Sandwich Structures – an Overview

By

K.Kalaichelvan

Associate Professor,

Dept of Production Technology,

MIT Campus, Anna University Chennai

Outline of Presentation

Sandwich structure – basics

Design requirement

Case study I

Applications

Case Study II

Conclusion

Sandwich Structure

Sandwich structure, consists of high strength facings or skins, being adhesively bonded to the low density core.

As per ASTM C274-53

A laminar construction comprising a combination of alternating dissimilar simple or composite materials assembled and intimately fixed in relation to each other so as to use the properties of each to attain specific structural advantages for the whole assembly

Core: A centrally located layer of a sandwich construction, usually low density, which separates and stablises the facings and transmits shear between the facings and provides most of the shear rigidity of the construction.

Facing (skin/face/face sheet):The outermost layer, generally thin and of high density, which resists of most of the edgewise loads and flatwise bending moments.

Adhesives:The adhesives are used to bind the Core and Facing.

Terminology of Sandwich Structures

Sandwich Construction Thin composite skins bonded to thicker,

lightweight core. Large increase in second moment of area

without weight penalty. Core needs good shear stiffness and strength. Skins carry tension and compression loads.

honeycombadhesive layer

face sheet

Sandwich panels are a very efficient way of providing high bending stiffness at low weight. The stiff, strong facing skins carry the bending loads, while the core resists shear loads. The principle is the same as a traditional ‘I’ beam:

Stiffness increase and weight reduction of sandwich composites

Composite sandwich structures are stiffer and stronger than the same weight single skin panels.

Selection of Core material

Foam core comparison

PVC (closed cell)

- ‘linear’ – high ductility, low properties- ‘cross-linked’ – high strength and stiffness, but brittle- ~ 50% reduction of properties at 40-60oC- chemical breakdown (HCl vapour) at 200oC

Foam core comparison

PU

- inferior to PVC at ambient temperatures- better property retention (max. 100oC)

Phenolic

- poor mechanical properties- good fire resistance- strength retention to 150oC

Foam core comparison

Syntactic foam

- glass or polymer microspheres- used as sandwich core or buoyant filler- high compressive strength

Balsa

- efficient and low cost- absorbs water (swelling and rot)- not advisable for primary hull and deck structures; OK for internal bulkheads

Sandwich Panels

Sandwich panels contd..

Design requirements

To find optimum Thickness of core (c) and Thickness of Face sheet (t)

for a specific stiffness ,minimum weight is to be obtained

Design Procedure

1.Core thickness ‘c’ == ‘d’ (1)

2.Core Modulus = Ec* ( * indicates property of the foam rather

than the material of foam made from)

3. Density of Core = ρc*

4. Core Shear Modulus = Gc*

5. Density of Face sheet = ρf

6. Longitudinal Stiffness of face sheet = Ef

7. Equivalent Flexural stiffness = (EI)eq

(EI)eq = Ef[Second moment of area of face sheet]

+

Ec[second moment of area of core]

= Ef [(bt3/6) + (btd2 /2)] + Ec[bc3/6]

------- (2)

The first, second and third terms describe the stiffness of the two face sheets and the core, while the second term adds the stiffness of the faces about the neutral axis of the beam. In a good beam design, the second term is substantially larger than the first and third terms and using equation (1) then

(EI)eq = Ef (btc2 /2) ----- (3)

8.Equivalent Shear Rigidity = (AG)eq

(AG)eq = Gc*bc ------ (4)

Total deflection = bending + shear

Bending depends on the skin properties; shear depends on the core

The stiff, strong facing skins carry the bending loads, while the core resists shear loads.

Contd..

9.When load ‘P’ acting on the end of Cantilever , the total deflection is sum of bending and shear components

δ = δb + δs = (Pl3/B1 EI) + (Pl/B2AG) using equation (3) and (4)

δ = (2l3/B1 Ef btc2) + (l/B2bcGc*) ------- (5)

10. For minimising weight: W = 2ρfgblt + ρc*gblc ------- (6)

Contd. To find optimum core thickness ‘c’ and face sheet

thickness ‘t’, two parameters (c/l) and (t/l) are considered.

Two constraints namely stiffness constraint and weight constraint are constructed from equation (5) and equation (6) respectively

11. From equation (5), Stiffness constraint is

------ (7)

Contd..

12. From equation (6), weight constraint is

----- (8)

For a specific stiffness (P/ δ) and a series of parallel lines for successively increasing weights(W) a graph is constructed

The optimum design point is found where objective function line is tangent to the stiffness constraint. At this point the optimum (minimum weight) values of face sheet thickness ‘t’ and core thickness ‘c’ for a specific stiffness can be taken from graph

The discontinuity of the stress at the skin/core boundary is a clear indication that the fiberglass is absorbing far more tension and compression then the core. The same applies to the simple 'I' beam.

                           So far, it has been only shown that bonding a strong, tensile material to the core will relieve it of a lot of stress yet make the entire sandwich core stronger.

Yield Stress at face for the given load P

Yield Stress at Core for the given load P

Mode of Loading B1 B2 B3 B4

Cantilever, end load (P) 3 1 1 1

Cantilever, uniformly distributed load (P/l)

8 2 2 1

Three point Bend, central load (P)

48 4 4 2

Three point Bend, uniformly distributed load (P/l)

384/5 8 8 2

Ends built in, central load (P)

192 4 8 2

Ends built in, uniformly distributed load (P/l)

384 8 12 2

Case Study

Fabrication of 3D – spacer fabric Sandwich structure

Development of Loom Methods

- Novel Stitching method

- Automatic stitching method

Effect of stitching in sandwich panels

There is the difficulty of obtaining a strong bond line between the skin and the core, which can often lead to reduction in performance or failure of the component when subjected to impact conditions.

The predominant failure is the delamination of core between the bonding interfaces.

Effect of stitching in sandwich panels (Contd.) The durability of these panels becomes

degraded due to de-lamination at the bonding interfaces and stiffness.

In order to overcome the shortcomings of conventional sandwich structures, the reinforcement to thickness of sandwich structures through the stitching process is one of the solutions to the problem of averting de-lamination and minimizing the degradation of stiffness.

Construction Material Specification

Skin Glass fabric 10-mil Bi-directionally

Woven Cloth.

Resin Epoxy / Hardener Dobeckot 520F / H758

Foam(Core Material) Polyurethane foam

Thermocole or extended

polystyrene

Thermocole with glass

tubes

Thermofoam

Density 65-70 kg/m3

Density 10kg/m3

Density 20kg/m3

Density 25kg/m3

Stitched fibre Glass yarn (Twisted) Distance, 8TPI

It is a bar of 52cm length, with 45 needles placed at 1cm gap.

The glass yarns are tied at one end and they are passed through the needle and feed from the other end.

DOWNWARD MOVEMENT(Z-AXIS) OF THE NEEDLE

BOBBIN ROTATION(1 REVOLUTION PER STITCH)

CROSSWISE MOVEMENT OF THE TABLE CARRYING THE STITCHING MECHANISM

LENGTHWISE DISPLACEMENT OF THE TABLE FOR VARYING THE PILE DENSITY

Needle bar arrangement.

Bobbin arrangement with stopper and lock.

Loop formation in Automatic stitching

Automatic stitching

The 10-mil bi-directionally woven glass fabrics were inserted above and below the transverse angle section, throughout the work area

It was firmly held in the roller rods, such that the fabric was held in tension

The top and the bottom decks were assembled and both of the decks were placed at the left bottom corner.

The absence frictional forces were confirmed by providing proper lubrication in the supporting bush for needle bar.

Automatic stitching (contd.)

The glass thread was wound in the both in bobbin and thread holder for the needle thread.

The glass fabric should be clean from foreign particles.

The stitching mechanism should be activated.

CURING OF RESIN

The resin used was epoxy resin –CY230

The hardener used is HY951

The resin and hardener mixing ratio 100 : 9

Curing time is 7-10 hrs.

Polyurethane Foam

PREPARATION OF CORE

Polyurethane foam is injected inside the 3D space.

Polyal and isocyanide is mixed in equal proportion.

Curing time is 45sec.

3D Spacer fabric without Foam

3D Spacer Fabric with Foam

3D spacer Fabric (Top View)

WITHOUT GLASS TUBES WITH GLASS TUBES

FLEXURAL PROPERTIES OF SANDWICH COMPOSITES

ASTM standard C-393

Static Bending test.

Load at a constant rate of 5mm/min

DIMENSION OF THE SPECIMEN

Total Length : 200mm

Span Length : 150mm

Width : 60mm

Depth : 20.6mm

MID-SPAN LOADING

QUARTER-POINT LOADING

Flexural Stiffness Parameter D and Core Shear Modulus G

CORE MATERIALS The flexural stiffness of thermocole with glass tubes 2x1 areal pile

density showed an increase of 213.72% and core shear modulus showed an increase of 84.79% compared to that of the polyurethane foam

The introduction of glass tubes have a significant effect on the flexural stiffness

Thermocole specimen with glass tubes also showed excellent core compressive strength compared to other specimens.

The flexural stiffness of thermocole with glass tubes increased by 176.8% and the core shear modulus increased by 657.05% compared to that of the thermofoam.

AREAL PILE DENSITYThe thermofoam cored specimen with 1x1 cm areal pile

density showed an increase of 495.8% in terms of flexural stiffness and an increase of 550.79% in terms of core shear modulus

The areal pile density increases the bending strength of the sandwich composite.

The damage propagation (i.e. delamination) is reduced by increasing the areal pile density.

The core shear rigidity increases with the increase in the areal pile density.

FLEXURE CREEP Thermocole with glass tubes showed a dominancy of 16.73% compared to

thermocole without glass tubes

Thermofoam has predominant compressive load carrying capacity and retains its original shape and size after the application of the forces.

 

Applications

Panels manufactured by Chempharm on the latest Sheet thickness : 0.5 mm to 0.8 mmmachines in India with following SpecificationSize: 1.2 Sandwich / Composite Mtr widths x 6.2 Mtr heightsMaterial of construction: / SS 304 / SS 316Overall thickness of the panel : 50 mm to 200 mmFilling Material : PUF / Mineral wool / Honey comb paper / EPS Application / Usage:Clean Room # 100 to # 100,000 for Pharmaceutical and Electronic industriesOffice Partitions for Industry High bay Warehouses, Storage facilitiesCold storages /Cold rooms and CA systemsRoof structure for Industrial Shed Important Note :The Mineral panel is biologically inert and recyclable. Mineral Panels are used for Fire retardant Application.

Contd..

Fan blade of modern gas turbine jet engines :

Two titanium alloy skins (the front and rear surface of the blade) are separated and bonded to a honeycomb structure made from titanium sheet

Hulls of modern racing yachts :

Two thin layers of composite skin (glass/carbon/kevlar in vinyl ester/epoxy) are separated by a nomex core (nomex is a combination aluminium honeycomb filled with rigid polyurethane foam).

Honeycomb Sandwich structure – a Case study

Component Material Specification

Core Glass ‘E’ fabricEpoxyHardener

Fiber – 220 GSM, Plain weaveLY 556HY 951Cell shape – regular hexagonCell size – 8, 16, 20, 25 mmCell thickness – 0.2 mmHeight – 8 mm

Facing Glass ‘E’ fabricEpoxyHardener

Fiber – 220 GSM, plain weaveLY 556HY 951Thickness – 1 mm

Adhesive Epoxy Flox adhesive

• Matrix used is epoxy resin LY 556 of density 20 g/m3

• Hardener HY 951 of density 0.972 to 0.992 g/ m3.• Resin and hardener is mixed in the weight ratio of 10:1.

• Matrix used is epoxy resin LY 556 of density 20 g/m3

• Hardener HY 951 of density 0.972 to 0.992 g/ m3.• Resin and hardener is mixed in the weight ratio of 10:1.

Selection of Core and Face materials

1. Basic raw materials glass cloth, Epoxy resin LY 556 and Hardener HY951 2. Glass and resin ratio is 65:35. The hardener is 10% of the resin percentage.3. Composites split honeycomb molding tool is made for special purpose fabrication 4. Take the resin and hardener, mix thoroughly and impregnate the glass cloth with the resin mixture and spread over the mould. 5.Then place the mandrel in the respective locations. Subject the system to vacuum for 30 minutes up to 450-500 Hg/mm2 3/2 – 2 hours.6. After ambient curing, release the mould (first half), similarly fabricate the second half 7. By getting two halves, join in together (which forms hexagonal structured cell). Similarly successive honeycomb cells are fabricated and integrated 8. Cut the honeycomb structure to the required standards. Take two plies of glass cloth impregnate the resin mixture, spread it on the honeycomb open structure and similarly apply the vacuum pressure.9. After ambient curing, subject the honeycomb structure to post curing.10.This is achieved by hot-air oven for 100 0C up to 2 hours

1. Basic raw materials glass cloth, Epoxy resin LY 556 and Hardener HY951 2. Glass and resin ratio is 65:35. The hardener is 10% of the resin percentage.3. Composites split honeycomb molding tool is made for special purpose fabrication 4. Take the resin and hardener, mix thoroughly and impregnate the glass cloth with the resin mixture and spread over the mould. 5.Then place the mandrel in the respective locations. Subject the system to vacuum for 30 minutes up to 450-500 Hg/mm2 3/2 – 2 hours.6. After ambient curing, release the mould (first half), similarly fabricate the second half 7. By getting two halves, join in together (which forms hexagonal structured cell). Similarly successive honeycomb cells are fabricated and integrated 8. Cut the honeycomb structure to the required standards. Take two plies of glass cloth impregnate the resin mixture, spread it on the honeycomb open structure and similarly apply the vacuum pressure.9. After ambient curing, subject the honeycomb structure to post curing.10.This is achieved by hot-air oven for 100 0C up to 2 hours

VACUUM BAG MOULDING VACUUM BAG MOULDING

Fabrication of Honeycomb core

11 22

33 44

Fabrication of Sandwich

11 22

33

Fabrication of Sandwich contd…

44 55

66 77

Summary of Case study II

Honeycomb sandwich structure was developed by varying cell height

Sandwich having Cell size of 8mm obtains better results when compared with other cell sizes.

Core shear modulus relation is established with respect to density ratio.