Fiber Reinforced Cores
Fiber Reinforced Cores achieve high performance by incorporation of the most cost effective FRP structural element of glass fibers in combination with low cost closed cell foams, plus a process that permits specific geometry variables of fiber weight angle and frequency to be specified as required.
Composite Materials are commonly referred to as Fiber Reinforced Plastics, and Glass fiber dominates all other reinforcements due to an unchallenged ratio of properties to cost. Large composite structures typically incorporate technical fabrics and sandwich structures in order to optimize structural performance and weight. Nonetheless, Core Materials have most often been based upon unreinforced resins such as PVC Foam and Polypropylene Honeycomb.
The question may be considered; If glass fiber reinforced plastics are so trusted for composite structures, why are core materials not similarly fiber reinforced? It may be recognized that for structures created of traditional materials, both flange and web are commonly produced of the same metal, as joists and planking may be each of wood, and beams of steel reinforced concrete incorporate the rebar throughout both flange and web.
The use of reinforcement fibers to provide the principle load bearing demands of core materials have actually been the object of significant development over the past decade. The challenge of how best to achieve the fiber geometry that can most efficiently address the load requirements at low density, and also be suitable for economical processing methods has been addressed by many researchers and commercial organizations.
Patents provide insight as each new revindication challenges previous thinking or addresses shortcomings of preceding inventions. Fundamental among the patents related to using fiber reinforced cores are the fiber geometries established in the Z Direction, how these fiber architectures are created, and how finished composite structures are processed. Not surprisingly, much work in this field has roots in aerospace structures, and is also based upon aerospace materials and processing methods.
The use of Z-Pins is a method of pre-cured pins made of pultruded carbon fiber which may be inserted through prepregs and foam, and then co-cured. Analysis of inclination angles with insertion at both plus and minus angles, with the pins being skew to both the X-Z and Y-Z planes, are used to consider geometrical forms including tetrahedral (four sided pyramids), truncated pyramids. Definition is made of anisotropic materials constructed along a single axis, or along both the X and Y axes simultaneously to produce an isotropic material.
Further work identifies that the truss network established by the Z Pins are able to support the structure of a panel entirely in the absence of the foam subsequently removed that was used to support the creation of the structure. Nonetheless, the processing with Z-Pins in practice has been expensive to position, and the technique is principally limited to autoclave processing of aerospace structures.
As process technology of Closed Molding has advanced, it is used for increasingly large objects, and with more economical tooling for smaller series part runs as well. Closed Molding is increasingly used for sandwich structures in a wide variety of commercial applications. These process developments are enabling technologies for a new generation of fiber reinforced core materials based upon dry fibers and liquid molding with the use of vacuum to assure impregnation.
Fiber Reinforced Core materials intended for liquid resin molding incorporate low density closed cell foams. The foam must provide the support to assure the geometry of the dry fibers, and also assure the displacement of resin when molding with vacuum. In addition, for molding with a single sided tool and a vacuum bag, the foam must provide sufficient compression resistance so as to not collapse under pressure up to one atmosphere. The most cost effective types of foams that meet this requirement include Polyurethane (PU) and extruded Polystyrene (XPS). More fire resistant foams that meet these same requirements include Polyisocyanurate (PIR) and Phenolic (Ph) foams. Each of these rigid foams is produced in large quantities principally for thermal insulation.
The rigid foams identified above can each provide sufficient resistance to vacuum pressure for foam densities as low as 32 Kg/m3 (2pcf). Due to poor mechanical properties of these low density foams, panel structures are typically produced with studs to provide the required shear-web properties, so the foam is incorporated into the panel for thermal properties alone. When fibers are incorporated into the foam, and infused with resin to provide composite properties of FRP, the fiber architecture is designed to eliminate the use of studs and perimeter frames. Not only can the fiber reinforced foam structure actually have better thermal properties by elimination of the thermal shunt associated with studs, but the global structure may be stronger, lighter, and even more cost effective than the original panel. This resulting construction with fiber reinforced cores is well adapted for a wide variety of cost performance competitive applications.
When molding with a fixed cavity tool, such as for vacuum assisted Light RTM, a soft compliant closed cell foam may be incorporated. In this case the tool may have vacuum applied without losing cross section, so the previous requirement that the foam be rigid for use with vacuum infusion, is not a restriction with LRTM. However, it would otherwise be possible for resin injection to cause collapse of a low density foam, which identifies the enabling technology of LRTM when injection pressure is controlled by automated monitoring of cavity pressure. The ability to use a soft compliant closed cell foam not only provides the benefit that the core material can easily conform to shaped parts, but also adapt to variable cross sections that include tapered edges and style lines. The most cost effective closed cell foam is made of Polyethylene (PE), and is produced in large quantities for use in packaging. Other thermoplastic foams include Polypropylene (PP) as well as specialty blends and cross linked foams that can provide features of increased temperature resistance while still being compliant for molding complex shapes.
Methods of incorporating dry fibers into closed cell foams have used various textile processes. This has included the use of fabrics placed between adjacent pieces of foam, or filament winding of dry fiber wrapped helicoidally about strips of foam, either of which will create webs. The fibers within these webs are oriented in positive and negative angles to provide shear strength. Alternatively, fiber insertion techniques such as stitching or tufting can be used to place fiber tows as discreet pins or struts that join opposing surfaces. These may also be inserted in rows with positive and negative angles to create a fiber architecture analogous to truss designs. Angles of 45 degrees will optimize shear capacity for flexural stiffness, which is most often the preferred choice. Increasing angles may provide an increase in compression with some loss of shear, which may be appropriate for severely loaded objects of short spans. Products made with orthogonal (90 degree) orientations suffer from shear performance little better than the base foam, and may also suffer damage to the skin laminates when heavily loaded in compression. Figure 1 is a Section of NidaFusion STO with foam removed identifying process variables of angle and step length.
It should be noted that vacuum infusion is often sited for the benefit of consolidating fiber volumes to achieve very high fiber to resin ratios. However, these compaction forces act only in plane, and materials introduced as webs do not realize this same benefit. Care must be taken to maintain the tight packing of adjacent pieces to assure that efficient web structures are created. Fiber insertion methods provide direct control of each fabrication variable including the dimension of the individual tow, the angle and the frequency of insertion. In addition, when the fiber insertion methods are used with the flexible foams, there is no need for scoring and the associated weight gain of excess resin filling wedge shaped gaps is prevented. Figure 2 demonstrates NidaFusion STF applied to a LRTM Counter-Mold.
Consideration must be given of how the web materials join the surface materials. Early methods with fabric to create webs did not achieve sufficient stress transfer, and other techniques may still be reliant upon first layer adhesion. The fiber insertion method provides an advantage that each tow of the resulting truss network is made to traverse the first layer surface material and become trapped between it and the layers of surface lamination. This can achieve a high level of mechanical integration which avoids the risk of surface layer delamination and provides exceptional damage tolerance.
These methods of parallel webs or trusses confer directional properties termed anisotropic. Care must be taken to account for this orientation in designing applications much as joists are oriented in a deck. The structure may be efficient when oriented as designed, but the material does not permit a random placement like that of typical structural core materials that have similar properties in all directions termed isotropic.
In some cases where it would be beneficial to have isotropic properties, bidirectional orientations of the fiber insertions may be used. Although the methods of creating webs with fabrics or winding may not be conducive to cross directional fabrication, the methods may be combined so that the fiber insertion method is used for the second axis. This is not without draw backs, notably increased density and cost, due to the virtual doubling of fiber insertions and processing. The higher cost of isotropy has heretofore penalized the commercialization of fiber reinforced core made accordingly.
At the most recent JEC of 2009, 3M presented a new Fiber Reinforced Core which achieves isotropy at a lower cost and density than for the fully bidirectional technique. This material is proposed both for Infusion based upon rigid foams as well as for LRTM using flexible foams. These products are identified as NidaFusion SXO and SXF respectively. Figure 3 is a Top View of NidaFusion SXO. Figure 4 is a comparison of Nida-Fusion Density per Step Length for either standard nonisotropic, isotropic, and bidirectional products. Figure 5 is a schematic representation of the isotropic core structure demonstrating fiber insertions on three axis creating a truncated trihedral pattern.
Fiber Reinforced Cores incorporate the most cost effective FRP structural element of glass fibers in combination with low cost closed cell foams associated with high volume applications of insulation or packaging, plus a process that permits specific geometry variables of fiber weight angle and frequency to be specified as required. These materials are adapted to the two most widely adopted closed molding processes. Response to the original question is unequivocal: Fiber Reinforced Cores can be trusted to provide excellent properties at low cost.
Nida-Fusion STO Complex
Used for production of planar sandwich structures, Nida-Fusion STO complex is made of :
- Rigid foam core with closed cells to prevent resin absorption,
- Fibres reinforcement on each side of the foam,
- Fiberglass roving, which goes through them obliquely, forming triangulations Truss Network.
These foams have an excellent insulation factor: between 0.018 and 0.023 W/m°K
Polyurethane foams (PU)
These are the most commonly used on the market..
- Length : from 1.5 to 3.2 meters, according to customers needs
- Width : 1.25 meter
- Thickness : 10 to 65 mm, according to customers needs
Polyisocyanurate foams (PI)
With the same dimensions as the polyurethane foam, they have a good fire resistance :
- Standard NF 92501 : M1
- Standard DIN 4102 : B2
- Standard ISO 3582 ou BS 4735 : max.mm 10
Brankkennziffer : 5.3
Nida-Fusion STO Complex before and after impregnation with cavity partial of foam.
Phenolic foams (PH)
This kind of foam has a very high fire resistance without any toxic fumes.
- Standard NF 92501 : M1, (F : under approval)
- Standard DIN 4102 : B2
- Standard iso 3582 or BS 4735 : max mm 10
This type of foam is easily damaged and must be handled with care before impregnation.
- Length : 1.00 meter
- Width : 1.25 meter
- Thickness : from 20 to 65 mm, according to customers needs
All the reinforcements available on the market can be used (fibreglass, aramid, carbon) :
- Woven fabrics,
- Non woven fabrics,
The Nida-Fusion STO complex is produced with only one layer of reinforcement on each side.
The triangulations of the sandwich material are characterised by :
The spacing corresponds to the distance between two stitches that have the same orientation. It can vary from 10 to 60 mm.
More the spacing of triangulations will be small, better will be the mechanical properties. However, this core will be heavier.
Two possibilities :
All the stitching lines are parallel to the length of the sheet.
Flexural strength is good in the length side of the sheet.
Off Axis orientation
All the stitching lines are oriented with a 30 degrees angle to the length of the sheet.
This configuration gives good bending resistance across the width of the core as well as along the length.
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