CCT Core Materials & Sandwich Structures 2 — Questions and Answers
Question 1: What is the main advantage of end-grain balsa wood core compared to closed-cell foam cores in marine composite applications?
- Higher temperature resistance up to 300°F
- Excellent compressive strength and stiffness at relatively low cost (Correct answer)
- Resistance to water absorption without any sealing
- Lower density than all foam alternatives
Correct answer: Excellent compressive strength and stiffness at relatively low cost
End-grain balsa provides high compressive strength and stiffness per unit cost, making it widely used in marine sandwich construction where load distribution is critical.
Question 2: What foam core type offers the best performance in applications exposed to elevated temperatures above 250°F (121°C)?
- PVC (polyvinyl chloride) cross-linked foam
- SAN (styrene acrylonitrile) foam
- PMI (polymethacrylimide) foam such as Rohacell (Correct answer)
- EPS (expanded polystyrene) foam
Correct answer: PMI (polymethacrylimide) foam such as Rohacell
PMI foams such as Rohacell maintain mechanical properties at elevated temperatures and are compatible with high-temperature autoclave cure cycles up to 350°F.
Question 3: Which process is used to form flat honeycomb core into a curved shape without cutting and re-bonding?
- Resin transfer molding
- Flex-core expansion or OML machining (Correct answer)
- Wet layup reshaping at room temperature
- Compression molding with matched tooling
Correct answer: Flex-core expansion or OML machining
Flex-core (also called over-expanded core) or CNC OML machining allows honeycomb to conform to curved geometries without slitting or dicing the core.
Question 4: What structural failure occurs when the core itself fails in shear under out-of-plane loading in a sandwich panel?
- Face sheet delamination
- Core shear failure (Correct answer)
- Panel buckling
- Adhesive peel failure
Correct answer: Core shear failure
Core shear failure occurs when transverse shear stresses exceed the core shear strength, causing the core to fracture, often appearing as a diagonal crack through the core.
Question 5: Why must foam core materials be properly vented or perforated before autoclave processing?
- To allow resin to flow into the core cells for better bonding
- To prevent trapped air or volatiles from expanding and causing face sheet disbonds or blistering (Correct answer)
- To reduce the overall weight of the panel by removing foam material
- To allow thermocouple wires to pass through the panel
Correct answer: To prevent trapped air or volatiles from expanding and causing face sheet disbonds or blistering
Trapped air and gas in closed-cell foam expand under autoclave heat and pressure, potentially causing face sheet disbonds or blowouts if not vented.
Question 6: What is the correct terminology for the tapered transition at the edge of a sandwich panel where the core thickness is gradually reduced to zero?
- Core runout or core ramp (Correct answer)
- Ply drop-off
- Scarfed edge
- Edge band
Correct answer: Core runout or core ramp
A core runout or core ramp is the engineered taper at a panel edge that transitions load smoothly from the sandwich section to the solid laminate without stress concentration.
Question 7: What is the minimum recommended film adhesive weight (per square foot) typically used to bond honeycomb core to prepreg face sheets in aerospace applications?
- 0.01 psf (0.05 kg/m²)
- 0.03 to 0.06 psf (0.15 to 0.30 kg/m²) (Correct answer)
- 0.15 psf (0.73 kg/m²)
- 0.50 psf (2.44 kg/m²)
Correct answer: 0.03 to 0.06 psf (0.15 to 0.30 kg/m²)
Film adhesives in the 0.03–0.06 psf range provide sufficient fillet formation into honeycomb cell walls without adding excessive weight, which is critical for aerospace panels.
CCT Core Materials & Sandwich Structures 2
What is the main advantage of end-grain balsa wood core compared to closed-cell foam cores in marine composite applications?