Bjorn’s Corner: Aircraft Structures Part 8. Composite Fibers.

July 3, 2026, ©. Leeham News: We do a series on aircraft structures and how they have shaped the way our airliners transport us around the world today. We started looking at composites last week with the first used composite in aeronautics, after wood, being glass fibers embedded in a plastic matrix. Now we compare all fibers used in aircraft structures, Figure 1. We use MPa (Mega Pascal) and GPa (Giga Pascal) in Figure 1 instead of ksi; MPa’s the SI unit for force per unit area, whereas we used the Imperial ksi before (kilo pounds per square inch). To go from MPa to ksi, divide the MPa by 6.9. I have included the data for Al 2024T3 and for the typical steel used (4043), e.g., in landing gear structures, in Figure 1. Caution: Their values are not directly comparable to the fiber values in the table, as this is for the complete isotropic materials (i.e., equally strong in all directions) compared with fiber-only data, which is for the very thin fiber and only in the pull direction (tensile strength and modulus). Next week we will look at what happens when the fibers are embedded in a plastic matrix and loaded in different directions. For the fibers, we started with glass fibers last week, which, when embedded in a plastic matrix, are called Fiberglass. We can see from the table that the commonly used E-Glass is not the least strong fiber; Kevlar 29 is. But an aircraft designer is not only looking at absolute strength; he’s also comparing strength-to-mass ratio, or Specific Strength, and there, Kevlar 29 beats E-Glass. At an absolute level, it’s equally stiff (Young’s Modulus), but once again, it is stiffer per kg used, or specific stiffness. Kevlar is an Aramid fiber used in applications that require high strength and toughness. It’s why Kevlar composites are used, for instance, in Turbofan fan cases to stop fan blade outs. The property toughness, or capability to absorb a high force without fracturing (i.e., to absorb energy by deformation, first with Young’s Modulus then plastically), is indicated by the Modulus. A low Modulus indicates the material flexes when hit but does not fracture as easily as a high Modulus material. Carbon fibers have a high absolute and specific strength. But carbon fibers are also very stiff; they have a high Modulus. While this gives the material low deformation under force, it also makes it brittle. Exceed its tensile strength limit, and it fractures instantly; there is no plastic deformation. The high strength and low toughness create problems for the aircraft designer. When designing a wing, the wingbox spars and covers are made of carbon fiber, whereas the leading edge uses a tougher material, such as fiberglass or an aluminum alloy, in the Boeing 787 (Figure 2). Observe how Boeing also uses lower-cost Fiberglass for lower-strength form structures such as wing-body fairings and covers for spoiler/flaps/ rudder mechanisms/hinges. The weather radar radome is, of course, made with E-Glass Fiberglass. The brittleness of carbon fibers led Airbus to build the cockpit area in aluminum alloys rather than composites for the A350 (Figure 3), as this structure must withstand hail and bird strikes. Airbus uses a carbon composite sandwich instead of Fiberglass for the wing fairing and mechanism/hinge covers. The low toughness of Carbon Fiber Reinforced Polymer (CFRP) was a problem during the development of the 787 and A350. For highly loaded parts like the wingbox, horizontal/vertical tail boxes, etc., the loads made the CFRP parts thick enough to withstand loads such as ice and snow, normal ramp rash, etc. The fuselage has lower loads. There, the lower flight loads resulted in CFRP part thicknesses for fuselage skins, etc., that were so low that they would withstand the aerodynamic and flight loads but not withstand ramp rash or other side impacts. How to overcome this and design the skins with stringers and frames as reinforcements and additional layers, such as for thunder-strike electrical dissipation, was a major investigation for both Boeing and Airbus. It was the primary source of program delays for the A350, according to Airbus program management. Miltary aircraft ans Airbus uses quartz matrl radomes for better radar waves transmission. The LEAP-1 fan blades are made of “knitted carcon fiber” hence not limitd by epoxi shear strenght Well done Claes. Traditional unidirectional fiber, which is the predominant fiber orientation of prepreg tape, is heavily dependent on its epoxy/matrix material for its compressive strength. The key factor in play is the interlaminar shear value between the fiber and the matrix. The higher this value, the higher the compressive load the laminate will sustain. The choice of the matrix allows for some fracture toughness to be inserted and manipulated thru the use of plasticizers with rubber like properties that delay shearing of the matrix and introduce compression and rebound qualities to the laminate.Its a very complex chemistry world and the advent of thermoplastic as the matrix vs thermometer will grant designers more freedoms not known recently. Claes observation that the fabric fiber orientation can be complicated is spot on. The textiles available today are far more varied than in the past and allow for the design of things today that were pipe dreams when the 787 was created.