Structural element and body structure including the same

The present invention relates to a structural element and a body structure including the structural element.

FIGS. 50 to 52 show an outer panel for a side structure of a conventional railroad car. An outer panel 101 (side structure) is provided with an window opening 102 , and is provided with door pocket parts 107 and 108 and door openings 103 and 104 arranged at fixed intervals on the opposite sides of the window opening 102 , respectively. The door pocket parts 107 and 108 are provided with reinforcing members 105 A and 105 B provided with vertical members 105 a having a substantially U-shaped cross section, and fixed in place by welding. Second reinforcing members 105 C and 105 D provided with vertical members 105 b having a substantially U-shaped cross section are disposed in upper parts (parts at a height corresponding to the height of the window opening 102 ) of a part between the first reinforcing member 105 A and the window opening 102 and a part between the first reinforcing member 105 B and the window opening 102 , respectively.

Three parallel third reinforcing members 105 E to 105 G having a substantially hat-shaped cross section are extended longitudinally in a lower part, extending under the window opening 102 , of a part extending between the first reinforcing members 105 A and 105 B. Two vertical fourth reinforcing members 106 A and 106 B having a substantially Z-shaped cross section are extended perpendicularly to the third reinforcing members 105 E through 105 G in the lower part extending under the window opening 102 . Side posts 106 C and 106 D, i.e., rim members, having a substantially Z-shaped cross section and a height greater than that of the fourth reinforcing members 106 A and 106 B are set on the opposite sides of the window opening 102 , respectively. A fifth reinforcing member 106 E having a substantially Z-shaped cross section is extended longitudinally under the window opening 102 and above the third reinforcing members 105 E to 105 G. Vertical door posts 106 F and 106 G having a substantially Z-shaped cross section are set vertically on the opposite sides of each of the door openings 103 and 104 .

The side posts 106 C and 106 D are welded to a part between the one of the opposite sides of the window opening 102 and the door pocket part 107 and a part between the other side of the window opening 102 and the door pocket part 108 , respectively, of the outer panel 101 , and the door posts 106 F and 106 G are welded to a part between the door pocket part 107 and the door opening 103 and a part between the door pocket 108 and the door opening 104 , respectively, of the outer panel 101 .

Those frame members including the side posts and the door posts maintain the shape of the side structure, and the plurality of reinforcing members maintain the shapes of parts, between the frame members, of the outer panel.

Those reinforcing members 105 A to 105 G, 106 A, 106 B and 106 E are welded to the outer panel 101 to secure the buckling strength of the outer panel 101 of the body structure of the car. Various body structures have been proposed.

(1) A side structure of an electric passenger car proposed in JP61-220962A is constructed by welding corrugated reinforcing plates to the inner surface of a side outer plate, and filling up spaces between the reinforcing plate and the side outer plate with a core material. (2) A side structure proposed in Japanese Patent No. 3015020 is formed by processing a stainless steel double-skin panel having outer and inner panels each serving as a load-bearing member. The outer and the inner panel have substantially the same thickness and are formed of the same material. This side structure is provided with core members formed by bending a stainless steel plate.

To fabricate the side structure mentioned in JP61-220962A, reinforcing plates of different sizes respectively corresponding to those of regions to be reinforced of the side outer panel need to be made, which requires troublesome processes because the reinforcing plates are corrugated. Since welding amount necessary for welding the reinforcing plates to the outer panel of a side structure is the same as that necessary for constructing a conventional frame-skin structure, the prior art body structure has no effect on reducing welding distortion. In filling up the spaces with a filling material (core material), the reinforcing plates need to have a thickness sufficient to withstand pressure due to the foaming of the filling material. Consequently, the weight of the reinforcing plates and the filling material is greater than that of a conventional buckling-preventing reinforced structure.

The technique mentioned in Japanese Patent No. 3015020 uses both the outer and the inner panel for bearing load. Therefore, the inner panel has a thickness substantially equal to that of the outer panel, is a high-strength member formed of the same material as the outer panel, and hence is considerably heavy. In constructing the side structure, it is necessary to bond together not only the outer panels but also the inner panels to bear load by the inner panels as well as the outer panels, which needs complicated welding construction. Spaces between the outer and the inner panel cannot effectively be used for thermal insulation, wiring and piping.

It is an object of the present invention to provide a structural element that enables weight reduction through the reduction of the number of reinforcing members, and the reduction of welding man-hour and welding distortion through the reduction of welding amount, and to provide a body structure employing the structural element.

According to the present invention, a structural element to be combined with other structural members to construct a body structure comprises: an outer panel serving as a structural member; an inner panel respectively having areas smaller than that of the outer panel; a rim member interposed between the outer panel and the inner panel, and joined to the outer and the inner panel; and a core member filling up a space defined by the outer panel, the inner panel and the rim member; wherein the core member lies between the outer panel and the inner panel and is adhesively bonded or fusion welded at least to the outer and the inner panel.

According to the present invention, the rim member joined to the outer and the inner panel is extended along the edge of the inner panel, and the outer panel is supported elastically by filling up the spaces defined by the outer panel, the inner panel and the rim member with the core member (elastic core member). The elastic support of the outer panel enhances the buckling strength of the outer panel and realizes the reduction of members, welding amount and welding distortion.

According to the present invention, the core member is a structural member formed of a plastic foam or wood.

Since the plastic foam and wood have a low density, the body structure can be easily simplified and lightened by properly selecting the pitches of the rim members and the thickness and density of the core member. When a plastic foam capable of serving as a heat-insulating material is used for forming the core member, the space between the outer and the inner panel can be effectively used to provide a heat-insulating effect without using complicated structures.

According to the present invention, the inner panel is a structural member of a fiber-reinforced plastic material or an aluminum alloy, or a stainless steel panel of a thickness not greater than ⅓ of the thickness of the outer panel.

Since the adjacent inner panels are not mechanically connected together and hence most part of load placed on the body structure is born by the outer panel, and the inner panel is loaded scarcely, the inner panel may be a lightweight member (a fiber reinforced plastic member, an aluminum alloy member or a stainless steel panel of a thickness not greater than ⅓ of the thickness of the outer panel).

The use of the lightweight material for forming the inner panel provides an advantageous effect of reducing the weight of the body structure.

According to the present invention, the rim member is a structural member formed of a fiber reinforced plastic material or an aluminum alloy, or a stainless steel plate thinner than the outer panel.

According to the present invention, the outer panel is formed of a metal, such as an aluminum alloy or a stainless steel.

According to the present invention, a body structure comprises: a structural element to be combined with other structural elements, comprising an outer panel serving as a structural member, inner panels respectively having areas smaller than that of the outer panel, rim members interposed between the outer panel and the inner panels, and joined to the outer panel and the inner panels, and core members filling up spaces defined by the outer panel, the inner panels and the rim members, the core members lying between the outer panel and the inner panels, and being adhesively bonded or welded at least to the outer panel and the inner panels; and a frame having reinforcing frame members arranged in the longitudinal and transverse directions; wherein sections of the inner panels of the structural element are placed in openings in the frame element, respectively.

The structural element is used for forming a body structure of a railroad car or the like.

According to the present invention, one of the openings of the frame element of the body structure is not covered with the structural element and is used as a window opening.

According to the present invention, one of the openings of the frame element of the body structure is not covered with the structural element and are used as a doorway opening.

According to the present invention, when the outer panel of the structural element has an outward convex bend, elastic support parts each including the core member, the rim members and the inner panel are arranged separately and are demarcated by the outward convex bend.

Since the core members are thus demarcated by the outward convex bend of the outer panel, the core members, the rim members and the inner panels do not need to be bent and the structural element can be constructed by using flat cores, inner panels and linear rim members. Thus, the body structure is not complicated.

According to the present invention, it is desirable that the distance b between a frame member of the frame element, and a rim member of the structural element adjacent and parallel to the frame member, meets Expression (1).

b ≤ t ⁢ π 2 ⁢ E 3 ⁢ ( 1 - v 2 ) ⁢ σ c1 ( 1 )

t: Thickness of the outer panel π: Ratio of the circumference of a circle to its diameter, E: Young's modulus of the outer panel, υ: Poisson's ratio of the outer panel, and σ c1 : Compressive stress parallel to a frame member induced in a part of the outer panel between the frame member and a rim member of the structural element parallel to the frame member, when the body structure is loaded

Although only the outer panel of the structural element extends between the frame member of the frame element and the rim member of the structural element, the outer panel secures a necessary buckling strength with respect to a direction parallel to the frame member.

According to the present invention, when the direction of the compressive stress induced in the outer panel is perpendicular to the frame member, it is desirable that the distance b between the frame member of the frame element, and the rim member of the structural element adjacent and parallel to the frame member, meets-Expression (2).

b ≤ t ⁢ π 2 ⁢ E 12 ⁢ σ c2 ( 2 )

t: Thickness of the outer panel π: Ratio of the circumference of a circle to its diameter E: Young's modulus of the outer panel ν: Poisson's ratio of the outer panel σ c2 : Compressive stress in a direction perpendicular to a frame member induced in a part of the outer panel between the frame member of the frame element and a rim member of the structural element adjacent and parallel to the frame member, when the body structure is loaded

Thus, the outer panel of the structural element extending between the frame member of the frame element and the rim member of the structural element secures a necessary buckling strength with respect to a direction parallel to the frame member.

FIGS. 1 ( a ) and 1 ( b ) are schematic views of structural elements according to the present invention;

FIG. 2 is a typical view of the structural element (elastic support system) shown in FIG. 1 ( a );

FIGS. 3 ( a ), 3 ( b ) and 3 ( c ) are schematic views of structural elements in other embodiments according to the present invention;

FIG. 4 is a view of assistance in explaining a method of calculating the buckling strength of a part of an outer panel extending between a rim member of a structural element and a frame member of a frame element;

FIG. 5 is a graph of assistance in explaining buckling coefficient;

FIG. 6 ( a ) is a front elevation of a structural element employed in buckling strength analysis;

FIG. 6 ( b ) is a sectional view taken on line A—A in FIG. 6 ( a );

FIG. 7 is a graph showing the relation between the bending rigidity EI per unit width of a panel and buckling strength;

FIG. 8 is a graph showing the relation between the square root of spring constant per unit area and buckling strength;

FIG. 9 is a graph showing the relation between the weight per unit area of a panel and the buckling strength of the panel;

FIGS. 10 ( a ), 10 ( b ) and 10 ( c ) are views of assistance in explaining constraining conditions for buckling strength analysis of buckling with respect to an X-direction, buckling with respect to a Y-direction and shear buckling, respectively;

FIG. 11 is a view showing a deformation mode when buckling with respect to the X-direction occurs;

FIG. 12 is a view of assistance in explaining stress induced in an outer panel when buckling with respect to the X-direction occurs;

FIG. 13 is a view of assistance in explaining stress induced in an inner panel when buckling with respect to the X-direction occurs;

FIG. 14 is a view of assistance in explaining stress induced in a core when buckling with respect to the X-direction occurs;

FIG. 15 is a view showing a deformation mode when buckling with respect to the Y-direction occurs;

FIG. 16 is a view of assistance in explaining stress induced in an outer panel when buckling with respect to the Y-direction occurs;

FIG. 17 is a view of assistance in explaining stress induced in an inner panel when buckling with respect to the Y-direction occurs;

FIG. 18 is a view of assistance in explaining stress induced in a core when buckling with respect to the Y-direction occurs;

FIG. 19 is a view showing a deformation mode when shear buckling occurs;

FIG. 20 is a view of assistance in explaining stress induced in an outer panel when shear buckling occurs;

FIG. 21 is a view of assistance in explaining stress induced in an inner panel when shear buckling occurs;

FIG. 22 is a view of assistance in explaining stress induced in a core when shear buckling occurs;

FIG. 23 is a front elevation of a side structure according to the present invention as viewed from the inner side;

FIG. 24 is a sectional view taken on line A—A in FIG. 23 ;

FIG. 25 is a sectional view taken on line B—B in FIG. 23 ;

FIG. 26 is a sectional view taken on line C—C in FIG. 23 ;

FIG. 27 is a sectional view taken on line D—D in FIG. 23 ;

FIG. 28 is an enlarged view of a part E in FIG. 24 ;

FIG. 29 is an enlarged view of a part F in FIG. 26 ;

FIG. 30 is a front elevation of another side structure according to the present invention as viewed from the inner side;

FIG. 31 is a sectional view taken on line A—A in FIG. 30 ;

FIG. 32 is a sectional view taken on line C—C in FIG. 30 ;

FIG. 33 is an enlarged view of a part E in FIG. 31 ;

FIG. 34 is an enlarged view of a part F in FIG. 32 ;

FIG. 35 is a view, similar to FIG. 23 , of a third side structure according to the present invention;

FIG. 36 is a sectional view taken on line A—A in FIG. 35 ;

FIG. 37 is a sectional view taken on line B—B in FIG. 35 ;

FIG. 38 is a sectional view taken on line C—C in FIG. 35 ;

FIG. 39 is a sectional view taken on line D—D in FIG. 35 ;

FIG. 40 is an enlarged view of a part E in FIG. 36 ;

FIG. 41 is an enlarged view of a part F in FIG. 36 ;

FIG. 42 is a front elevation of a structural element according to the present invention;

FIG. 43 is a sectional view taken on line A—A in FIG. 42 ;

FIG. 44 is a sectional view taken on line B—B in FIG. 42 ;

FIG. 45 is a sectional view taken on line C—C in FIG. 42 ;

FIG. 46 is a front elevation of a frame element according to the present invention;

FIG. 47 is a sectional view taken on line A—A in FIG. 46 ;

FIG. 48 is a sectional view taken on line B—B in FIG. 46 ;

FIG. 49 is a sectional view taken on line C—C in FIG. 46 ;

FIG. 50 is a view, similar to FIG. 23 , of a conventional side structure;

FIG. 51 is a sectional view taken on line A—A in FIG. 50 ; and

FIG. 52 is a sectional view taken on line C—C in FIG. 50 .

Preferred embodiments of the present invention will be described with reference to the accompanying drawings.

FIGS. 1 ( a ) and 1 ( b ) are schematic views of a structural element in a preferred embodiment according to the present invention.

Referring to FIG. 1 ( a ), the structural element according to the present invention includes an outer panel 1 forming the longitudinally and vertically extending outer surface of a railroad car, and a core (core member) 2 having a rectangular cross section and adhesively bonded to the inner surface of the outer panel 1 . Rim members 3 attached to the outer panel 1 surround the core 2 . An inner panel 4 is attached to the rim members 3 and is bonded to the inner surface of the core 2 so as to cover the core 2 . The rim members 3 are arranged at pitches P.

The core 2 thus supports the outer panel 1 elastically to enable the outer panel 1 to enhance buckling strength. Since any reinforcements for preventing buckling, which are necessary for the conventional structural element, are not necessary, the number of component members, the amount of welding and welding distortion can be reduced.

The core 2 may be formed of a heat-insulating material, i.e., a soft structural material. The thickness of the core 2 is determined so that the core 2 has a necessary heat-insulating ability. The core 2 maybe formed of a plastic foam, such as a polyurethane foam, a polyvinyl chloride foam, such as KLEGECELL®, a polyethylene foam or a phenol foam.

The specific weight of the core 2 of a plastics foam is in the range of {fraction (1/20)} to {fraction (1/80)} or below of that of a stainless steel plate forming the outer panel 1 . The very light core 2 has an advantageous effect of constructing the structural element in a lightweight structure by selectively properly determining the thickness L 1 and the density of the core 2 .

The rim members 3 are formed by bending plates, such as stainless steel plates. Each of the rim members 3 has a joining part 3 a welded to the inner surface 1 a , i.e., a surface on the side of the passenger room, of the outer panel 1 , a vertical wall part 3 b extending from an edge of the joining part 3 a substantially perpendicularly to the joining part 3 a , and a holding part 3 c extending from an inner edge of the vertical wall part 3 b in parallel to the joining part 3 a in a direction opposite a direction in which the joining part 3 a extends from the outer edge of the wall part 3 b . The holding part 3 c presses only the core 2 or the core 2 and the inner panel 4 against the outer panel 1 .

The joining part 3 a of the rim member 3 does not need necessarily welded to the inner surface 1 a of the outer panel 1 , but may be bonded adhesively to the inner surface 1 a of the outer panel 1 .

The structural elements shown in FIGS. 1 ( a ) and 1 ( b ) were modeled by an elastic support system shown in FIG. 2 , assuming that the outer panel 1 was a beam 11 , the rim members 3 (reinforcing members) were supporting points 12 A and 12 B, and the core 2 was a plurality of springs 13 to examine the effect of the Young's modulus of the core 2 on the buckling strength of the outer panel 1 .

In modeling the structural elements, it was assumed that the rigidity of the inner panel 4 was far higher than that of the core 2 , and the inner panel 4 was a rigid member supporting the springs 13 .

Expression (3) is a equilibrium equation representing the beam 11 of the model of an elastic support system shown in FIG. 2 .

EI ⁢ ⅆ   ⁢ 4 ⁢ y ⅆ x 4 = - k ⁢   ⁢ y ( 3 )

y: Deflection E: Young's modulus of the beam I: Geometrical moment of inertia of the beam k: Spring constant of the elastic support

The spring constant k per unit area of the core 2 of the structural element shown in FIGS. 1 ( a ) and 1 ( b ) is represented by Expression (4).

k = E 1 L 1 ( 4 )

E 1 : Young's modulus of the elastic member (the core 2 ) L 1 : Effective depth (thickness) of the elastic member (core 2 )

Expression (5), which is similar to Expression (1), is a equilibrium equation representing the symmetrical deformation of a cylindrical body.

D ⁢ ⅆ   ⁢ 4 ⁢ y ⅆ x 4 = - Et r 2 ⁢ y ( 5 )

D: Flexural rigidity of the plate t: Thickness of the plate r: Radius of the cylindrical body y: Radial displacement of the cylindrical body

The flexural rigidity D of the plate in Expression (5) is expressed by Expression (6).

D = Et 3 12 ⁢ ( 1 - v 2 ) ( 6 )

E: Young's modulus of the plate ν: Poisson's ratio

Assuming that Expressions (3) and (5) are equivalent, the following relation can be derived from the respective right sides of Expressions (3) and (5).

k = Et r 2 ( 7 )

Assuming that the beam 11 of the elastic support system is the cylindrical body, the equivalent radius r is represented by Expression (8).

r = Et k = EtL1 E1 ( 8 )

It is known that Expression (9) represents the axial compressive buckling stress σ cr of a cylindrical body.

σ cr = E 3 ⁢ ( 1 - v 2 ) · t r ( 9 )

The equivalent radius r in Expression (9) is a function of the spring constant of the elastic support system as represented by Expression (8). Substituting Expression (8) into Expression (9), Expression (10) is obtained.

σ cr = E 3 ⁢ ( 1 - v 2 ) × t r = Etk 3 ⁢ ( 1 - v 2 ) ( 10 )

Thus, the buckling stress σ cr in the elastic support system is proportional to the square root of the spring constant k of the elastic support system.

Incidentally, the necessary buckling strength of the side structure of an ordinary railroad car is on the order of 15 kgf/mm 2 at the maximum. Therefore, the spring constant k of the elastic support system is:

k = σ cr 2 × 3 × ( 1 - v 2 ) Et = 15 2 × 3 × ( 1 - 0.3 2 ) 19700 × 1.5 = 0.02 ⁢   ⁢ kgf ⁢ / ⁢ mm ( 11 )

E: Young's modulus of stainless steel (19700 kgf/mm 2 ) ν: Poisson's ratio (0.3) t: Thickness of the plate (1.5 mm)

The Young's modulus E 1 of the core of a thickness L 1 =30 mm is:

E 1 =k·L 1 =0.02×30=0.6 kgf/mm 2 .

Since the Young's modulus of the foregoing plastic form is about 1 kgf/mm 2 or above, the plastic foam generally used as a heat-insulating material can be used for forming the core.

The use of the structural elements of the present invention shown in FIGS. 1 ( a ) and 1 ( b ) in parts, which need buckling strength, of a body structure enhances the buckling strength of body structure.

Buckling strength analysis of structural elements shown in Table 1 was performed using a finite element method, which is a known method of analyzing stresses induced in the body structures of railroad cars, to examine the rigidities of the inner panel and the rim member. A finite element method divided each member into a plurality of meshes, formed a stress analysis model by using each mesh as a plate element (CQUAD4, CTRIA3), applied a predetermined load to the stress analysis model and calculated stress induced in the stress analysis model.

A structural element as shown in FIGS. 6 ( a ) and 6 ( b ) was used as a test sample. The structural element as a test sample was constructed by holding a flat core 2 (30 mm in thickness) of a plastic foam by four rim members 3 - 1 to 3 - 4 each having a joining part of L 13 =24 mm, a vertical wall part of L 14 =30 mm and a holding part of L 15 =25 mm, adhesively bonding the core 2 to an outer plate 1 of L 11 =1290 mm, L 12 =748 mm and t 11 =1.5 mm, and adhesively bonding a stainless steel inner panel 4 of t 12 =0.5 mm to the rim members 3 - 1 to 3 - 4 and the core 2 . This structural element corresponds to a section under a window opening of a side structure.

The stainless steel has a Young's modulus of 19700 kgf/mm 2 and a Poisson's ratio of 0.3, the GFRP has a Young's modulus of 1410 kgf/mm 2 , the plastic foam has a Young's modulus of 3 kgf/mm 2 (approximate value when the density is 50 kgf/m 3 .)

The buckling strength σ cr of a panel is proportional to the flexural rigidity of the panel (refer to a broken line in FIG. 7 ). However, it was verified that the structural elements of all the types excluding the structural element of the type F 2 do not meet the proportional relation as shown in FIG. 7 when the cores are formed in a thickness sufficient for the cores to serve as a heat-insulating member, such as 30 mm. It is considered that the proportional relation shown in FIG. 7 is not met because the load is born mainly by the outer panel and the inner panel is loaded indirectly through the core and the rim members, and the core is a structural member having a considerably low Young's modulus.

It is inferred that the same phenomenon occurred also in the structural element of the type F 2 . However, since the GFRP forming the inner panel has a Young's modulus of 1410 kgf/mm 2 , which is about {fraction (1/10)} of the Young's modulus of 19700 kgf/mm 2 of the stainless steel of the type S 2 and the Young's modulus of 7200 kgf/mm 2 of the aluminum alloy of the type A 2 , the calculated flexural rigidity of the plane is small and the type F 2 lies near the broken line shown in FIG. 7 . This is supported by a fact that the buckling strength of the structural element of the type F 2 is proportional to the square root of the spring constant (√{square root over (k)}) of the antiplane deflection of the core.

Since the structural elements of the types A 2 , S 2 and F 2 respectively provided with thick cores behave as elastic support plates when buckles as shown in FIG. 8 , the values for the types A 2 , S 2 and F 2 lie near a straight line represented by Expression (10) which is good when the buckling strength is proportional to the square root of the spring constant (√{square root over (F)}) of the antiplane deflection of the core (refer to broken line in FIG. 8 ).

Since the core may be formed of a soft material, such as a heat-insulating material, the core has a small density. Therefore, the buckling strength σ cr increases remarkably while the weight of the core changes very slightly, when the thickness of the core is increased from 10 mm to 30 mm (FIG. 9 ).

Thus, it is advantageous to use a soft core having a small density and an inner panel having the least possible thickness.

The buckling mode of the structural element of the type S 2 shown in Table 1 will be examined.

Unit loads Fx and Fy were applied in an X- and a Y-direction to the structural elements as shown in FIGS. 10 ( a ) and 10 ( b ), respectively, to study buckling, and a unit load F was applied in the X-direction and a load F(a/b) was applied in the Y-direction to the structural elements as shown in FIG. 10 ( c ) to study shear buckling.

Stresses induced in meshes determined by stress analysis were classified into a plurality of levels represented respectively by different colors to enable the recognition of stress distribution in the model at a glance ( FIGS. 11 to 14 , 15 to 18 and 19 to 22 ).

(A) Buckling with Respect to the X-direction

Buckling load F cr =27886 kgf, Outer panel sectional area A=1122 mm 2 , Mean buckling strength σ cr =F/A=24.9 kgf/mm 2

FIG. 11 shows a deformation mode. Stress distributions in the outer panel, the inner panel and the core of a plastic foam are shown in FIGS. 12 to 14 , respectively.

(B) Buckling with Respect to the Y-Direction

Buckling load F cr =45358 kgf, Outer panel sectional area A=1935 mm 2 , Mean buckling strength σ cr =F/A=23.4 kgf/mm 2

FIG. 15 shows a deformation mode. Stress distributions in the outer panel, the inner panel and the core of a plastic foam are shown in FIGS. 16 to 18 , respectively.

(C) Shear Buckling

Buckling load F cr =49377 kgf, Outer panel sectional area A=1935 mm 2 , Mean buckling strength σ cr =F/A=25.5 kgf/mm 2

FIG. 19 shows a deformation mode. Stress distributions in the outer panel, the inner panel and the core of a plastic foam are shown in FIGS. 20 to 22 , respectively.

It is known from the results of analysis that the mean buckling strength of 24.9 kgf/mm 2 with respect to the X-direction is practically sufficiently high because a necessary buckling strength with respect to the X-direction is on the order of 15 kgf/mm 2 at the maximum.

FIGS. 3 ( a ), 3 ( b ) and 3 ( c ) show body structures according to the present invention each including a structural element and frame members. Frame members 5 , 5 A, 5 B and 5 C are joined to outer panels 1 A, and 1 , and sections (elastic support parts) including inner panels 4 a , 4 b and 4 ′ and rim members 3 A and 3 ′ are formed between the frame members 5 , 5 A, 5 B and 5 C. Since the inner panels 4 a , 4 b and 4 ′ are discrete members for the sections, most part of load placed on the body structures is born by the outer panels 1 A and 1 and hence the inner panels 4 a , 4 b and 4 ′ are scarcely loaded. Therefore, the inner panels 4 a , 4 b and 4 ′ maybe structural members of a lightweight material, such as a fiber reinforced plastic material (FRP) or an aluminum alloy, or a stainless steel panels of a thickness not greater than ⅓ of the thickness of the outer panels. Since the inner panels 4 a , 4 b and 4 ′ are adhesively bonded to the cores 2 A and 2 so as to cover the cores 2 A and 2 , the cores 2 A and 2 are not exposed to air and hence the body structures are fire-resistant.

The rim members of the structural elements are not limited to rim members 3 and 3 ′ having a substantially Z-shaped cross section and may be rim members having a substantially U-shaped cross section. Rim members having a substantially Z-shaped cross section and those having a substantially U-shaped cross section may used in combination. The rim members may be of any suitable shapes provided that the same are capable of holding peripheral parts of the core 2 .

For example, a rim member 3 A having a substantially U-shaped cross section as shown in FIG. 3 ( a ) may be used instead of the rim members having a substantially Z-shaped cross section.

The rim member 3 A has an outer joining part 3 Aa attached to the inner surface 1 Aa of an outer panel 1 A, a vertical wall part 3 Ab extending from an edge of the joining part 3 Aa substantially perpendicularly to the joining part 3 Aa, and an inner joining part 3 Ac extending from an inner edge of the vertical wall part 3 Ab in parallel to the joining part 3 Aa in the direction of extension of the joining part 3 Aa and attached to the inner surface 4 Aa of an inner panel 4 A. The rim member 3 A has a substantially U-shaped cross section opening outward, i.e., in a direction away from the core 2 A. Peripheral parts of the inner panel 2 A substantially coincide with the joining parts 3 Aa and 3 Ab. Thus, the edges of the inner panel 2 A do not project from the rim members 3 A. However, the peripheral parts do not need necessarily coincide with the joining parts.

The frame members 5 are formed by bending plates (stainless steel plates). Each of the frame members 5 has an outer joining part 5 a attached to the inner surface 1 Aa of the outer panel 1 A, a vertical wall part 5 b extending from an edge of the joining part 5 a substantially perpendicularly to the joining part 5 a , and an inner joining part 5 c extending from an inner edge of the vertical wall part 5 b in parallel to the outer joining part 5 a in a direction opposite a direction in which the outer joining part 5 a extends. It is desirable to place the frame member 5 as close as possible to the rim member 3 A to avoid the buckling of a part between the frame member 5 and the rim member 3 A.

As shown in FIG. 3 ( b ), a rim member 3 A having a substantially U-shaped cross section and a rim member 3 ′ having a substantially Z-shaped cross section may be used in combination. The rim members 3 ′ having a substantially Z-shaped cross section has an outer joining part 3 a ′ joined to the inner surface 1 Aa of the outer panel 1 A, a vertical wall part 3 b ′ extending from an edge of the joining part 3 a ′ substantially perpendicularly to the joining part 3 a ′, and an inner joining part 3 c ′ extending from an inner edge of the vertical wall part 3 b ′ in parallel to the outer joining part 3 a ′ in a direction opposite a direction in which the outer joining part 3 a ′ extends from the outer edge of the wall part 3 b′.

The body structure is provided with a frame member 5 A having a substantially U-shaped cross section. The frame member 5 A has an outer joining part 5 Aa joined to the inner surface 1 Aa of the outer panel 1 A, a vertical wall part 5 Ab extending from an edge of the outer joining part 5 Aa perpendicularly to the outer joining part 5 Aa, and an inner joining part 5 Ac extending from the inner edge of the vertical wall part 5 Ab in parallel to the outer joining part 5 Aa in the direction of extension of the outer joining part 5 Aa.

A frame member 5 B of a substantially L-shaped cross section having an outer joining part 5 Ba and a vertical wall part 5 Bb, and another frame member 5 C of a substantially hat-shaped cross section having a main part 5 Ca of a substantially U-shaped cross section, and two outer joining parts 5 Cb and 5 Cc respectively extending in opposite directions as shown in FIG. 3 ( c ) may be used.

A body structure according to the present invention is a flat structure in which only an outer panel is extended between a rim member and a frame member, and this part must have a necessary buckling strength.

Conditions for securing buckling strength with respect to a direction parallel to the frame member will be explained.

The buckling strength σ cr of a flat panel shown in FIG. 4 is represented generally by Expression (12).

σ cr = K ⁢ π 2 ⁢ E 12 ⁢ ( 1 - v 2 ) ⁢ ( t b ) 2 ( 12 )

E: Young's modulus of the panel ν: Poisson's ratio π: Ratio of the circumference of a circle to its diameter, t: Thickness of the panel b: width of the panel a: Length of the panel K: Buckling coefficient ( FIG. 5 )

Buckling coefficient K is dependent on the constraining condition around the panel and the value of a/b. However, if the length a of the panel is sufficiently greater than the width b of the panel supported around its edges (a/b≧1), it is possible to consider on the basis of data shown in FIG. 5 that the buckling coefficient K=4. Therefore, Expression (1) is obtained on an assumption that K=4.

b ≤ t ⁢ π 2 ⁢ E 3 ⁢ ( 1 - v 2 ) ⁢ σ c1 ( 1 )

t: Thickness of the outer panel π: Ratio of the circumference of a circle to its diameter, E: Young's modulus of the outer panel, ν: Poisson's ratio of the outer panel, and σ c1 : Compressive stress parallel to the frame member induced in a part of the outer panel between the frame member and the rim member of the structural element parallel to the frame member, when the body structure is loaded

Although only the outer panel of the structural element extends between the frame member of the frame element and the rim member of the structural element, the outer panel is able to secure a necessary buckling strength with respect to a direction parallel to the frame member, provided that the width b meets Expression (1).

When the outer panel is formed of a stainless steel and has a thickness of 1.5 mm, and a compressive stress σ c1 =15 kgf/mm 2 is induced in a part of the outer panel extending between the frame member and the rim member adjacent and parallel to the frame member in a direction parallel to the frame member, the width (which corresponds to the distance between the frame member and the rim member adjacent and parallel to the frame member) b must be 103 mm or below.

b ≤ t ⁢ π 2 ⁢ E 3 ⁢ ( 1 - v 2 ) ⁢ σ c1 = 1.5 × π 2 × 19700 3 × ( 1 - 0.3 2 ) × 15 = 103 ⁢   ⁢ mm

Conditions for securing buckling strength with respect to a direction perpendicular to the frame member will be explained. The buckling strength of a flat panel can be approximated by the buckling of a beam having a unit width along a direction perpendicular to the direction of stress on an assumption that the length a of the panel is sufficiently smaller than the width b of the same (a/b≦0.5). The buckling strength of a beam supported at both ends is represented by Expression (13).

σ cr = π 2 ⁢ EI l 2 ⁢ A ( 13 )

E: Young's modulus π: Ratio of the circumference of a circle to its diameter I: Geometrical moment of inertia of the beam l: Length of the beam A: Sectional area of the beam

The geometrical moment of inertia I of a flat panel of a unit width and thickness t is equal to t 3 /12 and the sectional area A=t. By considering L to be the width b which corresponds to the distance between the frame member and the rim member.

b ≤ t ⁢ π 2 ⁢ E 12 ⁢   ⁢ σ c2 ( 2 )

t: Thickness of the outer panel π: Ratio of the circumference of a circle to its diameter E: Young's modulus of the outer panel ν: Poisson's ratio of the outer panel σ c2 : Compressive stress in a direction perpendicular to the frame member induced in a part of the outer panel between a frame member of a frame element and a rim member of a structural element adjacent and parallel to the frame element when the body structure is loaded

Thus, the part, between the frame member of the frame element and the rim member of the structural element, of the outer panel is able to secure a necessary buckling strength with respect to a direction parallel to the frame member when the width b of the part of the outer panel has meet the condition represented by Expression (2).

When the outer panel is formed of a stainless steel and has a thickness of 1.5 mm, and a compressive stress σ c2 =15 kgf/mm 2 is induced in a part of the outer panel extending between the frame member and the rim member adjacent and parallel to the frame member in a direction perpendicular to the frame member, the width b must be 49 mm or below.

b ≤ t ⁢ π 2 ⁢ E 12 ⁢   ⁢ σ c2 = 1.5 × π 2 × 19700 12 × 15 = 49 ⁢   ⁢ mm

A railroad car including the foregoing body structure will be concretely described with reference to the drawings by way of example.