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The grooves, perforations, and scores of a typical infusion core, designed to enable resin flow, are filled with a media (resin) having remarkably different physical and mechanical characteristics from either the skin or the core. These regions exhibit divergent stress characteristics leading to a knockdown in the specific-density shear properties of the core. The use of a three-dimensional spacer fabric at the skin to core interface to enable infusion flow facilitates the use of a plain core while improving the skin to core bond.
(Published on March - April 2008 – JEC Magazine #39)
PATRICKMACK, CHIEF TECHNOLOGIST, POLYNOVA COMPOSITES
Despite their many advantages in applications ranging from wind energy to yacht construction to civil engineering and aerospace, fibre reinforced plastic (FRP) laminates are commonly inferior to low-density materials such as wood when considering a part’s ability to resist bending moments. For example, since the stiffness of a panel is dependent not only on the material’s flexural modulus, but also on the thickness of the panel, one approach for stiffening an FRP panel would simply be to make it thicker. However, this approach will yield a further increase in weight and potentially unnecessary strength characteristics, construction difficulties, and prohibitive costs in consideration of the final desired structure.
Sandwich construction for greater stiffness
A preferred technique to increase the stiffness of an FRP panel is the use of a sandwich construction. In a laminate, sandwich construction offers the comparable advantages of an I-beam configuration, but instead of the web and flanges of a typical Ibeam, it makes use of a lightweight core material placed between layers of reinforcement, commonly referred to as skins. The role of the skins in the composite structure is to withstand the bending moments on the panel or beam by resisting the compressive and tensile loading set up in the opposite skins when the panel is subjected to bending load forces.
In order to resist the bending moments, the skins must be rigidly held in a predetermined position relative to the neutral axis of the sandwich (the centreline) and be prevented from moving relative to each other. It is the task of the selected core material and of the strength of the bond line between the skins and the core to provide In order to resist the bending moments, the skins must be rigidly held in a predetermined position relative to the neutral axis of the sandwich (the centreline) and be prevented from moving relative to each other. It is the task of the selected core material and of the strength of the bond line between the skins and the core to provide
The use of three-dimensional spacer fabrics designed for vacuum infusion, such as Polynova Composites’ Polybeam® Infusion Flow Reinforcement™ (IFR™), provides an opportunity to maximize the skin-to-core bond while maintaining the physical integrity of the core. IFR™ identifies a class of textiles that both facilitate infusion flow and contribute to the laminate as a constituent material. The Polybeam® family of spacer fabrics applies the wide design latitude of this textile family to solving the needs of the composite marketplace as an IFR™.
Three-dimensional spacer fabrics are a class of textiles comprised of two parallel “X/Y” planes of fibre fascia that are separated to a consistent “free-form” thickness by columns of Zdirectional fibres. Figure 1 schematically illustrates the three-dimensional spacer fabric in a free or relaxed form. As shown in this illustration, a pair of outer fabric layers (1, 2), generally woven or knitted and usually lying in the respective X - Y planes, are separated by a plurality of fibres (3) generally lying in a “Z” direction.
The overall thickness dimension (d) of the fabric may most usefully be between about 1 or 2 mm up to about 25 or 30 mm, and in some cases even thicker. Frequently, the fibre density for the “Z” fibres may be only a minor fraction of the fibre density of the outer layers (1, 2), but this may also vary according to the architecture and fibre population of the outer layers. The outer layers may range from an open honeycomb structure to a more tightly woven or knitted structure. The IFR™ spacer fabrics for infusion are generally designed to collapse under vacuum pressure, as illustrated in Figure 2, where d is reduced to d’. However, even though collapsed, they are designed such that there remains sufficient mean free path within the collapsed cross-section to facilitate the rapid distribution of resin throughout the structure, including the surrounding and adjacent plies of the entire laminae. The spacer fabric is in the collapsed state at the end of the infusion process and remains collapsed in the final part.
When an IFR™ spacer fabric having a tightly woven or knitted X,Y fascia and resilient Z-direction fibres is used as a lamina between the skin and the core, the resilient nature of the Z-fibres enables “spring back” that enhances the interfacial planarity of the bond line between the core and adjacent laminae, as illustrated in Figure 3.
Water ski application
By way of example, Liquid Access of Melbourne, Florida, USA, a leader in adaptive water ski design, manu-facturing, and instruction, turned to Polybeam® IFR™ enabled sandwich construction to meet the demands of a unique water ski in a cost-effective manner.
Developed in collaboration with World and National Disabled Water Ski Champion, Ann O’Brine-Satterfield, the company’s Sit-Skis enable men and women with a variety of disabilities to pursue the full sport of water skiing from beginner through jump and competition slalom. To date, several hundred skis have been produced and successfully fielded using this technology.
In manufacturing the Sit-Skis, the gelcoat is first sprayed into a standard female mould with a 5-cm flange. As shown in Figure 4, unidirectional carbon fibre is laid into the mould, followed by the Polybeam® IFR™.
A plain sheet of 1.3-cm foam core precut to fit the mould is then laid in, followed by another ply of the Polybeam® IFR™. Next, the biaxial fibreglass lamina and a semi-rigid gelcoated floating counter tool (float tool) are placed over the ply stack. The term “floating” refers to the free position of the counter tool within the vacuum envelope. The IFR™ extends beyond the float tool to accommodate the resin feed and vacuum port lines. The resin feed and vacuum ports are then appropriately placed and the infusion proceeds. The ski is released at the end of the cycle and trimmed for delivery.
Enveloping the core with the spacer fabric lends numerous benefits to the sandwich composite beyond enabling the infusion process. The use of the spacer fabric between the core and adjacent lamina enhances the bond strength at this critical interface by rendering the two surfaces planar while accommodating interfacial inconsistencies and allowing efficient energy transfer through the Z fibres.
The spacer fabric also assures resin flow on both sides of the laminae. A key benefit derived from the use of the IFR™ spacer fabric in this application is that it allows the use of a plain core over an infusion (grooved, perforated, scored) core. This not only eliminates the weight gain and cost associated with an infusion core, but also assures the core properties are maintained. Derived from a recent study using Airex C70.75 core and Ashland AME6000 INF vinyl ester resin, Figure 5 graphically depicts the specific density shear strength knockdown achieved when using resin-infused double cut core versus a plain core of like density. Here, we can see that plain core and post-infused double cut core of equal “pre-infused density” yield comparable shear properties; however, the resin more than doubles the density of the double cut core. When comparing similar density cores, the plain version will have superior shear properties.
As a further consideration, the grooves, perforations, and scores of an infusion core are filled with a medium (resin) having remarkably different mechanical and strength characteristics from either the skin or the core, resulting in a region of divergent stress characteristics – a concern for Liquid Access due to the fatigue and impact loading of the application.
The experience with Liquid Access may have implications in other applications, such as in the design of large wind turbine blades. Research conducted at the Technical University of Denmark suggests that, as wind turbine blades continue to grow in size, the application of sandwich construction over traditional single-skin composites in the flanges of the load-carrying spar has the potential to offer weight reductions upwards of 22% with increased buckling capacity. Although this evolutionary step in design shows great potential, the study assumes perfect bonding of the laminae and further indicates sensitivity to imperfections. The use of an IFR™ spacer fabric at the skin-to-core interface provides a viable strategy for assuring bond integrity at the critical interface, while eliminating the inherent imperfections of the infusion core.