Patent Publication Number: US-11046137-B2

Title: Fiber-reinforced resin structure body

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority from Japanese Patent Application No. 2018-176223 filed on Sep. 20, 2018, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND 
     The disclosure relaters to a fiber-reinforced resin structure body. 
     These days, fiber-reinforced resins such as carbon fiber-reinforced plastics (CFRPs) are used in various structure bodies in order to improve strength while reducing weight. A fiber-reinforced resin structure body that is a structure body made of such a fiber-reinforced resin may include two or more fiber-reinforced resin members. In fields regarding a fiber-reinforced resin structure body including two or more fiber-reinforced resin members, technologies for improving the mechanical properties of the fiber-reinforced resin structure body are proposed. 
     For example, Japanese Unexamined Patent Application Publication No. 2000-108232 discloses a technology in which, in sandwich structure bodies each of which includes a core material and an FRP skin plate disposed on both surfaces of the core material and ends of both of which are butt-joined together, an FRP linking layer spreading over surfaces of both end areas is provided and a layer including a resin diffusion medium is provided between the butted end surfaces, in order to provide sandwich structure bodies of which ends can be easily and inexpensively joined together into one body without using a fastening member or the like, of which the joint can be provided with sufficiently high strength and rigidity, and which are excellent in external appearance. 
     SUMMARY 
     An aspect of the disclosure provides a fiber-reinforced resin structure body including a first member made of a fiber-reinforced resin, a second member made of a fiber-reinforced resin and forming a closed space by being joined to the first member, and a joint joining the first member and the second member together. One or both of the first member and the second member comprise a side wall having two side surfaces located on both sides in a direction of a load loaded on a swing end of the fiber-reinforced resin structure body. The joint is provided more inward than an outer wall surface located on an opposite side to the closed space out of the two side surfaces is. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the disclosure and are incorporated in and constitute a part of this specification. The drawings illustrate example embodiments and, together with the specification, serve to explain the principles of the disclosure. 
         FIG. 1  is a schematic diagram illustrating an example of a configuration of a suspension system including a lower arm according to an embodiment of the disclosure; 
         FIG. 2  is a disassembled perspective view of a lower arm and a suspension cross member; 
         FIG. 3  is a perspective view illustrating an example of the lower arm according to the embodiment; 
         FIG. 4  is an explanatory diagram for describing stress occurring in the lower arm when the lower arm receives a load; 
         FIG. 5  is an explanatory diagram for describing stress occurring in the lower arm when the lower arm receives a load; 
         FIG. 6A  is a cross-sectional view illustrating an example of a configuration of a lower arm according to a comparative example; 
         FIG. 6B  is a cross-sectional view illustrating a situation where the lower arm according to the comparative example receives a load; 
         FIG. 7  is a cross-sectional view illustrating an example of a configuration of a lower arm according to the embodiment; 
         FIG. 8A  is a cross-sectional view illustrating an example of a configuration of a lower arm according to a first modified embodiment; 
         FIG. 8B  is a cross-sectional view illustrating an example of a configuration of a lower arm according to the first modified embodiment; 
         FIG. 8C  is a cross-sectional view illustrating an example of a configuration of a lower arm according to the first modified embodiment; 
         FIG. 8D  is a cross-sectional view illustrating an example of a configuration of a lower arm according to the first modified embodiment; 
         FIG. 9  is a cross-sectional view illustrating an example of a configuration of a lower arm according to a second modified embodiment; 
         FIG. 10  is a cross-sectional view illustrating an example of a configuration of a lower arm according to a third modified embodiment; 
         FIG. 11  is a cross-sectional view illustrating an example of a configuration of a lower arm according to a fourth modified embodiment; 
         FIG. 12  is a cross-sectional view illustrating an example of a configuration of a lower arm according to a fifth modified embodiment; and 
         FIG. 13  is a cross-sectional view illustrating an example of a configuration of a lower arm according to a sixth modified embodiment; 
     
    
    
     DETAILED DESCRIPTION 
     In the following, some embodiments of the disclosure are described in detail with reference to the accompanying drawings. Note that sizes, materials, specific values, and any other factors illustrated in respective embodiments are illustrative for easier understanding of the disclosure, and are not intended to limit the scope of the disclosure unless otherwise specifically stated. Further, elements in the following example embodiments which are not recited in a most-generic independent claim of the disclosure are optional and may be provided on an as-needed basis. Throughout the present specification and the drawings, elements having substantially the same function and configuration are denoted with the same reference numerals to avoid any redundant description. Further, elements that are not directly related to the disclosure are unillustrated in the drawings. The drawings are schematic and are not intended to be drawn to scale. Meanwhile, also in regard to structural parts of an automobile body, constituent parts using fiber-reinforced resins such as carbon fiber-reinforced plastics (CFRPs) are becoming used for the weight reduction of the car body. For example, a fiber-reinforced resin structure body that includes two or more fiber-reinforced resin members and that has a joint between the fiber-reinforced resin members may be used as such a structural part of an automobile body. Here, a structural part of an automobile body may receive a relatively large load that generates stress such as tensile stress or compressive stress. When such stress has occurred in a structural part having a joint, breaking may occur generally from a joint with weak strength as a starting point. Thus, in the case where a fiber-reinforced resin structure body having a joint is used as a structural part of an automobile body, it is presumably desirable to improve the strength of the fiber-reinforced resin structure body against stress. 
     Thus, according to an embodiment of the disclosure, a new and improved fiber-reinforced resin structure body capable of improving strength against stress can be provided. 
     1. Suspension Apparatus 
     First, a suspension apparatus  1  including lower arms  20  of a vehicle each corresponding to a fiber-reinforced resin structure body according to the embodiment is described with reference to  FIG. 1  to  FIG. 3 .  FIG. 1  is a schematic diagram illustrating an example of the configuration of a suspension apparatus  1  of front wheels of a vehicle including lower arms  20 , and  FIG. 2  is a disassembled perspective view of a lower arm  20  and a suspension cross member  8 .  FIG. 3  is a perspective view illustrating an example of the lower arm  20 . 
     As illustrated in  FIG. 1 , in the suspension apparatus  1 , the left and right of an engine room  2  are partitioned by front wheel aprons  3  that are constituent elements of car body frames. The front wheel aprons  3  are joined to a pair of left and right side frames  5  extending in the front-rear direction of the car body. Strut towers  6  are formed on the rear side of the front wheel aprons  3 . A strut suspension  7  is housed in the strut tower  6 . The upper side of the suspension  7  is supported by a strut support  6   a  formed on the upper side of the strut tower  6 , via a strut upper mount  7   a.    
     A suspension cross member  8  is provided on the lower side of the engine room  2 . Upper surfaces of the suspension cross member  8  at both ends in the vehicle width direction are fixed to the side frames  5  via fasteners such as bolts and nuts. The rear of a not-illustrated engine is installed on the upper surface of the suspension cross member  8  via an engine mount. Arm supports  9  are protruded from lower surfaces of the suspension cross member  8  at both ends in the vehicle width direction. As illustrated in  FIG. 2 , the left and right arm supports  9  include pairs of brackets  9   a  and  9   b  facing each other in the right and left and backward and forward directions with prescribed spacings, and a bolt insertion hole  9   c  is drilled in each of the brackets  9   a  and  9   b.  A cylindrical first base  21  provided at one root end of the lower arm  20  is disposed between the brackets  9   a  and  9   b.    
     The lower arm  20  has a substantially T-like or L-like planar shape that continues from the first base  21  serving as one root end to a tip  23 , branches from a central area to extend to the rear side, and continues to a second base  22  serving as another root end. A not-illustrated circular cylindrical member is press-fitted in the first base  21  of the lower arm  20 . The shaft of a bolt  12  inserted from the outside into the bolt insertion holes  9   c  drilled in the brackets  9   a  and  9   b  is caused to pierce the circular cylindrical member, and the shaft of the bolt  12  is fastened by a nut  13 . 
     A not-illustrated circular cylindrical member is press-fitted in the second base  22 . The second base  22  is axially supported at the side frame  5  via the circular cylindrical member. A not-illustrated circular cylindrical member is press-fitted in the tip  23  serving as a swing end. The tip  23  is linked to a not-illustrated ball joint via the circular cylindrical member, and a not-illustrated wheel hub configured to fix a front wheel  11  is supported in a freely rotatable manner. Thereby, the lower arm  20  supports the lower side of the suspension  7  via a not-illustrated hub housing, and is supported at the suspension cross member  8  and the side frame  5  in a swingable manner. 
     As illustrated in  FIG. 3 , the lower arm  20  includes the first base  21  linked to the suspension cross member  8 , the second base  22  linked to a side frame, and the tip  23  to which a ball joint is linked. A circular cylindrical member  27  is press-fitted in the first base  21 . A circular cylindrical member  28  is press-fitted in the second base  22 . A circular cylindrical member  29  is press-fitted in the tip  23 . The circular cylindrical member  27  press-fitted in the cylindrical first base  21  has a center axis substantially agreeing with the front-rear direction of the vehicle, and enables upward and downward swinging of the tip  23 . The circular cylindrical member  28  press-fitted in the second base  22  has a center axis running along a substantially vertical direction, and enables horizontal swinging of the tip  23 . 
     The lower arm  20  according to the embodiment is a fiber-reinforced resin structure body in which two members each made of a fiber-reinforced resin are joined to each other. More specifically, as illustrated in  FIG. 3 , the lower arm  20  is obtained by a first member  210  made of a fiber-reinforced resin and a second member  220  made of a fiber-reinforced resin being joined together at a joint  260 . 
     In the case where a member having a three-dimensional shape is molded using a fiber-reinforced resin like in the lower arm  20 , generally there is a case where molding by pressing is difficult. Thus, as described later, each of the first member  210  and the second member  220  can be produced by, for example, sticking a laminated fiber-reinforced resin sheet to a molding surface of a mold and hardening the sheet. In each of the first member  210  and the second member  220  thus produced, the processing accuracy of the surface on the molding surface side is relatively high. Thus, the first member  210  and the second member  220  produced in this way are joined to each other such that their molding surfaces appear on the surface, and a lower arm  20  having a three-dimensional shape is produced. 
     As described later, the lower arm  20  described above may receive a relatively large load that generates stress such as tensile stress or compressive stress. The lower arm  20  according to the embodiment has the joint  260  in a position where such stress relatively does not concentrate. Thereby, according to the embodiment, even when such a lower arm  20  receives a relatively large load, the occurrence of breaking from the joint  260  as a starting point can be prevented. In the following, stress occurring in the lower arm  20  is described, and then details of the lower arm  20  capable of improving strength against stress are described. 
     2. Lower Arm 
     2-1. Stress of Lower Arm 
     First, stress occurring in the lower arm  20  is described with reference to  FIG. 4  and  FIG. 5 .  FIG. 4  and  FIG. 5  are explanatory diagrams for describing stress occurring in the lower arm  20  when the tip  23  of the lower arm  20  receives a load.  FIG. 4  and  FIG. 5  illustrate schematic diagrams of the lower arm  20  illustrated in  FIG. 3  as viewed from the second member  220  side. 
       FIG. 4  illustrates a state where a load directed to the car body rear side is acting on the tip  23 . Arrow F 10  in  FIG. 4  illustrates the direction of the load acting on the tip  23 . As illustrated in  FIG. 4 , the load acting on the tip  23  has a component in the direction from the first base  21  toward the second base  22 . In the lower arm  20 , the first base  21  and the second base  22  are fixed to the car body. Therefore, when a load directed to the car body rear side is acting on the tip  23  as illustrated in  FIG. 4 , bending deformation in which the tip  23  warps toward the car body rear side occurs in the lower arm  20 . Consequently, tensile force is loaded on a side wall  240   a  on the car body front side located between the tip  23  and the first base  21 , and stress occurs in the side wall  240   a.  On the other hand, compressive force is loaded on a side wall  240   b  on the car body rear side located between the tip  23  and the second base  22 , and stress occurs in the side wall  240   b.    
       FIG. 5  illustrates a state where a load directed to the car body front side is acting on the tip  23 . Arrow F 20  in  FIG. 5  illustrates the direction of the load acting on the tip  23 . As illustrated in  FIG. 5 , the load acting on the tip  23  has a component in the direction from the second base  22  toward the first base  21 . When a load directed to the car body front side is acting on the tip  23  as illustrated in  FIG. 5 , bending deformation in which the tip  23  warps toward the car body front side occurs in the lower arm  20 . Consequently, compressive force is loaded on the side wall  240   a  on the car body front side, and stress occurs in the side wall  240   a.  On the other hand, tensile force is loaded on the side wall  240   b  on the car body rear side, and stress occurs in the side wall  240   b.    
     Here, a joint of a lower arm according to a comparative example in which, unlike the lower arm  20  according to the embodiment, a joint is provided in a position where stress concentrates is described.  FIG. 6A  is a cross-sectional view for describing an example of the configuration of a lower arm  90  according to the comparative example.  FIG. 6B  is a cross-sectional view for describing a situation where the lower arm  90  according to the comparative example receives a load.  FIG. 6A  and  FIG. 6B  illustrate cross sections in the tip of the lower arm  90  according to the comparative example, and correspond to the A-A cross section, which is a cross section along the direction of the load at the tip  23  of the lower arm  20  illustrated in  FIG. 4  and  FIG. 5 . 
     As illustrated in  FIG. 6A , the lower arm  90  according to the comparative example is obtained by a first member  910  and a second member  920  being joined together at a joint  960 . The first member  910  is a member made of a fiber-reinforced resin and having a concavity opened on the second member  920  side, and has a side  940   a  on the car body front side and a side  940   b  on the car body rear side. The second member  920  is a member made of a fiber-reinforced resin and having a concavity opened on the first member  910  side, and has a side  940   c  on the car body front side and a side  940   d  on the car body rear side. The concavity side of the first member  910  and the concavity side of the second member  920  face each other, and an end of the side  940   a  and an end of the side  940   c  are joined together at a joint  960   a  on the car body front side. Further, an end of the side  940   b  and an end of the side  940   d  are joined together at a joint  960   b  on the car body rear side. 
     Thereby, a lower arm  90  having a side wall  940   ac  located on the car body front side and a side wall  940   bd  located on the car body rear side, and a closed space  930  formed between the first member  910  and the second member  920  is formed. In the side wall  940   ac , a joint  960   a  that continues from the outer wall surface of the side wall  940   ac  located on the opposite side of the side wall  940   ac  from the closed space  930  to the inner wall surface of the side wall  940   ac  located on the closed space  930  side is formed. In the side wall  940   bd , a joint  960   b  that continues from the outer wall surface of the side wall  940   bd  located on the opposite side of the side wall  940   bd  from the closed space  930  to the inner wall surface of the side wall  940   bd  located on the closed space  930  side is formed. That is, in the lower arm  90  according to the comparative example, parts of the joint  960  are exposed on outer wall surfaces of a side wall  940 . 
     The side wall  940   ac  on the car body front side illustrated in  FIG. 6A  is located in an area of the lower arm  90  between the tip and a first base, and the side wall  940   bd  on the car body rear side is located in an area of the lower arm  90  between the tip and a second base. In a state where a load directed to the car body rear side is acting on the tip of the lower arm  90  as described with reference to  FIG. 4 , stress occurs in the side wall  940   ac  on the car body front side and the side wall  940   bd  on the car body rear side illustrated in  FIG. 6A . Further, in a state where a load directed to the car body front side is acting on the tip of the lower arm  90  as described with reference to  FIG. 5 , stress occurs in the side wall  940   ac  on the car body front side and the side wall  940   bd  on the car body rear side illustrated in  FIG. 6A . 
     In general, a position of an object where the maximum stress occurs when bending deformation occurs in the object is an outer layer of the object. Therefore, in a state where a load directed in the car body front-rear direction is acting on the tip of the lower arm  90 , a position of the lower arm  90  where stress concentrates is outer wall surfaces of the side wall  940 . 
     Further, in general, when stress occurs in an object having a joint, a position of the object that has weak strength and is likely to be a starting point of breaking is the joint. Therefore, in the lower arm  90  according to the comparative example in which parts of the joint  960  are exposed on outer wall surfaces of the side wall  940  where stress concentrates, when a load directed in the car body front-rear direction acts on the tip and stress has occurred in the lower arm  90 , breaking may occur from the joint  960  as a starting point, as illustrated in  FIG. 6B . 
     2-2. Configuration of Lower Arm 
     Next, the configuration of the lower arm  20  according to the embodiment is described with reference to  FIG. 7 .  FIG. 7  is a cross-sectional view for describing an example of the configuration of the lower arm  20  according to the embodiment.  FIG. 7  is specifically a cross-sectional view in the A-A cross section, which is a cross section along the direction of the load at the tip  23  of the lower arm  20  according to the embodiment illustrated in  FIG. 4  and  FIG. 5 . 
     As illustrated in  FIG. 7 , the lower arm  20  according to the embodiment is obtained by the first member  210  and the second member  220  being joined together at the joint  260 . As illustrated in  FIG. 7 , the first member  210  is a member made of a fiber-reinforced resin in which a cross section along the direction of the load at the tip  23  of the lower arm  20  is formed in a substantially U-like shape. The first member  210  has a side wall  240  substantially orthogonal to the direction of the load and a bottom surface  250  substantially parallel to the direction of the load. The second member  220  is a member made of a fiber-reinforced resin in which a cross section along the direction of the load is formed substantially in a straight line. The second member  220  forms a closed space  230  by being joined to the first member  210 . 
     Each of the first member  210  and the second member  220  can be produced by various production methods. For example, each of the first member  210  and the second member  220  is produced by a method in which a fiber-reinforced resin sheet in which reinforcing fiber is impregnated with a matrix resin is stacked on, for example, a molding surface of a mold made of a metal, a fiber-reinforced resin, or the like, the resulting fiber-reinforced resin stacked body is covered with a covering material, a vacuum pump is used to depressurize the space between the covering material and the mold and thereby the fiber-reinforced resin sheet is stuck to the molding surface of the mold, and the fiber-reinforced resin sheet is hardened. 
     Each of the first member  210  and the second member  220  may be produced also by a method in which a fiber-reinforced resin sheet is stacked on a molding surface of a mold, the resulting fiber-reinforced resin stacked body is covered with a covering material, a fixed member is fixed above the covering material, air or vapor is fed to the space between the covering material and the fixed member to pressurize the space and thereby the fiber-reinforced resin sheet is stuck to the molding surface of the mold via the covering material, and the fiber-reinforced resin sheet is hardened. The space between the covering material and the fixed member fixed above the covering material may be pressurized while being heated using an autoclave apparatus. 
     Each of the first member  210  and the second member  220  may be produced also by a method in which the depressurization of the space between the covering material and the mold and the pressurization of the space between the covering material and the fixed member fixed above the covering material are performed in parallel. 
     The fiber-reinforced resin sheet serving as a molding material is formed by impregnating reinforcing fiber with a matrix resin. The reinforcing fiber used is not particularly limited; for example, the reinforcing fiber may be carbon fiber, glass fiber, aramid fiber, or the like, or these reinforcing fibers may be used in combination. Among them, carbon fiber has high mechanical properties, and allows strength design to be made easily; thus, the reinforcing fiber preferably contains carbon fiber. 
     The reinforcing fiber may be continuous fiber continuing from one end to the other end of the fiber-reinforced resin sheet, or may be short fiber shorter than the length from one end to the other end of the fiber-reinforced resin sheet. Continuous fiber and short fiber may coexist in one fiber-reinforced resin sheet. The fiber-reinforced resin sheet stacked in the production process of each of the first member  210  and the second member  220  may be a laminated fiber-reinforced resin sheet; and may include a fiber-reinforced resin sheet in which fibers are orientated in one direction, and may include a fiber-reinforced resin sheet in which reinforcing fibers are disposed in a plurality of directions. By equalizing the orientation directions of reinforcing fibers of each fiber-reinforced resin sheet, the strength to the orientation direction of the resulting first member  210  or second member  220  can be effectively improved. Further, by varying the orientation directions of reinforcing fibers of one or both of the fiber-reinforced resin sheets laminated together, anisotropy can be provided to the strength of the resulting first member  210  or second member  220 . 
     A thermoplastic resin or a thermosetting resin is used as the matrix resin of the fiber-reinforced resin sheet. Examples of the thermoplastic resin include a polyethylene resin, a polypropylene resin, a polyvinyl chloride resin, an ABS resin, a polystyrene resin, an AS resin, a polyamide resin, a polyacetal resin, a polycarbonate resin, a thermoplastic polyester resin, a polyphenylene sulfide (PPS) resin, a fluorine resin, a polyetherimide resin, a polyether ketone resin, a polyimide resin, and the like. 
     One or a mixture of two or more of these thermoplastic resins may be used as the matrix resin. Alternatively, the matrix resin may be a copolymer of these thermoplastic resins. In the case where the matrix resin is a mixture of these thermoplastic resins, a compatibilizer may be used in combination. Further, the matrix resin may contain a bromine-based fire retardant, a silicon-based fire retardant, red phosphorus, or the like as a fire retardant. 
     In this case, examples of the thermoplastic resin used include polyolefin-based resins such as polyethylene and polypropylene, polyamide-based resins such as nylon 6 and nylon 66, polyester-based resins such as polyethylene terephthalate and polybutylene terephthalate, and resins such as a polyether ketone, a polyether sulfone, and an aromatic polyamide. In particular, the thermoplastic matrix resin is preferably at least one selected from the group consisting of a polyamide, polyphenylene sulfide, polypropylene, a polyether ether ketone, and a phenoxy resin. 
     Examples of the thermosetting resin that can be used as the matrix resin include an epoxy resin, an unsaturated polyester resin, a vinyl ester resin, a phenolic resin, a polyurethane resin, a silicon resin, and the like. One or a mixture of two or more of these thermosetting resins may be used as the matrix resin. In the case where any of these thermosetting resins is used for the matrix resin, an appropriate hardener or reaction accelerator may be added to the thermosetting resin. 
     The fiber-reinforced resin sheet is produced by, for example, a method in which reinforcing fiber is impregnated with a matrix resin while the reinforcing fiber is continuously delivered, by a common process such as a film impregnation method or a melt impregnation method. The fiber-reinforced resin sheet is cut to a desired size, and thereby a fiber-reinforced resin sheet as a molding material is obtained. Ends in the width direction of a plurality of fiber-reinforced resin sheets each cut in a desired size may be joined to each other by an adhesive or the like, and thereby a fiber-reinforced resin sheet with a desired width and a desired length may be formed. The thickness of the fiber-reinforced resin sheet may be a value in the range of 0.03 to 1 mm, for example. 
     The side wall  240  of the first member  210  has two side surfaces located on both sides in the direction of the load at the tip  23  of the lower arm  20 . Specifically, an outer wall surface located on the opposite side of the side wall  240  from the closed space  230  and an inner wall surface located on the closed space  230  side are formed on the side wall  240 . An outer wall surface located on the car body front side and an inner wall surface located on the car body rear side are formed on the side wall  240   a  on the car body front side, and an outer wall surface located on the car body rear side and an inner wall surface located on the car body front side are formed on the side wall  240   b  on the car body rear side. 
     The joint  260  joining the first member  210  and the second member  220  together is provided more on the inside than the outer wall surface of the side wall  240  is. For example, as illustrated in  FIG. 7 , the second member  220  is disposed on the inside of the first member  210 , and is joined to the first member  210 . More specifically, the ends of the second member  220  located on both sides in the direction of the load at the tip  23  of the lower arm  20  are joined to the inner wall surfaces of the side wall  240  of the first member  210  by an adhesive, and thereby the joint  260  is formed more on the inside than the outer wall surface of the side wall  240  is. That is, in the lower arm  20  according to the embodiment, the joint  260  is not exposed on the outer wall surface of the side wall  240 . 
     At this time, as the adhesive that can be used for the joining of the first member  210  and the second member  220 , an epoxy resin-based, acrylic resin-based, or urethane resin-based adhesive, or the like may be used, as appropriate. However, the method for joining the first member  210  and the second member  220  together is not limited to a method using an adhesive, and various methods may be employed, such as vibration melt pressure bonding and hot melt pressure bonding as typical examples. 
     In a state where a load directed to the car body rear side is acting on the tip  23  of the lower arm  20  as described with reference to  FIG. 4 , stress occurs in the side wall  240   a  on the car body front side and the side wall  240   b  on the car body rear side illustrated in  FIG. 7 . Further, in a state where a load directed to the car body front side is acting on the tip  23  of the lower arm  20  as described with reference to  FIG. 5 , stress occurs in the side wall  240   a  on the car body front side and the side wall  240   b  on the car body rear side illustrated in  FIG. 7 . 
     As described above, in general, a position of an object where the maximum stress occurs when bending deformation occurs in the object is an outer layer of the object. Therefore, in a state where a load directed in the car body front-rear direction is acting on the tip  23  of the lower arm  20 , a position of the lower arm  20  where stress concentrates is outer wall surfaces of the side wall  240 . Further, in general, when stress occurs in an object having a joint, a position of the object that has weak strength and is likely to be a starting point of breaking is the joint. 
     The lower arm  20  according to the embodiment has a structure in which the joint  260  is not exposed on the outer wall surface of the side wall  240 . That is, in the lower arm  20  according to the embodiment, the joint  260  does not exist in a position where stress concentrates, but is provided in a position where stress relatively does not concentrate. Thereby, in the lower arm  20  according to the embodiment, when a load directed in the car body front-rear direction acts on the tip  23  and stress has occurred in the lower arm  20 , the fear that breaking will occur from the joint  260  as a starting point can be reduced. 
     In the lower arm  20  according to the embodiment, the joining surface of the joint  260  is provided in a direction crossing the direction of the load at the tip  23  of the lower arm  20 . More specifically, as illustrated in  FIG. 7 , the joining surface of the joint  260  is formed along the inner wall surface of the side wall  240  substantially orthogonal to the direction of the load. 
     In general, when an object receives a load and experiences bending deformation, tensile stress or compressive stress may occur in part of the object. For example, in a state where a load directed to the car body rear side is acting on the tip  23  of the lower arm  20  as described with reference to  FIG. 4 , tensile stress may occur on the car body front side of a joint  260   a  on the car body front side illustrated in  FIG. 7 , and compressive stress may occur on the car body rear side of the joint  260   a . Further, tensile stress may occur on the car body front side of a joint  260   b  on the car body rear side, and compressive stress may occur on the car body rear side of the joint  260   b.  Further, in a state where a load directed to the car body front side is acting on the tip  23  of the lower arm  20  as described with reference to  FIG. 5 , compressive stress may occur on the car body front side of the joint  260   a  on the car body front side illustrated in  FIG. 7 , and tensile stress may occur on the car body rear side of the joint  260   a.  Further, compressive stress may occur on the car body front side of the joint  260   b  on the car body rear side, and tensile stress may occur on the car body rear side of the joint  260   b.    
     Here, in the case where the same material is used and the same amount of bending deformation has occurred, the difference between the magnitude of the maximum tensile stress and the magnitude of the maximum compressive stress occurring in the joint  260  is influenced by the length of the joint  260  in a direction along the direction of the load at the tip  23  of the lower arm  20 . Specifically, in the case where the length of the joint  260  along the direction of the load is long, the difference between the magnitude of the maximum tensile stress and the magnitude of the maximum compressive stress occurring in the joint  260  is large. On the other hand, in the case where the length of the joint  260  along the direction of the load is short, the difference between the magnitude of the maximum tensile stress and the magnitude of the maximum compressive stress occurring in the joint  260  is small. In the case where the difference between the magnitude of the maximum tensile stress and the magnitude of the maximum compressive stress occurring in the joint  260  is small, the fear that breaking will occur from the joint  260  as a starting point can be reduced. 
     The joining surface of the joint  260  of the embodiment is formed along the inner wall surface of the side wall  240  substantially orthogonal to the direction of the load at the tip  23  of the lower arm  20 . That is, the length of the joint  260  along the direction of the load is short. Thereby, in the lower arm  20  according to the embodiment, when a load directed in the car body front-rear direction acts on the tip  23  and stress has occurred in the lower arm  20 , the fear that breaking will occur from the joint  260  as a starting point can be reduced even more. 
     3. MODIFIED EMBODIMENTS 
     In the above, an example of the configuration of the lower arm according to the embodiment is described; however, the lower arm according to the embodiment may have various modifications. For example, in the case where a first member and a second member are joined together at a joint, a positioning structure that determines the position of the joint may be provided in one or both of the first member and the second member, and thereby the position of the joint can be easily determined while a closed space is formed. Some modified embodiments of such a lower arm will now be described. 
     3-1. First Modified Embodiment 
       FIG. 8A  to  FIG. 8D  are cross-sectional views for describing examples of the configuration of a lower arm  30  according to a first modified embodiment.  FIG. 8A  to  FIG. 8D  illustrate cross sections in the tip of the lower arm  30  according to the first modified embodiment, and correspond to the A-A cross section, which is a cross section along the direction of the load at the tip of the lower arm illustrated in  FIG. 4  and  FIG. 5 . 
     The lower arm  30  according to the first modified embodiment has a first member  310 , a second member  320 , a closed space  330 , a side wall  340 , a bottom surface  350 , and a joint  360 . The first member  310 , the second member  320 , the closed space  330 , the side wall  340 , the bottom surface  350 , and the joint  360  correspond to the first member  210 , the second member  220 , the closed space  230 , the side wall  240 , the bottom surface  250 , and the joint  260  of the lower arm  20  according to the embodiment, respectively. 
     The first modified embodiment differs from the embodiment in that a protrusion or a level difference is provided on one or both of the first member  310  and the second member  320 . In the example illustrated in  FIG. 8A , a protrusion  380  is formed from the bottom surface  350  of the first member  310  toward the second member  320  side. The tip of the protrusion  380  is in contact with the second member  320 , and determines the position of the second member  320  with respect to the first member  310 . 
     In the example illustrated in  FIG. 8B , a protrusion  382  is formed from part of the second member  320  toward the first member  310  side. The tip of the protrusion  382  is in contact with the first member  310 , and determines the position of the second member  320  with respect to the first member  310 . 
     In the example illustrated in  FIG. 8C , a protrusion  384  protruding from the side wall  340  to the closed space  330  side is formed on part of the side wall  340  of the first member  310 . The second member  320  side of the protrusion  384  is in contact with the second member  320 , and determines the position of the second member  320  with respect to the first member  310 . 
     In the example illustrated in  FIG. 8D , a level difference  390  that is set back with respect to the side wall  340  from the closed space  330  side to the opposite side to the closed space  330  is formed at an end on the opening side of the side wall  340  of the first member  310 . The position of the second member  320  with respect to the first member  310  is determined by the second member  320  being fitted in the level difference  390 . 
     The position where each of the protrusions  380 ,  382 , and  384  and the level difference  390  is provided is not limited to the example illustrated in any of  FIG. 8A  to  FIG. 8D , and may be any position whereby the position of the joint  260  can be easily determined. Further, the shape of each of the protrusions  380 ,  382 , and  384  and the level difference  390  is not limited to the example illustrated in any of  FIG. 8A  to  FIG. 8D , and may be any shape whereby the position of the joint  260  can be easily determined. Further, the contacts between the protrusion  380  and the second member  320 , the protrusion  382  and the first member  310 , the protrusion  384  and the second member  320 , and the level difference  390  and the second member  320  may be joined by, for example, an adhesive, or may not be joined. 
     Thus, in the first modified embodiment, a protrusion or a level difference is provided on one or both of the first member  310  and the second member  320 . Thereby, in the case where the first member  310  and the second member  320  are joined together at the joint  360 , the position of the joint  360  can be easily determined. Further, similarly to the embodiment, the joint  360  joining the first member  310  and the second member  320  together is provided more on the inside than the outer wall surface of the side wall  340  is. Thereby, when a load directed in the car body front-rear direction acts on the tip of the lower arm  30  and stress has occurred in the lower arm  30 , the fear that breaking will occur from the joint  360  as a starting point can be reduced. 
     3-2. Second Modified Embodiment 
       FIG. 9  is a cross-sectional view for describing an example of the configuration of a lower arm  40  according to a second modified embodiment.  FIG. 9  illustrates a cross section in the tip of the lower arm  40  according to the second modified embodiment, and corresponds to the A-A cross section, which is a cross section along the direction of the load at the tip of the lower arm illustrated in  FIG. 4  and  FIG. 5 . 
     The lower arm  40  according to the second modified embodiment has a first member  410 , a second member  420 , a closed space  430 , a side wall  440 , a bottom surface  450 , and a joint  460 . The first member  410 , the second member  420 , the closed space  430 , the side wall  440 , the bottom surface  450 , and the joint  460  correspond to the first member  210 , the second member  220 , the closed space  230 , the side wall  240 , the bottom surface  250 , and the joint  260  of the lower arm  20  according to the embodiment, respectively. 
     The second modified embodiment differs from the embodiment in that an inclination is provided on the side wall  440  of the first member  410 . In the example illustrated in  FIG. 9 , the side wall  440  of the first member  410  is formed as a surface that becomes farther from the facing side wall  440  with distance from the bottom surface  450  toward the second member  420  side. More specifically, a side wall  440   a  on the car body front side is formed as an inclined surface that becomes farther from a facing side wall  440   b  on the car body rear side with distance from the bottom surface  450  toward the second member  420  side. The side wall  440   b  on the car body rear side is formed as an inclined surface that becomes farther from the facing side wall  440   a  on the car body front side with distance from the bottom surface  450  toward the second member  420  side. 
     The second member  420  is formed with dimensions substantially coinciding with the dimensions of the opening side of the first member  410 . More specifically, as illustrated in  FIG. 9 , the length of the second member  420  along the direction of the load at the tip of the lower arm  40  substantially coincides with the length from the opening side of the inner wall surface of the side wall  440   a  to the opening side of the inner wall surface of the side wall  440   b.    
     The second member  420  is disposed on the inside of the first member  410 , and is held at the first member  410  in a position where the dimensions of the opening side of the first member  410  and the dimensions of the second member  420  coincide. Thereby, the position of the second member  420  with respect to the first member  410  is determined. 
     Thus, in the second modified embodiment, an inclination is provided on the side wall  440  of the first member  410 . Thereby, in the case where the first member  410  and the second member  420  are joined together at the joint  460 , the position of the joint  460  can be easily determined. Further, similarly to the embodiment, the joint  460  joining the first member  410  and the second member  420  together is provided more on the inside than the outer wall surface of the side wall  440  is. Thereby, when a load directed in the car body front-rear direction acts on the tip of the lower arm  40  and stress has occurred in the lower arm  40 , the fear that breaking will occur from the joint  460  as a starting point can be reduced. 
     3-3. Third Modified Embodiment 
       FIG. 10  is a cross-sectional view for describing an example of the configuration of a lower arm  50  according to a third modified embodiment.  FIG. 10  illustrates a cross section in the tip of the lower arm  50  according to the third modified embodiment, and corresponds to the A-A cross section, which is a cross section along the direction of the load at the tip of the lower arm illustrated in  FIG. 4  and  FIG. 5 . 
     The lower arm  50  according to the third modified embodiment has a first member  510 , a second member  520 , a closed space  530 , a side wall  540 , a bottom surface  550 , and a joint  560 . The first member  510 , the second member  520 , the closed space  530 , the side wall  540 , the bottom surface  550 , and the joint  560  correspond to the first member  210 , the second member  220 , the closed space  230 , the side wall  240 , the bottom surface  250 , and the joint  260  of the lower arm  20  according to the embodiment, respectively. 
     The third modified embodiment differs from the embodiment in that the lower arm  50  includes a cover member covering the first member  510  and the second member  520 . In the example illustrated in  FIG. 10 , a cover member  580  is formed over the side wall  540  and the bottom surface  550  of the first member  510  and the second member  520 . The cover member  580  is formed of, for example, a fiber-reinforced resin member containing reinforcing fiber. 
     A procedure in which the position of the second member  520  with respect to the first member  510  is determined by the cover member  580  will now be described. First, the second member  520  is joined to part of the cover member  580  formed in a flat surface shape. Subsequently, the cover member  580  is disposed on the opening side of the first member  510  so that the second member  520  joined to the cover member  580  is fitted into the opening of the first member  510 . In a state where the second member  520  is fitted in the opening of the first member  510 , the cover member  580  is bent so as to cover the side wall  540  and the bottom surface  550  of the first member  510 , and is joined to the first member  510 . Thus, the position of the second member  520  with respect to the first member  510  can be easily determined by using the cover member  580 . 
     Further, when bending deformation has occurred in the lower arm  50 , the cover member  580  strengthens the rigidity of the first member  510  and the second member  520  against tensile force loaded on the side wall  540 . The cover member  580  is specifically a unidirectional fiber-reinforced resin member in which reinforcing fiber is orientated along the direction of tensile force loaded on the side wall  540 . Here, tensile strength to the orientation direction of the fiber-reinforced resin member can be effectively improved by equalizing the orientation directions of reinforcing fibers contained in the fiber-reinforced resin member. Thus, rigidity to the direction of tensile force loaded on the side wall  540  can be strengthened by using, as the cover member  580 , a unidirectional fiber-reinforced resin member in which reinforcing fiber is orientated along the direction of tensile force loaded on the side wall  540 . 
     The cover member  580  does not necessarily need to be formed over the entire periphery of the first member  510  and the second member  520 . For example, part of the side wall  540  of the first member  510  and the second member  520  may be covered with the cover member  580 . However, by the cover member  580  being formed over the entire periphery of the first member  510  and the second member  520 , the unity of the lower arm  50  as a member is ensured even more, and therefore the rigidity of the lower arm  50  can be strengthened more effectively. 
     Thus, in the third modified embodiment, the lower arm  50  includes the cover member  580  covering the first member  510  and the second member  520 . Thereby, in the case where the first member  510  and the second member  520  are joined together at the joint  560 , the position of the joint  560  can be easily determined. Further, rigidity to the direction of tensile stress occurring in the side wall  540  can be strengthened. Further, similarly to the embodiment, the joint  560  joining the first member  510  and the second member  520  together is provided more on the inside than the outer wall surface of the side wall  540  is. Thereby, when a load directed in the car body front-rear direction acts on the tip of the lower arm  50  and stress has occurred in the lower arm  50 , the fear that breaking will occur from the joint  560  as a starting point can be reduced. 
     3-4. Fourth Modified Embodiment 
       FIG. 11  is a cross-sectional view for describing an example of the configuration of a lower arm  60  according to a fourth modified embodiment. FIG.  11  illustrates a cross section in the tip of the lower arm  60  according to the fourth modified embodiment, and corresponds to the A-A cross section, which is a cross section along the direction of the load at the tip of the lower arm illustrated in  FIG. 4  and  FIG. 5 . 
     The lower arm  60  according to the fourth modified embodiment has a first member  610 , a second member  620 , a closed space  630 , a side wall  640 , a bottom surface  650 , and a joint  660 . The first member  610 , the second member  620 , the closed space  630 , the side wall  640 , the bottom surface  650 , and the joint  660  correspond to the first member  210 , the second member  220 , the closed space  230 , the side wall  240 , the bottom surface  250 , and the joint  260  of the lower arm  20  according to the embodiment, respectively. 
     The fourth modified embodiment differs from the embodiment in that a partition is provided between the first member  610  and the second member  620 . In the example illustrated in  FIG. 11 , a partition  680  is formed in a substantially U-like shape, and a substantially U-like opening is disposed between the first member  610  and the second member  620  so as to be directed in a direction parallel to the direction of the load at the tip of the lower arm  60 . One end of the partition  680  is in contact with the bottom surface  650  of the first member  610 , and the other end is in contact with the second member  620 ; thereby, the position of the second member  620  with respect to the first member  610  is determined. At this time, the partition  680  exhibits also the effect of increasing the second moment of area of the lower arm  60  in a cross section along the direction of the load at the tip of the lower arm  60 . 
     The position where the partition  680  is provided is not limited to the example illustrated in  FIG. 11 , and may be any position whereby the position of the joint  660  can be easily determined. Further, the shape of the partition  680  is not limited to the example illustrated in  FIG. 11 , and may be any shape whereby the position of the joint  660  can be easily determined. However, the shape is desirably a shape whereby the second moment of area of the lower arm  60  can be effectively increased. Further, the contacts between the partition  680  and the first member  610 , and the partition  680  and the second member  620  may be joined by, for example, an adhesive, or may not be joined. 
     Thus, in the fourth modified embodiment, the partition  680  is provided between the first member  610  and the second member  620 . Thereby, in the case where the first member  610  and the second member  620  are joined together at the joint  660 , the position of the joint  660  can be easily determined, and the second moment of area of the lower arm  60  can be increased. Further, similarly to the embodiment, the joint  660  joining the first member  610  and the second member  620  together is provided more on the inside than the outer wall surface of the side wall  640  is. Thereby, when a load directed in the car body front-rear direction acts on the tip of the lower arm  60  and stress has occurred in the lower arm  60 , the fear that breaking will occur from the joint  660  as a starting point can be reduced. 
     3-5. Fifth Modified Embodiment 
       FIG. 12  is a cross-sectional view for describing an example of the configuration of a lower arm  70  according to a fifth modified embodiment.  FIG. 12  illustrates a cross section in the tip of the lower arm  70  according to the fifth modified embodiment, and corresponds to the A-A cross section, which is a cross section along the direction of the load at the tip of the lower arm illustrated in  FIG. 4  and  FIG. 5 . 
     The lower arm  70  according to the fifth modified embodiment has a first member  710 , a second member  720 , a closed space  730 , a side wall  740 , a bottom surface  750 , and a joint  760 . The first member  710 , the second member  720 , the closed space  730 , the side wall  740 , the bottom surface  750 , and the joint  760  correspond to the first member  210 , the second member  220 , the closed space  230 , the side wall  240 , the bottom surface  250 , and the joint  260  of the lower arm  20  according to the embodiment, respectively. 
     The fifth modified embodiment differs from the embodiment in that a core material is embedded in the closed space  730  of the lower arm  70 . In the example illustrated in  FIG. 12 , a core material  780  is embedded in the entire closed space  730  of the lower arm  70 . As the core material  780 , for example, a granular resin material or a foamable material may be used. The core material  780  is laid on the inside of the first member  710 , and the second member  720  is mounted on the laid core material  780 ; thereby, the position of the second member  720  with respect to the first member  710  is determined. 
     Thus, in the fifth modified embodiment, a core material is embedded in the closed space  730  of the lower arm  70 . Thereby, in the case where the first member  710  and the second member  720  are joined together at the joint  760 , the position of the joint  760  can be easily determined. Further, similarly to the embodiment, the joint  760  joining the first member  710  and the second member  720  together is provided more on the inside than the outer wall surface of the side wall  740  is. Thereby, when a load directed in the car body front-rear direction acts on the tip of the lower arm  70  and stress has occurred in the lower arm  70 , the fear that breaking will occur from the joint  760  as a starting point can be reduced. 
     3-6. Sixth Modified Embodiment 
       FIG. 13  is a cross-sectional view for describing an example of the configuration of a lower arm  80  according to a sixth modified embodiment.  FIG. 13  illustrates a cross section in the tip of the lower arm  80  according to the sixth modified embodiment, and corresponds to the A-A cross section, which is a cross section along the direction of the load at the tip of the lower arm illustrated in  FIG. 4  and  FIG. 5 . 
     The lower arm  80  according to the sixth modified embodiment has a first member  810 , a second member  820 , a closed space  830 , a side wall  840 , a bottom surface  850 , an upper surface  852 , and a joint  860 . 
     The sixth modified embodiment differs from the embodiment in that a cross section of each of the first member  810  and the second member  820  along the direction of the load at the tip of the lower arm  80  is formed in a substantially L-like shape. The first member  810  is a member made of a fiber-reinforced resin and having a side wall  840   b  substantially orthogonal to the direction of the load at the tip of the lower arm  80  and the bottom surface  850  substantially parallel to the direction of the load. In the first member  810 , a cross section along the direction of the load is formed in a substantially L-like shape as illustrated in  FIG. 13 . The second member  820  is a member made of a fiber-reinforced resin and having a side wall  840   a  substantially orthogonal to the direction of the load and the upper surface  852  substantially parallel to the direction of the load. The second member  820  forms the closed space  830  by being joined to the first member  810 . In the second member  820 , a cross section along the direction of the load is formed in a substantially L-like shape as illustrated in  FIG. 13 . 
     In the case where the second member  820  is joined to the first member  810 , the inner wall surface of the side wall  840   b  of the first member  810  and an end of the upper surface  852  of the second member  820  are joined together at a joint  860   b . Further, the inner wall surface of the side wall  840   a  of the second member  820  and an end of the bottom surface  850  of the first member  810  are joined together at a joint  860   a.  At this time, the first member  810  is disposed in such a manner that the side wall  840   b  of the first member  810  is located on the lower side, and the second member  820  is mounted on the first member  810  in such a manner that the side wall  840   a  of the second member  820  is located on the upper side; thereby, the position of the second member  820  with respect to the first member  810  is determined. 
     Thus, in the sixth modified embodiment, a cross section of each of the first member  810  and the second member  820  along the direction of the load at the tip of the lower arm  80  is formed in a substantially L-like shape. Further, the joint  860  is provided on the inner wall surface of the side wall  840  of each of the first member  810  and the second member  820 . Thereby, in the case where the first member  810  and the second member  820  are joined together at the joint  860 , the position of the joint  860  can be easily determined. Further, similarly to the embodiment, the joint  860  joining the first member  810  and the second member  820  together is provided more on the inside than the outer wall surface of the side wall  840  is. Thereby, when a load directed in the car body front-rear direction acts on the tip of the lower arm  80  and stress has occurred in the lower arm  80 , the fear that breaking will occur from the joint  860  as a starting point can be reduced. 
     4. CONCLUSIONS 
     As described hereinabove, the lower arm  20  according to the embodiment includes the first member  210  made of a fiber-reinforced resin and the second member  220  made of a fiber-reinforced resin and forming the closed space  230  by being joined to the first member  210 . The first member  210  has the side wall  240  substantially orthogonal the direction of the load at the tip  23 , which is a swing end of the lower arm  20 . The side wall  240  has an outer wall surface and an inner wall surface located on both sides in the direction of the load. The joint  260  joining the first member  210  and the second member  220  together is provided more on the inside than the outer wall surface of the side wall  240  is. 
     Thereby, when a load directed in the car body front-rear direction acts on the tip  23  and stress has occurred in the lower arm  20 , the fear that breaking will occur from the joint  260  as a starting point can be reduced. Therefore, the strength of the lower arm  20  against stress can be improved. 
     Although the preferred embodiments of the disclosure have been described in detail with reference to the appended drawings, the disclosure is not limited thereto. It is obvious to those skilled in the art that various modifications or variations are possible insofar as they are within the technical scope of the appended claims or the equivalents thereof. It should be understood that such modifications or variations are also within the technical scope of the disclosure. Further, also forms in which some or all of the embodiment and the modified embodiments described above are combined should be seen as within the technical scope of the disclosure, as a matter of course. 
     For example, although in the above an example in which the first member  210  is disposed on the lower side and the second member  220  is disposed on the upper side is described, the positional relationship between the first member  210  and the second member  220  is not limited to this example. The first member  210  may be disposed on the upper side, and the second member  220  may be disposed on the lower side. 
     Further, although in the above an example in which the fiber-reinforced resin structure body of the embodiment of the disclosure is used for a lower arm is described, the disclosure is not limited to this embodiment. The disclosure can be applied also to, for example, any other structure such as an upper arm as long as it is a fiber-reinforced resin structure body in which two or more members each made of a fiber-reinforced resin are joined together at a joint.