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• face sheet failure

  • face sheets fail by yielding or fracture

  • face sheet material exceeds allowable stress/strain

• core shear failure

  • core fails in shear, usually with cracks at 45 deg. to midplane

  • core carries almost all transverse load - mainly subjected to shear

  • honeycomb - cell wall buckling (not always visible when load is removed)

• core crushing

  • face sheets move toward each other - bending/thickness loads

  • core has insufficient compressive strength

• core tensile failure

  • core has insufficient tensile strength

• face sheet to core debonding

  • bond has insufficient shear/peel/tensile strength

• local indentation

  • point loads - fittings, corners, joints

  • loaded face sheet bends independently to opposite sheet - if stress exceeds core’s compressive strength, core will fail

  • can be avoided by spreading load over large area

• face sheet wrinkling

  • buckling face sheet, accompanied by core crushing, core tearing, face sheet to core debonding

  • prevalent with thin face sheets and low density core

• face sheet dimpling (aka intracell buckling)

  • local instability - buckling of face sheet into or out of confines of cell

  • thin face sheets and large cell size

• general buckling

  • resembles classical buckling, face sheets and core remain intact

• shear crimping

  • instability that occurs when wavelength of each buckle is same order as cell size

  • local core shear failure, lateral dislocation of face sheets

  • can occur when core shear modulus is low

Transition to solid laminate

• face sheet changes direction: core subject to flatwise tension/compression

  • tension: adhesive bond must be strong enough

  • compression: crushing strength must be enough

• normal shear force at maximum at fastener centerline

• ramp region: core must have adequate shear strength

Fabrication of Sandwich Structures

Materials

Face Sheets

co-curing face sheets

• dimpling

  • result of co-curing

  • thin face sheets and large celled honeycomb cores

• mechanical properties of a co-cured face sheet may be lower (because of reasons like dimpling, waviness)

  • can test properties by co-curing face sheets with core, machining away core, and testing face sheets to determine their properties after co-curing

• permeability of face sheets can be an issue (interconnecting network of voids, porosity, microcracks, etc. that give inside access to environment)

  • damage, weight increase, higher susceptibility to impact damage

  • films like Tedlar or Mylar can be used during lay-up to eliminate permeability - cause other servicing issues

Adhesives

honeycomb

• adhesive must flow a little into cells to form fillets, but must not be too much

evaluating bonding

• test similar but higher density core to stress adhesive and find deficiencies

• testing alternative materials with less contact area - higher stress for evaluation

film adhesives

• bonding face sheet to core

• loosely woven polyester, glass, or nylon mesh for handling (called scrim/carrier)

• if carrier is on the surface of the film, it must face the core

• paper side to face sheet

• can come in unsupported form

  • extremely lightweight

Surfacing and Sealing

• used for issues with composite face sheets

• similar to film adhesives but with less density, better surface appearance, sandability

• cured with prepreg, may reduce core crush

• already cured parts

  • resin wash - low viscosity resin, can smooth surface and seal pinholes

    • can be used to prepare for paint - increases weight

    • warm part to better draw resin into discontinuities

Processes

Core

• handle with care: infrequently, with gloves, prevent contaminationl/twisting

Cleaning

metallic honeycomb - spray/immersion in solvent, wipe with solvent

Forming

metal honeycomb

• brake or rolls

  • protect from direct contact using a thin sheet

nonmetallic core

• heat forming - if core is flexible to not be damaged by the process

  • higher density core must be thinner

Splicing

• locally changing core for different properties

• can be done prior to bonding or during

• insert is cut larger than the hole for a snug fit, good practice to match ribbon direction

• criteria for honeycomb edge

  Image Added

Potting

• reinforcing sections of core for fasteners

• light loads - foaming adhesives, higher density core

• heavier loads

  • synthetic foam - lightweight

  • epoxy with chopped fibers

  • solid laminates or metal inserts

• core may be removed from areas to be potted - make sure core nearby is not distorted

• clean core prior to potting, potting compound should be room temp

• apply potting compound using injection gun, spatula, trowel - protect nearby core

  • clean sheet of thin aluminum with a cutout for potting region

• if co-curing - make sure curing cycles are compatible

Core Stabilization for Machining

• polyethylene glycol solutions, vacuum chucks, ice