Aircraft Composite Structures Spokane Community College

Composite structures in Aircraft Composite: combinations of two or move materials that differ in composition or form. In composites, the individual elements do not merge into each other as in plastics; each element brings something to the table and can be easily identified Why use composites: Excellent elastic properties Ability to be customized in strength and stiffness Damage tolerance characteristics Sensitivity to environmental factors High Strength to weight ratio Excellent formability – complex contour and shapes Elements of composite structures Reinforcing material – structural strength Matrix material – bonding substance Core material – saves weight and designates shape Slide

Fiberglass Strand of spun molten silica glass woven into cloth Disadvantage – more weight/less strength Two types E-glass – electric glass – resistant to electricity S-Glass – magnesia-aluminasilicate – high tensil strength Originally used for fairings/etc. now improved

Aramid 1970’s – Dupont created aramid – KEVLAR Advantages High tensile strength Exceptional flexibility High tensile stiffness Low compressive properties excellent toughness Excellent vibration dampening characteristics Disadvantage – stretches Drilling and cutting is a problem – fizzy’s – can wick moisture, may also screw up holes

Carbon/Graphite Produced in an inert atmosphere by the pyrolysis of organic fibers Carbon and graphite are different – Carbon lower temp for production and less carbon 93-95% Graphite – Higher temp – 99% graphite Advantages High compression strength and stiffness Disadvantages Promotes galvanic corrosion when bonded with aluminum or steel – must have barrier and corrosion resistant treatment BORON – Boron fibers deposited onto thin fibers of tungsten Excellent compressive strength and stiffness, extremely hard Hazardous to work with and expensive - no generally used in civilian aircraft Ceramic Fibers – high temp applications – handle temps up to 2,200 degrees F. Space shuttle and some firewalls

Fiber Science Arrangement of the fibers in the fabric designates usage and properties Tinsel strength – fabric only, lessens with matrix added Lay up of fabric direction designates properties Strength always runs parallel to the direction of the thread run Fabric orientation (slide) Warp – Threads that run the length of the fabric – 0 degrees Interweaving colored thread often designates warp Markings on pre-preg as well can designate warp Weft or fill – Perpendicular to the warp Create reinforcing of the cloth Selvage edge = Tightly woven edge to keep from unraveling Not for use – not same characteristics as the material Bias – 45degree angle Allows manipulation of the fabric – allows for movement unlike warp or Weft

Fabric Styles Fabric styles Unidirectional Multidirectional Mat Unidirectional – Slide Strength in a single direction Occasional cross threads to hold the bundles in place Bidirectional/multi directional Warp usually outnumber the waft - more strength in the warp direction Must be aligned in repair to give strength characteristics designated by the manufacturer Intraply Hybird – different fibers woven together giving different strength and flexibility characteristics

Mats Chopped fibers compressed Used in combination with woven and unidirectional fabrics – mats are not as strong, cheap filler

Fabric Weaves Resistant to fiber breakout, damage and delamination vs. Unidirectional Composites are available in many weaves based on characteristics SLIDE Physical stability decreases, lightness increases and pliability increases from plain weave to eight weave – Explain slide

Matrix Systems Matrix – holds reinforcing fibers together – the ability of the matrix to transfer stress is the key to the strength of a composite structure. RESIN – organic polymer – original Polyester resin – not sufficient strength and brittle – not for structural members NOW – Resin matrix systems Thermo plastic – not strong enough for structural applications by itself – must have other materials introduced Heat formed – not structural due to lack of strength If reheated will soften and perhaps change shape – Windshield on light aircraft Thermosetting Resins – heat to form and assumes irreversible shape of the part Types of Thermosetting resins – Polyester resins – Used with fiberglass for non-structural parts Give fiberglass cohesiveness and rigidly but not strength for structural members Also –shrinks when cured – possible warpage Epoxy Resins – Thermosetting plastic resins – two part – resin and catalyst - catalyst acts as the curing agent beginning the chemical action for hardening the epoxy. Advantages – Excellent adhesion, useful for dissimilar material bonding – must be well designed and good preparation of the surfaces Low shrinkage High strength to weight ratio Exceptional chemical resistance Adheres to almost any material Can be used in place of polyester in almost every application Long shelf life Short pot life NOT EVERY type of resin is acceptable for every application – check the book QUESTIONS - END