Composite Inspection Methods

Composite Inspection Methods

• Composite inspection techniques and non-destructive testing (NDT) methods involve using multiple methods to accurately determine the airworthiness of a structure.
• Metal inspection and NDT methods transfer to composite applications.
• Composite structures need ongoing inspections at regular intervals alongside non-scheduled damage inspections and testing.
• Damaged composite structures require thorough inspection to determine the extent of the damage, which may extend beyond the immediately apparent defect.
• Proper inspection and testing methods classify damage, determining if it's repairable or requires part replacement.
• Classifying damage determines the repair method.
• Manufacturer's structural repair manual outlines inspection procedures, damage classification factors, and recommended repair methods.

Visual Inspection

• Visual inspection is the most frequently used inspection method in aviation.
• Pilots, ground crew, and maintenance technicians visually inspect the aircraft daily.
• Visual inspection detects resin-rich areas, resin starvation, edge delamination, fibre break-out, cracks, blistering, and surface irregularities.
• Strong light and a magnifying glass are useful tools.
• A small microscope helps determine if fibres in a cracked surface are broken or if the crack affects only the resin.

Tap Testing

• Tap testing inspection effectively detects good bond lines or air pockets/inclusions causing disbonds in finished jobs/repairs.

Ultrasonic Inspection

• Ultrasonic inspection is a common instrumental NDT method for composites.
• Useful for detecting internal damage like delaminations, core crush, and subsurface defects.
• Two common methods: pulse echo and through-transmission.
• Pulse echo method: the tester sends ultrasonic pulses through the part and receives the return echo, displaying echo patterns on an oscilloscope, requiring access to only one side of the structure.
• Through-transmission method: two transducers are used.
• One emits ultrasonic waves, and the other receives them, located on opposite sides of the part.
• Defects at multiple levels are easily detected through reduced sound waves on the receiver side.

Radiographic Inspection

• Radiography (x-ray inspection) detects differences in thickness or physical density compared to surrounding composite material.
• Detects surface/internal cracks, entrapped water in honeycomb core cells, foreign objects, and the extent/size of damage, unlike ultrasonic or tap testing.

Thermography

• Thermography detects flaws by identifying temperature variations on the surface of a damaged part; heat is applied, and temperature gradients are measured using an infrared camera.
• Requires knowledge of the thermal conductivity of the test specimen and a reference standard for comparison.
• Thermography can identify water ingress into aircraft structures by imaging the aircraft in flight or after flight.

Damage Classifications

• Three categories of damage classification:
  • Negligible - damage that can be corrected by a simple repair procedure with no restrictions on flight operations
  • Repairable - damage to the skin, bond, or core that cannot exist without restrictions on flight operations, but can be repaired
  • Non-repairable - damage to the structure/component that can't be repaired (component must be replaced).
• Areas of condensation after cold-soaking at altitude may indicate water ingress and should be investigated.

Cosmetic Defects

• Cosmetic defects are present on the outer surface skin without damage to structural reinforcing fibres.

Impact Damage

• Impact damage occurs when a foreign object strikes the part, ranging from slight to severe.
• Careless handling during transportation, storage, or standing parts on their edge without adequate protection is the common cause.
• Thin face sheets on sandwich panels are susceptible to impact damage.
• Inspect for delamination around the impacted area on a subject area.
• Nicking, chipping, cracking, or breaking pieces of the edge/corner can be caused by improper handling.

Delamination

• Delamination is the separation of fabric layers in a laminate which can occur with no visible outer skin indications.
• Often accompanies other types of damage, especially impact damage.
• Results from several causes, including impact, moisture ingress, or lightning strikes.

Disbonds

• Disbonding is the separation of composite material from another material to which it's adhesively bonded.
• Differs from delamination, which is separation between plies/layers of the composite.
• Separation between skin and core of a composite sandwich structure is a core disbond.
• Disbonds consequence may stem from poor adhesion, service loading, or impact damage.
• May not be externally visible, hard to detect using NDT methods if tight/weakly bonded called a kissing bond, and important to avoid in joins like end connections.

Cracks

• Cracks occur in composite structures as in metallic ones, arising in manufacture or service conditions.
• Cracking can have a significant effect on the composite's integrity, allowing environmental ingress and extended damage.
• Cracking is often associated with the final stages of in-service failure.
• Sometimes detectable visually, but may need more advanced non-destructive inspection (NDI) methods.
• A thorough inspection should determine the extent of each crack.

Resin Matrix Damage

• Resin matrix damage can be caused by fire, excessive heat, UV rays, paint stripper, or impacts.

Water and Aircraft Fluid Intrusion

• Water and other fluid intrusion pose a problem for honeycomb cores.
• Causes weight gain, contamination of bond joints, corrosion in aluminum honeycomb, and disbonds if water freezes and expands.
• Water intrusion is a common problem with high-temperature repairs.
• The curing heat causes trapped water to turn to steam, disbonding face sheets around the repair, thus converting a small damage area into a large one.

Hole Damage

• Holes may occur from impact damage, over-torqued fasteners, or fastener pull-through; holes drilled in the wrong location, wrong size, or wrong number may be hole damages.
• Holes caused by lightning strikes may burn off resins, leaving bare cloth.
• Tiny holes (pinholes) in the skin surface may lead to more extensive damage if moisture gets into the core structure, causing a small delamination that grows.

Composite Repairs

• Procedures for repairing laminated composite structures depend on the type of damage.
• Damage ranges from simple surface scratches to damage extending through all internal plies and core honeycomb material.
• Four basic types of composite repairs:
  • Bolted metal/cured composite patches
  • Bonded metal/cured composite patches
  • Resin injections
  • New repair plies laminated to the damage
• Bolted and bonded surface patches aren't usually recommended because they don't restore the original strength characteristics; a bolted/bonded patch also causes undesirable aerodynamic changes.
• Resin injection repairs fill holes/voids by injecting resin; typically done on non-structural parts.
• Injected resin doesn't restore strength and expands delamination in some instances.
• Lamination of new repair plies is the best permanent repair to composite structures, removing damaged plies and layering new ones.

Composites Damage Assessment and Repair Process

• Basic steps of damage assessment and repair:
  • Find the damage - clean damaged area and remove surface coatings.
  • Assess the extent of the damage - use visual and NDI techniques and then mark out the repair.
  • Define the repair procedure.
  • Fabricate the repair.
  • Prepare the repair site.
  • Lay-up install the repair.
  • Clean up the repair site.
  • Inspect the repair site for structural integrity - may use NDI again.
  • Document the repair.

Potted Repairs

• Potted repairs use filler to complete the composite repair process.
• Alternative to installing a core material plug but don't provide same strength as core material.
• Filling with a resin-and-filler mixture adds weight and decreases flexibility.
• Applicability is for honeycomb core sandwich structures with damage up to 1 inch in diameter.
• Drilling a small hole into a delaminated area and injecting resin can strengthen it.

Injection Repairs

• Sometimes, minor internal delamination is repaired with a potting compound, it can be repaired by simply injecting resin into the cavity caused by ply separation.
• Severe delamination requires removal and repair or replacement.
• Checking the manufacturer's repair limitations is important.
• Injecting resin into the delamination, clamping the edge, and allowing resin to cure repairs minor edge delamination.
• Edges damaged by crushing/puncture are repaired by scarf-cutting and installing new plies.

Laminate Damage to One Surface

• Single-surface laminate damage repairs require removing and replacing damaged laminate plies.
• Fiber damage to one surface, use a 1" overlap, 1/2" step cut typical to repair