Question 6 of 8: Processing routes for aerospace FRP laminated components

Question 6: Processing routes for aerospace FRP laminated components (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 is turned into 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. In Canadian aerospace practice the choice is further constrained by certification: primary structure certified to Transport Canada Civil Aviation requirements, following the composite guidance harmonised with FAA AC 20-107B, must be made by a process whose statistical allowables are established from coupon and element testing, which strongly favours the mature, well-characterised routes for anything flight-critical.

(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 pressure of 0.6–0.7 MPa and a programmed temperature cycle to 180 ℃. The pressure collapses entrapped air and dissolves residual volatiles back into the resin.

Its advantage is decisive and is why it remains the reference process for wing skins, spars and fuselage barrels: fibre volume fractions of 60–65 %, void contents below 1 % and tightly controlled ply orientation give the highest and, more importantly, the least scattered mechanical properties of any route, so the design allowables can be set close to the mean. 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 limit the whole factory, the prepreg itself must be shipped and stored frozen with a finite out-time, and part size is capped by the vessel. Cost per kilogram is the highest of the four.

(b) Liquid composite moulding: resin transfer moulding (RTM) and its vacuum-assisted variant (VARTM). A dry fibre preform, often stitched 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, then cured.

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 out-of-autoclave curing removes the bottleneck vessel entirely. VARTM in particular scales to very large parts. Against that, fibre volume fractions typically reach only 50–60 %, void content is harder to control than under 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. RTM tooling for a closed mould is expensive and slow to develop, so the route pays for itself only over a substantial production run.

(c) Filament winding. Continuous tow is drawn through a resin bath and wound onto a rotating mandrel along computer-controlled helical, hoop or polar paths, then cured, and the mandrel is extracted or dissolved.

Where the geometry suits it, nothing competes: it is fast, highly automated, uses the cheapest possible form of fibre, and places each tow under controlled tension along precisely the trajectory the load demands, so pressure vessels, drive shafts, engine-casing rings and rocket motor cases achieve outstanding specific strength. Fibre volume fractions of 60 % and above are routine. The limitation is geometric and absolute: the process makes surfaces of revolution and convex shapes, cannot produce a concave surface or a flat panel with a good finish, and cannot easily place fibre in the axial direction of a small-diameter part. Mandrel extraction constrains the design, the outer surface is rough unless the part is subsequently machined or shrink-taped, and interlaminar properties depend heavily on winding tension and bandwidth control.

(d) 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, capable of steering and of dropping individual tows) onto the tool, compacting with a heated roller. The result is either taken to an autoclave or, with thermoplastic matrices and laser heating, consolidated in situ.

This is the route that made large composite airframes economic. 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 that let the laminate 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 only justified at rate, deposition slows sharply on tightly curved or highly contoured surfaces where the head must decelerate, and small or intricate parts are still made by hand. Unless in-situ consolidation is used, the autoclave bottleneck remains.