Preform technologies are advancing with an almost unlimited degree of freedom to orient fiber tows in three dimensions and weave them to near net shape. Advanced three-dimensional preforms offer increased stiffness and strength in the through-thickness direction (at the expense of in-plane properties), improved damage tolerance, and potential for improved ballistic performance for composite armor.
Mechanical property models exist for the prediction of thermoelastic properties of these various fiber architectures. However, the ability to predict damage evolution and long-term durability is not sufficiently robust and is currently dependent on expensive testing programs. Future research should be directed toward establishing reliable models to relate fiber, matrix, and interface properties, processing effects, and fiber architecture to damage mechanisms and life prediction.
Historically, the development of structural fibers has been driven by two competing factors: performance and cost. In general, fibers with higher strength and modulus are more expensive than fibers with more moderate properties. Early in the development of structural fibers, the selection of a fiber for a particular application&#;especially a military application&#;was driven primarily by performance requirements. With the emergence of non-aerospace, non-military applications, cost&#;given an acceptable level of performance&#;has become the driver for fiber selection. Cost has also become a major concern of DoD when making material selections for future military applications. Despite this concern about cost, however, DoD continues to require a high degree of fiber consistency. Demonstration of this consistency (by the fiber producer) drives up material costs.
There are several important factors controlling fiber cost: the cost of the raw material; processing costs; and&#;for military applications&#;the cost of qualifying a material for a given application, including the quality control and reporting that are required for fibers to meet military specifications. Commercially available fibers do not have this third cost and, as a result, are significantly cheaper.
It is important to recognize that material costs, and specifically fiber costs, are just one factor affecting the cost of a composite part. The recurring costs associated with composite processing, component assembly, and product inspection can be significant. In some applications, material costs are a small fraction of the total cost of the system. In addition, nonrecurring costs may be related to material qualification of the fiber, of the resin, and of the fiber and resin combination in the composite, at both the coupon and the structural levels.
The magnitudes of composite material and component qualification or certification efforts and costs vary widely and depend on many factors, including specific product requirements, the criticality of the application, and the degree to which human safety is involved. In military aircraft, for example, the process of design development, material qualification, process development, structural analysis, and testing can be extremely complex and costly due to mission criticality, the need for high reliability under extreme conditions, and survivability requirements. With increasingly stringent requirements and more sophisticated composite products, qualification and certification programs have become more and more complex over the years, and with this complexity has come increased cost. Therefore, if new materials have only marginal (though desirable) benefits compared to existing systems, end users may be unwilling to expend the resources necessary to qualify them if qualification costs remain at present levels. Conversely, for DoD to take advantage of incremental improvements in current fibers or lower-cost commercial fibers, more efficient approaches to material qualification will be needed.
In the past, research on composites has been viewed as a collection of issues, such as fiber development, matrix studies, interfacial bonding, processing, and finally structural mechanics. Traditionally, matrices have been thought to protect the fibers. However, it is now understood that the matrix will influence the entirety of process-structure-property relationships for the composite system while leaving the fibers unchanged. Further, the properties and structure of the resin matrix are different when reinforced, and this presents a major challenge. Only recently has the industry matured to the point where the fiber, resin matrix, interface, and surface are routinely seen as a system.
With the development of computers capable of modeling structures at the molecular scale, the time is approaching when it will be possible to design the optimum composite for a given system performance and cost. This systems perspective will involve modeling the structure and the manufacturing processes
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Composite core materials are used to complete the function of engineered structures by making up for a type of design or performance need, whether it&#;s weight reduction, providing energy absorption, impact resistance or adding stiffness. It&#;s important to select the right composite core material that performs as intended and provides the properties specified for your application. However, with so many viable material options available, it&#;s imperative to first identify the criteria the material needs to meet to help narrow down the decision making process.
In this whitepaper, General Plastics will cover the many factors one should consider when selecting the right composite core material. We&#;ll also specifically discuss application considerations, important processes and material compatibility elements.
The role of a composite core material is to provide strength, stiffness and continuity throughout a structure without drastically increasing weight. Whether bonded between two composite skins to form a sandwich panel or over-molded with self-skinning flexible foam or decorative cover, core material serves as the central member or foundation of a structure.
The aerospace industry is one of the primary users of composite materials[i], commonly used as interior floor and ceiling panels, cargo liners, overhead stowage bins, panels, window surrounds, lavatory modules, galleys, food and drink trolleys and bulkheads/class dividers. However, the use of composites is growing across other various industries including marine, transportation, medical and more. The U.S. composite end products market was valued at $26.7 billion in and is forecasted to grow at 3.8% compound annual growth rate (CAGR) in the next five years to reach $33.4 billion in .[ii]
Typically known for its exceptional strength-to-weight ratio, composite core materials need to be evaluated for other qualities and characteristics such as durability, resistance to corrosion, moisture and chemicals, and other environmental exposures. However, these qualities depend on the material used, which makes analyzing and choosing the right core material to meet your structural demands exceptionally important. Core selection begins with careful consideration of the material qualities, manufacturing process and the desired performance.
Common Composite Core Materials
Balsa: End grain balsa&#;s closed-cell structure consists of elongated, prismatic cells with a length (grain direction) that is approximately 16 times the diameter. Balsa is available in sheet form for flat panel construction or in a scrim-backed block arrangement that conforms to complex curves.
Honeycomb: Honeycomb materials include paper, aluminum, phenolic resin impregnated fiberglass, polypropylene and aramid fiber phenolic-treated paper. The physical properties vary to a large degree with the specific material and density. Fabrication of extremely lightweight panels is possible with honeycomb cores.
PVC Foam: Polyvinyl chloride (PVC) foam cores are manufactured by combining a polyvinyl copolymer with stabilizers, plasticizers, cross-linking compounds and blowing agents. PVC foams offer a good combination of strength and weight with densities ranging from 4 to 30 lbs/ft&#;.
Polyurethane Foam: PU foams are available in blocks, sheets or other shapes. They are sometimes also individually molded into discrete part-shapes. These foams can be useful (depending on formulation) to temperatures range of 275° &#; 350°F, depending on the formulation, while retaining a substantial portion of their strength and toughness. This allows them to be used in panel applications along with high-temperature curing prepregs, cured in ovens or autoclaves.
Design engineers typically focus on the core material&#;s design or performance needs, property requirements needed to fit those needs and the end cost. For instance, honeycomb, which is commonly used in composite core panel applications, has excellent strength to weight ratio, but can be susceptible to moisture and is also on the expensive end of the spectrum. Narrowing down the properties specifically needed for the application while also reviewing cost is a vital first step.
Typical types of composite core applications can be separated into two main groups: Sandwich Panels and Shaped Cores.
Sandwich Panels are used in a number of applications in various markets such as aerospace and automotive. A typical sandwich panel consists of upper and lower skins with a much thicker core in the middle. The main purpose of the core in the center of the sandwich panel is to provide as much spacing between the two skins as possible to create the panel stiffness, adding as little weight as possible, while resisting the forces created when the structure is loaded.[iii]
Important Core Material Properties to Consider
Compressive Strength
A material&#;s compressive strength describes how much force a material can take before it yields to the force and deforms or breaks. Some materials fracture at their compressive strength limit; others deform irreversibly, so a given amount of deformation may be considered as the limit for a compressive load.
Flexural Strength
The stress a material can sustain just before it yields in a flexure test. Also known as bend strength, it helps to measure how rigid a material is.
Flexural Modulus
The ratio of stress to strain in flexural deformation or the tendency for a material to resist bending. Flexural modulus is an important calculation as it relates to the resistance of a panel to deflect when used as a structural member.
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Shear Strength
Measures the maximum amount of stress that can be sustained by a material before it tears or breaks when layers shift and slide.
Shear Modulus
The ratio of shear stress to the shear strain. This property tells us in advance how resistant a material is to shearing deformation. If a material is very resistant to attempted shearing, then it will transmit the shear energy very quickly[iv].
Peel Strength
Used to measure the adhesive strength of two or more materials that have been bonded together, such as the two face sheets and core material within a sandwich panel.
Application Environment Considerations
What type of environment will the panel be exposed to? Will it come in contact with moisture? Mechanical properties are important, but there are critical environmental design parameters that could cause an issue with a core material&#;s life expectancy, if not addressed.
Moisture Resistance
Any panel exposed to outdoor elements and high amounts of condensation or humidity, needs a core material that is water-resistant to avoid saturation, softening, swelling, rot or bowing of the core. For instance, balsa and other wood core materials may eventually rot, swell or degrade with long-term exposure. It is also important to select a core material that does not support fungal growth.
Thermal Conductivity
Thermal conductivity measures how a material conducts or resists heat transfer, which is important to analyze if the panel needs to provide a thermal break or insulating properties.
Chemical and Corrosion Resistance
If exposed to industrial cleaners, solvents or chemicals, look for a material that is resistant to expected conditions. It should not support fungus or attract rodents and insects.
Flammability
Sandwich panels are used frequently for aircraft interiors, therefore, meeting aerospace&#;s stringent standards for flame, smoke and toxicity (FST) and heat release is very important. If your panel could potentially be exposed to fire, it needs to meet a number of requirements to guarantee that it will not propagate a fire if exposed to a flame. Is the material flame-retardant and self-extinguishing? Does it intumesce to form a protective carbonaceous char layer that assists self-
extinguishment, insulates and protects the material from continued burning?
Weight
In aerospace and automotive markets, lightweighting has become increasingly important to OEMs, Tier 1 and 2 suppliers. Manufacturers are pressed to create new products that meet these industries&#; growing needs and new government standards. It&#;s imperative to identify the right lightweight core material that also doesn&#;t sacrifice the other performance characteristics you need.
The second group of core applications, shaped core, uses a core material as the base structure that is then over-molded or covered with a cosmetic or performance-enhancing material. The core material is typically molded or machined into the desired shape or geometry required by the application. Winglets, automobile bumpers, car dashboards, glare shields, crash pads, automotive seatbacks and headliners would all be examples of shaped core applications.
All of the factors listed for Sandwich Panels should be considered for Shaped Core, however, there are additional impact mitigation and energy absorption-related questions that need to be analyzed:
Solidifying a bulletproof design and selecting the right materials are critical to your project&#;s success. However, selecting the right processing method for your materials can impact time-to-market, expenses, overall product quality and your bottom line. Common manufacturing methods involved when making sandwich panels or shaped core parts include:
Once you&#;ve decided on your core material and manufacturing process, you must verify that it is the most cost-effective option for your budget. There are a number of core materials available that offer a product that may be considered over-engineered for the application. Instead of paying a premium for capabilities you do not need, focus on a material that provides the properties and capabilities that fit the necessary function for your project.
For instance, Honeycomb is typically used in aerospace applications since it is one of the lightest weight core options available, and has excellent fire-resistant properties. However, the exposed edges and cutouts of a honeycomb panel are susceptible to ingress of moisture unless they are sealed with rigid foam edge closeouts or labor-intensive potting compounds to seal the edges. In addition, mounting and fastener locations across the panel surface need a substrate with suitable fastener pullout strength properties. A more cost-effective material that may be considered is polyurethane foam, especially when material cost is factored in. Polyurethane foam is typically offered in a wide range of densities and has excellent thermal properties. Some PMI and PVC foams may also be considered, however, these exhibit some changes in shape when exposed to humidity over time. Ultimately, the core material switch may reduce a company&#;s processing and materials costs.
General Plastics Manufacturing Company has been manufacturing flexible and rigid polyurethane materials, aircraft assemblies, molded parts and other specialty foam products for over 70 years. Our LAST-A-FOAM® closed- and open-cell polyurethane foam products provide versatile solutions in the aerospace, defense, nuclear, construction, marine and Tooling & Molds industries stemming from chemistry-based solutions and a commitment to excellence and quality.
General Plastics offers core material and finished panels to meet the specific needs of each customer. Our composite core products are flame-retardant, deliver long-lasting performance and uniformity over time and offer an excellent alternative to wood, honeycomb and other core materials. Our product line includes core material that passes fire, smoke and toxicity requirements and heat release standards for interior aerospace applications. LAST-A-FOAM® products are non-decaying polyurethane composite cores that support fiberglass laminating, are flame-retardant, strong and lightweight, durable, resistant to chemicals and extremely cost-effective. LAST-A-FOAM® composite core materials have been used for years in the aerospace and automotive industries in place of aluminum, as well as for sports equipment and insulated doors and windows. In the marine industry, General Plastics&#; composite core foams provide a non-decaying alternative to wood that supports fiberglass laminating production processes.
From the impact resistance required for a hockey stick blade to the durability of a wing tip, General Plastics&#; LAST-A-FOAM® offers high-strength, low-weight solutions for diverse manufacturing processes. Whether you need board stock or machined parts, we can meet your exacting specifications.
Technical Papers
Composite Core: Factors to Consider When Choosing Core Material for Your Application
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General Plastics&#; Aerospace-Grade Core Materials and Built-to-Print Polyurethane Parts For the Business Aviation Industry
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General Plastics to Spotlight Lightweight Composite Core and Dielectric Materials at SAMPE, May 22-23
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General Plastics to Feature its FST/OSU-Compliant Rigid Foam Series, Lightweight Core Materials and Custom Molded Flyaway Parts at Aircraft Interiors Expo
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