JP2014145368A - Crash box made of steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inexpensive and light crash box made of a steel sheet, which copes with variations of crash angles and has excellent impact absorption properties in the axial collapse direction, by working out plans of improving a closed cross-sectional shape and imparting a taper to the crash box in the axial-length direction.SOLUTION: In the crash box made of the steel sheet having a hat-shaped closed cross sectional structure where a hat-shaped member bent into a hat shape and a member of a closing plate having different bending rigidity are stacked up to form a closed cross section structure and stacked sites of flanges protruded outside from the contour of the closed cross section are joined together, v-shaped recesses are formed in the inside direction of the closed cross section, on a web part of the hat-shaped member and on a central part region of the closing plate in the direction behind the web region, respectively, and the cross section shape has a shape where a cross sectional area is gradually increased rearward from the collapsing face side in the axial-length direction.

Description

  The present invention relates to a steel plate crash box that is disposed at the front end of an automobile frame, particularly a front frame or a rear frame, for efficiently absorbing energy at the time of a collision.

In the event of a collision at low speed, only the crash box deforms and absorbs energy, so that the vehicle body and other vehicle parts are not damaged, and the repair is completed simply by replacing the crash box and bumper reinforcement members. The shock absorbing member installed for the purpose of reducing this is a crash box. Therefore, as a crash box, it is important how to effectively absorb the impact energy by the structural members constituting the crash box.
Crush boxes are generally made of steel plate pressed products with a closed cross-section with a monaca structure. When impact loads are applied in the axial length direction (longitudinal direction), the impact energy is absorbed by buckling in a bellows shape. Designed to. At this time, in order to improve the shock absorption characteristics, the peak load directly connected to the difficulty of the first buckling is lowered, and the buckling deformation is regularly generated in a bellows shape without being influenced by the fluctuation factors causing disturbance. It is important to absorb impact energy stably and efficiently.

  For example, a crash box arranged at the top of an automobile is arranged in parallel with the fender panel in a state where the base end is fixed to the front frame and the tip is fixed with a bumper reinforcing member and a bolt or the like. When a car collides head-on at the time of a collision accident, an impact load is applied to the tip of the crash box, causing buckling deformation in the axial direction, but the load at the beginning of the collision tends to be relatively large. In a frontal collision, the driver feels danger and turns the steering wheel reflectively, so the collision angle is likely to be oblique.

  Therefore, at the time of a light collision of 15 km / h or less, the peak load in the initial buckling is lowered to prevent other structural members arranged behind the cabin including the front frame from being damaged first (hereinafter, this specification) In this document, this required performance is described as “damageability”. Furthermore, even when a collision angle fluctuation within ± 10 ° occurs, orderly plastic deformation progresses in order from the collision surface side to the front frame side. It is buckled and deformed in a bellows shape (hereinafter, this required performance is referred to as “robustness”), and almost all of the impact energy associated with the collision can be absorbed only by the crash box (hereinafter, this requirement is referred to in this specification). It is required that the performance be described as “shock absorbing ability”. In other words, in order to minimize vehicle damage, the crash box needs to be designed so that the impact performance can be exhibited as a buffer member in consideration of the fluctuation factors.

  Conventionally, in order to solve the above-described problem, for example, as seen in Patent Document 1, a measure has been taken to arrange a depression called a crush bead that gives a starting point of deformation. In the structural member proposed in Patent Document 1, a plurality of weakly crushed beads on the structural surface are arranged at equal intervals with respect to the axial length direction of the member and the direction thereof is orthogonal to the axial length direction. It is designed to preferentially plastically deform the part, induce initial buckling of the target member with a lower load, and to buckle and deform regularly in a bellows shape from the collision surface side. .

  In addition, as seen in Patent Document 2, the peak load at the beginning of the collision is reduced by gradually increasing the thickness from the front to the rear on the collision surface side, and the specific strength (unit density of the material is higher than that of the steel plate). Also, a cylindrical structural member has been proposed that enables a significant improvement in shock absorption capacity per unit mass of a member by using a fiber reinforced resin having a high tensile strength ratio relative to the material.

JP-A-55-136660 JP-A-6-264949

However, the measure proposed by Patent Document 1 for arranging so-called crushed beads is very effective for the reproducibility and stabilization of buckling deformation, while the crushed beads are arranged to absorb impact energy that can be absorbed by the entire member. It will be greatly reduced compared to when not. For this reason, when the desired shock absorption capacity cannot be obtained, it is necessary to increase the thickness of the member, which is disadvantageous in that it increases the mass of the member.
Further, in order to secure the shock absorbing ability, there is a means of replacing the material with a high-strength plain steel plate. In addition, because the press formability such as elongation of the steel sheet itself also decreases, the material cracks when providing multiple crushed beads by press forming, the place where the crushed bead was formed during buckling deformation due to automobile collision, and the spot welded place There is also a drawback that cracking is likely to occur.

On the other hand, the structural member disclosed in Patent Document 2 that gradually increases the wall thickness and uses fiber reinforced resin as a material is difficult to achieve by simply increasing the strength of the steel sheet. Damage ability and shock absorption per unit mass of the member are difficult. Although it is very excellent in that it is relatively easy to achieve the improvement of two collision performances such as performance, there are many problems in terms of material cost and manufacturing time, etc. There is a problem in that it is limited to luxury cars.
The present invention has been invented in view of these current conditions, and by improving the closed cross-sectional shape and adding taper to the axial length direction, the peak load at the beginning of a collision at the time of an automobile collision is small, and An object of the present invention is to provide a low-cost and lightweight steel-made crash box that has improved impact absorption capacity per unit mass of the member and can cope with fluctuations in the collision angle.

  In order to achieve the object, the steel plate crash box of the present invention has a closed cross-sectional structure formed by stacking a hat-shaped member bent into a hat shape and a member having different bending rigidity of the closing plate, from the outline of the closed cross-section. It is a steel-made crush box with a hat-type closed cross-section structure that joins the overlapping parts of the flanges protruding outward, with respect to the web part of the hat-type member and the central part region of the closing plate in the direction that hits behind it. It has a V-shaped dent on the inner side of the closed cross-section that becomes the contour, and the cross-sectional area is gradually increased from the collision surface side toward the rear in the axial direction to form a divergent taper.

  According to the present invention, in a structural member having a hat-type closed cross-sectional structure, the closed cross-sectional shape is the inside of the closed cross-section with respect to the web part of the hat-type member and the central region of the closing plate in the direction of hitting behind the web part. The cross-sectional area is gradually increased by forming a V-shaped dent in the direction and tapering the closed cross-sectional shape size from the collision surface side toward the rear in the axial length direction. By adopting such a shape, it is possible to provide a lightweight steel steel crash box that is excellent in shock absorption characteristics in the axial crushing direction and that can cope with fluctuations in the collision angle.

Explanatory drawing of hat-shaped closed cross-sectional shape of various crash boxes used as specimens Explanatory drawing of the difference in buckling form of various crash boxes with different cross-sectional shapes Outline diagram of crush box with double concave cross section with taper Conceptual diagram explaining the drop weight test method within the scope of this specification A diagram conceptually showing the load-displacement curve obtained in the drop weight test Schematic diagram showing buckling deformation of specimen (Condition 4, Collision angle = 0 °) Transition diagram of buckling deformation obtained by axial crush FEM analysis under the same conditions as the drop weight test

  The inventors of the present invention have made various studies on improving the performance of a crash box that requires high shock absorption characteristics and the like. In the process, the cross-sectional shape is a so-called hat-shaped closed cross-section, and the cross-sectional shape and the partial quenching method are devised to improve the axial crushing strength and improve the shock absorption characteristics at a low cost. It turns out that it is obtained. These findings are proposed in Japanese Patent Application Laid-Open No. 2011-178179.

By the way, when an automobile collides with another object, an impact load is hardly applied straight in the axial length direction of the crash box. That is, rather than the case where the crash boxes on both sides placed at the top of the car collide with each other due to a frontal collision, the one that collides partially and begins to collapse from one crash box, and the other crashes sequentially with a time difference. There are many. In this case, the crash box does not receive an impact load straight in the axial direction, but receives an impact load in a direction to which a slight angle is added. As described above, even when the vehicle collides at various angles, it is required to have a stable shock absorbing characteristic.
Therefore, the present inventors, in a steel plate crash box having a hat-shaped closed cross section, in addition to the excellent damage ability, even when the collision angle fluctuates, the shock absorption capacity varies. The present inventors have studied a cross-sectional shape that can exhibit high robustness and impact absorption ability that is not easily generated and is not affected, and reached the present invention.

The following is an explanation from the background of the study.
In the above-mentioned JP2011-178179A, the inventors of the present invention have a hat-type closed cross section made of a steel plate crush box, for example, compared to a simple □ cross section cross section made of a steel plate crush box as shown in FIG. Reported that a single-concave cross-section steel plate crush box with a V-shaped dent in the inner direction of the closed cross-section in the web portion as shown in FIG. Yes.

Since the performance can be improved by adopting a single concave cross-sectional shape, a V-shaped dent recessed in the inner direction of the closed cross section is also formed in the central region of the closing plate of the hat closed cross section as shown in FIG. I decided to make it a so-called double concave cross-sectional shape.
In addition, we decided to make the columnar crush box with a double concave cross section tapered so that the cross section at the collision end is small and the cross-sectional area gradually increases toward the end on the vehicle body side.
In addition, the steel plate crush box of various shapes is obtained by melt-joining a hat-shaped cross-section shaped steel plate bent into a hat shape and a superposed flange portion of the steel plate constituting the closing plate by spot welding or laser welding.

The impact absorbing ability of various types of steel plate crash boxes was examined by changing the collision angle.
The details will be given in the embodiments described later, but in the case of a steel plate crash box having a closed cross-section structure with a hat-shaped member and a closing plate, in addition to the hat-shaped member web portion, the closing plate in the direction hitting behind it A V-shaped dent is also formed in the inner region of the closed cross-section, and the cross-sectional shape gradually increases in cross-sectional area from the collision surface side toward the rear in the axial length direction, so that the taper shape expands toward the end. It can be seen that the shape that has become larger can reduce the initial peak load, and the impact absorption capability can exhibit a high level with reproducibility without being affected by disturbance factors such as collision angle fluctuations. It has been reached. That is, according to the present invention, in addition to excellent damage ability, a steel plate crash box having high robustness and shock absorption capability can be provided.

In addition to the hat-shaped member web portion, a crash box with a so-called double-concave cross-sectional shape in which a V-shaped dent is formed in the inner direction of the closed cross-section also in the central region of the closing plate in the direction of hitting behind the web The reason why the absorption ability is improved is considered as follows.
That is, if it has a concave cross-sectional shape with a V-shaped recess formed in the center of the hat-shaped member web portion (FIG. 2 (b)), the curved surface is larger than a simple □ -shaped cross-section (FIG. 2 (a)). Many ridgeline areas increase from four to seven. Furthermore, if it is a double concave cross-sectional shape (FIG. 2 (c)) in which a V-shaped dent is also formed in the central region of the closing plate, the ridge line region is greatly increased to ten locations.

  In addition, the part that mainly absorbs impact energy, that is, the part that generates a large plastic strain is more than the two-dimensional bending deformation part of the flat part shown in the one-dot broken line ellipse in FIG. As indicated by an arrow in the figure, it is located on a curved surface where two flat portions intersect such as a so-called corner R portion that induces a complicated three-dimensional bending deformation. Furthermore, as an incidental effect, the plastic strain rate naturally increases in the portion where the plastic strain is high because the increase rate of the plastic strain per unit time is large. It is considered that the increase in the ridgeline region combined the above effects to greatly increase the axial crushing strength and to obtain a high impact absorption capability (impact absorption energy).

By the way, the initial peak load of buckling directly linked to the performance of damageability is determined by the size / thickness (ie, cross-sectional area) of the crush box cross section and the material characteristics. In order to improve the damageability so that the initial damage caused by the automobile collision at a low speed does not occur in the structural member arranged behind the front frame or the like, it is effective to reduce the initial peak load, It is not preferable to reduce the plate thickness or use a low-strength material because the impact absorption capacity of the crush box is lowered.
However, the idea of increasing the ridgeline region of the present invention can provide a crash box that can obtain a high shock absorption capacity even if the initial peak load is reduced.

  Next, regarding the superiority of the columnar crush box whose cross-sectional shape is enlarged in a taper shape that widens toward the rear from the collision surface side in the axial length direction, details are also described in the embodiment. However, by increasing the taper shape, the buckling deformation regularly proceeds in a state of being sequentially folded in a bellows shape from the collision surface side having a small cross section toward the rear side having a large cross section. As a result, it is considered that even if the collision angle changes variously, the buckling deformation proceeds regularly and exhibits a stable shock absorbing ability without variation, that is, the robustness is improved.

As the material steel plate from which each specimen was manufactured, a workable cold-rolled high-tensile steel plate for automobiles having a plate thickness of 1.0 mm or 1.4 mm and a tensile strength of 440 MPa class and 980 MPa class was used. It was.
A crush box for a test was produced from the material steel plate with the specifications and shapes shown in Table 1, and a falling weight test was performed.
In Table 1, the specimen having a double-concave cross section with the cross-sectional shape in the axial length direction extending toward the end has the shape and size shown in FIG. Various specimens are joined by spot welding at intervals of 25 mm at the flange. The length of each specimen in the axial length direction (longitudinal direction) is 135 mm.

Table 1 shows the dynamic yield strength of materials obtained by carrying out a tensile test using a high-speed tensile tester and a JIS 7 test piece. The tensile test conditions were assumed to be the same level as the strain rate at the time of automobile collision, and the test speed was set to 24.6 m / s (equivalent plastic strain rate≈10 ⁻³ / s).
Further, in order to advance understanding of approximately the value of the initial peak load obtained in the falling weight test, since the dynamic buckling resistance hefty of BS _d of the specimen was calculated by the following equation, this value is also shown together in the table.
BS _d = (YS _d ÷ 1000) × A = (YS _d ÷ 1000) × {W ÷ (ρ × H)} (1)
Here, YS _d : dynamic yield strength (MPa) of the material, A: average cross-sectional area (mm ³ ) of the cross section of the specimen, W: mass (kg) of the specimen, ρ: density of steel (= 7.84 × 10 ⁻⁶ kg / mm ³ ), H: length (mm) in the axial length direction of the specimen.

The falling weight test was performed in the manner shown in FIG. In addition, in the falling weight test method shown in FIG. 4, the specimen 11 and the upper and lower wedge-shaped plate 12 are continuously joined by arc welding.
The falling weight test was performed in three modes using a wedge-shaped patch plate 12 with an adjusted apex angle, assuming the case of collision at angles of −10 °, 0 °, and + 10 °. The falling weight condition was set such that the weight of the weight was 640 kg, and the falling height of the weight was set so that the initial collision speed to the specimen was 15 km / h.
Such a falling weight test was performed, and a load-displacement curve as shown in FIG. 5 was obtained for each of the various specimens, and the occurrence of buckling was observed.
Moreover, the initial peak value and impact absorption energy of each specimen were calculated based on the load-displacement curve. Here, the impact energy (or shock absorption) absorbed by the specimen when the area enclosed in the load-displacement curve shown in FIG. 5 obtained by integrating the load for each arbitrary displacement is crushed to 70 mm. ). The obtained results are summarized in Table 2.

  The average value of the initial peak load shown in Table 2 is the dynamic buckling strength of the specimen calculated in advance (that is, it causes plastic deformation when the specimen is axially crushed at a speed corresponding to an automobile collision). Therefore, it is obvious that the initial peak load is almost determined by the average cross-sectional area of the specimen cross section and the material characteristics, as described above. And in order to suppress an initial peak load to 220 kN or less so that structural members other than a crash box may not be buckled initially, it turned out that the dynamic buckling strength of a specimen needs also to be 220 kN or less.

  On the other hand, as is apparent from the results shown in Table 2, under the conditions (conditions 3 to 7) having the double concave cross section, the impact absorption energy increases dramatically because the ridge line area is increased. For example, in the conditions 2, 3, and 9 using the same steel plate, the ridge line regions are 4, 10, and 7, respectively. That is, there is no great difference in the average value of the initial peak load, but the impact absorption energy can be greatly increased by increasing the ridge line region.

As described above, according to the above procedure, the two characteristics of the initial peak load and the shock absorption ability can be designed to be individually desired characteristics to some extent in an independent state.
Furthermore, the proposed structure (conditions 4 to 7) having a taper at the end is slightly lower in impact absorption capacity than condition 3 in which no taper is formed, but the taper is reproducible and stable in buckling deformation. From this, it can be seen that it is effective in suppressing variations in the characteristic values due to the collision angle fluctuation, that is, it is excellent in robustness.

An example of the change over time of the buckling deformation in the specimen provided with the taper is shown in FIG. About the specimen shown by the conditions 4 in Table 1, the situation of the buckling deformation observed by the high-speed video when the falling weight test was carried out at a collision angle of 0 ° is schematically shown. It turns out that it buckles and deforms in a bellows shape from the collision surface side.
Further, FIG. 7 shows the analysis results under the following four conditions side by side so that the influence of the taper on the specimen can be compared with the time-dependent change in the buckling deformation of the crash box obtained by the FEM analysis. (A) in FIG. 7 is a case where the specimen of Condition 3 to which no taper is applied is crushed under the condition of a collision angle of 0 °, and (b) is a collision angle of +10 with respect to the same specimen. It is an analysis result when changing to °. FIG. 7 (c) shows a case where the test specimen of condition 4 with a taper is crushed under the condition of a collision angle of 0 °, and (d) shows a collision angle of + 10 ° with respect to the same specimen. It is an analysis result when changing to.
Condition 3 in which the cross-sectional area of the hat-shaped closed cross section is constant in the axial length direction is not reproducible because the first buckling occurrence location is not determined, but in the case of condition 4, by providing a taper, It can be seen that the buckling deformation progresses regularly in a state in which the buckling deformation is sequentially folded in a bellows shape from the collision surface side having a short circumference to the rear side having a long circumference.

Claims (1)

  A hat-shaped closed section in which a hat-shaped member bent into a hat shape and a member with different bending rigidity of the closing plate are overlapped to form a closed section structure, and the overlapping portion of the flange protruding outward from the outline of the closed section is joined. A steel steel crash box with a V-shaped dent on the inner side of the closed cross section, which is the contour of the web part of the hat-shaped member and the central area of the closing plate in the direction of hitting behind it. And a steel-made crash box characterized in that a taper is formed by gradually increasing the cross-sectional area from the collision surface side toward the rear in the axial direction.

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