Composite Materials and Manufacturing Processes Notes

Aircraft Structural Materials and Joints

  • Advanced composites are discussed, including technological processes.
  • References:
    • Marques AT. Composite Systems: Design and Manufacture for Durability, University of Porto, September 2019.
    • Miravete A. Materials, Composites Design Workshop XIX, Stanford University, July 2020.
    • Gay D, Hoa SV, Tsai SW. Composite Materials Design and Applications. CRC Press LLC, 2003.
    • Jones RM. Mechanics of Composite Materials, 2nd Ed. Taylor & Francis Group, LLC, 1999.

Manufacturing Processes for Thermoset Matrix Composites

  • Thermoset matrices are the most used for structural applications.
  • Process:
    • Fibrous reinforcement (pre-)impregnation
    • Curing (pressure + temperature)
    • Consolidated composite system
  • Pressure and temperature are applied to optimize the structural performance of the composite.

Vacuum Bagging and Autoclave Curing - Prepreg Processing

  • To achieve the highest properties, ply thickness and resin distribution are controlled using pre-impregnated material (prepreg concept).

Prepreg Processing: Material Placement and Hand Lay-Up

  • The prepreg material is placed over a tool (mould) to form the geometric shape of the part.
  • Hand lay-up:
    • Rate: 1 kg/hr
    • Suitable for small parts and complex shapes.
    • High labour costs.
    • Unidirectional, plain weave, and harness satin weave architectures are used.
    • Combinations, such as plain weave and unidirectional, are also utilized.

Classical (Manual) Process

  • Pre-form (tape, prepreg) cutting
  • Positioning and placing, potentially using a laser projection system

Vacuum Bagging and Curing

  • After positioning, vacuum bagging is applied.
  • Curing is done in an autoclave.

Automated Fibre Placement (AFP) and Automated Tape Laying (ATL)

  • Material is placed over a tool (mould) to form the geometric shape of the part.
    • Hand lay-up: 1 kg/hr
    • Automated fibre placement: 5 kg/hr, tape width from 3.175 mm (1/8”) to 12.7mm (1/2”)
    • Automated tape laying: 20 kg/hr, tape width up to 300 mm (12”)

Automated Processes for Thermoset Matrix Composites

  • Automated processes involve:
    • Pre-form (tape, prepreg)
    • (Vacuum bagging)
    • Curing

Automated Fibre Placement (AFP)

  • Automated Fibre Placement (AFP) is an automated process of heating and compacting narrow unidirectional prepreg tapes.
  • Suitable for medium to large complex curvature monolithic parts.

Automated Tape Laying (ATL)

  • Automated tape laying (ATL) allows rapid deposition of composite prepreg tapes to large monolithic parts with low to medium curvature.
  • ATL is a well-established automated manufacturing technique for composites.
  • Unidirectional prepreg tapes are laid onto a part mould/tool using different process variants and manufacturing routes.

Vacuum Bagging and Autoclave Usage in Prepreg Processing

  • The assembly is vacuum bagged before being transferred to the autoclave (or oven) for curing.
  • To optimize performance and minimize void content, both temperature and pressure are applied during curing (autoclave).

Fast Curing Prepregs and Alternative Cure Cycles

  • Fast curing prepregs:
    • Curing cycles below 7 min at 120ºC and pressures over 5 bar.
    • Alternative cure cycles can be used.

Oven Curing

  • An oven (only temperature is applied) may also be used - out-of-autoclave prepregs – currently still restricted to non-aerospace applications.

Liquid Moulding Processes

  • Resin infusion involves:
    1. Dry reinforcements
    2. Resin impregnation

Resin Transfer Moulding (RTM)

  • Mould and counter-mould are used.
  • Suitable for highly structural parts.
  • Pressure: 4 bar (up to 120 bar)
  • Matrix viscosity: ≈200 cP
  • Injection time: ≈ minutes
  • Darcy’s law (1D) governs the injection process: t:injectiontimet: injection time
    L:wet−outlengthL: wet-out length
    ΔP:drivingpressureincrement(Pa−Pb)\Delta P: driving pressure increment (Pa - Pb)
    Pa:injectionpressurePa: injection pressure
    Pb:atmosphericpressurePb: atmospheric pressure
    η:viscosityoftheresin\eta: viscosity of the resin
    K:permeabilityofthefabricK: permeability of the fabric

Application of Resin Transfer Moulding (RTM) - Airbus A320 Spoilers

  • New advanced composite spoilers for the Airbus A320 Family are produced using Resin Transfer Moulding (RTM) technology.
  • The redesigned spoiler is produced using a highly-automated, out-of-autoclave process.
  • Results in improved manufacturing efficiencies and reduced energy and costs compared to the original design.

Vacuum Assisted RTM (VARTM)

  • Process involves a vacuum bag, resin inlet, and vacuum outlet to assist resin infusion.

Thermoplastic Composite Manufacturing

  • Relevant aspects include:
    • Curing is not necessary.
    • Residual stresses are related to material properties and processing temperatures.
    • Reduced tendency for delamination.
    • Fibre movement during “thermoforming” and welding.
    • Increased tendency for resin rich areas.
    • Variation in crystalline morphology variations of the mechanical properties.

Automated Lay-Up and Consolidation

  • Process:
    • Automated lay-up
    • Consolidation
    • Heating
    • Hot stamping
  • Composite tapes are used for tailored layup and preform.

In Situ Consolidation

  • Parameters: Pressure, temperature, time.

Cutting of Reinforcements and Trimming of Laminates

  • Ply drops: Some components need a large number of layers with different formats.
  • Cutting and trimming should:
    • Respect the shape of the cut.
    • Respect the orientation specified by design.
    • Minimize waste.
  • Methods:
    • CNC cutting tables
    • Laser beams

Finishing and Machining Operations

  • Trimming the edges is the first and generally mandatory machining operation after demoulding of composite parts.
  • Drilling operations usually follow.
  • Machining of composite parts is done mainly by conventional machining techniques or abrasive water jet machining.
  • Typical defects caused by trimming operations:
    • Fibre pull-out and uncut fibres that remain at the cut edge
    • Delamination
    • Resin degradation, leading to better surface quality and lower tool wear
  • Typical defects caused by drilling operations:
    • Decohesion of the first ply at the hole entry
    • Fibre pull-out and thermal-mechanical degradation at the hole wall
    • Delamination of the last ply at the hole exit

Types of Defects in Composite Systems

  • Porosity
  • Microcracking
  • Contamination
  • Fibre breakage, misalignment, and undulation
  • Nicks, scratches, and gouges
  • Dents
  • Distortion
  • Degree of cure variations of fibre/resin ratio
  • Stacking sequence
  • Prepreg failures and joints
  • Delaminations and disbonds
  • “Honeycomb” core damage (sandwich structures)
  • Misplacement of filling compound
  • Gap / Overlap, Wrinkle, Fold, Lack of material, Positioning, Intrusions

Effect of Defects

  • Void content:
    • Low: < 0.9%
    • Medium: 0.9-3.0%
    • High: > 3.0%
  • Variation of interlaminar shear strength with the void content and preconditioning.

Effect of Inclusions

  • Effect of inclusions, at half thickness, on interlaminar shear strength

Quality Control

  • Composite material control begins at the supplier level.
  • Process Control Documents (PCD) are used to regulate the manufacture of a given composite material.
  • Statistical Process Control (SPC) is used to monitor the data generated during manufacture of the composite material.
  • Lot release testing (supplier level): A set of tests is performed on the material to assure that the new batch is comparable to the material originally qualified.
  • Lot acceptance/receiving inspection (user level): The composite material user typically prepares material specifications which define incoming material inspection procedures and supplier controls that ensure the materials used in composite construction will meet the engineering requirements.
  • Shelf life and out time control (user level):
    • B-staged materials change relatively rapidly when exposed to room temperature, so the out-time is controlled.
    • Once parts have been laid up with B-staged materials, they may be allowed some additional time at room temperature before beginning cure.
    • Because B-staged materials continue to change during refrigerated storage, the shelf life is also controlled.
    • After the shelf life has been exceeded, extensions may be allowed by the controlling specification.
  • The quality of a composite part is to a significant extent determined by the manufacturing process.
  • Quality control during manufacturing is crucial to obtain satisfactory results.

Non-Destructive Inspection (NDI) Techniques

  • Mechanical: Bending, vibration, tap tests, lamb wave
  • Acoustic: Ultrasounds / acoustic sensing
    • Widely used to detect delamination, foreign object inclusions and porosity.
  • Radiology: X-ray / computer tomography
  • Thermal: Thermography
  • Electrical: Eddy current / dielectric analysis
    • Often applied to monitor curing
  • Visual: Visual inspection / holography / shearography / optical fibres
  • The integration of emerging NDI technologies with existing ones and numerical simulation is the focus of current major research.

Destructive Inspection Techniques

  • Cross-sectioning:
    • Polishing the edge of a cut through the part and observed on an optical microscope to identify fibre distribution, fibre orientation, fibre waviness, fibre volume content, void content, porosity, matrix cracks, and delaminations.
  • Resin tests:
    • To determine the extent of cure of the resin through the glass transition temperature (via thermomechanical analysis, TMA) and the residual heat of reaction (via differential scanning calorimetry, DSC).
  • Mechanical tests:
    • Conducted on the full part or on smaller coupons or elements cut from the part.
  • Resin ingestion:
    • Burning off the resin from a sample of the laminate to determine fibre and void fractions.
  • Deply:
    • Heating a laminate to allow the individual plies to be pulled apart and visually identify fibre breakage within each ply.