Title: Machining, joining and repair
1Machining, joining and repair
2Machining 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.
3Machining 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.
4Machining 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
5Machining 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
6Machining 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.
7Machining - 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?
8Joining 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
9Joining 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
10BigHead Bonding Fasteners images from
http//www.bighead.co.uk/
- Extended heads to spread load
11Joining 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
12Correct joint design ... redrawn from diagrams
in The Permabond Engineers Guide to Adhesives
- Compression good Shear OK
??
?
13Wrong joint design... redrawn from diagrams in
The Permabond Engineers Guide to Adhesives
- Peel (one flexible) Cleavage (two rigid)
x
14Bonding - 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.
15Joining 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
16Painting/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
21Repair
- 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)Â - appropriate machining techniques
- to remove the failed material
- rebuild the laminate
22Repair - 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.
23Self-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
24Summary
- Machining
- cutting
- abrasive water jet
- laser
- Joining
- fasteners
- adhesive bonding
- welding thermoplastic
- painting/surface coating
- Repair