Patent Publication Number: US-2011049916-A1

Title: Impact absorbing member for vehicle

Description:
The disclosure of Japanese Patent Application No. 2009-202178 filed on Sep. 2, 2009 including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an impact absorbing member for a vehicle, and particularly to a technology in which impact energy absorbing performance is stably obtained when an impact absorbing member for a vehicle includes an asymmetric taper portion. 
     2. Description of the Related Art 
     An impact absorbing member for a vehicle, which includes a hollow cylindrical body, is known. The cylindrical body has a plurality of side walls in the form of flat plates, and has a polygonal section. The impact absorbing member is disposed between a vehicle body-side member and a bumper member in a manner such that an axial direction of the cylindrical body coincides with a front-rear direction of a vehicle. The impact absorbing member absorbs impact energy by being crushed into an accordion shape when the impact absorbing member receives a compression load in the axial direction. A device described in Japanese Patent Application Publication No. 2006-123887 (JP-A-2006-123887) is an example of the impact absorbing member for a vehicle. The device includes a cylindrical body with a substantially octagonal section, and the cylindrical body is constituted by paired half bodies each of which has a shape obtained by dividing the cylindrical body into upper and lower halves. While both of side end edge portions of one of the paired half bodies are overlapped with both of side end edge portions of the other of the paired half bodies, respectively, to form a cylindrical shape, both of the side end edge portions of the paired half bodies are discontinuously joined at points located at predetermined intervals in an axial direction, by spot welding. In the impact absorbing member for a vehicle, stress concentration portions are provided so that the stress concentration portions and joining points, at which the side end edge portions are joined to each other by spot welding, are alternately positioned in the axial direction. When the impact absorbing member is crushed into the accordion shape, the stress concentration portions serve as starting points at which plastic deformation starts, so that the impact absorbing member is smoothly crushed. Each of the stress concentration portions is a recessed groove that extends in a direction perpendicular to the axial direction, and that is inwardly recessed. Each of the stress concentration portions extends to straddle two adjacent ridge lines of the cylindrical body with the polygonal section. 
     One kind of the impact absorbing member for a vehicle includes an asymmetric taper portion in which at least one of paired side walls, which are located on sides opposite to each other, and face each other, is inclined in the axial direction, depending on, for example, the sizes of the bumper member and the vehicle body-side member, and a positional relation between the bumper member and the vehicle body-side member in a top-bottom direction. For example, in the case where the size of the bumper member in the top-bottom direction is smaller than that of the vehicle body-side member, while an upper side wall that is located at an upper position and extends in a front-rear direction of the vehicle is substantially horizontal, a lower side wall located at a lower position is inclined upward in a direction toward the bumper member. 
     However, when the impact absorbing member includes the taper portion, imbalance in load transmission occurs, and crush behavior becomes unstable when the impact absorbing member is crushed into the accordion shape. Thus, impact energy absorbing performance may be impaired. It is effective to provide the stress concentration portion to stabilize the crush behavior when the impact absorbing member is crushed into the accordion shape. However, in the device described in Japanese Patent Application Publication No. 2006-123887, because each of the stress concentration portions is provided to include the ridge line portions of the cylindrical body with the polygonal section, influence on the impact energy absorbing performance is large. Therefore, a plate thickness needs to be large to ensure desired impact energy absorbing performance. In addition, it is difficult to tune the position, size, shape, and the like of the stress concentration portion to stabilize the crush behavior while ensuring the impact energy absorbing performance. 
     SUMMARY OF THE INVENTION 
     The invention has been made under such circumstances and an object of the invention is to stabilize crush behavior to stably obtain predetermined impact energy absorbing performance when a cylindrical body includes an asymmetric taper portion, and to make it possible to easily tune a stress concentration portion to stabilize the crush behavior. 
     The object indicated above can be achieved according to the first aspect of the invention, which provides an impact absorbing member for a vehicle, which includes a hollow cylindrical body that has a plurality of side walls in a form of flat plates and has a polygonal section, wherein the impact absorbing member is disposed between a vehicle body-side member and a bumper member, in a manner such that an axial direction of the cylindrical body coincides with a front-rear direction of a vehicle, and the impact absorbing member absorbs impact energy by being crushed into an accordion shape when the impact absorbing member receives a compression load in the axial direction, the cylindrical body including, (a) paired side end edge portions being overlapped with each other by a predetermined width in a first side wall in a form of a flat plate, and integrally joined to each other along the axial direction, whereby forming the closed polygonal section; (b) an asymmetric taper portion in which a second side wall, which is located on a side opposite to the first side wall to face the first side wall, is inclined in the axial direction more greatly with respect to the axial direction than the first side wall; and (c) a stress concentration portion provided at an overlapping portion at which the side end edge portions are overlapped with each other in the first side wall such that the stress concentration portion does not reach a ridge line of the cylindrical body with the polygonal section, and serving as a starting point at which plastic deformation starts when the impact absorbing member is crushed into the accordion shape. 
     The object indicated above can be achieved according to the second aspect of the invention, which provides the impact absorbing member for a vehicle according to the first aspect of the invention, wherein (a) in the taper portion, at least the second side wall is inclined with respect to the axial direction so that a distance between the first side wall and the second side wall decreases in a direction from the vehicle body-side member toward the bumper member; (b) the paired side end edge portions in the first side wall are discontinuously joined to each other at joining points located at predetermined intervals in the axial direction by spot welding; and (c) the stress concentration portion is provided at least a position between a first joining point that is closest to an end portion on a side of the bumper member, and a second joining point that is the second closest to the end portion, among a plurality of joining points at which the paired side end edge portions are joined to each other by the spot welding. 
     The object indicated above can be achieved according to the third aspect of the invention, which provides the impact absorbing member for a vehicle according to the first or second aspect of the invention, wherein (a) the cylindrical body is constituted by paired half bodies each of which has a shape obtained by substantially symmetrically dividing the cylindrical body in half along a direction parallel to the axial direction, and the cylindrical body is formed by overlapping both of the side end edge portions of one of the paired half bodies with the both of the side end edge portions of other of the paired half bodies, respectively, to form a cylindrical shape, and integrally joining the both of the side end edge portions of the one of the paired half bodies to the both of the side end edge portions of the other of the paired half bodies, respectively, by spot welding; and (b) two side walls constituted by the both of the side end edge portions of the paired half bodies joined by the spot welding are the first side wall and the second side wall. 
     The object indicated above can be achieved according to fourth aspect of the invention, which provides the impact absorbing member for a vehicle according to any one of the first to third aspects of the invention, wherein (a) the cylindrical body includes a substantially horizontal upper side wall that is located at an upper position and extends in the front-rear direction of the vehicle, and a lower side wall that is located at a lower position, and is inclined upward over an entire length in the axial direction so that a distance between the upper side wall and the lower side wall decreases in a direction toward the bumper member, and (b) the upper side wall is the first side wall, the lower side wall is the second side wall, and the entire upper side wall and the entire lower side wall form the taper portion. 
     According to the first aspect of the invention, in the impact absorbing member for a vehicle, the stress concentration portion is provided at the overlapping portion at which the paired side end edge portions are overlapped with each other in the first side wall such that the stress concentration portion does not reach the ridge line of the cylindrical body with the polygonal section. Therefore, as compared to the case where the stress concentration portion is provided to include the ridge line of the cylindrical body with the polygonal section, or the stress concentration portion is provided in a side wall portion constituted by one plate material, influence on the impact energy absorbing performance is small. Thus, it is possible to easily tune the position, size and shape, and the like of the stress concentration portion to stabilize the crush behavior, while ensuring desired impact energy absorbing performance. The first side wall and the second side wall constitute the asymmetric taper portion, and the inclination of the first side wall is comparatively small or 0 (parallel to the axial direction), and thus, a large compression load is applied to the first side wall. Therefore, it is possible to appropriately change the crush behavior by tuning the stress concentration portion provided in the first side wall. Thus, in the cylindrical body with the taper shape whose crush behavior is easy to be unstable due to the imbalance in the load transmission, the crush behavior can be easily stabilized by tuning the stress concentration portion, and desired impact energy absorbing performance can be stably obtained. 
     According to the second aspect of the invention, the impact absorbing member includes the taper portion in which at least the second side wall is inclined upward so that the distance between the first side wall and the second side wall decreases in the direction from the vehicle body-side member toward the bumper member. The paired side end edge portions in the first side wall are discontinuously joined to each other at the joining points located at the predetermined intervals in the axial direction by spot welding. The stress concentration portion is provided at least between the first joining point that is closest to the end portion of the bumper member side and the second joining point that is the second closest to the end portion of the bumper member side among the plurality of joining points at which the side end edge portions are joined to each other by spot welding. Therefore, the stress concentration portion is plastically deformed at the initial stage of collision, and thus, the crush of the cylindrical body into the accordion shape smoothly starts at a low load. Thus, predetermined impact energy absorbing performance can be stably obtained from the initial stage of collision. 
     According to the third aspect of the invention, the cylindrical body is constituted by the paired half bodies each of which has a shape obtained by substantially symmetrically dividing the cylindrical body in half along the direction parallel to the axial direction. The cylindrical body is formed by overlapping the both of the side end edge portions of the paired half bodies, and joining them integrally with each other, by spot welding. The two side walls constituted by the both of the side end edge portions of the paired half bodies joined to each other by spot welding correspond to the first and second side walls, respectively. Thus, the crush behavior can be easily stabilized so that desired impact energy absorbing performance can be obtained, by appropriately tuning the stress concentration portion provided in the first side wall. 
     According to the fourth aspect of the invention, the cylindrical body includes the substantially horizontal upper side wall and the lower side wall that is inclined upward over the entire length in the axial direction so that the distance between the upper side wall and the lower side wall decreases in the direction toward the bumper member. The upper and lower side walls correspond to the first and second side walls, respectively. The entire upper side wall and the entire lower side wall form the taper portion. Even when there is the imbalance in the load transmission between the upper side wall and the lower side wall, the crush behavior can be easily stabilized, and desired impact energy absorbing performance can be stably obtained, by appropriately tuning the stress concentration portion provided in the upper side wall to which a high compression load is applied. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and further objects, features and advantages of the invention will become apparent from the following description of example embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein: 
         FIGS. 1A and 1B  are diagram each explaining an impact absorbing member for a vehicle according to an embodiment of the invention,  FIG. 1A  is a schematic plane view showing an example of a manner in which the impact absorbing members for a vehicle are disposed in a vehicle, and  FIG. 1B  is a plane view showing the enlarged impact absorbing member for a vehicle at a right side in  FIG. 1A ; 
         FIG. 2  is an enlarged view of a section taken along a line II-II in  FIG. 1A ; 
         FIGS. 3A and 38  are diagram each explaining the impact absorbing member for a vehicle in  FIGS. 1A and 1B ,  FIG. 3A  is a perspective view of a cylindrical body, and  FIG. 3B  is a sectional view of the cylindrical body taken at a cutout portion (stress concentration portion); and 
         FIGS. 4A and 4B  are diagrams showing results of a study on characteristics of changes in an axial compression load and absorbed energy with respect to a compression stroke in a product according to the invention in which the cutout is provided, and a comparative product in which the cutout is not provided, by conducting a weight-drop test. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     An impact absorbing member for a vehicle according to the invention can be applied to an attachment portion of a bumper member attached to a front side of a vehicle, and an attachment portion of a bumper member attached to a rear side of the vehicle. However, the impact absorbing member for a vehicle according to the invention may be applied to only one of the bumper members attached to the front side and rear side of the vehicle. 
     For example, in a front bumper, the shape of the bumper member in a longitudinal direction, that is, the shape of the bumper member in a plane view seen from above the vehicle is preferably a smoothly curved shape whose center portion protrudes forward. However, the bumper member may have various shapes. For example, the bumper member may have a substantially linear shape, or only both end portions of the bumper member may be inclined rearward or curved rearward. The impact absorbing member for a vehicle according to the invention is disposed in a manner such that, for example, an axial direction of a cylindrical body coincides with a front-rear direction of the vehicle. However, the impact absorbing member for a vehicle according to the invention need not necessarily be disposed in a manner such that the axial direction of the cylindrical body strictly coincides with the front-rear direction of the vehicle, and may be disposed in a manner such that the cylindrical body is inclined in a right-left direction, or in a top-bottom direction, depending on the shape and the like of the bumper member. 
     The impact absorbing member for a vehicle according to the invention includes the cylindrical body and, for example, paired attachment plates that are integrally fixed to both ends of the cylindrical body in the axial direction. For example, (a) a section of the cylindrical body, which is perpendicular to the axial direction of the cylindrical body, has a flat polygonal shape, and the number of sides of the polygonal shape is an even number equal to or larger than four; and (b) a recessed groove, which is inwardly recessed, is formed along the axial direction in each of paired side walls constituting the two sides of the polygonal section, which are parallel to each other. The section of the cylindrical body may have a simple polygonal shape, such as a square shape, or the cylindrical body may have a side wall in which a portion, for example, a corner portion is curved into an arc shape or the like. 
     For example, the cylindrical body is formed as follows. Paired half bodies are formed by pressing a thin plate material. Each of the paired half bodies has a shape obtained by dividing the cylindrical body in half along a direction substantially parallel to the axial direction. Each of the paired half bodies has, for example, a substantially angular U-shaped section or a substantially M-shaped section. The paired half bodies are integrally joined to each other while both of side end edge portions at an open side of one of the paired half bodies are overlapped with both of side end edge portions at an open side of the other of the paired half bodies. Thus, the cylindrical body is formed. The cylindrical body may be formed by bending one thin plate material to form a predetermined polygonal section, and then, overlapping and integrally joining both side end edge portions with each other. 
     It is appropriate to employ spot welding, as means for joining the overlapped side end edge portions to form the cylindrical body. However, other welding means, such as arc welding, may be employed. Further, it is possible to join the side end edge portions using a joining member such as a rivet. The side end edge portions may be discontinuously joined to each other at joining points located at predetermined intervals in the axial direction. In the case where, for example, the arc welding is employed, it is possible to continuously join the side end edge portions in the axial direction. In the case where the side end edge portions are discontinuously joined to each other at the joining points by spot welding or the like, the interval between the joining points may be constant, or may be continuously increased or decreased in the axial direction, for example. 
     It is appropriate to provide, for example, a cutout that extends from a side end edge of one of the paired side end edge portions in a direction substantially perpendicular to the axial direction of the cylindrical body, as a stress concentration portion provided at an overlapping portion at which the side end edge portions are overlapped with each other. However, a recessed groove or a through-hole may be provided as the stress concentration portion. It is preferable that the cutout should be in the form of a slit, that is, the cutout should have a U-shape, an angular U-shape, or the like. However, the cutout may have a relatively small V-shape. Through-holes with various shapes may be employed. For example, a simple circular hole, an elliptic hole, and a long hole, such as a rectangular hole and an oval hole, may be employed. 
     The stress concentration portion is provided, for example, at one of the paired side end edge portions, which is located at an inner side of the cylindrical body. However, it is possible to provide the stress concentration portion at the side end edge portion located at an outer side of the cylindrical body. It is also possible to provide the stress concentration portions at both of the paired side end edge portions. In this case, when the cutouts are formed so that the cutouts are overlapped with each other, or the through-holes are formed so that the through-holes are overlapped with each other, a portion of the cylindrical body is open, and water or the like may enter the inside of the cylindrical body. However, it is possible to prevent the entry of water or the like, by closing the open portion using a seal member or the like with low strength, as needed. Although the stress concentration portion is provided at the overlapping portion at which the side end edge portions are overlapped with each other, the stress concentration portion may be provided to extend beyond the overlapping portion, as long as the stress concentration portion is located within the side wall in the form of a flat plate, that is, the stress concentration portion does not reach a ridge line of the cylindrical portion with the polygonal section. 
     The second aspect of the invention relates to a case where the cylindrical portion includes a taper portion in which at least a second side wall is inclined so that a distance between a first side wall and the second side wall decreases in a direction from a vehicle body-side member toward the bumper member. When implementing the first aspect of the invention, the taper shape may be such that the distance between the first side wall and the second side wall decreases in a direction from the bumper member toward the vehicle body-side member. The cylindrical body may have the taper portion in a manner such that the cylindrical body has a taper shape over an entire length thereof, or a portion of the cylindrical body, which is located on the side of the bumper member, has a taper shape. The taper portion has an asymmetric shape with respect to the axial direction. The axial direction coincides with a direction in which a compression load is applied, and generally coincides with the front-rear direction of the vehicle, which is substantially horizontal. 
     In the second aspect of the invention, the stress concentration portion is provided at least a position between a first joining point that is closest to an end portion on the side of the bumper member, and a second joining point that is the second closest to the end portion. The stress concentration portion may be provided at an intermediate portion between each of all the pairs of adjacent joining points in the axial direction, or at an intermediate portion between each of several pairs of the adjacent joining points in the axial direction. Thus, the second aspect of the invention can be realized in various forms. 
     In the case where two side walls constituted by both of the side end edge portions of the paired half bodies are the first side wall and the second side wall as in the third aspect of the invention, the stress concentration portion may be provided in only the first side wall, or in both of the first side wall and the second side wall. 
     In the fourth aspect of the invention, while an upper side wall is substantially horizontal, a lower side wall is inclined upward so that a distance between the upper side wall and the lower side wall decreases in the direction toward the bumper member. Thus, imbalance in load transmission occurs between the upper side wall and the lower side wall, resulting in unstable crush behavior. Therefore, the stress concentration portion is provided in the upper side wall to which a large compression load is applied. However, when implementing the other inventions, to the contrary, the lower side wall may be substantially horizontal, and the upper side wall may be inclined downward so that the distance between the upper side wall and the lower side wall decreases in the direction toward the bumper member. In this case, the lower side wall is the first side wall. Thus, the paired side end edge portions are overlapped with each other, and integrally joined to each other, in the lower side wall, and the stress concentration portion(s) is(are) provided in the side end edge portion(s). Similarly, the invention may be applied to the other side walls, such as right and left side walls. That is, the invention is applied to paired side wall portions between which the imbalance in load transmission occurs. 
     Embodiment 
     Hereinafter, an embodiment of the invention will be described in detail with reference to the drawings.  FIGS. 1A and 113  are diagrams each explaining an impact absorbing member for a vehicle according to an embodiment of the invention.  FIG. 1A  is a schematic plane view of a portion near a bumper beam  10  at a front side of a vehicle, which is seen from above the vehicle. Crush boxes  14 R and  14 L, each of which serves as the impact absorbing member for a vehicle, are disposed at front end portions of right and left side members  12 R and  12 L. Right and left end portions of the bumper beam  10  are fixed to the crush boxes  14 R and  14 L, respectively.  FIG. 1B  is a plane view showing an enlarged crush box  14 R at the right side. The crush box  14 R includes a hollow cylindrical body  20  that has a plurality of side walls in the form of flat plates, and has a polygonal section; and paired attachment plates  22  and  24  that are integrally welded and fixed to the respective end portions of the cylindrical body  20  in the axial direction. The crush box  14 R is fixed to the side member  12 R and the bumper beam  10  using bolts or the like (not shown) through the attachment plates  22  and  24  in a manner such that the axial direction of the cylindrical body  20  is substantially parallel to the front-rear direction of the vehicle. 
     When an impact is applied from ahead of the vehicle, and the crush box  14 R receives an axial compression load, the cylindrical body  20  is crushed into an accordion shape. At this time, the deformation absorbs the impact energy, and reduces the impact applied to structural members of the vehicle, such as the side member  12 R. The cylindrical body  20  is crushed into the accordion shape, because a plurality of portions of the cylindrical body  20  are sequentially buckled (that is, a plurality of portions of the cylindrical body  20  are sequentially bent into a V-shape). In general, buckling starts from a portion of the cylindrical body  20 , which is located on the side of the bumper beam  10 , that is, an input-side portion of the cylindrical body  20 . Then, buckling proceeds in a direction toward a vehicle body, as time passes. The bumper beam  10  functions as a reinforcement member and an attachment member of a bumper. A bumper body  16  made of synthetic resin or the like is integrally attached to the bumper beam  10 . The bumper beam  10  corresponds to the bumper member. The side members  12 R and  12 L correspond to the vehicle body-side member. The crush box  14 L at the left side is symmetrical to the crush box  14 R at the right side, and the crush box  14 L has the same advantageous effects as those of the crush box  14 R. Therefore, in the following description, the crush box  14 R at the right side will be specifically described. 
       FIG. 2  is an enlarged view of a section taken along a line II-II in  FIG. 1A .  FIGS. 3A and 313  are diagrams each showing only the crush box  14 R.  FIG. 3A  is a perspective view obliquely seen from ahead of the vehicle.  FIG. 3B  is a sectional view of the cylindrical body  20  taken along a plane perpendicular to the axial direction (i.e., a sectional view seen from ahead of the vehicle). The section of the cylindrical body  20  perpendicular to the axial direction is long in the top-bottom direction, and has a flat polygonal shape (an octagonal shape in the embodiment). Recessed grooves  34  and  36  are formed at center portions of paired right and left side walls  30  and  32  with large width, which constitute two sides of the polygonal section, which are parallel to each other. Each of the recessed grooves  34  and  36  is inwardly recessed, and extends in the axial direction (in a precise sense, the recessed groove is slightly inclined in the top-bottom direction as shown in  FIG. 2 ). As evident from  FIG. 2 , the size of the bumper beam  10  in the top-bottom direction is smaller than the size of the side member  12 R in the top-bottom direction. Therefore, while an upper side wall  38 , which is located at an upper portion of the cylindrical body  20 , and extends in the front-rear direction of the vehicle, is substantially horizontal, a lower side wall  40 , which is located at a lower portion of the cylindrical body  20 , is inclined upward so that a distance between the upper side wall  38  and the lower side wall  40  decreases in a direction toward the bumper beam  10 , in accordance with the difference in the size. That is, while the upper side wall  38  is substantially horizontal, the lower side wall  40  is inclined upward in a direction from an end portion on the side of the side member  12 R toward an end portion on the side of the bumper beam  10 , and thus, the cylindrical body  20  has a taper shape. In a side view shown in  FIG. 2 , the upper side wall  38  and the lower side wall  40  over an entire length thereof in the axial direction form an asymmetric taper portion, with respect to the axial direction that is substantially horizontal. In the embodiment, the substantially horizontal upper side wall  38  corresponds to the first side wall. The lower side wall  40 , which is inclined with respect to the axial direction (horizontal direction) more greatly than the upper side wall  38 , corresponds to the second side wall. 
     As evident from  FIGS. 3A and 3B , the cylindrical body  20  is constituted by paired half bodies  26  and  28 . Each of the paired half bodies  26  and  28  has a shape obtained by substantially symmetrically dividing the cylindrical body  20  into right and left halves along a direction substantially parallel to the axial direction. Thus, each of the paired half bodies  26  and  28  has a substantially M-shaped section. Each of the half bodies  26  and  28  is formed, for example, by pressing and bending a thin plate material. Side end edge portions  26   a  and  28   a  at the open sides of the paired half bodies  26  and  28  are overlapped with each other by a predetermined width, and integrally joined to each other. Also, side end edge portions  26   b  and  28   b  at the open sides of the paired half bodies  26  and  28  are overlapped with each other by the predetermined width, and integrally joined to each other. Thus, a closed polygonal section is formed. The paired side end edge portions  26   a  and  28   a  constitute the upper side wall  38 , and the paired side end edge portions  26   b  and  28   b  constitute the lower side wall  40 . The paired side end edge portions  26   a  and  28   a  are discontinuously joined to each other at joining points located at predetermined intervals in the axial direction by spot welding, and the paired side end edge portions  26   b  and  28   b  are discontinuously joined to each other at joining points located at predetermined intervals in the axial direction by spot welding. In  FIG. 1B  and  FIG. 3A , joining points Pa 1  to PaX, and Pb 1  to PbX (joining points Pb 3  to PbX are not shown) indicated by circle marks are positions at which the side end edge portions are joined to each other by the spot welding. In the embodiment, the side end edge portions are joined to each other at the joining points located at substantially equal intervals. The joining point Pa 1  is the first joining point closest to the bumper beam  10 . The joining point Pa 2  is the second joining point that is second closest to the bumper beam  10 . In the embodiment, the number X of joining points is 6. In each of the upper side wall  38  and the lower side wall  40 , spot welding is performed at 6 joining points. The positions of the joining points in the upper side wall  38  in the axial direction are substantially the same as the positions of the joining points in the lower side wall  40  in the axial direction. The number X of the joining points and the interval between the joining points are appropriately set according to, for example, the length of the cylindrical body  20 . 
     In the upper side wall  38  constituted by the paired side end edge portions  26   a  and  28   a , a stress concentration portion  42  is provided such that the stress concentration portion  42  does not reach a ridge line of the cylindrical body  20  with the polygonal section. When the cylindrical body  20  is crushed into the accordion shape, the stress concentration portion  42  serves as a starting point at which plastic deformation starts. The stress concentration portion  42  is a cutout in the form of a slit, which is provided at the side end edge portion  26   a  located at the inner side of the upper side wall  38 . The stress concentration portion  42  is formed in a U-shape at an intermediate position between the joining points Pa 1  and Pa 2 , and extends from the side end edge of the side end edge portion  26   a  in a direction substantially perpendicular to the axial direction. The stress concentration portion  42  is provided to extend to cross a joining line that connects the joining points Pa 1  to PaX. In the embodiment, the stress concentration portion  42  is provided in only the upper side wall  38  to which a relatively high load is applied. However, it is possible to provide a similar stress concentration portion in the lower side wall  40  as well, as needed. 
     Because the stress concentration portion  42  is provided, local strength at the stress concentration portion  42  is low as compared to the joining point Pa 1  ahead of the stress concentration portion  42 , and the joining point Pa 2  behind the stress concentration portion  42 . Therefore, when the axial compression load is applied, a portion near the stress concentration portion  42  is plastically deformed so that the portion is bent into a V-shape. The plastically deformed portion serves as a starting point at which deformation starts. Thus, the crush of the cylindrical body  20  into the accordion shape smoothly starts at a low load. Strength is relatively high at portions near the joining points Pa 1  to PaX, and Pb 1  to PbX. Therefore, portions at the intermediate positions between the adjacent joining points among the joining points Pa 1  to PaX, and P 111  to PbX in the axial direction are sequentially bent into a V-shape, from the portions closest to the bumper beam  10 . Thus, the crush proceeds, and the cylindrical body  20  is crushed into the accordion shape. The crush proceeds in a direction toward the side member  12 R in a chain reaction, due to an impact caused by buckling (crush) of a front portion in which the stress concentration portion  42  is provided. An additional stress concentration portion (such as a cutout) may be provided at least one intermediate position between adjacent joining points among the joining points Pa 2  to PaX, as needed. In this case, buckling is accelerated or stabilized. It is possible to provide a stress concentration portion (such as a cutout) at least one intermediate position between adjacent joining points among the joining points Pb 1  to PbX in the lower side wall  40 . In the embodiment, while the upper side wall  38  is substantially horizontal, the lower side wall  40  is inclined upward in the direction toward the bumper beam  10 , and thus, the upper side wall  38  and the lower side wall  40  form an asymmetric taper shape. Therefore, imbalance in load transmission is easy to occur between the side walls  38  and  40 , and thus, crush behavior is easy to be unstable. However, it is possible to stabilize the crush behavior while ensuring desired impact energy absorbing performance, by appropriately tuning (adjusting) the position, size, shape, and the like of the stress concentration portion  42  provided in the upper side wall  38 . 
     As described above, in the crush box  14 R in the embodiment, the stress concentration portion  42  is provided at the overlapping portion at which the side end edge portions  26   a  and  28   a  are overlapped with each other in the upper side wall  38  in the form of a flat plate such that the stress concentration portion  42  does not reach the ridge line of the cylindrical body  20  with the polygonal section. Therefore, as compared to the case where the stress concentration portion is provided to include the ridge line of the cylindrical body with the polygonal section, or the stress concentration portion is provided in a side wall portion constituted by one plate material (for example, a right side wall  32  or a left side wall  30 ), influence on the impact energy absorbing performance (i.e., the axial crush strength of the cylindrical body  20 ) is small. Thus, it is possible to easily tune the shape, size, and the like of the stress concentration portion  42  to stabilize the crush behavior, while ensuring desired impact energy absorbing performance. The upper side wall  38  and the lower side wall  40  constitute the asymmetric taper portion, and the inclination of the upper side wall  38  with respect to the axial direction is substantially 0, and thus, a large compression load is applied to the upper side wall  38 . Therefore, it is possible to appropriately change the crush behavior by tuning the stress concentration portion  42  provided in the upper side wall  38 . Thus, in the cylindrical body  20  with the asymmetric taper shape whose crush behavior is easy to be unstable due to the imbalance in the load transmission, the crush behavior is easily stabilized by tuning the stress concentration portion  42 , and desired impact energy absorbing performance is stably obtained. 
     In the embodiment, the lower side wall  40  is inclined upward so that the distance between the upper side wall  38  and the lower side wall  40  decreases in the direction from the side member  12 R toward the bumper beam  10 . The side end edge portions  26   a  and  28   a  in the upper side wall  38  are discontinuously joined to each other at the joining points located at the predetermined intervals in the axial direction by spot welding. The stress concentration portion  42  is provided between the first joining point Pa 1  that is closest to the bumper beam  10  and the second joining point Pat that is the second closest to the bumper beam  10  among the plurality of joining points Pa 1  to PaX at which the side end edge portions  26   a  and  28   a  are joined to each other by spot welding. Therefore, the stress concentration portion  42  is plastically deformed at the initial stage of collision, and thus, the crush of the cylindrical body  20  into the accordion shape smoothly starts at a low load. Thus, predetermined impact energy absorbing performance is stably obtained from the initial stage of collision. 
     In the embodiment, the cylindrical body  20  is constituted by the paired half bodies  26  and  28  each of which has a shape obtained by substantially symmetrically dividing the cylindrical body  20  in half along the direction parallel to the axial direction. The cylindrical body  20  is formed by overlapping and joining the side end edge portions  26   a  and  28   a  of the paired half bodies  26  and  28  integrally with each other, and overlapping and joining the side end edge portions  26   b  and  28   b  of the paired half bodies  26  and  28  integrally with each other, by spot welding. The upper side wall  38  constituted by the side end edge portions  26   a  and  28   a  joined to each other by spot welding corresponds to the first side wall. The lower side wall  40  constituted by the side end edge portions  26   b  and  28   b  joined to each other by spot welding corresponds to the second side wall. Thus, the crush behavior can be easily stabilized so that desired impact energy absorbing performance can be obtained, by appropriately tuning the stress concentration portion  42  provided in the upper side wall  38  to which a relatively high load is applied. 
     In the embodiment, the cylindrical body  20  includes the substantially horizontal upper side wall  38  and the lower side wall  40  that is inclined upward over the entire length in the axial direction so that the distance between the upper side wall  38  and the lower side wall  40  decreases in the direction toward the bumper beam  10 . Thus, the cylindrical body  20  has the asymmetric taper shape. The entire upper side wall  38  and the entire lower side wall  40  form the taper portion. Even when there is the imbalance in the load transmission between the upper side wall  38  and the lower side wall  40 , the crush behavior can be easily stabilized, and desired impact energy absorbing performance can be stably obtained, by appropriately tuning the stress concentration portion  42  provided in the upper side wall  38  to which a high compression load is applied. 
     For comparison, a weight-drop test was conducted by freely dropping a weight under a test condition described below, using the crush box  14 R in the embodiment, and a comparative product in which the stress concentration portion  42  is not provided. Thus, the characteristics of changes in the axial compression load and absorbed energy with respect to a compression stroke were studied.  FIGS. 4A and 4B  show obtained results. 
     Test condition 
     Weight of a weight body: 1100 kg 
     Height from which the weight body is dropped: 1.11 m 
     Collision speed: 17 km/h 
     In each of  FIGS. 4A and 4B , a solid line labeled “WITH CUTOUT” shows the characteristic of the product according to the invention (i.e., the crush box  14 R), and a dash line labeled “WITHOUT CUTOUT” shows the characteristic of the comparative product. In the case of the comparative product (in which the cutout is not provided), the axial compression load at the initial stage of collision is low, particularly the second load peak value is low, and a difference between the second load peak value and the third load peak value is large. Thus, repeated buckling wave forms are unstable. This makes the subsequent repeated buckling unstable, resulting in a decrease in the amount of absorbed energy. In contrast, in the case of the product according to the invention (in which the cutout is provided), the second load peak value is substantially equal to the third load peak, and thus, the wave forms are stable. Further, the amount of absorbed energy is increased, as compared to the comparative product in which the cutout is not provided. Note that the absorbed energy in  FIG. 4B  corresponds to the integral of the axial compression load in  FIG. 4A . 
     Although the embodiment of the invention has been described in detail with reference to the drawings, the embodiment is merely an example, and the invention can be implemented in various forms obtained by altering or modifying the embodiment based on the knowledge of those skilled in the art.