Chapter 6: Composite Damage: Detection and Repair

Defects in composites

• Delamination — separation between plies. Caused by impact, manufacturing faults, moisture, heat or overload; it greatly reduces compressive strength.
• Disbond — separation of a skin from its core or of bonded parts, often from moisture ingress or poor bonding.
• Voids and porosity — trapped air from manufacture or repair.
• Impact damage — a dropped tool, hail, bird strike or ground equipment. Barely visible impact damage (BVID) can hide wide delaminations beneath a small dent.
• Moisture — water entering honeycomb through damaged skins or edge seals; it freezes and expands, and corrodes aluminium core.
• Heat damage — discoloured, blistered or soft resin near engines, APU exhausts or from fire.
• Lightning strike — burned or puncture holes at attachment points, damaged protection mesh.
• Erosion — rain and sand wear on leading edges and radomes; cracks and fibre breakage.

Detection of defects

• Visual inspection first, with light at a low angle to show dents and surface ripples.
• Tap test (coin, tap hammer or electronic tapper) — a clear, sharp ring means sound material; a dull, flat sound means delamination or disbond. Works best on thin skins and near the surface; mark the boundary of the dull area.
• Ultrasonic — pulse-echo or through-transmission, and dedicated bond testers, map the size and depth of delaminations and disbonds.
• Thermography — an infrared camera shows how heat spreads; water and disbonds stand out.
• Radiography — reveals water in honeycomb cells and crushed or damaged core.
• Moisture meters, shearography and laser methods are also used.

Repair and inspection procedures

Damage is measured (including the hidden extent found by NDT) and classified against the SRM: allowable, repairable, or beyond limits. Typical repair types:

• Filler or potted repairs — small dents or holes in honeycomb panels filled with resin compound.
• Bolted (mechanically fastened) doublers — for thick laminates or field repairs; use titanium or stainless fasteners in carbon structure.
• Bonded repairs — the damaged area is removed and the edges scarfed (a long, shallow taper, often 30:1 to 50:1 or as specified) or stepped (each ply cut back in steps). Replacement plies of the same material, number and orientation are laid in, usually starting with the smallest ply and finishing with an extra (sacrificial) ply on top.
• Core damage — the damaged core is cut out and a core plug bonded in with core-splice adhesive before the skin plies are restored. Wet core is dried first.

Bagging and curing

• Surfaces are cleaned, sanded and kept free of contamination (no silicone, no fingerprints); materials are checked for shelf life and out-time.
• The lay-up is covered with peel ply, release film, bleeder (absorbs excess resin), breather (lets air flow) and a vacuum bag sealed with tape. Vacuum consolidates the plies and draws out air.
• Heat is applied with a heat blanket or lamps controlled by a hot bonder, following the specified cure cycle (ramp rate, cure temperature — often around 120 °C or 180 °C for prepregs — dwell time and cool-down), monitored by thermocouples.
• Afterwards the repair is inspected: visual, tap test or ultrasonic for voids and disbonds, and Barcol hardness to confirm resin cure. Lightning-protection mesh, conductive finishes and paint are restored.