Question 5 of 8: Three consolidation routes for aerospace FRP laminates, and where each may fly

Question 5: Three consolidation routes for aerospace FRP laminates, and where each may fly (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Consolidation is the step in which a stack of fibre and resin becomes a void-free solid laminate, and it is the step that decides whether a composite part is fit for primary structure. Three variables govern it: the fibre volume fraction achieved, the void content left behind, and the fibre alignment preserved. Every process below is a different bargain among those three and the commercial realities of rate, size and tooling cost.

The distinction the question turns on is a certification one, so it is worth stating before the processes are described. Primary structure is structure whose failure would cause loss of the aircraft — wing skins and spars, fuselage barrels and frames, the empennage torque boxes, engine pylons. Secondary structure carries load but its failure is survivable: fairings, access panels, control-surface skins, radomes, interior floor panels. Primary structure certified to Transport Canada Civil Aviation requirements, following the composite guidance harmonised with FAA AC 20-107B, must be produced by a process whose statistical design allowables have been established from coupon, element and sub-component testing, and whose process parameters are frozen and monitored. That is a heavy burden, and it is the single biggest reason a route may be excellent yet still be used only for secondary parts.

(a) Autoclave curing of pre-impregnated tape or fabric. Layers of prepreg — fibre already impregnated with a partially advanced (B-staged) epoxy at a controlled resin content — are laid onto a mould, vacuum-bagged, and cured under simultaneous vacuum, external gas pressure of 0.6–0.7 MPa and a programmed temperature cycle to 180 ℃. The pressure collapses entrapped air and drives residual volatiles back into solution in the resin.

Applicability. This is the reference process for primary structure and is equally usable for secondary parts, though it is rarely economic for them. Its advantage is decisive: fibre volume fractions of 60–65 per cent, void contents below one per cent and tightly controlled ply orientation give the highest and, more importantly, the least scattered mechanical properties of any route, so B-basis allowables can be set close to the mean and the designer is not forced to knock the material down to cover process variability. The disadvantages are equally decisive. Cycle times run to six or eight hours; the autoclave is a very large pressure vessel whose capital cost and throughput govern the whole factory and whose internal dimensions cap part size; the prepreg must be shipped and stored frozen and has a finite out-time once thawed; and cost per kilogram is the highest of the three. Geometrically it suits panels, skins and mildly double-curved surfaces; deep concave features are hard to bag without bridging.

(b) Liquid composite moulding: resin transfer moulding (RTM) and its vacuum-assisted variant (VARTM). A dry fibre preform, often stitched, woven or braided to near-net shape, is placed in a closed matched-metal tool (RTM) or under a vacuum bag on a single-sided tool (VARTM), and low-viscosity resin is injected or infused through it and then cured, out of autoclave.

Applicability. Mainstream for secondary structure, and accepted for primary structure only case by case, on programmes that have invested in qualifying the specific preform, resin and tool — the A380 rear pressure bulkhead and the A400M spars are the standard counter-examples showing it can be done. The attractions are cost and geometry. Dry fabric is far cheaper than prepreg and needs no frozen storage; complex three-dimensional shapes, thick sections and integrally stiffened parts can be made in one shot, eliminating fasteners and the assembly labour that goes with them; and removing the autoclave removes the factory bottleneck. VARTM in particular scales to very large parts, since atmospheric pressure costs nothing however big the bag. Against that, fibre volume fractions typically reach only 50–60 per cent, void content is harder to control without autoclave pressure, and the process is sensitive to preform permeability — a dry spot or a race-tracked flow front is a rejectable defect that is not always visible from outside. Closed RTM tooling is expensive and slow to develop, so the route repays itself only over a substantial production run.

(c) Automated tape laying and automated fibre placement (ATL/AFP). A numerically controlled head lays prepreg tape (ATL, wide tape on gently contoured surfaces) or several narrow slit tows independently (AFP, able to steer and to drop individual tows) onto the tool, compacting each course with a heated roller. The result is then either taken to an autoclave or, with thermoplastic matrices and laser heating, consolidated in situ as it is laid.

Applicability. The route of choice for large primary structure — it is how the 787 fuselage barrels and A350 wing skins are made — and equally capable on secondary parts, though seldom justified for them. It removes hand labour and its variability from the laying operation, achieves very high and repeatable deposition accuracy, supports ply drop-offs and steered fibre paths so the laminate can be tailored locally to the load field, and scales to fuselage-barrel size. Its costs are capital and time: the machines and their programming are extremely expensive and are justified only at rate; deposition slows sharply on tightly curved or highly contoured surfaces where the head must decelerate and where narrow tows must be used; and small or intricate parts are still laid by hand. Unless in-situ consolidation is used, the autoclave bottleneck remains behind it.