Abstract:
A bumper reinforcement member including a front reinforcement member and a back reinforcement member, wherein the front reinforcement member includes a front intermediate face, which becomes a bumper front face, and a front groove arranged above and below the front intermediate face and that recesses from the bumper front face side towards a bumper rear face side; the back reinforcement member includes a back intermediate face positioned on a front side than bottom faces of both upper and lower front grooves, and a back groove arranged above and below the back intermediate face and that recesses from the bumper front face side towards the bumper rear face side; and the front reinforcement member and the back reinforcement member have the front groove contained in the back groove without bringing at least the bottom faces of the front groove and the back groove into surface contact with each other.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a bumper reinforcement member configuring a bumper of a vehicle. 
     2. Description of the Related Art 
     A bumper of a vehicle is configured by a bumper reinforcement member for absorbing impact, and a bumper cover for covering the bumper reinforcement member. The bumper reinforcement member absorbs impact by deforming a hollow section (plastic deform of a face that forms the section). In general, a superior bumper has a high maximum value (peak load) in a three-point bending test applying a load at one point and supports a test piece at the other two points, a larger deformed amount and a larger value of integral (in other words, impulse or gross amount of energy absorption). An energy absorption property in the bumper reinforcement member can be enhanced by thickening each face that is plastic-deformed, using a high strength material, or making the cross section complicating. 
     JP2005-170234A discloses a bumper reinforcement member (impact absorption member) including a plurality of vertical walls having different heights (width in a front and back direction of a vehicle) and a horizontal wall that couples the vertical walls to each other. The bumper reinforcement member is formed with a step at the horizontal wall (4, 5, 9 in FIG. 1 of JP2005-170234A). The vertical walls are arranged in a positional relationship of interfering with each other when buckled or crushed by impact. JP2005-170234A says that, according to such a bumper reinforcement member, the peak load that occurs first (initial peak load) becomes small and the energy absorption amount can be enhanced since the impact is applied in a step wise manner. 
     The bumper reinforcement member disclosed in JP2005-170234A has a problem in that the deformation of each vertical wall is not uniform as the vertical wall is long. Therefore, the vertical walls may not necessarily interfere when buckled or crushed. Even if the vertical walls are assumed to interfere with each other, the vertical walls are believed to not interfere with each other unless the respective vertical walls are greatly buckled or crushed since each vertical wall is greatly spaced apart. Furthermore, as it is clear from the fact that the bumper reinforcement member has a plurality of peak loads (see graph of FIG. 5 of JP2005-170234A), a stable energy absorption property may not be obtained as a whole. Moreover, since the peak loads appear in plural times, the energy absorption amount inevitably becomes small as a whole. 
     JP2003-237507A discloses a bumper reinforcement member including a main reinforcement member  1  supported to a vehicle body side, and a supplementary reinforcement member  3  to be attached to a front face of the main reinforcement member. The supplementary reinforcement member has a substantially concaved sectional portion  2  and the main reinforcement member has a concaved groove portion  5 . The substantially concaved. sectional portion  2  is in contact with the concaved groove portion  5 . The bumper reinforcement member disclosed in JP2003-237507A does not require special members. The structure is also easy to process. Changes in material and special production facilities are unnecessary, and increase in cost can be suppressed. Furthermore, the energy absorption amount is enhanced while suppressing local buckling. 
     The bumper reinforcement member of JP2003-237507A is designed so that a high peak load appears only once. In this configuration, a stable energy absorption property and a large energy absorption amount are obtained, compared to JP 2005-170234A. However, since the supplementary reinforcement member is attached to the front face of the main reinforcement member, and the supplementary reinforcement member greatly projects out from the front face of the main reinforcement member, the bumper reinforcement member inevitably becomes large. Increase in weight also becomes a problem. Variety of design is also limited. Thus, the bumper reinforcement member disclosed in JP2003-237507A is difficult to use in light automobiles to which weight limits and layout limits are strictly imposed and automobiles having high designability. 
     JP2004-074834A, discloses a bumper reinforcement member including a main reinforcement member  3  of a rear face opened sectional structure with a front face  9 , an upper lateral face  10  and a lower lateral face  11 , and an supplementary reinforcement member bridged to the upper and lower lateral faces from the front face of the main reinforcement member. The supplementary reinforcement member has a mountain folded portion  14  on the front face  9 . The main reinforcement member  3  includes, on the front face, a front face groove  2  having a concaved cross section with a groove bottom face  12  and a groove lateral face  11 , and the supplementary reinforcement member  1  is bridged to the upper and lower lateral faces from the front face groove. The bumper reinforcement member disclosed in JP2004-074834A has a rear face opened sectional structure (structure in which the rear face is opened). The bumper reinforcement member has a high peak load and a large energy absorption amount that are not inferior to the bumper reinforcement member of a closed sectional structure. 
     The bumper reinforcement member disclosed in JP2004-074834A is not as enlarged as the bumper reinforcement member disclosed in JP2003-237507A since the supplementary reinforcement member is arranged inside the main reinforcement member. Therefore, the bumper reinforcement member of JP2004-074834A does not impose a design limitation. JP2004-074834A gives importance to controlling the deformation of an opened sectional structure. In other words, the energy absorption property of the same extent as the bumper reinforcement member of the closed sectional structure is merely ensured. Therefore, the bumper reinforcement member disclosed in JP2004-074834A does not increase the peak load or increase the energy absorption amount compared to the bumper reinforcement member of the closed sectional structure. 
     The energy absorption property in the bumper reinforcement member can be enhanced by thickening each face that is plastic-deformed, using a high strength material, or making the cross section complicating. However, the bumper reinforcement member is enlarged and the weight becomes excessively large if each face is thickened. The manufacturing cost becomes high if the high strength material is used. Making the cross section complicating deprives variety of design of the bumper reinforcement member and has a possibility of influencing the design of the entire vehicle. Therefore, miniaturization, reduction in manufacturing cost, and simplification of the cross section become important issues in enhancing the energy absorption property. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a bumper reinforcement member including a front reinforcement member and a back reinforcement member, wherein the front reinforcement member includes a front intermediate face, which becomes a bumper front face, and a front groove that is arranged above and below the front intermediate face and that recesses from the bumper front face side towards a bumper rear face side; the back reinforcement member includes a back intermediate face positioned on a front side than bottom faces of both upper and lower front grooves, and a back groove that is arranged above and below the back intermediate face and that recesses from the bumper front face side towards the bumper rear face side; and the front reinforcement member and the back reinforcement member have the front groove contained in the back groove without bringing at least the bottom faces of the front groove and the back groove into surface contact with each other. 
     The front reinforcement member and the back reinforcement member may satisfy a following condition
 
 B =( 1/10 to ½) L  
 
0≦A&lt; 1/10L
 
where B is a depth of the front groove from the bumper front face to the bottom face of the front groove, L is a depth of the back groove from the bumper front face to the bottom face of the back groove, and A is a smallest distant from a corner, which connects an upper lateral face or a lower lateral face and the bottom face of the front groove, to an upper lateral face or a lower lateral face of the back groove.
 
     Here, “front groove depth B” refers to the smallest orthogonal distance between the surface of the bumper front face and the surface of the bottom face of the front groove. The “smallest distance A” from the corner to each of the upper lateral face or the lower lateral face of the back groove refers to the smallest orthogonal distance of the surface of the corner and the upper lateral face or the lower lateral face of the back groove to which the corner faces. The bumper reinforcement member of the present invention has the back intermediate face projected to the front side than the bottom faces of the front grooves, and hence the smallest distance A of four areas can be set with the corners at four areas connecting the upper lateral face or the lower lateral face and the bottom face of each of the upper and lower front grooves facing the upper lateral face or the lower lateral face of each of the upper and lower back grooves. 
     In the bumper reinforcement member of the present invention, the back intermediate face positioned on the front side than the bottom faces of the front grooves comes into contact with the front intermediate face and starts to integrally deform when a force is applied from the bumper front face. In the present invention, a high peak load thereby appears. The S-shaped cross section formed when the lateral face of the back groove deforms. Also the S-shaped cross section formed when the lateral face of the front groove deforms. In the present invention, S-shaped cross sections formed by the back groove deformation and the front groove deformation face each other in a shifted manner. Thus, the respective expansion collide with each other thereby suppressing the deformation of the lateral face of the back groove and the lateral face of the front groove, and gradually lowering the change in energy absorption property followed from the peak load. The present invention increases the energy absorption amount of the bumper reinforcement member by the high peak load and the gradual energy absorption property. 
     When a force is applied from the bumper front face, the peak load rises until the front intermediate face and the back intermediate face are in contact with each other and start to integrally deform. When the back intermediate face and the front intermediate face start to integrally deform, about first half of the respective lateral faces of the front groove and the back groove inwardly expands to narrow the groove width, and about last half of the lateral faces outwardly expands to widen the groove width. Bach lateral face thus becomes an S-shaped cross section. Therefore, if the front groove depth B of each front groove is smaller than or equal to ½ of the back groove depth L of the back groove containing the front groove, the expansion of the last half of the lateral face of the front groove and the expansion of the first half of the lateral face of the back groove can be brought to interfere thereby suppressing the respective deformation. However, the lateral face of the front groove becomes difficult to deform if the front groove depth B is too small, and hence the front groove depth B is greater than or equal to 1/10 of the back groove depth L. 
     In this case, when the first half of the lateral face of the back groove starts to deform, the last half of the lateral face of the front groove preferably starts to deform outwardly without delay, so that the lateral face of the front groove and the lateral face of the back groove interfere as fast as possible. Therefore, the smallest distance A from the corner connecting the upper lateral face or the lower lateral face and the bottom face of the front groove to the upper lateral face or the lower lateral face of the back groove fitted with the front groove is most preferably “0 (zero)”. On the contrary, the deformation of the first half of the lateral face of the back groove and the collision with the last half of the lateral face of the front groove delay as the smallest distance A becomes greater. When the smallest distance A becomes too large, the effect in which the front groove and the back groove suppress the respective deformation may be lost. Therefore, the smallest distance A is smaller than or equal to 1/10 of the back groove depth L. 
     As described above, in the bumper reinforcement member of the present invention, the front intermediate face and the back intermediate face comes into contact with each other and start to integrally deform when the force is applied from the bumper front face, whereby high peak load is achieved. Therefore, the back intermediate face is preferably brought into contact with the front intermediate face from the bumper rear face side and integrated from the beginning to enhance the peak load to a maximum. In this case, a portion of high rigidity is formed at the middle in the vertical direction of the bumper reinforcement member. The peak load of the bumper reinforcement member thus can be enhanced to a maximum. Furthermore, since the portion of high rigidity is deformed at the early step of applying a force to the bumper front face, fluctuation of load occurring when deforming the portion of weak rigidity, or from other reasons is thus less likely to occur. Thus, the bumper reinforcement member of the present invention in which the back intermediate face is brought into contact with the front intermediate face from the bumper rear face side realizes higher peak load and more gradual energy absorption property. 
     For example, the bumper reinforcement member of the present invention has the front reinforcement member and the back reinforcement member configured as sheet metal members of separate bodies. Specifically, the front reinforcement member has a front upper flange extending from a front end of an upper lateral face of the upper front groove, a front lower flange extending from a front end of a lower lateral face of the lower front groove, and a face including the front upper flange, the front intermediate face, and the front lower flange as the bumper front face; and the back reinforcement member has a back upper flange extending from a front end of an upper lateral face of the upper back groove, the back upper flange face being brought into surface contact and joined with the front upper flange from the bumper rear face side, a back lower flange extending from a front end of a lower lateral face of the lower back groove, the back lower flange face being brought into surface contact and joined with the front upper flange from the bumper rear face side, the upper lateral face of the upper back groove as an upper bumper lateral face, the lower lateral face of the lower back groove as a lower bumper lateral face, and a face including the bottom faces of the back grooves as the bumper rear face. 
     When the front reinforcement member and the back reinforcement member are configured as separate bodies, the front upper flange and the back upper flange as well as the front lower flange and the back lower flange are in surface contact and joined, respectively. According to this configuration, the rigidity at each front end (base end at which the front groove and the back groove deform) of the upper lateral face or the lower lateral face of the front groove and the back groove is enhanced. In addition, if the front intermediate face and the back intermediate face are brought into contact, a site of high rigidity is formed. In this configuration, the front groove and the back groove are sandwiched by the sites of high rigidity (namely, above-said front end adjacent to the flanges and the contacting front and back intermediate faces), whereby the front groove and the back groove (especially, each lateral face) can relatively easily deform with respect to the bumper front face. The operation of the present invention for opposing and suppressing the deformation of the front groove and the back groove thus can be easily exhibited. The front reinforcement member and the back reinforcement member, which are sheet metal members of separate bodies, can be respectively manufactured by various types of conventionally known methods (press molding method, roll molding method, and the like). When the front intermediate face and the back intermediate face are brought into contact, such intermediate faces may be joined. 
     In the bumper reinforcement member of the present invention, the front reinforcement member and the back reinforcement member may be configured as an integrated sheet metal member. Specifically, the front reinforcement member and the back reinforcement member have, a front upper flange extending from a front end of an upper lateral face of the upper front groove folded downward to form a back upper flange to be overlapped on the front upper flange, and the back upper flange continued to a front end of an upper lateral face of the upper back groove; a front lower flange extending from a front end of a lower lateral face of the lower front groove folded upward to form a back lower flange to be overlapped on the front lower flange, and the back lower flange continued to a front end of a lower lateral face of the lower back groove; and a face including the front upper flange, the front intermediate face, and the front lower flange as the bumper front face, the upper lateral face of the upper back groove as an upper bumper lateral face, the lower lateral face of the lower back groove as a lower bumper lateral face, and a face including the bottom faces of the back grooves as the bumper rear face 
     When the front reinforcement member and the back reinforcement member are configured as integrated bodies, the front upper flange and the back upper flange as well as the front lower flange and the back lower flange are formed by folding the plate material. Namely, the plate material of the back upper flange, which extends from the front upper flange, is overlapped behind the front upper flange by folding the plate material . The lower flange is formed in the same way of folding the plate material. According to this configuration, the rigidity at each front end (base end at which the front groove and the back groove deform) of the upper lateral face or the lower lateral face of the front groove and the back groove is enhanced. In addition, if the front intermediate face and the back intermediate face are brought into contact, a site of high rigidity is formed. In this configuration, the front groove and the back groove are sandwiched by the sites of high rigidity (namely, above-said front end adjacent to the flanges and the contacting front and back intermediate faces), whereby the front groove and the back groove (especially, each lateral face) can relatively easily deform with respect to the bumper front face. The operation of the present invention for opposing and suppressing the deformation of the front groove and the back groove thus can be easily exhibited. The front reinforcement member and the back reinforcement member, which are an integrated sheet metal member, preferably form the front intermediate face or the back intermediate face with the end edges of the plate material abutted to each other. The front reinforcement member and the back reinforcement member, which are an integrated sheet metal member, can be traversably integrally manufactured by various types of conventionally known methods (roll molding method, and the like). When the front intermediate face and the back intermediate face are brought into contact, such intermediate faces may be joined. 
     According to the present invention, enlargement and increase in manufacturing cost can be prevented. The bumper reinforcement member that enhances the energy absorption property can be provided without affecting the designability of the bumper in the vehicle. Specifically, the peak that is generated at first is increased by bringing the front intermediate face of the front reinforcement member and the back intermediate face of the back reinforcement member closer or into contact with each other, the deformation of the front groove of the front reinforcement member and the back groove of the back reinforcement member are opposed to gradually lower the change from the peak load, and the stable and large energy absorption property is achieved. Furthermore, the present invention achieves higher peak load and more gradual energy absorption property by bringing the back intermediate face into contact with the front intermediate face from the bumper rear face side. 
    
    
     
         FIG. 1  is a perspective view of a bumper reinforcement member configured by a front reinforcement member and a back reinforcement member of separate bodies (present example); 
         FIG. 2  is a cross-sectional view of the bumper reinforcement member of the present example; 
         FIG. 3  is a cross-sectional view corresponding to  FIG. 2  of the bumper reinforcement member of another example in which a smallest distance A is 0 (zero); 
         FIG. 4  is a cross-sectional view corresponding to  FIG. 2  of the bumper reinforcement member of another example in which a front intermediate face and a back intermediate face are separated; 
         FIG. 5  is a perspective view illustrating another example of a bumper reinforcement member configured by an integrated front reinforcement member and back reinforcement member; 
         FIG. 6  is a cross-sectional view corresponding to  FIG. 2  illustrating the bumper reinforcement member of the present example in which a load F is applied to a bumper front face deforming the same; 
         FIG. 7  is a schematic view illustrating a test device for a three-point bending load test for applying the load F in the example and comparative examples; 
         FIG. 8  is a cross-sectional view of the example; 
         FIG. 9  is a cross-sectional view of a first comparative example; 
         FIG. 10  is a cross-sectional view of a second comparative example; 
         FIG. 11  shows a force-stroke curve indicating the results of the three-point bending load test of the example and each of the comparative examples; 
         FIG. 12  is a graph showing a relationship between the smallest distance A and a peak load; 
         FIG. 13  is a graph showing a relationship between the smallest distance A and an energy absorption amount; 
         FIG. 14  is a graph showing a relationship between a front groove depth B and the peak load; and 
         FIG. 15  is a graph showing a relationship between the front groove depth B and the energy absorption amount. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The best modes for carrying out the present invention are described below with reference to the drawings . As illustrated in  FIG. 1  and  FIG. 2 , a bumper reinforcement member  1  of the present invention is configured by assembling a front reinforcement member  2  and a back reinforcement member  3 , which are separate bodies made of sheet metal. The bumper reinforcement member  1  is supported by a vehicle frame (not illustrated) through a supporting stay  4  for supporting the back reinforcement member  3 . The supporting stay  4  is connected to bottom faces  323 ,  333  of upper and lower back grooves  32 ,  33  of the back reinforcement member  3  configuring a bumper rear face. The bumper reinforcement member  1  is normally bridged across a pair of right and left supporting stays  4 ,  4 . The supporting stay  4  may be a square pipe having rigidity, a pipe-shaped member having impact absorbing function, or a member separately incorporating an impact absorbing device. 
     The front reinforcement member  2  has a configuration in which upper and lower front grooves  22 ,  23  recessed from a bumper front face side toward a bumper rear face side. The upper and lower front grooves  22 ,  23  are arranged above and below a front intermediate face  21  parallel to a perpendicular direction. The front grooves  22 ,  23  have a channel cross section configured by upper lateral faces  221 ,  231  and lower lateral faces  222 ,  232  parallel to a horizontal direction, as well as bottom faces  223 ,  233  parallel to the perpendicular direction. The front reinforcement member  2  of the present example has a front upper flange  24  extending upward in the perpendicular direction from a front end of the upper lateral face  221  of the upper front groove  22 , and a front lower flange  25  extending downward in the perpendicular direction from .a front end of the lower lateral face  232  of the lower front groove  23 . A face including the front upper flange  24 , the front intermediate face  21 , and the front lower flange  25  is the bumper front face. 
     The back reinforcement member  3  has a configuration in which upper and lower back grooves  32 ,  33  recessed from the bumper front face side towards the bumper rear face side. The upper and lower back grooves  32 ,  33  are arranged above and below a back intermediate face  31 , which is positioned on the front side than the bottom faces  223 ,  233  of both front grooves  22 ,  23  and which is parallel to the perpendicular direction. The back grooves  32 ,  33  have a channel cross section configured by upper lateral faces  321 ,  331  and lower lateral faces  322 ,  332  parallel to the horizontal direction, as well as bottom faces  323 ,  333  parallel to the perpendicular direction. The back reinforcement member  3  of the present example has a back upper flange  34  extending upward in the perpendicular direction from a front end of the upper lateral face  321  of the upper back groove  32 , and a back lower flange  35  extending downward in the perpendicular direction from a front end of the lower lateral face  332  of the lower back groove  33 . The upper lateral face  321  of the upper back groove  32  becomes the upper bumper lateral face, the lower lateral face  332  of the lower back groove  33  becomes the lower bumper lateral face, and a face including each of the bottom faces  323 ,  333  of the back grooves  32 ,  33  becomes the bumper rear face. 
     The front reinforcement member  2  and the back reinforcement member  3  of the present example have the back intermediate face  31  in contact with the front intermediate face  21  from the bumper rear face side, the back upper flange  34  in contact with the front upper flange  24  from the bumper rear face side, and the back lower flange  35  in contact with the front lower flange  25  from the bumper rear face side. The bumper reinforcement member  1  is configured by joining the front intermediate face  21  and the back intermediate face  31 , the front upper flange  24  and the back upper flange  34 , as well as the front lower flange  25  and the back lower flange  35  by spot welding or arc welding. 
     A front groove depth B from the bumper front face (=front intermediate face  21 ) of the front groove  22  to the bottom face  223  is assumed to be 1/10 to ½ of a back groove depth L from the bumper front face of the back groove  32  to the bottom face  323  (B= 3/10 L in the present example). The front groove depth B is a parameter directly related to both a peak load and an energy absorption amount, where the peak load and the energy absorption amount are made large at 1/10 to ½ of the back groove depth L. Therefore, the front groove depth B is preferably 1/10 to ½ of the back groove depth L and determined in the range of ¼ to 3/10 of the back groove depth L, if possible, although subjected to restriction such as vehicle design. The back groove depth L is determined from a depth DA (see  FIG. 8  to be described later) obtained for the bumper reinforcement member  1 . These are the same for the front groove depth B from the bumper front face (=front intermediate face  21 ) of the front groove  23  to the bottom face  233 . 
     A smallest distance A from a corner  224 ,  225  connecting the upper lateral face  221  or the lower lateral face  222  and the bottom face  223  of the front groove  22  to the upper lateral face  321  or the lower lateral face  322  of the back groove  32  is smaller than 1/10 of the back groove depth L (A= 1/20 L in the present example). The smaller the smallest distance A, in other words, the closer the corner  224 ,  225  connecting the upper lateral face  221  or the lower lateral face  222  of the front groove  22  and the bottom face  223  is from the upper lateral face  321  or the lower lateral face  322  of the back groove  32 , the greater the peak load and the energy absorption amount become. Therefore, the corner  224 ,  225  may be closely attached to the upper lateral face  321  or the lower lateral face  322  of the back groove  32 , as in the bumper reinforcement member  1  of another example in  FIG. 3 . This is the same for the lower front groove  23  and lower back groove  33 . The smallest distance A is from a corner  234 ,  235  connecting the upper lateral face  231  or the lower lateral face  232  and the bottom face  233  of the front groove  23  to the upper lateral face  331  or the lower lateral face  332  of the back groove  33 . A width of the back groove  32  or the back groove  33  (opposing distance of the upper lateral face  321 ,  331  to the lower lateral face  322 ,  332 ) is a numerical value obtained by adding the smallest distance A to the width of the front groove  22  or the front groove  23 . 
     The bumper reinforcement member  1  of the present example uses the same material for the front reinforcement member  2  and the back reinforcement member  3 , and has the same plate thickness. The present invention merely needs to be able to oppose the deformation of the back grooves  32 ,  33  and the deformation of the front grooves  22 ,  23 . The front reinforcement member  2  and the back reinforcement member  3  may be made of different materials or may have different plate thicknesses as long as the back grooves  32 ,  33  and the front grooves  22 ,  23  deform at the same amount at the same timing. Furthermore, the corners connecting each face are all formed to an arcuate cross-sectional shape, but the size of the radius of the arc does not influence the effect of the present invention. Thus, the corner connecting the front intermediate face  21  and the lower lateral face  222  of the front groove  22 , for example, may be formed to a right angle, if possible. 
     The bumper reinforcement member  1  of the present invention may be configured with the front intermediate face  21  and the back intermediate face  31  separated as in  FIG. 4  as long as the back intermediate face  31  is positioned on the front side than each bottom face  223 ,  233  of the front groove  22 ,  23 . Thus, an effect of increasing the energy absorption amount of the present invention can be achieved by opposing the deformation of the lower lateral face  222  of the upper front groove  22  to the deformation of the lower lateral face  322  of the upper back groove  32 , and opposing the deformation of the upper lateral face  231  of the lower front groove  23  and the deformation of the upper lateral face  331  of the lower back groove  33 , and suppressing the respective deformation. 
     The bumper reinforcement member  1  of the present invention may be configured with the front reinforcement member  2  and the back reinforcement member  3  as an integrated sheet metal member, as in  FIG. 5 . Specifically, the front reinforcement member  2  and the back reinforcement member  3  have the front upper flange  24  extending from the front end of the upper lateral face  221  of the upper front groove  22  folded downward to form the back upper flange  34  to be overlapped on the front upper flange  24 , the back upper flange  34  continued to the front end of the upper lateral face  321  of the upper back groove  32 , the front lower flange  25  extending from the front end of the lower lateral face  232  of the lower front groove  23  folded upward to form the back lower flange  35  to be overlapped on the front lower flange  25 , the back lower flange  35  continued to the front end of the lower lateral face  332  of the lower back groove  33 , and the end edges of the plate material is brought into surface contact and abutted to the front intermediate face  21  from the bumper rear face side thereby forming the back intermediate face  31 . 
     The bumper reinforcement member  1  of another example described above has a face including the front upper flange  24 , the front intermediate face  21  and the front lower flange  25  as the bumper front face, the upper lateral face  321  of the upper back groove  32  as the upper bumper lateral face, the lower lateral face  332  of the lower back groove  33  as the lower bumper lateral face, and a face including each of the bottom faces  323 ,  333  of the back grooves  32 ,  33  as the bumper rear face. Besides the bumper reinforcement member  1  of another example has the front upper flange  24  and the back upper flange  34 , and the front lower flange  25  and the back lower flange  35  respectively continued, the bumper reinforcement member is spot-welded or arc-welded for reinforcement. The end edge of the plate material forming the back intermediate face  31  is spot-welded or arc-welded to the front intermediate face  21 . 
     When the bumper reinforcement member  1  of the present example ( FIG. 1  and  FIG. 2 ) is subjected to a load F at the bumper front face, the front upper flange  24 , the front intermediate face  21  and the front lower flange  25 , which configure the bumper front face, deform in the extending direction (only the portion applied with the load F is recessed), as illustrated in  FIG. 6 , thereby deforming the front grooves  22 ,  23  and the back grooves  32 ,  33 , respectively, and receding the bumper front face. In this case, the bumper reinforcement member  1  of the present example has the front upper flange  24  integrated with the back upper flange  34 , the front intermediate face  21  integrated with the back intermediate face  31 , and the front lower flange  25  integrated with the back lower flange  35 . Therefore, the rigidity of the bumper front face is enhanced, and the peak load that enhances the force-stroke property (load-displacement property) is high. 
     When the front upper flange  24 , the front intermediate face  21 , and the front lower flange  25  configuring the bumper front face deform in the extending direction, they recede in the direction of the bumper rear face, and the first half of the opposing lateral faces ( 221  and  222 ,  231  and  232 ) of the front groove  22  and the front groove  23  recess so as to approach each other. The last half expands so as to move away from each other. Since the back upper flange  34 , the back intermediate face  31 , and the back lower flange  35  also recede in the direction of the bumper rear face, the first half of the opposing lateral faces ( 321  and  322 ,  331  and  332 ) of the back groove  32  and the back groove  33  recess so as to approach each other. The last half expands so as to move away from each other. 
     The opposing lateral faces ( 221  and  222 ,  231  and  232 ) of the front groove  22  and the front groove  23 , and the opposing lateral faces ( 321  and  322 ,  331  and  332 ) of the back groove  32  and the back groove  33  are thus all the same in deforming to an S-shaped cross section. However, the bumper reinforcement member  1  of the present example has the front groove depth B of 3/10 of the back groove depth L (B= 3/10 L). Therefore, even if the front grooves  22 ,  23  and the back grooves  32 ,  33  all deform to an S-shaped cross section, the last half of the opposing lateral faces ( 221  and  222 ,  231  and  232 ) of the front groove  22  and the front groove  23  that expand to move away from each other and the first half of the opposing lateral faces ( 321  and  322 ,  331  and  332 ) of the back groove  32  and the back groove  33  that recess to approach each other collide at a colliding point C as in  FIG. 6 . The deformation of the front grooves  22 ,  23  and the back groove  32 ,  33  is thus respectively suppressed, and a large energy absorption amount is achieved. 
     EXAMPLE 
     A three-point bending load test was conducted to evaluate the bumper reinforcement member  1  of the present invention. The test device used in the three-point bending load test is a generally known device. Specifically, as illustrated in  FIG. 7 , the long bumper reinforcement member  1  is supported at bilaterally symmetric positions on the bumper rear face  13  side by supporting members  5 ,  5  having a distal end face of an arcuate cross section. The bumper front face is then pushed at the center in the extending direction with an application member  6  having a distal end, face of an arcuate cross section to apply the load F. 
     In the example, a 980 MPa material having a plate thickness of 1.2 mm was used for the front reinforcement member and the back reinforcement member, as illustrated in  FIG. 8 . The height WA of the bumper reinforcement member is 80 mm, the depth DA of the bumper reinforcement member is 40 mm, the smallest distance A is 1 mm, the front groove depth B is 9 mm, and the length in the extending direction (length in the orthogonal direction in the plane of the drawing) is 1200 mm. In the example, the bumper reinforcement member is supported by the supporting members  5 ,  5  arranged at a spacing of 880 mm. 
     A first comparative example corresponds to JP2003-237507 as illustrated in  FIG. 9 , and has a structure in which a supplementary reinforcement member having a depth of 17 mm is attached to a front face of a main reinforcement member having a depth of 28 mm. The main reinforcement member and the supplementary reinforcement member respectively use the 980 MPa material having a plate thickness of 1.2 mm. A groove formed in the main reinforcement material has a width MW of 28 mm and a depth ML of 14.8 mm, and a groove formed in the supplementary reinforcement member has a width SW of 16 mm and a depth SL of 20.8 mm. 
     A second comparative example corresponds to JP2004-074834 as illustrated in  FIG. 10 , and has a structure in which a supplementary reinforcement member is attached to an inner side of a main reinforcement member. The main reinforcement member and the supplementary reinforcement member respectively use the 980 MPa material having a plate thickness of 1.2 mm. A groove formed in the main reinforcement material has a width MW of 23 mm and a depth ML of 5 mm, and a groove formed in the supplementary reinforcement member has a width SW of 36.9 mm and a depth SL of 16.4 mm. A projection site, which is the characteristic of the second comparative example, is formed to an arcuate cross section of a radius of 3 mm at a position spaced apart by 5 mm from the back face of the bumper front face. 
     The test results of the example, the first comparative example, and the second comparative example are shown in  FIG. 11 . The applied load is converted to load per unit mass in order to exclude influences of the amount of the used material in the force-stroke property of  FIG. 11 . As apparent from the force-stroke property, the load is overall high and the energy absorption amount (area surrounded by the curve) is large in the example compared to the first comparative example and the second comparative example. Furthermore, the change after reaching the peak load is also gradual, no particular fluctuation in the load is found, and the energy absorption property is stable. 
     The relationship between the smallest distance A and the peak load, the smallest distance A and the energy adsorption amount, the front groove depth B and the peak load, and the front groove depth B and the energy absorption amount was checked for the example. The configuration of the test device and the configuration of the example are as described above. The energy absorption amount is the measurement data until the displacement amount from when the application member  6  collides with the bumper front face of the example reaches 80 mm. 
     In the three-point bending load test for examining the relationship between the smallest distance A and the peak load, and between the smallest distance A and the energy absorption amount, the front groove depth B was fixed at 10 mm (about ¼ L) and the smallest distance A was changed in the range of 0 mm to 6 mm (about 1/7 L). As a result, as shown in  FIG. 12  and  FIG. 13 , the peak load is the highest and the energy absorption amount is also large when the smallest distance A is “0 (zero)”, that is, when the corner connecting the lateral face and the bottom face of the front groove is in contact with the lateral face of the back groove. The energy absorption amount gradually reduces as the smallest distance A becomes larger. The peak load starts to greatly lower when starting to exceed the smallest distance A=2 mm, that is, about 1/20 L, and the lowering starts to stop when exceeding the smallest distance A=4 mm, that is, about 1/10 L. Therefore, the peak load becomes high and the energy absorption amount becomes large if the smallest distance A is smaller than or equal to 1/10 L. 
     In the three-point bending load test for examining the relationship between the front groove depth B and the peak load, and between the front groove depth B and the energy absorption amount, the smallest distance A was fixed at 1 mm (about 1/40 L) and the front groove depth B was changed in the range of 0 mm to 40 mm (about 1 L). As a result, as shown in  FIG. 14  and  FIG. 15 , it was found that both the peak load and the energy absorption amount become maximum when the front groove depth B is 10 mm, that is, about ¼ L. Furthermore, both the peak load and the energy absorption property become small when the front groove depth B becomes small or large. The energy absorption property has a relatively gradual change even when the front groove depth B becomes large. However, the peak load becomes high when the front groove depth B is from 1/10 L to ½ L.