Machining, joining and repair

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Machining, joining and repair

• John Summerscales

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Machining health and safety

• machining of composites is probably of
  greater risk than the potentially toxic
  chemicals (if the latter are handled with due
  respect) used in composites manufacture
• dust and decomposition products arise
• essential to minimise this risk by
• extraction at source, or
• entrapment in a stream of gas or water.

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Machining of composites

• heterogeneous, anisotropic structure
• hence greater similarity to wood than to Fe/Al
• low heat dissipation
• low coefficient of thermal expansion
• hot tool expands more rapidly then work-piece.
• coefficient of thermal expansion for a holein an
  unconstrained plate of materialis the same as
  for the material containing the hole.

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Machining of composites

• cutting composite materials/structures wears
  cutting tools more rapidly than cutting
  traditional engineering materials.
• tool durability and initial cost
• high speed steel ltlt carbide lt boron nitride (BN)
  or polycrystalline diamond (PCD)
• more expensive cutting tool is cost-efficient
• when costs calculated over tool-life
• surface finish will be smoother

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Machining water-jet

• normally conducted with abrasive powder
• e.g. garnet injected into the jet.
• for abrasive water jet cutting (AWC),
• flow speeds of 850 m/s
• 4-8 litres/min through a 0.8 mm diameter hole
• cut is 0.5-2.50.4 mm wide and tapered
• water absorption may be an issue, especially for
• materials with weak fibre-matrix interfaces
• aramid composites

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Machining LASER

• CO2 LASER cutting
• beam of 0.1-1.0 mm focussed diameter
• co-axial inert gas
• depth of focussed field is proportional to
  spot-size
• tolerance is typically 0.5 mm.
• Ease of cutting
• aramids are easily machined with lasers
• glass is intermediate, and
• carbon is difficult because of its high thermal
  conductivity.

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Machining - aramids

• special tools and techniques are appropriate
• e.g. band saw
• fine tooth blade (550-866 teeth/m)
• straight-set or raker-set teeth
• operate at high speed to stretch and shear
• to minimise the production of fuzzand keep the
  teeth from snagging fibresrun the blade in
  reverse (teeth pointing upwards)
• HS issues with sub-diameter particles
• also relevant for natural fibre composites?

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Joining fasteners

• in general, double lap jointspreferable to
  single lap shear joints
• fasteners should normally be
• 2 - 4 diameters from edge, and
• 3 - 4 diameters from adjacent fasteners
• Stress analysis dependent on
• any pre-load
• stacking sequence
• free-edge effects, etc

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Joining fasteners

• typical failures include
• bearing failure,
• shear-out,
• cleavage, and
• direct failure of substrate or fastener material
• important considerations in joint design
• matrix creep torque applies compressive
  stress in the unreinforced direction of the
  laminate
• galvanic corrosion
• C and Al at opposite ends of the electrochemical
  corrosion series
• thin fibreglass layer minimises such corrosion

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BigHead Bonding Fasteners images from
http//www.bighead.co.uk/

• Extended heads to spread load

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Joining adhesive bonding

• adhesive joints
• spread load over more uniform area than fasteners
  
• result in a lower stress concentration
• good joint design isessential for
  highly-stressed applications
• joints
• best loaded in compression
• acceptable performance in shear
• avoid tension, especially peel and cleavage

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Correct joint design ... redrawn from diagrams
in The Permabond Engineers Guide to Adhesives

• Compression good Shear OK

??
?
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Wrong joint design... redrawn from diagrams in
The Permabond Engineers Guide to Adhesives

• Peel (one flexible) Cleavage (two rigid)

x
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Bonding - surface preparation

• Surface preparation is crucial to achievement of
  a good bond
• for composites normallydegrease-abrade-degrease-d
  ry sequence
• shot-blasting the surface is inappropriateit
  tends to remove too much substrate
• plastic bead blasting (or similar blast media)
  permits greater control of material removal
• aerospace industry avoids silicone release
• material transfer to the part surface can cause
  significant weakening of the subsequent bond.

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Joining welding thermoplastics

• joining of thermoplastic matrix composites
• heat - compress - intermolecular diffusion - cool
• variety of techniques to heat the substrates
• hot-plate
• resistance heating/induction heating
• infrared/laser
• dielectric/microwave
• friction-inertia/vibration welding
• ultrasonic welding
• solvent welding also possible
• beware health and safety and solvent entrapment

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Painting/surface coatings

• painting of composite substrates
• surface preparation as for adhesive bonding
• current trend towards in in-mould coating
• eliminates solvents in the workplace
• reduces labour required
• more uniform coating thickness
• but only on horizontal surfaces in compression
  moulding
• PU research funded by
• DTI Technology Programme/Zero Emission
  Enterprises
• EU REA grant FP7-SME-2011-1-286520.

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Gel-coat application

• By hand-painting or spray onto the open mould.
• The process releases volatile organic compounds
  (VOC) into the workplace and the environment.
• By mould-opening and flow into the space
• horizontal surfaces increase by the required
  distance
• vertical surfaces see no increase in space

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New in-mould process

• InGeCt IMGC (in-mould gel-coating)
• applicable to RTM, RIFT and similar processes
• mould cavity divided by a separator layer
• separator has texture to
• provide stand-off from mould surface
• enhance physical bond to laminate and gel-coat
• IPR protected by British Patent GB 2 432 336A
• InGeCt IMS (in-mould surfacing)
• silicone shim defines space for gel coat
• mould laminate remove shim inject gelcoat

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IMGC concept

• as for RTM, but with two injection ports

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InGeCt double tetrahedron mould
challenging geometry for test mould tool
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Repair

• before repair, non-destructive evaluation
• to determine full extent of damage
• design the repair
• for a general repair
• the hole is normally tapered at ten times the
  depth
• for an aerospace repair
• the hole is normally tapered at fifty-times the
  depthor at 12.7 mm/ply (half-inch/ply) 
• appropriate machining techniques
• to remove the failed material
• rebuild the laminate

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Repair - sandwich panels

• it may be practical to
• replace just one laminate skin, or
• replace one skin and the core,leaving the second
  face intact.
• foaming adhesive used to bond-in replacement
  honeycomb.

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Self-healing composites

• proposed use of hollow glass fibrescontaining
  uncured resin
• low viscosity resin systems generally do not
  achieve the highest mechanical properties
• high viscosity resin systems would require some
  form of pressure to facilitate flow
• how to mix and flow with no applied pressure ?
• University of Delaware Center for Composite
  Materials is developing biomineralisation as a
  route to the repair of the fibre network

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Summary

• Machining
• cutting
• abrasive water jet
• laser
• Joining
• fasteners
• adhesive bonding
• welding thermoplastic
• painting/surface coating
• Repair