model 101. In the collision analysis, the initial speed of vehicle model 101 was set to 64 km / h.

[0077] Next, the axial force ratio F between the crash box 21 and the front side member 11 was calculated for each combination of plate thickness and tensile strength shown in No. 1 to No. 10 in Table 1. xThe relationship between the axial force ratio and the intrusion amount at the intrusion amount evaluation portion determined by crash analysis of vehicle model 101 was plotted. The critical axial force ratio was then determined from the plotted relationship between the axial force ratio and the intrusion amount. Figure 6 shows a graph plotting the relationship between the axial force ratio and the dash panel intrusion amount. As shown in Table 1 and Figure 6, in the region where the axial force ratio was greater than 0.69 (Nos. 2 and 4 in Table 1), the dash panel intrusion amount tended to decrease as the axial force ratio decreased. Furthermore, in the region where the axial force ratio was 0.69 or less (Nos. 1, 3, and 5 to 10 in Table 1), the dash panel intrusion amount was almost constant, within a ±20% range, regardless of the axial force ratio. Based on this, the critical axial force ratio for the dash panel intrusion amount was determined to be 0.69.

[0078] In Table 1, the crash boxes 21 and the front side members 11 having 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 However, this was a value equal to or less than the critical axial force ratio determined from Fig. 6. Therefore, the vehicle front body structure 1 having the crash boxes 21 and the front side members 11 with these combinations of plate thickness and tensile strength was within the scope of the present invention (invention example).

[0079] In contrast, the crash boxes 21 and front side members 11 with the combinations of plate thickness and tensile strength shown in No. 2 and No. 4 in Table 1 have an axial force ratio F x was a value greater than the critical axial force ratio determined from Fig. 6. Therefore, the vehicle body front structure 1 having the crash boxes 21 and the front side members 11 with these combinations of plate thickness and tensile strength was outside the scope of the present invention (Comparative Example).

[0080] Table 1 also shows the results of calculating the weight difference between the crash box 21 and the front side member 11 for each combination of plate thickness and tensile strength No. 2 to No. 10, using the weight when the plate thickness No. 1 is set for the crash box 21 and the front side member 11 as the base weight.

[0081] In the invention examples shown in No. 3 and No. 6 to No. 9, the tensile strength of the crash boxes 21 and the front side members 11 was increased and the plate thickness was reduced compared to No. 1, thereby reducing the weight of the crash boxes 21 and the front side members 11 without reducing the amount of dash panel intrusion.

[0082] FIG. 11 shows an example of a load-stroke curve obtained by a collision analysis of an offset collision of vehicle model 101 in which the combinations of plate thickness and tensile strength of the crash box 21 and the front side member 11 are No. 3 (Example 2 of the invention) and No. 4 (Comparative Example 2) shown in Table 1.

[0083] 12 is a graph summarizing the results of dash panel intrusion, foot brake intrusion, and crash energy absorption by the crash boxes (crash box energy absorption) for the invention examples (Nos. 1, 3, and 10) and the comparative example (No. 4). In FIGS. 11 and 12, the crash box EA is the amount of crash energy absorbed by the compressive deformation of the crash boxes 21 up to the distance and time at which buckling (bending deformation) of the front side member 11 begins.

[0084] When the combination of plate thickness and tensile strength of the crash box 21 and the front side member 11 was No. 1 (Example 1) shown in Table 1, the crash box EA amount was 58.5 kN m up to a distance of 668.8 mm and a time of 0.041 s, at which buckling of the front side member 11 began, as shown in Figures 11(a) and 12. In addition, the dash panel intrusion amount was 22.5 mm, and the foot brake intrusion amount was 15.0 mm.

[0085] The combination of the plate thickness and tensile strength of the crash box 21 and the front side member 11 in No. 3 (Example 2) in Table 1 has a higher axial force ratio F than No. 1 (Example 1). x The axial force ratio F of No. 3 (Example 2) was large (No. 1: 0.50, No. 3: 0.69). xis equal to the critical axial force ratio (=0.69). As shown in Figure 11(a), smooth bellows-like compressive deformation of the crash boxes 21 was promoted before the front side member 11 began to buckle (fracture). This resulted in a 6.6% improvement in crash box EA (No. 3: 62.4 kN m, No. 1: 58.5 kN m) compared to No. 1 (Example 1), a 9% reduction in dash panel intrusion (No. 3: 20.4 mm, No. 1: 22.5 mm), and a 20% reduction in foot brake intrusion (No. 3: 12.0 mm, No. 1: 15.0 mm). Furthermore, the axial force F of the crash boxes 21 of No. 3 (Example 2) was significantly lower than that of No. 1 (Example 1). x1 was 53% larger (No. 1: 304.9 kN, No. 3: 466.1 kN), and due to sufficient compressive deformation of the crush box 21, the crush box EA amount of No. 3 (invention example) was improved by 6.6% (No. 3: 62.4 kN m, No. 1: 58.5 kN m) compared to No. 1 (invention example 1).

[0086] The combination of the thickness and tensile strength of the crash box 21 and the front side member 11 of No. 4 (Comparative Example 2) in Table 1 is x11B, the distance until the onset of buckling (bending deformation) of the front side member 11 was 18% shorter than that of No. 1 (Example 1) (No. 4: 550.6 mm, No. 1: 668.8 mm), preventing sufficient progress of compressive deformation of the crash boxes 21. For this reason, in No. 4 (Comparative Example 2), the tensile strength of the steel plate used for the crash boxes 21 was 1180 MPa, and the axial force was 87% higher than that of No. 1 (Example 1) (No. 4: 568.8 kN, No. 1: 304.9 kN), but the EA amount of the crash boxes was 12.8% lower than that of No. 1 (Example 1) (No. 4: 51.0 kN m, No. 1: 58.5 kN m). Furthermore, in No. 4 (Comparative Example 1), the amount of compressive deformation of the crash boxes 21 until the front side members 11 began to fold was reduced, resulting in an increase in dash panel intrusion by 73% (No. 4: 38.9 mm, No. 1: 22.5 mm) and an increase in foot brake intrusion by 58% (No. 4: 23.7 mm, No. 1: 15.0 mm).

[0087] The combination of the plate thickness and tensile strength of the crash box 21 and the front side member 11 shown in No. 10 (Example 8) in Table 1 has a higher axial force ratio F than No. 1 (Example 1). x The tensile strength of No. 10 (Example 8) was reduced by 11% (No. 10: 20.0 mm, No. 1: 22.5 mm) and the foot brake intrusion was reduced by 18% (No. 10: 12.3 mm, No. 1: 15.0 mm) compared to No. 1 (Example 1). Because No. 10 (Example 8) used steel plates with lower tensile strength for the crash boxes 21 than No. 1 (Example 1) (No. 1: 440 MPa, No. 10: 270 MPa), the crash box EA was reduced by 57% (No. 10: 25.3 kN m, No. 1: 58.5 kN m).

[0088] In this example, the axial force ratio and the weight of the front side member 11 and the crash box 21 were calculated when the combination of plate thickness and tensile strength was changed, and the optimal combination of plate thickness and tensile strength was determined from the perspective of reducing the weight of the front body structure 1.

[0089] Table 2 shows the results of determining the axial force ratio for various combinations of plate thickness and tensile strength. Table 3 also shows the weight difference between the front side member 11 and the crash box 21 and the combined weight difference for various combinations of plate thickness and tensile strength, using the crash box 21 and front side member 11 according to No. 1 (Example 1) in Table 1 as a reference.

[0090]

[0091]

[0092] Regarding the axial force ratios in Table 2 and the total weight differences between the front side members 11 and the crash boxes 21 in Table 3, the grayed-out columns indicate combinations of plate thickness and tensile strength for which the axial force ratio was equal to or less than the critical axial force ratio (≦0.69) shown in FIG. 6 .

[0093] From the results shown in Tables 2 and 3, when the plate thickness of the front side member 11 is set to 1.0 mm for the front side member inner 11a (inner) and 1.6 mm for the front side member outer 11b (outer), and the tensile strength is set to 1470 MPa class, and when the plate thickness of the crash box 21 is set to 1.2 mm and the tensile strength is set to 1470 MPa class, the absolute value of the difference in total weight between the front side member 11 and the crash box 21 is the largest, at -2151.2 g, and it was shown that this is the optimal combination of plate thickness and tensile strength shown in Tables 2 and 3.

[0094] In addition, the combination of plate thickness and tensile strength that maximizes the amount of collision energy absorbed by the crash box 21 (crash box EA amount) is the axial force F calculated for the crash box 21 among the combinations that are equal to or less than the critical axial force ratio (≦0.69) shaded in gray in the column of Table 2. x1In this embodiment, the plate thickness of the front side member 11 can be determined as follows: front side member inner 11a (inner) 1.2 mm, front side member outer 11b (outer) 1.8 mm, and tensile strength of 1470 MPa class; and the plate thickness of the crash box 21 can be determined as follows: 1.4 mm, and tensile strength of 1470 MPa class.

[0095] As described above, the present invention has demonstrated that in the event of a frontal collision, the front side members can bend after the crash boxes are sufficiently compressed, thereby suppressing the amount of intrusion of the body parts into the passenger compartment to a predetermined level or less and ensuring sufficient collision energy absorption. Furthermore, it has been demonstrated that the weight of the vehicle body can be reduced without compromising collision performance (amount of collision energy absorption and amount of intrusion of the body parts into the passenger compartment).

[0096] According to the present invention, it is possible to provide a vehicle front body structure that can achieve compactness while satisfying predetermined collision performance in the event of a frontal collision of the vehicle, a design method for the vehicle front body structure, and a manufacturing method for the vehicle front body structure.

[0097] REFERENCE SIGNS LIST 1 Vehicle body front structure 11 Front side member 11a Front side member inner 11b Front side member outer 13 Folded bead 21 Crash box 21a Crash box inner 21b Crash box outer 23 Bead 31 Dash lower panel 31a Intrusion amount evaluation portion 33 Bumper beam 101 Vehicle model 103 Collision body

Claims

1. A vehicle front structure including a pair of left and right front side members each having a closed cross-sectional structure extending in the longitudinal direction of the vehicle body, and a crash box each having a closed cross-sectional structure extending forward from the tip of each front side member, the crash box having a closed cross-sectional structure, x1 and the axial force F of the cross section where the cross-sectional line length of the closed cross-sectional structure of the front side member is the smallest. x2 and the ratio F x1 / F x2 a plate thickness and a tensile strength of the metal plates used for the front side members and the crash boxes are set so that a critical axial force ratio is equal to or less than a predetermined critical axial force ratio, so that in the event of a frontal collision of the vehicle, the crash boxes are sufficiently compressed and deformed, and then the front side members are broken and deformed.

2. A method for designing a vehicle front structure including 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 crash boxes extending from the tips of the front side members toward the front of the vehicle body and having a closed cross-sectional structure, the method being designed so that the crash boxes are sufficiently compressed and deformed in the event of a frontal collision of the vehicle, and then the front side members are folded and deformed, the method comprising: a vehicle model acquisition step of acquiring a vehicle model having the vehicle front structure, and setting an intrusion amount evaluation portion for evaluating an intrusion amount of a vehicle part into the passenger compartment in the event of a frontal collision of the vehicle model; a cross-sectional line length minimum cross-section determination step of determining a cross-sectional line length minimum cross-section at which the cross-sectional line length is minimum for each of the crash boxes and the front side members in the acquired vehicle model; and an axial force ratio calculation step of calculating an axial force at the cross-sectional line length minimum cross-section when various plate thicknesses and tensile strengths are set for each of the crash boxes and the front side members in the vehicle model, and calculating a ratio between the axial force calculated for the crash boxes and the axial force calculated for the front side members. a body part passenger compartment side intrusion amount calculation step of performing a collision analysis of a frontal collision of the vehicle model in which the various plate thicknesses and tensile strengths are set, and calculating an intrusion amount of the body part into the passenger compartment side at the intrusion amount evaluation portion set in the vehicle model at the time of a 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 at a substantially constant value, from a relationship between an 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 body part into the passenger compartment side at the intrusion amount evaluation portion calculated by the collision analysis of the vehicle model in which various plate thicknesses and tensile strengths are set in the body part passenger compartment side intrusion amount calculation step; and 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, from among the axial force ratios calculated for combinations of various plate thicknesses and tensile strengths.

3. A design method for a front vehicle body structure as described in claim 2, wherein in the axial force ratio calculation step, the weights of the crash boxes and the front side members for the various combinations of plate thickness and tensile strength are calculated, and in the plate thickness and tensile strength determination step, a combination of plate thickness and tensile strength that results in an axial force ratio equal to or less than the critical axial force ratio is determined, among the combinations of plate thickness and tensile strength that result in a minimum total weight of the crash boxes and the front side members calculated in the axial force ratio calculation step.

4. A design method for a vehicle front structure as described in claim 2, wherein in the plate thickness and tensile strength determination step, a combination of plate thickness and tensile strength that maximizes the axial force calculated for the crash box is determined from among combinations of plate thickness and tensile strength that result in an axial force ratio that is equal to or less than the critical axial force ratio.

5. By using the design method for a vehicle front structure according to any one of claims 2 to 4, a combination of plate thickness and tensile strength is determined for each of the front side members and crash boxes of the vehicle front structure, and the axial force F of the cross section that minimizes the cross-sectional line length of the closed cross-sectional structure in the crash box is determined by using metal plates having the determined combination of plate thickness and tensile strength. x1 and the axial force F of the cross section where the cross-sectional line length of the closed cross-sectional structure of the front side member is the smallest. x2 and the ratio F x1 / F x2 a vehicle front structure including the front side member and the crash box, wherein the axial force ratio is equal to or less than the critical axial force ratio.

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