Patent Publication Number: US-2020298501-A1

Title: Reinforcement structure and producing method of reinforcement structure

Description:
TECHNICAL FIELD 
     The present invention relates to a reinforcement structure and a method for producing a reinforcement structure. 
     BACKGROUND ART 
     It has been conventionally known that an adherend such as metal plate used for various industrial products is reinforced by a reinforcement sheet. 
     As such a reinforcement sheet, for example, a steel plate reinforcement sheet including glass cloth and a thermosetting resin layer that is laminated on the glass cloth has been proposed (ref: for example, Patent Document 1). 
     In the steel plate reinforcement sheet, after the thermosetting resin layer is attached to a surface of the adherend along it, and the glass cloth is positioned at a fixed interval (with the same width) to the surface of the adherend, the thermosetting resin layer is cured by heating, so that the adherend is reinforced. 
     CITATION LIST 
     Patent Document 
     Patent Document 1: Japanese Unexamined Patent Publication No. 2010-58394 
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     The adherend has various shapes in accordance with industrial products, and may have, for example, a first surface and a second surface extending in a different direction and a corner portion that connects the first surface to the second surface. In this case, when an external force is applied to the adherend, a stress is concentrated on the corner portion. Then, intensive reinforcement of the corner portion of the adherend is considered. 
     In the steel plate reinforcement sheet described in Patent Document 1, however, the glass cloth is positioned at a fixed interval (with the same width) to the surface of the adherend in a state of being attached to the adherend, so that the intensive reinforcement of the corner portion cannot be carried out, so that there is a limit in achieving improvement of the strength of the adherend. 
     The present invention provides a reinforcement structure in which a reinforcement sheet is capable of intensively reinforcing a first corner portion of an adherend, and improvement of the strength is capable of being achieved, and a method for producing a reinforcement structure. 
     Means for Solving the Problem 
     The present invention [1] includes a reinforcement structure including an adherend and a reinforcement sheet adhering to the adherend and reinforcing the adherend, wherein the adherend includes a first surface extending in a first direction, a second surface extending in a second direction crossing the first direction, and a first corner portion connecting the first surface to the second surface; the reinforcement sheet includes a front layer containing a plurality of fibers and positioned at spaced intervals to the first surface, the second surface, and the first corner portion and a core material layer containing a resin and disposed between the front layer and the adherend; the front layer includes a first front layer portion positioned at spaced intervals to the first surface in the second direction and extending in the first direction, a second front layer portion positioned at spaced intervals to the second surface in the first direction and extending in the second direction, and a third front layer portion positioned at spaced intervals to the first corner portion in a third direction crossing both directions of the first direction and the second direction and connecting the first front layer portion to the second front layer portion; and a gap between the first surface and the third front layer portion in the second direction gradually increases as it gets closer to the first corner portion in the first direction with respect to a gap between the first surface and the first front layer portion in the second direction, and a gap between the second surface and the third front layer portion in the first direction gradually increases as it gets closer to the first corner portion in the second direction with respect to a gap between the second surface and the second front layer portion in the first direction. 
     According to the structure, the third front layer portion of the front layer is positioned at spaced intervals to the first corner portion in the third direction; the gap between the first surface and the third front layer portion in the second direction gradually increases as it gets closer to the first corner portion in the first direction with respect to the gap between the first surface and the first front layer portion in the second direction; and the gap between the second surface and the third front layer portion in the first direction gradually increases as it gets closer to the first corner portion in the second direction with respect to the gap between the second surface and the second front layer portion in the first direction. 
     Thus, compared to a case where a gap between the surface of the adherend and the third front layer portion is fixed (the same width), the reinforcement sheet can intensively reinforce the first corner portion of the adherend, and accordingly, improvement of the strength of the reinforcement structure can be achieved. 
     The present invention [2] includes the reinforcement structure described in the above-described [1], wherein the core material layer includes a first portion along the first surface, a second portion along the second surface, and a third portion along the first corner portion; and a thickness of the third portion along the second direction is thicker than a thickness of the first portion along the second direction and a thickness of the third portion along the first direction is thicker than a thickness of the second portion along the first direction. 
     According to the structure, the third portion of the core material layer is along the first corner portion, the thickness of the third portion along the second direction is thicker than that of the first portion along the second direction, and the thickness of the third portion along the first direction is thicker than that of the second portion along the first direction. 
     That is, the third portion that is positioned near the first corner portion is thicker than both portions of the first portion along the first surface and the second portion along the second surface. 
     Thus, the reinforcement sheet can more intensively reinforce the first corner portion of the adherend, and accordingly, the improvement of the strength of the reinforcement structure can be surely achieved. 
     The present invention [3] includes the reinforcement structure described in the above-described [2], wherein the first portion, the second portion, and the third portion are integrated. 
     According to the structure, the first portion, the second portion, and the third portion are integrated, so that compared to a case where the first portion, the second portion, and the third portions are separate pieces, a reduction of the number of components can be achieved. Also, the improvement of relative position accuracy of the first portion, the second portion, and the third portion can be achieved, and the reinforcement sheet can more stably intensively reinforce the first corner portion. 
     The present invention [4] includes the reinforcement structure described in the above-described [2] or [3], wherein the adherend further includes a third surface facing the first surface at spaced intervals thereto in the second direction and a second corner portion connecting the third surface to the second surface; the core material layer further includes a fourth portion along the third surface and a fifth portion along the second corner portion; and a thickness of the fifth portion along the second direction is thicker than a thickness of the fourth portion along the second direction and a thickness of the fifth portion along the first direction is thicker than a thickness of the second portion along the first direction. 
     According to the structure, the fifth portion of the core material layer is along the second corner portion, the thickness of the fifth portion along the second direction is thicker than that of the fourth portion along the second direction, and the thickness of the fifth portion along the first direction is thicker than that of the second portion along the first direction. 
     That is, the fifth portion that is positioned near the second corner portion is thicker than both portions of the fourth portion along the third surface and the second portion along the second surface. 
     Thus, even when the adherend has the first corner portion and the second corner portion, the reinforcement sheet can intensively reinforce both of the first corner portion and the second corner portion, and accordingly, the improvement of the strength of the reinforcement structure can be furthermore surely achieved. 
     The present invention [5] includes the reinforcement structure described in any one of the above-described [1] to [4] further including an adhesive layer disposed between the core material layer and the adherend. 
     According to the structure, the adhesive layer is disposed between the core material layer and the adherend, so that the core material layer can surely adhere to the adherend. Thus, the reinforcement sheet can stably reinforce the adherend. 
     The present invention [6] includes the reinforcement structure described in any one of the above-described [1] to [4], wherein the core material layer is in direct contact with each of the first surface, the second surface, and the first corner portion. 
     According to the structure, the core material layer is in direct contact with each of the first surface, the second surface, and the first corner portion, so that compared to a structure including the adhesive layer, a reduction of the number of components can be achieved, and the first portion, the second portion, and the third portion can be disposed so as to be surely along the first surface, the second surface, and the first corner portion. 
     The present invention [7] includes a method for producing a reinforcement structure including the steps of preparing a reinforcement sheet including a core material layer containing a resin and a front layer disposed at one side in a thickness direction of the core material layer and containing a plurality of fibers; preparing an adherend including a first surface extending in a first direction, a second surface extending in a second direction crossing the first direction, and a first corner portion connecting the first surface to the second surface; and attaching the reinforcement sheet to the adherend so as to position the core material layer between the adherend and the front layer and position the front layer at spaced intervals to the first surface, the second surface, and the first corner portion, wherein in the step of attaching the reinforcement sheet to the adherend, a first front layer portion in the front layer is disposed so as to be positioned at spaced intervals to the first surface in the second direction and extend in the first direction; a second front layer portion in the front layer is disposed so as to be positioned at spaced intervals to the second surface in the first direction and extend in the second direction; a third front layer portion in the front layer is disposed at spaced intervals to the first corner portion in a third direction crossing both directions of the first direction and the second direction; and the third front layer portion is disposed so that a gap between the first surface and the third front layer portion in the second direction gradually increases as it gets closer to the first corner portion in the first direction with respect to a gap between the first surface and the first front layer portion in the second direction, and a gap between the second surface and the third front layer portion in the first direction gradually increases as it gets closer to the first corner portion in the second direction with respect to a gap between the second surface and the second front layer portion in the first direction. 
     According to the method, in the step of attaching the reinforcement sheet to the adherend, the third front layer portion is disposed so that the gap between the first surface and the third front layer portion in the second direction gradually increases as it gets closer to the first corner portion in the first direction with respect to the gap between the first surface and the first front layer portion in the second direction, and the gap between the second surface and the third front layer portion in the first direction gradually increases as it gets closer to the first corner portion in the second direction with respect to the gap between the second surface and the second front layer portion in the first direction. 
     That is, in the step of attaching the reinforcement sheet to the adherend, the third front layer portion in the front layer can be disposed as described above, so that the reinforcement structure having the improved strength can be smoothly produced. 
     The present invention [8] includes the method for producing a reinforcement structure described in the above-described [7], wherein in the step of attaching the reinforcement sheet to the adherend, the core material layer flows so that in the core material layer, a thickness along the second direction of a third portion along the first corner portion is thicker than a thickness along the second direction of a first portion along the first surface, and a thickness of the third portion along the first direction is thicker than a thickness along the first direction of a second portion along the second surface. 
     According to the method, in the step of attaching the reinforcement sheet to the adherend, the core material layer containing the resin flows, so that the thickness along the second direction of the third portion along the first corner portion is thicker than the thickness along the second direction of the first portion along the first surface, and the thickness of the third portion along the first direction is thicker than the thickness along the first direction of the second portion along the second surface. 
     Thus, the third portion of the core material layer can be smoothly formed so as to have the above-described thickness by an easy method. 
     The present invention [9] includes the method for producing a reinforcement structure described in the above-described [7] or [8], wherein in the step of attaching the reinforcement sheet to the adherend, the reinforcement sheet is pressed toward the adherend by a mold having a shape corresponding to the adherend, and the mold includes a first mold surface extending in the first direction corresponding to the first surface, a second mold surface extending in the second direction corresponding to the second surface, and a third mold surface connecting the first mold surface to the second mold surface and extending in a direction crossing the third direction. 
     According to the method, the third mold surface of the mold connects the first mold surface to the second mold surface, and extends in the direction crossing the third direction. 
     Thus, when the reinforcement sheet is pressed toward the adherend by the mold, space for disposing the third front layer portion of the core material layer can be ensured between the first corner portion and the third mold surface. As a result, in the step of attaching the reinforcement sheet to the adherend, the third front layer portion can be smoothly disposed as described above. 
     Effect of the Invention 
     In the reinforcement structure of the present invention, a reinforcement sheet can intensively reinforce a first corner portion of an adherend, and improvement of the strength can be achieved. 
     In the method for producing a reinforcement structure of the present invention, the reinforcement structure having the improved strength can be smoothly produced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a cross-sectional view of a first embodiment of a reinforcement structure of the present invention. 
         FIG. 2A  shows an explanatory view for illustrating a one embodiment of a method for producing a reinforcement structure of the present invention, and shows a step of preparing a reinforcement sheet. 
         FIG. 2B , subsequent to  FIG. 2A , shows a step of preliminarily shape the reinforcement sheet. 
         FIG. 2C , subsequent to  FIG. 2B , shows a step of preparing a metal plate. 
         FIG. 3A , subsequent to  FIG. 2C , shows a step of attaching the reinforcement sheet to the metal plate. 
         FIG. 3B , subsequent to  FIG. 3A , shows a step of removing a mold from the reinforcement structure. 
         FIG. 4  shows a cross-sectional view of a second embodiment of a reinforcement structure of the present invention. 
         FIG. 5  shows a cross-sectional view of a third embodiment of a reinforcement structure of the present invention. 
         FIG. 6  shows a cross-sectional view of a fourth embodiment of a reinforcement structure of the present invention. 
         FIG. 7  shows a side cross-sectional view of a modified example (embodiment in which a pre-adhesion core material layer has a thick portion) of the reinforcement sheet shown in  FIG. 2A . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
     1. Schematic Configuration of Reinforcement Structure 
     A reinforcement structure  1  that is a first embodiment of a reinforcement structure of the present invention is described with reference to  FIG. 1 . 
     As shown in  FIG. 1 , the reinforcement structure  1  includes a metal plate  2  as one example of an adherend and a reinforcement sheet  3 , and is configured from the metal plate  2  that is reinforced by the reinforcement sheet  3 . 
     In  FIG. 1 , the up-down direction on the plane of the sheet is referred to as a first direction, the upper side on the plane of the sheet is referred to as one side in the first direction, and the lower side on the plane of the sheet is referred to as the other side in the first direction. In FIG.  1 , the right-left direction on the plane of the sheet is referred to as a second direction perpendicular to the first direction, the left side on the plane of the sheet is referred to as one side in the second direction, and the right side on the plane of the sheet is referred to as the other side in the second direction. 
     The metal plate  2  has a hat shape when viewed from the cross-sectional view when being cut in the first direction and the second direction, and integrally includes a recessed portion  20  and a flange portion  21 . 
     The recessed portion  20  has a recessed shape having an opening toward one side in the first direction as a cross-sectional shape when being cut in the first direction and the second direction. The inner surface of the recessed portion  20  has a first surface  20 A, a second surface  20 B, a third surface  20 C, a first corner portion  20 D, and a second corner portion  20 E. 
     The first surface  20 A is the inner-side surface of the recessed portion  20 , and is positioned at one end portion in the second direction at the inner surface of the recessed portion  20 . The first surface  20 A extends in the first direction. 
     The second surface  20 B is the bottom surface of the recessed portion  20 , and is positioned at the other end portion in the first direction at the inner surface of the recessed portion  20 . The second surface  20 B extends in the second direction. 
     The third surface  20 C is the inner-side surface of the recessed portion  20 , and is positioned at the other end portion in the second direction at the inner surface of the recessed portion  20 . The third surface  20 C faces the first surface  20 A at spaced intervals thereto in the second direction. The third surface  20 C extends in the first direction. 
     The first corner portion  20 D connects the first surface  20 A to the second surface  20 B. To be more specific, the first corner portion  20 D is a continuous portion from the other end portion in the first direction of the first surface  20 A to one end portion in the second direction of the second surface  20 B. In the first corner portion  20 D, the other end portion in the first direction of the first surface  20 A and one end portion in the second direction of the second surface  20 B form an angle of about 90°. 
     The second corner portion  20 E connects the second surface  20 B to the third surface  20 C. To be more specific, the second corner portion  20 E is a continuous portion from the other end portion in the second direction of the second surface  20 B to the other end portion in the first direction of the third surface  20 C. In the second corner portion  20 E, the other end portion in the second direction of the second surface  20 B and the other end portion in the first direction of the third surface  20 C form an angle of about 90°. The second corner portion  20 E is positioned al spaced intervals to the first corner portion  20 D at the other side in the second direction. 
     The flange portion  21  is continuous from one end portion in the first direction at the outer surface of the recessed portion  20 , and extends outwardly in the second direction. 
     The metal plate  2  is a metal plate used for various industrial products, and is not particularly limited. Examples of the metal plate  2  include metal plates (for example, pillar (A pillar, B pillar, C pillar), side sill, roof rail, roof, fender, hood, trunk, quarter panel, door, or the like for automobiles) used for transportation machines and metal plates used for household electric appliances. Preferably, a metal plate used for transportation machines is used. 
     The reinforcement sheet  3  adheres to the metal plate  2  and reinforces the metal plate  2 . The reinforcement sheet  3  is disposed at the inside of the recessed portion  20  in the metal plate  2 , and adheres along the inner surface of the recessed portion  20 . The reinforcement sheet  3  includes a core material layer  30 , a front layer  31 , and an adhesive layer  32 , and preferably consists of the core material layer  30 , the front layer  31 , and the adhesive layer  31   
     The core material layer  30  is disposed between the inner surface of the recessed portion  20  and the front layer  31 , and is along the first surface  20 A, the first corner portion  20 D, the second surface  20 B, the second corner portion  20 E, and the third surface  20 C. The core material layer  30  has a U-shape having an opening toward one side in the first direction as a cross-sectional shape when being cut in the first direction and the second direction. The core material layer  30  contains at least a resin, and preferably contains a filler. In the following, the resin contained in the core material layer  30  is referred to as a core material resin. 
     Examples of the core material resin include polyolefin resin, polyamide resin, polyacetal resin, polycarbonate resin, polyphenylene ether resin, polybutylene terephthalate resin, polysulfide resin, polyphenylene sulfide resin, polyether ether ketone resin, polyimide resin, polyether imide resin, liquid crystal polymer, fluorine resin, epoxy resin, phenol resin, melamine resin, urea resin, unsaturated polyester, thermosetting polyimide resin, polyurethane resin, alkyd resin, and benzoxazine resin. Preferably, an epoxy resin is used. 
     To be specific, examples of the epoxy resin include unmodified epoxy resin, rubber-modified epoxy resin, urethane-modified epoxy resin, and silicone-modified epoxy resin. These core material resins can be used alone or in combination of two or more. 
     Of these core material resins, preferably, an unmodified epoxy resin and a rubber-modified epoxy resin are used. 
     The unmodified epoxy resin is an epoxy resin that is not modified by a modifier (for example, a rubber component (described later) or the like) having a functional group that reacts with an epoxy group. Examples of the unmodified epoxy resin include bisphenol epoxy resin (for example, bisphenol A epoxy resin, bisphenol F epoxy resin, bisphenol S epoxy resin, hydrogenated bisphenol A epoxy resin, or the like), novolak epoxy resin (for example, phenol novolak epoxy resin, cresol novolak epoxy resin, or the like), aromatic epoxy resin (for example, biphenyl epoxy resin, naphthalene epoxy resin, or the like), nitrogen-containing cyclic epoxy resin (for example, triglycidyl isocyanurate, hydantoin epoxy resin, or the like), alicyclic epoxy resin (for example, dicyclo ring-type epoxy resin or the like), glycidyl ether epoxy resin, and aliphatic epoxy resin. These unmodified epoxy resins can be used alone or in combination of two or more. 
     Of these unmodified epoxy resins, preferably, a bisphenol epoxy resin is used, more preferably, a bisphenol A epoxy resin is used. 
     The rubber-modified epoxy resin is an epoxy resin containing a rubber component, and is a reaction product of the epoxy resin with the rubber component. 
     As the epoxy resin used for the rubber-modified epoxy resin, for example, the above-described unmodified epoxy resin is used, preferably, a bisphenol epoxy resin is used, more preferably, a bisphenol A epoxy resin and a bisphenol F epoxy resin are used. 
     The rubber component has the functional group that reacts with the epoxy group. Examples of the rubber component include terminated carboxyl group butadiene-acrylonitrile copolymer (CTBN), butadiene rubber, acrylic rubber, and styrene-butadiene elastomer. These rubber components can be used alone or in combination of two or more. 
     Of these rubber components, preferably, a terminated carboxyl group butadiene-acrylonitrile copolymer (CTBN) is used. That is, as the rubber-modified epoxy resin, preferably, a CTBN-modified epoxy resin that is modified with a CTBN is used. 
     To prepare the rubber-modified epoxy resin, for example, the above-described unmodified epoxy resin is mixed with the above-described rubber component, and a catalyst is added thereto as needed to react (be modified) at, for example, 100 to 180° C. In this manner, the functional group and the epoxy group of the rubber component react, thereby preparing the rubber-modified epoxy resin. 
     The unmodified epoxy resin and the rubber-modified epoxy resin are preferably used in combination. More preferably, the core material resin consists of the unmodified epoxy resin and the rubber-modified epoxy resin. When the unmodified epoxy resin and the rubber-modified epoxy resin are used in combination, improvement of compatibility and adhesive properties of the core material resin can be achieved compared to a case where the unmodified epoxy resin is mixed with a synthetic rubber (for example, polybutene rubber or the like). 
     When the unmodified epoxy resin and the rubber-modified epoxy resin are used in combination, a mass ratio (rubber-modified epoxy resin: unmodified epoxy resin) of the rubber-modified epoxy resin to the unmodified epoxy resin is, for example, 0.1:99.9 to 99.9:0.1, preferably 1:99 to 99:1, more preferably 10:90 to 90:10, further more preferably 60:40 to 40:60. 
     The filler is a hard filler, and is, for example, uniformly dispersed in the core material layer  30 . Examples of the filler include calcium carbonate (for example, heavy calcium carbonate, light calcium carbonate, Hakuenka, or the like), calcium oxide, talc, mica, clay, mica powder, bentonite, silica (for example, hydrophobic silica or the like), alumina, aluminum silicate, aluminum hydroxide, titanium oxide, barium titanate, ferrite, carbon black, acetylene black, aluminum powder, glass powder, glass balloon, and chopped strands (for example, glass fiber, carbon fiber, aramid fiber, or the like). These fillers can be used alone or in combination of two or more. 
     Of these fillers, preferably, calcium carbonate is used, more preferably, heavy calcium carbonate is used. 
     A content ratio of the filler in the core material layer  30  is, for example, 15 mass % or more, preferably 40 mass % or more, more preferably 50 mass % or more, further more preferably 60 mass % or more, particularly preferably 70 mass % or more, and for example, below 85 mass %, preferably 80 mass % or less. 
     When the content ratio of the filler in the core material layer  30  is the above-described lower limit or more, a Young&#39;s modulus of the core material layer  30  can be adjusted within a preferable range (described later), so that the improvement of the maximum bending testing force of the reinforcement structure  1  can be achieved. When the content ratio of the filler in the core material layer  30  is below the above-described upper limit, the filler can be uniformly dispersed in the core material layer  30 , and the core material layer  30  can be surely formed. 
     The core material layer  30  preferably further contains a latent curing agent as an optional component. 
     The latent curing agent is solid at room temperature (23° C.), and cures the core material resin at a predetermined temperature. The latent curing agent has, for example, activity at 80° C. or more and 200° C. or less. 
     Examples of the latent curing agent include urea compound, amine compound, acid anhydride compound, amide compound, cyano compound, dihydrazide compound, imidazole compound, and imidazoline compound. 
     Examples of the urea compound include 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), N′-phenyl-N,N-dimethylurea, and 1,1′-(methyl-m-phenylene) bis(3,3′-dimethylurea). 
     Examples of the amine compound include ethylenediamine, propylenediamine, diethylenetriamine, triethylenetetramine, and amine adduct thereof, methaphenylenediamine, diaminodiphenyl methane, and diaminodiphenyl sulfone. 
     Examples of the acid anhydride compound include phthalic anhydride, maleic anhydride, tetrahydrophthalic anhydride, dodecenyl succinic anhydride, dichloro succinic anhydride, benzophenone tetracarboxylic anhydride, and chlorendic anhydride. 
     An example of the amide compound includes polyimide. 
     An example of the cyano compound includes dicyandiamide. 
     An example of the hydrazide compound includes dihydrazide such as adipic acid dihydrazide. 
     Examples of the imidazole compound include methylimidazole, 2-ethyl-4-methylimidazole, ethylimidazole, isopropylimidazole, 2,4-dimethylimidazole, phenylimidazole, undecylimidazole, heptadecylimidazole, and 2-phenyl-4-methylimidazole. 
     Examples of the imidazoline compound include methylimidazoline, 2-ethyl-4-methylimidazoline, ethylimidazoline, isopropylimidazoline, 2,4-dimethylimidazoline, phenylimidazoline, undecylimidazoline, heptadecylimidazoline, and 2-phenyl-4-methylimidazoline. 
     These latent curing agents can be used alone or in combination of two or more. 
     Of these latent curing agents, preferably, a urea compound and a cyano compound are used, more preferably, a urea compound and a cyano compound are used in combination, further more preferably, DCMU and dicyandiamide are used in combination. 
     The content ratio of the latent curing agent with respect to 100 parts by mass of the core material resin is, for example, 0.5 parts by mass or more, preferably 1 part by mass or more, and for example, 30 parts by mass or less, preferably 20 parts by mass or less, more preferably 15 parts by mass or less. 
     Furthermore, in addition to the above-described components, as an optional component, for example, a known additive can be also added to the core material layer  30  at an appropriate ratio. Examples of the known additive include toughness imparting agent (for example, thermoplastic resins such as rubber, elastomer, urethane, and polyamide; core-shell-type acrylic polymer rubber particles, or the like), tackifier (for example, rosin resin, terpene resin, coumarone-indene resin, petroleum resin, or the like), lubricant (for example, stearic acid or the like), stabilizer, antiaging inhibitor, oxidation inhibitor, ultraviolet absorber, colorant, flame retardant, antistatic agent, conductivity imparting agent, sliding property imparting agent, and surfactant. 
     When the core material resin is in a cured state, the Young&#39;s modulus of the core material layer  30  is, for example, 600 MPa or more, preferably 1000 MPa or more, more preferably 2000 MPa or more, and for example, 7000 MPa or less. The Young&#39;s modulus can be calculated by the following formula (1) by using the result measured by the following tensile test (hereinafter, the same). 
     Tensile test: a sample (the core material layer) is cut out into a size of a width of 10 mm×a length of 100 mm. Then, the minimum thickness of the central portion of the sample is measured. Next, the sample is measured under the following test conditions. 
     Test conditions: distance between holder of 50 mm, tensile rate of 1 mm/min, room temperature of 23° C. 
     
       
         
           
             
               
                 
                   E 
                   = 
                   
                     
                       
                         Δ 
                          
                         
                             
                         
                          
                         F 
                       
                       S 
                     
                     × 
                     
                       L 
                       
                         Δ 
                          
                         L 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     ΔF: increase in load (0.1→0.4 mm displacement), S: cross-sectional area (thickness×width), L: initial length (50 mm), ΔL: increased amount of length 
     When the Young&#39;s modulus of the core material layer  30  is the above-described lower limit or more, the improvement of reinforcement properties of the reinforcement sheet  3  with respect to the metal plate  2  can be surely achieved, and the improvement of the maximum bending testing force of the reinforcement structure  1  can be surely achieved. Thus, a reduction in thickness of the core material layer  30  can be achieved, and to be specific, when a thickness of the core material layer  30  is set within the following range, the maximum bending testing force of the reinforcement structure  1  can be sufficiently ensured. 
     The front layer  31  is positioned at the opposite side of the inner surface of the recessed portion  20  with respect to the core material layer  30 , and is positioned at spaced intervals to the first surface  204 , the first corner portion  20 D, the second surface  20 B, the second corner portion  20 E, and the third surface  20 C. The front layer  31  is disposed on the front surface of the core material layer  30 . 
     The front layer  31  contains at least a plurality of fibers, and preferably, further contains a resin. In the following, the resin contained in the front layer  31  is referred to as a front layer resin. 
     Examples of the fiber include carbon fiber and glass fiber. These fibers can be used alone or in combination of two or more. Of these fibers, preferably, a carbon fiber is used. 
     The mass (basis weight) per unit area of the plurality of fibers in the front layer  31  is, for example, 10 g/m 2  or more, preferably 80 g/m 2  or more, and for example, 500 g/m 2  or less, preferably 200 g/m 2  or less. 
     The front layer resin is not particularly limited, and an example thereof includes a thermosetting resin. To be specific, examples of the thermosetting resin include epoxy resin, urethane resin, melamine resin, and phenol resin. These front layer resins can be used alone or in combination of two or more. 
     Of these front layer resins, preferably, an epoxy resin is used. As the epoxy resin used for the front layer resin, for example, the same epoxy resin as that used for the core material resin is used. These epoxy resins can be used alone or in combination of two or more. 
     The content ratio (resin content ratio) of the front layer resin in the front layer  31  is, for example, 10 mass % or more, preferably 20 mass % or more, and for example, 60 mass % or less, preferably 40 mass % or less. 
     The front layer  31  preferably contains the above-described latent curing agent at an appropriate ratio as an optional component. 
     The front layer  31  preferably contains a plurality of unidirectional fiber resin composite sheets, more preferably consists of a plurality of unidirectional fiber resin composite sheets. 
     Each of the plurality of unidirectional fiber resin composite sheets contains the above-described plurality of fibers and the above-described front layer resin. Each of the plurality of unidirectional fiber resin composite sheets is formed by subjecting the front layer resin to impregnation treatment of a fiber substrate consisting of the plurality of fibers. 
     As shown in  FIG. 2A , before the reinforcement sheet  3  adheres to the metal plate  2 , and in a state in which the reinforcement sheet  3  has a flat plate shape, the plurality of fibers extend in the same direction perpendicular to a thickness direction of the reinforcement sheet  3  (hereinafter, referred to as a fiber direction) in each of the plurality of unidirectional fiber resin composite sheets. The plurality of fibers are arranged in the direction perpendicular to the fiber direction so as to be generally parallel with each other in each of the unidirectional fiber resin composite sheets. 
     The plurality of unidirectional fiber resin composite sheets are laminated so that the fiber directions of the unidirectional fiber resin composite sheets that are next to each other cross (for example, are perpendicular to) each other. 
     When the fiber direction is in a state of being orientated in a tensile direction, a tensile strength per one layer of the plurality of unidirectional fiber resin composite sheets is, for example, 100 MPa or more, preferably 300 MPa or more, and for example, 7000 MPa or less. The tensile strength can be calculated by measuring the maximum testing force (the maximum value of a load in a tensile test) of a sample (the unidirectional fiber resin composite sheet), and dividing the obtained maximum testing force by the cross-sectional area (thickness×width) of the sample in conformity with the above-described tensile test (hereinafter, the same). 
     When the tensile strength of each of the unidirectional fiber resin composite sheets is the above-described lower limit or more, the improvement of the reinforcement properties of the reinforcement sheet  3  with respect to the metal plate  2  can be furthermore surely achieved, and the improvement of the maximum bending testing force of the reinforcement structure  1  can be furthermore surely achieved. 
     Each of the unidirectional fiber resin composite sheets has a thickness of, for example, 0.01 mm or more, preferably 0.05 mm or more, and for example, 0.5 mm or less, preferably 0.3 mm or less. 
     The number (number of lamination) of the plurality of unidirectional fiber resin composite sheets is, for example, two or more, preferably three or more, and for example, below 10. 
     When the number (number of lamination) of the plurality of unidirectional fiber resin composite sheets is the above-described lower limit or more, the reinforcement properties of the reinforcement sheet  3  with respect to the metal plate  2  can be ensured. When the number (number of lamination) of the plurality of unidirectional fiber resin composite sheets is the above-described upper limit or less, the number of lamination of the plurality of unidirectional fiber resin composite sheets can be reduced, and a reduction in production cost of the front layer  31  can be achieved. 
     As shown in  FIG. 1 , the adhesive layer  32  is disposed between the core material layer  30  and the inner surface of the recessed portion  20 . The adhesive layer  32  allows the core material layer  30  to adhere to the inner surface of the recessed portion  20 . That is, the core material layer  30  adheres to the recessed portion  20  via the adhesive layer  32 . The adhesive layer  32  is in direct contact with the first surface  20 A, the first corner portion  20 D, the second surface  20 B, the second corner portion  20 E, and the third surface  20 C, and is provided along those. The adhesive layer  32  is, for example, formed from a known adhesive or a double-coated adhesive tape. 
     Examples of an adhesive that forms the adhesive layer  32  include epoxy adhesive, urethane adhesive, and acrylic adhesive. These adhesives can be used alone or in combination of two or more. 
     The double-coated adhesive tape that forms the adhesive layer  32  includes, for example, a substrate, and the above-described adhesive that is laminated on both surfaces of the substrate. An example of the double-coated adhesive tape that forms the adhesive layer  32  includes a thermosetting double-coated tape NA590 manufactured by NITTO DENKO CORPORATION. The adhesive layer  32  is preferably formed from a double-coated adhesive tape. 
     2. Details of Core Material Layer and Front Layer 
     Next, the details of the core material layer  30  and the front layer  31  are described. 
     The core material layer  30  integrally includes a first portion  30 A along the first surface  20 A, a second portion  30 B along the second surface  20 B, a third portion  30 C along the first corner portion  20 D, a fourth portion  30 D along the third surface  20 C, and a fifth portion  30 E along the second corner portion  20 E. That is, the first portion  30 A, the second portion  30 B, the third portion  30 C, the fourth portion  30 D, and the fifth portion  30 E are integrated. 
     The first portion  304  is positioned at the opposite side of the first surface  20 A with respect to the adhesive layer  32 . The first portion  30 A is positioned between the first surface  20 A and a first front layer portion  31 A (described later). Of the core material layer  30  adhering to the first surface  20 A by the adhesive layer  32 , the first portion  30 A is a portion at one side in the first direction, and is a portion that is overlapped with the first front layer portion  31 A (described later) when projected in the second direction. 
     The second portion  30 B is positioned at the opposite side of the second surface  20 B with respect to the adhesive layer  32 . The second portion  30 B is positioned between the second surface  20 B and a second front layer portion  31 B (described later). Of the core material layer  30  adhering to the second surface  20 B by the adhesive layer  32 , the second portion  30 B is a central portion in the second direction, and is a portion that is overlapped with the second front layer portion  31 B (described later) when projected in the first direction. 
     The third portion  30 C is disposed between the first portion  30 A and the second portion  30 B. In an enlarged view of  FIG. 1 , the first portion  30 A, the second portion  30 B, and the third portion  30 C are shown by being defined by a phantom line for convenience. The third portion  30 C is continuous to the first portion  30 A and the second portion  30 B, and to be more specific, is continuous to the other end portion in the first direction of the first portion  30 A and one end portion in the second direction of the second portion  30 B. The third portion  30 C is positioned closer to the first corner portion  20 D compared to the first portion  30 A in the first direction, and is positioned closer to the first corner portion  20 D compared to the second portion  30 B in the second direction. The third portion  30 C is positioned between the first corner portion  20 D and a third front layer portion  31 C (described later). 
     A thickness of the third portion  30 C along the second direction is thicker than that of the first portion  30 A along the second direction. To be more specific, in the first direction, the thickness of the third portion  30 C along the second direction gets thicker from a continuous portion of the first portion  30 A toward the other side in the first direction, and thereafter, is fixed when it reaches a continuous portion to the second portion  30 B. The thickness of the third portion  30 C along the second direction is a size of the third portion  30 C in the second direction in the cross-section when the reinforcement structure  1  is cut in the first direction and the second direction. 
     A thickness of the third portion  30 C along the first direction is thicker than that of the second portion  30 B along the first direction. To be more specific, the thickness of the third portion  30 C along the first direction gets thicker from the continuous portion of the second portion  30 B toward one side in the second direction, and thereafter, is fixed when it reaches the continuous portion to the first portion  30 A. The thickness of the third portion  30 C along the first direction is a size of the third portion  30 C in the first direction in the cross-section when the reinforcement structure  1  is cut in the first direction and the second direction. 
     The fourth portion  30 D is positioned at the opposite side of the third surface  20 C with respect to the adhesive layer  32 . The fourth portion  30 D is positioned between the third surface  20 C and a fourth front layer portion  31 D (described later). The fourth portion  30 D is positioned at spaced intervals to the first portion  30 A at the other side in the second direction. Of the core material layer  30  adhering to the third surface  20 C by the adhesive layer  32 , the fourth portion  30 D is a portion at one side in the first direction, and is a portion that is overlapped with the fourth front layer portion  31 D (described later) when projected in the second direction. 
     The fifth portion  30 E is disposed between the second portion  30 B and the fourth portion  30 D. The fifth portion  30 E is continuous to the second portion  30 B and the fourth portion  30 D, and to be more specific, is continuous to the other end portion in the second direction of the second portion  30 B and the other end portion in the first direction of the fourth portion  30 D. The fifth portion  30 E is positioned closer to the second corner portion  20 E compared to the fourth portion  30 D in the first direction, and is positioned closer to the second corner portion  20 E compared to the second portion  30 B in the second direction. The fifth portion  30 E is positioned between the second corner portion  20 E and a fifth front layer portion  31 E (described later). 
     A thickness of the fifth portion  30 E along the second direction is thicker than that of the fourth portion  30 D along the second direction. To be more specific, the thickness of the fifth portion  30 E along the second direction gets thicker from a continuous portion of the fourth portion  30 D toward the other side in the first direction, and thereafter, is fixed when it reaches the continuous portion to the second portion  30 B. 
     A thickness of the fifth portion  30 E along the first direction is thicker than that of the second portion  30 B along the first direction. To be more specific, the thickness of the fifth portion  30 E along the first direction gets thicker from the continuous portion of the second portion  30 B toward the other side in the second direction, and thereafter, is fixed when it reaches the continuous portion to the fourth portion  30 D. The fifth portion  30 E has a shape linearly symmetrical with respect to the third portion  30 C. 
     The front layer  31  integrally includes the first front layer portion  31 A, the second front layer portion  31 B, the third front layer portion  31 C, the fourth front layer portion  31 D, and the fifth front layer portion  31 E. 
     The first front layer portion  31 A is positioned at spaced intervals to the first surface  20 A in the second direction. The first front layer portion  31 A is positioned at the opposite side of the first surface  20 A with respect to the first portion  30 A, and is disposed on the first portion  30 A. 
     The first front layer portion  31 A extends in the first direction. Extension of the first front layer portion  31 A in the first direction includes a case where the direction in which the first front layer portion  31 A extends is parallel with the first direction and a case where the direction in which the first front layer portion  31 A extends inclines with respect to the first direction at an angle of, for example, 1° or more and 30° or less. That is, an angle made between the direction in which the first front layer portion  31 A extends and the first direction is, for example, 0° or more and 30° or less. 
     The second front layer portion  31 B is positioned at spaced intervals to the second surface  20 B in the first direction. The second front layer portion  31 B is positioned at the opposite side of the second surface  20 B with respect to the second portion  30 B, and is disposed on the second portion  30 B. 
     The second front layer portion  31 B extends in the second direction. The extension of the second front layer portion  31 B in the second direction includes a case where the direction in which the second front layer portion  31 B extends is parallel with the second direction and a case where the direction in which the second front layer portion  31 B extends inclines with respect to the second direction at an angle of, for example, 1° or more and 30° or less. That is, an angle made between the direction in which the second front layer portion  31 B extends and the second direction is, for example, 0° or more and 30° or less. 
     The third front layer portion  31 C is positioned at spaced intervals to the first corner portion  20 D in the third direction crossing both directions of the first direction and the second direction. The third front layer portion  31 C is positioned at the opposite side of the first corner portion  20 D with respect to the third portion  30 C, and is disposed on the third portion  30 C. The third front layer portion  31 C connects the first front layer portion  31 A to the second front layer portion  31 B. To be more specific, the third front layer portion  31 C connects the other end portion in the first direction of the first front layer portion  31 A to one end portion in the second direction of the second front layer portion  31 B. 
     The third front layer portion  31 C extends in a direction perpendicular to the third direction so as to cross over the first corner portion  20 D when viewed in the third direction. To be more specific, the third front layer portion  31 C inclines from one side toward the other side in the second direction as it goes from one side toward the other side in the first direction. 
     The direction in which the third front layer portion  31 C extends greatly inclines with respect to the first direction compared to the direction in which the first front layer portion  31 A extends. An angle made between the direction in which the third front layer portion  31 C extends and the first direction is, for example, 20° or more, preferably above 30°, and for example, below 90°. 
     The direction in which the third front layer portion  31 C extends greatly inclines with respect to the second direction compared to the direction in which the second front layer portion  31 B extends. An angle made between the direction in which the third front layer portion  31 C extends and the second direction is, for example, 20° or more, preferably above 30°, and for example, below 90°. 
     A gap between the first surface  20 A and the third front layer portion  31 C in the second direction gradually increases as it gets closer to the first corner portion  20 D in the first direction with respect to a gap between the first surface  20 A and the first front layer portion  31 A in the second direction. 
     The gap between the first surface  20 A and the third front layer portion  31 C in the second direction is a size of the gap between the first surface  20 A and the third front layer portion  31 C in the cross section when the reinforcement structure  1  is cut in the first direction and the second direction. 
     A gap between the second surface  20 B and the third front layer portion  31 C in the first direction gradually increases as it gets closer to the first corner portion  20 D in the second direction with respect to a gap between the second surface  20 B and the second front layer portion  31 B in the first direction. 
     The gap between the second surface  20 B and the third front layer portion  31 C in the first direction is a size of the gap between the second surface  20 B and the third front layer portion  31 C in the cross section when the reinforcement structure  1  is cut in the first direction and the second direction. 
     The fourth front layer portion  31 D is positioned at spaced intervals to the third surface  20 C in the second direction. The fourth front layer portion  31 D is positioned at the opposite side of the third surface  20 C with respect to the fourth portion  30 D, and is disposed on the fourth portion  30 D. The fourth front layer portion  31 D is disposed at spaced intervals to the first front layer portion  31 A at the opposite side in the second direction. 
     The fourth front layer portion  31 D extends in the first direction. The extension of the fourth front layer portion  31 D in the first direction includes a case where the direction in which the fourth front layer portion  31 D extends is parallel with the first direction and a case where the direction in which the fourth front layer portion  31 D extends inclines with respect to the first direction at an angle of, for example, 1° or more and 30° or less. That is, an angle made between the direction in which the fourth front layer portion  31 D extends and the first direction is, for example, 0° or more and 30° or less. 
     The fifth front layer portion  31 E is positioned at spaced intervals to the second corner portion  20 E in the direction crossing both directions of the first direction and the second direction. The fifth front layer portion  31 E is positioned at the opposite side of the second corner portion  20 E with respect to the fifth portion  30 E, and is disposed on the fifth portion  30 E. The fifth front layer portion  31 E connects the fourth front layer portion  31 D to the second front layer portion  31 B. To be more specific, the fifth front layer portion  31 E connects the other end portion in the first direction of the fourth front layer portion  31 D to the other end portion in the second direction of the second front layer portion  31 B. The fifth front layer portion  31 E has a shape linearly symmetrical with respect to the third front layer portion  31 C. 
     The direction in which the fifth front layer portion  31 E extends greatly inclines with respect to the first direction compared to the direction in Which the fourth front layer portion  31 D extends. An angle made between the direction in which the fifth front layer portion  31 E extends and the first direction is, for example, 20° or more, preferably above 30°, and for example, below 90°. 
     The direction in which the fifth front layer portion  31 E extends greatly inclines with respect to the second direction compared to the direction in which the second front layer portion  31 B extends. An angle made between the direction in which the fifth front layer portion  31 E extends and the second direction is, for example, 20° or more, preferably above 30°, and for example, below 90°. 
     3. Producing Method of Reinforcement Structure 
     Next, a method for producing the reinforcement structure  1  as a first embodiment of a method for producing a reinforcement structure of the present invention is described with reference to  FIGS. 2A to 3B . The direction of the reinforcement structure  1  shown in  FIG. 1  is reverse to that of the reinforcement structure  1  produced in  FIGS. 2A to 3B . Thus, in  FIGS. 2A to 3B , the lower side on the plane of the sheet is one side in the first direction, the upper side on the plane of the sheet is the other side in the first direction, the right side on the plane of the sheet is one side in the second direction, and the left side on the plane of the sheet is the other side in the second direction. 
     The method for producing the reinforcement structure  1  includes a step of preparing the reinforcement sheet  3  (ref:  FIGS. 2A and 2B ), a step of preparing the metal plate  2  (ref  FIG. 2C ), a step of attaching the reinforcement sheet  3  to the metal plate  2  (ref  FIG. 3A ), and a step of removing a mold  4  from the reinforcement structure  1  (ref:  FIG. 3B ). 
     In the method for producing the reinforcement structure  1 , first, as shown in  FIG. 2A , the reinforcement sheet  3  including the core material layer  30  and the front layer  31  is prepared. The reinforcement sheet  3  before adhering to the metal plate  2  is distinguished from the reinforcement sheet  3  after adhering to the metal plate  2  as a pre-adhesion reinforcement sheet  3 X. The core material layer  30  included in the pre-adhesion reinforcement sheet  3 X is defined as a pre-adhesion core material layer  30 X, and the front layer  31  included in the pre-adhesion reinforcement sheet  3 X is defined as a pre-adhesion front layer  31 X. 
     The pre-adhesion reinforcement sheet  3 X has a flat plate shape, to be specific, has a predetermined thickness, extends in a predetermined direction perpendicular to the thickness direction, and has a flat front surface and a flat rear surface. 
     The pre-adhesion reinforcement sheet  3 X includes the pre-adhesion core material layer  30 X, the pre-adhesion front layer  31 X, and the above-described adhesive layer  32 . The pre-adhesion reinforcement sheet  3 X preferably further includes a release layer  33 . 
     The pre-adhesion core material layer  30 X has a thin layer shape, and has a flat front surface and a flat rear surface. The core material layer  30  contains at least the above-described core material resin, preferably further contains the above-described filler, and if necessary, further contains the above-described optional component. The core material resin contained in the pre-adhesion core material layer  30 X is desirably in an uncured state in the case of the thermosetting resin. 
     The pre-adhesion core material layer  30 X has a fixed thickness over the whole layer. The pre-adhesion core material layer  30 X has a thickness of, for example, 0.1 mm or more, preferably 1 mm or more, and for example, 20 mm or less, preferably 10 mm or less, more preferably 2 mm or less. 
     The pre-adhesion front layer  31 X is disposed at one side in the first direction of the pre-adhesion core material layer  30 X (one side in the thickness direction). The pre-adhesion front layer  31 X has a thin layer shape, and has a flat front surface and a flat rear surface. The pre-adhesion front layer  31 X contains at least the above-described plurality of fibers, preferably further contains the above-described front layer resin, and if necessary, further contains the above-described optional component. The front layer resin contained in the pre-adhesion front layer  31 X is desirably in an uncured state in the case of the thermosetting resin. 
     The pre-adhesion front layer  31 X has a thickness of, for example, 0.02 mm or more, preferably 0.15 mm or more, and for example, 5 mm or less, preferably 1 mm or less, more preferably 0.5 mm or less. 
     The adhesive layer  32  is disposed at the other side in the first direction of the pre-adhesion core material layer  30 X (the other side in the thickness direction). 
     The adhesive layer  32  has a thickness of, for example, 0.05 mm or more, preferably 0.1 mm or more, and for example, 1 mm or less, preferably 0.5 mm or less. 
     The release layer  33  is disposed at the opposite side of the adhesive layer  32  with respect to the pre-adhesion front layer  31 X. The release layer  33  is releasably attached to the front surface of the adhesive layer  32  so as to protect the adhesive layer  32  until the pre-adhesion reinforcement sheet  3 X is used for the production of the reinforcement structure  1 . 
     The attached surface of the release layer  33  is subjected to release treatment as needed. Examples of a material for the release layer  33  include resin materials such as polyester (for example, polyethylene terephthalate (PET) or the like), polyolefin (for example, polyethylene, polypropylene, or the like), and fluorine resin (for example, polytetrafluoroethylene (PTFE) or the like) and resin coat paper. Preferably, polyethylene terephthalate is used. 
     The release layer  33  may not be provided on the front surface of the adhesive layer  32 . In this case, the adhesive layer  32  is exposed. 
     Although not shown, a release layer may be provided on the front surface of the pre-adhesion front layer  31 X. 
     Next, as shown in  FIG. 2B , the pre-adhesion reinforcement sheet  3 X is preliminarily shaped as needed. The pre-adhesion reinforcement sheet  3 X is preliminarily shaped by, for example, the mold  4  having a shape corresponding to the metal plate  2 . 
     The mold  4  integrally includes a base portion  40  and a protruding portion  41 . 
     The base portion  40  has a flat plate shape extending in the second direction, The protruding portion  41  protrudes from the base portion  40  toward the other side in the first direction. The protruding portion  41  has a shape that is capable of being inserted into the recessed portion  20  in the metal plate  2 , and has a first mold surface  41 A, a second mold surface  41 B, a third mold surface  41 C, a fourth mold surface  41 D, and a filth mold surface  41 E. 
     The first mold surface  41 A is one end surface in the second direction of the protruding portion  41 , and extends in the first direction corresponding to the first surface  20 A. The second mold surface  41 B is the other end surface in the first direction of the protruding portion  41 , and extends in the second direction corresponding to the second surface  20 B. 
     The third mold surface  41 C connects the first mold surface  41 A to the second mold surface  41 B, and extends in a direction perpendicular to the third direction (ref:  FIG. 1 ). To be more specific, the third mold surface  41 C connects the other end portion in the first direction of the first mold surface  41 A to one end portion in the second direction of the second mold surface  41 B. The third mold surface  41 C inclines from the other side toward one side in the second direction as it goes from the other side toward one side in the first direction. 
     The fourth mold surface  41 D is the other end surface in the second direction of the protruding portion  41 , and extends in the first direction corresponding to the third surface  20 C. The fifth mold surface  41 E connects the fourth mold surface  41 D to the second mold surface  41 B, and extends in a direction crossing both directions of the first direction and the second direction. To be more specific, the fifth mold surface  41 E connects the other end portion in the first direction of the fourth mold surface  41 D to the other end portion in the second direction of the second mold surface  41 B. The fifth mold surface  41 E inclines from the other side toward one side in the second direction as it goes from one side toward the other side in the first direction. 
     To preliminarily shape the pre-adhesion reinforcement sheet  3 X by the mold  4 , the reinforcement sheet  3  is pressed along the protruding portion  41  to be preliminarily heated so that the front layer  31  is positioned at the side of the protruding portion  41 . 
     A preliminarily heating temperature is, for example, 30° C. or more, preferably 40° C. or more, and for example, 120° C. or less, preferably 80° C. or less. The preliminarily heating time is, for example, 15 seconds or more, and for example, 5 minutes or less. 
     In this manner, the reinforcement sheet  3  is preliminarily shaped into a generally U-shape having an opening toward one side in the first direction. 
     Thereafter, the release layer  33  is peeled from the adhesive layer  32  to expose the adhesive layer  32 . 
     Next, as shown in  FIG. 2C , the above-described metal plate  2  is prepared. 
     Next, as shown in  FIG. 3A , the reinforcement sheet  3  is attached to the metal plate  2  so that the core material layer  30  is positioned between the inner surface (the first surface  20 A, the first corner portion  20 D, the second surface  20 B, the second corner portion  20 E, and the third surface  20 C) of the recessed portion  20  and the front layer  31 , and the front layer  31  is positioned at spaced intervals to the inner surface (the first surface  20 A, the first corner portion  20 D, the second surface  20 B, the second corner portion  20 E, and the third surface  20 C) of the recessed portion  20 . 
     To be more specific, first, the reinforcement sheet  3  is heated at an attachment temperature. 
     The attachment temperature is a temperature at which the latent curing agent substantially reacts or more, and is, for example, 80° C. or more, preferably 120° C. or more, and for example, 250° C. or less, preferably 200° C. or less. 
     The metal plate  2  is pressed toward the mold  4  so that the reinforcement sheet  3  and the protruding portion  41  are fitted into the recessed portion  20 . In other words, the reinforcement sheet  3  is pressed toward the metal plate  2  by the mold  4  having a shape corresponding to the metal plate  2 . 
     At this time, the core material layer  30  is pressed by the metal plate  2  and flows so as to be along the inner surface (the first surface  20 A, the first corner portion  20 D, the second surface  20 B, the second corner portion  20 E, and the third surface  20 C) of the recessed portion  20 , while the adhesive layer  32  adheres to the inner surface of the recessed portion  20 . 
     In this manner, the first portion  30 A is formed between the first mold surface  41 A and the first surface  20 A; the second portion  30 B is formed between the second mold surface  41 B and the second surface  20 B; the third portion  30 C is formed between the first corner portion  20 D and the third mold surface  41 C; the fourth portion  30 D is formed between the fourth mold surface  41 D and the third surface  20 C; and the fifth portion  30 E is formed between the second corner portion  20 E and the fifth mold surface  41 E. 
     In other words, the core material layer  30  flows so that in the core material layer  30 , the thickness along the second direction of the third portion  30 C along the first corner portion  20 D is thicker than the thickness along the second direction of the first portion  30 A along the first surface  20 A, and the thickness of the third portion  30 C along the first direction is thicker than the thickness along the first direction of the second portion  30 B along the second surface  20 B. 
     Also, the first front layer portion  31 A is disposed so as to be positioned at spaced intervals to the first surface  20 A in the second direction, and extend in the first direction; the second front layer portion  31 B is disposed so as to be positioned at spaced intervals to the second surface  20 B in the first direction, and extend in the second direction; the third front layer portion  31 C is disposed at spaced intervals to the first corner portion  20 D; the fourth front layer portion  31 D is disposed so as to be positioned at spaced intervals to the third surface  20 C in the second direction, and extend in the first direction; and the fifth front layer portion  31 E is disposed at spaced intervals to the second corner portion  20 E. 
     The gap between the first surface  20 A and the third front layer portion  31 C in the second direction is formed so as to gradually increase as it gets closer to the first corner portion  20 D in the first direction with respect to the gap between the first surface  20 A and the first portion  30 A in the second direction. 
     The gap between the second surface  20 B and the third front layer portion  31 C in the first direction is formed so as to gradually increase as it gets closer to the first corner portion  20 D in the second direction with respect to the gap between the second surface  20 B and the second front layer portion  31 B in the first direction. 
     In other words, the third front layer portion  31 C is disposed so that the gap between the first surface  20 A and the third front layer portion  31 C in the second direction and the gap between the second surface  20 B and the third front layer portion  31 C in the first direction are the description above. 
     Thereafter, the above-described attachment temperature is retained for a predetermined attachment time. 
     The attachment time is, for example, one second or more, preferably 10 seconds or more, and for example, 60 minutes or less, preferably 30 minutes or less. 
     In this manner, each of the core material resin of the core material layer  30  and the front layer resin of the front layer  31  is cured (is brought into a cured state) in the case of an uncured state before heating, while the adhesive layer  32  is cured. 
     In this manner, the reinforcement sheet  3  adheres to the metal plate  2  to reinforce the metal plate  2 , and the reinforcement structure  1  including the metal plate  2  and the reinforcement sheet  3  is produced. 
     Thereafter, as shown in  FIG. 3B , if necessary, the reinforcement structure  1  is cooled, and thereafter, the mold  4  is removed from the reinforcement structure  1 . 
     As shown in  FIG. 1 , in the reinforcement structure  1 , the third front layer portion  31 C of the front layer  31  is positioned at spaced intervals to the first corner portion  20 D in the third direction; the gap between the first surface  20 A and the third front layer portion  31 C in the second direction gradually increases as it gets closer to the first corner portion  20 D in the first direction with respect to the gap between the first surface  20 A and the first front layer portion  31 A in the second direction; and the gap between the second surface  20 B and the third front layer portion  31 C in the first direction gradually increases as it gets closer to the first corner portion  20 D in the second direction with respect to the gap between the second surface  20 B and the second front layer portion  31 B in the first direction. 
     Thus, the reinforcement sheet  3  can intensively reinforce the first corner portion  20 D of the metal plate  2 , and accordingly, the improvement of the strength of the reinforcement structure  1  can be achieved. 
     To be specific, the maximum bending testing force at room temperature (23° C.) of the reinforcement structure  1  is, for example, 10 kN or more, preferably 12 kN or more. The maximum bending testing force can be measured in conformity with the method described in Examples (hereinafter, the same). 
     The third portion  30 C of the core material layer  30  is along the first corner portion  20 D, the thickness of the third portion  30 C along the second direction is thicker than that of the first portion  30 A along the second direction, and the thickness of the third portion  30 C along the first direction is thicker than that of the second portion  30 B along the first direction. 
     That is, the third portion  30 C that is positioned near the first corner portion  20 D is thicker than both portions of the first portion  30 A along the first surface  20 A and the second portion  30 B along the second surface  20 B. 
     Thus, the reinforcement sheet  3  can more intensively reinforce the first corner portion  20 D of the metal plate  2 , and accordingly, the improvement of the strength of the reinforcement structure  1  can be surely achieved. 
     In the core material layer  30 , the first portion  30 A, the second portion  30 B, the third portion  30 C, the fourth portion  30 D, and the fifth portion  30 E are integrated. Thus, compared to a case where each of the portions of the core material layer  30  is a separate piece, a reduction of the number of components can be achieved. Also, the improvement of relative position accuracy of each of the portions of the core material layer  30  can be achieved, and the first corner portion  20 D can be more stably intensively reinforced. 
     The fifth portion  30 E of the core material layer  30  is along the second corner portion  20 E, the thickness of the fifth portion  30 E along the second direction is thicker than that of the fourth portion  30 D along the second direction, and the thickness of the fifth portion  30 E along the first direction is thicker than that of the second portion  30 B along the first direction. 
     That is, the fifth portion  30 E that is positioned near the second corner portion  20 E is thicker than both portions of the fourth portion  30 D along the third surface  20 C and the second portion  30 B along the second surface  20 B. 
     Thus, even when the metal plate  2  has the first corner portion  20 D and the second corner portion  20 E, the reinforcement sheet  3  can intensively reinforce both of the first corner portion  20 D and the second corner portion  20 E, and accordingly, the improvement of the strength of the reinforcement structure  1  can be furthermore surely achieved. 
     The adhesive layer  32  is disposed between the core material layer  30  and the metal plate  2 . Thus, the core material layer  30  can surely adhere to the metal plate  2 . As a result, the reinforcement sheet  3  can stably reinforce the metal plate  2 . 
     As shown in  FIG. 3A , in the method for producing the reinforcement structure  1 , in the step of attaching the reinforcement sheet  3  to the metal plate  2 , the third front layer portion  31 C is disposed so that the gap between the first surface  20 A and the third front layer portion  31 C in the second direction gradually increases as it gets closer to the first corner portion  20 D in the first direction with respect to the gap between the first surface  20 A and the first front layer portion  31 A in the second direction, and the gap between the second surface  20 B and the third front layer portion  31 C in the first direction gradually increases as it gets closer to the first corner portion  20 D in the second direction with respect to the gap between the second surface  20 B and the second front layer portion  31 B in the first direction. 
     That is, in the step of attaching the reinforcement sheet  3  to the metal plate  2 , the third front layer portion  31 C in the front layer  31  can be disposed as described above, so that the reinforcement structure  1  having the improved strength can be smoothly produced. 
     In the step of attaching the reinforcement sheet  3  to the metal plate  2 , the core material layer  30  containing the resin flows, so that the thickness along the second direction of the third portion  30 C along the first corner portion  20 D is thicker than the thickness along the second direction of the first portion  30 A along the first surface  20 A, and the thickness of the third portion  30 C along the first direction is thicker than the thickness along the first direction of the second portion  30 B along the second surface  20 B. 
     Thus, the third portion  30 C of the core material layer  30  can be smoothly formed so as to have the above-described thickness by an easy method. 
     The third mold surface  41 C of the mold  4  connects the first mold surface  41 A to the second mold surface  41 B, and extends in a direction crossing the third direction in which the first corner portion  20 D faces the third front layer portion  31 C. 
     Thus, when the reinforcement sheet  3  is pressed toward the metal plate  2  by the mold  4 , space for disposing the third front layer portion  31 C can be ensured between the first corner portion  20 D and the third mold surface  41 C. As a result, in the step of attaching the reinforcement sheet  3  to the metal plate  2 , the third front layer portion  31 C can be smoothly disposed as described above. 
     Second Embodiment 
     Next, a reinforcement structure  10  as a second embodiment of a reinforcement structure of the present invention is described with reference to  FIG. 4 . The same reference numerals are provided for members and steps corresponding to each of those in the above-described first embodiment, and their detailed description is omitted. 
     As shown in  FIG. 1 , the reinforcement structure  1  includes the adhesive layer  32 . However, the reinforcement structure of the present invention may not include the adhesive layer. As shown in  FIG. 4 , the reinforcement structure  10  does not include the adhesive layer  32 , and the core material layer  30  is in direct contact with the inner surface (each of the first surface  20 A, the first corner portion  20 D, the second surface  20 B, the second corner portion  20 E, and the third surface  20 C) of the recessed portion  20 . 
     According to the second embodiment, the core material layer  30  is in direct contact with the inner surface of the recessed portion  20 , so that compared to a structure including the adhesive layer  32 , a reduction of the number of components can be achieved, and the arrangement can be achieved so that the core material layer  30  (each of the first portion  30 A, the second portion  30 B, the third portion  30 C, the fourth portion  30 D, and the fifth portion  30 E) is surely along the inner surface (each of the first surface  20 A, the second surface  20 B, the first corner portion  20 D, the third surface  20 C, and the second corner portion  20 E) of the recessed portion  20 . 
     In the second embodiment, the same function and effect as that of the first embodiment can be achieved. 
     Third Embodiment 
     Next, a reinforcement structure  11  as a third embodiment of a reinforcement structure of the present invention is described with reference to  FIG. 5 . The same reference numerals are provided for members and steps corresponding to each of those in the above-described first embodiment, and their detailed description is omitted. 
     As shown in  FIG. 1 , in the reinforcement structure  1 , in the first corner portion  20 D, the other end portion in the first direction of the first surface  20 A and one end portion in the second direction of the second surface  20 B form an angle of about 90°. However, the structure of the first corner portion is not limited to this. 
     As shown in  FIG. 5 , in the reinforcement structure  11 , the first corner portion  20 D has a circular arc shape. To be more specific, the first corner portion  20 D is continuous from the other end portion in the first direction of the first surface  20 A to curve toward the other side in the second direction as it goes toward the other side in the first direction. Then, the first corner portion  20 D is connected to one end portion in the second direction of the second surface  20 B. Also, the second corner portion  20 E may have a circular arc shape as in the same manner as the first corner portion  20 D. 
     In the third embodiment, the same function and effect as that of the first embodiment can be achieved. 
     Fourth Embodiment 
     Next, a reinforcement structure  12  as a fourth embodiment of a reinforcement structure of the present invention is described with reference to  FIG. 6 . The same reference numerals are provided for members and steps corresponding to each of those in the above-described first embodiment, and their detailed description is omitted. 
     In the first to third embodiments, the third portion  30 C is along the first corner portion  20 D, and the third portion  30 C is in tight contact with the first corner portion  20 D via the adhesive layer  32 . However, the present invention is not limited to this. 
     In the fourth embodiment, the third portion  30 C is not along the first corner portion  20 D, and space S is formed between the first corner portion  20 D and the third portion  30 C (to be more specific, the adhesive layer  32 ). The thickness of the third portion  30 C (thickness in the third direction) is generally the same as the thickness in the second direction of the first portion  30 A and the thickness in the first direction of the second portion  30 B. 
     In the fourth embodiment, as described above, when each of the gap between the first surface  20 A and the third front layer portion  31 C in the second direction and the gap between the second surface  20 B and the third front layer portion  31 C in the first direction is configured so as to gradually increase as it gets closer to the first corner portion  20 D, the same function and effect as that of the above-described first embodiment can be achieved. In view of improvement of the strength of the reinforcement structure  1 , the first embodiment is more preferable. 
     Also, space may be formed between the second corner portion  20 E and the fifth portion  30 E in the same manner as the description above. 
     Modified Examples 
     In each of the modified examples below, the same reference numerals are provided for members and steps corresponding to each of those in the above-described first embodiment, and their detailed description is omitted. 
     As shown in  FIG. 2A , in the first embodiment, the pre-adhesion core material layer  30 X has a fixed thickness over the whole layer. However, the pre-adhesion core material layer  30 X is not limited to this. 
     As shown in  FIG. 7 , the pre-adhesion core material layer  30 X may also include a thick portion  35  and a thin portion  36 . 
     The thick portion  35  is positioned in the center in the second direction in the pre-adhesion core material layer  30 X. The thickness of the thick portion  35  is thicker than that of the thin portion  36 . The thin portion  36  is positioned at both sides in the second direction with respect to the thick portion  35 . 
     When the pre-adhesion reinforcement sheet  3 X includes the pre-adhesion core material layer  30 X, in the step of attaching the reinforcement sheet  3  to the metal plate  2 , as referred to  FIG. 3A , the thick portion  35  is sandwiched between the second surface  20 B of the recessed portion  20  and the second mold surface  41 B of the protruding portion  41 , and a portion of the thick portion  35  smoothly flows toward the space between the first corner portion  20 D and the third mold surface  41 C and/or the space between the second corner portion  20 E and the fifth mold surface  41 E. 
     In this manner, the third portion  30 C and/or the fifth portion  30 E can be smoothly formed. 
     In the first embodiment, the first surface  20 A, the second surface  20 B, and the third surface  20 C extend linearly. However, the shape of each of the surfaces is not limited to this, and may be curved. 
     In the first embodiment, the first front layer portion  31 A, the second front layer portion  31 B, the third front layer portion  31 C, the fourth front layer portion  31 D, and the fifth front layer portion  31 E extend linearly. However, the shape of each of the portions is not limited to this, and may be curved. 
     In the first embodiment, the reinforcement sheet  3  adheres to the entire inner surface of the recessed portion  20 . However, the adhesive region of the reinforcement sheet  3  is not limited to this, and the reinforcement sheet  3  may adhere to the inner surface of the recessed portion  20  so that a portion of the inner surface of the recessed portion  20  (for example, a one-side portion in the first direction of the first surface  20 A and/or the third surface  20 C or the like) is exposed. For example, in the first embodiment, the end surface of the reinforcement sheet  3  is flush with the one-side surface in the first direction of the flange portion  21 . Alternatively, the end surface of the reinforcement sheet  3  may be positioned at the other side in the first direction with respect to the one-side surface in the first direction of the flange portion  21 . 
     In the first embodiment, the metal plate  2  has a hat shape when viewed from the cross-sectional view. However, the shape of the metal plate  2  is not limited to this. The metal plate  2  may also have an L-shape when viewed from the cross-sectional view having only the first surface  20 A, the second surface  20 B, and the first corner portion  20 D. 
     In the first to fourth embodiments, the metal plate  2  is used as one example of an adherend. However, the adherend is not limited to this, example thereof includes concrete. 
     Also, a substrate may be disposed between the front layer  31  and the core material layer  30  and/or between the adhesive layer  32  and the core material layer  30 . Examples of the substrate include glass cloth, carbon cloth, synthetic resin non-woven fabric (for example, polypropylene resin non-woven fabric, polyethylene resin non-woven fabric, ester resin non-woven fabric, or the like), metal foil, carbon fiber of short fiber, glass fiber of short fiber, and polyester film. 
     An adhesive layer that is configured in the same manner as that of the adhesive layer  32  can be also disposed between the front layer  31  and the core material layer  30 . 
     In the modified examples, the same function and effect as that of the first embodiment can be achieved. 
     The first to fourth embodiments and the modified examples can be appropriately used in combination. 
     EXAMPLES 
     Next, the present invention is further described based on Examples and Comparative Examples shown below The present invention is however not limited by these Examples and Comparative Examples. The specific numerical values in mixing ratio (content ratio), property value, and parameter used in the following description can be replaced with upper limit values (numerical values defined as “or less” or “below”) or lower limit values (numerical values defined as “or more” or “above”) of corresponding numerical values in mixing ratio (content ratio), property value, and parameter described in the above-described “DESCRIPTION OF EMBODIMENTS”.  
     Example 1 
     1. Preparation of Front Layer 
     Unidirectional fiber resin composite sheets (four pieces) (manufactured by TORAY INDUSTRIES INC., thickness of 0.10 mm) were prepared. The unidirectional fiber resin composite sheet included a plurality of carbon fibers (CFUD, basis weight of 100 g/m 2 ) that extended in the same fiber direction and were arranged in a direction perpendicular to the fiber direction so as to be generally parallel with each other, and an epoxy resin (front layer resin, content ratio of 33 mass %) that was impregnated in the plurality of carbon fibers. When the front layer resin was in a cured state, the tensile strength of each of the unidirectional fiber resin composite sheets was 2500 MPa. 
     The plurality of unidirectional fiber resin composite sheets were laminated so that the fiber directions of the unidirectional fiber resin composite sheets that were next to each other were perpendicular to each other. Thereafter, the plurality of unidirectional fiber resin composite sheets were pressed so as to be sandwiched from the outside in the thickness direction, thereby preparing the front layer (pre-adhesion front layer) 
     2. Preparation of Core Material Layer 
     The following components were kneaded with a mixing roll, thereby preparing a resin composition. The components included 40 parts by mass of an unmodified epoxy resin (core material resin, uncured state, trade name: JER 828, epoxy equivalent of 184 g/eq. to 194 g/eg., manufactured by Mitsubishi Chemical Corporation); 23 parts by mass of a first CTBN-modified epoxy resin (core material resin, uncured state, trade name: EPR2000, epoxy equivalent of 215 g/eq., manufactured by ADEKA CORPORATION); 37 parts by mass of a second CTBN-modified epoxy resin (core material resin, uncured state, trade time: HypoxRA840, epoxy equivalent of 350 g/eq., manufactured by CVC Specialty Chemicals Inc.); 321 parts by mass of a filler (calcium carbonate); 2 parts by mass of a first latent curing agent (urea compound, trade name: DCMU-99, manufactured by Hodogaya Chemical Co., Ltd.); and 5 parts by mass of a second latent curing agent (dicyandiamide, trade name: DDA-50, manufactured by CVC Specialty Chemicals Inc.). 
     Thereafter, the resin composition was formed into a core material layer (pre-adhesion core material layer) having a thick portion shown in  FIG. 7 . The core material layer included the thick portion positioned in the center in the second direction, and a thin portion positioned around it. The core material resin in the core material layer was in an uncured state. 
     3. Preparation of Adhesive Layer 
     Each of the following components was blended to be kneaded with the mixing roll, thereby preparing a kneaded product (thermosetting resin composition). In the kneading, first, the epoxy resin, the rubber-modified epoxy resin, the filler, and the toughness imparting agent were kneaded with the mixing roll that was heated at 120° C. and thereafter, the kneaded product was cooled to 60 to 100° C. Furthermore, the latent curing agent was added thereto to be kneaded with the mixing roll, thereby obtaining the kneaded product. 
     Bisphenol A epoxy resin (trade name: Epikote (registered trademark) #834, epoxy equivalent of 270 g/eq., manufactured by Mitsubishi Chemical Corporation) . . . 50 parts by mass 
     Liquid CTBN-modified epoxy resin (trade name: EPR 1415-1, epoxy equivalent of 400 g/eq., manufactured by ADEKA CORPORATION) . . . 35 parts by mass 
     Solid CTBN-modified epoxy resin (trade name: HypoxRK84L, epoxy equivalent of 1200 to 1800 g/eq., manufactured by CVC Specialty Chemicals Inc.) . . . 15 parts by mass 
     Urea compound: 3-(3,4-dichlorophenyl)-1,1-dimethylurea (trade name: DCMU, manufactured by Hodogaya Chemical Co., Ltd.) . . . 2 parts by mass 
     Dicyandiamide (trade name: DDA-50, manufactured by CVC Specialty Chemicals Inc.) . . . 5 parts by mass 
     Hydrophobic silica: dimethyl silicone oil surface-treated silica (trade name: AEROJIL RY200, average particle size of 12 μm, manufactured by NIPPON AEROSIL CO., LTD.) . . . 2 parts by mass 
     Glass powder: glass powder (trade name: PF70E-001, specific gravity of 2.58, average fiber length of 10.5 μm, manufactured by Nitto Boseki. Co., Ltd.) . . . 20 parts by mass 
     Next, the obtained kneaded product in a state of being sandwiched between the release paper was rolled to have a thickness of 0.1 mm by the press molding, thereby forming the thermosetting resin layer. 
     Thereafter, the release paper on the one-side surface of the thermosetting resin layer was peeled, and the thermosetting resin layer was attached to both surfaces of the non-woven fabric (trade name: 8004, manufactured by NISSEI Co., Ltd.) having a thickness of 130 μm by heat press (50° C.) so as to have a total thickness of the thermosetting resin layer and the substrate of 0.2 mm, thereby preparing the double-coated adhesive tape. 
     4. Attaching of Front Layer and Adhesive Layer to Core Material Layer 
     Next, the front layer was disposed on the front surface of the core material layer to be pressed so that the front layer went toward the core material layer. In this manner, the front layer was attached to the core material layer. 
     The double-coated adhesive tape was disposed on the rear surface of the core material layer to be pressed so that the double-coated adhesive tape went toward the core material layer. In this manner, the double-coated adhesive tape was attached to the core material layer, thereby forming the adhesive layer. 
     In this way, the reinforcement sheet (pre-adhesion reinforcement sheet) including the core material layer, the front layer, and the adhesive layer was prepared. 
     5. Preparation of Metal Plate 
     A metal plate having a hat shape when viewed from the cross-sectional view shown in  FIG. 2C  was prepared. The metal plate had a thickness of 0.8 mm. The size (size in a depth direction of the recessed portion) in the first direction of the metal plate was 20 mm, the size (size including the flange portion and the recessed portion) in the second direction of the metal plate was 60 mm, and the size in the direction perpendicular to both directions of the first direction and the second direction of the metal plate was 150 mm. The size in the second direction of the recessed portion of the metal plate was 30 mm. 
     6. Attachment of Reinforcement Sheet to Metal Plate 
     First, as shown in  FIG. 2B , the reinforcement sheet was heated at 60° C. (preliminarily heating temperature) to be preliminarily shaped by the mold having a shape corresponding to the recessed portion. The mold had a shape shown in  FIG. 2B , and had the first mold surface, the second mold surface, the third mold surface, the fourth mold surface, and the fifth mold surface. 
     Next, as shown in  FIG. 3A , the reinforcement sheet that was preliminarily shaped was heated at 175° C. (attachment temperature), and thereafter, the metal plate was pressed toward the mold so that that reinforcement sheet was fitted into the recessed portion. 
     Next, 175° C. (attachment temperature) was retained for 150 seconds, and heating was carried out at 175° C. for 35 minutes with a hot air dryer, so that the adhesive, the core material resin, and the front layer resin were cured. 
     In this manner, the reinforcement sheet adhered to the inner surface (the first surface, the second surface, the third surface, the first corner portion, and the second corner portion) of the recessed portion, thereby producing the reinforcement structure. Thereafter, the mold was removed from the reinforcement structure. 
     The reinforcement structure of Example 1 had the same structure as that of the first embodiment shown in  FIG. 1 . 
     Example 2 
     A reinforcement structure was produced in the same manner as that of Example 1, except that as shown in  FIG. 2A , the core material layer (pre-adhesion core material layer) had a fixed thickness over the whole layer, and did not have the thick portion. The thickness of the core material layer was the same as that of the thin portion of the core material layer in Example 1. 
     The reinforcement structure of Example 2 had the same structure as that of the fourth embodiment shown in  FIG. 6 . 
     Comparative Example 1 
     The reinforcement structure was produced in the same manner as that of Example 1, except that the mold did not have the third mold surface and the fifth mold surface, the first mold surface was directly connected to the second mold surface, and the second mold surface was directly connected to the fourth mold surface. 
     In the reinforcement structure of Comparative Example 1, the entire front layer was positioned at fixed intervals (with the same width) to the inner surface (the first surface, the second surface, and the third surface) of the recessed portion. 
     Comparative Example 2 
     The above-described unidirectional fiber resin composite sheets (12 pieces) were prepared, and laminated so that the fiber directions of the unidirectional fiber resin composite sheets that were next to each other were perpendicular to each other. Thereafter, the plurality of unidirectional fiber resin composite sheets were pressed so as to be sandwiched from the outside in the thickness direction, thereby preparing the front layer. 
     Next, the above-described double-coated adhesive tape was attached to the front layer, thereby preparing the reinforcement sheet including the front layer and the adhesive layer, and without including the core material layer. 
     Next, the reinforcement sheet adhered to the inner surface of the recessed portion in the same manner as that of Example 1, thereby producing the reinforcement structure. 
     Evaluation 
     Measurement of Maximum Bending Testing Force 
     The maximum bending testing force (the maximum value of a load in a bending test) of each of the reinforcement structures obtained in Examples and Comparative Examples was measured under the following test conditions with a bending testing machine (trade name: Technograph TG-100kN (load cell: TC3D-100kN), manufactured by Minebea Co., Ltd.). 
     Test conditions: three-point bending method, distance between supporting points=100 mm, test rate of 5 mm/min, room temperature (23° C.) 
     The results are shown in the following. 
     Example 1: 12.7 kN, Example 2: 11.9 kN, Comparative Example 1: 9.9 kN, Comparative Example 2: 7.8 kN 
     While the illustrative embodiments of the present invention are provided in the above description, such is for illustrative purpose only and it is not to be construed as limiting the scope of the present invention. Modification and variation of the present invention that will be obvious to those skilled in the art is to be covered by the following claims. 
     INDUSTRIAL APPLICABILITY 
     The reinforcement structure of the present invention can be applied for reinforcement of an adherend used for various industrial products, and can be, for example, preferably used for reinforcement of a metal plate used in transportation machines, a metal plate used in household electric appliances, or the like. The method for producing a reinforcement structure of the present invention can be applied for various industrial products, and can be, for example, preferably used for transportation machines, household electric appliances, or the like. 
     DESCRIPTION OF REFERENCE NUMERALS 
       1  Reinforcement structure 
       2  Metal plate 
       3  Reinforcement sheet 
       4  Mold 
       10  Reinforcement structure 
       11  Reinforcement structure 
       12  Reinforcement structure 
       20 A First surface 
       20 B Second surface 
       20 C Third surface 
       20 D First corner portion 
       20 E Second corner portion 
       30  Core material layer 
       30 A First portion 
       30 B Second portion 
       30 C Third portion 
       30 D Fourth portion 
       30 E Fifth portion 
       31  Front layer 
       31 A First front layer portion 
       31 B Second front layer portion 
       31 C Third front layer portion 
       32  Adhesive layer 
       41 A First mold surface 
       41 B Second mold surface 
       41 C Third mold surface