Carbon Fibre Reinforced Polymers (CFRP): Synthesis, Manufacturing & Microstructure Design

Carbon Fibre Reinforced Polymers (CFRP) provide maximum strength-to-weight ratios, making them the primary structural material for modern aerospace (Boeing 787, Airbus A350), wind energy, and performance automotive components. In India, companies like Tata Advanced Systems and Hindustan Aeronautics Limited (HAL) fabricate CFRP panels to reduce weight and increase fuel efficiency. Understanding carbon fiber precursor chemistry, weave patterns, and laminating processes is a core competency for advanced composites engineers.

• Trace the synthesis of carbon fibres from Polyacrylonitrile (PAN) and pitch precursors.
• Compare carbon fibre grades (Standard Modulus, High Modulus, Ultra High Modulus).
• Explain CFRP manufacturing methods (autoclave prepreg, resin transfer moulding - RTM, pultrusion).
• Calculate laminate ply orientation configurations and directional elastic properties.
• Reference international composite testing standards including ASTM D3039 and ISO 527.

3.1 Carbon Fibre Precursor Chemistry

Over 90% of carbon fibres are synthesised from Polyacrylonitrile (PAN) precursors through three thermal steps:

1. Oxidative Stabilization: Heating PAN fibres to 200–300°C in air to form a ladder polymer structure (infusible cyclic ring).
2. Carbonization: Heating to 1000–1500°C in an anaerobic nitrogen atmosphere to drive off non-carbon elements (H, N, O), leaving aligned carbon sheets.
3. Graphitization: Heating above 2000°C to align graphite crystallites, increasing elastic modulus.

3.2 CFRP Manufacturing Methods

• Prepreg Autoclave Curing: Partially cured (B-staged) pre-impregnated plies are stacked, vacuum bagged, and cured under high pressure (0.6 MPa) and temperature. Yields the highest $V_f > 60\%$ and minimum voids ($< 1\%$).
• Resin Transfer Moulding (RTM): Dry fiber preforms are placed in a closed metal mould, and liquid epoxy is injected under pressure, followed by curing. Ideal for complex geometric shapes.
• Filament Winding: Continuous carbon rovings are wound onto a rotating mandrel to make pressure vessels (Type IV CNG/Hydrogen tanks).

3.3 Elastic Modulus of CFRP (Rule of Mixtures)

For unidirectional continuous CFRP loaded along the fibres:

For standard PAN carbon fibre ($E_f = 230$ GPa) and epoxy matrix ($E_m = 3.5$ GPa) with $V_f = 0.60$:

Problem: A structural engineer designs a CFRP component. The candidate PAN carbon fibre has a density $\rho_f = 1.80$ g/cm³ and tensile strength $\sigma_f = 3500$ MPa. The epoxy matrix has a density $\rho_m = 1.20$ g/cm³ and tensile strength $\sigma_m = 80$ MPa. The laminate has a fibre volume fraction $V_f = 60\%$. Calculate:

1. The overall density of the CFRP composite.
2. The longitudinal tensile strength ($\sigma_c$) using the simplified rule of mixtures.
3. Compare the specific strength (strength/density) of this CFRP with structural steel (strength = 450 MPa, density = 7.85 g/cm³).

Solution:

1. Calculate composite density $\rho_c$:

1. Calculate longitudinal tensile strength $\sigma_c$ (assuming isostrain state):

Where $\sigma_m^*$ is the matrix stress at fiber failure (\approx. 50 MPa). Using the conservative fiber-dominated approximation:

1. Calculate specific strengths:

• Specific strength of CFRP: $\frac{2100 \text{ MPa}}{1.56 \text{ g/cm}^3} = \mathbf{1346.15 MPa/(g/cm}^3\mathbf{)}$
• Specific strength of Steel: $\frac{450 \text{ MPa}}{7.85 \text{ g/cm}^3} = \mathbf{57.32 MPa/(g/cm}^3\mathbf{)}$

Interpretation: The CFRP composite is 23.4 times structurally more efficient in tension than structural steel per unit weight, demonstrating why carbon fibre is preferred in weight-sensitive aerospace structures.

Hindustan Aeronautics Limited (HAL) uses advanced carbon/epoxy prepregs in their composite manufacturing division (Bengaluru) to produce the fuselage, tail fin, and wings of the Tejas LCA fighter jet, achieving weight savings and high maneuverability.

• PAN: Polyacrylonitrile; the primary polymer precursor used to synthesize carbon fibers.
• Carbonization: Anaerobic heat treatment removing non-carbon atoms, forming pure carbon sheets.
• Prepreg: Partially cured resin-impregnated fiber sheets ready for lay-up and autoclave curing.
• Specific Strength: Tensile strength divided by density; measures material structural weight efficiency.
• RTM: Resin Transfer Moulding; liquid composite molding process injecting resin into dry fiber preforms.

1. ASTM D3039 — Standard Test Method for Tensile Properties of Polymer Matrix Composite Materials
2. ISO 527-5 — Determination of tensile properties of unidirectional fibre-reinforced plastic composites
3. ASTM D3529 — Resin solids and volatile content of prepregs

1. Describe the chemical structural changes that occur during the oxidative stabilization of PAN precursors. Draw the molecular ladder structure.
2. Why do carbon fibre laminates require symmetric and balanced ply lay-ups (e.g., $[0/\pm45/90]_s$)? Describe the curing defects that occur in unsymmetric laminates.
3. Calculate the transverse modulus ($E_2$) of a carbon/epoxy composite with $V_f=0.55$, $E_f=22$ GPa, and $E_m=3.5$ GPa.