Machining n Joining

8/8/2019 Machining n Joining

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MACHINING AND JOINING OF

COMPOSITES

PRESENTED BY

SYED BASITH MUZAMMIL

1ST SEMESTER M.E

ADVANCED MATERIAL TECHNOLOGY



Machining: health and safety

machining of composites isprobably of greater risk thanthe 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 byo extraction at source, or

o entrapment in a stream of gas or water.



Machining of composites

heterogeneous, anisotropic structure

o hence greater similarity to wood than to Fe/Al

low heat dissipation

low coefficient of thermal expansion

o hot tool expands more rapidly then work-piece.

o coefficient of thermal expansion for a hole

in an unconstrained plate of materialis the same as for the material containing the hole.



Machining of composites

cutting composite materials/structureswears cutting tools more rapidly

than cutting traditional engineering materials.

tool durability and initial cost:

o high speed steel << carbide < boron nitride (BN)or polycrystalline diamond (PCD)

more expensive cutting tool is cost-efficient o when costs calculated over tool-life

o surface finish will be smoother





Machining: LASER

CO2 LASER cutting

o beam of 0.1-1.0 mm focussed diameter

o co-axial inert gas

o depth of focussed field is proportional to spot-size

o tolerance is typically ±0.5 mm.

Ease of cutting:

o aramids are easily machined with lasers

o glass is intermediate, and

o carbon is difficult

because of its high thermal conductivity.



Machining - aramids

special tools and techniques are appropriate e.g. band saw:

o fine tooth blade (550-866 teeth/m)

o straight-set or raker-set teetho operate at high speed to stretch and shear

o 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?



Joining: fasteners

in general, double lap jointspreferable to single lap shear joints

fasteners should normally be:

o 2 - 4 diameters from edge, and

o 3 - 4 diameters from adjacent fasteners

Stress analysis dependent on:

o any pre-load

o stacking sequence

o free-edge effects, etc



Joining: fasteners

typical failures include:o bearing failure,

o shear-out,

o cleavage, and

o direct failure of substrate or fastener material

important considerations in joint design:

o matrix creep: torque applies compressive stress

in the unreinforced direction of the laminate

o galvanic corrosion:

C and Al at opposite ends of the electrochemicalcorrosion series

thin fibreglass layer minimises such corrosion



BigHead Bonding Fastenersimages from: http://www.bighead.co.uk/

Extended heads to spread load:



Joining: adhesive bonding

adhesive joints

o spread load over more uniform area than fasteners

o result in a lower stress concentration

good joint design isessential for highly-stressed applications

joints:

o best loaded in compression

o acceptable performance in shear

o avoid tension, especially peel and cleavage



Correct joint design

...redrawn from diagrams in The [Permabond]

Engineers Guide to Adhesives

Compression good Shear OK

substrateadhesiveKEY:



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 iscrucial to achievement of a good bond

for composites normally

degrease-abrade-degrease-dry sequenceo 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:

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:

ohot-plate

o resistance heating/induction heating

o infrared/laser

o dielectric/microwave

o friction-inertia/vibration welding

o 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

o eliminates solvents in the workplace

o reduces labour required

o more uniform coating thickness

but only on horizontal surfaces in compression moulding current research at UoP funded by DTI

Technology Programme/Zero Emission Enterprises call



Repair

before repair, non-destructive evaluationo to determine full extent of damage

design the repair

o for a general repair the hole is normally tapered at ten times the depth

o 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:

o replace just one laminate skin, or

o replace one skin and the core,

leaving the second face intact.

foaming adhesiveused to bond-in replacement honeycomb.



Self-healing composites

proposed use of hollow glass fibrescontaining uncured resin:

o low viscosity resin systems generally do not achieve

the highest mechanical propertieso high viscosity resin systems would require some

form of pressure to facilitate flow

o how to mix and flow with no applied pressure ? University of Delaware Center for Composite

Materials is developing biomineralisation as aroute to the repair of the fibre network



Summary

Machiningo cutting

o abrasive water jet

o laser

Joining

o fasteners

o adhesive bonding

o welding thermoplastic

o painting/surface coating

Repair

Documents

Machining n Joining