8- AVS 4720 Adv Structures Damage Detection and Removal

Which would not be used with Carbon Fiber?

X-Ray

T/F: A disbond is an adhesive failure to the core material.

True

Which would not be used with Carbon Fiber?

X-Ray

T/F: A disbond is an adhesive failure to the core material.

True

T/F: Damage to the matrix is always visible to the naked eye.

False

Define low level/energy impact

A light hit that may not break the surface but causes hidden damage like delamination.

Define Medium level/energy impact

A harder hit that can make a small dent or hole.

like, local crushing, delamination, or backside fiber fracture

Types of Damages

- Holes and Punctures

-Delaminations

-Disbonds

-Core Damage

-Resin Damage

-Water Ingression or Intrusion

-Lightning, Fire, and Heat Damage

Define a disbond

Separation between bonded materials due to adhesive failure.

what can cause Disbond

Manufacturing defects, impact damage, or thermal expansion

Define core damage

When the inside the core of a composite gets weak or broken.

what can cause core damage

Improper vacuum bagging, handling damage or impacts

Explain fire damage

When a material is exposed to high heat or flames, it can cause resin and fibers in composites to burn or degrade

Explain Results of Fire Damage

burn or weaken the resin and fibers, making the material brittle. reduces the strength and some damage un-visible.

What type of processes are allowed for paint removal with composites?

chemical stripping, hand sanding, and blasting

Types of Blasting

-sandblasting

-aluminum oxide blasting

-plastic media blasting

What are some of the cautions when removing the paint with composites?

Avoid damaging the composite surface,

Chemical strippers must be compatible,

control heat buildup,

Limit repeated blasting

When removing damage from a composite structure, what shape should the cut out be? Why?

tapered (scarf) or stepped, typically circular or oval

because it helps in evenly distributing stress and maintaining the original strength

When replacing/repairing material, what is the ideal process to use?

adhesive bonding with matching materials, ensuring the same fiber orientation, core type, and curing method to restore strength

Provide examples of Primary structures

Heavily loaded such as Aircraft wing spar, fuselage frames, or landing gear beams

Provide example of secondary structures

intermediate load and safety criteria, such as Fairings, access panels, and floor panels

Define High level/energy impact

A very strong hit that completely breaks through the material.

example of Cosmetic repair

Fills small scratches with epoxy-based filler (e.g., surface scuffs on fairings)

Resin Injection repair

Injects resin into small delaminations (e.g., minor wing skin damage).

Mechanically Fastened Repair

Uses bolts or rivets to attach a patch (e.g., titanium patch on carbon fiber)

Adhesive Bonded Repair

Bonds a patch with glue (e.g., fuselage panel repair)

Flush Repair

Replaces damage with a patch that sits level (e.g., smooth aircraft skin repair).

Resin Infusion Repair

Uses vacuum to pull resin into dry fabric (e.g., fixing honeycomb panels).

Define the taper/scarf distance for repair per ply of composite laminate

Typically 10:1 to 50:1 (length-to-thickness ratio), depending on repair requirements

ow is the scarf angle determined?

Calculated using the required taper ratio

Scarf Angle= Scarf Distance​/Laminate Thickness

What percent of original undamaged strength will be provided in an exact ply-by-ply replacement?

typically restores 80-100%

What is the result if you add more ply's?

Increased stiffness but uneven load transfer

Potential stress concentration

Extra weight

Explain hot bonder

Controls heat and time for curing repairs.

Explain thermocouples

Measure and monitor temperature.

Explain Heat blanke

Heats the repair area evenly.

Explain caul plate

Presses the repair for a smooth finish.

What is the last thing you do with a repair?

Last step is final inspection to ensure proper curing, strength, and surface finish before returning the part to service.

Provide example of tertiary structures

lightly loaded or non-critical such as Interior cabin panels and decorative trim

Holes and Punctures

-caused by high or medium level impacts

-easily detected

-tools in pockets

Delaminations

-most common damage and is the separation of the materials

-usually caused by low energy impacts such as glancing bird strikes

-can be visible if near the surface

-can be caused by manufacturing defects, resin starvation in a laminate and moisture ingression

Resin Damage

-hard to detect and how much damage there actually is

-fire or excessive heat, UV rays, paint stripper, impacts

Water Ingression or Intrusion

-greater loss in compression strength than tension strength

-Causes: weight gain, disbonds, corrosion in aluminum honeycomb

-big problem in high temp cures because the water turns to steam

Lightning, Fire, and Heat Damage

-fire or excessive heat may char or burn resin and/or fibers

-difficult to determine how badly damaged or how deep the damage extends

How to detect damage in composites

Pulse Echo, Ultrasound & Ultrasonic

Tap Testing

requires knowledge of underlying structure

Xray

better with fibberglass and aramid fiber than carbon fiber because carbon fiber is dark in color

Thermal Imaging

Can provide subsurface image of entire structure

Ultrasonic

-pulse-echo

-through-transmission

-phased by array UT

Laser Shearography

used in production and during mx inspections

Holographic Laser Interferometry

non-contact optical technique that provides visual representation of out-of-plane deformations

Pulse Echo

-one transducer generates and receives the sound wave w/in 2 inches of good part

-usually limited to detecting first occurring defect-sound wave echoes back preventing detection of anything beyond

-more sensitive to misalignment between transducer and part

-better at detecting thin film inclusions in composites and something that doesn't belong

-can determine depth of defect by knowing speed of the sound wave through the composite

Through-Transmission

-uses two transducers

-defects located in the sound path between the two transducers will interrupt the sound transmission

-interruptions are analyzed to determine size and location of defects

-less sensitive to small defects that pulse-echo

-cannot determine depth of defect

-better at detecting defects in multilayered structures and in quantizing porosity

-can tolerate greater transducer misalignment with part (up to 10 degrees)

-requires access to both sides of damaged part

Phased Array UT

-excellent detection of small defects

-ability to determine both depth and location of defects

-soundwaves can be focussed which examines depth

-soundwaves can be steered which examines left, right, at an angle, etc

Repairs Should Use Same

-materials

-fiber orientation

-core orientation

-stacking sequence

-nesting

-curing temperature

-sealing of original part

Must Consider for Repairs

-matrix resins

-fibers/fabrics

-core materials

-lightning strike materials

-sealants

Chemical Stripping

You can not use most of these to remove paint...very few are okay and must be monitored closely

Plastic Media

-cleaning process designed to remove paint chips and solid matter

-particles become dull with reuse and can be damaged by oil

Skin Removal Processes

1. Cutting and Peeling- chisel/knife

2. Routing - best if used w/template

3. Grinding- die grinder gently

can be any combination of these

Core Removal Processes

-cutting and peeling

-chiseling

-routing and grinding

Contamination

contaminated surfaces must be removed and treated in a way that will allow the resigns and adhesives of repair to bond

-prepreg films, water, oils, hydraulic fluid

Drying

-all moisture affected composite materials must be dry prior to repair

-drying cycles are specified in repair manuals

-often times, it will take days to dry at a long time, low temperature

Design Goal of Repair

to return the structure to its original strength, stiffness, shape, and surface finish

Engineering Support Involvement

large repairs

The perfect repair

replacement of the part

Composite Design Considerations

-temporary or permanent

-bonded or bolted

-flush or external doublers

-single-sided or double-sided

-wet lay-up or prepreg

-appearance desired

-access to part and/or damaged area

-details (shape, heat sinks, etc)

Composite Design Practicalities

-time

-environment

-skills

-equipment

-materials

-regulatory and organizational requirements

-failure consequences (cosmetic fairing or primary structure)

Composite Design Parameters

-strength

-stiffness

-weight

-shape/contours

-appearance/surface finish