The present invention relates to a composite material and a synthetic cross tie using the composite material. 
Fiber reinforced foam thermosetting resin molded goods, which resemble native wood in appearance and show performances in physical properties equal to or more than native wood, are used as constitute materials including building material, structural material, cross tie, and board material used for waterish places. 
In general, the foam thermosetting resin molded goods of this type have a plate-like or bar-like molded main body which is formed by the foam thermosetting resin liquid being foamed and cured, as disclosed by Japanese Patent Publication No.  Sho 52-2421 and Japanese Laid-open Patent Publication No.  Hei 5-23947.  In an interior of the main body, glass fibers having long fibers are paralleled in the longitudinal direction as reinforced fibers and dispersed in generally parallel. 
This conventional type of foam thermosetting resin molded goods have a sufficient strength against a bending stress exerted in a direction orthogonal to the longitudinal direction in that the reinforced fibers are paralleled in the longitudinal direction and dispersed generally parallel in the interior of the molded main body.  However, they have the disadvantage that for example, when nailed, they are easily cracked or fractured in a direction parallel to the reinforced fibers or the disadvantage that they have a low unnailing strength. 
In view of this, there was proposed a composite material in which surface layers, which comprises foam thermosetting resin and in which reinforced fibers are paralleled in the longitudinal direction and dispersed in generally parallel, are laminated on both surfaces of a core layer comprising foam thermosetting resin in which not more than 50 weight % of filler is dispersed (Cf.  JP Laid-open Patent Publication No.  Hei 5-138797). 
In short, this composite material is made to have a sandwich structure in which the core layer of excellent in compressive strength is sandwiched between the surface layers to thereby produce improved compression strength and nailing performance. 
However, since this composite material has, as shown in FIG.  17, the structure that thermosetting resin layer 300 exists between the filler 200 of the core layer 100 and the filler 200, it has the disadvantage that its compression strength and nailing performance are insufficiently improved. 
Also, it suffers from the disadvantage that when the composite material is deflected largely or bent repeatedly, the thermosetting resin layer 300 is destroyed. 
In the consideration of these circumstances, the present invention has been made with the aim to provide a composite material capable to further improve its compression strength and nailing performance and a synthetic cross tie using the composite material. 
To achieve this object, a composite material according to the present invention as set forth in claim 1 (hereinafter it is referred to as xe2x80x9cthe composite material of claim 1xe2x80x9d) comprises a core layer comprising filler and synthetic resin and containing the filler having a weight 0.7 times or more the product of volume of the core layer and bulk density of the filler; and a surface layer comprising a thermosetting resin reinforced by long fibers extending parallel in a longitudinal direction thereof, the surface layer being laminated on the core layer to cover at least one surface of the core layer with respect to a thickness direction thereof. 
A composite material according to the present invention as set forth in claim 2 (hereinafter it is referred to as xe2x80x9cthe composite material of claim 2xe2x80x9d) comprises a core layer comprising filler and synthetic resin and containing the filler having a weight 0.7 times or more the product of volume of the core layer and bulk density of the filler; and a surface layer comprising a thermosetting resin including lightweight filler reinforced by long fibers extending parallel in a longitudinal direction thereof, the surface layer being laminated on the core layer to cover at least one surface of the core layer with respect to a thickness direction thereof. 
The composite material according to the present invention as set forth in claim 3 (hereinafter it is referred to as xe2x80x9cthe composite material of claim 3xe2x80x9d) features that in the composite material of claim 1 or 2, the surface layer has a density of 0.3 g/cm3 or more to 1.5 g/cm3 or less. 
The composite material according to the present invention as set forth in claim 4 (hereinafter it is referred to as xe2x80x9cthe composite material of claim 4xe2x80x9d) features that in the composite material of any one of claims 1 through 3, it comprises a surface layer having a bending modulus of 6,000 MPa or more and a bending strength of 100 MPa or more. 
The composite material according to the present invention as set forth in claim 5 (hereinafter it is referred to as xe2x80x9cthe composite material of claim 5xe2x80x9d) features that in the composite material of any one of claims 1 through 4, the filler having an average particle size of 0.5 mm or more is used. 
The composite material according to the present invention as set forth in claim 6 (hereinafter it is referred to as xe2x80x9cthe composite material of claim 6xe2x80x9d) features that in the composite material of any of claims 1 through 5, the filler has two or more peak areas that constitute 8 volume % or more on a particle size distribution curve plotting particle size in abscissa and a volume ratio of filler per particle size to all fillers in ordinate and also has the size distribution that most frequent particle size values in the smaller peak area of 8 volume % or more is 0.7 or less of most frequent particle size values in the larger peak area of 8 volume % or more next to the smaller peak area. 
The composite material according to the present invention as set forth in claim 7 (hereinafter it is referred to as xe2x80x9cthe composite material of claim 7xe2x80x9d) features that in the composite material according to any of claims 1 through 6, thermosetting resin is used as the synthetic resin forming the core layer. 
The composite material according to the present invention as set forth in claim 8 (hereinafter it is referred to as xe2x80x9cthe composite material of claim 8xe2x80x9d) features that in the composite material of any of claims 1 through 6, thermoplastic resin is used as the synthetic resin forming the core layer. 
The composite material according to the present invention as set forth in claim 9 (hereinafter it is referred to as xe2x80x9cthe composite material of claim 9xe2x80x9d) features that in the composite material of any of claims 1 through 7, thermosetting polyurethane resin foam of polyol equivalent of 230 or more to 1,500 or less or thermosetting polyurethane resin foam having a density of 0.3 g/cm3 or more and polyol equivalent of 1,500 or less is used as the synthetic resin forming the core layer. 
A composite material according to the present invention as set forth in claim 10 (hereinafter it is referred to as xe2x80x9cthe composite material of claim 10xe2x80x9d) comprises a core layer comprising filler and synthetic resin and a surface layer comprising synthetic resin foam and laminated on the core layer to cover at least one surface of the core layer with respect to a thickness direction thereof, wherein a variation, curve of bending stress of the core layer that varies with the bending and deflection has a singular point at which slope of the tangent line decreasing gradually from the point in time at which the bending is started increases again before becoming negative. 
The composite material according to the present invention as set forth in claim 11 (hereinafter it is referred to as xe2x80x9cthe composite material of claim 11xe2x80x9d) features that in the composite material of claim 10, thermosetting resin foam reinforced by long fibers extending parallel in a longitudinal direction thereof is used as the synthetic resin foam. 
The composite material according to the present invention as set forth in claim 12 (hereinafter it is referred to as xe2x80x9cthe composite material of claim 12xe2x80x9d) features that in the composite material of claim 10 or 11, the core layer has deflection of 0.8% or less at the singular point. 
The composite material according to the present invention as set forth in claim 13 (hereinafter it is referred to as xe2x80x9cthe composite material of claim 13xe2x80x9d) features that in the composite material of any of claim 10 through 12, the core layer has the bending modulus of 800 MPa or more when further deflected from deflection at the singular point. 
The composite material according to the present invention as set forth in claim 14 (hereinafter it is referred to as xe2x80x9cthe composite material of claim 14xe2x80x9d) features that in the composite material of any of claim 1 through 13, the core layer is formed by a plurality of core layer forming composition layers. 
The composite material according to the present invention as set forth in claim 15 (hereinafter it is referred to as xe2x80x9cthe composite material of claim 15xe2x80x9d) features that in the composite material of claim 14, one of the core layer forming composition layers is formed of thermosetting resin reinforced by long fibers extending parallel in a longitudinal direction thereof or thermosetting resin including lightweight filler reinforced by long fibers extending parallel in a longitudinal direction thereof. 
A composite material according to the present invention as set forth in claim 16 (hereinafter it is referred to as xe2x80x9cthe composite material of claim 16xe2x80x9d) comprises a core layer comprising synthetic resin as a main component and a surface layer comprising foam thermosetting resin reinforced by long fibers extending parallel in a longitudinal direction thereof or elastic synthetic resin reinforced by long fibers extending parallel in a longitudinal direction thereof and laminated on the core layer to cover both surfaces of the core layer with respect to a thickness direction thereof, wherein the core layer and the surface layer have the relation that satisfies the equations of CSaxe2x89xa7xc2xdxc3x97CSb, Ea less than Eb, and ESaxe2x89xa7xc2xdxc3x97ESb (where CSa represents yield strain in compression of the core layer; CSb represents yield strain in compression of the surface layer; Ea represents a tension elasticity modulus of the core layer; Eb represents a tension elasticity modulus of the surface layer; ESa represents yield strain in tension of the core layer; and ESb represents yield strain in tension of the surface layer). 
The composite material according to the present invention as set forth in claim 17 (hereinafter it is referred to as xe2x80x9cthe composite material of claim 17xe2x80x9d) features that in the composite material of claim 16, it follows that 0.005xe2x89xa6CSa, 50 MPaxe2x89xa6Ea, 0.005xe2x89xa6ESa, 0.01xe2x89xa6CSb, 5,000 MPaxe2x89xa6Ebxe2x89xa618,000 MPa, and 0.01xe2x89xa6ESb. 
The composite material according to the present invention as set forth in claim 18 (hereinafter it is referred to as xe2x80x9cthe composite material of claim 18xe2x80x9d) features that in the composite material of any of claims 1 through 17, the core layer has a compression shear strength DBa of 5 MPa or more. 
The composite material according to the present invention as set forth in claim 19 (hereinafter it is referred to as xe2x80x9cthe composite material of claim 19xe2x80x9d) features that in the composite material of any of claims 1 through 18, the core layer and the surface layer are integrally adhesive bonded to each other through an intermediate layer comprising non-foam thermosetting resin or low-power foam resin. 
The composite material according to the present invention as set forth in claim 20 (hereinafter it is referred to as xe2x80x9cthe composite material of claim 20xe2x80x9d) features that in the composite material of claim 19, an intermediate layer portion has the compression shear strength of 6 MPa or more, or the surface layer and the core layer both have the compression shear strength of 6 MPa or more, when compressive force is applied to the composite material in a direction parallel to the fiber extending direction of the long fibers of the surface layer so that a breaking surface can be formed in the intermediate layer portion, and wherein the composite material has the physical property that either the surface layer or the core layer is first broken when the compressive force is applied to the composite material in the direction parallel to the fiber extending direction of the long fibers of the surface layer so that the breaking surface can be formed in the intermediate layer portion. 
The composite material according to the present invention as set forth in claim 21 (hereinafter it is referred to as xe2x80x9cthe composite material of claim 21xe2x80x9d) features that in the composite material of claim 19 or 20, a resin-impregnated sheet-like material is interposed in the intermediate layer. 
The composite material according to the present invention as set forth in claim 22 (hereinafter it is referred to as xe2x80x9cthe composite material of claim 22xe2x80x9d) features that in the composite material of claims 1 through 7 and claims 9 through 21, polyurethane resin foam is used as the synthetic resin of the core layer and polyurethane resin foam is used as the synthetic resin of the surface layer. 
The composite material according to the present invention as set forth in claim 23 (hereinafter it is referred to as xe2x80x9cthe composite material of claim 23xe2x80x9d) features that in the composite material of any of claims 1 through 22, which has a total thickness of 100 mm or more and a ratio between a thickness of the core layer and a sum total of thickness of the surface layer covering the core layer in the thickness direction is within the range of 9/1 to 1/1. 
The composite material according to the present invention as set forth in claim 24 (hereinafter it is referred to as xe2x80x9cthe composite material of claim 24xe2x80x9d) features that in the composite material of claim 15 or 23, the core layer has at least two core layer forming composition layers (A) comprising filler and synthetic resin and at least one core layer forming composition layer (B) comprising thermosetting resin reinforced by long fibers interposed between two core layer forming compositions (A),(A) of the at least two core layer forming composition layers (A) and extending parallel in a longitudinal direction of the composite material, and a ratio between a sum total of thickness of the core layer forming composition layer (A) and a sum total of thickness of the core layer forming composition layer (B) is within the range of 95/5 to 50/50. 
The composite material according to the present invention as set forth in claim 25 (hereinafter it is referred to as xe2x80x9cthe composite material of claim 25xe2x80x9d) features that in the composite material of any of claims 1 through 24, the surface layer is laminated on the core layer to cover at least two surfaces of the corer layer with respect to a thickness direction thereof; the composite material has a total thickness of 100 mm or more with respect to a thickness direction thereof; a thickness of the surface layer on the side thereof on which a pulling force is exerted when the composite material is bent in the thickness direction is 5% or more to 25% or less of the total thickness; and the thickness of the surface layer on the side thereof on which a compressive force is exerted is 1.5% or more to 15% or less of the total thickness. 
The composite material according to the present invention as set forth in claim 26 (hereinafter it is referred to as xe2x80x9cthe composite material of claim 26xe2x80x9d) features that in the composite material of any of claims 1 through 25, the surface layer surrounds four surfaces of the core layer and constitutes 10 volume % or more to 65 volume % or less of the total of the composite material. 
A synthetic cross tie according to the present invention as set forth in claim 27 (hereinafter it is referred to as xe2x80x9cthe cross tie of claim 27xe2x80x9d) uses a composite material according to any of claims 1 through 26. 
In the following, the constitution of the composite materials of the respective Claims will be described in detail. 
While the synthetic resins which may be used for the core layer in the composite materials of claims 1 through 6 include thermosetting resins as in the composite material of claim 7 and thermoplastic resin as in the composite material of claim 8, the mixture of thermosetting resin and thermoplastic resin may be used. 
While no particular limitation is imposed on the thermosetting resins used for the core layer, the thermosetting resins which may be used include the resins which are in liquid form or powder form before reaction and are of foamable, including, for example, polyurethane resin, phenol resin, unsaturated polyester resin, diallyl phthalate resin, vinyl ester resin, epoxy resin, urea resin, melamine resin, polyimide resin, polyamide-imide resin, acrylic resin, natural rubber, and synthetic rubber.  These may be used in combination of two or more. 
While no particular limitation is imposed on the thermoplastic resins used for the core layer, the thermoplastic resins which may be used include, for example, polystyrene, syndiotactic polystyrene, high-density polyethylene, low-density polyethylene, linear low-density polyethylene, polypropylene, rigid polyvinyl chloride, acrylic resin, ABS resin, aliphatic polyamide (nylon) resin, polyethylene terephthalate, polybutylene terephthalate, polyoxymethylene, polycarbonate, polyarylate, polysulfone, polyether sulfone, polyether ether ketone, and polyphenylene sulfide, or copolymer thereof and blend thereof.  Also, these may be of foamable. 
Further, of these thermoplastic resins, crystalline resins have preferably a melting point of 80xc2x0 C. or more, or further preferably 120xc2x0 C. or more.  On the other hand, non-crystalline resins have preferably a glass transition point of 80xc2x0 C. or more, or further preferably 100xc2x0 C. or more.  With the melting point and the glass transition point lower than these temperatures, there is the possibility that the bending properties and the heat resisting properties may reduce. 
The filler having a coefficient.  of thermal expansion approximating to that of the long fibers for use in the surface material should preferably be used. 
While no particular limitation is imposed on the thermosetting resins.  used for the surface layer in the composite materials of claims 1 through 4, the thermosetting resins which may be used include the resins which are in liquid form or powder form before reaction and are of foamable, including, for example, polyurethane resin, phenol resin, unsaturated polyester resin, diallyl phthalate resin, vinyl ester resin, epoxy resin, urea resin, melamine resin, polyimide resin, polyamide-imide resin, and acrylic resin. 
The foam thermosetting resins used for the surface layer include heat decomposable foaming agents, such as azo compound and sodium bicarbonate, solvent foaming agents, such as fleon, carbon deoxide and pentane, and foam thermosetting resin liquids including those from which gas is formed as by-product in the reaction and curing.  For example, rigid or semi-rigid polyurethane foam, phenol foam, low-power polyester foam can be cited. 
Polyurethane foam, in particular, is preferably used in that it has a relatively high mechanical strength and can easily form closed cells, when foamed, and thus has excellent unabsorbent. 
While no particular limitation is imposed on the lightweight filler used in the surface layer in the composite material of claim 2, the lightweight fillers which may be used include, for example, powder/granular material, foam particle and hollow particle, such as glass hollow particle, silica balloon, fly ash balloon, shirasu balloon, porous glass, expanded shale, porous ceramics, perlite, pumice, vermiculite and synthetic resin.  Synthetic resins which may be used as the lightweight filler include the same thermosetting resin curing material and rosslinked rubber as those used for the surface layer, and further include crystalline thermoplastic resin having a higher melting point than a temperature at which the thermosetting resin is cured and non-crystalline thermoplastic resin having a glass transition point.  Those having closed cells are preferably used to reduce percentage of absorption of the composite material.  The lightweight filler may be surface-treated with a silane coupling agent and the like. 
In the composite material of claim 1 or 2, the surface layer has preferably a density of 0.3 g/cm3 or more to 1.5 g/cm3 or less.  The reason therefor is that a too small density of the surface layer causes the bending strength of the composite material to reduce, while on the other hand, a too large density of the surface layer causes the composite material to be easily cracked when nailed.  The preferable density range varies depending on the intended use of the composite material.  For example, for using the composite material to a railway sleeper, the surface layer has further preferably a density of 0.6 g/cm3 or more to 1.5 g/cm3 or less.  For using the composite material to loading platform material or flooring material of a floating bridge, a track and a boat, the surface layer has further preferably a density of 0.3 g/cm3 or more to 0.8 g/cm3 or less. 
In the composite material of claims 1 through 5, the long fiber is not limited to any particular configuration, as far as it has the capability as the reinforced fiber.  The long fibers which may be used include, for example, mono-filament, fibril (a feathered fiber) chemical cellulose and weaving yarn.  The materials thereof include organic materials, such as glass, carbon and synthetic resin.  Glass or carbon which produces a large reinforcing effect is of preferable.  These may be used singularly or in combination of two or more. 
While the percentage of the long fibers contained in the surface layer is not particularly limited, 5 volume % or more to 40 volume % or less is of preferable.  A less than 5 volume % of long fibers produce no reinforcing effects such as the bending strength.  On the other hand, an excess of 40 volume % of long fibers may produce a possible fracture running parallel to the fibers when the composite material is nailed. 
When the thermosetting resin of the surface layer is foamable material, the resin density is preferably 0.2 g/cm3 or more.  A less than 0.2 g/cm3 resin density provides undesirable reduction of the bending strength.  No particular upper limit is specified.  An upper limit of a resin density of the surface layer is substantially equal to that of the foam thermosetting resin that can substantially be produced. 
In the composite material of claim 3, the surface layer preferably contains 20 volume % or more to 50 volume % or less of lightweight filler, in order to satisfy the above-noted proportion of the long fibers and the resin density of the thermosetting resin of foamable material.  With a less than 20 volume % of lightweight filler, the surface layer is easily cracked when nailed.  On the other hand, with an excess of 50 volume % of lightweight filler, the lightweight fillers are not uniformly dispersed in the thermosetting resin, so that there is the possibility that the physical properties, such as the bending strength, may be reduced. 
In the composite material of claims 1 through 3, the bending modulus of the surface layer is preferably 6,000 MPa or more, as in the composite material of claim 4, further preferably 7,000 MPa or more, or still further preferably 8,000 MPa or more.  The reason is that a less than 6,000 MPa bending modulus can cause reduction of the bending modulus of the entire composite material, so there is the possibility that, for example, when used to railway sleepers, the composite materials may deflect largely to easily cause deviation of a rail track.  No particular upper limit of the bending modulus is specified.  An upper limit of the bending modulus of the surface layer is substantially equal to that of the surface layer that can substantially be produced. 
In the composite material of claims 1 through 3, the bending strength of the surface layer is preferably 100 MPa or more, as in the composite material of claim 4, or further preferably 120 MPa.  The reason is that with a less than 100 MPa bending strength, there is the possibility that when used to cross ties, the composite materials may easily reduce in long-term bending durability. 
The bending modulus and the bending strength are measured in accordance with the method prescribed by JIS Z 2101.  The bending load is applied to a test piece in a direction vertical to a longitudinal direction of the test piece with the long-fiber extending direction as the longitudinal direction. 
In the composite material of claims 5 through 9, the same resins as those used in the composite material of claims 1 through 4 can be used as the resin for use in the surface layer. 
In the composite material of claim 5, the average particle size of the filler except the fibers is limited to 0.5 mm or more.  The reason is that with a less than 0.5 mm average particle size of the filler, when the filler having a weight 0.7 times or more the product of volume of the core layer and bulk density of the filler is tried to be contained, sufficient dispersion is not achieved in the mixing process of the filler and the synthetic resin, so that the resin cannot adhere to the filler uniformly and thus there is the possibility that satisfactory physical properties, such as the bending strength, cannot be obtained.  The upper limit of the particle size of the filler is preferably made to be substantially smaller than the thickness of the core layer.  If the filler having a diameter that will be substantially larger than the thickness of the core layer is used, then there is the possibility that the nailing property varies so largely, depending on the places to be nailed, that the material cannot withstand continued use. 
In the present invention, the particle size can be obtained by sifting with a standard screen prescribed by JIS Z 8801.  Combination of basic dimensions of meshes of typical screens for use in the sifting is selected from:
4.00 mm, 2.80 mm, 2.00 mm, 1.40 mm, 1.00 mm, 850 xcexcm, 500 xcexcm, 300 xcexcm, 212 xcexcm, 106 xcexcm, and 75 xcexcm.  Additional screens with different dimensions of meshes may properly be added. 
The value of the particle size of the filler is expressed by the basic dimensions of meshes of the screens.  The average particle size of the filler is a value obtained by summing the products of volume ratios of the fillers remaining in the respective screens to the total fillers except the fibers and the values of particle sizes over all the screens. 
While no particular limitation is imposed on the filler in the composite material of the present invention, the fillers which may be used include, for example, amorphous powders, such as powder/granular rock, glass powder, quartz powder, calcium silicate, ground cement concrete, river sand, sea sand and silica sand, inorganic short fiber powders, such as wollastonite, inorganic powder/granular material having cells, such as expanded shale, pumice and glass foam, pulverized resins, such as pulverized polyvinyl chloride, pulverized fiber-reinforced resin and pulverized fiber-reinforced foam polyurethane, inorganic powder having a relatively small particle size, such as calcium carbonate, fly ash, mica, talc, clay, alumina, vermiculite and sludge dry powder/granular material, and granulated materials thereof previously bonded by resin.  These may be used singularly or in combination of two or more. 
The fibrous fillers which may be used include inorganic fibers, such as glass fiber, carbon fiber and boron fiber, and organic short fibers, such as vinylon fiber, polyester fiber, aliphatic polyamide fiber and aromatic polyamide. 
Also, the above-noted inorganic fillers as surface-treated by silane coupling agent may be used.  When the polyurethane resin is used for the resin in the surface layer, silane coupling agent having a fimctional group which reacts with an isocyanate group, such as a mercapto group, an amino group and an imino group, is preferably used. 
In the composite material of claim 6, the filler used has two or more peak areas that constitute 8 volume % or more on a particle size distribution curve plotting particle size in abscissa and a volume ratio of filler per particle size to all fillers in ordinate and also has the size distribution that most frequent particle size values in the smaller peak area of 8 volume % or more is 0.7 or less of most frequent particle size values in the larger peak area of 8 volume % or more next to the smaller peak area.  The reason is that the mixture of fillers of different particle sizes can facilitate the mixture and impregnation into the synthetic resin, especially the thermosetting resin, and further can facilitate increase of amount of filler added, to significantly enhance the laminating effect of the core layer and the surface layer.  In addition to this, the size distribution peaks at two or more points, and as such can allow the resin to moderately filled in between the fillers.  As a result of this, the composite material, when nailed, is resiliently compressed without the core layer being destroyed and the resilience provides improved nail holding ability and durability against the repeatedly applied unnailing force. 
For the composite material of claim 5, the size distribution curve in the composite material of claim 6 is obtained by plotting a volume ratio of the filler per particle size to all fillers except the fibers obtained in the same manner with respect to the each particle size.  The volume ratio shows a volume ratio between the fillers screened with neighboring screens of basic dimensions. 
The phrase of xe2x80x9cthe larger peak area of 8 volume % or more next to the smaller peak areaxe2x80x9d is intended to mean that if a peak area of less than 8 volume % exists between the large and small peak areas, such a peak area of less than 8 volume % is not taken as the larger peak area of 8 volume % or more next to the small peak area. 
The volume % of the peak area is obtained as a percentage of an area surrounded by the distribution curve extending between its boundaries intersecting the abscissa axis and the abscissa, in the case of its boundaries not intersecting the abscissa, an area surrounded by the distribution curve extending between minimum value points and the abscissa (in the case of using the minimum values, an area surrounded by the distribution curve, the abscissa, and perpendicular lines dropped from the minimum value points to the abscissa) to a total area bounded.  by the whole distribution curve and the abscissa, as shown in FIG.  18. 
If the peak area has a tabletop peak extending parallel to the abscissa, a center of the parallel extending part is taken as the most frequent particle size value. 
In the composite material of claim 6, no particular limitation is imposed on the fillers providing larger values of the most frequent particle sizes.  The fillers include, for example, amorphous granular materials, such as granular rock, granular glass, calcium silicate, ground cement concrete, river sand, sea sand and silica sand, inorganic short fiber powders, such as wollastonite, inorganic powder/granular material having cells, such as expanded shale, pumice and glass foam, inorganic powder/granular materials having a relatively small particle size, such as pulverized vinyl chloride, pulverized fiber-reinforced resin, calcium carbonate and fly ash, and sludge dry powder/granular material, and granulated materials thereof previously bonded by resin.  On the other hand, no particular limitation is imposed on the fillers providing smaller values of the most frequent particle sizes.  The preferable fillers include, for example, powdered silica sand, quartz, mica, talc, clay and alumina and additionally include those belonging to sludge dry powder/granular material and inorganic powder/granular material.  These may be used singularly or in combination of two or more.  Further, the above-noted inorganic fillers as surface-treated by silane coupling agent may be used.  When the polyurethane resin is used for the resin in the surface layer, silane coupling agent having an active hydrogen which reacts with an isocyanate group, such as a mercapto group, an amino group and an imino group, is preferably used. 
In the composite material of claim 9, the polyol equivalent means a value calculated from the following equation (1).  The measuring method is as follows.  After the resin is hydrolyzed, amine originating from isocyanate component is removed through ion exchange resin, alkaline cleaning and the like so that polyol component can be recovered, and then the hydroxyl value of the recovered polyol component is measured. 
Polyol equivalent=Molecular weight of KOHxc3x971,000/Hydroxyl value of Polyol (mgKOH/g)xe2x80x83xe2x80x83(1)
When a foam urethane resin having the polyol equivalent of 230 or more to 1500 or less is used as the thermosetting resin forming the core layer, the density of 0.25 g/cm3 or more to 0.6 g/cm3 is of preferable. 
On the other hand, when a foam urethane resin having the density of 0.3 g/cm3 or more and the polyol equivalent of 1,500 or less is used as the thermosetting resin forming the core layer, the polyol equivalent of 110 or more to 1,200 or less is of preferable. 
With the polyol equivalent of less than 110, the resin in the core layer may become too rigid, so that there is the possibility that flexibility of the composite material itself may become insufficient.  On the other hand, with the polyol equivalent of more than 1,500, the resin in the core layer may become too soft, so that there is the possibility that the nailing performance of the composite material may become insufficient. 
The density of polyurethane resin can be adjusted by adjusting a ratio between the fillers and the resin and foaming the polyurethane resin in between the fillers.  The foaming is performed by use of foaming agent.  The foaming agent may be selected properly for the resin used. 
The foaming agents which may be used include, for example, physical foaming agents (volatile foaming agents), such as fleon, carbon deoxide and pentane, decomposition-type foaming agents, such as azo compound and sodium hydrogen carbonate, and reaction-type foaming agents, such as carbon dioxide produced by reaction of isocyanate and water. 
For polyurethane, carbon dioxide produced by reaction of isocyanate and water should preferably be used because fleon can deplete the ozone layer.  Also, the foaming agent should be previously mixed with the resin. 
In the composite material of claim 10, the flexibility can be calculated from the following equation (2), using the deflection at a center of span=xcex94y, which corresponds to a permissible capacity at rated load center distance in the evaluation of the bending strength.  The bending strength is measured in accordance with the method prescribed by JIS Z 2101. 
Deflection (%)=6xc3x97(thickness of a test piece)xc3x97xcex94y/(span)2xc3x97100xe2x80x83xe2x80x83(2)
The measurement may be made of the physical properties by cutting out the core layer from the composite material or by producing the core layer having the same construction. 
The composite material is bent in the direction vertical to the longitudinal direction of the test piece.  The relation between the deflection and the bending stress is shown in a graph plotting the deflection in abscissa and the bending stress in ordinate. 
In the composite material of claims 10 through 13, the singular point means a non-differentiable bent back point, a point of inflection at which a curved line changes from concave to convex or conversely, and the like. 
In the composite materials of claims 10 and 11, the core layer has deflection of 0.8% or less, or preferably 0.7% or less, at the singular point, as in the composite material of claim 12.  The reason is that with the deflection of the core layer of more than 0.8% at the singular point, the composite material, when bent or compressed, may be broken before the fillers are brought into contact with each other, so that there is the possibility of providing reduced strength. 
In the composite materials of claims 10 through 12, it is preferable that the core layer has the bending modulus of 800 MPa or more, or preferably 950 MPa, when further deflected from deflection at the singular point, as in the composite material of claim 13.  The reason is that the bending modulus of less than 800 MPa may reduce the effect that the fillers in the core layer are brought into contact to each other when the composite material is bent or compressed, so that there is the possibility of providing reduced strength. 
In the case of the singular point being the bent back point, the bending modulus is the slope of the tangent line found from the large deflection direction. 
In the composite materials of claims 10 through 13, either thermosetting resin which can be set under heat or at room temperature or thermoplastic resin which can be plasticized under heat may be used as the synthetic resin used for the core layer. 
The thermosetting resins which may be used include the resins which are in liquid form or powder form before reaction and are of foamable, including polyurethane resin, phenol resin, unsaturated polyester, diallyl phthalate resin, vinyl ester resin, epoxy resin, urea resin, melamine resin, polyimide resin, polyamide-imide resin, acrylic resin, natural rubber, and synthetic rubber.  These may be used in combination of two or more. 
On the other hand, the thermoplastic resins which may be used include polystyrene, syndiotactic polystyrene, high-density polyethylene, low-density polyethylene, linear low-density polyethylene, polypropylene, rigid polyvinyl chloride, acrylic resin, ABS resin, aliphatic polyamide resin, polyethylene terephthalate, polybutylene terephthalate, polyoxymethylene, polycarbonate, polyarylate, polysulfone, polyether sulfone, polyether ether ketone, and polyphenylene sulfide, or copolymer thereof and blend thereof.  Also, these may be of foamable.  Further, the thermosetting resin and thermoplastic resin may be used in combination. 
In the composite materials of claims 10 through 13, no particular limitation is imposed on the fillers.  The fillers which may preferably used include, for example, amorphous granular materials, such as powder/granular rock, granular glass, calcium silicate, ground cement concrete, river sand, sea sand and silica sand, inorganic short fiber powders, such as wollastonite, inorganic powder/granular material having cells, such as expanded shale, pumice and glass foam, inorganic powder/granular materials having a relatively small particle size, such as pulverized vinyl chloride, pulverized fiber-reinforced resin, pulverized fiber-reinforced rigid foam urethane, calcium carbonate and fly ash and hollow particles thereof, sludge dry powder/granular material, mica, talc, clay, alumina, vermiculite, and glass short fibers.  In addition, inorganic fibers, such as carbon fiber and boron fiber, organic short fibers, such as vinylon fiber, polyester fiber, aliphatic polyamide fiber and aromatic polyamide, and granulated materials thereof previously bonded by resin can be cited as the fillers.  These may be used in combination of two or more.  Further, the above-noted inorganic fillers as surface-treated by silane coupling agent may be used.  When the polyurethane resin is used for the resin in the surface layer, silane coupling agent having an active hydrogen which reacts with an isocyanate group, such as a mercapto group, an amino group and an imino group, is preferably used. 
The sludge dry powder/granular materials include high-temperature dry solids content produced from a sludge treatment facility.  The pulverized fiber-reinforced resins include pulverized fiber-reinforced plastic (FRP) and pulverized fiber-reinforced rigid foam urethane.  Further, fibrous ones include needle-like or shavings-like chips produced by scraping the fiber reinforced resin having unidirectionally aligned fibers in the fiber extending direction. 
In the composite materials of claims 10 through 13, it is preferable that the core layer contains the filler having a weight 0.7 times or more the product (weight) of volume of the core layer and bulk density of the filler. 
With the amount of filer of less than 0.7 times the product (weight) of volume of the core layer and bulk density of the filler, the thermosetting resin layer is allowed to exist between the fillers and accordingly the proportion of the fillers being not brought into direct contact with each other is increased.  This makes it difficult to present the singular point and also may cause the compression strength and the nailing performance to be insufficient. 
In the composite materials of claims 10 through 13, it is preferable that the average density of the core layer is in the same range as in the composite material of claim 1. 
In the composite material of claim 10, either short fibers or long fibers may be used as the reinforced fiber used for the surface layer, though the long fibers are of preferable as in the composite material of claim 11.  As far as the long fibers can reinforce the surface layer at least in the longitudinal direction thereof, any of mono-filament, fibril synthetics and weaving yarn, and unidirectional reinforcing one, such as roving, bidirectional reinforcing one, such as a mat, and tridirectional reinforcing one, such as sewed mats, may selectively be used.  These may be used singularly or in combination of two or more.  The same reinforced fibers as those in the composite materials of claims 10 and 11 may be used for the reinforced fibers of the surface layers of the composite materials of claims 12 and 13. 
Either thermosetting resin which can be set under heat or at room temperature or thermoplastic resin which can be plasticized under heat may be used as the synthetic resin used for the surface layer. 
The thermosetting resins which may be used include the resins which are in liquid form or powder form before reaction and are of foamable, including polyurethane resin, phenol resin, unsaturated polyester, diallyl phthalate resin, vinyl ester resin, epoxy resin, urea resin, melamine resin, polyimide resin, polyamide-imide resin, acrylic resin, natural rubber, and synthetic rubber.  These may be used in combination of two or more. 
On the other hand, the thermoplastic resins which may be used include polystyrene, syndiotactic polystyrene, high-density polyethylene, low-density polyethylene, linear low-density polyethylene, polypropylene, rigid polyvinyl chloride, acrylic resin, ABS resin, aliphatic polyamide resin, polyethylene terephthalate, polybutylene terephthalate, polyoxymethylene, polycarbonate, polyarylate, polysulfone, polyether sulfone, polyether ether ketone, and polyphenylene sulfide, or copolymer thereof and blend thereof.  Also, these may be of foamable. 
Further, of these thermoplastic resins, crystalline resins have preferably a melting point of 80xc2x0 C. or more, or further preferably 120xc2x0 C. or more.  On the other hand, non-crystalline resins have preferably a glass transition point of 80xc2x0 C. or more, or further preferably 100xc2x0 C. or more.  With the melting point and the glass transition point lower than these temperatures, the bending properties and the heat resisting properties may reduce. 
Further, the thermosetting resin and the thermoplastic resin may be used in combination. 
In the composite materials of claims 10 through 13, the foaming agents used may selectively be used in accordance with the types of resins.  For example, physical foaming agents, such as fleon, carbon deoxide and pentane, decomposition-type foaming agents, such as azo compound and sodium hydrogen carbonate, and reaction-type foaming agents, such as carbon dioxide produced by reaction of isocyanate and water can be cited. 
For example, when polyurethane is used as the resin, carbon dioxide produced by reaction of isocyanate and water should preferably be used because fleon can deplete the ozone layer.  These may be used singularly or in combination of two or more.  Also, the foaming agent should preferably be previously mixed with the resin. 
In the composite materials of claims 10 through 13, it is preferable that the density of the surface layer is in the same range as in the composite material of claim 1, though no particular limitation is imposed thereon. 
In the composite materials of claims 14 and 15, the same surface layer as those in the composite materials of claims 1 through 13 may be used. 
When the composite material of the present invention is used for synthetic cross ties, the long fibers extending parallel to the longitudinal direction of the surface layer are preferably used. 
In the composite materials of claims 14 and 15, it is preferable that the core layer forming material layers on the compression side thereof on which they are compressed when bent in the thickness direction are so constituted that they contain the fillers having a weight 0.7 times or more the product of volume of the core layer forming material layers and bulk density of the fillers so that the fillers in the core layer forming material layers, when compressed, can be brought into contact with each other to provide high elasticity and high strength, while also the core layer forming material layers on the tension side are so constituted that they are formed of the foam polyurethane resin having the polyol equivalent of 230 or more to 1,500 or less or the foam polyurethane resin having the density of 0.3 g/cm3 or more and the polyol equivalent of 1,500 or less so that they can follow the expansion resulting from the deflection. 
Also, it is preferable that the core layer forming material layers on the tension side thereof contain elastic members, such as rubber chips or springs, therein.  The elastic members contained can provide vibration absorption to the composite material, while maintaining the high bending modulus by the surface layer and the core layer forming material layers on the compression side. 
In the composite material of claim 15, the same as those of the surface layers of the composite materials of claims 1 through 13 may be used as the core layer forming material layer (hereinafter, it is referred to as xe2x80x9cthe intermediate fiber reinforced layerxe2x80x9d) which is interposed between the core layer forming layer comprising filler and synthetic resin (hereinafter it is referred to as xe2x80x9cthe filler containing layerxe2x80x9d) and the filler containing layer and is formed of foam thermosetting resin reinforced by the long fibers extending parallel in the longitudinal direction. 
In the composite materials of claims 14 and 15, it is necessary that the core layer forming material layers bordering on each other are bonded together.  If those layers are not bonded adequately, there is the possibility that the peel may be caused in the interface therebetween to cause the destruction of the entire composite material. 
While no particular limitation is imposed on the bonding method, the bonding methods include, for example, the method of simultaneously molding the mutually bordering core layer forming material layers, the method of adhesive bonding the molded core layer material layers to each other by use of epoxy adhesive or urethane adhesive and the method of molding additional core layer forming material layer on the molded core layer forming material layer. 
In the composite material of claim 16, it is necessary that the core layer and the surface layer have the relation that satisfies the equations of CSaxe2x89xa7xc2xdxc3x97CSb, Ea less than Eb, and ESaxe2x89xa7xc2xdxc3x97ESb (where CSa represents yield strain in compression of the core layer; CSb represents yield strain in compression load of the surface layer; Ea represents a tension elasticity modulus of the core layer; Eb represents a tension elasticity modulus of the surface layer; ESa represents tensile yield strain of the core layer; and ESb represents yield strain in tension of the surface layer).  The reason is that if that relation is not satisfied, the performance (bending) of the fiber reinforced surface layer is not brought out, so that the composite material is not allowed to have the performance (bending) equivalent to or more than the material comprising only the thermosetting resin reinforced by the long fibers extending parallel in the longitudinal direction as in the surface layer or the thermosetting resin including the lightweight fillers reinforced by the long fibers extending parallel in the longitudinal direction.  Specifically, as shown in FIG.  11, even when the core layer is on the side (upper side in FIG.  11) on which compression is generated by a load applied to cause the longitudinal bending along the fiber extending direction of the surface layer, if the relation of CSaxe2x89xa7xc2xdxc3x97CSb is satisfied, the compression strength is improved.  Also, even when the surface layer is on the side (lower side in FIG.  11) on which tension force is generated, if the relation of Ea less than Eb and ESaxe2x89xa7xc2xdxc3x97ESb is satisfied, the improvement of the bending strength can be expected. 
In the composite material of claim 16, it is preferable that it follows that 0.005xe2x89xa6CSa, 50 MPaxe2x89xa6Ea, 0.005xe2x89xa6ESa, 0.01xe2x89xa6CSb, 5,000 MPaxe2x89xa6Ebxe2x89xa618,000 MPa, and 0.01xe2x89xa6ESb, as in the composite material of claim 17.  The reason is as follows. 
In the core layer, the yield strain in compression CSa of 0.005 or more can provide reinforced compression property of the surface layer.  The tension elasticity modulus Ea of 50 or more and the tensile yield strain ESa of 0.005 or more can produce the effect of following the deflection of the surface layer, to produce the composite material having performance (bending) equivalent to that of a single material of the surface layer and durability against a repeated load equivalent to that of the single material of the surface layer. 
In the composite material of claims 16 and 17, it is preferable that the compression modulus of elasticity Ca of the core layer in the longitudinal direction is 300 MPa or more to 12,000 MPa or less and the compression modulus of elasticity Cb of the surface layer is 2,000 MPa or more to 8,000 MPa or less, in that Ea and Eb are well balanced to further improve the durability. 
Further, the composite material having the compression modulus of elasticity Cb of the surface layer of 2,000 MPa or more to 8,000 MPa or less, the Eb of 5,000 MPa or more to 18,000 MPa or less and the ESb of 0.01 or more can provide further sufficient strength as the structural member resembling the woods. 
In the composite materials of claims 16 and 17, no particular limitation is imposed on the foam thermosetting resin forming the surface layer.  For example, rigid or semi-rigid polyurethane foam, phenol foam, low-power foam polyester foam can be cited as those foam thermosetting resins, including the heat decomposable foaming agents, the solvent foaming agents, such as fleon, and the foam thermosetting resin liquids including those from which gas is formed as by-product in the reaction and curing. 
Polyurethane foam, in particular, is preferably used in that it has a relatively high mechanical strength and can easily form closed cells, when foamed, and thus has excellent unabsorbent. 
On the other hand, while no particular limitation is imposed on the elastic synthetic resin forming the surface layer, the elastic synthetic resins include a resin belonging to the category of rubber, such as elastomer, flexible PVC and plastic polyvinyl alcohol.  Resins having comparatively low elasticity, such as Polyolefin resin and high-impact ABS, are also included, as far as they have a specified elasticity. 
In the composite material of claims 16 and 17, the long fiber used in the surface layer is not limited to any particular configuration, as far as it has the capability as the reinforced fiber.  The same long fibers as those used for the surface layers of claims 1 through 15 may be used. 
While the percentage of the long fibers contained in the surface layer is not particularly limited, 5 volume % or more to 40 volume % or less is of preferable.  A less than 5 volume % of long fibers produce no reinforcing effects such as the bending strength.  On the other hand, an excess of 40 volume % of long fibers may produce a possible fracture running parallel to the fibers when the composite material is nailed. 
In the composite materials of claims 16 and 17, no particular limitation is imposed on the core layer.  While the core layer is in general formed by the mixture of the fillers in the synthetic resin, it may be formed by two or more filler containing layers being laminated as in claims 14 and 15 or by the intermediate finer-reinforced layer being interposed between the filler containing layer and the filler containing layer. 
In the composite materials of claims 16 and 17, while no particular limitation is imposed on the synthetic resin which is a main component of the core layer or of the filler containing layer, the thermosetting resin or the thermoplastic resin may be used as that synthetic resin. 
The thermosetting resins which may be used include the resins which are in liquid form or powder form before reaction and are of foamable, including, for example, polyurethane resin, phenol resin, unsaturated polyester resin, diallyl phthalate resin, vinyl ester resin, epoxy resin, urea resin, melamine resin, polyimide resin, polyamide-imide resin, acrylic resin, natural rubber, and synthetic rubber.  These may be used in combination of two or more. 
On the other hand, the thermoplastic resins which may be used include polystyrene, syndiotactic polystyrene, high-density polyethylene, low-density polyethylene, linear low-density polyethylene, polypropylene, rigid polyvinyl chloride, acrylic resin, ABS resin, aliphatic polyamide resin, polyethylene terephthalate, polybutylene terephthalate, polyoxymethylene, polycarbonate, polyarylate, polysulfone, polyether sulfone, polyether ether ketone, and polyphenylene sulfide, or copolymer thereof and blend thereof.  Also, these may be of foamable. 
Further, of these thermoplastic resins, crystalline resins have preferably a melting point of 80xc2x0 C. or more, or further preferably 120xc2x0 C. or more.  On the other hand, non-crystalline resins have preferably a glass transition point of 80xc2x0 C. or more, or further preferably 100xc2x0 C. or more.  With the melting point and the glass transition point lower than these temperatures, there is the possibility that the bending properties and the heat resisting properties may reduce. 
Further, the thermosetting resin and the thermoplastic resin may be used in combination. 
For use of the foamable synthetic resin, those having the closed cells are preferable to prevent water absorbing property. 
The same fillers as those used for the core layer in the composite materials of claims 1 through 15 may be used. 
The same as the one used in the surface layer may be used as the intermediate fiber-reinforced layer. 
In the composite materials of claims 1 through 17, it is preferable that the core layer has a compression shear strength of 5 MPa or more, as in the composite material of claim 18.  The reason is as follows. 
If the compression shear strength DBa of the core layer is less than 5 MPa or more, then the composite material cannot be allowed to have the bending strength equivalent to the material comprising only the thermosetting resin reinforced by the long fibers extending parallel in the longitudinal direction as in the surface layer or the thermosetting resin including the lightweight fillers reinforced by the long fibers extending parallel in the longitudinal direction.  As a result, there is the possibility that the shear failure may be caused by the bending. 
To obtain the compression shear strength DBa of the core layer of 5 MPa or more, it is preferable to treat the fillers with silane coupling agent or add the short fibers, pulverized fiber reinforced plastics, pulverized fiber reinforced rigid foam urethane, or fibrous ones including needle-like or shavings-like chips produced by scraping the fiber reinforced resin having unidirectionally aligned fibers in the fiber extending direction. 
When the inorganic filler is used, the specific gravity of 0.5 or more is of preferable.  Preferably, the inorganic filler having the specific gravity of 0.5-1.5 in all fillers is 50 volume % or less of the total core layer. 
Further, in the composite materials of claims 16 through 18, in the case of possible occurrence of friction shearing in the surface, it is preferable that at least two longitudinal surfaces of the core layer are surrounded by the surface layer and/or the volume of the core layer is 50% or more to less than 65% of the total volume of the composite material.  The composite material thus constituted can provide an improved bending strength, as compared with the one comprising the single material of the surface layer, and is of advantageous in reduction of the material cost. 
In the composite materials of claims 1 through 18, no particular limitation is imposed on the production method.  The composite material may be produced by either a batch process or a continuous process. 
For reference""s sake, an example of the batch production process is the process disclosed, for example, by Japanese Laid-open Patent Publication No.  Hei 5-138797, in which after material preformed to form the core layer and material preformed to form the surface layer are preformed and then are set in casting molds, a mixture of the long fibers and the thermosetting resin, or a mixture of the filler and the thermosetting resin, or molding material thereof, which is to form the surface layer or the core layer, is filled in a casting mold before the preformed material is cured, and then the thermosetting resin is set by heating. 
On the other hand, an example of the continuous process is as follows.  A number of long fibers to be the reinforced fibers are aligned parallel with predetermined interval while they are traveled in one direction.  Then, a foam thermosetting resin liquid is sprayed from over the group of long fibers as aligned parallel on the travelling way.  Thereafter, the foam thermosetting resin liquid thus sprayed is impregnated in between the fibers forming the respective long fibers. 
Then, an extrusion shaping die is placed to confront a center part of the group of long fibers impregnated with the foam thermosetting resin liquid, and the mixture of the filler and thermosetting resin to form the core layer is shaped to enclose the core layer by the group of long fibers while it is continuously extruded from the extrusion shaping die.  Thereafter, they are continuously fed into a cylindrical molding passage to thermally set the foam thermosetting resin liquid in the molding passage, so as to form the core layer and the surface layer simultaneously. 
As an alternative method thereto, the following method may by taken.  The mixture of the filler and foam thermosetting resin to form the core layer is fed in between the groups of two-tiered long fibers impregnated with the foam thermosetting resin liquid and then is pressed by endless belts and the like to be shaped into a specified section form enclosed by the group of long fibers.  Thereafter, the layer thus shaped is continuously fed in the uncured condition or after foamed and thermally set in the cylindrical molding passage.  It is then continuously fed into the cylindrical molding passage as it stands or after its surface is ground, for example, by sanding, so that the foam thermosetting resin liquid is foamed and thermally set in the molding passage, so as to form the core layer and the surface layer simultaneously or sequentially. 
In the composite material of claim 19, the same synthetic resin, foam synthetic resin, filler and long fiber as those used in claims 1 through 18 may be used for the core layer and the surface layer. 
In the composite material in claim 19, no particular limitation is imposed on the non-foam thermosetting resin and low-power foam resin used for the intermediate layer, as far as they have the adhesion properties for allowing the surface layer and the core layer to adhere to each other.  For example, polyurethane resin, epoxy resin, phenol resin, unsaturated polyester resin, urea resin, melamine resin, polyimide resin, polyamide-imide resin, acrylic resin, natural rubber, and synthetic rubber can be cited as the examples.  If necessary, catalyst, foam stabilizer, foam assistant, filler, reinforcing short fiber, coloring agent, ultraviolet absorbent, antioxidant, crosslinking agent, stabilization agent, plasticizer, fire retardant, etc.  may be added (always excepting foaming agent).  For reference""s purpose, in the case where polyurethane resins are used for the surface layer and the core layer, the same polyurethane resin should preferably be used for obtaining high adhesion property. 
In use of the polyurethane resin, even if no foaming agent such as water is added, the polyurethane resin reacts with moisture in the air or moisture from the surface layer or core layer to form some foaming, but such degree of foaming is of negligible. 
Further, the intermediate layer is formed between the core layer and the surface layer in order to integrate the core layer and the surface layer.  No particular limitation is imposed on the thickness of the intermediate layer.  If improvement is desired of the physical properties, such as the bending elasticity and the unnailing strength, then the non-foam thermosetting resin or low density foam resin of high elasticity modulus may be formed on the core layer to a larger thickness. 
In the composite material of claim 20, the intermediate layer provided between the surface layer and the core layer is limited to 6.0 MPa or more in the compression shear strength when compressive force is applied to the composite material in a direction parallel to the fiber extending direction of the long fibers of the surface layer.  Preferably, the intermediate layer has the compression shear strength of 7.0 MPa or more, or further preferably 7.5 MPa or more.  The compression shear strength can be measured in accordance with the shearing test method prescribed by JIS Z 2101. 
The reason why the shear strength of the intermediate layer with respect to the fiber extending direction of the long fibers of the surface layer is limited to 6.0 MPa or more is as follows.  With the shear strength of the intermediate layer of less than 6 MPa, the durability against the repeated bending is reduced, which tends to destruction in the intermediate layer in the form of destruction.  The destruction of the intermediate layer in the form of destruction diffuses progressively over the destruction surface since the point of time of destruction, and the reduction of strength makes rapid progress.  As a result of this, this composite material can no longer be used for e.g. structural material and cross ties which require high elasticity, bending strength and durability against the repeated fatigue. 
No particular limitation is imposed on the method of providing the shear strength of the intermediate layer of 6 MPa or more.  For example, the method of interposing a high-strength and high-elasticity resin, such as epoxy resin, between the surface layer and the core layer; the method of making the resin density of the intermediate layer 1.1 times or more of the resin density of the surface layer and core layer when the shear strength of the surface layer with respect to the fiber extending direction of the long fibers and the shear strength of the core layer are 6 MPa or more; and the method of arranging a resin impregnated sheet-like material in the intermediate layer, as in the composite material of claim 21, can be cited as the examples. 
The methods of making the density of the resin between the surface layer and the core layer higher than the density of resin of the surface layer and core layer include, for example, the method of applying epoxy resin or foam urethane resin or non-foam resin to the surface of the core layer or the intermediate layer when the multi-layered material is produced. 
In the composite material of claim 21, the resin impregnated sheet-like material is the one that is impregnated with non-foam thermosetting resin or low-power foam resin and is used for the convenience sake to provide the intermediate layer between the core layer and the surface layer.  No particular limitation is imposed on the resin impregnated sheet-like material, as far as it can be impregnated with non-foam thermosetting resin or low-power foam resin.  For enhancing high strength of the intermediate layer in itself, a high strength sheet-like material may be used. 
No particular limitation is imposed on the resin impregnated sheet-like material, as far as it can be impregnated with non-foam thermosetting resin liquid or low-density foam resin liquid.  The resin impregnated sheet-like materials which may be used include, for example, non-woven fabric comprising inorganic glass fibers or synthetic resin fibers (e.g. polyester non-woven fabric (SUPAN BONDO E 1050 available from ASAHI CHEMICAL INDUSTRIAL CO.,  LTD.  and vinylon non-woven fabric (e.g. BINIRON SUPAN REESU available from KURARAY CO.,  LTD.)  and woven fabric.  In addition, a porous sheet, having a number of holes, of a synthetic resin sheet, paper and metal fiber cloth may be used. 
When the shear strength of the core layer is 6 MPa or the shear strength of the surface layer is 6 MPa with respect to the fiber extending direction, the resin impregnated sheet-like material can allow the intermediate layer to be 6 MPa or more in the compression shear strength with respect to the direction parallel to the fiber extending direction by the density of the resin between the surface layer and the resin impregnated sheet-like material and the density of the resin between the core layer and the resin impregnated sheet-like material being made to be 1.1 times or more of the density of the resin of the core layer and surface layer. 
When polyurethane resin is used as the resin of the core layer and surface layer and the intermediate layer is interposed between the core layer and the surface layer, the rein impregnated sheet-like material, of which raw material has a chemical/physical affinity for the resin of the core layer and surface layer, such as vinylon fiber or glass fiber subjected to the silane coupling treatment, and has excellent adhesion properties, can allow the density of the resin between the surface layer and the sheet-like material and the density of the resin between the core layer and the sheet-like material to be 1.05 times or more of the density of resin of the core layer and surface layer, for example when the shear strength of the core layer is 6 MPa or the shear strength of the surface layer is 6 MPa with respect to the fiber extending direction. 
In the composite materials of claims 1 through 7 and claims 9 through 21, it is preferable that the foam polyurethane resin is used as the synthetic resin of the core layer and the synthetic resin of the surface layer, as in the composite material of claim 22. 
The reason why the foam polyurethane resin is used for the core layer is that it has a relatively high mechanical strength and is capable to form closed cells easily when foamed, and excellent unabsorbent. 
The foam polyurethane resins which may widely be used include known foam polyurethane resins obtained by the reaction with polyol and polyisocyanate. 
The polyols, having at least two hydroxyl groups at the molecular end thereof, include, for example, polyether polyol, such as polypropylene oxide, polyethylene oxide and polytetramethylene glycol, and copolymers thereof, polyester polyols, such as polycondensate of aliphatic dicarboxylic acid, such as adipic acid, and glycol having not more than 12 carbons, such as ethylene glycol, propylene glycol, butylene glycol and hexamethylene glycol, polyester polyol which is polycondensate of hydroxycarboxylic acid, such as poly xcex5-caprolactone, and copolymer thereof, and polymer polyols which are graft copolymers of the polyols and polymer of monomer having vinyl group.  These may be used singularly or in combination of two or more.  The polyisocyanates which may be used have at least two isocyanate groups and include, for example, hydrogenated materials of 4,4xe2x80x2-methylene-diphenyl-diisocyanate, 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, isophorone diisocyanate, hexamethylene diisocyanate, and 4,4xe2x80x2-methylene-diphenyl-diisocyanate, apocytes thereof, and apocytes of isomers thereof.  These may be used singularly or in combination of two or more.  In terms of safety, reactivity and convenience in handling, the mixture of 4,4xe2x80x2-methylene-diphenyl-diisocyanate and apocyte of isomer thereof (hereinafter it is referred to as xe2x80x9cthe polymeric MDIxe2x80x9d) is of preferable. 
Examples of foaming agents used in the reaction are heat decomposable foaming agents, pneumatogen, such as fleon, and water.  Further, by-products, such as decomposed gas, produced in the reaction of the heat decomposable foaming agents may be used.  Since fleon can deplete the ozone layer, carbon dioxide produced by reaction of isocyanate and water should preferably be used.  Also, the foaming agent should be previously mixed with the resin. 
If necessary, catalyst, foam stabilizer, foam assistant, filler, reinforcing short fiber, coloring agent, ultraviolet absorbent, antioxidant, crosslinking agent, stabilization agent, plasticizer, fire retardant, etc.  may be added to the foam urethane resin. 
While no particular limitation is imposed on the catalyst, for example organotin catalyst such as dibutyltin dilaurate, amine catalyst, and temperature sensitive catalyst may be used. 
The composite materials of claims 19 through 22 may be produced by either a batch process or a continuous process. 
Preferably, the curing timing of the respective layers should be in most possible exact with each other.  This seems to be because this can provide increased bonding force to contribute to improvement of interfacial strength if chemical bonds are formed between respective layers. 
For reference""s sake, reference is given to the batch production.  For example, material preformed to form the core layer and material preformed to form the surface layer are preformed, first, and then are set in casting molds.  In this process, the resin impregnated sheet-like material in which non-foam thermosetting resin was impregnated in advance is rested on a surface of the thus-set preformed material on the side thereof on which the core layer and the surface layer are laminated.  Before the preformed material is cured, the mixture of long fiber and foam synthetic resin which is to form the surface layer, or the mixture of filler and synthetic resin which is to form the core layer, or an additional preformed material which is to form the surface layer or the core layer, is filled in a casting mold and then the synthetic resin of the core layer and the surface layer and the non-foam thermosetting resin is cured by heating. 
Referring now to the continuous process, for example, a number of long fibers which are to be the reinforced fibers are aligned parallel with predetermined interval while they are traveled in one direction in two vertical levels.  Then, a foam polyurethane resin liquid is sprayed from over the groups of long fibers which were aligned parallel in two vertical levels on the travelling way, so that the sprayed foam polyurethane resin liquid is impregnated in between the fibers forming the respective long fibers. 
Further, in the state in which the resin impregnated sheet-like material in which non-foam thermosetting resin liquid or low-power foam resin liquid was impregnated in advance is arranged between the groups of two-tiered long fibers impregnated with the foam thermosetting resin liquid, the mixture of the filler and foam thermosetting resin to form the core layer is fed into and shaped by endless belts and the like, so that they are shaped into a specified section form enclosed by the group of long fibers and then are continuously fed in the as-uncured condition.  Thereafter, they are continuously fed into a cylindrical molding passage so that the foam thermosetting resin liquid can be foamed and thermally set in the molding passage, so as to form the core layer, the surface layer and the intermediate layer simultaneously. 
The composite material of claim 23 has a total thickness of 100 mm or more and a ratio between a thickness of the core layer and a sum total of thickness of the surface layer covering the core layer in the thickness direction is within the range of 9/1 to 1/1.  The reason for this requirement is that with the ratio of more than 9/1, the bending strength becomes insufficient, while on the other hand, with the ratio of less than 1/1, there is the possibility that either of the compression strength and the nail holding performance resulting from the addition of the fillers may not be satisfied. 
In the composite material of claim 24, the core layer has at least two core layer forming composition layers (A) comprising filler and synthetic resin and at least one core layer forming composition layer (B) comprising thermosetting resin reinforced by long fibers interposed between two core layer forming compositions (A),(A) of the at least two core layer forming composition layers (A) and extending parallel in a longitudinal direction of the composite material, and a ratio between a sum total of thickness of the core layer forming composition layer (A) and a sum total of thickness of the core layer forming composition layer (B) is within the range of 95/5 to 50/50.  The reason for this requirement is that with the ratio of more than 95/5, the nail holding performance of the core layer forming composition layer (B) which is the intermediate fiber-reinforced layer is reduced, while on the other hand, with the ratio of less than 50/50, there is the possibility that compression elasticity limit may be reduced. 
In the composite material of claim 25, the surface layer is laminated on the core layer to cover at least two surfaces of the core layer with respect to a thickness direction thereof; the composite material has a total thickness of 100 mm or more with respect to a thickness direction thereof; and a thickness of the surface layer on the side thereof on which a pulling force is exerted when the composite material is bent in the thickness direction is 5% or more to 25% or less of the total thickness and the thickness of the surface layer on the side thereof on which a compressive force is exerted is 1.5% or more to 15% or less of the total thickness.  The reason is as follows. 
When the thickness of the surface layer on the side thereof on which a pulling force is exerted is too small, a sufficient bending strength may not be presented.  On the other hand, when the thickness of the surface layer on the side thereof on which the pulling is exerted is too large, the effect of reduction of material cost resulting from the provision of the core layer may be reduced.  When the thickness of the surface layer on the side thereof on which a compressive force is exerted is too small, the core layer may be buckled due to a deformation, thereby, a sufficient bending strength may not be presented.  On the other hand, when the thickness of the surface layer on the side thereof on which the compressive force is exerted is too large, the effect of reduction of material cost resulting from the provision of the core layer may be reduced. 
In the composite material of claim 26, the surface layer surrounds four surfaces of the core layer and constitutes 10 volume % or more to 65 volume % or less of the total of the composite material.  The reason for this requirement is that with less than 10 volume %, the bending strength becomes insufficient, while on the other hand, with more than 65 volume %, there is the possibility that either of the compression strength and the nail holding performance resulting from the addition of the fillers may not be satisfied. 
The composite material of the present invention is suitable for structural material.  It can suitably be used as a substitution of wood and also for intended uses for weight reduction of concrete products, such as lids used in a water treatment plant, pressure bearing boards used in the slop or equivalent places, sheathings for use in the Shield Earth Retaining Wall method (hereinafter it is referred to as xe2x80x9cthe SEW construction methodxe2x80x9d) and cross ties as cited in claim 27. 
It is to be noted that the SEW construction method is the construction method in which a high strength and high durability wall is incorporated in an earth retaining wall at a part thereof through which a shield machine passes so that the shield machine can directly cut that part of the wall to travel through it from a starting point to a terminal end, without any need for the cutting of the wall circularly by humans and machines. 
In the composite material and cross tie of the present invention, the surface layer and the core layer may be provided, at their exposed sides, with a decoration layer, a weatherproof layer, and a waterproof layer, if required. 
While no particular limitation is imposed on the weatherproof layer, for example a coating film formed by the application of weatherproof paint can be cited as the weatherproof layer. 
While no particular limitation is imposed on the waterproof layer, it may be formed of a waterproof sheet and a waterproof board, made of rubber, synthetic resin, metal sheet or combination thereof, and a coating film formed by the application of a non-immersible paint and the equivalent formed by the application or impregnation of water-repellent material of oily material such as paraffin and petroleum jelly. 
While the composite material of the present invention has an excellent nail holing performance, the nail holding performance may be further enhanced by forming prepared holes in nailing points, inpouring adhesive in the prepared holes, and then striking the nails therein.