S — Aircraft StructuresChapter 3 · 55 practice questions

Chapter 3: Composite Structures - Fiberglass, Carbon Fiber, Kevlar, Honeycomb, Vacuum Bagging, Bonded Repairs

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Composite Structures - Fiberglass, Carbon Fiber, Kevlar, Honeycomb, Vacuum Bagging, Bonded Repairs

Overview

This chapter covers the fundamental principles of composite materials used in aircraft structures, including fiberglass, carbon fiber, and Kevlar reinforcements, as well as honeycomb core constructions. It addresses material properties, damage mechanisms, inspection techniques, and repair procedures. Understanding the behavior of resin systems, the importance of environmental control, and proper repair methodologies is essential for maintaining the structural integrity of composite aircraft components.

Diagram — Composite Structures - Fiberglass, Carbon Fiber, Kevlar, Honeycomb, Vacuum Bagging, Bonded Repairs Composite Structures - Materials & Repair Fiberglass Glass fibers in epoxy/polyester Non-conductive, corrosion resistant Radomes, fairings, secondary struct. Carbon Fiber High strength, high stiffness Electrically conductive Wing skins, control surfaces Kevlar (Aramid) Organic fiber, high tensile strength Toughness, impact resistant Ballistic protection, leading edges Reinforcement Fibers Honeycomb Core Construction Core (Nomex/Aluminum) Top Skin (Fiberglass/Carbon) Bottom Skin Adhesive bond between skin and core Vacuum Bagging Process Vacuum Bag (nylon film) Breather Cloth Release Film (perforated) Laminate (prepreg layers) Release Film (solid) Tool (mold surface) To vacuum pump Bonded Repair Process 1. Damage assessment 2. Remove damaged material 3. Prepare repair patch (scarf joint) 4. Apply adhesive film 5. Cure (heat/vacuum) Typical scarf ratio: 20:1 to 30:1 for structural repairs Galvanic Corrosion Note Carbon fiber + aluminum = corrosion risk Isolation layer (fiberglass ply) required between carbon & Al — Resin systems: Epoxy (most common), Polyester, Phenolic Cure cycles: Temperature & time critical for mechanical properties

Key Concepts

Composite Material Systems

Reinforcement Fibers

Reinforcement Fibers — Comparison Matrix Reinforcement Fibers: Glass, Carbon, Kevlar Properties and Risks — Aerospace Composite Materials GLASS FIBER WOVEN PROPERTIES Good mechanical strength Electrical insulation Low cost Non-corrosive USES • Fairings, radomes • Non-structural panels SENSITIVITIES ⚠ Edge moisture absorption ⚠ UV degradation ELECTRICAL INSULATOR No galvanic risk ⌀ CARBON FIBER CARBON PROPERTIES High specific strength High stiffness Electrical conductivity Impact brittle USES • Primary structures • Wings, control surfaces, fuselage GALVANIC RISK ⚠ Contact with aluminum → galvanic corrosion ISOLATION LAYER REQUIRED Between carbon and aluminum ⚡ KEVLAR (ARAMID) ARAMID PROPERTIES Impact resistance Lightweight Hygroscopic (absorbs moisture) USES • Fairings, protections • Helicopter blades ABRASIVE MATERIAL ⚠ Difficult to machine ⚠ Delamination possible SPECIAL TOOLS REQUIRED Carbide drills + backing support ⚙ Chapter S-STRUCTURES — Composite Materials | AC 43.13-1B Standards | Verify material compatibility before repair
  • Fiberglass: The most common composite reinforcement, offering good strength, corrosion resistance, and electrical transparency. Used in radomes, fairings, and secondary structures. Glass fibers are non-conductive and do not cause galvanic corrosion when in contact with metals.
  • Carbon Fiber: High-strength, high-stiffness reinforcement used in primary structures such as wing skins, control surfaces, and empennage components. Carbon fiber is electrically conductive, which creates a galvanic corrosion risk when in contact with aluminum or other dissimilar metals. An isolation layer (typically fiberglass ply) must be used between carbon fiber and aluminum.
  • Kevlar (Aramid): An organic fiber known for high tensile strength, toughness, and impact resistance. Kevlar is hygroscopic (absorbs moisture from the air), which is a critical consideration during repairs. It is also abrasive and difficult to cut, requiring specialized tools such as carbide-tipped blades or hot knives to prevent fraying.

Resin Systems

  • Epoxy: The most common aerospace resin system, offering excellent mechanical properties, chemical resistance, and adhesion. Epoxy is a two-part system (resin and hardener) that cures through a chemical reaction.
  • Polyester: Used primarily in non-structural applications and some secondary structures. Less common in aerospace due to lower mechanical properties.
  • Prepreg (Pre-impregnated): Reinforcement fabric pre-impregnated with partially cured resin. Requires freezer storage at manufacturer-specified temperatures (typically 0°F/-18°C) to prevent premature curing. Must be thawed in its sealed bag before use to prevent moisture condensation.

Honeycomb Core

Honeycomb Core Sandwich Construction Honeycomb Sandwich Structure SANDWICH VIEW SKIN (aluminum / fiber) adhesive film CORROSION of Al core powder adhesive film SKIN CORE MATERIALS: Aluminum 5052 / 5056 (3.2 mm cell) Nomex (phenolic-impregnated aramid paper) MOISTURE INGRESS MECHANISM crack corrosion spread Moisture ingress via unsealed edges or damaged skins CORE SPLICE — SYNTACTIC FOAM WHY SYNTACTIC FOAM? • Seals the core against moisture ingress • Transfers loads between core sections • Glass microspheres + epoxy resin • Machinable to restore original profile unsealed edge TYPICAL DEFECTS Spongy feel → moisture in core Aluminum honeycomb corrosion (loss of stiffness, white powder exudation) Core crushing (impact, excessive clamping) Skin/core delamination (tap test: dull sound) Water ingress via edges → freezing → disbonding
  • Aluminum Core: Used in sandwich panels for high strength-to-weight ratio. Susceptible to corrosion, particularly when moisture intrudes through damaged edges or face sheets.
  • Nomex (Aramid) Core: Non-metallic core offering corrosion resistance and good mechanical properties.
  • Core Splicing: When repairing honeycomb core, a film adhesive with foam carrier (syntactic foam) is used to bond new core sections to existing structure.

Damage Mechanisms and Inspection

Composite Inspection — Tap Test and Ultrasonics Composite Inspection — Tap Test and Ultrasound 1. Visual Inspection Crack Impact Limitation: Does not detect deep internal defects. 2. Tap Test (Percussion Test) 5¢ Clear / Sharp Sound = Sound structure Dull / Flat Sound = Delamination or disbond Tool: Coin or specialized lightweight hammer. 3. Ultrasound (Internal Detection) Delamination (Invisible) Plies Probe A-Scan Display Defect Peak Principle: Echo returned by the defect interface. 4. Infrared Thermography Heat Source Composite Surface IR Camera Defect = Change in thermal conductivity (Hot or cold zone depending on method) Advantage: Rapid inspection of large areas.

Environmental Degradation

Composite Environmental Degradation — Comparison Environmental Degradation of Composites Degradation Mechanisms — AME Training Transport Canada 1. Moisture Intrusion Composite laminate H₂O Delamination Discoloration Effects: • Surface discoloration • Delamination between plies • Loss of structural stiffness • Spongy sound on tap test Detection: • Visual inspection • Tap test (dull sound) • Moisture meter • Ultrasound (NDT) 2. UV Degradation UV radiation Gel coat Crazing (chalking) Effects: • Gel coat chalking (powdery appearance) • Surface crazing • Gel coat micro-cracks • Onset of moisture intrusion Prevention: • Protective paint • Edge sealing • Waxes / UV products • Protected storage 3. Chemical Attack SKYDROL LD-4 Epoxy resin Softening (chemical degradation) CH Effects: • Resin softening • Loss of fiber/resin bond • Localized discoloration • Reduced mechanical properties Aggressive fluids: • Hydraulic fluids (Skydrol) • Solvents (MEK, acetone) • Fuels, acids • Check resin compatibility 4. Thermal Damage Heat (fire, friction) Degradation without visible delamination Burn Detection: thermocouple, NDT Effects: • Resin degradation without external signs • Exceeding polymerization temperature • Silent loss of mechanical strength Limits: • Tolerance ±5°F (±3°C) • Immediate corrective action • Full NDT inspection required AC 43.13-1B — Chapter 3: Composite Structures — Inspection and Repair Techniques
  • Moisture Intrusion: The most common cause of composite degradation in service. Water enters through cut edges, damaged gel coat, or unsealed fastener holes. Symptoms include white discoloration, spongy feel, and eventual delamination. Edge sealing is critical for all composite structures.
  • UV Degradation: Causes surface chalking (powdery, faded appearance) and crazing (network of fine cracks in the gel coat). These are typically cosmetic issues unless they penetrate the laminate.
  • Chemical Attack: Hydraulic fluids, solvents, acids, and alkalis can attack the resin matrix, causing softening, swelling, and discoloration. Chemical compatibility must be verified before any fluid contact.
  • Thermal Damage: Localized heat sources (heat guns, lightning strikes) can degrade the resin matrix without visible delamination. Discoloration is a warning sign requiring NDT evaluation.

Mechanical Damage

  • Impact Damage: Can cause delamination, fiber breakage, and core crushing. Internal damage may exist without visible surface indication. Ultrasonic testing is the primary NDT method for detecting internal delamination.
  • Edge Delamination: Typically caused by moisture intrusion at cut edges. Proper edge sealing during manufacture and maintenance is essential.
  • Fastener Hole Damage: Delamination near fastener holes can propagate under load. Stop-drilling is not effective for delamination; proper repair per manufacturer instructions is required.

Inspection Methods

  • Visual Inspection: Detects surface damage, discoloration, chalking, crazing, and exposed fibers.
  • Tap Test: Produces a dull sound over disbonded or delaminated areas. Used for initial assessment but requires follow-up NDT to determine full extent.
  • Ultrasonic Testing: The most effective NDT method for detecting internal delamination, disbonds, and core damage in composite structures.
  • Moisture Meter: Used to verify dryness before bonding repairs. Provides quantitative measurement of moisture content.

Repair Procedures

Composite Repair — Vacuum Bag Layup Sequence Composite Repair — Vacuum Bagging Stack Sequence Parent Laminate Parent Laminate Vacuum Bag Breather / Bleeder (Absorption & Vent) Release Film Peel Ply Repair Plies (Orientation 0/90/±45) Primer / Adhesive Scarf Ratio Standard 20:1 Atmospheric Pressure Sequence (surface → out): 1. Part / Repair 2. Peel ply → Release film 3. Breather/Bleeder → Bag Animation: Vacuum compression

Surface Preparation

The most critical step in any bonded repair is surface preparation. The sequence is:

  1. Abrade the surface to remove the resin-rich layer and expose fibers for mechanical bonding
  2. Clean with solvent to remove loose particles and contaminants
  3. Dry if moisture is present (critical for Kevlar)
  4. Apply primer if specified by manufacturer

Wet Lay-Up Repairs

Resin Mixing and Working Properties

Resin Working Properties — Pot Life, Gel Time, Cure Resin Working Time: Pot Life, Gel, Cure Effect of temperature on the polymerization kinetics of epoxy resins Mixing (T₀) End of Pot Life Gel End of Cure POT LIFE (Working Time) Working time after mixing resin + hardener GEL TIME Liquid → solid transition No more working possible POLYMERIZATION (CURE) Development of final mechanical properties → → → EFFECT OF TEMPERATURE ↑ HIGH TEMPERATURE • Reduced pot life (faster reaction) • Lower viscosity (fluid resin) ⚠ Risk of premature gel ↓ LOW TEMPERATURE • Extended pot life (slow reaction) • High viscosity (thick resin) • Possible incomplete cure °C STORAGE LIFE (SHELF LIFE) Store in freezer at 0°F (-18°C) — Thaw in sealed packaging to prevent condensation Limited working life at room temperature once thawed TYPICAL VALUES Pot life: 20 min @ 25°C Cure: 250°F (121°C) 2 hours Tolerance: ±5°F (±3°C) Low temperature: ← extended pot life → slower cure Pot life Gel Cure Shelf life AC 43.13-1B Chapter 3 — Composite Structures | Transport Canada — AME Training (S-STRUCTURES)
  • Pot Life: The time after mixing during which resin remains workable. Temperature-dependent - higher temperatures reduce pot life, lower temperatures increase viscosity and extend pot life.
  • Cure Time: The time required for resin to fully harden after application.
  • Gel Time: When resin starts to thicken and become unworkable.
  • Shelf Life: Storage life of uncured materials before mixing.

Critical Factors

  • Mixing Ratio: Must be precise - incorrect ratio is the most common cause of incomplete curing
  • Temperature: Affects viscosity and pot life; manufacturer's recommended application temperature range must be followed
  • Moisture: Kevlar repairs require thorough drying before bonding to prevent porosity and blistering during cure

Vacuum Bagging

Material Sequence (from part surface outward)

  1. Peel Ply: Fabric that leaves a textured surface for bonding after removal
  2. Release Film: Prevents resin from sticking to breather cloth
  3. Breather Cloth: Allows air removal and uniform vacuum distribution
  4. Vacuum Bag: Seals the assembly

Component Functions

  • Bleeder Cloth: Absorbs excess resin to maintain correct fiber-to-resin ratio (placed between peel ply and breather)
  • Breather: Provides air path for vacuum; prevents bag from blocking airflow
  • Radius Fillers: Applied at sharp corners to prevent bag tearing or bridging

Vacuum Requirements

  • Target vacuum: 25-28 in-Hg
  • Leak rate test: If vacuum drops more than 2-3 in-Hg in 5 minutes, locate and seal leaks
  • Common leak sources: Bag punctures, sealant tape failures

Elevated Temperature Curing

Heat Blankets

  • Standard method for localized heating when parts cannot be moved to an oven
  • Must use thermocouple feedback for temperature control
  • Thermocouple placement: At the bondline (between patch and structure) for accurate adhesive temperature measurement

Cure Cycle Control

Cure Cycle Control — Heat Blanket and Thermocouple Polymerization Cycle Control Cross-section — vacuum bag stack Vacuum bag Breather fabric Release film Peel ply Composite laminate (plies) Bondline TC Base structure / substrate Heating blanket Heating Thermocouple (measurement) Temperature Controller Current Temp 248°F Setpoint: 250°F ± 5°F OK Heat AL Alarm ▲ ▼ Power Polymerization Tolerance Window 230°F 240°F 245°F 250°F 255°F 265°F 280°F 248°F Range ±5°F (±3°C) Out of tolerance Corrective Action Procedure — Temperature Overrun 1. ALARM Temperature out of tolerance detected by thermocouple 2. CUT OFF Immediately cut off power to the heating blanket 3. ASSESS Assess resin state — possible overheating (deterioration) 4. DOCUMENT Record deviation and notify supervisor / eng. ⚠ TEMPERATURE OVERRUN = RESIN DETERIORATION — IMMEDIATE CORRECTIVE ACTION REQUIRED
  • Temperature tolerance: Typically ±5°F (±3°C)
  • Exceeding cure temperature degrades resin and reduces mechanical properties
  • Immediate corrective action required for temperature excursions

Scarf Repairs

Scarf Repair — Ratio and Ply Orientation Scarf Repair: Ratio and Ply Orientation Cross-section view — correct 20:1 ratio Original laminate Bond line 20 units 1 unit 2 mm (example) Ratio too low (10:1) — insufficient surface 10 units ✗ Insufficient bond surface Ply orientation — reproducing the laminate 0° 90° +45° −45° Typical sequence: [0/90/±45]s Each repair ply must reproduce the original orientation and stacking order. Key points — AC 43.13-1B standards 20:1 ratio = 20 mm of scarf length for each 1 mm of thickness A higher ratio (30:1, 50:1) improves strength but removes more material Ply orientation must match the original laminate (0/90/±45) A ratio too low (10:1) reduces bond surface → risk of structural failure ✓ 20:1 ✗
  • Standard ratio for carbon fiber/epoxy: 20:1 (20 units horizontal for every 1 unit thickness)
  • Must replicate original ply orientation and stacking sequence
  • Used for large-area delamination and through-thickness damage

Drilling Composite Materials

  • Kevlar: Sharp carbide-tipped drill with backing block to prevent delamination and fraying
  • Carbon Fiber/Fiberglass: Sharp carbide-tipped drill with drill guide for accuracy
  • High speeds generate heat and cause melting/fraying in Kevlar

Important Regulations and Procedures

AC 43.13-1B: The primary reference for acceptable composite repair practices, covering:

  • Surface preparation requirements
  • Material storage and handling
  • Vacuum bagging procedures
  • Cure cycle specifications
  • Inspection criteria

CARs Standard 571.06: Requires repairs to be based on acceptable data (manufacturer's instructions or approved alternative)

Manufacturer Limitations: Must be strictly followed:

  • Maximum allowable damage sizes
  • Repair material specifications
  • Ply orientation requirements
  • Temperature and pressure parameters

Common Relationships Between Concepts

Temperature Effects

  • Higher temperature → Shorter pot life → Faster working time required
  • Lower temperature → Higher viscosity → Difficult spreading
  • Temperature excursion → Resin degradation → Reduced mechanical properties

Moisture Relationships

  • Cut edges → Moisture intrusion → Edge delamination
  • Kevlar hygroscopic nature → Moisture absorption → Porosity during cure
  • Honeycomb core → Water ingress → Core corrosion → Spongy feel

Material Compatibility

  • Carbon fiber + Aluminum → Galvanic corrosion → Isolation layer required
  • Epoxy + Hydraulic fluid → Chemical attack → Softening/swelling
  • Prepreg + Freezer storage → Extended shelf life → Thaw before use

Damage Progression

  • UV exposure → Chalking/crazing → Surface degradation
  • Impact → Visible dent → Internal delamination (may not be visible)
  • Chemical spill → Surface softening → Resin degradation → Structural weakness

Repair Sequence Relationships

  • Surface preparation → Bond strength → Repair integrity
  • Vacuum bagging → Uniform pressure → Proper consolidation
  • Cure cycle control → Resin properties → Structural performance

Practice this chapter

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