National Institute of Aviation Research (NIAR): “The use of composites in modern aircraft designs is accelerating and will underpin a quadrupling of the aerospace composites market over the next 20 years.”
While the many benefits of composite aircraft construction continue to be extolled in the press, it is human nature which makes it difficult for us to accept the unfamiliar or unknown. Despite the incredible advancement and growth of composites in aerospace over the past 20 years, many people within and outside the industry remain skeptical of the technology. It’s time we all learn about this extremely durable and versatile class of materials and what makes them so invaluable to aerospace.
First of all, what is a composite? A composite is a macroscopic (visible to the naked eye) combination of two or more materials which results in a material possessing structural properties none of the constituent materials possess individually. Because the materials are not soluble in one another, they retain their identity. Chemical compounds such as alloys therefore are not composites. The fact is we are surrounded by products meeting this description; automobile bumpers, bathtubs, cardboard boxes, diving boards, exercise equipment, fuel tanks, golf clubs, etc. Why not airplanes?
Like their metal counterparts, composite aircraft utilize a variety of materials and construction methods. The most basic unit of composite construction is the laminate. The laminate is analogous to sheet metal and consists of one or more plies or lamina as they are called. A lamina combines a fiber component usually of fiberglass, carbon fiber, or aramid (Kevlar is DuPont’s aramid product) and a surrounding matrix material, epoxy resin being the most common in aerospace. The fibers in the laminate may be chopped (short fibers with random orientation), woven cloths (long fibers in specific directions), or tapes (long fibers in a single direction) but in all cases must be saturated by the matrix material.
Boeing, on the 787 Dreamliner: “Engineers are discovering that their composites are tougher than they initially imagined … maintenance costs will be about 30 percent lower than for aluminum airplanes … corrosion and fatigue benefits are going to be astounding.”
Laminates vs. sandwich construction
More laminas result in stronger, thicker but heavier laminates. The principle advantages of laminates over sheet metals of equivalent strength are: less weight, high fatigue and corrosion resistance, and that they are formed in mold tools resulting in fewer, larger aerodynamically smooth parts designed and built to handle specific loading. The advantages of sheet metals over laminates are that metals are isotropic in nature (they are uniformly strong in all directions), less expensive, and generally have better part stiffness characteristics than laminates of equal strength.
To increase their isotropic characteristics, laminates are usually made with the long fibers of multiple laminas running in two or more directions within the laminate. To overcome laminate stiffness limitations, a second type of composite construction is used, sandwich structure.
Sandwich structure introduces a core material bonded between two laminates or basesheets. The loading mechanics of sandwich structure act similarly to truss or beam construction. The core material carries and transfers the imposed loads to the basesheets perpendicular to the applied load just as stringers and frames do. Typical core materials include high density polymer foams, honeycombs (a hexagonal cell material usually made of aluminum or aramid oriented perpendicular to the basesheets), and cross cut balsa woods. Large composite parts frequently utilize both laminates and sandwich structures in their lay-ups.
When a core material is sandwiched between two basesheets, the part may become as much as 40 times stronger than the combined strength of the basesheets. It is the core’s material and thickness which determine its ultimate strength. Because of the thin base-sheets, sandwich structures have low impact tolerance, and once punctured are highly susceptible to moisture ingress.
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