Guidelines for Property Testing

Introduction

Sandwich structure: face sheets, core, connection material

• Face takes bending loads (one compression and the other tension) and sometimes in plane shear loads

  • Compressive, tensile, shear strengths/moduli

• Core design properties: compressive strength/modulus, shear strength/modulus

Evaluation of Core Materials

Mechanical Properties

• Compressive strength/modulus

• Tensile strength

• Shear strength/modulus

Environmental Effects

Honeycomb

• Mechanical properties vary with core density, cell dimensions, cell direction

Temperature/wetness

• Core loses strength at elevated temperatures and when wet 

  • Amount of strength loss depends on type of core

• Strengths and moduli are higher in cold temperatures

Test Methods

Shear properties

• balsa/foam: same in both directions

• Honeycomb: different in both directions depending on cell geometry and thickness (L = ribbon, W = transverse)

  • Hexagonal: L = 2W

Bare compressive strength test

• Core tested without face sheets

  • With face sheets: stabilized compression test

    • Higher strength and stiffness values

• Quality control test

• Obtains the honeycomb compressive modulus

Plate shear test

• Shear strength and modulus of core

• Bond core to steel plates and load plates in tension/compression with load line going through diagonal corners of the core

• Important test because core takes shear load

Evaluation of Core-to-Face Sheet Bonds

Core-to-face bond

• Enables face sheets and core to work together

• Not the desired failure mode

• Attachment methods: adhesive bonding, prepregs, brazing, fusion

• Cell size is important in testing

Environmental Effects

Moisture: when present during cure, can produce porous bonds

Test Methods

Tests

• Mil-A-25463

  • Climbing drum peel, flatwise tensile, flexural strength, creep

  • Conducted at various temperatures/moisture levels 

• MMM-A-132

  • Tensile lap shear, creep rupture, T-peel, blister detection

  • Conducted at various temperatures/moisture levels

Flatwise tensile, climbing drum peel: evaluating core-to-face sheet bond

• Cleavage test for thicker faced panels

• Flatwise tensile

  • Cut small (usually 2x2 in) piece and bond to metal 

  • Fix specimen to test machine and pull apart - record max load and mode of failure

    • Failure mode reveals if panel was made properly: core tearing, core to face adhesion failure, interfacial adhesive failure

    • Block and panel bond failure - not valid, adhesion to core/face sheet - there is some contamination and cleaning process should be fixed

    • High strength core, high temperatures - core failure is not achievable

• Climbing drum peel

  • Peel one face sheet from the panel

  • Works with thin face sheets, same modes of failure as above

Evaluation of Face Sheet Properties

Face sheets: in plane loading

• Bending: one panel in compression and the other in tension

• Column: both panels are in compression

• Shear panels: face sheets take in-plane shear loads

Core: resists out of plane shear loads

Mechanical Properties

compressive/tensile strengths/moduli - most important mechanical properties

Environmental Effects

Hygrothermal expansion

• Ply orientation, fiber CTE, modulus, volume fraction 

• Zero CTEs and CME are possible in certain directions but not at the same time

Test Methods

Open

Evaluation of Sandwich Panels

• Sandwich panels can be designed to be lightweight/stiff/strong

• Bending stiffness/strength can increase with sandwich panels compared to plates

  • Depends on thickness, core depth/density, adhesive bond strength

Mechanical Properties

Bending strength: depends on thickness, compressive/tensile strength

Bending stiffness: depends on thickness, compressive/tensile modulus

Out of plane shear strength: depends on thickness, shear strength of core

Out of plane shear rigidity: depends on thickness and shear modulus

Environmental Effects

High temperature, moisture, fluids degrade sandwich properties

• Caused by poor core sealing and porous face sheets

Damage Resistance

Damage resistance: resistance from various forms of damage from specific events

Not resistant to impacts

• thicker/tougher face sheets and core provide better impact resistance

Damage Tolerance

Damage tolerance: structure’s ability to sustain loads with some damage and perform functions

Most sandwiches not designed for damage tolerance until recently

• Design to restrict area of potential damage to acceptable size

Repair

During manufacturing - protect exposed corners with temporary covers

Repair procedures

• Equalling strength and stiffness of original part

• Minimum increase in weight/aerodynamics/electrical properties

• Replace damaged material with identical material

• Avoid abrupt changes in cross sectional area

  • Taper joints, make small patches circular instead of rectangular, rounding corners of large repairs, fading repairs into original contours

Test Methods

Open

Evaluation of Inserts and Fasteners

Introduction 

Attachments - concentrated stresses

• Lightly stressed parts - unreinforced fastener holes are enough

• Local reinforcements are needed most of the time

Test Methods

Sub-element or sub-component tests: dependence on specific configuration, environmental effects usually incorporated

Mechanical Properties

Important properties

• Core shear strength/modulus

• Face sheet compression (local bearing) strength and modulus

Evaluation of Other Features

Introduction

Sandwich parts joined to framing members: continuous-edge reinforcement

• Consider loads to be transferred, type of face sheets/core, attachment fittings, smoothness of surface

• Crushed low density honeycomb should be resin stabilized

• Edge treatments can be a moisture seal

• Openings in sandwiches for inspection, filler holes, adjusting fittings are often needed

  • Requires consideration in design (ratio of opening to panel size)

  • High strength core inserts, edge treatments

Test Methods

Edge reinforcements - sub element/sub component tests

Material Data

Cores

• Separation, support, stabilization for face sheets for desired bending rigidity 

• Carries major out of place/transverse shear loads

• thermal/acoustic insulation

• Lowest density part of sandwich

Honeycomb Cores

• Can be formed to moderate amounts of single curvature

  • Specialized cell configurations exist for compound or severe curvature

  • Nonmetallic cores can be formed using heat

  • Hexagonal and bisected hexagonal - less formable

  • Overexpanded, under expanded, flexible - severe single curvatures, moderate compound curvatures

• Different properties depending on sheet material, sheet thickness, cell size, cell shape

• Cell size determined by diameter of circle inscribed in cell 

  • Aircraft: 1/16 - 7/16 in.

• Can add density by under expanding or crushing cells together

  • Properties increase in proportion to density increase

• Nonmetallic cores - better thermal insulation

Cross-banded Core

• Sheets assembled with corrugations in adjacent sheets perpendicular to each other

  • Corrugation flutes are 45 deg. To face sheets

• Not as strong in transverse direction as honeycomb, but more compressive strength

• High stiffness - bad for curves, curves can be machined from blocks of cross banded core

Corrugated Core

• Form sheet of metal foil or resin-treated glass cloth to sine wave corrugations

  • Corrugation flutes run parallel to face sheets

• Form to single curvature

Waffle Type Core

• Sheets of resin-treated glass fiber formed to waffle type core, thin metal sheets dimpled into waffle configuration

• Bad for tapered core thickness

Foam Cores

• Developed to overcome disadvantages of natural cores

• Controlling expansion process for cores can get desired core properties

• Not as strong/stiff as honeycomb

• Core joint elimination, thin/uniform bonding later between face sheets and core, can use pre molded face sheets, good electrical properties, lower cost, thermal insulation

Wood Cores

• Balsa wood (density 6-15 lb/ft3)

  • Grain parallel or perpendicular to face sheets (perpendicular = end grain application)

• Points of attachment, exposed edges - require high strength insert

  • Eng grain mahogany, aluminum extrusions

Core Properties

• Elastic moduli/strength increases with density

  • Linear extrapolation to get properties at different densities

  • Properties of cores + several densities are known - curvilinear relationship

• • W_c = core density

  • W_o = density of foil/ribbon material

  • T = thickness of foil/ribbon material

  • S = cell size 

Face Sheets

• Carry major applied loads, stiffness, stability, configuration, strength

• Must be properly bonded to core

Description of Face Sheets

Adhesively bonded pre fabricated face sheets

• use sheet material properties for structural sizing, in plane material properties not dependent on bonding operation

• metallic face sheets, pre-cured composite sheets

• film adhesive or paste adhesive

Co-cured or co-bonded face sheets with adhesive

• Sandwich co-cure: Face sheets attached to core while uncured and everything cures together

• Sandwich co-bond: one face sheet is pre-cured, other is cured with adhesive that bonds to core

  • Controlling overall thickness and finish

  • Two cure cycles

• Curing face sheet materials to core can result in waviness/dimpling of plies adjacent to core

Self adhesive face sheets

• Assembled with core in an uncured state

• Relies on face sheet resin material to bond face sheet to core, no separate adhesive

• Prepreg face sheets: higher resin content

Adhesives

• Purpose: attacking face sheets and core, reacting shear/peeling forces to face sheets/core, enabling materials to work together, bond face sheets to fittings, reinforce plates, edge strips

Description of Adhesives

synthetic/polymer based - controlled temperature to cure

Adhesive forms/types and uses

Primary considerations

• Strength, temperature range, face sheet interface (honeycomb), compatibility of cure parameters

Resins from self-adhesive face sheets

• resin from prepreg bonds face sheets to core material

• cheaper, lower weight

Film adhesive

• easy to handle and control during layup - controlled thickness/amount of adhesive

• carbon fiber & metallic core

  • degree of separation to minimize galvanic corrosion problems

• knitted carriers - higher peel strength

• mat carriers - limit co-mingling of prepreg resin and film adhesive, lower peel strength

Paste adhesives

• semi-solid, single part or two part compounds

• short working life, inexpensive

• require means to control bondline thickness

• generally not used for bonding face sheets to core; used to bond parts into sandwich, fill areas of core for stiffening/strengthening

Liquid Resins

• bond face sheets to core

  • core must be relatively solid and non-porous (balsa, foam, etc.)

  • honeycomb: seal at face sheet interface and then use liquid resin so cells don’t get filled up

Foaming Adhesives

• splice core sections when size exceeds standard core stock sizes, filling voids/unsupported face sheet material

• bonding replacement sections of damaged core

• not usually used for bonding

Adhesive Chemistries

Epoxy

• conversion from liquid resin to hardened adhesive

• thermosetting resins

• most common forms: two part paste and one part frozen film

  • two part: cure under ambient conditions

  • films: require elevated temperatures

• advantages: high strength/modulus, low levels of volatiles, low cure shrinkage, good chemical resistance, ease of processing, adhesion to range of substrate materials

• disadvantages: mixing requirements, limited pot life, brittleness, moisture causes reduction of properties, curing is slower that polyester resins, higher cost

• typical cure temps: 250-350 F

Bismaleimide (BMI Resins)

• high temp applications - excellent physical property retention with moisture and temperatures

• increased toughness, thermal stability, reduced moisture absorption

• typical cure temp: 350-400 F, 440 F leads to optimal properties

Phenols

• thermosetting resins, good fire resistance, high temp performance, long term durability, resistance to hydrocarbon/chlorinated solvents

• used with honeycomb when fire resistance is required

• brittle, volatiles sometimes generated during cure, health safety issues

Polyester

• thermosetting, inexpensive, fast processing (compared to epoxy), good fatigue resistance, UV stability, good performance with moisture

• good adhesion to glass fibers, bad with carbon fiber

• tougher than epoxies for a given thermal stability

Polyimide

• thermoset/thermoplastic, high temp applications, high cost

• cure temp: 550 F

  • require high temp bagging films, bleeder, breather

Adhesive Property Chart

Design and Analysis of Sandwich Structures

Introduction

• design must account for core shear deformation

• include effects of core shear properties on deflection, stress, and instability of sandwich in design

• face sheet and core bond must be strong enough to resist shear/tensile stresses

• core material chosen to be compatible with face sheet and adhesives

Design and Certification

Basic Design Principles

• space strong/thin face sheets far enough apart for high ratio of bending stiffness to weight

  • core: strength to withstand load and stiffness to stabilize face sheets in a configuration

  • adhesive must support and transfer all design loads

Important issues to consider

• core shear/crushing/buckling, face sheet dimpling/wrinkling/buckling, strength of core to face attachment, hardpoints (inserts/attachment points, ramps (transition form sandwich to solid laminate)

basic principles for successful design

• face sheets should be thick/strong enough to withstand chosen loads

• core should have enough shear rigidity/strength

• face sheet should be stiff enough so no wrinkling

• cell size/corrugation spacing so dimpling does not occur with loads

• core-face sheet attachment should be strong enough

Design Process

• account for impact damage and fluids

  • some impacts leave damage not visible on the surface (ex: crushed core)

  • face sheets can develop microcracks (flexing, impacts, etc.) that let in fluid

    • can debond face sheets and core, accumulate in core with cells

Sandwich Panel Failure Modes

• 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

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