Sandwich Panels

Sandwich Panel Design (structures/composites/sandwich-panels)

Use when the task is sandwich panel design or analysis for aerospace structures: face sheet stress, core shear, face wrinkling, equivalent bending stiffness, deflection, and honeycomb versus foam core selection.

Domain quick reference

• A sandwich panel is two thin, stiff face sheets bonded to a lightweight core. The faces carry the bending moment as a tension-compression couple; the core carries the transverse shear; the large face separation gives high bending stiffness per unit weight.
• Face centroid distance d = c + t, overall thickness h = c + 2t, where t is one face thickness and c is the core thickness. The couple arm is d, not h.
• Equivalent bending stiffness per unit width, D = Eftd^2/(2*(1-nu^2)) + Eft^3/(6(1-nu^2)); the first term (face couple) dominates when t is small relative to d.
• Face stress from a bending moment M per unit width: sigma = M/(d*t), compression on the loaded face, tension on the opposite face.
• Core shear stress from a shear load V per unit width: tau = V/(b*d). The core carries essentially all transverse shear; a shear margin of tau_allow/|tau| >= 1 is required.
• Face wrinkling is local buckling of a face sheet into the core: sigma_wr = 0.5*(EfEcGc)^(1/3). Honeycomb cores resist wrinkling better than soft foams; foam is more impact tolerant and forms to curved tooling.
• Sandwich beam deflection adds a core shear term to the bending term: delta = 5qL^4/(384D) + qL^2/(8Gcd) for a simply supported beam under a uniform load q per unit width. The shear term is significant for soft cores and short spans.
• Core selection: honeycomb wins on specific shear stiffness (Gc/rho, typically 3-10x foam); foam wins on impact tolerance, moisture immunity, and cost on contoured parts.

Workflow

1. Collect the configuration: face modulus Ef and poisson ratio nu, face thickness t, core thickness c, core modulus Ec, core shear modulus Gc, and the loads (moment M, shear V, or distributed load q over span L).
2. Compute the couple arm d = c + t and the equivalent bending stiffness D with bending_stiffness.
3. Compute the face stresses from the moment with face_stress and check them against the face allowable with face_stress_margin.
4. Compute the core shear stress from the shear load with core_shear_stress and check the core with core_shear_margin.
5. Check face wrinkling with wrinkling_stress against the face allowable (wrinkling is often the limiting face failure mode).
6. Compute the bending and shear deflection terms with sandwich_beam_deflection; check the total against the deflection requirement.
7. Select the core type with select_core: weight-critical flat panels favor honeycomb, impact- or moisture-critical or contoured parts favor foam.

Pitfalls

• Using the overall thickness h instead of the couple arm d in the stress and stiffness formulas; the error is large when t is not negligible relative to c.
• Ignoring the core shear deflection term for short spans or foam cores; it can exceed the bending deflection.
• Checking only face stress and missing face wrinkling, which is driven by the core moduli, not the face strength.
• Applying a face allowable to the core shear stress or vice versa; the two failure modes use different allowables.
• Treating honeycomb as universally better: moisture intrusion and curved tooling favor foam.
• Reading the wrinkling exponent as 1/2 instead of 1/3.

Behavior contract (gate 3)

The bending stiffness, face stress, core shear, wrinkling, deflection, margin, and core selection logic is exercised by the gate 3 contract test: scripts/test_sandwich_panels.py against scripts/sandwich_panels_logic.py (stdlib unittest, offline). Run: python3 scripts/test_sandwich_panels.py

Compliance

• FAR-25 (14 CFR Part 25, US government work, public domain) and CS-25 (EASA, free download) frame the transport aeroplane structural certification context; both are referenced only, summary-not-copy per standards-map.yaml. The formulas here are generic sandwich mechanics, not text from either standard.
• compliance: STANDARDS-REF, gated: false.