CASA06A-AIRCRAFT MATERIALS: COMPOSITE AND NON-METALLIC

What is a composite structure?

Two or more materials combined to form a structure much stronger than the individual components

How do composite structures differ from metallic structures?

Excellent elastic properties, customisable strength and stiffness, different damage tolerance and sensitivity to environmental factors. Require different approaches to design, fabrication, assembly, quality control and maintenance

What is the main advantage of composites over metal structures?

High strength-to-weight ratio. Also allows complex aerodynamic shapes reducing drag and extending aircraft range

What are the three main parts of a fibre-reinforced composite?

Fibre, matrix and interface (boundary between individual elements)

What are the four most common reinforcing fibres in commercial aircraft composites?

Fibreglass, aramid (Kevlar), carbon/graphite and ceramic

What is fibreglass made from and what are its limitations?

Small strands of molten silica glass at about 1260 degrees spun and woven into cloth. Weighs more than other composite fibres and has less strength. Historically brittle with polyester resin. Newer matrix formulas have improved its benefits

What are E and S glass types of fibreglass known for?

High tensile strength fibreglass

What is aramid (Kevlar) and what are its key properties?

Organic aromatic-polyamide polymer introduced by DuPont in the early 1970s. Tensile strength approximately 4 times greater than aluminium alloy. Non-conductive with no galvanic reaction with metals. Excellent vibration damping, shatter and fatigue resistance. Very light

What is the main disadvantage of aramid?

It stretches which causes problems when cutting. Drilling can cause fibres to stretch to breaking point if the drill bit grabs a fibre

What are the advantages and disadvantages of carbon fibre?

Advantages: high compressive strength and stiffness. Disadvantage: cathodic so promotes galvanic corrosion when bonded to aluminium or steel without corrosion barriers

How is galvanic corrosion prevented between carbon fibre and aluminium?

Fibreglass placed at interfaces as corrosion barrier. Aluminium surfaces anodised, primed and painted before assembly with carbon material

What is ceramic fibre used for?

High-temperature applications. Retains strength and flexibility up to 1200 degrees. Used in space shuttle tiles, some firewalls and gas turbine inlet guide vanes

What is fibre science?

The selective placement of fibres to obtain maximum strength in various directions based on the loads a component will experience. Strength and stiffness depend on orientation of plies to the load direction

What is the warp direction in composite fabric?

Threads running the length of the fabric as it comes off the roll. Designated 0 degrees. Typically more threads in warp direction making it stronger. Identified by a different colour or type of thread inserted at periodic intervals

What is the weft/fill direction?

Threads running perpendicular at 90 degrees to the warp fibres. Interwoven with warp threads to create the reinforcing cloth

What is the selvage edge?

The tightly woven edge parallel to the warp direction. Prevents unravelling but has different strength characteristics from the body of the fabric. Removed before use

What is the bias in composite fabric?

Fibre orientation at 45 degrees diagonal to warp threads. Allows manipulation of fabric to form contoured shapes. Fabric can often be stretched along the bias but seldom along the warp or fill

What is a unidirectional fibre orientation?

All major fibres run in one direction. Not woven together so no fill fibres. Maximum strength in a single direction. Small cross threads may hold fibre bundles in place but are not considered woven fibres

What is a bidirectional/multi-directional fabric orientation?

Fibres run in two or more directions. Warp threads usually outnumber weft so more strength in warp direction

What is the plain weave and what are its properties?

Most simple weave. Warp and fill yarns interlaced alternately over and under each other. Good stability, porosity and least yarn slippage for a given yarn count

What is a satin weave?

Warp floats or skips over as many as 12 fill yarns before being woven in. Notable for smooth surface created by relatively long warp yarn floats

What is a twill weave?

Repeats on three or more warp and fill yarns. Has a distinctive diagonal line on the surface. Comes in many variations

What are the two most common weaves in advanced composite aircraft construction?

Plain weave and satin weave

What is the function of the matrix in a composite?

Completely surrounds the fibre giving it extra strength. Transfers stress to the reinforcing fibres

What are resin matrix systems classified as?

A type of plastic. Also referred to as fibre-reinforced plastics

What are the two general categories of plastic matrix systems?

Thermoplastic and thermosetting

What is a thermoplastic resin?

Uses heat to form desired shape. Not permanent — if heated a second time it will flow to form another shape. Examples: cellulose acetate and acrylic such as Lucite, Plexiglas and Perspex

Why has cellulose acetate largely been replaced by acrylic?

Cellulose acetate tends to shrink and turn yellow. Acrylic is stiffer, more transparent and colourless

What is a thermosetting resin?

Usually liquid or low-melting-point solid initially. Cured by catalyst, heat or combination of both. Once cured cannot be converted back to liquid or reshaped. Softens when heated but does not melt

What are the two common structural thermosetting resins in aircraft?

Polyester resin and epoxy resin

What is polyester resin used for?

Non-structural applications with fibreglass such as fairings, spinners and aircraft trim. Gives fibreglass cohesiveness and rigidity

What is the critical safety warning when using polyester resin?

Never mix accelerators directly with catalysts. They will violently react and catch fire or explode if not diluted by resin first

How is polyester resin mixed and what affects cure time?

Mixed by volume 1 to 5% catalyst. Hot day requires less catalyst to prolong cure time. Thick layer cures faster than thin layer

What are the key properties of epoxy resins?

Outstanding adhesion, strength and resistance to moisture and chemicals. Two-part system: base resin and hardener. Hardener mixed in larger amounts than polyester catalyst

Can all epoxy resins be used for all repairs?

No. Always use the resin specified in the manufacturer's repair manual

What is a film adhesive?

Pre-blended resin and catalyst on a thin film of plastic. Requires refrigeration to slow cure rate. Used to bond patches to repair areas. Starts curing at room temperature if left out

What is foam adhesive used for?

Splicing replacement honeycomb core segments to existing cores. Foams up and expands into crevices when heat is applied. Can also permanently install fasteners

What are pre-pregs?

Fabrics pre-impregnated with the correct amount and weight of resin matrix. Eliminates mixing and application issues. Fibres completely encapsulated. Must be stored at -18 degrees C

What are the advantages of pre-pregs over raw fabric?

Correct matrix amount pre-applied. Fibres fully encapsulated. Eliminates weighing and mixing. Reduces problems with wetting fabric. Saves time and reduces mixing ratio errors

What are the disadvantages of pre-pregs in maintenance?

Must be stored frozen at -18 degrees C. Limited shelf life even frozen. More expensive than raw fabric. Moisture, dust or contaminants compromise bond durability

What is the warning when opening pre-preg material?

Open only in a controlled environment and not until the material has thawed to room temperature

What are fillers (thixotropic agents) used for?

Control viscosity and weight. Reduce pot life and cured strength. Make application easier. Increase resin volume making it less dense and less susceptible to cracking

What are microballoons and what do they do?

Small plastic or glass spheres. Provide greater resin concentration at edges and corners, reduce overall weight and lower stress concentrations. Do not add strength

What is flox and when is it used?

Fuzzy fibre taken from fabric strands. Used when added strength is desired. Mixed with resin to fill holes provides more strength than pure resin. Pure epoxy resin produces brittle heavy plugs

What is shelf life?

Time span a product remains useful. Listed on label. Storage temperature affects shelf life

What is pot life/gel time?

Time before resin begins to gel. Governed by catalyst proportion and ambient temperature. Higher temperature equals shorter pot life. For polyester the mixing ratio is 1 to 5% by volume

What is the note about epoxy resin pot life?

Unlike polyester resin, epoxy resin pot life cannot be changed by reducing the amount of hardener

What is hardening time?

Time for the resin to set. Has not achieved full strength at this point

What is maturing time?

Further period after hardening during which the moulding gains maximum strength, hardness, chemical resistance and stability

What happens if mix ratio is incorrect?

Even a slightly improper mix ratio can make a tremendous difference in the strength of the final composite

How must time-expired polyester resin be disposed of?

Mix with appropriate catalyst, allow to cure then dispose per local regulations. Do not dispose of unmixed uncured product in general waste

How must catalyst-soaked rags be disposed of?

Not in general waste bins as rags could spontaneously combust. Contact local hazardous material collection agencies for large quantitie