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 used in composites manufacture if the latter are handled with due respect 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 hole?

4. Coefficient ofthermal expansion for a hole • in an unconstrained plate of material, the material expands as normal • the air in the hole has ‘near zero’ mechanical properties • CTE of the hole is the sameas for the material containing the hole.

5. 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 << carbide < 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

6. 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 ~0.5-2.5±0.4 mm wide and tapered • water absorption may be an issue, especially for • materials with weak fibre-matrix interfaces • aramid composites

7. 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.

8. 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) • H&S issues with sub-diameter particles • also relevant for natural fibre composites?

9. 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

10. 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 endsof the electrochemical corrosion series • thin fibreglass layer minimises such corrosion

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

12. 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

13. Correct joint design ... redrawn from diagrams in The [Permabond] Engineers Guide to Adhesives KEY: adhesive substrate • Compression good Shear OK  

14. Wrong joint design... redrawn from diagrams in The [Permabond] Engineers Guide to Adhesives x • Peel (one flexible) Cleavage (two rigid) x

15. Bonding - surface preparation • Surface preparation is crucial to achievement of a good bond • for composites normallydegrease-abrade-degrease-dry sequence • shot-blasting the surface is inappropriate:it 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.

16. 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

17. 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.

18. 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

19. 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

20. IMGC concept Separator layer Gel-coat injection Mould tool Gel-coat Laminate Mould tool as for RTM, but with two injection ports: Laminate injection

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

22. 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

23. 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.

24. 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

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