Getting To The Core Of Composite Laminates
A wealth of low-cost core solutions are available for high-performance sandwich structures.
By Sara Black, Technical Editor | October 2003
Although the largest market for core is still aerospace — where high-performance aluminum and aramid honeycomb cores are used in aircraft primary structure, as well as in interior panels and floors — honeycombs, foam and balsa wood play a significant role in structural parts for the marine, wind energy and transportation markets. Lightweight, low in density and available at relatively low cost, core materials provide the foundation for incredibly strong and stiff sandwich structures, when placed between skins made with reinforcing fibers and resin. Given the right combination of core, composite skins and adhesive, composite manufacturers can deliver cost-effective sandwich structures to customers in virtually any non-aerospace market where high stiffness and low weight are design priorities.
Source: DIAB
Foam core materials come in a wide range of densities and thicknesses, scored and unscored, for many types of high-performance applications. Design requirements of the entire finished laminate guide core material selection.
With the multitude of core types available, in variable densities and many types of surface finish, core selection begins with careful consideration of the manufacturing process and the desired performance of the overall laminate, says Alex Gutierrez, business development manager for the DIAB Group's (Laholm, Sweden) DeSoto, Texas operation. "It's a total engineered solution that must consider the core as well as the skins and the adhesion between them," he explains. "You need to analyze the demands on the overall structure to make your core decisions."
WHY A CORED SANDWICH?
Composite designers determined early on that sandwiching a low-density, lightweight core material between thin face sheets can dramatically increase a laminate's stiffness with little added weight. A sandwich structure is cost-effective because the relatively low-cost core replaces more expensive composite reinforcement material and can be cocured with the skins in one-shot processes like resin infusion. And the stiffer but lighter sandwich panel requires less supporting structure than a solid laminate.
Source: DIAB
Low-density foam core materials are a mainstay in marine fabrication.
In a sandwich panel, the core functions like the connecting web of an I-beam, separating the face skins at a constant distance, while the skins themselves function as the I-beam flanges (see illustration, p. 26). The sandwich panel's bending stiffness is proportional to the core thickness, in the same way an I-beam is stiffer as the distance between the flanges increases. Doubling the core thickness yields a panel six times stronger and 12 times stiffer, with very little weight increase, says Dr. Brent Strong of Brigham Young University (Provo, Utah, U.S.A.).
Source: DIAB
DIAB supplies core kits already cut and beveled to the customer's specification, ready to place in the mold.
Core also helps distribute loads and stresses on the skins, which makes a cored sandwich an excellent design for absorbing impact stresses. While honeycomb has a higher strength-to-weight ratio, solid foam or balsa cores are in contact with 100 percent of the skin area, which spreads the impact over a larger area than honeycomb. A core's compression strength prevents the thin skins from wrinkling (buckling) failure, while its shear modulus keeps the skins from sliding independent of each other when subjected to bending loads. Equally important is the adhesive that bonds the core to the skins — it must be strong enough to withstand the constant compression/tension forces of dynamic loading, such as the forces on a boat hull. The coefficients of thermal expansion (CTE) of the core, the laminate material and the adhesive must be compatible to ensure that thermal cycling doesn't cause debonding.
With the explosion of closed molding methods over the last few years, new grooved core forms have been developed that can be used as the resin infusion media, without the need for special sacrificial resin flow media layers. Because laminates are processed and cured in a single shot, infusing thick monolithic laminates is risky because of the potential for high exotherm. Cores help reduce the thickness of the infused laminate, giving fabricators greater control over their processes. Plus, cored panels offer excellent heat insulation, sound abatement, fire proofing and vibration damping.
A number of nonwoven "bulker ply" core products are also available, like Lantor Composites' (Veenedaal, The Netherlands) Coremat product or nonwovens containing microspheres like those from spheretex America Inc. (Pointe Vedra Beach, Fla., U.S.A.). These very lightweight, core-like mats, although not covered in this article, can offer structural performance as well as improved cosmetics (by preventing print-through of woven fiber laminates), vibration and noise dampening, weight reduction and lower labor costs (because of ease of handling). Fabricators say they like the improved impact resistance and elongation as well as weight savings compared to traditional cores, depending on design, and they are certainly very viable alternatives to the core types discussed below.
FOAM CORES
Structural foam cores are manufactured from a number of thermoset and thermoplastic polymers including polyvinyl chloride (PVC), polyurethane (PU), polystyrene (PS), styrene acrylonitrile (SAN), polyetherimide (PEI) and polymethacrylimide (PMI). Foams (with the exception of polyurethanes) are produced by mixing liquid polymers and blowing agents, then pouring the mixture into metal molds and allowing a partial cure under high heat and pressure. The resulting rubbery mass, sometimes called an amoeba or an embryo, is demolded, then placed in a second mold and heated again (with hot water or steam) in an expansion chamber, which activates the blowing agent and controls the gas expansion pressure. The result is a roughly 4-ft by 8-ft by several-inch-thick block of foam, containing closed, gas-filled bubbles or cells. Foams can be manufactured in densities ranging from 2 lb/ft3 (30 kg/m3) up to 20 lb/ft3 (300 kg/m3) by varying the ratio of the polymer ingredients to blowing agents and adjusting gas pressure. Polyurethane foam, a thermoset that generates gas when an isocyanate is mixed with a polyol, is either made in batches ("bun casting") or a continuous foaming process.
Source: General Plastics
Economical polyurethane foam is often specified for boat transoms that do not take heavy fatigue load cycles.
Of the various structural foam core types, perhaps the most commonly used is PVC, which is actually a hybrid of PVC and polyurea. Two types of PVC foam are available. Crosslinked, or semi-rigid, foams are relatively stiff and strong, can perform at temperatures up to 120°C/250°F and are resistant to styrene, so they can be used with polyester and vinyl ester resins. Linear or ductile PVC foams, made with a different polymer formulation, are more elastic than crosslinked varieties and are widely used in marine applications, where they offer high deflection before failure and excellent impact resistance. While linear PVC is easier to heat-form around complex curves, the tradeoff is somewhat lower mechanical properties and reduced temperature resistance compared to the crosslinked version. Both offer good properties in fatigue resistance.
Because foams like PVC contain gas under pressure, outgassing can occur over time — that is, the gas escapes from the closed cells and migrates to voids or unbonded areas in the laminate. In some instances, outgassing has been blamed for delaminations and blistering in marine construction, especially in parts made at elevated cure temperatures with epoxy resin systems or finished in dark colors. But core experts contend the problem is caused primarily by improper laminating technique and poor skin-to-core bond. Most core manufacturers offer "stabilized" products that minimize the problem.
Source: DIAB
A composite sandwich structure functions similarly to the connecting web of an I-beam, with the core separating the skins at a constant distance, while the skins themselves function as the I-beam flanges.
DIAB is the largest structural foam core manufacturer, with production facilities in Sweden, Italy and the U.S. It produces Divinycell and Klegecell, both semi-rigid, crosslinked PVC foams. Both are used extensively in the marine, transportation, wind and general industrial products markets, says Gutierrez. The company also produces a linear PVC foam, Divinycell HD, used mainly in military applications.
The toughened, high-density Divinycell HD was recently selected as the core material below the waterline for the Visby-class military corvette ship, built by Swedish marine fabricator Kockums for the Swedish Royal Navy. The 300-ft long minehunter/anti-submarine vessel is constructed entirely of resin-infused carbon fiber/vinyl ester cored composite panels for extreme light weight and stealth characteristics. Kockums' composites designers selected the Divinycell PVC product for its high elongation and impact resistance, to counteract the high-speed ship's slam loads encountered in rough seas as well as the forces of underwater mine explosions.
