Question 1

What is the difference between core shear failure and face wrinkling in sandwich panels?

Accepted Answer

Core shear failure occurs when the shear stress in the core exceeds its shear strength (τ_core > τ_c). It is the dominant mode for long spans or soft cores. Face wrinkling (also called inter-cell buckling) is a local instability where the compressed face sheet buckles away from the core. The wrinkling stress is approximately σ_w = 0.5·(Ef·Ec·Gc)^(1/3), where Ef is the face-sheet modulus and Ec, Gc are the core's elastic and shear moduli. Lightweight foam cores are most susceptible to face wrinkling.

Question 2

How is the equivalent bending stiffness of a sandwich panel calculated?

Accepted Answer

The approximate equivalent bending stiffness is D ≈ Ef·tf·(tc + tf)²/2, neglecting the face-sheet self-bending terms. The full expression is D = Ef·tf³/6 + Ef·tf·d²/2 + Ec·tc³/12, where tf is face-sheet thickness, tc is core thickness, d is half the face-centroid separation, Ef is face-sheet modulus, and Ec is core modulus. When tc ≥ 10·tf the simplified formula is accurate to within about 1%.

Question 3

When should I choose Nomex honeycomb over foam core?

Accepted Answer

Nomex honeycomb offers exceptional specific stiffness and strength, making it the standard for aerospace and motorsport structures where minimum weight is critical. Its drawbacks include susceptibility to local impact damage (with difficult repair) and higher material cost. Foam cores (PMI, PVC, polyurethane) are less expensive, easier to repair, and provide isotropic properties, but have lower stiffness and strength. The choice depends on the trade-off between weight savings, impact tolerance, repairability, and budget.

Question 4

How do you calculate the face-sheet stress in a sandwich beam?

Accepted Answer

For a simply supported beam of span L under uniformly distributed load q, the maximum bending moment is M = qL²/8. The face-sheet stress is σf = M·d / (2·D), where d is the distance between face-sheet centroids and D is the equivalent bending stiffness. The simplified version is σf ≈ M / (Af·d), where Af is the face-sheet cross-sectional area per unit width. The maximum core shear stress at the mid-plane is τc = Q / (tc·b), where Q is the shear force and b is the panel width.

Sandwich Panel Analysis (Equivalent Stiffness, Core Shear, Failure Modes)

Bending and Shear Stiffness

Midspan Deflection (Simply Supported)

Face Wrinkling (Hoff-Mautner)

Core Shear Stress and Face Stress