Lecture 4: Raw Materials for part fab: reinforcements 2

Filament

Single fiber

Strand

Bundle or group of untwisted filaments

Tow

Untwisted bundle of continuous filaments, with a specific count

Yawn

Twisted bundle of continuous filaments

Roving

Group of fiberglass filaments

Tape

Collection of parallel filaments, held together by a binder (matrix)

Woven Fabric

Planar, interlaced yawns or tows in various specific patterns. Weight distribution is evener across all directions, at least compared to one direction fabrics.

Nonwoven (noncrimp) fabric

Yarns are placed parallel to each and then stitched together using polyester thread. Most of fiber’s weight is in one direction.

The amount of fiber in different directions is controlled by what?

The weave pattern. In one directional fabric, most of the fiber weight can be in that direction, for a weaved fabric weight distribution is equally distributed. There are hybrid versions that combine both.

Drape

Ability of a material to conform to contour

Tack

Stickiness of prepreg material

Warp fibers

Run in machine / longitudinal direction

Fill fibers

Interweave above and below warp fibers

Plain weave

-interlaced yarns in alternating over and under pattern

-Stable and resistant to in-plane shear

-Applications include flat laminates and tooling

Basket weave

-Two interlaced yarns in alternating over and under pattern

-less stable but more pliable / better drape

Twill weave

-Fill fibers pass over one and under two or more warp fibers

-Better wet out / drape

-Slightly less stable slightly more strong

Satin or harness weave

-One warp yarn passes over four+ fill yarns before going underneath one

-Better drape / stretch

-Worst stability, wet out and air removal

-Good for highly contour surfaces (airplanes)

Crowfoot satin weave

-One warp yarn carried over three then under one fill yawn

-more fiber-direction strength yet good flexibility

-Fishing rods, diving boards

Leno weave

-Two parallel warp yarns twisted around each fill yarn, providing locked effect

-Locks fibers in place

-Used for tooling and repairs

Nonwoven / noncrimp fabrics

Fibers kept parallel and stitched together

Nonwoven / noncrimp advantages

-Greater flexibility

-Fibers can be laid at any angle, then stitched to make multiaxial stitched piles

-Greater strength because fibers remain straight

Warp unidirectional fabrics

fibers primarily at 0 degrees

Weft unidirectional fabrics

Fibers primarily at 90 degrees

Other non-woven directions

-Warp triaxial (0, +- 45)

-Weft triaxial (90, +- 45)

-QUadraxial fabrics (quasi isotropic, provides strength in all 4 fiber axi directions)

Each gives different properties

Non-woven fabric (mat)

-flat sheet made from non-woven fibers bonded together by heat chemicals, or pressure instead of being woven or knitted

-33-50% as strong as fabric laminates (a composite made by stacking multiple layers [called plies], some of which are woven, of reinforcement and bonding them together with resin.) because fibers aren’t as oriented

-Cheaper the woven fabrics

-Types include chopped strand mat and continuous strand mat (latter having better properties)

Yield number

Yd / lb, corresponds to roving (group of fiberglass filamebts) size

What does higher yd/lb mean?

More yards of fiber can be obtained from one pound → different properties, including improved strength and reduced weight

Bi-ply fabric

Woven fabric + strand mat stitched together

Prepreg

-Resin-impregnated fiber, fabric or mat

-Can be unidirectional, woven fabric, or rovings

-Epoxy-based prepregs are very common

How are prepregs classified?

-Resin types (thermoset vs thermoplastic)

-Reinforcement form (filament, tape, woven fabric, mat)

-Reinforcement material (carbon, glass, etc)

Prepreg advantages

-Consistent fiber/resin ratio and properties

-You can control fiber orientation which controls properties

-Higher Vf

-Better mechanical performance per weight (high stiffness, strength, corrosion resistance)

Prepreg disadvantages

-High cost, difficult repair, difficult inspection

Prepreg applications

-Aerospace, sporting, medical components, circuit boards

thermoset

-Strong covalent bonds between polymer chains

-Remain hard when heated

thermoplastic

-Weak intermolecular forces between polymer chains

-Soften when heated

Thermoset Prepregs (TSPs)

-Most common

-Easy to drape

-Epoxy heavily used as resin

-Must be stored in low T environment and limited shelf life

-Requires longer processing time

What processes are thermoset prepregs used for?

-Hand lay-up

-Roll wrapping

-Compression molding

-Automatic lay-up

How are thermoset prepregs made?

-Full curing requires high pressure and high temperature

-Solvent impregnation (like thermoplastics)

-Hot melt technology

Thermoplastic Prepreg (TPPs)

-Unlimited shelf life

-Cycle time much faster

-Common resins include nylon, PEEK, polyimide

How are thermoplastic prepregs made?

-Requires even more heat and pressure then thermoset prepregs

-Solvent impregnation (like thermosets)

-Hot melt coating technique

-Film stacking

Thermoplastic prepreg advantages over thermoset prepregs

-Recyclability

-Good solvent and chem resistance

-Reduced process time

-Indefinite shelf life w/ no refrigeration

-Better flexibility, toughness, and impact resistance

Disadvantages of thermoplastic prepregs

-Higher processing temperature and pressure

-Poor drape capabilities and no tack

Preform

Reinforcement in the form of a thick 2-D or 3-D fiber architecture, aka reinforcement in the shape of final product. They can be woven fabric, knitted, stitched, braided, short fiber, non-woven mats.

Prepreg vs preform

Prepreg: resin already there

Preform: resin comes later

How are preforms made?

-Placed in the mold cavity and then resin is injected into cavity to obtain composite part

-Can also be made by braiding and filament winding of dry fibers over a mandrel

-They are also feedstock for RTM and structural reaction injection molding (SRIM)

Preform applications

Airplane fuselage, automative high-performance structural elements, wind turbine blades

Advantages of 2d preform woven fabrics

-Needed for high in-plane stiffness and strength

-Gives good cutting accuracy

-Ply drop-off possible

-Many fiber / weave options

-Good for thinner structures

Advantages of 3d woven preform fabrics

-Improve preform handling

-Improve delamination resistance

-Carry loads through complex / out of plane paths

Braided preforms

Helical and longitudinal fiber yarns or tows interlaced to give a single / multiple layers of braided tubular preform

Braided preform advantages

-Stable

-Good specific strength / torsional load capability

-Can be made over any mandrel

-Biaxial / triaxial structures possible

-Good interlaminar properties with 3D architectures

Stitched laminates (preform)

Stitching runs through the reinforcement layers and sews the laminate [stacked layers of reinforcements bonded by resin] together

Stitched laminates (preform) benefits

-Improved compression after impact

-Increased fracture toughness

-Provides mechanical connection between preforms

-Keeps preform from shifting

-Debulks fiber preform