ACS Composite Materials & Repair 2 — Questions and Answers
Question 1: What is the primary purpose of a peel ply in composite layup?
- To provide a sacrificial surface layer that creates a bondable texture when removed (Correct answer)
- To seal the vacuum bag against air leaks
- To distribute resin evenly across the laminate
- To reinforce the outer skin of the composite panel
Correct answer: To provide a sacrificial surface layer that creates a bondable texture when removed
Peel ply is a woven fabric applied as the last layer during layup; when peeled off after cure it leaves a clean, textured surface ready for secondary bonding.
Question 2: Which type of damage to a composite structure is MOST difficult to detect visually?
- Surface scratches and paint chips
- Delamination beneath an intact surface skin (Correct answer)
- Impact craters that dent the outer ply
- Frayed edges at fastener holes
Correct answer: Delamination beneath an intact surface skin
Delamination (separation of plies) can exist beneath an undamaged-looking outer skin and requires tap testing or NDI methods to detect.
Question 3: During a tap test on a composite panel, a dull or hollow sound compared to surrounding areas indicates:
- Proper resin consolidation
- A subsurface void, delamination, or disbond (Correct answer)
- Excess resin in that area
- Correct fiber orientation
Correct answer: A subsurface void, delamination, or disbond
A dull or dead sound during tap testing indicates a loss of bond or internal void, while a solid ringing sound indicates good laminate integrity.
Question 4: What is the correct fiber orientation for a quasi-isotropic composite layup?
- All plies at 0°
- Alternating plies at 0° and 90° only
- Plies at 0°, +45°, -45°, and 90° (Correct answer)
- Plies at 0°, 45°, and 90° with no -45°
Correct answer: Plies at 0°, +45°, -45°, and 90°
A quasi-isotropic layup uses plies at 0°, +45°, -45°, and 90° to provide approximately equal stiffness and strength in all in-plane directions.
Question 5: When performing a scarf repair on a composite laminate, the scarf ratio is typically expressed as:
- Depth of damage divided by the panel thickness
- Horizontal taper length divided by the laminate thickness (Correct answer)
- Number of plies removed divided by total ply count
- Repair patch diameter divided by damage diameter
Correct answer: Horizontal taper length divided by the laminate thickness
Scarf ratio is the ratio of the taper length to the laminate thickness (e.g., 20:1 means 20 mm of taper per 1 mm of thickness), determining how gradually load is transferred.
Question 6: Which adhesive film storage requirement is MOST critical to maintaining its properties?
- Store at room temperature away from UV light
- Store refrigerated or frozen and track out-time carefully (Correct answer)
- Store in a dry cabinet at low humidity only
- Store at elevated temperature to keep it pliable
Correct answer: Store refrigerated or frozen and track out-time carefully
Adhesive films and prepregs must be stored frozen (typically at 0°F or below) to arrest curing, and out-time at room temperature must be logged and not exceeded.
Question 7: When removing paint before a composite repair, the preferred method over mechanical sanding is:
- High-pressure water blasting
- Solvent stripping with MEK
- Chemical paint stripper rated for composites (Correct answer)
- Open-flame heat gun softening
Correct answer: Chemical paint stripper rated for composites
Chemical strippers formulated for composites remove paint without abrading fibers or introducing heat that could damage the matrix resin.
ACS Composite Materials & Repair 2
What is the primary purpose of a peel ply in composite layup?