Sandwich structures

Definition (what it is)

A sandwich structure is a layered construction in which two thin, stiff face sheets (skins) are bonded to a relatively thick, lightweight core. The core keeps the faces separated and transfers shear between them, while the faces carry most of the in-plane and bending loads. By increasing the distance between the faces with a light core, the structure achieves very high bending stiffness and strength at low areal density compared with a monolithic material of the same thickness. This is often described as stressed-skin construction and is functionally analogous to an I‑beam (faces act like flanges; core acts like the web).

Function and purpose (key technical characteristics)

  • Structural efficiency: High stiffness-to-weight and strength-to-weight ratios arise from the large second moment of area created by face separation. For a given mass, bending stiffness can be orders of magnitude higher than a solid plate.
  • Load distribution: Under bending, one face mostly sees tension and the other compression; the core carries transverse shear and stabilizes the faces against local buckling (wrinkling).
  • Out-of-plane properties: Flatwise tensile and compressive strengths depend on core architecture and bond quality; through-thickness stiffness is much higher than that of a single thin face sheet.
  • Thermal and acoustic behavior: Many cores (foams, honeycombs) provide thermal insulation, vibration attenuation, and acoustic damping; faces can be tailored for heat spreading or electromagnetic properties.
  • Tailorability: Performance can be tuned via face material/thickness, core thickness/density/cell size or foam grade, adhesive/bondline, curvature, and local reinforcements (inserts, potting). Graded or hybrid cores can tailor crash response and local stiffness.
  • Limitations: Concentrated loads, sharp radii, point impacts, and fastener-bearing loads require local reinforcement (inserts, potting, doublers). Moisture ingress, elevated temperatures, and poor bonding can degrade performance.

Common damage and failure modes

  • Face wrinkling and overall panel buckling under compression.
  • Core shear failure or shear crimping.
  • Core crushing (flatwise compression) and local indentation.
  • Face yielding/fracture.
  • Face–core debonding (delamination) and peel failure.
  • Through-thickness cracking of faces or bondline fatigue under cyclic loading.

Design must check these modes under combinations of bending, shear, compression, impact, and environmental exposure.

Components

  • Face sheets (skins): Thin, high-stiffness/strength layers that carry in-plane and bending loads; selected for modulus, strength, durability, surface finish, and forming/paint compatibility. Typical thickness: metals ~0.2–2 mm; FRP laminates ~0.3–1.5 mm.
  • Core: Lightweight layer that provides thickness and shear stiffness/strength at minimal mass; selected for density, shear modulus/strength, compression strength, thermal/acoustic properties, and processing temperature. Typical thickness: ~3–100+ mm; densities ~20–200 kg/m³ for foams/honeycombs.
  • Adhesive/bondline or metallurgical joint: Transfers loads between faces and core; thickness, toughness, and surface preparation are critical to static and fatigue performance.
  • Edge close-outs and inserts: Perimeter closures, potting, and hard points used to distribute attachment loads, protect exposed cores, and seal against moisture ingress.

Typical materials

  • Face sheets:
    • Metals: Aluminum alloys (5xxx/6xxx common; 2xxx/7xxx for high performance), steel (galvanized, stainless), titanium; magnesium less common.
    • Fiber-reinforced polymers (FRP): Glass, carbon, or aramid fiber in thermoset resins (epoxy, polyester, vinyl ester) or thermoplastics (PA, PPS, PEEK, PET).
    • Hybrids: Fiber–metal laminates for improved fatigue and damage tolerance; natural fibers (flax, basalt) for interior or low-load uses.
  • Cores:
    • Honeycomb: Aluminum, aramid paper (Nomex), and thermoplastic (PP, PET); steel for high-temperature/high-load environments.
    • Foams: PVC, PMI, PET, PU (thermoset or thermoplastic), PEI; phenolic foams where fire/smoke/toxicity performance is critical.
    • Natural/wood: End-grain balsa for high specific compressive properties.
    • Architected cores: Corrugated, folded-sheet, lattice/truss cores via stamping, additive manufacturing, or brazed assemblies; syntactic foams (hollow microspheres in a matrix); metal foams.

Bonding and manufacturing methods

  • Adhesive bonding: Film, paste, or liquid adhesives (epoxy, polyurethane, phenolic) cured under pressure; bondline control and surface preparation are essential.
  • Composite processing:
    • Prepreg lay-up with core, cured in autoclave or out-of-autoclave vacuum bagging.
    • Liquid molding (infusion/RTM/VARTM), placing core between wet face laminates; edge close-outs and inserts integrated pre- or post-cure.
  • Thermoplastic processing:
    • Compression molding of organosheets onto foam or thermoplastic honeycomb cores; twin-sheet thermoforming; overmolding to add ribs, bosses, and attachment features.
  • Metal sandwich fabrication:
    • Roll bonding, adhesive bonding with press forming, brazing or diffusion bonding; aluminum honeycomb panels bonded with high-temperature adhesives; welded corrugated cores.
  • In-situ foaming and cavity filling:
    • Expandable or injected structural foams used in closures or local reinforcements for stiffness/NVH and crash energy management.
  • Edge treatments and integration:
    • Potting compounds, perimeter close-outs, and inserts for fasteners; sealing to prevent moisture ingress; trimming and NDI (e.g., ultrasound) for quality assurance.

Design variables and considerations

  • Geometry: Face and core thicknesses, panel curvature, cut-outs, and beadings; minimum bend radii to avoid core damage.
  • Interfaces: Adhesive selection and thickness; peel and shear strength; surface prep (cleaning, abrasion, priming).
  • Loads and environments: Bending, shear, compression, impact; fatigue; temperature excursions; moisture/fuel/chemical exposure; fire/smoke/toxicity where required.
  • Durability: Moisture ingress and freeze–thaw effects, galvanic corrosion in metal–composite hybrids, adhesive aging, creep/relaxation in polymeric cores, thermal cycling.
  • Joints and attachments: Embedded inserts, bonded studs, edge frames; avoid crushing cores under bolt preload by using potting or load-spreading features.
  • Optimization: Trade-offs among stiffness, strength, energy absorption, NVH, thermal performance, cost, cycle time, and recyclability; use of graded cores and hybrid faces to tailor local responses.

Testing and standards (examples)

  • Flexural properties: ASTM C393 (sandwich beam flexure).
  • Core shear: ASTM C273.
  • Core compression: ASTM C365.
  • Flatwise tensile: ASTM C297.
  • Edgewise compression: ASTM C364.
  • Peel/adhesion: ASTM D1781 (climbing drum peel), ASTM D3167 (T-peel, where applicable).
  • Environmental: ASTM C272 (water absorption of core); application-specific fire/smoke/toxicity and thermal tests; OEM- or industry-specific NVH, crash, and durability protocols.

Applications and relevance

  • Cross-industry: Aerospace control surfaces and fairings; marine decks/hulls; wind turbine blades; rail and bus floors; architectural/cladding panels; refrigerated truck bodies; sports equipment and protective structures.
  • Automotive and EV relevance:
    • Mass reduction: Sandwich panels in roofs, hoods/tailgates, floors, parcel shelves, and underbody shields reduce mass while meeting stiffness targets, improving range and efficiency.
    • Battery protection and pack structures: Sandwich floors/trays and lids offer high bending stiffness, intrusion resistance, puncture resistance, and thermal insulation; can integrate heat spreaders, fire barriers, and venting strategies.
    • NVH (noise, vibration, harshness): Cores provide damping and acoustic isolation, enabling quieter cabins with thin-gauge lightweight panels.
    • Thermal management: Low-conductivity cores help thermally isolate cabins and battery packs; metal-faced sandwiches can embed cooling channels or heat-spreading layers.
    • Crash energy management: Tailored cores (graded foams, corrugated/folded metals) absorb impact energy at controlled loads for side-pole, underride, and pedestrian protection scenarios.
    • Manufacturing at scale: Thermoplastic sandwiches and adhesive-bonded metal sandwiches are compatible with high-rate press lines; in-situ foaming supports local reinforcement without full panel replacement.

Synonyms and related terms

  • Synonyms/near terms: Sandwich panel, sandwich plate, sandwich-structured composite, cored laminate, multilayer panel.
  • Related structures: Honeycomb panel, foam-cored laminate, balsa-cored laminate, corrugated-core/lattice/truss-core panel, paper honeycomb panel, steel or aluminum sandwich plate.
  • Distinct from: Stiffened panels with discrete ribs/stringers (not a continuous core), monolithic laminates without a thick core, and simple I- or box-beams (functionally similar in bending but not skin–core composites).

Sustainability and end-of-life

  • Metals (e.g., aluminum-faced sandwiches) are readily recyclable; adhesive residues may need management.
  • Thermoplastic faces and cores improve reprocessability and potential for welding/bonding without thermosets.
  • Thermoset–honeycomb systems can be durable but are harder to recycle and typically require specialized repair.
  • Design for disassembly (mechanical inserts, separable adhesives) and material pairing can improve circularity.

Typical limitations and good practices

  • Protect core edges and penetrations with close-outs/potting to prevent moisture ingress and crushing.
  • Use inserts or load spreaders for fasteners and hinges to avoid local face/core damage.
  • Validate peel and shear in bondlines; toughness can be more critical than peak strength for impact/durability.
  • Consider thermal expansion mismatches and galvanic couples in hybrid metal–composite sandwiches.
  • Apply appropriate NDI and process controls to ensure bond quality and detect disbonds or core defects.