JP7385121B2 - Design method for vehicle body structural components - Google Patents

Description

The present invention relates to a method for designing a vehicle body structural member.

Conventionally, a hollow member made of a steel plate and having a predetermined cross-sectional shape has been used as a vehicle body structural member. These vehicle body structural members are required to be lightweight and have sufficient bending strength when subjected to impact such as a collision. For this reason, in recent years, high-strength steel plates having high strength are sometimes used as materials.

Patent Document 1 below discloses that in order to realize a member for vehicle body structure that is lighter and has higher axial compressive bending strength, the surface that undergoes compressive deformation is curved outward in a convex manner. The technology is described.

Japanese Patent Application Publication No. 2005-186777

The technology described in Patent Document 1 merely curves the surface of the member that undergoes compressive deformation outward in a convex manner, and the cross-sectional shape including the plane continuous with the curved surface is the same as that of the entire member. The effect on bending strength is not considered. Furthermore, thinning and increasing the strength of materials used for vehicle body structural members can reduce the elastic buckling stress of the members. For this reason, elastic buckling may occur before the yield stress of the material is reached at the portions subjected to the bending load, particularly in the flat portions, which may reduce the bending strength.
However, conventional techniques, including the technique described in Patent Document 1, do not set the optimal cross-sectional shape of the member to obtain high bending strength.

Therefore, the present invention has been made in view of the above problems, and an object of the present invention is to provide a new and improved method of designing a vehicle body structural member that can exhibit high bending strength. It's about doing.

（２）上記（１）に記載の車体構造部材の設計方法では、前記第一影響度αを、前記車体構造部材の降伏応力σ_ｙから引張応力σ_ＴＳまでの加工硬化度合いを示すパラメータｃの関数ｇ（ｃ）で算出し、前記第二影響度βを、前記車体構造部材の弾性座屈応力σ_{ｃｒ（Ｒ）}と降伏応力σ_ｙとの比率の関数ｆ（σ_{ｃｒ（Ｒ）}／σ_ｙ）で算出し、前記モーメント効率Ｍ_ｍａｘ／Ｍ_ｙを下記の（３）式により算出してもよい。

（３）上記（２）に記載の車体構造部材の設計方法では、前記関数ｇ（ｃ）がＶｏｃｅの式に基づき導出されてもよい。
（４）上記（１）～（３）のいずれか一項に記載の車体構造部材の設計方法では、前記断面のうち前記（１）式と（２）式を満たす断面が、前記長手方向の全長の５０％以上に存在するように前記車体構造部材を設計してもよい。
Specific embodiments of the present invention are as follows.
(1) A first aspect of the present invention is a method for designing a vehicle body structural member extending in the longitudinal direction, which comprises determining the section modulus Z, maximum bending moment M _max , and yield moment M in a cross section perpendicular to the longitudinal direction. moment efficiency M _max /M _y which is the ratio to _y , a first degree of influence α indicating the degree of influence on the maximum bending moment M _max due to work hardening at the time of collision, and bending extending perpendicularly to the longitudinal direction. The second degree of influence β, which indicates the degree of influence on the maximum bending moment M _max due to the development of the plastic deformation region in the direction intersecting the reference line, satisfies the following formulas (1) and (2). This is a method for designing a vehicle body structural member, which designs the cross-sectional shape and/or material of the vehicle body structural member.

(2) In the method for designing a vehicle body structural member described in (1) above, the first degree of influence α is determined by a parameter c indicating the degree of work hardening from the yield stress σ _y to the tensile stress σ _TS of the vehicle body structural member. The _second degree of influence β is calculated using the function f(σ _{cr (R )} /σ _y ), and the moment efficiency M _max /M _y may be calculated using the following equation (3).

(3) In the method for designing a vehicle body structural member described in (2) above, the function g(c) may be derived based on Voce's equation.
(4) In the method for designing a vehicle body structural member according to any one of (1) to (3) above, the cross section satisfying the formulas (1) and (2) in the longitudinal direction is The vehicle body structural member may be designed to extend over 50% of the total length.

According to the aspect of the present invention described above, while reliably avoiding elastic buckling, the influence of work hardening upon collision on the maximum bending moment M _max and the development of the plastic deformation region in the direction intersecting the bending reference line can be improved. The vehicle body structural member can be designed so as to exhibit a high bending strength by considering the degree of influence on the maximum bending moment _Mmax .

FIG. 2 is a schematic diagram showing an example of a structural member, in which (a) is a perspective view, and (b) is a cross-sectional view taken along the line A-A' in (a). It is a graph showing both the change in the section modulus Z and the change in the moment efficiency M _max /M _y due to a design change in the radius of curvature R of the curved portion. It is a graph showing a change in maximum bending moment M _max as the radius of curvature R increases. It is a graph which shows the relationship between the parameter c which shows the degree of work hardening from yield stress _σy to tensile stress _σTS, and function g(c). It is a graph plotting the relationship between the ratio of elastic buckling stress σ _cr to yield stress σ _y and moment efficiency M _max /M _y . FIG. 2 is a diagram showing an example of an automobile frame to which the structural member is applied.

In the following description, the axial direction of the vehicle body structural member, that is, the direction in which the axis extends, will be referred to as the longitudinal direction.
"Cross-sectional length" means the length along the circumferential direction in a cross section perpendicular to the longitudinal direction of the vehicle body structural member.
The term "flat portion" refers to a linear portion in a cross section perpendicular to the longitudinal direction of the vehicle body structural member, specifically, a portion having a radius of curvature larger than the maximum external dimension of the cross section. The maximum external dimension means the length of a straight line that provides the maximum distance between any two points on the cross section.
“Curved portion” is a portion of a cross section perpendicular to the longitudinal direction of a vehicle body structural member excluding a flat portion, that is, a portion where the radius of curvature is less than or equal to the maximum external dimension of the cross section, and is located toward the outside or inside of the vehicle body structural member. It means an arc-shaped part that is convex in the direction. Therefore, the rounded end of the curved portion means the boundary between the flat portion and the curved portion.
"Moment efficiency" means the ratio of the maximum bending moment M _max to the yield moment M _y . That is, the moment efficiency is an index calculated by M _max /M _y , and it can be considered that when the moment efficiency exceeds 1, plastic buckling occurs, and when it is less than 1, elastic buckling occurs.

The radius of curvature of the curved portion is obtained as follows. That is, in a cross section perpendicular to the longitudinal direction of the vehicle body structural member, there are two rounded stops at both ends of the curved part, and a bend located equidistantly along the surface from the two rounded stops on the curved part of the surface. Find the center point and the three points. By determining the curvature from these three points using a known mathematical method, the radius of curvature of the curved portion can be obtained. Note that the above-mentioned surface is the surface on the outside of the bending of the plate material.

Hereinafter, a method for designing a vehicle body structural member 1 according to the present embodiment will be described using a vehicle body structural member (hereinafter referred to as a structural member 1) shown in FIG. 1 as an example.
FIG. 1A is a perspective view of the structural member 1. FIG. The structural member 1 is a structural member of the vehicle body, in other words, a skeleton member. The vehicle body is, for example, the body of an automobile.
As shown in FIG. 1(a), the structural member 1 is a hollow cylindrical rectangular tube member extending in the longitudinal direction. FIG. 1(b) shows a cross section perpendicular to the longitudinal direction (AA' cross section in FIG. 1(a)) at the longitudinal center of the structural member 1. As shown in FIG.

The structural member 1 can be formed by applying various known processing techniques to plate materials such as steel plates. As an example, the structural member 1 may be formed by forming a blank material into a predetermined shape by cold or hot pressing and joining the ends.
The structural member 1 preferably has a tensile strength of 1180 MPa or more, more preferably 1500 MPa or more, in a planar portion, which will be described later.
Note that the plate material may be, for example, a steel plate with a thickness of 0.4 mm or more and 1.6 mm or less, from the viewpoint of shock absorption properties and weight reduction required for the structural member 1.
The material of the plate material may be metal such as aluminum in addition to steel.

The structural member 1 is provided on a vehicle body so that it bends when subjected to a load during a collision. Bending is applied so that one side is a bending compression side and the other side is a bending tension side, centering on a bending reference line L extending in a direction perpendicular to the longitudinal direction. In the example shown in FIG. 1B, the upper side of the bending reference line L is the bending compression side, and the lower side of the bending reference line L is the bending tension side.

As shown in FIG. 1(b), the cross section of the structural member 1 has a cross section perpendicular to the longitudinal direction that includes a pair of first plane parts 11a, 11a parallel to the bending reference line L and a cross section perpendicular to the bending reference line L. It has a pair of second plane parts 11b, 11b. The pair of first plane parts 11a, 11a and the pair of second plane parts 11b, 11b are connected via the four curved parts 12, thereby forming a closed cross section.

The pair of first plane parts 11a, 11a extend in parallel with the bending reference line L in between, as shown in FIG. 1(b). The pair of first plane parts 11a, 11a are spaced apart from each other by a distance h.
The first plane part 11a is located farther from the bending reference line L than the second plane part 11b, and is therefore a part where elastic buckling is likely to occur due to bending load. Therefore, the smaller the cross-sectional length _bf of the first plane portion 11a, the more elastic buckling can be suppressed. On the other hand, if the cross-sectional length _bf of the first flat portion 11a is too small, the section modulus decreases and the bending strength of the entire member decreases.
In addition, in this embodiment, since the cross-sectional shape is line-symmetrical about the bending reference line L as an example, the pair of first plane parts 11a, 11a have the same cross-sectional length, but the cross-sectional shape is not line-symmetrical. In this case, the cross-sectional length of the plane portion that is most likely to undergo elastic buckling in the cross-section is defined as the cross-sectional length _bf .

The second plane portion 11b is a portion extending in a direction perpendicular to the bending reference line L. In the second plane portion 11b, the farther away from the bending reference line L the position is, the higher the compressive stress or tensile stress is generated. The smaller the cross-sectional length _hf of the second flat portion 11b, the less likely elastic buckling will occur, but if the cross-sectional length _hf is too small, the section modulus will decrease and the bending strength of the entire member will decrease. .
In addition, in this embodiment, since the cross-sectional shape is line-symmetrical with respect to the direction perpendicular to the bending reference line L, the pair of second plane parts 11b, 11b have the same cross-sectional length; If the cross-sectional shape is not, the cross-sectional length of the plane portion having the longest maximum distance from the bending reference line L is defined as the cross-sectional length _hf .
Further, in this embodiment, the second plane portion 11b exists along the direction perpendicular to the bending reference line L, but the second plane portion 11b only needs to intersect the bending reference line L.

The curved portion 12 has higher rigidity against bending loads than the first flat portion 11a and the second flat portion 11b, and is therefore a portion that is less likely to undergo elastic buckling.
In the cross section shown in FIG. 1(b), all four curved portions 12 have the same radius of curvature R, but if the cross sections have different radii of curvature, the curvature of the curved portion with the maximum radius of curvature Let the radius be the radius of curvature R.

The present inventor has focused on the fact that a design change in the radius of curvature R of the curved portion 12 in the structural member 1 affects the section modulus Z and the moment efficiency M _max /M _y .
FIG. 2 is a graph showing both the change in the section modulus Z and the change in the moment efficiency M _max /M _y when the maximum external dimension is fixed and the design of the radius of curvature R of the curved portion 12 is changed.
As shown in this graph, the section modulus Z decreases as the radius of curvature R increases, while the moment efficiency M _max /M _y increases as the radius of curvature R increases.
The change in the maximum bending moment M _max accompanying an increase in the radius of curvature R of the curved portion 12 is expressed as a wavy (decrease→increase→decrease) curve as shown in the graph of FIG.

Here, a change (inclination) in the maximum bending moment M _max due to a change in the radius of curvature R of the curved portion 12 is determined as follows.
First, since the maximum bending moment M _max is a value obtained by multiplying the moment efficiency M _max /M _y by the section modulus and the yield stress, the maximum bending moment M _max can be expressed by the following equation (101).

Among these, the moment efficiency M _max /M _y and the section modulus Z are values that depend on the radius of curvature R of the curved portion 12 , and the yield stress σ _y is a value that does not depend on the radius of curvature R of the curved portion 12 . Therefore, by differentiating the equation (101) with the radius of curvature R of the curved portion 12, the following equation (102) is obtained.

Since the yield stress σ _y is a positive value, if the value of the formula in parentheses on the right side of the above equation (102) is greater than 0, the maximum bending moment increases as the radius of curvature R of the curved portion 12 increases. M _max will always increase (ie, the upward-sloping region of the graph in FIG. 3). Therefore, if the section modulus Z and moment efficiency M _max /M _y are designed to satisfy the following formula (1), a vehicle body structural member that can ensure high bending strength can be obtained.

Here, the value of moment efficiency M _max /M _y under conditions where plastic buckling occurs is determined by the first influence α, which is the influence on the maximum bending moment M _max due to work hardening at the time of collision, and the second plane It is calculated by taking into account the second degree of influence β, which is the degree of influence on the maximum bending moment M _max due to the development of the plastic deformation region in the cross-sectional length direction (that is, the direction intersecting the bending reference line L).
Specifically, it can be expressed by the following formula (2).

Since the condition for plastic buckling to occur is maximum bending moment M _max > yield moment M _y , moment efficiency M _max /M _y is greater than 1.0. Therefore, the first term on the right side of the above equation (2) is set to 1.0.
The moment efficiency M _max /M _y in the range exceeding 1.0 is determined by multiplying the first influence degree α and the second influence degree β, so the second influence degree on the right side of the above equation (2) The terms are α and β.

Therefore, in order to satisfy the above equations (1) and (2), that is, while avoiding elastic buckling, the degree of influence (first influence degree α) on the maximum bending moment M _max due to work hardening at the time of collision is The vehicle body structural members are designed so that the optimum bending strength can be expressed by taking into consideration the degree of influence (second degree of influence β) on the maximum bending moment M _max due to the development of the plastic deformation region in the direction intersecting the bending reference line L. By designing, it is possible to design a structural member with high bending strength.

Hereinafter, the first degree of influence α and the second degree of influence β will be explained in more detail.

(First influence α)
The first degree of influence α is a parameter representing the degree of work hardening from yield stress σ _y to tensile stress σ _TS of the material of the structural member 1, and c is a parameter representing the characteristics of true stress σ and plastic strain ε ^p of the material. It is derived from the FEM results based on the hardening law.
As the work hardening law, known formulas such as Voce's formula, Swift's formula, and n-th power hardening law formula can be adopted.
As an example, when Voce's equation is employed, the true stress σ from the yield stress σ _y to the tensile stress σ _TS can be expressed by the following equation (103).

Here, the first degree of influence α can be expressed as a function g(c) as a function of a parameter c indicating the degree of work hardening from yield stress σ _y to tensile stress σ _TS .
FIG. 4 is a graph plotting the relationship between the parameter c and the function g(c) based on the FEM results based on the above equation (103). From this graph, the following equation (104) is derived.

Although the above equation (104) is derived from the FEM results based on Voce's equation, it may also be derived from the FEM results based on Swift's equation or the n-th power hardening law equation.

(Second influence degree β)
As the elastic buckling stress σ _cr becomes larger than the yield stress σ _y , the moment efficiency M _max /M _y increases due to work hardening of the material and development of the plastic deformation region in the direction intersecting the bending reference line L. Figure 5 shows the yield stress σ y based on the FEM results conducted with the parameter c = 0 in equation (104), g(c) = 0, that is, α = 0.15, and the first influence α fixed _. 3 is a graph plotting the relationship between the ratio of the elastic buckling stress σ _cr to the moment efficiency M _max /M _y ;
When the bending moment increases and the deformation of the structural member 1 further progresses, not only the portion far from the bending reference line L but also the portion close to the bending reference line L becomes a plastic deformation region. Finally, the entire cross section of the member becomes a plastic deformation region.
Therefore, the second degree of influence β can be expressed as a function f(σ _cr(R) /σ _y ) using equation (105) below.

Although the above equation (105) was derived from the FEM results using the radius of curvature R as a variable, it may also be derived from the FEM results using the plate thickness and cross-sectional size as variables.

As described above, the first degree of influence α can be expressed by the function g(c), and the second degree of influence β can be expressed by the function f(σ _{cr (R)} /σ _y ). Therefore, the moment efficiency M _max /M _y in the above equation (1) can be calculated using the following equation (3).

In this way, the first degree of influence α is calculated by the function g(c) of the parameter c that indicates the degree of work hardening from the yield stress σ _y to the tensile stress σ _TS , and the second degree of influence β is calculated using the elastic buckling stress By calculating the moment efficiency M _max / _{M y using the} above equation (3) _{, the} optimum Therefore, it is possible to more accurately design the cross-sectional shape and/or material of the vehicle body structural member that can exhibit a bending strength.
In particular, it is preferable to derive the function g(c) based on Voce's equation because the first influence degree α can be calculated more accurately.

Hereinafter, for the structural member 1, a specific method of determining d(M _max /M _y )/dR, Z, and dZ/dR in the above equation (1) will be explained.

d(M _max /M _y )/dR is determined as follows.
First, by replacing the function f(σ _cr(R) /σ _y ) in the above equation (3) with the above equation (105), the following equation (106) is obtained.

Since the function g(c) and the yield stress σ _y do not depend on the radius of curvature R, by differentiating the above equation (106) with the radius of curvature R, the following equation (107) is obtained.

Here, σ′ _cr(R) (that is, dσ _cr(R) /dR, which is the derivative of σ _cr(R) ) is derived as follows.
First, the elastic buckling stress σ _{cr (R)} of the structural member 1 is defined as the elastic buckling stress σ _cr of the first plane portion 11a, (a) the Young's modulus E of the first plane portion 11a, the plate thickness t, and The Poisson's ratio ν increases as the Poisson's ratio ν increases, and (b) it increases as the cross-sectional length _bf of the first plane portion 11a decreases.
In addition, _bf is the outer diameter (mm) of the cross section. Further, k is a coefficient affected by the radius of curvature of the ridge line, and is a value determined from a differential equation for buckling of a flat plate and an eigenvalue determined by a flexure shape that satisfies the differential equation. When the radius of curvature of the ridge lines at both ends of the first plane portion 11a is 0 mm, k=4.0. For example, k can be determined by the formula k=4.0exp (-28R×t/b _f ×b), where b is the length of the bending reference line in the cross section.

In the case of a rectangular tube shape like the structural member 1, the section modulus Z is expressed by the following formula (109) and the following formula (110), where the moment of inertia I is the length h which is the length h _f +2R of the second plane part 11b. It can be obtained by the formula. Note that θ is the angle formed by the first plane portion 11a and the second plane portion 11b (that is, 90° in the example of this embodiment).

Therefore, dZ/dR in equation (1), that is, the section modulus Z differentiated by the radius of curvature R of the curved portion 12, can be calculated using equation (111) below.

Furthermore, dI/dR in the above equation (111) can be calculated using the following equation (112).

The preferred embodiments of the present invention have been described above in detail. An application example of the structural member of the present invention will now be described with reference to FIG. FIG. 6 is a diagram showing an automobile frame 2 as an example to which the structural member is applied. The structural members may constitute the automobile frame 2 as a cabin frame or a shock absorbing frame.

Application examples of structural members include roof centering force 201, roof side rail 203, B pillar 207, side sill 209, tunnel 211, A pillar lower 213, A pillar upper 215, kick reinforcement 227, floor cross member 229, and under reinforcement. 231, front header 233, etc. Furthermore, examples of application of structural members as shock absorbing skeletons include the rear side member 205, the apron upper member 217, the bumper reinforcement 219, the crash box 221, the front side member 223, and the like. In addition to the above, the structural member may be applied to a door impact beam or the like as a reinforcing member provided inside an automobile door. In short, the structural member of the present application can be applied to any part where a bending load can be applied.

According to the structural member design method according to the present embodiment, while avoiding elastic buckling, the degree of influence on the maximum bending moment M _max due to work hardening at the time of collision and the plastic deformation region in the direction intersecting the bending reference line can be improved. By designing a vehicle body structural member in such a way that it can exhibit an optimal bending strength by considering the degree of influence on the maximum bending moment M _max due to the evolution of , it is possible to design a structural member having a high bending strength.

Although the structural member 1 according to one embodiment of the present invention has been described above, the technical scope of the present invention is not limited to the above embodiment, and various changes can be made without departing from the spirit of the present invention. Is possible.

Moreover, although the structural member 1 according to the above embodiment has a uniform cross-sectional shape over its entire length, it does not have to have a uniform cross-sectional shape over its entire length. In this case, it is preferable that the cross section satisfying the above formula (1) and the above formula (2) exists in 50% or more of the total length in the longitudinal direction, and more preferably 80% or more. In other words, it is preferable to design the vehicle body structural member so that the cross section satisfying the above formula (1) and the above formula (2) exists in 50% or more of the total length in the longitudinal direction. It is further preferred to design the structural member.

According to the present invention, it is possible to provide a method for designing a vehicle body structural member that can ensure high bending strength.

1 Structural member 11a First plane part 11b Second plane part 12 Curved part

Claims (4)

A method for designing a longitudinally extending vehicle body structural member, the method comprising:
In a cross section perpendicular to the longitudinal direction,
Section modulus Z,
Moment efficiency M _max /M _y , which is the ratio between the maximum bending moment M _max and the yield moment M _y ,
a first degree of influence α indicating the degree of influence on the maximum bending moment M _max due to work hardening at the time of collision, and
a second degree of influence β indicating the degree of influence on the maximum bending moment M _max due to the development of the plastic deformation region in a direction intersecting the bending reference line extending perpendicularly to the longitudinal direction;
A method for designing a vehicle body structural member, characterized in that the cross-sectional shape and/or material of the vehicle body structural member is designed so that the following formulas (1) and (2) are satisfied.

Calculating the first degree of influence α using a function g(c) of a parameter c indicating the degree of work hardening from yield stress σy to tensile stress _σTS of the vehicle body structural member,
The second degree of influence β is calculated by a function f (σ _{cr (R)} /σ _y ) of the ratio of the elastic buckling stress σ _{cr (R)} and the yield stress σ _y of the vehicle body structural member,
2. The method of designing a vehicle body structural member according to claim 1, wherein the moment efficiency M _max /M _y is calculated using the following equation (3).

3. The method of designing a vehicle body structural member according to claim 2, wherein the function g(c) is derived based on Voce's equation. Claims 1 to 3 , wherein the vehicle body structural member is designed such that a cross section that satisfies the formulas (1) and (2) among the cross sections is present at 50% or more of the total length in the longitudinal direction. The method for designing a vehicle body structural member according to any one of the above.

Images