Vehicle body front structure, design method of vehicle body front structure, and manufacturing method of vehicle body 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 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 each front side member toward the front side of the vehicle body, a method for designing the vehicle front body structure, and a method for manufacturing the vehicle front body structure. [Background technology]

[0002] In recent years, electric vehicles, which emit less CO2 than gasoline-powered vehicles while driving, have become more common as an environmental measure. Increasing the battery load is an effective way to extend the driving range of electric vehicles, but this increases the vehicle weight, which increases the collision energy in the event of a vehicle collision. Therefore, electric vehicles and other vehicles are required to have a body structure that can fully absorb collision energy. Conventionally, a vehicle front structure is known that absorbs collision energy by compressing and deforming (axially crushing) the front side member in the longitudinal direction during a frontal collision (head-on collision). In a frontal collision, the front side of such a front side member is compressed and deformed in the longitudinal direction of the vehicle body, and the rear side is bent and bent so as to protrude toward the outside of the vehicle in the width direction of the vehicle body and downward in the vertical direction of the vehicle body. This absorbs the collision energy input to the vehicle front structure and prevents a deterioration in occupant protection performance (occupant injury value) caused by vehicle parts such as the dash panel intruding into the passenger compartment due to on-board objects such as the engine and motor.

[0003] In general, when comparing compressive deformation and bending deformation in vehicle body parts, compressive deformation can absorb more collision energy even with the same stroke. For this reason, for example, Patent Document 1 discloses a front vehicle body structure in which a rigidity difference is provided between the front side and rear side of a front side frame (corresponding to the front side member of the present application) and rigidity differences are provided alternately in the vehicle front-rear direction for each region. In this technology, compressive deformation starts from the front side of the front side frame, which has a relatively low rigidity, during a vehicle frontal collision, and then compresses and deforms the rear side of the front side frame, which has a relatively high rigidity. This enables the front side member to be compressed and deformed smoothly in the bellows shape over the entire longitudinal direction, so that collision energy can be sufficiently absorbed. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2021-66202 Summary of the Invention [Problem to be solved by the invention]

[0005] In some vehicle front body structures, crash boxes are provided in front of the front side members, and in the event of a frontal collision, the crash boxes are compressively deformed in the fore-and-aft direction of the vehicle, making it possible to further absorb collision energy. In such a front body structure equipped with a front side member and a crash box, a steel plate with a relatively low strength of about 440 MPa has been used for the crash box in order to allow sufficient compressive deformation. Furthermore, a steel plate with a relatively high strength of tensile strength of 780 MPa or more has often been used for the front side member. In response to the demand for increased absorption of collision energy in the event of a frontal collision of an electric vehicle, the use of a steel plate with a tensile strength of 980 MPa or more (ultra-high tensile material) for the crash box has been considered.

[0006] In such cases, it is important to compressively deform the front sides of the crash boxes and the front side members to obtain the required amount of collision energy absorption. However, simply increasing the strength of the steel plate used for the crash boxes can result in bending deformation in the front side members before sufficient compressive deformation occurs in the crash boxes. In such cases, there is a problem that the crash boxes cannot fully absorb the collision energy, and the vehicle body deformation amount and other required collision performance cannot be met. Therefore, in order to increase the strength of the crash box, it is important that the crash box undergoes sufficient compressive deformation before the front side member undergoes bending deformation. To achieve this, it is considered necessary to appropriately determine the thickness and material strength of the metal plates, such as steel plates, used for the crash box and the front side member.

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

[0008] In addition, electric vehicles equipped with batteries are required to have a compact front body structure in order to ensure a certain level of collision energy absorption performance as well as to secure space for the battery. To achieve a compact front body structure, it is required that the crash boxes and front side members are able to sufficiently absorb collision 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 vehicle front collision by the amount of the lowered rigidity of the front side member. Therefore, in order to ensure the amount of collision energy absorption by compressive deformation during a front collision, it was necessary to lengthen the front side member so that the amount of compressive deformation would increase. Therefore, the technology of Patent Document 1 could not simultaneously ensure the collision energy absorption performance and compactness in the vehicle body front structure of an electric vehicle.

[0010] The present invention has been made to solve the above-mentioned problems, and has an object to provide a vehicle body front structure that is capable of achieving compactness while satisfying specified collision performance in the event of a vehicle front collision, a method for designing a vehicle body front structure, and a method for manufacturing a vehicle body front structure. [Means for solving the problem]

[0011] (1) A vehicle front structure according to the present invention includes a pair of left and right front side members each extending in a fore-and-aft direction of a vehicle body and having a closed cross-sectional structure, and crash boxes each extending from a tip of each front side member toward a front side of the vehicle body and having a closed cross-sectional structure, 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 the smallest 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 plate thickness and tensile strength of the metal plates used for the front side member and the crash boxes are set so that the 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 member is broken and deformed.

[0012] (2) A method for designing a vehicle front structure 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 extending in a vehicle longitudinal direction, and crash boxes each having a closed cross-sectional structure extending from a tip of each front side member towards the front side of the vehicle, the method being designed 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, a vehicle model acquisition step of acquiring a vehicle model having the vehicle body front structure and setting an intrusion amount evaluation portion for evaluating an intrusion amount of a vehicle body part into a passenger compartment at the time 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 in which a cross-sectional line length is minimum for each of the crash box and the front side member in the acquired vehicle model; 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 vehicle body part passenger compartment side intrusion amount 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 vehicle body part into a passenger compartment side at the intrusion amount evaluation portion set in the vehicle model when the vehicle model hits the frontal collision; a critical axial force ratio determination step of determining a critical axial force ratio at which the intrusion amount is saturated at a substantially constant value, based on a 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 crash 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; The method is characterized by including a plate thickness and tensile strength determination step for 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 axial force ratios calculated for various combinations of plate thickness and tensile strength.

[0013] (3) In the above (2), In the axial force ratio calculation step, weights of the crash boxes and the front side members for various combinations of plate thicknesses and tensile strengths are calculated, The plate thickness and tensile strength determination step is characterized in that a 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, and that 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.

[0014] (4) In the above (2), The plate thickness and tensile strength determination step is characterized in that 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.

[0015] (5) A method for manufacturing a vehicle body front structure according to the present invention includes the steps of: 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 any one of the design methods of the vehicle body front structure according to any one of (2) to (4) above; Using the metal plate having the determined combination of plate thickness and tensile strength, the axial force F of the cross section in which the cross-sectional line length of the closed cross-sectional structure in the crash box is minimized is calculated. 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 and manufacturing a vehicle front body structure including the front side member and the crash box, in which the axial force ratio is equal to or less than the critical axial force ratio. Effect of the Invention

[0016] According to the present invention, in the event of a frontal collision of the vehicle, the front side member can bend and deformed after the crash box has 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 body can 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 the amount of intrusion of body parts into the passenger compartment, thereby contributing to improving the marketability of the automobile. [Brief description of the drawings]

[0017] [Figure 1] 1 is a diagram showing an example of a configuration of a vehicle body front structure according to an embodiment of the present invention; [Diagram 2] 1A to 1C are diagrams illustrating compressive deformation of a front side member of a vehicle body front structure and bending deformation of a crash box during a frontal collision of the vehicle. [Diagram 3] 4 is a graph that shows a schematic load-stroke curve of a vehicle body front structure during a frontal collision of the vehicle. [Figure 4] 1 is a graph showing a schematic diagram of changes over time in loads acting on a crash box and a front side member during a frontal collision of a vehicle having a vehicle front structure in which only the crash box is reinforced. [Diagram 5] 1 is a graph showing a schematic representation of the change over time in loads generated in a crash box and a front side member during a frontal collision of a vehicle having a front body structure in which the front side members are made stronger in accordance with the increased strength of the crash boxes. [Figure 6] FIG. 13 is a plot showing the relationship between the axial force ratio, which is the ratio between the axial force of the crash box and the axial force of the front side member, and the dash panel intrusion amount, which is the amount of intrusion of the dash lower panel into the passenger compartment due to deformation thereof during a frontal collision of the vehicle, and a diagram illustrating the critical axial force ratio, which is the axial force ratio at which the amount of intrusion of the dash panel saturates at a substantially constant value in the plot. [Figure 7]1 is a diagram showing an example of a cross-sectional shape perpendicular to the vehicle front-rear direction of a front side member and a crash box of a vehicle front body structure according to an embodiment of the present invention. FIG. [Figure 8] 1 is a flowchart showing a process flow in a design method for a vehicle body front structure according to an embodiment of the present invention. [Figure 9] 1 is a diagram for explaining a collision analysis of an offset collision as a frontal collision of a vehicle in an embodiment. FIG. [Figure 10] 1 is a diagram showing positions at which the dash panel intrusion amount, which is the amount of deformation of the dash lower panel toward the passenger compartment due to a frontal collision of the vehicle, was measured in the embodiment. FIG. [Figure 11] 1 shows load-stroke curves of a vehicle front structure obtained by crash analysis during a vehicle frontal collision in the examples ((a) example of the invention (No. 3), (b) comparative example (No. 4)). [Figure 12] This is a graph showing the dash panel intrusion amount, foot brake intrusion amount, and collision 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 collision analysis at the time of a frontal collision of a vehicle in the examples. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[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 front-rear direction, the width direction, and the up-down direction of the vehicle body.

[0019] <How the present invention was arrived at> FIG. 2 is a schematic diagram showing a vehicle body front structure 1 including a front side member 11 and a crash box 21 to which the present invention is directed.

[0020] As shown in FIG. 1, the front side member 11 extends in the vehicle body longitudinal direction, and its end on the rear side of the vehicle body is connected to a dash lower panel 31 that separates the cabin (passenger compartment) of the vehicle from the vehicle body front structure 1. In a frontal collision of the vehicle, the front side member 11 undergoes compressive deformation (axial crushing) in the longitudinal direction (vehicle body longitudinal direction) and also undergoes folding deformation (bending crushing) in a direction perpendicular to the longitudinal direction (outside in the vehicle body lateral direction or downward in the vehicle body vertical direction). In addition, since the front side member 11 is provided with a folding bead 13 as shown in FIG. 2, folding deformation is induced starting from the folding bead 13 in a frontal collision of the vehicle. In addition, 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 boxes 21 extend from the tips of the front side members 11 toward the front side of the vehicle body, and undergo bellows-like compressive deformation in the longitudinal direction (axial crush) during a frontal collision of the vehicle. In order to induce bellows-like compressive deformation in the longitudinal direction of the vehicle body (longitudinal direction) 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, for example. Also, as shown in Fig. 7(c), the crash boxes 21 have a closed cross-sectional structure perpendicular to the longitudinal direction.

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

[0023] In a vehicle front body structure 1 in which a folding bead 13 is provided to a front side member 11 as in Patent Document 1, the folding bead 13 causes folding deformation and significantly reduces the load, thereby inducing smooth compressive deformation in the front side member 11. However, in order to obtain the same amount of collision energy absorption as a vehicle front structure 1 (a conventional vehicle front structure) in which the front side member 11 does not have the foldable bead 13, it is necessary to increase the overall length of the front side member 11 and the crash box 21 and increase the stroke (amount of deformation) until the front side member 11 is broken (x in FIG. 3). A →x A ').

[0024] In contrast, from the viewpoint of weight reduction, by using a high-strength metal plate (e.g., a high-tensile steel plate) for the crash box 21, the strength at the time of a frontal collision (F → F' in FIG. 3) and the subsequent load are also improved. This is considered to reduce the stroke (amount of deformation) required to obtain the same amount of collision energy absorption as the conventional 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 breaks 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] Therefore, the inventors have conducted extensive research into this issue, and have come up with the idea of ​​optimizing the balance of the axial forces of the crash boxes 21 and the front side members 11 so as to increase the strength of the front side members 11 in accordance with the increase in strength of the crash boxes 21. The axial force means 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 the formula (1) described below.

[0027] It was then discovered that by optimizing the balance of axial forces between the crash boxes 21 and the front side members 11 and increasing the strength of the crash boxes 21, it is possible to promote smooth bellows-like compressive deformation after the start of buckling deformation of the crash boxes 21. As a result, it was found that the crash boxes 21 can adequately absorb the collision energy, and furthermore, the intrusion of the vehicle body parts into the passenger compartment can be kept below a predetermined level, improving the safety of the cabin. The optimization of the balance of the 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 structure 1 including the front side member 11 and the 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 show a schematic representation of the change in the load received by the front side member 11 relative to the deformation (stroke) of the vehicle front structure 1 in the vehicle longitudinal direction.

[0029] 4, when a collision load is applied to the vehicle body front structure 1 during a frontal collision of the vehicle, the crash boxes 21 first undergo slight elastic (compressive) deformation in the fore-and-aft direction of the vehicle. Then, the crash boxes 21 buckle under load F1 and begin to undergo compressive deformation (axial crushing) (deformation amount d1), and then become completely crushed and the crushing ends (deformation amount d2). Next, the front side member 11 undergoes transitional elastic deformation, and when it receives the transmitted load F3, it buckles and begins to undergo bending deformation (deformation amount d3), and then reaches its yield strength (maximum load) and completes the bending deformation (deformation amount d4).

[0030] Here, if only the crash box 21 is made of high-tensile steel sheet 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. Thus, 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 by which the deformation amount of the crash box 21 due to the compressive deformation becomes smaller (= d2 - d3'). As a result, the crash box 21 cannot sufficiently absorb the collision energy.

[0031] On the other hand, when the front side member 11 is also made of high-tensile steel sheet 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, since the crash box 21 can be sufficiently compressed and deformed until it is completely crushed and the crushing is completed (deformation amount d2), and then the bending deformation of the front side member 11 can be started (deformation amount d3 > d2), the compressive deformation of the crash box 21 is not inhibited.

[0032] Also, even if the thickness of the closed cross-section structure of the crash box 21 is increased or the sectional line length of the closed cross-section structure is increased so that the crash box 21 is 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 crash box 21 is made of high-tensile steel sheet, the compressive deformation of the crash box 21 is inhibited.

[0033] Thus, 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 have considered how to optimize the balance of the strength between 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 strength of the crash box 21 and the front side member 11. The axial force is an index corresponding to the design allowable load (withstand load) in the longitudinal direction of the member. Note that, when the design allowable load is the buckling load, the yield strength may be used instead of the tensile strength in formula (1). Axial force (N) = cross-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 F x1 Axial force F of front side member 11 x2 Ratio to (=F x1 / F x2 ) is the axial force ratio (F X ) was used.

[0036] Based on the axial force ratio thus specified, a study was conducted to determine whether the balance of axial forces in the crash boxes 21 and the front side members 11 could be appropriately adjusted. Thus, the plate thicknesses and tensile strengths of the crash boxes 21 and the front side members 11 were changed to various combinations, and an investigation was conducted into 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 of the vehicle. As a result, as shown in Fig. 6, it was found that when the axial force ratio is equal to or less than a certain value, the intrusion amount of the vehicle body part becomes saturated at a certain level.

[0037] In this application, the axial force ratio between the crash box 21 and the front side member 11 at the point where 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 described later are set for the front side member 11 and the crash box 21. x 13 is a graph plotting the dash panel intrusion amount, which is the intrusion amount of a vehicle body part into the passenger compartment, against the intrusion amount of the vehicle body part into the passenger compartment.

[0038] In this way, it has been found that the balance of the 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 to be equal to or less than the critical axial force ratio.

[0039] That is, it was found that the balance of the strength of the crash box 21 and the front side member 11 can be optimized by determining the plate thickness and tensile strength (or yield strength) of the crash box 21 and the front side member 11 so that the axial force ratio between the crash box 21 and the front side member 11 is an appropriate value equal to or less than the critical axial force ratio. 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, the front side member 11 can be broken and deformed after the crash box 21 is sufficiently compressively deformed during a frontal collision of the vehicle. This has led to the finding that the amount of intrusion of the vehicle body parts into the passenger compartment can be suppressed to a predetermined level or less, and the collision energy can be sufficiently absorbed by the compressive deformation of the crash box 21. The present invention has been made based on the above-mentioned investigation results, and the specific configuration thereof will be described below.

[0040] <Front body structure> FIG. 1 shows, as an example, a vehicle body front structure 1 according to an embodiment of the present invention. The vehicle body front structure 1 comprises a pair of left and right front side members 11 extending in the fore-and-aft direction of the vehicle body and having a closed cross-sectional structure, and crash boxes 21 extending from the tip of each front side member 11 towards the front side of the vehicle body and having a closed cross-sectional structure.

[0041] 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.

[0042] The crash box 21 has a closed cross-sectional structure formed by joining a crash box inner 21a having a U-shaped cross section and a crash box outer 21b having a U-shaped cross section, as shown in Fig. 7. In the vehicle body front structure 1, a bumper beam 33 extending in the vehicle body width direction is connected to the tip of each crash box 21, as shown in Fig. 1.

[0043] In the vehicle body front structure 1, the axial force F x1 and the axial force F of the front side member 11 x2 and the ratio F x1 / F x2 The sheet thickness and tensile strength of the metal sheets used for the front side members 11 and the crash boxes 21 are set so that the axial force ratio is equal to or less than a predetermined critical axial force ratio. Here, the axial force F of the crash box 21 x1 is given at the cross section where the cross-sectional line length of the closed cross-sectional structure in the crash box 21 is the smallest. In addition, the axial force F of the front side member 11 x2 is given at a cross section where the cross-sectional line length of the closed cross-sectional structure of the front side member 11 is minimum.

[0044] 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 broken and deformed.

[0045] Thus, according to the vehicle body front structure 1 according to the present embodiment, in the event of a frontal collision of the vehicle, the crash boxes 21 can be sufficiently compressed and then the front side members 11 can be folded and deformed. This makes it possible to keep the intrusion amount 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-rear direction, and allows the crash boxes 21 to sufficiently absorb the collision energy.

[0046] Furthermore, in the present invention, if 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 makes it possible to reduce the weight and size of the vehicle body without compromising collision performance, such as absorption of collision energy during a frontal collision of the vehicle and intrusion of vehicle body parts into the passenger compartment, thereby contributing to improving the marketability of the automobile.

[0047] 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.

[0048] In addition, as for the lower limit of the axial force ratio, so 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) practically used for the front side member 11 and the crash box 21, it will not cause a significant difference in the amount of intrusion into the passenger compartment.

[0049] For example, in the vehicle body front 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 class to 1470 MPa class. In this case, the axial force F 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 for which this ratio is maximum have a plate thickness of 1.0 mm (outer) and 1.6 mm (inner), and a tensile strength of 1470 MPa class. Therefore, the axial force ratio of 0.15 given by the combination of the plate 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 member 11 broke and deformed after the crash box 21 had sufficiently collapsed axially, and no increase in the amount of intrusion of the body parts into the passenger compartment was observed.

[0050] <Design method for vehicle front structure> A design method for 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 design method for a vehicle body front structure according to the present embodiment is for designing the front side members 11 to bend and deform after the crash boxes 21 are sufficiently compressed and deformed during a frontal collision of the vehicle.

[0051] The design method for the vehicle body front structure according to the present 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, as shown in Fig. 8. Furthermore, the design method for the vehicle body front structure according to the present embodiment includes a critical axial force ratio determination step S9, and a plate thickness and tensile strength determination step S11, as shown in Fig. 8. Hereinafter, the above steps will be described with reference to FIGS. 8 and 9 with respect to the case of designing the vehicle body front structure 1 shown in FIG.

[0052] <Vehicle model acquisition steps> In the vehicle model acquisition step S1, a vehicle model having a vehicle body front structure 1 is acquired, and a portion of the vehicle model 101 shown in FIG. 9 for evaluating the intrusion amount of a vehicle body part into the passenger compartment during a frontal collision is set. In this embodiment, as a frontal collision of the vehicle model 101, an offset collision with a collision object 103 as shown in FIG. In this embodiment, the vehicle body part for evaluating the amount of intrusion into the passenger compartment in a frontal collision of vehicle model 101 is dash lower panel 31 (FIG. 1). Then, as shown in FIG. 10, intrusion amount evaluation parts 31a are set in dash lower panel 31, and the amount of intrusion into the passenger compartment at each intrusion amount evaluation part 31a is calculated.

[0053] <Step for determining minimum section line length> In the cross-section line length minimum cross-section determination step S3, a cross-section with a 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 this embodiment, for each of the crash boxes 21 and front side members 11 of the vehicle body front structure 1 shown in Figure 7(a), the cross sections AA (Figure 7(c)) and BB (Figure 7(b)) in which the bead 23 and the folded bead 13 are provided, respectively, were determined as the cross sections in which the cross-sectional line lengths in cross sections perpendicular to the longitudinal direction are shortest.

[0054] <Axial force ratio calculation step> In the axial force ratio calculation step S5, first, the axial force F at the cross section with the minimum cross section 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). 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 Here, 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 (e.g., 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.

[0055] <Steps for calculating the amount of intrusion of body parts into the passenger compartment> In the body part passenger compartment side intrusion amount calculation step S7, first, a collision analysis is performed regarding a frontal collision of the vehicle model 101 in which various combinations of plate thickness and tensile strength of the metal plates used for the front side members 11 and the crash boxes 21 are set. Then, the intrusion amount of the body part (dash lower panel 31) into the passenger compartment at the intrusion amount evaluation portion 31a set in the vehicle model 101 at the time of the frontal collision of the vehicle model 101 is calculated.

[0056] 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.

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

[0058] In the critical axial force ratio determination step S9, the relationship between the axial force ratio and the intrusion amount of the vehicle body part can be expressed as shown in FIG. As shown in Fig. 6, the penetration amount of the car body part 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 certain range (within ±20%). From this, it can be determined that the critical axial force ratio is 0.69.

[0059] <Step to determine plate thickness and tensile strength> In the plate thickness and tensile strength determining step S11, a combination of plate thickness and tensile strength of the metal plate 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 determining step S9 is determined. When determining the combination of plate thickness and tensile strength of the metal plates to be used for the front side member 11 and the crash box 21, it is advisable to select from various combinations of plate thickness and tensile strength for which the axial force ratio is calculated in the axial force ratio calculation step S5.

[0060] As described above, according to the design method for a vehicle body front structure according to the embodiment of the present invention, the vehicle body front structure 1 can be designed so that, in the event of a frontal collision of the vehicle, the crash boxes 21 are sufficiently compressed and deformed, and then the front side members are folded and deformed. Then, in the event of a frontal collision of a vehicle equipped with the vehicle body front structure 1 designed in this way, the intrusion amount 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-aft direction, and the collision energy can be sufficiently absorbed by the crash boxes 21.

[0061] In the plate thickness and tensile strength determination step, any 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 determining step, for example, the plate thickness and tensile strength may be determined using weight as an index, as described below.

[0062] In this case, first, in the axial force ratio calculation step S5, for various combinations of 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] Then, in the plate thickness and tensile strength determination step S11, the 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, and that 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. This makes it possible to design a vehicle body front structure 1 that achieves weight reduction of the vehicle body while ensuring collision performance.

[0064] In the above description, the optimum plate thickness and tensile strength are determined using weight as an index, but the rigidity of the vehicle front body structure or the amount of collision energy absorption may also be used as an index. When determining a combination of the 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 amount 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 smaller than the critical axial force ratio is determined. x Among the combinations of plate thickness and tensile strength, the axial force F calculated for the crash box 21 is x1 Determine the combination of plate thickness and tensile strength that maximizes This promotes sufficient compressive deformation of the crash boxes 21, while maximizing the strength of the crash boxes 21 and maximizing the amount of collision energy absorption (the value obtained by integrating the load over the distance (stroke) in the load-stroke curve).

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

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

[0067] <Manufacturing method for vehicle body front structure> The manufacturing method of the vehicle body front structure according to the embodiment of the present invention determines 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 by the design method of the vehicle body front structure according to the embodiment of the present invention described above. Then, using metal plates having the determined combination of plate thickness and tensile strength, 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 21 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 smallest. x2 and the ratio F x1 / F x2 The vehicle body front structure 1 including the front side members 11 and the crash boxes 21 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.

[0068] According to the manufacturing method of the vehicle body front structure according to the embodiment of the present invention, the vehicle body 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 and then the front side members 11 are bent and deformed. In the event of a frontal collision of a vehicle equipped with the vehicle body 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 energy can be sufficiently absorbed by the crash boxes 21 without increasing the length of the front side members 11 in the vehicle front-rear direction. EXAMPLES

[0069] An analysis was carried out to verify the effects of the present invention, which will be described below. In the analysis, as shown in Fig. 9, a vehicle model 101 having a vehicle body front structure 1 (Fig. 1) was used, and an offset collision was targeted in which a collision body 103 collided with the front of the vehicle model 101 while being shifted from the center of the vehicle body in the 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 for causing the front side member 11 to bend after the crash boxes 21 in the vehicle body front structure 1 were 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 portion was set to evaluate the intrusion amount of the vehicle body parts into the passenger compartment during a frontal collision of the vehicle model 101. In this embodiment, the dash lower panel 31 and the foot brake (not shown) are vehicle body parts for evaluating the amount of intrusion into the passenger compartment. Six intrusion amount evaluation sites 31a are set for the dash lower panel 31, as shown in Fig. 10. The foot brake itself is set as the intrusion amount evaluation site.

[0071] Next, for each of the crash boxes 21 and 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 length were determined to be cross section AA (Figure 7(c)) and cross section BB (Figure 7(b)), respectively.

[0072] Next, for each of the crash box 21 and the front side member 11, the axial force F x1 and F x2 Calculate the axial force ratio F x (=F x1 / F x2 ) is calculated. Here, the axial force ratio F x The calculations were performed 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. The axial force of each of the crash box 21 and the front side member 11 was calculated using PRIMER 1.80 of ARUP software manufactured by JSOL Corporation. In addition, the weights of the crash boxes 21 and the front side members 11 were calculated for various combinations of the plate thickness and tensile strength of the metal plates used for the front side members 11 and the crash boxes 21.

[0073] 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 the crash box 21 and the front side member 11 calculated for each combination. x Shows. [Table 1]

[0074] Next, a crash analysis of an offset crash was performed on the vehicle model 101 with various combinations of plate thickness and tensile strength set, and the amount of intrusion of the body parts into the passenger compartment was calculated at the intrusion amount evaluation portion set on the vehicle model 101. In the crash analysis, the initial speed 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 was calculated for each combination of plate thickness and tensile strength shown in No. 1 to No. 10 in Table 1. x A relationship between the axial force ratio and the intrusion amount at the intrusion amount evaluation portion obtained by the collision analysis of the vehicle model 101 was plotted. Then, the critical axial force ratio was 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 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. 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 was within the range of ±20% regardless of the axial force ratio, and was 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 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 equal to or less than the critical axial force ratio determined from Fig. 6. Therefore, the vehicle front 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).

[0077] In contrast, the crash box 21 and the front side member 11 having 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 However, this was a value larger 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).

[0078] 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, based on the weight when the plate thickness No. 1 is 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, the tensile strength of the crash box 21 and the front side member 11 is made higher and the plate thickness is made thinner than in No. 1, thereby reducing the weight of the crash box 21 and the front side member 11 without reducing the amount of penetration of the dash panel.

[0080] FIG. 11 shows an example of a load-stroke curve obtained by a collision analysis of an offset collision of a 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 (invention example 2) and No. 4 (comparison example 2) shown in Table 1.

[0081] Furthermore, FIG. 12 shows a graph summarizing the results of dash panel intrusion amount, foot brake intrusion amount, and collision energy absorption amount by the crash box (crash box EA amount) for the invention examples (No. 1, No. 3, and No. 10) and the comparative example (No. 4). 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 begins.

[0082] 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 EA amount of the crash box 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, was 58.5 kN m, 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.

[0083] The combination of the plate thickness and tensile strength of the crash box 21 and the front side member 11 of 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). x is equal to the critical axial force ratio (=0.69). As shown in Fig. 11(a), smooth bellows-like compressive deformation of the crash boxes 21 was promoted before the front side member 11 started to buckle (fracture deformation). As a result, compared to No. 1 (invention example 1), the crash box EA amount was improved by 6.6% (No. 3: 62.4 kN m, No. 1: 58.5 kN m), the dash panel intrusion amount was reduced by 9% (No. 3: 20.4 mm, No. 1: 22.5 mm), and the foot brake intrusion amount was reduced by 20% (No. 3: 12.0 mm, No. 1: 15.0 mm). Furthermore, the axial force F of the crash box 21 of No. 3 (Example 2) is smaller than that of No. 1 (Example 1). x1was 53% larger (No. 1: 304.9 kN, No. 3: 466.1 kN), and due to sufficient compressive deformation of the crash box 21, the EA amount of the crash box 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).

[0084] The combination of the plate 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 the axial force ratio F x was greater than the critical axial force ratio of 0.69 (=0.84), and as shown in FIG. 11(b), the distance until the start of buckling (fracture deformation) of the front side member 11 was 18% shorter than that of No. 1 (Example 1 of the invention) (No. 4: 550.6 mm, No. 1: 668.8 mm), and the compressive deformation of the crash box 21 could not be advanced sufficiently. For this reason, in No. 4 (Comparative Example 2), although the tensile strength of the steel plate used for the crash box 21 was set to 1180 MPa and the axial force was increased by 87% compared to No. 1 (Invention Example 1) (No. 4: 568.8 kN, No. 1: 304.9 kN), the EA amount of the crash box was reduced 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), the amount of compressive deformation of the crash box 21 until the front side member 11 began to break was reduced, resulting in an increase in the dash panel intrusion amount of 73% (No. 4: 38.9 mm, No. 1: 22.5 mm) and an increase in the foot brake intrusion amount of 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 (Example 8) in Table 1 has a lower axial force ratio F than No. 1 (Example 1). x(No.1: 0.50, No.10: 0.19), which promoted smooth bellows-like axial crushing of the crash boxes before the front side member 11 started to buckle (fracture deformation). As a result, compared to No.1 (Invention Example 1), No.10 (Invention Example 8) reduced the dash panel intrusion amount by 11% (No.10: 20.0 mm, No.1: 22.5 mm) and the foot brake intrusion amount by 18% (No.10: 12.3 mm, No.1: 15.0 mm). Note that No.10 (Invention Example 8) used a steel plate with a lower tensile strength for the crash boxes 21 than No.1 (Invention Example 1) (No.1: 440 MPa class, No.10: 270 MPa class), and therefore reduced the crash box EA amount by 57% (No.10: 25.3 kN m, No.1: 58.5 kN m).

[0086] In this embodiment, the axial force ratio and the weight of the front side member 11 and the crash box 21 were further 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 viewpoint of reducing the weight of the front body structure 1.

[0087] Table 2 shows the results of determining the axial force ratio for various combinations of plate thickness and tensile strength. Table 3 also shows, for various combinations of plate thickness and tensile strength, the weight difference between the front side member 11 and the crash box 21, and the combined weight difference, based on the crash box 21 and front side member 11 relating to No. 1 (Example 1 of the invention) in Table 1 described above.

[0088] [Table 2]

[0089] [Table 3]

[0090] In the axial force ratios in Table 2 and the total weight difference between the front side member 11 and the crash box 21 in Table 3, the columns shaded in gray 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.

[0091] 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), with a tensile strength of 1470 MPa, and when the plate thickness of the crash box 21 is set to 1.2 mm and with a tensile strength of 1470 MPa, 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, which is the optimal combination of plate thickness and tensile strength shown in Tables 2 and 3.

[0092] 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. x1 In this embodiment, the plate thickness of the front side member 11 is set to 1.2 mm for the front side member inner 11a (inner), 1.8 mm for the front side member outer 11b (outer), and the tensile strength is set to 1470 MPa class, and the plate thickness of the crash box 21 is set to 1.4 mm and the tensile strength is set to 1470 MPa class.

[0093] As described above, according to the present invention, in the event of a frontal collision of a vehicle, it is possible to cause the front side members to fold after the crash boxes are sufficiently compressed, and it has been shown that the intrusion amount of the body parts into the passenger compartment can be kept below a predetermined level, and the amount of collision energy absorption can be ensured. Furthermore, it has been demonstrated that the weight of the body can be reduced without compromising the collision performance (the amount of collision energy absorption and the amount of intrusion of the body parts into the passenger compartment). [Explanation of symbols]

[0094] 1. Front body structure 11 Front side member 11a Front side member inner 11b Front side member outer 13 Broken bead 21 Crash Box 21a Crash Box Inner 21b Crash Box Outer 23 Bead 31 Dash lower panel 31a Penetration evaluation area 33 Bumper beam 101 Vehicle Model 103 Collider

Claims

1. A vehicle body front structure including a pair of left and right front side members each extending in a vehicle body longitudinal direction and having a closed cross-sectional structure, and crash boxes each extending from a tip of each front side member toward a vehicle body front side and having a closed cross-sectional structure, 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 the smallest x1 and the axial force F of the cross section at which 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 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 from a tip of each front side member toward a vehicle body front side, the vehicle front structure being designed so that the crash boxes are sufficiently compressed and deformed and then the front side members are folded and deformed in the event of a frontal collision of the vehicle, the method comprising: a vehicle model acquisition step of acquiring a vehicle model having the vehicle body front structure and setting an intrusion amount evaluation portion for evaluating an intrusion amount of a vehicle body part into a passenger compartment at the time 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 in which a cross-sectional line length is minimum for each of the crash box and the front side member in the acquired vehicle model; 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 vehicle body part passenger compartment side intrusion amount 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 vehicle body part into a passenger compartment side at the intrusion amount evaluation portion set in the vehicle model when the vehicle model hits the frontal collision; a critical axial force ratio determination step of determining a critical axial force ratio at which the intrusion amount is saturated at a substantially constant value, based on a 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 crash 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 results in an axial force ratio 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 various combinations of plate thickness and tensile strength.

3. In the axial force ratio calculation step, weights of the crash boxes and the front side members for various combinations of plate thicknesses and tensile strengths are calculated, 3. The design method for a vehicle front structure according to claim 2, wherein, in the plate thickness and tensile strength determination step, a combination of plate thickness and tensile strength that minimizes the total weight of the crash boxes and the front side members calculated in the axial force ratio calculation step 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.

4. 3. The design method for a vehicle front body structure according to claim 2, characterized in that, 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. A method for designing a vehicle body front structure according to any one of claims 2 to 4, 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, Using the metal plate having the determined combination of plate thickness and tensile strength, the axial force F of the cross section in which the cross-sectional line length of the closed cross-sectional structure in the crash box is minimized is calculated. x1 and the axial force F of the cross section at which 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 of the front side member and the crash box is equal to or less than the critical axial force ratio.

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