Machining joining and repair John Summerscales Machining health

Machining, joining and repair John Summerscales

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.

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? •

Coefficient of thermal 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 same as for the material containing the hole.

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 •

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

Machining: LASER • CO 2 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. •

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 fuzz and keep the teeth from snagging fibres run the blade in reverse (teeth pointing upwards) • • H&S issues with sub-diameter particles • also relevant for natural fibre composites?

Joining: fasteners • in general, double lap joints preferable 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 •

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

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

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

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

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

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

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

Painting/surface coatings • painting of composite substrates o surface preparation as for adhesive bonding • current trend towards in in-mould coating eliminates solvents in the workplace o reduces labour required o more uniform coating thickness o • • but only on horizontal surfaces in compression moulding PU research funded by • DTI Technology Programme/Zero Emission Enterprises • EU REA grant FP 7 -SME-2011 -1 -286520.

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

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

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

In. Ge. Ct double tetrahedron mould challenging geometry for test mould tool

Repair • before repair, non-destructive evaluation o to determine full extent of damage • design the repair o for a general repair • o the hole is normally tapered at ten times the depth for an aerospace repair • the hole is normally tapered at fifty-times the depth or at 12. 7 mm/ply (half-inch/ply) • appropriate machining techniques o to remove the failed material • rebuild the laminate

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

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

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