Drawing No. EH–MT–007 // Engineering Materials
Carbon / Epoxy Composite Properties
Reviewed September 2026
IM7/8552 is a high-performance aerospace carbon/epoxy system. The displayed 0° properties demonstrate why composites must be described by fiber direction, layup, cure condition and environment rather than one isotropic material constant.
Representative properties
The stated basis, temperature and direction are part of the value. Switch units without changing the physical reference.
What changes these properties?
Engineering variability
- 0°, 90°, shear and laminate-level properties can differ by an order of magnitude or more.
- Moisture and elevated temperature reduce matrix-dominated properties.
- Layup, fiber volume, cure cycle, void content and defects control performance.
- There is no conventional homogeneous yield point or melting point for the cured thermoset composite.
Selection note
Representative of aerospace primary-structure composites where high specific stiffness and strength dominate, but laminate analysis and damage tolerance are mandatory.
Where a property is shown as qualitative rather than numeric, that is intentional: the selected source or the material physics does not justify a single comparable scalar.
Carbon fibre epoxy in practice
A material designed at the same time as the part, which is why its data sheet means less than its layup.
Where it is specified
Aerospace primary and secondary structure (the 787 and A350 fuselage and wings are majority composite), Formula 1 and high-end automotive monocoques, wind turbine blade spar caps, pressure vessels and COPV tanks, sports equipment, and satellite structures where dimensional stability under thermal cycling matters.
It is chosen where specific stiffness governs. Carbon fibre epoxy can reach several times the stiffness-to-weight of aluminium along the fibre direction — but only along it.
Chosen over what
Against aluminium: far better specific stiffness and strength, no fatigue endurance problem in well-designed laminates, and near-zero thermal expansion if the layup is tailored. Against that: an order of magnitude more expensive, anisotropic, difficult to inspect, and unrepairable by conventional means.
Against glass fibre: roughly three times the stiffness at 30% lower density, at several times the price. Wind blades use carbon only in the spar caps where stiffness pays, and glass elsewhere.
Against titanium: lighter and stiffer, but titanium is isotropic, tolerant of point loads and bolted joints, and predictable in fire — which is why the two are used together, with titanium fittings at load introduction points.
How it fails
Delamination is the characteristic mode. The matrix, not the fibre, carries interlaminar load, so through-thickness strength is a small fraction of in-plane strength. Barely visible impact damage is the design driver in aerospace: a tool dropped on a panel can leave almost no surface mark while producing internal delamination that reduces compressive strength by 40–60%. Design allowables are set by compression-after-impact testing for exactly this reason.
Galvanic corrosion of adjacent metals. Carbon fibre is cathodic and will drive aggressive corrosion of aluminium or steel fasteners in contact with it; a glass-fibre isolation ply or sealant is standard practice.
Moisture absorption and hot-wet knockdown. Epoxy absorbs 1–2% moisture, which lowers the glass transition temperature and reduces matrix-dominated properties; design values are quoted hot-wet for this reason.
Free-edge and bolt-hole stresses, where the laminate cannot redistribute load plastically as a metal would.
Fabrication notes
The layup is the material. Fibre orientation, stacking sequence, symmetry and balance determine properties entirely. A quasi-isotropic laminate typically achieves only about half the axial modulus of a unidirectional one — the single scalar modulus in any property table is a directional reference, not a design value.
Cure control governs quality. Prepreg is autoclave-cured under vacuum and pressure on a controlled ramp; voids above 1–2% measurably reduce interlaminar strength. Out-of-autoclave and resin infusion processes trade some quality for cost and size.
Machining requires diamond-coated tooling; carbon fibre is highly abrasive, and drilling causes exit-side delamination unless backed and fed correctly. The dust is electrically conductive and a hazard to equipment.
Joining is by adhesive bonding or by bolting into thickened, reinforced regions; welding is impossible and bolted joints require careful bearing-bypass analysis.
Specifications and standards
ASTM D3039 (tension), D6641 (compression), D2344 (short-beam shear), D7137 (compression after impact), D5528 (Mode I interlaminar fracture); ISO 527-5 and ISO 14126; SACMA recommended methods; NCAMP and CMH-17 (Composite Materials Handbook, formerly MIL-HDBK-17) for statistically derived design allowables.
References
CMH-17 Composite Materials Handbook; Hexcel and Toray prepreg technical data; Daniel and Ishai, Engineering Mechanics of Composite Materials.
How to interpret this material
Stiffness
Young’s modulus describes elastic stiffness. For crystals and composites, orientation can make one scalar modulus only a directional reference.
Strength / failure
Ductile metals can use yield and tensile strength. Brittle ceramics/semiconductors and anisotropic composites require fracture, flexural or directional strength descriptions instead.
Thermal response
Conductivity, heat capacity and expansion are temperature dependent. The last column is a melting or transition descriptor only where that physical concept is appropriate.
Technical sources
The database favors manufacturer, industry, government or academic technical references and labels secondary condition-specific datasets when a primary source does not expose a complete property set.