Patent Application
20150246489
The invention relates to a composite material structures where the composite is composed of structural fibres embedded within a matrix polymer resin system. The invention particularly but not exclusively relates to methods and apparatus for manufacture of the structures. Use of the invention enables the construction of web type structures employing unidirectional fibres in which the webs pass through each other by the creation of a node. The invention also facilitates integration of the web structures into various different products and to products obtained by use of the methods.
Historically web type structures have been used to provide structural stiffness, for example in structures made of steel, cast iron, many alloys and plastics. Typical components incorporating web type structures include gear boxes, manhole covers, cabinet doors and open mesh grating. Processes such as moulding, machining from solids and casting can produce web structures effectively but in the case of steel and other materials supplied in lengths of constant cross section a method of connecting the webs is required. This incurs additional complexity, effort and cost.
Composite materials consisting of structural fibres such as glass or carbon fibres can be made efficiently by a pultrusion process. The sections made are typically of constant cross sectional shape. However it is difficult to efficiently make them into a finished product which effectively uses their structural properties. Ladders are a typical product made from pultruded glass fibre composite sections. However, they require additional effort in the form of drilling, bonding and mechanical fixing methods to complete a finished ladder.
The format of fibres typically found in composite products may fall into three main categories:
1. Random fibres either in chopped or chopped strand mat format.
2. Woven roving or stitch bonded fabrics.
3. Unidirectional fibres where all the fibres are in the same direction.
The format and quantity of fibres significantly influences the tensile strength of the product. In practice the fibre volume fraction is limited as each fibre should be coated with the matrix resin and the fibres have to be packed together. The packing density is influenced by the fibre format. Random fibres have the least packing density.
The process of manufacturing high strength composites typically found in the aeronautical industry involves taking woven laminates from which profiles are cut to predefined shapes. The laminates are then directly applied to a mould surface or assembled into a preform and then placed into a mould. The laminates may be pre-impregnated with a matrix resin and are then heat cured or contain only fibre and are infused with a matrix resin. The laminates typically contain fibres in two or three directions and are laid down to provide predetermined fibre orientations. Laminates of this type are referred to as bi-directional laminates and can have high tensile strength values, typically 300 Mpa for glass fibre and 440 Mpa for a carbon fibre laminate.
EP 1917401 discloses structures which achieve tensile strength values in the range 300 to 380 MPa.
Composite structures made from uni-directional fibres have significantly higher tensile strength values than bi-directional fibres, typically 800 Mpa for glass fibre and 1000 Mpa for carbon fibre. Products made from uni-directional fibres have been limited to items such as golf club shafts, fishing rods and spokes supporting cryogenic units. To date the technology to build more complex structures where elements of the structure are required to pass through each other has not been available.
To build the type of structures that this technology can facilitate requires creating fibre preforms of complex fibre architecture. It is the object of the present invention to provide a method of apparatus for automatically assembling uni-directional fibres into a preform of fibres than can be handled, placed into a mould or become part of a more complex preform.
According to a first aspect of this invention, a method of forming a fibre reinforced polymer structure comprises the steps of:
providing a first bundle of fibres arranged in an array and having a first fibre volume fraction (FVF);
forming a node region in the bundle wherein a transverse dimension of the first bundle is increased and a second perpendicular dimension of the bundle is decreased so that the first FVF is maintained at a substantially constant value;
providing a second bundle of fibres extending in angular relation to the first bundle, arranged in an array and having a second FVF;
forming a node region in the second bundle wherein a transverse dimension of the second bundle is increased and a perpendicular dimension of the second bundle is decreased so that the second FVF is maintained at an approximately constant value;
wherein the node region of the second bundle overlies the node region of the first bundle to form an assembly;
infusing the assembly with a polymer resin; and
curing the resin to form the structure.
Preferably, the step of overlying first and second bundles is repeated to form a structure having a multi-layered node.
The transverse dimension of each bundle is preferably increased from the transverse dimension or width of an internodal body of the bundle to a transverse nodal dimension or width, with an intermediate portion of increasing transverse dimension or width;
wherein the perpendicular dimension or height decreases from the perpendicular dimension or height of the internodal bundle to a perpendicular nodal dimension or height with an intermediate portion of decreasing transitional perpendicular dimension or height.
Advantageous methods include the step of providing an array of thermally fusible fibres during formation of a node region.
The fusible fibres may be used to secure the bundles together by microbonding to form a preform.
The array of fusible fibres may be laid upon the first bundle as a web or as a warp or waft of parallel or unidirectional fibres. Alternatively, the fusible fibres may be formed as a skeleton having a configuration of the completed structure. In a further alternative embodiment, a fusible sheet is applied between the first and second bundles.
The fusible fibres may be deployed on a first bundle before laying a second bundle onto the fusible fibres. This results in the fusible fibres being sandwiched between adjacent bundles.
Alternatively, the fusible fibres may be interleaved between rovings of each bundle, preferably extending transversely of the bundles. For example, the fusible fibres may be layed as weft fibres. Use of a woven arrangement has the advantage of binding the fibres together in the node region.
Alternatively, individual fusible fibres or bundles of fusible fibres may be laid parallel to a bundle or may be formed into a skeleton or thin sheet for application between adjacent bundles.
The method may further comprise the step of applying heat and pressure to the assembly to form a bonded node preform.
The assembly may be passed through a heated nip, for example between heated rollers or between a heated roller and support surface. Alternatively, a heated press may be used.
Preferably cooling means are provided for rapidly solidifying the fusible fibres. Air jets or other gaseous cooling means may be employed. Alternatively, the structure may be passed between cooled rollers. In an embodiment in which the assembly comprises an array of nodes, for example a grating, cooling gas may be passed through apertures between the nodes to promote rapid solidification of the composite structure.
According to a second aspect of the present invention, fibre reinforced polymer structure manufacturing apparatus comprises:
a first feeder for providing a first bundle of fibres in a first direction;
a second feeder for providing a second bundle of fibres in a second direction;
wherein the first and second directions are disposed in angular relation; and arranged so that the first and second bundles are overlaid in a node region;
the apparatus further comprising first and second dies, each having a variable lateral and perpendicular dimension, arranged so that the first and second bundles pass through a respective die at or adjacent the node region to control the lateral and perpendicular dimensions of the bundle;
wherein the node regions are superimposed to form an uncured assembly; and heating means arranged to provide heat and pressure to bond the assembly to form a fibre reinforced structure.
The structure may comprise a preform for an engineering component. The apparatus of this invention allows manufacture of performs with a high degree of structure and dimensional accuracy and consistency.
The die may advantageously comprise a channel together with means for adjusting a lateral and perpendicular dimension of the channel in the node region.
In a particularly preferred embodiment, the die comprises a rotatable cam, preferably a cylindrical rotatable cam having a circumferential channel with a variable lateral and perpendicular dimension. Conveniently the channel has variable width and depth.
A stepper motor or other reversibly controllable drive means may be arranged to rotate the cam in both a forward and reverse direction with respect to the direction of fibre feed so that a channel with variable dimensions is presented to the bundle as it reaches the node region.
In the preferred method, each die may be rotated either with or against the direction of travel of the bundle to alter the dimensions of the channel through which the bundle passes, the die being returned to a starting point after each node has been formed and laid upon the preceding bundle. The vertical dimension or depth of the channel may be selected so that a bundle is urged into engagement with the underlying bundle.
Alternatively, a die having an adjustable orifice may be employed.
Apparatus for manufacture of grating or lattice structures may have two or more arrays of feeders disposed in angular relation and arranged to lay bundles successively to build up a multi-layer node structure.
Structural fibres used in the production of high strength composite products are commonly supplied in creels which are wound from a continuous length of direct roving. A typical creel may have a weight of 20 Kg. The direct roving consists of a number of fibres lightly bonded together by a sizing agent and are normally measured in a unit such as Tex. For example, a direct roving defined as 1200 Tex would have a weight of 1200 grams per kilometre (g/km).
The invention may further comprise a fusible fibre feeder arranged to lay an array of fusible fibres upon each bundle of first and second fibres in the node region.
In a preferred embodiment, the bundle comprises a plurality of parallel rovings or tapes of fibres laid with an array of fusible fibres alternating above and below adjacent rovings. For example, a bundle having a width of 5 mm may comprise 12 rovings of 600 Tex. The fusible fibres may be interleaved or woven between adjacent tapes. The fusible fibres may be provided as a skeleton laid between adjacent tapes. The skeleton preferably has a width equal to or less than the width of the bundle.
The manufacturing process commences with laying dry fibres in alternate layers to create the webs. As the fibres enter the node they are spread into their appropriate shape necessary to pass through each node. The shape is maintained by either micro bonding or stitching the fibres together and in this way a preform of fibres is constructed. The preform is then placed in either a mould or may be assembled into a more complex preform which is then assembled into a mould. The mould is closed and sealed and then infused with catalysed resin. On completion of the curing stage the mould is opened and the component is removed. At this point the component will only require removal of flash and if required the addition of ancillary components.
The invention provides a construction method that allows webs of composite material to pass though each other by the creation of a node, within which bundles of structural fibres that form the webs and typically comprise unidirectional fibres, are permitted to change their cross sectional shape so that in both webs their cross sectional area and therefore their fibre volume fraction remains constant as each web passes through the node. In this manner the fibre volume fraction can be maintained at its desired value.
According to a third aspect of the present invention, a fibre reinforced polymer structure manufactured by a method in accordance with the first aspect of this invention or using apparatus in accordance with the second aspect of this invention comprises a plurality of first and second bundles of substantially unidirectional fibres;
a first bundle overlying a second bundle in angular relation to form a node;
wherein the fibre volume fraction (FVF) of each bundle remains approximately constant as it passes through the node.
This invention may provide a fibre reinforced polymer structure comprising a junction where two webs pass through each other that permits the tensile strength properties of each web to be retained.
The fibre volume fraction (FVF) may be defined as the volume of fibre as a proportion of the total composite volume or as the ratio of the cross-sectional areas. FVF may be determined by chemical matrix digestion, photomicroscopy or by calculation.
A structure in accordance with the present invention may have a FVF of about 40% to about 60%, preferably about 50% to about 60%.
In some embodiments of the invention, the FVF values of the first and second bundles are approximately the same. In this case the FVF of the node may be approximately the same as the FVF of each bundle. This avoids unnecessary interlaminar shear problems.
Alternatively, the FVF of the first bundle may be different to the FVF of the second bundle, with the total FVF of the node being a mean or intermediate value of the components.
For the purposes of simplicity, the composite structure will be described with horizontally extending bundles arranged in a vertical stack. However, overlying bundles may extend vertically or may extend in a curved direction, for example if the structure is assembled on a roller or other cylindrical surface. The bundles may also be assembled on a conical or spherical surface.
The FVF may change vertically across the node although the FVF of each bundle remains approximately constant. Preferably, the total FVF remains approximately constant across all bundles within a node.
By use of the terms substantially constant or approximately constant it is intended that the FVF may have a tolerance or margin of variation of ±10%, preferably ±5%, more preferably ±2%.
The angle between the first and second bundles is preferably 90°. Alternatively, an angle of 30°, 40°, 60° or other suitable angle may be employed.
The angle between adjacent bundles is preferably constant in the vertical direction from the top to the bottom of the node. However, second bundles may be overlaid at two or more different angles, for examples 30° and 60°, relative to the direction of the first bundles.
In order to maintain a constant FVF, the height of each bundle may decrease at the node with the dimension in the transverse direction or width increasing in inverse proportion. In a first embodiment in which two similar bundles are overlain at 90°, the height of each bundle may be reduced by 50% at the node with the width increasing by 100% in order to maintain a constant FVF. Preferably, a constant cross sectional area is maintained in each bundle.
The fibres of each bundle may be splayed or displaced laterally as the fibres approach the node. Each bundle of fibres may have a median, longitudinal axis, the displacement of each fibre being proportional to the distance from the axis. Fibres, particularly glass fibres, may be displaced at an angle up to 30°, typically 20°, without kinking or deformation.
The incremental angular displacement between adjacent fibres is small so that interlaminar shear forces are minimised. Preferably, the angular displacement is less than 5°, typically 1% or less. Small incremental angular displacements reduce interlaminar shear forces so that a value approaching zero is obtained.
In order to achieve a desired fibre structure at a junction where webs pass through it is necessary for fibres to make angular changes relative to each other.
These changes of angle can be made without loss of strength properties, providing that the change between individual fibres is small. For example, if a layer of fibres is 5 mm wide and the fibre architecture requires that the fibre of the outer layer on one side is to deviate by 20 degrees from the fibre at the opposite side, then the angular change can be evaluated. Assuming the fibre diameter is 17 micron, then across the width of the layer of fibres there will be 294 fibres. The angular change of each fibre relative to its adjacent fibre is equal to 20/294, producing an angular change of 0.068 degree between each fibre. This can be considered as an infinitesimally small change of angle.
In particularly preferred embodiments, the following requirements are be met;
In a particularly preferred embodiment, a node is characterised as a region in which all of the fibres of a first bundle pass over all of the fibres of a second bundle, wherein all of the fibres of each bundle are parallel, crimp free and have a constant fibre volume fraction (FVF) with no change in direction of each fibre relative to adjacent fibres in the bundle.
It is particularly advantageous if the change in direction of a fibre relative to an adjacent fibre of the bundle is less than 0.5°, especially less than 0.01°.
The distance from the axis of the fibres of a first bundle may increase in a direction towards the node until a region of overlap (defined as a node) with fibres of a second bundle is reached. The region of overlap or node may be rectangular or square in plan view for perpendicular bundles with the fibres of each bundle extending in parallel spaced relation perpendicular to the fibres of an adjacent bundle. Where the bundles are the same size, the region of overlap may be square.
The presence of transverse fibres for non-perpendicular arrangements or perpendicular fibres for rectilinear arrangements strengthens the node by providing a restoring force reducing the tendency of the fibres in a bundle to move closer when under tension. Interlaminar strains are relieved and the FVF within the node remains constant.
Various types of fibre may be employed, including aramid, carbon fibre, S-glass and E-glass. The matrix polymer resin may be a thermosetting polymer, or a thermoplastic polymer or blend having sufficient melt flow to permit the polymer to be shaped at an elevated temperature below the temperature at which it degrades. Thermosetting polymers may be selected from polyester, acrylic, polyurethane and hybrid resin combinations. Polyethers, polyetherketones and polyetheretherketones, for example as sold under the registered trade mark VICTREX may be used. An additional advantage of the invention is that webs can be formed from more than one fibre type allowing specific properties and economic criteria to be achieved.
Fibres may be provided as tapes comprising a monolayer of fibres or a thickness of a small number of fibres, for example 1 to 5 or 1 to 10 fibres. These tapes may be laid alternately to provide a closely integrated nodal structure, for example in which each row of fibres is adjacent rows of fibres extending in a different direction.
Co-extruded fibres such as DYNEEMA (registered trade mark of DuPont) may be employed. Such fibres may be thermally set in a moulding tool.
In a particularly preferred embodiment, the node further comprises an array of thermally fusible fibres located between first and second bundles. The fusible fibres may be arranged to bond the bundles, for example, by microbonding.
The fusible fibres may be laid as an array between the first and second bundles, for example as a skeleton, web, warp or weft during assembly of the node.
Alternatively, a bundle may incorporate a proportion of fusible fibres, for example located on an outer surface of the bundle and arranged to contact an adjacent bundle during assembly of the structure.
The fusible fibres may be composed entirely of a fusible material having a melting temperature in the range 60° C. to 160° C. Various polymers or blends such as polyester, copolyester or polyamide may be used. Alternatively, combination yarns may be used, comprising a high tenacity carrier thread coated with a fusible coating such as low melting point polyamide. Suitable fibres are manufactured by EMS-Chemie AG under the registered trade mark GRILON.
Functional devices may be incorporated into one or more of the bundles. For example, sensors or identity devices may be included to allow remote checking of the security or location of the structure, for example of a cover for an access chamber.
The dimensional configuration of the node may be precisely determined by the need to maintain the fibre volume fraction of the fibres in the webs as they pass over each other in the node and accommodate the changing width of the webs as they approach the node. If the webs are of different thickness the node shape may be changed accordingly. In the event that webs approach each other at an angle of less than 90° the node shape is modified so that the fibre volume fraction can be maintained at a constant value.
In preferred embodiments of the invention the fibre volume fraction of the web is maintained as it passes through the node. However individual design requirements may dictate that some webs do not have the same structural requirement and may employ a different fibre volume fraction compared to the web they pass through. In such a case, the node design may be adapted.
In a further embodiment of the invention it may be advantageous to incorporate a larger web in one direction and a smaller web in another direction and only where the smaller web passes through the larger web will a node be required.
The use of a node through which the fibres of both webs pass serves to maintain the structural properties of the web as there is no change in fibre volume fraction. There may be a deviation from linearity of the fibres as they approach the junction within the node where the fibres from both webs pass over each other. Tensile testing of webs has shown no reduction in strength of the webs and that the strength of the nodes benefits from the interaction of fibres from each web.
According to a fourth aspect of the present invention, an engineering component comprises a fibre reinforced polymer structure in accordance with the third aspect of this invention.
According to a fifth aspect of the present invention, a fibre preform for an engineering component comprises a fibre reinforced structure in accordance with the third aspect of this invention, wherein the bundles are bonded together. The bundles may be bonded, for example, by microbonding.
The engineering component may comprise a structural member or load bearing panel, joist, beam or support. A load bearing work surface or cover may comprise the structure together with a suitable surface layer.
Engineering components in accordance with this invention find applications in manufacture of automobile chassis members and structural components, aerospace components and civil engineering components. Marine or other water resistant components may be provided.
The engineering component may comprise an array of first and second bundles, each arranged in parallel spaced relation, for example, to form a lattice. Such gratings may be used for access chambers.
The component may comprise a grating having apertures between adjacent bundles.
The invention is further described, by means of example but not in any limitative sense, with reference to the accompanying drawings, of which:—
In each of the embodiments, the layers of fibres of the composite structure extending in the 0° direction overlap the layers of fibres extending 90° at a node point (11). In each of the embodiments the shape of the layers of fibres (7,8,9,10) is altered at the node point (11) to enable the layers to overlap each other without varying the fibre volume fraction. In
As the layers of fibre (7,8,9,10) progressively change shape as they approach the node (11) small resin rich areas (14) develop within the structure. In order to keep the size of these resin rich areas to a minimum, the thickness (12) of each fibre layer (7,8,9,10) is kept small, typically 0.5 mm, and hence at the node (11) the height of the resin rich area (14) may be 0.25 mm. It has been found that the fibre volume fraction of each layer remains constant as they approach the node and even though their shape changes.
In
The structure shown in
In
The structure shown in
A second bundle (41) overlies the first bundle (34). The second bundle (41) has intermediate regions (42,43) and a nodal region (35). In the nodal region the second bundle of fibres extend in parallel, perpendicular to the first bundle of fibres, to form a square overlapping node. The fusible fibres (40) are fusibly engaged and secure with the fusible fibres (44) of the second bundle in the nodal region, for example, at (45,46). The layers of fibres may be further secured when the preform is moulded into the finished engineering component or other product. The fibres of one bundle therefore provide perpendicular support to the fibres of adjacent bundles, relieving forces which may urge the fibres to move towards each other when the structure is under tension.
In the illustrated embodiment, the first and second bundles have the same dimensions so that the node (35) is square in plan view. The angle (47) between the parallel internodal fibres (35) and the fibres in the intermediate region (38) may be 20°-30°. This angle is preferably maintained at a minimum value to reduce interlaminar strains. However, nodal bonding (45) of adjacent fibres allows use of relatively large angles (47).
The apparatus of this invention may permit a number of bundles or layers of fibres to be assembled with the transverse and perpendicular dimensions changed at each node in order to achieve a desired configuration. As each layer of fibres is laid down, it may be microbonded to retain the shape relative to the previously laid down layers in order to form a complete fibre preform.
Two arrays of fibre bundle laying heads (63,64) are located along edges of the platen in the X and Y directions. The fibre bundles may be delivered from the heads in the form of tapes. The bundle laying process is carried out by positioning and raising the platen (60) under the row of X direction heads (64). The platen (60) is moved an appropriate distance in the Y direction to lay a series of parallel bundles (66) on the tray (61) to produce the Y axis webs of the preform. The bundles are then cut by cutting apparatus (not shown). The platen (60) is then lowered in the Z direction and a tray is positioned under the Y heads. The bundle laying process is then carried out in a similar manner to produce the X axis webs of the preform (65). The process may be repeated as many times as necessary to build up the layers of preform to a suitable thickness and configuration.
In an alternative embodiment, a single array of tape heads is employed with the platen being rotated through 90° between alternate bundle laying stages.
In
In
The direct rovings or fibres may need conditioning to remove crimping induced during manufacture of the creel. This can be done by passing the rovings through rollers.
The preform assembly (78) passes downstream from the guidewheel (67) (to the left as shown in
The structure of the skeleton (17) of fusible fibres is shown in
When the skeleton has been heat treated the structural fibres of bundle (78) are held in the correct position. The consecutive layers of bundles of the assembly may be bonded together to form a semi-rigid preform.
The rotation of the guidewheel (67) is synchronised to the linear movement of platen (60) and tray (61). This permits the fibre architecture to be configured in a precisely controlled predetermined manner to form a microbonded preform.
As the structural fibres of bundle (78) continue to be laid down as a result of the linear movement of the platen (in the direction of arrow C in
In
Hot shoe (81) includes heater elements (87) to maintain a controlled elevated temperature. Alternatively or in addition, an array of holes may be provided to permit circulation of heated air through the bundle (78 in order to provide a more rapid and controllably responsive force of heat.
When an appropriate length of bundle (78 has been laid, it is cut to length. The process is then repeated for the X and Y axes as necessary to build up a desired bundle of layers microbonded together and laid out in each direction.
The X, Y and Z motion of the platen (60 is provided by stepping motors (not shown). By this means, the discreet location of the platen (60) in all three axes is known and may be controlled by a microprocessor control unit during the manufacturing process. The angular orientation of the guidewheel (12) is also controlled by a stepping motor. Linking of the four motions by the control unit allows the cross-sectional dimensions of each bundle to be controlled at any particular location within the preform (79,80).
Specific programmes for the microprocessor and the number and location of the guidewheels may be changed to facilitate production of a wide variety of composite structures.
A typical gully grating made in accordance with the invention may provide a clear opening of 450×450 mm and be suitable to support a load of 400 KN as defined in European Standard E124:1994.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1215124.7 | Aug 2012 | GB | national |
| 13156456.9 | Feb 2013 | EP | regional |
| 1303183.6 | Feb 2013 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2013/067560 | 8/23/2013 | WO | 00 |
