Plastic Honeycomb: Definitions and Its Applications
You can also read this article in Thai language version by clicking on this link:
Coming soon!
Introduction
Before going to talk about a plastic honeycomb, we will focus on composite material, which defines as any material that consist of two or more components with different properties and exact boundaries between the components. This material can be classified into two main groups.
1. Filled Materials
The existence of some basic or matrix material whose properties are improved by filling it with some particles. The definition is referred to filled materials. The matrix volume typically contains materials and materials properties more than 50 percent, which is modified by the fillers spontaneously, and mostly ruled by the matrix. As mentioned earlier, filled materials can be classified as homogeneous and isotropic and we can be described their behavior by using conventional materials (Vasiliev and Morozov, 2013[1]).
2. Reinforced Materials
Lightweight, high-stiffness and/or high-strength materials are normally referred to reinforced materials or advanced composites, which normally used in the aerospace because of its lightweight property. Moreover, these materials not only used in the aircraft industry, but also used in the industry which require high-performance materials such as sports, automotives. Although application of plastics is focused on functional integration and mass production at low cost that commonly excludes advanced composites. Highly demanding applications that require lightweight and reliability are mostly used thermoset plastics. For example, there are aromatic polyamide fibers which called Aramid fibers. They are made from polymer solution into a chemical bath by using pressure. One of two forms: para- and meta-aramid, para-aramid crystals are formed into high stiffness and strength fibers, which commercially known as Kevlar. On the other hand, the meta-aramid that has lower strength and not align into a form of fiber are available as Nomex. It has superior resistance to thermal, chemical, and radiation. On top of that it is used for honeycomb sandwich structure as hexagonal honeycombs (Stokes, 2020[2]).
Reinforced plastics have an interesting property such as superior mechanical strength with lightness, corrosion resistance, thermal and electrical insulation when compared to other materials. For example, metals have higher mechanical properties compare to reinforced plastics. However, those plastics can be significantly improved by reinforcement technique. In comparison with unreinforced forms, the reinforced plastics offer higher mechanical strength, which may require improvement to enhance their impact strength. As well as all materials, the methodology for reinforced plastics molding and processing can have a remarkable influence on properties. (Murphy, 1994[3]).
There are plenty types of fiber geometries, which are used to produce high-performance and lightweight composites. However, this report will focus on sandwich structures with laminates bonded to honeycomb cores.
Honeycomb Sandwich Structure
A sandwich structure consists of two bonding materials and a core material, where the same or different materials are combined. So that, they can be used in products in the form of large structures or sub-structures with an irregular distribution of the different materials (Campbell, 2010[4]). As shown in Figure 1.1, the interior sandwich or core carries the shear loads in comparison with I-beam.
Sandwich construction is utterly structurally efficient, particularly in stiffness-critical applications, especially honeycomb core construction. The doubling of core thickness leads to over seven times in stiffness with only a three percent weight gain, while the quadrupling of core thickness enhances stiffness over 37 times with only a six percent weight gain as shown in Figure 1.2 (Campbell, 2010[4]). Typically, sandwich structures are used for structural, electrical, insulation, and/or energy absorption applications.
As shown in Figure 1.3, the details of a conventional honeycomb core panel are demonstrated. Typical face sheets include aluminum, glass, aramid, and carbon. Normally, structural film adhesives are used to bond the face sheets to the core. The adhesive which is the very important component provides a good fillet at the core-to-skin interface (Campbell, 2010[4]).
In figure 1.4, conventional honeycomb core terminology is shown. The honeycomb can be fabricated from a variety of materials such as aluminum, glass fabric, aramid paper, aramid fabric, or carbon fabric. Honeycomb created for use with organic matrix composites is bonded together with an adhesive, which called the node bond adhesive (Campbell, 2010[4]). The core ribbon direction is “L” direction, which is stronger than the width (node bond) or “W” direction. The thickness is indicated by “t,” and the cell size is the dimension across the cell, as shown in Figure 1.4.
Despite a variety of cell configurations are available, there are three prevalent cores: hexagonal core, flexible core, and overexpanded core as shown in figure 1.5. The most used conformation is hexagonal core, which is available all nonmetallic materials and aluminum.
· Hexagonal core: It is very efficient structure. It can even be made stronger by adding longitudinal reinforcement (reinforced hexagonal core) in the “L” direction along the nodes in the ribbon direction. The main drawback of the hexagonal structure is its limited formability; aluminum hexagonal core is typically roll-formed to shape, while nonmetallic hexagonal core must be heat-formed.
· Flexible core: It was developed to offer much better formability. This conformation provides for outstanding formability on compound contours without cell wall buckling. It can be formed around tight radii in both the “L” and “W” directions. Nevertheless, it must be held in place after forming or it will spring back to the flat condition on the release of pressure.
· Overexpanded core: It has better formability than hexagonal core, although not as good as that of flexible core. This conformation is hexagonal core that has been overexpanded in the “W” direction, providing a rectangular configuration that facilitates forming in the “L” direction. The length in the “W” direction is about twice that in the “L” direction. This configuration, compared to regular hexagonal core, increases the “W” shear properties but decreases the “L” shear properties.
Plastic Honeycomb
Plastic honeycomb has core materials, which made of thermoplastics. They are light in weight, offering useful properties. On top of that they can be bent, formed, shaped and have an ability to recycle. The main types are:
· Acrylonitrile butadiene styrene (ABS): for high rigidity, impact strength, toughness, surface hardness and dimensional stability
· Polycarbonate (PC): UV-stabilized, with excellent light transmission, good heat resistance, self-extinguishing properties
· Polypropylene (PP): for good chemical resistance
· Polystyrene (PS) or Polyethylene (PE): general-purpose low-cost core materials
· Ionomer: energy-absorbent core material
Honeycomb cores can be made a full range of thermoplastic and thermoplastic elastomer, and fusion bonded using proprietary technology, with glass, aramid and carbon reinforcement. It is claimed to be the only aerospace-type honeycomb with ‘memory, so that it can revert to its original shape without failure. The rate of return (immediate or gradual) can be specified, to suit the application (Murphy, 1994[3]).
Aluminum, glass, carbon, and aramid are commonly used as a face sheet material. A conventional sandwich structure has relatively thin facing sheets of 0.010 to 0.125 in. (0.25 to 3 mm) with core densities in the range of 1 to 30 pcf (pounds per cubic foot) (16 to 480 kg/m3). Metallic and nonmetallic honeycomb core, balsa wood, open and closed cell foams, and syntactics are used as a core material. A cost-performance comparison for core materials is demonstrated in Figure 2.1. Foam cores are more inexpensive than honeycomb cores but provide a decrease in performance. However, this explains why a variety of commercial applications use foam cores, which are also easier to work with, while aerospace applications prefer to the high-performance and more expensive like honeycombs. A relative strength-stiffness comparison of different core materials is demonstrated in Figure 2.2 (Campbell, 2010[4]).
Main properties of these cores are:
· Impact absorption
· Thermal insulation
· Vibration damping/isolation
· Acoustic absorption
Production
Honeycomb core is normally made by either the expansion or the corrugation process demonstrated in Figure 2.3.
· Expansion method: It is most suitable for low-density ≤10 pcf (≤160 kg/m3). Honeycomb core used for bonded assemblies. The foil is cleaned, corrosion-protected if it is aluminum, printed with layers of adhesive, cut to length, stacked, and then placed in a press under heat and pressure to cure the node bond adhesive. After curing, the block, or honeycomb before expansion, which called a HOBE. It is sliced to the correct thickness and expanded by clamping and then pulling on the edges. Expanded aluminum honeycomb retains its shape at this point because of yielding of the aluminum foil during the expansion method. Glass or aramid, which are nonmetallic cores, must be held in the expanded position and dipped in a liquid resin, which then must be cured before the expansion force can be released. Even though epoxy and polyester resin systems are possible, phenolic and to some extent polyimide for high-temperature applications are by far the most prevalent. Several dip and cure sequences can be necessitated to fabricate the requisite density. Since phenolics and polyimides are high-temperature condensation curing resins, it is important that they are entirely cured to drive off all volatiles. If the volatiles are not completely removed during core fabrication, they can evolve during sandwich curing, creating enough pressure to possibly split the node bonds. Therefore, after the initial cure, it is conventional practice to post cure the phenolic or polyimide core at higher temperatures to guarantee that the reactions are finish. (Campbell, 2010[4]).
· Corrugation method: It is a more expensive process and exclusive for materials that cannot be made by expansion or for high-density cores such as ≥10 pcf (≥160 kg/m3) cores. For example, high-temperature metallic core such as titanium is made by corrugation and then welded together at the nodes to make the completed core sections (Campbell, 2010[4]).
Applications
1. Aerospace Industry
Manufacturing airplanes at cheaper costs is another aspect of advanced RP structural applications. In many cases, the carbon and aramid fiber RP components compare approvingly with the cost of common component structures, despite the rather high material costs. An important aspect here is the feasible simplification of the design. For instance, the complicated leg fairing of the Airbus as demonstrated in figure 2.4 was replaced by an uncomplicated all-RP sandwich (honeycomb core) panel reinforced by two RP beams. Besides a weight savings of about 30%, the production hours were decreased by 27% (Rosato, 2005[5]).
Although RP component may cost more than other material, its weight savings can remarkably decrease fuel consumption; therefore, cost-efficiency results. Fuel efficiency is a function of the aircraft’s factors, including power plant, aerodynamics, and structural efficiency. With military aircraft, flight distance is gained is an important gain. The advancement of RP technology contributed to its use on stabilizers for practically all USA fighter aircraft since 1970 such as F-14, F-15, F-16, YF-17, F-18, etc., and these parts are generally giving long-life, trouble-free service. Fighter wing covers of F-18 and the covers and substructures of a V/STOL attack aircraft like A V-8B are made from graphite/epoxy advanced composite material as demonstrated in figure 2.5 (Rosato, 2005[5]).
2. Marine Industry
Plastic boats have been designed and manufactured since at least the 1940s. Anyone can now inspect that almost all boats, at least up to 9 m (30 ft) are made from RPs that are usually hand lay-up moldings from glass roving, chopper glass spray-ups, and/or glass fiber mats with TS polyester resin matrices. Because of the superb performance of many plastics in flesh and seawater, they have been used in almost all-structural and nonstructural applications from ropes to tanks to all kinds of instrument containers (Rosato, 2005[5]).
By far the most major application of RP in marine structures, specifically with respect to volume consumed, has been in boat manufacturing. This has emerged in both civilian and military markets. Growth continues where it already possesses the small boats with the larger boat market growing. In Europe, Catana, a French boat builder, had interruption to collision damage in mind in selecting Twaron for its latest Catana 52 catamarans as demonstrated in figure 2.6. A composite sandwich structure on these yachts combines aramid in the outer hull laminates, as well as triaxial glass fiber. Various cores, including PVC foam and balsa, used in the sandwich are 20–40 mm of thickness, regarding to purpose and location. Shaped foam core is used below the waterline, though where the hull bottoms could take the ground, the laminate is solid. Weight saving carbon/honeycomb bulkheads are bonded into the hulls while they are still in the mold (Rosato, 2005[5]).
References
[1] Vasiliev, V. v., & Morozov, E. v. (2013). Advanced Mechanics of Composite Materials and Structural Elements: Third Edition. https://doi.org/10.1016/C2011-0-07135-1
[2] Stokes, V. K. (2020). Introduction to plastics engineering. https://doi.org/10.1002/9781119536550
[3] Murphy, J. (1994). The reinforced plastics handbook. Elsevier Advanced Technology. https://doi.org/10.1016/C2009-0-11287-4
[4] Campbell, F. C. (2010). Structural Composite Materials. USA: ASM International.
[5] Rosato, D. (2005). Reinforced Plastics Handbook. https://doi.org/10.1016/B978-1-85617-450-3.X5000-2
