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Machining, joining and repair

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Machining, joining and repair John Summerscales – PowerPoint PPT presentation

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Title: Machining, joining and repair


1
Machining, joining and repair
  • John Summerscales

2
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.

3
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.

4
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

5
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

6
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.

7
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?

8
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

9
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

10
BigHead Bonding Fasteners images from
http//www.bighead.co.uk/
  • Extended heads to spread load

11
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

12
Correct joint design ... redrawn from diagrams
in The Permabond Engineers Guide to Adhesives
  • Compression good Shear OK

??
?
13
Wrong joint design... redrawn from diagrams in
The Permabond Engineers Guide to Adhesives
  • Peel (one flexible) Cleavage (two rigid)

x
14
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.

15
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

16
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.

17
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

18
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

19
IMGC concept
  • as for RTM, but with two injection ports

20
InGeCt double tetrahedron mould
challenging geometry for test mould tool
21
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

22
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.

23
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

24
Summary
  • Machining
  • cutting
  • abrasive water jet
  • laser
  • Joining
  • fasteners
  • adhesive bonding
  • welding thermoplastic
  • painting/surface coating
  • Repair
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