CCT Core Materials & Sandwich Structures 1 — Questions and Answers
Question 1: What is the primary structural purpose of core material in a sandwich composite panel?
- Increase panel weight for stability
- Separate face sheets to increase bending stiffness with minimal weight gain (Correct answer)
- Provide electrical conductivity between face sheets
- Replace adhesive film between plies
Correct answer: Separate face sheets to increase bending stiffness with minimal weight gain
Core material separates the face sheets, dramatically increasing bending stiffness (I-beam effect) while adding very little weight.
Question 2: Which core material is most commonly used in aerospace structural sandwich panels due to its high strength-to-weight ratio and fire resistance?
- End-grain balsa wood
- PVC closed-cell foam
- Nomex aramid honeycomb (Correct answer)
- Aluminum honeycomb with polyester face sheets
Correct answer: Nomex aramid honeycomb
Nomex aramid honeycomb offers excellent strength-to-weight ratio, fire resistance, and low moisture absorption, making it the preferred aerospace core.
Question 3: What is the term for the failure mode in a sandwich panel where the face sheet buckles locally due to insufficient core support?
- Delamination
- Face sheet wrinkling (Correct answer)
- Core shear failure
- Interlaminar tension failure
Correct answer: Face sheet wrinkling
Face sheet wrinkling is a local buckling mode caused by inadequate core density or thickness failing to support the compressive face sheet.
Question 4: What is the 'ribbon direction' of honeycomb core, and why does it matter?
- The direction perpendicular to cell walls; it has no structural significance
- The direction in which cells are aligned and bonded; it has the highest shear modulus (Correct answer)
- The direction of resin flow during infusion; it determines permeability
- The direction of autoclave pressure application; it governs cure quality
Correct answer: The direction in which cells are aligned and bonded; it has the highest shear modulus
The ribbon direction (L-direction) is where honeycomb cell bonds run, giving the highest shear modulus and strength, so orientation must match design loads.
Question 5: What is the purpose of 'potting compound' used around inserts in composite sandwich panels?
- To seal panel edges from moisture ingress
- To locally fill and solidify the core to distribute insert loads and prevent crushing (Correct answer)
- To bond face sheets to the core during secondary bonding
- To provide electrical grounding for the panel
Correct answer: To locally fill and solidify the core to distribute insert loads and prevent crushing
Potting compound fills the core cells around hardware inserts to distribute fastener loads over a larger area and prevent honeycomb crushing under pull-out forces.
Question 6: Which non-destructive testing (NDT) method is most commonly used to detect disbonds between the face sheet and core in a sandwich structure?
- Radiography (X-ray)
- Coin tap testing or ultrasonic through-transmission (Correct answer)
- Eddy current inspection
- Dye penetrant inspection
Correct answer: Coin tap testing or ultrasonic through-transmission
Coin tap testing and ultrasonic through-transmission testing are the standard methods for detecting face-to-core disbonds, with tap testing being the simplest field technique.
Question 7: When splicing honeycomb core panels edge-to-edge, what material is typically used to fill the joint and restore continuity?
- Dry woven fiberglass tape
- Film adhesive strips only
- Syntactic foam or potting compound filled into the gap (Correct answer)
- Additional layers of prepreg
Correct answer: Syntactic foam or potting compound filled into the gap
Syntactic foam or potting compound is used to fill core splice gaps, providing a smooth, solid transition that prevents stress concentrations and moisture trapping.
CCT Core Materials & Sandwich Structures 1
What is the primary structural purpose of core material in a sandwich composite panel?