ENERGY ABSORBING STRUCTURE, ESP. FOR SKIS

Abstract: The invention relates to an integral laminated sand­ wich structure with a low specific weight and with im­ proved damping capacity. The structure comprises a honeycomb structure as a core layer [1] which is co­ vered on at least one side with a bonding layer [4] and/­ or [5] of rubberlike material in which twisted fiber bundles [6] are embedded and wherein the so covered core layer is bonded at both sides with a rigid plate [8] and/or [9], e.g. a metal plate. The twisted fiber bundles [6] are preferably steel cords. A ski is also described as a specific embodiment of the invention.

Description: ENERGY ABSORBING STRUCTURE The present invention relates to an energy absorbing laminated sandwich structure with a low specific weight and with an increased damping capacity for mechanical vibrations. Damping of mechanical vibrations is often desirable e.g. to suppress nuisance by noise due to vibrations and often favours the fatigue resistance of the damped structures. The invention relates further particularly to a sliding shoe such as a ski comprising said laminated structure as a load bearing structure. It is generally known to make light weight laminated structures comprising a honeycomb structure as a core layer which is covered on both sides with a rigid covering plate adhering thereto and consisting e.g. of plastic, either reinforced or not, veneer or metal. It is now an object of the invention to provide an integral laminated structure of this type with improved capacity of damping mechanical vibrations by providing a layer of rubber-like material as a bonding layer between the light weight core layer and the rigid plate or sheet disposed on at least one side of the said core and wherein the bonding layer comprises twisted fiber bundles embedded therein and extends over at least a part of the surface of this covered side. A honeycomb structure is preferably used as core layer. However a more general concept of light-weight core layer is applicable : it can generally consist of a structure comprising a number of strips or foils mutually connected in certain areas and delimiting thereby intermediate spaces between them which spaces extend across the core thickness. By bonding the honeycomb to the covering plates by means of'the rubberlike material instead of the usual curable adhesives, the strength of the structure decreases. This is due to the fact that the rubber is subject to a larger deform mation under the same shearing stress as e.g. a cured (rigid) bonding layer on the base of epoxy resins would be. The incorporation of twisted fiber bundles in this rubberlike layer compensates to a substantial degree for this decrease of strength. As already described in the Belgian patent 879.030 of Applicant, the absorption of the mechanical vibrations is partially due to the intrinsic damping capacity of the rubberlike material and partially to the friction and deformation of the fibers embedded in said material. In this patent is also mentioned that steel cords, embedded in rubber provide excellent damping properties in such laminated structures and that said properties still improve when a metal plate is bonded to the core structure. An additional advantage of the invention resides in its simple manufacturing method compared to presently known laminated structures wherein the bonding layer is mostly prepared on the base of a substrate comprising an incompletely cured (B-stage) epoxy resin. Further the rubberlike bonding layer according to the invention is often substantially cheaper and maintains its elastomeric (hence damping) behaviour after the bonding (e.g. by vuldanising) is accomplished. The invention will now be illustrated by the description of the basic composition of a light weight lami nated structure and of the composition and manufacture of a ski presenting a particular embodiment of the invention. Figure 1 shows a cross section of the basic composi tion of a platelike light weight laminated structure. Figure 2 is a schematic view in cross section of the composition of a ski. The laminated structure according to figure 1 consists of a core layer 1, e.g. a honeycomb structure comprising a number of strips 2 which are mutually connected to each other in the areas 7. Intermediate spaces 3 are located between subsequent strips 2. The core layer 1 is provided on at least one side with a rubberlike bonding layer 4 in which twisted fiber bundles 6 are incorporated in a mutual parallel arrangement. A similar bonding layer 5 is applied to the other side of the core layer. This layer 5 can also contain twisted fiber bundles 6 when desirable. The layers 4, resp. 5 are covered with and bonded to a rigid plate 8, resp. 9, which is often a metal plate. The strips 2 can in principle consist of any plate or sheetlike material with a certain rigidity such as paper (cardboard), plastic (either reinforced or not), metal foil or metal sheet etc. The shape and cross dimensions of the free spaces or cells 3 can be chosen in view of the required thickness, strength, cost and/ or weight of the core layer. The dimensions of cells 3 should not be uniform ; they may be different in predetermined areas of the laminated structure. They may be filled, at least partially, with e.g. foam. The strips can be arranged obliquely with respect to the planes of the laminated structure or even parallel thereto in certain areas i.s.o. perpendicular thereto. The composition of the bonding layers 4, resp. 5 shall be determined to provide a good adhesion to the strip portions with which they have contact as well as to the fiber bundles and to the contact planes with the rigid covering plates 8, resp. 9. Further, at least one of the layers 4 or 5 comprising the embedded fiber bundles should maintain its elastomeric character to obtain the damping effect contemplated. Hence, when a rubber sheet is used as elastomeric bonding layer, it may be necessary to coat it with a specific adhesion promoting agent versus the plates 8 or 9. The adhesive power and the flexibility of the bonding layers will preferably remain active in a wide temperature range, e.g. between -400C and 120eC. Elastomeric plastics can also be used, such as e.g. elastic adhesives on the basis of epoxy resins and with a high elongation capacity. To compensate for the decrease in strength (by the application of rubberlike materials i.s.o. the usual bonding substances which are cured) it will often be desirable to reinforce the rubberlike material additionally, e.g. with a fiberlike filler material. The twisted fiber bundles in the bonding layer preferably have a high tensile strength ( > 2000 N/mm2) and a high tensile modulus of elasticity ( ) 7.104 N/mm2). Suitable fibers thus are glass fibers, carbon fibers, boron fibers, polyamide fibers, polyaramid fibers, polyester fibers and/or metal fibers or filaments in the form of cords or cables. The tensile modulus shall preferably be chosen to substantially correspond (same order of magnitude) with that of the plate bonded thereto. The thickness of the bonding layer will be between 1,33 and 4 times the thickness or diameter of the fiber bundles. The twisted fiber bundles, e.g. steel cords can extend over the whde or part of the whole surface of the laminated structure. When fiber bundles are used in both the upper, resp. lower bonding layer 5 resp. 4, they can be oriented in a different direction. The direction of the cords in layer 4 can e.g. be perpendicular to that of the cords in layer 5. In certain cases it might be useful to choose cords with a substantial elongation at rupture ( 2 5 %). The cords can also be applied in the form of a woven fabric, whereby running in warp and/or weft direction. The cords can also improve the strength of the laminated structure, when the other constituents have a relatively low strength. For this purpose it is also possible to incorporate two cord layers in one or both bonding layers with different cord orientations. The incorporation of the cords has further the additional effect that the borders of the strips 2 into contact with layers 4 and 5 do not cut therethrough during the compression of the covering plates 8 and 9 thereon. The continuity of the bond over the whole surface between plates 8 and 9 and core structure 1 is thus ensured. The light weight laminated structure according to the invention can also have other shapes than that of a flat plate, e.g. it can be a curved or undulated plate, a plate with changes in thickness over its surface, a profiled lath or a tube. These structures can e.g. be used as bearing frames or elements for vibration sensitive instruments and devices e.g. in engine rooms, in airplanes or ships, for loudspeaker boxes and other sport goods (skate boards, tennisrackets) etc. In general the light weight laminated structure according to the invention will be very well suited in applications wherein damping requirements are made besides specific requirements regarding strength and weight. The concept of the invention is particularly advantageous if it is desired to realize an object or structure with a certain springiness under bending load as e.g. in skis and next to the combination of requirements strength/weight/damping. The invention is also useful for application in other sports goods such as tennis rackets and skate boards. Example In figure 2 a cross section is shown of a ski with improved damping properties as a specific example of an embodiment of the invention. The ski has as a bearing structure a light weight laminated structure comprising a honeycomb structure 1 from aluminium foil (origin : Hexel type 1/8 - 5052-0007) as a core layer and covered on its underside with a rubber strip 4 with a thickness of about 0,8 mm. In the rubber strip 4 having a width (70 mm) equal to that of the ski, 50 longitudinally arranged steel cords 6 are embedded which cords have a construction 4 x 0,175 (i.e. 4 brass coated steel filaments each having a diameter of 0,175 mm), a tensile modulus of 20.104 N/mm2, s tensile-strength of 2700 N/mm2 and an elongation at rupture of about 1,5 %. The upper side of the honeycomb structure is covered with a similar rubber sheet 5. To the thus coated core layer is then bonded on the underside, resp. the upper side a plate 8, resp. 9 from an aluminium-alloy t type ZICRAL or type 7075T6 and having a thickness of about 0,6 to 1 mm (tensile strength about 550 N/mm2) or from steel with a thickness of about 0,25 to 0,35 mm (tensile 2 strength about 1600 N/mm2). The honeycomb structure is cut beforehand to a shape close to the ultimate shape of the ski. Its thickness in the middle of the ski amounts to about 1,5 cm. When rapping on this bearing structure, it sounds dull, which indicates excellent damping. The manufacture of the ski in a mould as described hereinafter and with rubber sheets on the core layer offers the advantage that the shape of the core layer, as obtained by cutting should not be very pree cise because the rubber like material compensates for the irregularities in the cut shape during the bonding treatment. The ski is built up and consolidated in a mould to a solid structure with the desired shape in a very simple and cheap manner. Conventional preformed steel edges 14 and between them a polyethylene runner 12 cut to the appropriate size are placed in the bottom of the mould. On top of the steel edges 14 and runner 12 a rubber sheet 10 (eventually reinforced with e.g. cords) is disposed and on top thereof is placed the aluminium or steel plate 8 which on its turn is covered with the rubber layer 4 comprising the steel cords. Next, rubber strips or temperature resistant plastic strips 11 are arranged against the longitudinal upright walls of the mould. Further the honeycomb structure 1 (bezel) is placed interbetween these strips 11. The whole struc ture is again covered with a rubber sheet 5 comprising steel cords 6 and further covered'with a second alumi nium or steel plate 9. This plate 9 can be imprinted, painted or otherwise coated with a finishing layer or design if desired. The mould is subsequently closed and heated for a sufficient time at about 1500C (depending from the rubber composition) to bond the different layers to each other. After this consolidation treat ment the ski can be removed from the mould. It is also possible to have the rubber elements 5 and 11 (or 4 and 11) assembled in the form of one single sheet comprising a longitudinal area 4, resp. 5 in its center (and in which the steelcords 6 are embedded), which area is provided at both sides with two longitudinal strips 11 (preferably somewhat thicker) said strips having a width corresponding to the height of the upright walls of the ski. In this case the manufacturing process can start by putting plate 9 in the bottom of the mould and covering it with the rubbersheet comprising elements 5, 6 and 11 with the strips 11 upright against the longitudinal walls of the mould. Next the honeycomb structure is placed in the mould and the assembly is covered with sheet 4, 6, plate 8 and sheet 10. Finally the steel edges 14 are placed in the upper lateral corners of the mould and the runner 12 between them. The mould is then closed. The rubber sheets comprising elements 4, 6 and 5, 6 or 4, 6, 11 resp. 5, 6, 11 can be obtained in the usual manner by extrusion through a die slot. A number of parallel steel cords 6 is then embedded therein during extrusion. The steel cords 6 can also be supplied in the form of a woven fabric (steelcord in warp direction or in weft or both in warp and weft if desirable). To increase the strength of the ski, it can be useful to reinforce the ski side portions (11) in another manner. It is in general also possible to incorporate steel cords only in a part of the surface of sheet 4 (or 5) or otherwise to apply local reinforcements in another form than twisted fiber bundles. To provide an excellent bonding of the rubber layer to the core layer it can be advantageous to provide a composite rubber layer with a foam rubber structure on the rubber layer side facing the core layer. This can be achieved by co-extruding a normal rubber composition (while embedding fiber bundles therein) next to a rubber layer containing a foam generting agent. The foaming then occurs in this composite rubber layer during the bonding treatment by vulcanisation. In this way the anchoring of the rubber layer occurs through a greater depth in the surface portions of the honeycomb structure.

Claims: CLAIMS 1. An integral laminated structure with a low specific weight comprising a core layer (1), sandwiched between two outer layers, at least one surface of the core layer being covered with a layer (4) resp. (5) of rubberlike material in which twisted fiber bundles (6) are embedded, characterized in that said core layer (1) comprises a number of strips (2) locally connected to each other and delimiting intermediate spaces (3) therebetween which spaces extend across the thickness of the core and that a rigid plate (8) resp. (9) is bonded to the layer of rubberlike material on its side opposite to the core layer side. 2. Structure according to claim 1, wherein the core layer (1) comprises a honeycomb structure. 3. Structure according to claim 1 or 2, wherein the modulus of elasticity of the rigid plate and of the embedded fiber bundles have the same order of magnitude. 4. Structure according to claim 3, wherein this modulus of the embedded fiber bundles is at least 7.104 N/mm2 whereas their tensile strength is at least 2000 N/mm2. 5. Structure according to claim 4, wherein the twisted fiber bundles are steel cords. 6. Structure according to any of the previous claims, wherein the rigid plate is a metal plate. 7. Structure according to any of claims 1 to 5, wherein the rigid plate is a glass fiber reinforced resin sheet. 8. Structure according to any of the previous claims wherein the thickness of the layer of rubberlike material is between 1,33 and 4 times the fiber bundle diameter. 9. Structure according to any of the previous claims wherein the layer of rubber like material at its side facing the core layer has a foam structure. 10. Sliding shoe, such as a ski, characterized in that it comprises as a bearing structure an integral laminated structure according to any of the previous claims. 11. Sliding shoe according to claim 10, wherein the twisted fiber bundles (6) are embedded over at least a part of the surface of the sliding shoe in at least one of the layers (4) or (5). 12. Sliding shoe according to claim 11, wherein the twisted fiber bundles are steel cords.

Inventors: WIEME, ANDRE, Wieme, André
Applicants: N.V. BEKAERT S.A.
Application year: 1981
Main classification: A63C5
IPC codes: A63C5