3.2 Composite Damage Assessment, Repair Methods & Shop Safety

Composite Damage Assessment, Repair Methods & Shop Safety

FAA Airframe Mechanic Standard: Composite structural maintenance requires rigorous damage evaluation, precise non-destructive inspection (NDI), calibrated scarfing geometry, controlled vacuum consolidation, and thermal cure cycle management in accordance with 14 CFR Part 43, FAA-H-8083-31A, and AC 43.13-1B Chapter 3.

---

1. Composite Damage Classifications

Damage in composite aircraft structures is classified by its physical mechanism and depth:

• Delamination: Separation of adjacent reinforcing fiber plies within a solid laminate or sandwich facesheet. Often caused by interlaminar shear stress or low-velocity impact.
• Disbond (Debond): Separation between two bonded structural elements, such as the composite facesheet lifting off the honeycomb core or a bonded doubler separating from a skin.
• Fiber Breakout / Fracture: Rupture of reinforcing fibers due to tensile overstress, puncture, or lightning strike burning.
• Core Crush / Buckling: Compressive collapse of honeycomb cell walls or crushed foam core resulting from blunt impact or high concentrated surface loads.
• Moisture Ingress: Water entering damaged sandwich panels through micro-cracks or pinholes. At high cruising altitudes, water freezes, expands, and tears the facesheet away from the core; at high speeds, trapped water causes core cell rupture during rapid cabin depressurization.
• Barely Visible Impact Damage (BVID): Subsurface internal damage (delamination, core shear failure) caused by low-velocity impact (dropped tools, hail, runway debris) that leaves little or no visible indentation on the outer mold line (OML).

---

2. Non-Destructive Inspection (NDI/NDT) Techniques

Before undertaking any repair, an AMT must establish the full geographic boundary of structural damage using approved NDI methods.

+-------------------------------------------------------------------------+
|               COMPOSITE NON-DESTRUCTIVE INSPECTION (NDI)                |
+-------------------+----------------------------+------------------------+
| Tap Testing       | Ultrasonic Inspection      | Radiography (X-Ray)    |
| - Metallic ring   | - Pulse-Echo / Thru-Trans  | - Detects core crush   |
|   = Sound         | - Precise depth & boundary | - Trapped water in     |
| - Dull thud       | - Standard shop NDI        |   honeycomb cells      |
|   = Delamination  |                            | - Heavy safety controls|
+-------------------+----------------------------+------------------------+

A. Tap Testing (Acoustic Ring)

• Procedure: Lightly tapping the composite surface along a grid using a specialized composite tap hammer, lightweight plastic rod, or a smooth steel coin (e.g., a quarter).
• Acoustic Signature:
  • Sound, well-bonded structure: Produces a sharp, crisp, high-pitched metallic ringing sound as acoustic energy transfers cleanly into the underlying structure.
  • Delaminated / Disbonded structure: Produces a flat, hollow, dull "thud" sound because the internal air gap prevents sound propagation.
• Limitations: Only effective for detecting shallow defects within the outer 2 to 3 plies or skin-to-core disbonds in thin skins ($< 0.080\text{ in}$). Cannot reliably detect deep internal delaminations in thick laminates.

B. Ultrasonic Inspection

• Pulse-Echo: A single piezoelectric transducer transmits high-frequency sound waves ($1\text{ to }10\text{ MHz}$) into the laminate and receives reflected echoes. An internal delamination reflects sound energy back prematurely, displaying a distinct flaw signal on the A-scan display and establishing exact defect depth.
• Through-Transmission: Requires two aligned transducers on opposite sides of the part (one transmitting, one receiving). A defect attenuates or blocks the sound beam.

C. Radiography (X-Ray)

• High-energy X-rays pass through the composite component onto radiographic film or digital detector plates.
• Primary Application: Superior for identifying water ingress in honeycomb core cells, crushed or distorted core cell ribbons, sheared internal fasteners, and foreign object debris (FOD). Less effective for planar delaminations parallel to the X-ray beam.

D. Thermography (Infrared NDI) & Laser Shearography

• Flash Thermography: Heat lamps apply a brief thermal pulse to the surface; an infrared camera monitors surface temperature decay. Subsurface voids and disbonds impede heat transfer, appearing as warm localized hotspots.
• Laser Shearography: Illuminates the surface with coherent laser light while applying a slight mechanical or vacuum stress. An interferometer compares speckle patterns to detect minute out-of-plane surface gradient changes caused by subsurface delaminations.

---

3. Structural Repair Methodologies: Scarf vs. Step-Sanding

When structural composite damage exceeds allowable limits, the damaged plies must be machined away and replaced with an engineered repair patch.

                             SCARF REPAIR PROFILE (50:1 Taper)

 <--------------------------------- Scarf Width --------------------------------->
 
             Original Laminate                                   Original Laminate
 ======-----------------------------------------------------------------======
       \                                                               /
        \                                                             /
         \================== Replacement Plies =====================/
          \________________________________________________________/

A. Scarf (Tapered) Repair Geometry

In a scarf repair, damaged material is machined out in a continuous uniform bevel angle. The standard structural taper ratio specified in AC 43.13-1B and most aircraft Structural Repair Manuals (SRMs) is 50:1 (taper length to laminate thickness): $\text{Scarf Taper Ratio} = \frac{\text{Scarf Length per Side}}{\text{Laminate Thickness}} = \frac{50}{1}$

• Example: For a solid carbon fiber skin with a thickness of $0.060\text{ in}$, the scarf taper must extend: $\text{Taper Length} = 0.060\text{ in} \times 50 = 3.0\text{ inches per side}$
• Non-structural or secondary skins may permit a steeper 30:1 taper ratio, but critical primary flight structures strictly mandate 50:1 or greater.

B. Step-Sanded Repair

In step-sanding, each ply is carefully ground away to form a series of discrete, stepped concentric rings. Each step is typically machined $0.5\text{ inches}$ ($12.7\text{ mm}$) wide per ply. Step repairs require extreme precision to avoid cutting into underlying undamaged plies.

C. Ply Orientation & Ply Schedule Matching

During structural repair:

1. Every replacement ply must match the original ply in fiber material (carbon, fiberglass, Kevlar), weave style, and areal weight.
2. Warp Clock Alignment: Each replacement ply must align precisely with the original ply's orientation ($0^\circ, +45^\circ, -45^\circ, 90^\circ$). Altering ply orientation drastically alters torsional and bending stiffness.
3. Sacrifice / Cover Ply: An extra cover ply (usually fine-weave fiberglass or carbon extending $0.5\text{ to }1.0\text{ inch}$ beyond the scarf perimeter) is placed over the entire repair to protect against aerodynamic erosion and permit flush sanding.

D. Honeycomb Core Repairs

• Potted Repair: For small puncture damage ($< 1.0\text{ inch}$ diameter) with no structural core crush, the damaged cell area is cleaned, dried, and injected with an epoxy resin filled with glass microballoons (syntactic foam).
• Core Replacement Plug: For damage exceeding $1.0\text{ inch}$, the damaged core is routed out cleanly. A matching core plug (same material, cell size, density, and ribbon direction) is inserted and bonded to the surrounding core walls using an expanding structural foaming adhesive film.

---

4. Vacuum Bagging Assembly & Consumables Stack

Vacuum bagging consolidates laminate plies, removes trapped air and volatile gases, and extracts excess matrix resin to achieve an optimum fiber-to-resin ratio (typically $55%\text{ to }60%$ fiber by volume).

+-------------------------------------------------------------------------+
|                   VACUUM BAGGING STACK (Top to Bottom)                  |
+-------------------------------------------------------------------------+
| [1] Vacuum Bag Film (Nylon / Polyamide film; sealed with mastic tape)   |
| [2] Breather Cloth (Non-woven polyester; continuous vacuum pathway)     |
| [3] Solid Separator Film (Non-porous release film; isolates resin)      |
| [4] Bleeder Cloth (Absorbent felt; absorbs excess resin)                |
| [5] Perforated Release Film (Porous fluoropolymer; regulates bleeding)  |
| [6] Peel Ply (Textured nylon/polyester; creates clean bonding surface)  |
| [7] Composite Repair Laminate + Thermocouples                           |
| [8] Tool / Aircraft Mold Line Skin                                      |
+-------------------------------------------------------------------------+

Layer-by-Layer Components & Functions

1. Peel Ply (Release Ply): A tightly woven nylon or polyester fabric laid directly against the wet laminate. It allows resin and air to pass through and is peeled off after cure, leaving a clean, textured, contaminant-free surface ready for painting or secondary bonding without mechanical sanding.
2. Perforated (Porous) Release Film: A fluoropolymer (Teflon/ETFE) film with calibrated micro-perforations. It allows volatile gases and controlled quantities of excess resin to escape into the bleeder while preventing the bleeder cloth from sticking to the laminate.
3. Bleeder Cloth: An open, absorbent non-woven synthetic felt that soaks up excess resin bled from the laminate.
4. Solid (Non-Porous) Separator Film: A solid plastic barrier placed over the bleeder cloth to prevent liquid resin from migrating into the breather cloth.
5. Breather Cloth: A thick, fluffy non-woven polyester mat that covers the entire repair area. It provides a continuous, open path for air evacuation under the vacuum bag, preventing dead spots.
6. Thermocouples (J or K Type): Calibrated temperature-sensing wire junctions taped around the repair perimeter and under the heat blanket to provide real-time thermal feedback to the hot bonder.
7. Vacuum Bag & Mastic Sealant Tape: High-temperature flexible nylon film sealed around the repair perimeter using high-tack synthetic rubber mastic tape (butyl sealant tape).

Vacuum Pressure & Leak Checking

• Operating Vacuum: Structural hot-bond repairs typically require $22\text{ to }28\text{ inches of Mercury (inHg)}$ (equivalent to $\approx 10.8\text{ to }13.7\text{ psi}$ consolidating force).
• Vacuum Drop Test (Leak Check): After reaching full vacuum, the vacuum source is isolated. The bag must not lose more than $1.0\text{ inHg}$ of vacuum pressure over a 5-minute period. Any excessive leak must be located and sealed with mastic tape before initiating thermal cure.

---

5. Thermal Cure Cycles & Hot Bonder Operation

Structural repairs using $250^\circ\text{F}$ or $350^\circ\text{F}$ curing prepregs or film adhesives utilize microprocessor-controlled hot bonders paired with flexible silicone heating blankets.

  Temp (°F)
    ^
    |                        Soak / Dwell Period
    |                   +---------------------------+
350°|                  /                             \
    |                 /                               \
    |     Ramp-Up    /                                 \   Cool-Down Rate
    |    1-5°F/min  /                                   \    < 5°F/min
    |              /                                     \
    |             /                                       \
 70°+------------+-----------------------------------------+--------> Time (min)
    |<-- Heat -->|<---------- Hold at Temp --------------->|<- Cool ->|

The Three Critical Cure Stages

1. Ramp-Up Rate: Heating must occur at a controlled rate, typically $1^\circ\text{F}\text{ to }5^\circ\text{F}$ per minute. Heating too fast causes thermal shock, uneven resin viscosity, and traps volatiles before the resin can flow.
2. Soak / Dwell Temperature: The bonder holds the setpoint temperature (e.g., $250^\circ\text{F} \pm 10^\circ\text{F}$ or $350^\circ\text{F} \pm 10^\circ\text{F}$) for the manufacturer-mandated soak duration (typically $60\text{ to }120\text{ minutes}$) to complete polymer cross-linking.
3. Cool-Down Rate: Controlled cooling must not exceed $5^\circ\text{F}$ per minute. Continuous vacuum pressure must be maintained throughout cooling until the repair surface drops below $140^\circ\text{F}$ ($60^\circ\text{C}$). Releasing vacuum or removing heat blankets above $140^\circ\text{F}$ induces internal residual stress, micro-cracking, and part warpage.

---

6. Shop Safety, PPE & Chemical Hazards

• Respirable Particulates: Machining, trimming, and sanding carbon fiber and fiberglass generates fine airborne particulates that cause severe mechanical irritation to lung tissue. AMTs must operate in HEPA-filtered downdraft sanding booths and wear a NIOSH-approved N95 or P100 half-face particulate respirator and sealed safety goggles.
• Epoxy & Hardener Toxicity: Amine-based epoxy curing agents are potent chemical sensitizers. Repeated skin contact causes severe allergic contact dermatitis and permanent respiratory sensitization. AMTs must wear disposable chemical-resistant nitrile gloves (latex gloves are permeable to epoxy amines and solvents) and barrier creams.