ACS Composite Materials & Repair 3 — Questions and Answers
Question 1: What is the purpose of a breather layer in a vacuum bag layup?
- To provide the release surface between the part and the tool
- To allow air and volatiles to escape evenly across the part during cure (Correct answer)
- To add structural plies to the outer surface
- To maintain constant temperature across the laminate
Correct answer: To allow air and volatiles to escape evenly across the part during cure
The breather (bleeder) layer is a porous material placed over the part to allow entrapped air and cure volatiles to flow uniformly to the vacuum port.
Question 2: Carbon fiber composites must be handled with care around aluminum structure primarily because of:
- Carbon's higher coefficient of thermal expansion causing fit issues
- Galvanic corrosion of the aluminum where carbon and aluminum are in direct contact (Correct answer)
- Carbon dust clogging aluminum rivets
- Carbon fibers conducting heat into aluminum fittings
Correct answer: Galvanic corrosion of the aluminum where carbon and aluminum are in direct contact
Carbon fiber is cathodic relative to aluminum, and direct contact in the presence of moisture creates a galvanic cell that accelerates aluminum corrosion.
Question 3: Which nondestructive inspection (NDI) method is BEST suited for detecting delamination in thick composite laminates?
- Dye penetrant inspection
- Magnetic particle inspection
- Ultrasonic inspection (A-scan or C-scan) (Correct answer)
- Eddy current inspection
Correct answer: Ultrasonic inspection (A-scan or C-scan)
Ultrasonic inspection can penetrate through multiple plies and detect internal delaminations, voids, and disbonds that are invisible to surface methods.
Question 4: In a wet layup repair, the pot life of a two-part epoxy refers to:
- The total shelf life of the unmixed components
- The time after mixing during which the resin remains usable for application (Correct answer)
- The time required to reach full cure at room temperature
- The time needed before the part can be demolded
Correct answer: The time after mixing during which the resin remains usable for application
Pot life is the working time after mixing during which viscosity remains low enough to wet out fabric and achieve consolidation before the resin gels.
Question 5: What is the MINIMUM ply overlap required when adding repair plies in a stepped or scarf repair per typical SRM guidance?
- 0.25 inch
- 0.50 inch
- 1.0 inch per ply step (Correct answer)
- 2.0 inches per ply step
Correct answer: 1.0 inch per ply step
Most structural repair manuals require a minimum of 1 inch of overlap per stepped ply to ensure adequate load transfer from the repair patch to the original structure.
Question 6: When curing a composite repair using a heat blanket, which parameter is MOST important to monitor continuously?
- Ambient relative humidity at the repair site
- Cure temperature ramp rate and hold temperature at the part surface (Correct answer)
- Color change of the laminate during the cure cycle
- Vacuum bag pressure at the pump inlet only
Correct answer: Cure temperature ramp rate and hold temperature at the part surface
Incorrect ramp rates can cause residual stress or incomplete cure; hold temperature must be verified at the part surface because heat blankets can overshoot.
Question 7: Aramid (Kevlar) fiber composite panels present a unique machining challenge because:
- Aramid fibers are brittle and shatter when drilled
- Aramid fibers are tough and tend to fray or delaminate rather than cut cleanly (Correct answer)
- Aramid fibers conduct electricity, requiring insulated tooling
- Aramid fibers expand significantly when heated by cutting tools
Correct answer: Aramid fibers are tough and tend to fray or delaminate rather than cut cleanly
Aramid fibers are extremely tough and absorb energy rather than fracturing, causing fuzzing and delamination during drilling or trimming without special tooling.
ACS Composite Materials & Repair 3
What is the purpose of a breather layer in a vacuum bag layup?