Aircraft Materials (Composite and Non-metallic)

• Composite Structure


  The term composite is used to describe two or more materials that are combined to form a structure that is much stronger than the individual components.
• Composite structures differ from metallic structures in several ways: excellent elastic properties, customisability in strength and stiffness, damage tolerance characteristics and sensitivity to environmental factors. Consequently, composites require a vastly different approach compared to metals with regard to their design, fabrication and assembly, quality control and maintenance.
• Difference in design, fabrication, assembly, quality control and maintenance of composites

  Compared to metals
• One main advantage to using a composite over a metal structure is its high strength-to-weight ratio. Weight reduction is a primary objective when designing structures using composite materials.
• In addition, the use of composites allows the formation of complex, aerodynamically contoured shapes, reducing drag and significantly extending the range of the aircraft. Composite strength depends on the types of fibres and bonding materials used and how the part is engineered to distribute and withstand specific stresses.
• Composites in Aircraft Structures
• Composite materials are found in the manufacture of primary flight controls and high-integrity structural repairs in aircraft.
• Amount of composite materials used in A320


  • Composite materials used in primary flight controls
  • Composite materials used in high integrity structural repairs
• Composite Elements


  In aircraft construction, most currently produced composites consist of a reinforcing material to provide structural strength, joined with a matrix material to serve as the bonding substance. In addition, adding core material saves overall weight and gives shape to the structure.
• The three main parts of a fibre-reinforced composite are the fibre, the matrix and the interface or boundary between the individual elements of the composite.
• Reinforcing Fibres


  Reinforcing fibres provide the primary structural strength to the composite structure when combined with a matrix. Reinforcing fibres can be used in conjunction with one another (hybrids), woven into specific patterns (fibre science), combined with other materials such as rigid foams (sandwich structures) or simply used in combination with various matrix materials. Each type of composite combination provides specific advantages.
• Following are the four most common types of reinforcing fibres used in commercial aircraft composites:

  • Fibreglass (glass cloth)
  • Aramid
  • Carbon
  • Ceramic
• Fibreglass (Glass Cloth)


  Fibreglass is made from small strands of molten silica glass (about 1260 °C) that are spun together and woven into cloth. Many different weaves of fibreglass are available, depending on the particular application. Its widespread availability and its low cost make fibreglass one of the most popular reinforcing fibres.

  
• Fibreglass weighs more than most other composite fibres, but has less strength. In the past, it was used for non-structural applications; the weave was heavy and added polyester resins made the part brittle. Recently, however, newly developed matrix formulas have increased the benefits of using fibreglass.

  
• Fibreglass


  • E and S glass is high tensile strength fibreglass

  
• Aramid


  In the early 1970s, DuPont introduced Aramid, an organic aromatic-polyamide polymer commercially known as Kevlar®.
• Aramid


  • Exhibits high tensile strength, exceptional flexibility, high tensile stiffness, low compressive properties and excellent toughness. The tensile strength of Kevlar® composite material is approximately 4 times greater than alloyed aluminium. Aramid fibres are non-conductive and produce no galvanic reaction with metals. Another important advantage is its outstanding strength-to-weight ratio; it is very light compared to other composite materials. Aramid-reinforced composites also demonstrate excellent vibration-damping characteristics in addition to a high degree of shatter and fatigue resistance.

  
• Disadvantage of Aramid


  It stretches, which can cause problems when it is cut. Drilling Aramid can also be a problem if the drill bit grabs a fibre and pulls until it stretches to its breaking point.
• Carbon


  Advantages of carbon materials are their high compressive strength and degree of stiffness.
• However, carbon fibre is cathodic, while aluminium and steel are anodic. Thus, carbon promotes galvanic corrosion when bonded to aluminium or steel, and special corrosion-control techniques are needed to prevent this occurrence. Carbon materials are kept separate from aluminium components when sealants and corrosion barriers, such as fibreglass, are placed at the interfaces between composites and metals. To further resist galvanic corrosion, anodise, prime and paint any aluminium surfaces prior to assembly with carbon material.
• Carbon-fibre composites are used to fabricate primary structural components such as ribs and wing skins.
• Even very large aircraft can be designed with a reduced number of reinforcing bulkheads, ribs and stringers thanks to the high strength and high rigidity of carbon-fibre composites. Carbon fibre is stronger in compressive strength than Kevlar®, but it is more brittle.
• Ceramic Fibre


  Ceramic fibres are used in high-temperature applications. This form of composite will retain most of its strength and flexibility at temperatures up to 1200 °C. For example, tiles on the space shuttle are made of a special ceramic composite that dissipates heat quickly.
• Ceramic Fibre


  • Some firewalls are also made of ceramic-fibre composites.
  • The most common use of ceramic fibres in civilian aviation is in combination with a metal matrix for high-temperature applications.
• Fibre Science
• Fibre Science


  The selective placement of fibres needed to obtain the greatest amount of strength in various applications is known as fibre science. The strength and stiffness of a composite depends on the orientation of the plies to the load direction. A sheet-metal component will have the same strength no matter which direction it is loaded.

  
• For example, if a wing in flight bends upwards as well as twists, the part can be manufactured so one layer of fibres runs the length of the wing, reducing the bending tendency, and another layer runs at 45° and at 90° to limit the twisting. Each layer may have the major fibres running in a different direction. The strength of the fibres is parallel to the direction the threads run. This is how designers can customise fibre direction for the type of stress the part might encounter.
• In flight, the structure tends to bend and twist. The fibre layers are laid in a way that limits these forces, thereby customising a part to the type of stresses it will encounter.
• Fibre science


  The selective placement of fibres needed to obtain the greatest amount of strength in various applications
• Strength and stiffness of a composite

  • Depends on the orientation of the plies to the load direction
  • A sheet-metal component will have the same strength no matter which direction it is loaded
• Wing in flight


  • Bends upwards as well as twists
  • Part can be manufactured so one layer of fibres runs the length of the wing, reducing the bending tendency, and another layer runs at 45° and at 90° to limit the twisting
  • Each layer may have the major fibres running in a different direction
• The strength of the fibres is parallel to the direction the threads run
• Fibre direction customisation


  To reduce stress
• Fabric orientation


  Important for design, manufacturing and repair work
• Warp


  The threads in a section of fabric that run the length of the fabric as it comes off the roll or bolt, designated as 0°
• Weft/Fill


  The threads that run perpendicular (90°) to the warp fibres
• Selvage edge


  The tightly woven edge parallel to the warp direction, which prevents edges from unravelling
• Bias


  The fibre orientation that runs at a 45° angle (diagonal) to the warp threads, allows for manipulation of the fabric to form contoured shapes
• Fabric styles


  • Unidirectional
  • Bidirectional
  • Multidirectional
• Unidirectional fabric


  All of the major fibres run in one direction, giving the fabric the majority of its strength in a single direction