Composites Fabrication for Aerospace Structures
Aerospace composite parts are made by laying up prepreg or dry fabric on a tool, consolidating and curing under controlled pressure and temperature, then verifying the laminate with ultrasonic inspection and test coupons. Material control, especially prepreg out-time, and cure records matter as much as the layup itself.
Where the quality is decided
A composite part is made of material whose properties are created during your supplier's process, not delivered by a mill. That changes how engineers should buy it. The questions that matter are about material handling, cure control and how the laminate is verified, not just about dimensions.
For design data and test methodology the reference is CMH-17, the Composite Materials Handbook, published by SAE on behalf of the CMH-17 organization at Wichita State University. Volume 1 covers characterization, test planning, sampling, data reduction and statistics (revision H, 2022). Volume 2 holds statistically based polymer matrix property data and Volume 3 covers design, analysis and manufacturing guidance. Programs build their allowables and process specifications from this framework, so a supplier that cannot discuss it is unlikely to be qualified for structure.
Prepreg handling and out-time
Thermoset prepreg starts curing slowly as soon as it warms up. It is stored frozen at the temperature the material data sheet specifies, and the freezer itself should be monitored and alarmed. Out-time is the cumulative time out of the freezer, and it must be logged every time the roll comes out and goes back. Limits vary widely by resin system, and some resins allow only a few weeks at room temperature. The material data sheet or process spec is the authority.
Why it matters: a SERDP funded study found that the main cause of out-time defects was insufficient resin flow, caused by crosslinking during room temperature exposure, which shows up as porosity and poor consolidation. The same study found freezer storage did not significantly change resin Tg or rheology after 20 months, and that B-stage Tg measured by DSC could track accumulated room temperature time.
What to require: material certs with batch and manufacture date, freezer storage records, out-time logs per roll and per ply kit, and thaw procedures that keep rolls bagged until they reach room temperature so moisture does not condense on cold material.
Layup and cure methods
| Method | How it works | Where it fits | Watch for |
|---|---|---|---|
| Autoclave prepreg | Vacuum bagged layup cured under heat and pressure in a pressure vessel | Primary structure, highest consistency | Autoclave size limits part size, high capital cost |
| Out of autoclave prepreg | Vacuum bag only cure in an oven with resins formulated for low pressure | Large parts, lower tooling and equipment cost | Very sensitive to out-time and bagging quality |
| Resin transfer molding (RTM) | Dry preform in a closed matched mold, resin injected | Complex shapes, two good surfaces | Tooling cost, dry spots |
| Vacuum infusion | Dry fabric under a bag, resin drawn through by vacuum | Large secondary structure and tooling | Fiber volume control |
| Filament winding | Fiber wound on a rotating mandrel | Pressure vessel overwraps, tubes, motor cases | Limited to shapes of revolution |
Whatever the method, ply orientation, ply count, ply drops and splice locations must follow the ply table on the drawing. A ply book or laser projected layup with sign off per ply is normal for structure.
Cure records should include thermocouple traces from the part or tool, not just the oven air, plus vacuum and pressure traces. Lagging thermocouples on thick sections and leading ones on thin edges show whether the whole part saw the required cure. The heat treatment page explains furnace pyrometry, and many composite process specs borrow the same discipline for ovens and autoclaves.
Tooling
Tool material is chosen for thermal expansion match and durability. Invar is common for high temperature cure of carbon fiber parts because its low expansion is close to that of the laminate, which keeps dimensions and reduces residual stress. Composite tools are cheaper and lighter for prototypes and moderate rates. Aluminum is inexpensive and machines easily but expands far more than carbon laminates, so it suits low temperature cure or parts where dimensional accuracy at cure temperature is not critical. The controlled expansion alloys page covers Invar 36, and large tool machining is on the large part machining page.
Verification: NDI and coupon testing
Most structural laminates get ultrasonic inspection, usually through transmission or pulse echo C-scan, to find delaminations, porosity and inclusions. Acceptance comes from the drawing or process spec. Bonded joints and sandwich structures may also need tap testing or bond testers. NDT methods are covered on the nondestructive testing page.
Process control coupons, either cut from trim areas or laid up as a traveler panel cured with the part, are tested to standard methods:
| Test | Standard | What it tells you |
|---|---|---|
| Fiber, resin and void content | ASTM D3171 (current D3171-22) | Method I digests or burns off the matrix to calculate fiber, matrix and void volume. Method II estimates content from thickness and known fiber areal weight but cannot measure voids |
| Short beam strength | ASTM D2344 | Apparent interlaminar shear strength from a three point bend on a short beam, span to thickness ratio 4.0. Used mainly as a cure and void quality screen, not for design allowables |
| Glass transition temperature | ASTM D7028 (D7028-07, reaffirmed 2024) | DMA Tg in flexure, used as an indicator of upper service temperature and cure state. Results depend on apparatus and parameters, so compare like with like |
D3171 notes that higher void content tends to reduce fatigue resistance and increase moisture susceptibility, which is why void limits appear in most aerospace laminate specs.
Machining, drilling and bonding
Trimming and drilling carbon laminates uses diamond coated or PCD tooling and controlled feeds to avoid delamination and fiber breakout at exit. Backup material and peck strategies matter. Hole quality requirements, including delamination limits around the hole, should be on the drawing. Machining dust from carbon fiber is conductive and must be extracted.
Secondary bonding requires a controlled surface: peel ply or abrasion, solvent wipe and bonding within the time window the adhesive spec allows. Bond line thickness and cure are recorded like a laminate cure. Ask for traveler coupons for bonded joints on structure, often lap shear or similar tests per the program spec.
What the drawing and PO should state
- Material specification, including resin system, fiber, form and areal weight, and any approved product list
- Ply table with orientation, sequence, ply drops and allowable splice or overlap locations
- Process specification for layup and cure, with the required cure cycle reference rather than a cycle written on the drawing
- NDI method and acceptance class, and which zones need full coverage
- Coupon testing required per part, per lot or per cure load, with the test standards and limits
- Hole quality, edge sealing and surface finish requirements for trimmed and drilled edges
Leaving any of these open forces the fabricator to assume, and assumptions on structure tend to come back as findings at first article.
Get composite parts quoted
If you have composite layups, bonded assemblies, tooling or trim and drill work, send the drawing with the material and process specs. We source from qualified composite fabricators, flow down material control, cure record and inspection requirements, check the records and coupon data before release, and a person replies within one business day. We sign a mutual NDA before files. Export controlled data follows our controlled programs process. Back to the processes hub, or see inspection and metrology.
Questions
What is prepreg out-time and why does it matter?
Out-time is the cumulative time a thermoset prepreg spends above freezer temperature. The resin keeps slowly curing, which reduces flow during the final cure and can leave porosity or poor consolidation. Limits come from the material data sheet or process spec and vary widely, with some resins allowing only a few weeks at room temperature. Suppliers should log out-time per roll and per ply kit.
Autoclave or out of autoclave?
Autoclave cure gives the most consistent consolidation and is the default for primary structure, but part size is limited by the vessel and costs are high. Out of autoclave prepregs are formulated to consolidate under vacuum only and can cure in an oven, which suits large parts. They are more sensitive to out-time and bagging quality, so process control has to be tighter.
Which tests confirm a laminate was cured correctly?
Common checks are ultrasonic C-scan for internal defects, ASTM D3171 for fiber, resin and void content, ASTM D2344 short beam strength as a screen for cure and porosity, and ASTM D7028 DMA for glass transition temperature. Thermocouple and pressure records from the cure itself are the first line of evidence. Your process spec will say which tests apply and the limits.
Why is Invar used for composite tools?
Its thermal expansion is low and close to that of carbon fiber laminates. A tool that expands much more than the laminate during cure changes the part dimensions and can induce residual stress. Invar tools hold dimensions well through high temperature cures and survive many cycles. They are heavy and expensive, so composite or aluminum tools are often used for prototypes or low temperature cures.
What is CMH-17 used for?
CMH-17, the Composite Materials Handbook, is the standard reference for characterizing composite materials, generating statistically based property data, and design, analysis and manufacturing guidance. Volume 1 covers test planning and statistics, Volume 2 holds polymer matrix property data, and Volume 3 covers design and manufacturing. Programs use it to build allowables and process specifications.
Related
Sources
- Wichita State NIAR: about CMH-17
- SAE: Composite Materials Handbook Volume 1 Revision H
- ASTM D3171-22 constituent content of composite materials
- Instron: ASTM D2344 short beam strength testing
- ANSI Webstore: ASTM D7028-07(2024) DMA glass transition
- SERDP: prepreg out-time project
- Explore Composites: prepreg handling
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