Laminating with Graphite · Carbon Nylon Fiberglass Aramid (Kevlar) Polyethylene (Spectra)...
Transcript of Laminating with Graphite · Carbon Nylon Fiberglass Aramid (Kevlar) Polyethylene (Spectra)...
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Laminating with Graphite
Doug Taylor C.P. Northwestern University O & P Associates, Inc.
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Polymers
Monomers Chained Together Millions of Molecules
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Polymers
Thermoplastic (Reheatable) spaghetti like structure
Thermoset (Non-reheatable) three dimensional crosslinked network
which is permanent
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Polymers
Weak Compared to Metals Less Stiff Than Metals
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Composites (Laminants)
Reinforcement Matrix
Interface (Adhesion of Primary Importance)
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Materials Selection
Resin Type: Acrylic Epoxy
Vinyl Ester Polyester
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Materials Selection
Fiber Type: Carbon Nylon
Fiberglass Aramid (Kevlar)
Polyethylene (Spectra)
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Materials Form Selection
Fiber: Unidirectional
Woven Braided
Stockingette Random
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Fiber Materials
Principle load bearing component High strength but brittle and notch-
sensitive Small diameter
Used in bundles called tows
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Fiberglass
Ceramic fiber Inexpensive raw materials: sand,
coke, and coal
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Fiberglass Types
E-glass (most common) S-glass (stronger)
R-glass D-glass A-glass M-glass
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Fiberglass Superior tensile strength
Strong but not stiff Low cost
Tough Perfectly elastic (Obeying Hooke’s
Law)
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Fiberglass Very brittle
Highly notch-sensitive Surface defects from dust, water, and
touch greatly effect strength
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Fiberglass Very poor bond to polymer resins
Silane coupling agents used to improve adhesion but bond is still
poor
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Fiberglass Composites Design flexibility Low cost tooling
Lower cost materials Heavier composites
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Fiberglass Composites Static fatigue loading will decrease
ultimate strength Fiber pull out, debond and
delamination improves toughness by accumulating damage and dissipating
fracture energy
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Aramid Fiber
Trade Names: Kevlar Twaron
Technora
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Aramid Fiber (Kevlar) Aromatic polyamide thermoplastic
polymer Several grades
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Aramid Fiber (Kevlar)
Low density High specific strength
Good toughness Damage tolerant
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Aramid Fiber (Kevlar) Low compressive strength
Absorbs moisture (up to 3%) Poor adhesion to polymers
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Spectra
Ultra high molecular weight polyethylene fiber (UHMPE)
Thermoplastic fiber
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Spectra Very high tensile strength
Low weight Good abrasion resistance
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Spectra Very poor adhesion to polymers
Must be plasma treated to improve adhesion
Poor compressive strength
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Carbon Fiber
Two dimensionally covalently bonded material
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Carbon Fiber or Graphite
Precursor materials: Polyacrylonitrile (PAN)
Rayon Extruded pitch
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Carbon Fiber or Graphite
Well oriented fiber Stiff and strong in one plane Higher modulus (stiffness)
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Carbon Fiber or Graphite Linear stress-strain behavior
Elastic to failure Elongation to failure 2%
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Carbon Fiber or Graphite Creep resistant
Chemically inert Negative coefficient of thermal
expansion Does not absorb moisture
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Carbon Fiber or Graphite Brittle
Expensive Low impact strength
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Carbon Fiber or Graphite Surface treatments used to protect the
fibers and to improve adhesion
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Composite Properties Dependent On:
Fiber Type Fiber Volume Fraction (Vf)
Fiber Orientation Fiber Size
Fiber Adhesion to Resin Resin Type
Process Variables
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Reinforced Plastics (Low Strength)
Short Fibers Low Fiber Volume Fraction
Poor Fiber Orientation Weaker Fibers
Thermoset and Thermoplastic Resins
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Composites (Medium Strength)
Longer Fibers Moderate Fiber Volume Fraction
Good Fiber Orientation Strong Fibers
Thermoset and Thermoplastic Resin
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Advanced Composites (High Strength)
Long Fibers (7 cm minimum) Maximized Fiber Volume (50-80%) Superior Fiber Orientation (Fibers
Aligned with the Axis of Stress High Strength-High Stiffness Fibers
(Carbon) Thermoset and Thermoplastic Resins
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High Performance Composites Fiber orientation along the axis of
stress Fiber type strong and stiff
Fiber volume fraction 50-70% Void content or air bubbles minimal
Resin type having good strength Good compaction or consolidation of
layers
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High Performance Composites Design
Understanding laminate structural behavior vital
Adhesion of layers (plies) critical under multiple stress, strain, impact
load conditions Affected by fabrication method
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Component Design
Surface finish Fatigue life
Overall configuration Scrap or rework potential
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Overall Configuration Endoskeletal Sockets
Sockets with openings inherently weaker
Distal stresses are mostly out of the fiber plane
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Ply Lay Up Design Adhesion Strength Weight
Stiffness Operating temperature
Toughness
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Liquid Composite Molding Factors
Preform permeability Preform volume fraction Preform fiber orientation
Resin viscosity Resin injection rate
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Liquid Composite Molding Advantages
Excellent weight:performance ratios Cheap tooling
Design flexibility Noncorrosive parts Parts consolidation
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Vacuum Assisted Resin Transfer Molding (VARTM)
Voids 0-2% Thick near net-shape
Less post fabrication work (Peel ply removal and surface finishing)
Good surface detail and accuracy Can mold in fittings, hardware and
foam cores
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Vacuum Assisted Resin Transfer Molding (VARTM)
Volume fractions to 68% Less wasted material
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Woven Fabric Composites More balanced properties in fabric
plane Higher impact resistance than UD
Higher out-of-plane strength Easier handling (reduction in labor)
Reduced in-plane stiffness and strength
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Matrix “Weak link”- transfers load to fibers
Keeps fibers in orientation Provides resistance to crack
propagation and damage Provides ALL interlaminar shear
strength Protects fibers from abrasion and
chemical attack
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Resin Flow Depends On Resin viscosity
Preform permeability Part thickness
Part shape
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Resin Flow Depends On Tow shape Tow size
Fiber orientation Stacking sequence
Fiber volume fraction
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Resin Flow Flatter is better
Changes in direction should be smooth and gentle
Minimum radii two or three times the thickness
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Resin Flow Dry fiber flow Wet fiber flow Racetracking
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Open Weaves Better wettability
Handling more difficult Gap- space between yarns facilitates
resin flow
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Prosthetic Composites Combinations of Unidirectional and
Woven Carbon Fiber or Graphite Cloths or Braids
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Prosthetic Composites Recommendations:
Even and balanced reinforcement distribution
Small tow sizes (3K ) and spaces between fiber tows to facilitate resin
flow
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Prosthetic Composites Recommendations:
Maximum vacuum pressure Low viscosity resin with 30 minute
gel time Prevent bag bridging by keeping it
moist Seal off resin reservoir
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Prosthetic Composites Recommendations:
Use a thin fiberglass inner layer to protect the patient from brittle failures
(2 oz.) Use layers of fiberglass to reduce
compressive stress at fasteners Use a layer of fiberglass to protect aluminum from contact with carbon
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Prosthetic Composites Recommendations:
Use an external layer of fiberglass to protect against expected impact
damage Use of hybrid cloths with fiberglass
or Kevlar will reduce cost and increase impact resistance
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Prosthetic Composites Recommendations:
Sandwich unidirectional cloths between layers of plain weave cloths Do not sandwich dissimilar materials because it will cause a delamination
mode under fatigue loading
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Prosthetic Composites Recommendations:
Keep resin content as little as possible, the fiber should carry the
load Avoid resin-rich areas
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Prosthetic Composites Recommendations:
Use soft linings for protection from skin irritation and to facilitate reliefs
Use extra cloth over bony prominences and brims for extra
relief areas
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Prosthetic Composites Recommendations:
Use large amounts of unitape for structures that are not cylindrical in nature such as syme prostheses and AFO’s and orient some of them at +450 and -450 to reduce torsional
deformation
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Prosthetic Composites Recommendations:
Grinding operations should be done with large amounts of air flow (dust
collector) Wet sand ground areas with 300 grit
sand paper by hand Clean interfaces with acetone to
remove carbon residue
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Prosthetic Composites Recommendations:
Use large amounts of carbon fiber in off axis stress areas such as socket attachments and hip joint areas or anywhere high stress is expected
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Prosthetic Composites Recommendations:
Inspect structures regularly and modify layups accordingly
Spot repairs can be easily made
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Prosthetic Composites Recommendations:
Consider the main structure of the device first
Then deal with the cosmesis separately