Machining, bonding and repair.
PowerPoint
papers
Index
| Composites Design and Manufacture (Plymouth University teaching support materials) Machining, bonding and repair. |
Lecture PowerPoint |
Review papers |
Subject Index |
|
Adhesives Product Locator (Huntsman Araldite)
Adhesives selection and stress analysis software (Permabond PAL and P-STRESS)
Machining (see Åström [1] pp 309-324 and 443-444 and Abrate & Walton [2])
In respect of Health & Safety, the machining of composites is probably of greater risk than the potentially toxic chemicals (provided that the latter are handled with due respect) used in composites manufacture, especially in view of the dust and decomposition products arising during machining. It is essential to minimise this risk by extraction at source or entrapment in a stream of gas or water.
A key feature in the machining of composites is their heterogeneous, anisotropic structure and hence the greater similarity to wood than to steel/aluminium. Composite materials may have low heat dissipation and the tool may expand on heating more rapidly then the work-piece. Do note that the coefficient of thermal expansion for a hole in an unconstrained plate of material is the same as that for the material containing the hole.
Composite materials tend to wear the cutting tools more rapidly than traditional engineering materials. The tool durability increases from high speed steel << cemented carbide < polycrystalline diamond, but that sequence also applies to costs. In general, the more expensive the cutting tool the more cost-efficient it will be assuming that cost is calculated over the tool-life and the surface finish will be smoother (however, if for example only five rough shallow holes are ever to be drilled at a specific diameter then high speed steel will probably be most cost-effective !). Yang et al [3] have reported the determination of the optimal machining parameters for cutting glass fibre using the reliability analysis based on the Taguchi method.
ABRASIVE WATER-JET (AWJ) cutting is normally conducted with powder (eg garnet) injected into the jet. The AWJ operation is typically conducted at flow speeds of 850 m/s (4-8 litres/minute through a 0.8 mm diameter hole). The cut is normally 0.5-2.5±0.4 mm wide and tapered. Water absorption by the work-piece may be an issue, especially for materials with weak fibre-matrix interfaces or aramid composites. Hashish [4] has suggested that AWJ is the tool of choice for trimming and cutting carbon-fibre composites for technical, environmental and economic reasons.
Carbon dioxide LASER cutting normally uses a beam of 0.1-1.0 mm focussed diameter with co-axial inert gas. The depth of the focussed field is proportional to the spot-size. The tolerance on the cut is typically ±0.5 mm. Aramids are easily machined with lasers, glass is intermediate and carbon is difficult because of its high thermal conductivity.
Special tools and techniques are appropriate for machining aramids [5] and these may be more relevant than traditional routes for natural fibre composites. For example [5], the band saw should have a fine tooth blade (550-866 teeth/m) with straight-set or raker-set teeth and operate at high speed to stretch and shear the material. To minimise the production of fuzz and to keep the teeth from snagging fibres run the blade in reverse (teeth pointing upwards) [5]. "As shipped, KEVLAR aramid fiber products do not pose a hazard. KEVLAR staple and pulp contain a small amount of respirable fibers which may become airborne during opening, mixing, carding, or regrinding waste products containing KEVLAR. When mechanically working KEVLAR fiber or materials containing KEVLAR in operations such as cutting, machining, grinding, crushing or sanding, airborne respirable fibers may be formed. Repeated or prolonged inhalation of excessive concentration of respirable fibers may cause permanent lung injury" [6].
Books:
Fasteners (see Gutowski [7] pp 489-496)
In general double joints are preferable to single lap shear joints. Fasteners should be placed 2-4 diameters from the edge and 3-4 diameters from adjacent fasteners. Stress analysis will be dependent on any pre-loading, the stacking sequence, free-edge effects, etc. Typical failures include bearing failure, shear-out and cleavage as well as direct failure of the substrate or fastener materials. Two important considerations in fastened joint design are:
One company that specialises in fasteners (as illustrated below) for incorporation into composite structures is bigHead Bonding Fasteners.
Bonding (see Åström [1] pp 326-339 and Gutowski [7] pp 496-501, Lees [8] and Wahab [9])
This teaching support material complements that in lecture A11: Adhesives and bonded structures.
Adhesive joints spread the load over a more uniform area than fasteners and thus result in a lower stress concentration. Good joint design is essential for highly-stressed applications. Bonded joints are best loaded in compression and give acceptable performance in shear. Tension, especially peel (where at least one component is flexible) and cleavage (where rigid components are involved) should be avoided [10]. The PowerPoint presentation for lecture A11 (slides 12-15) provides illustrations of good and bad designs for adhesively bonded joints. Surface preparation is crucial to the achievement of a good bond and for composites normally includes a degrease-abrade-degrease-dry sequence. The use of shot-blasting to abrade the surface is inappropriate: it tends to remove too much substrate. Plastic bead blasting (or similar blast media) permits greater control of material removal. The aerospace industry generally avoids the use of silicone release materials in composite manufacture as material transfer to the part surface can cause significant weakening of the subsequent bond.
For adhesively bonded composite components, co-curing is often adopted: the substrate and the adhesive joint are cured simultaneously.
The National Physical Laboratory has an on-line Adhesive Design Toolkit.
The Araldite/Huntsman Advanced Materials website offers:
The Permabond Engineering Adhesives website offers pages on Adhesives Overview, Anaerobics, Cyanoacrylates and useful Application Tips:
Suppliers of adhesives
Reference:
Welding (see Åström [1] pp 335-336 and Gutowski [7] pp 501-509)
When the joining of thermoplastic matrix composites is required, this is normally achieved by thermal welding: heat - compress - allow intermolecular diffusion - cool. A variety of techniques may be used to heat the substrates:
Yousefpour et al [11] have recently reviewed the different fusion-bonding methods for thermoplastic composite components and presented recent developments in this area. The various welding techniques and the corresponding manufacturing methodologies, the required equipment, the effects of processing parameters on weld performance and quality, the advantages/disadvantages of each technique and the applications are described. The Edison Welding Institute website has a series of short introductions to the various techniques. Abrate [12] has published a useful list of relevant references.
Solvent welding is a potential alternative technique, but it is rarely used for composites for health and safety and for solvent entrapment reasons.
Painting/surface coatings
The painting of composite substrates has essentially similar requirements for surface preparation as for adhesive bonding (see above). Ryntz and Yaneff [13] have surveyed recent developments in the coating of polymers and plastics. Tracton has published a handbook of Coatings Technology [14].
Repair (see Åström [1] pp 340-346, Armstrong & Barrett [15] and NetComposites)
Before repair, it is first necessary to determine the full extent of the damage (often utilising non-destructive testing techniques) and then use appropriate machining techniques to remove the failed material. For a general repair, the hole is normally tapered at ten times the depth of the hole. For an aerospace repair the hole is normally tapered at fifty-times the depth of the hole or at 12.7 mm/ply.
Dorworth [16] has reviewed the lessons learned by aerospace companies, whilst increasing the scope and quality of repairs, to identify lessons learnt that may be transferred to other industries.
For sandwich panels, it may be practical to replace just one laminate skin, or one skin and the core, leaving the second face intact. A foaming adhesive is normally used to bond-in replacement honeycomb.
There is increasing interest in self-healing composites for aircraft [17] and space [18, 19] applications utilising hollow glass fibres which contain uncured resin. Potential problems with this technology are that low viscosity resin systems generally do not achieve the highest mechanical properties, while high viscosity resin systems would require some form of pressure to facilitate flow. Further, there are issues with retarding cure until such time as the resin has flowed into the fractured component. The University of Delaware Center for Composite Materials is developing biomineralisation as a route to the repair of the fibre network [20].
Composites may also be used to repair other material systems. Guidance documents appropriate to establishing the state of the substrate include:
References
Videos (no longer available in PU CSH Library):