A

Review Report

On

STUDIES ON SHEAR FLOW OF FOAM CORE -GLASS /EPOXY SKIN SANDWICH COMPOSITES IN FLEXURE

Masters of Technology in Manufacturing Engineering

May2015

ABSTRACT

Studies on understanding shear flow by varying geometry and skin to core weight ratio of sandwich composites help understand the crux of failure patterns. Glass epoxy skin foam core sandwich composites used in high end structural aerospace applications exhibit various types of failures occurring over time. The current study focuses on understanding the shear flow patterns in failure by comparing the flexural, compressive and tensile properties. Foams of low density are chosen to fabricate the sandwich panels with glass epoxy skin by maintaining a constant skin to core weight ratio of 4:1with various thickness 10mm, 25mm, 50mm for PUF and PIR foams of 125kg/cum density. Foam tensile test specimens are fabricated, and tested, compression test properties are used, and, the shift in neutral axes understood by a new novel design approach for the flexural specimens . Analytical calculations can be done to find ‘r’ ratio and shift in neutral axes.

Keywords: Rigid foam, Glass epoxy, polyisocyanurate, polyurethane, shear flow, shift in neutral axis ,skin to core weight ratio etc.

i

ACKNOWLEDGEMENTS

I register my special thanks to Dr. SK Sekar, Dean, SMBS, VIT University, for his support in the

completion of the project.

I convey my thanks to Dr.Ramanujan, Professor and Programme manager,Manufacturing

Engineering, for his boosting support and timely help in the completion of this project.

I thank my most respectful and esteemed guide Dr. K. Padmanabhan, Professor and

Assisstant director, Centre for Excellence in Nano Composites, Manufacturing division, for his

constant encouragement, timely help and insights which provided me the zeal in the completion

of the project work.

I would like to thank Prof. Kuppan, In charge of Advanced Materials and Processing Lab,

School of Mechanical and Building Sciences, who has provided the Instron machine for testing

the specimens.

I am grateful to Ramya M, Project Associate, School of Mechanical and Building

Sciences for her support and encouragement throughout the project. Her guidance has enabled

me to overcome the problems faced during the executions of the project.

I thank AR&DB (Aeronautics Research and Development Board) and VIT University for

all financial support and encouragement in pursuing a project of this magnitude and completing

it successfully.

Above all, I thank my family members for being supportive and being instrumental in the

successful completion of this project.

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CONTENTS

Chapter TitlePage

No.

Abstract i

Acknowledgements ii

List Of Figures vi

List Of Tables vii

Nomenclature x

Chapter 1 Introduction 1-13

1.1 Composites

1.11 Constituents Of Composites

1.2 Sandwich Composites

1.21 Components Of Sandwich Composites

1.3 Why Use Sandwich Composites

1.4 Acoustic Properties Of Sandwich Composites

1.4.1 Thermal And Electrical Properties

1.4.2 Mechanical And Structural Properties

1.5 Advantages And Disadvantages

1.6 Applications Of Sandwich Composites

Chapter 2 Literature Review 13-19

Chapter 3 Methodology 20-30

iii

3.1 Materials Used PUF And PIR And Glass Fabric Advantages

3.2 Calculation For Fabrication Process Of 125 Kg/M3 Density Foam

At4:1 Panels At Various Thickness 10mm,25mm,50mm

3.3 Vacuum Bagging Process For Sandwich Panels

3.4 Fabrication Of Tensile Test Specimens

Chapter 4 Testing 30-40

4.1 Testing Of Tensile Specimens

4.2 Flexural Test

4.3 Compression Test

4.3 Theoretical Calculations

4.4 Shear Flow Diagram

4.5 Comparison Between Compression And Tensile Test For Poisson’s

Ratio

4.6 Failure Modes Of Sandwich Beams

4.7 R Ratio And C/D Calculations

Chapter 5 Results And Discussions 41-60

5.1 Comparison Plots For Various Foam Thicknesses (PIR-125)

With Various Mechanical Properties

5.2 Comparison Plots For Various Foam Thicknesses (PUF-125) With

Various Mechanical Properties

5.3 Tensile Behaviour Of Foams Under Instron. Load Vs Extension Plots

5.4 Tensile Test Data

41-60

iv

5.5 Plots For Shear Flow With Various Mechanical Properties

5.6 Co- Relate Shear Flow With Shift In Neutral Axis

Chapter 6 Conclusions 60

Chapter 7 References 61

v

List Of Tables

Table No:

Title Page No.

3.1

Calculation For Fabrication Process Of 125 Kg/M3 Density

Foam At4:1 Panels With 10mm,25mm,50mm. 24

3.2 Foam Properties Of Puf & Pir125 25

4.1Poisons Ratio And Compressive Stress Values From Compression Test

37

4.2Poisons Ratio And Compressive Stress Values From Tensile

Test38

5.1

Mechanical Properties Of Puf & Pir125 At 4:1skin To

Weight Ratio With Thickness 10mm,25mm,50mm 40

5.2 Tensile Test Data 48

5.3 Comparison Of C/D Ratio In Tensile Foam Specimen For

Puf &Pir125

49

5.4

Shear Flow In Pir -125 And Puf -125(4:1skin To Weight

Ratio) 50

vi

List Of Figures

Figure No.

Title Page No.

1.1 Structure Of Sandwich Composite

3

1..2 Typical Sandwich Construction 4

3.1 Polyurethane Foam Board 21

3.2Poly Iso-Cynurate Foam

22

3.3 Glass Fabric (280 Gsm) 23

3.4 Glass Fabric (100 Gsm)

23

3.5 Cross Section Of Vacuum Bagging Process 26

3.6Pir -125 ,4:1(10mm) Sandwich Panels Kept In Sealed

Vacuum Bags For Curing27

3.7

Puf 125 ,4:1. (50mm)Vacuum Bagging Technique For

Fabrication Of Sandwich Composites 28

3.8 Fabrication Of Tensile Specimen After Hand Mill Grinding 28

3.9Grooves Made At The End Of Specimens ,Glass Laminates

Inserted Into Grooves29

4.1 Instron 8801 Machine For Testing 31

vii

4.2 Puf 125 Sample Before Tensile Test 32

4.3 Pir125 Sample Before Tensile Test 32

4.4 Testing Of PIR-125-4:1-16:1 Specimen 33

4.5 Testing Of PUF -125-4:1-16:1 Specimen 33

4.6 PUF-125 Foam Compression Test 33

4.7Experimental Set Up Of PUF& PIR Foams In Compression Test

34

4.8Foam Core Sandwich Construction Showing The

Architecture34

4.9 Shear Flow Diagram 37

4.10Tensile Fracture Of Face Sheets

38

4.11 Face Sheet Wrinkling

38

4.12Shear Failure In Core

39

4.13Delamination

39

5.1 Foam Thickness Vs Maximum Bending Stress 41

5.2 .Foam Thickness Vs. Normal Stress (N/Mm2) 41

5.3 Foam Thickness Vs. Shear Strength In Core 42

5.4. Foam Thickness Vs. Bending Shape Factor For Stiffness 42

5.5 Foam Thickness Vs Flexural Rigidity Per Unit Width 43

5.6 Foam Thickness Vs Shear Deflection 43

5.7 Foam Thickness Vs Shear Strain At Max Load 44

5.8 Load Vs Extension Plot For PIR -125 Sample-1 45

viii

5.9 ;Load Vs Extension Plot For PIR -125 Sample-2 45

5.10 Load Vs Extension Plot For PIR -125 Sample-3 46

5.11 ;Load Vs Extension Plot For PUF -125 Sample-1 46

5.12 Load Vs Extension Plot For PUF -125 Sample-2 47

5.13 Load Vs Extension Plot For PUF -125 Sample-3 47

5.14 Shear Flow Vs Maximum Bending Stress (N/Mm2) 51

5.15 Thickness Vs Shear Flow (N/Mm) 51

5.16 Shear Flow Vs Normal Stress 52

5.17 Shear Flow Vs Shear Deflection 52

5.18 Shear Flow Vs Shear Strain 53

5.19 Shear Flow Vs Flexural Rigidity Per Unit Width 53

5.20 Shear Flow Vs Max Bending Stress 54

5.21 Shear Flow Vs Normal Stress 54

5.22 Shear Flow Vs Flexural Rigidity Per Unit Width 55

5.23 Shear Flow Vs Bending Shape Factor 55

5.24 Shear Flow Vs Shear Deflection 56

5.25 Shear Flow Vs Shear Strain 57

5.26 Shift in neutral axis Vs Shear Flow for pir 125 58

5.27 Shift in neutral axisvs Shear Flow for puf 125 58

ix

NOMENCLATURE

h = Total height of sandwich

b = width of the specimen

t = thickness of the skin

Ef = Elastic modulus of the fabric

Ec = Elastic modulus of the core

d = Centroidal height of the specimen

c = Core thickness

W = maximum load

δ = deflection corresponding to maximum load

a = span of the specimen

M = Bending moment

y = distance from the neutral axis

D = Flexural Rigidity

δshear = Shear deflection

σb = Bending stress

B = Bending strength

τc = Maximum shear stress in core

γ = Shear strain

σx = Normal stress

If = Second moment of inertia

Φbe = Bending shape factor for stiffness

Φbf = Shape factor for failure in bending

GFRP = Glass Fibre Resin Polymer

GSM = Grams per square meter

FRP = Fibre-reinforced polymer

ASTM = American Society for Testing and Material

x

CHAPTER 1

1. INTRODUCTION

COMPOSITE MATERIALS:

The composite material is made from two or more constituent materials with significantly

different physical or chemical properties that remain separate and distinct at the macroscopic or

microscopic scale within the finished structure is called composite. One of the phases is usually

discontinuous, stiffer, and stronger and is called the reinforcement, whereas the less stiff and

weaker phase is continuous and is called the matrix. Sometimes because of chemical interactions

or other processing effect, an additional distinct phase called an interphase exists between the

reinforcement and the matrix. The area which separates the fibre from the matrix and may differ

from them chemically, physically and mechanically is called interface.

CONSTITUENTS OF COMPOSITES:

RESINS:Typically, most common polymer-based composite materials, including fibreglass,

carbon fibre, and Kevlar, include at least two parts, the substrate and the resin. Polyester resin

tends to have yellowish tint, and is suitable for most backyard projects. Its weaknesses are that it

is UV sensitive and can tend to degrade over time, and thus generally is also coated to help

preserve it. It is often used in the making of surfboards and for marine applications. Its hardener

is a peroxide, often MEKP (methyl ethyl ketone peroxide). When the peroxide is mixed with the

resin, it decomposes to generate free radicals, which initiate the curing reaction. Hardeners in

these systems are commonly called catalysts, but since they do not re-appear unchanged at the

end of the reaction, they do not fit the strictest chemical definition of a catalyst.

Vinyl ester resin tends to have a purplish to bluish to greenish tint. This resin has lower

viscosity than polyester resin, and is more transparent. This resin is often billed as being fuel

resistant, but will melt in contact with gasoline. This resin tends to be more resistant over time to

degradation than polyester resin, and is more flexible. It uses the same hardeners as polyester

resin (at a similar mix ratio) and the cost is approximately the same. Epoxy resin is almost totally

transparent when cured. In the aerospace industry, epoxy is used as a structural matrix material

or as structural glue.

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Shape memory polymer (SMP) resins have varying visual characteristics depending on

their formulation. These resins may be epoxy-based, which can be used for auto body and

outdoor equipment repairs; cyanate-ester-based, which are used in space applications and acryl

ate-based, which can be used in very cold temperature applications, such as for sensors that

indicate whether perishable goods have warmed above a certain maximum temperature. These

resins are unique in that their shape can be repeatedly changed by heating above their glass

transition temperature (Tg). When heated, they become flexible and elastic, allowing for easy

configuration. Once they are cooled, they will maintain their new shape. The resins will return to

their original shapes when they are reheated above their Tg. The advantage of shape memory

polymer resins is that they can be shaped and reshaped repeatedly without losing their material

properties. These resins can be used in fabricating shape memory composites.

REINFORCEMENT:

Fibre

Reinforcement usually adds rigidity and greatly impedes crack propagation. Thin fibres

can have very high strength, and provided they are mechanically well attached to the

matrix they can greatly improve the composite's overall properties.

Fibre -reinforced composite materials can be divided into two main categories normally

referred to as short fibre reinforced materials and continuous fibre-reinforced materials.

Continuous reinforced materials will often constitute a layered or laminated structure.

The woven and continuous fibre styles are typically available in a variety of forms, being

pre-impregnated with the given matrix (resin), dry, uni-directional tapes of various

widths, plain weave, harness satins, braided, and stitched.

The short and long fibres are typically employed in compression moulding and sheet

moulding operations. These come in the form of flakes, chips, and random mate (which

can also be made from a continuous fibre laid in random fashion until the desired

thickness of the ply / laminate is achieved).

Common fibres used for reinforcement include glass fibres, carbon fibres, cellulose

(wood/paper fibre and straw) and high strength polymers for example - aramid.

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SANDWICH COMPOSITES:

Sandwich structured composites are a special class of composite materials which have

become very popular due to high specific strength and bending stiffness. Low density of these

materials makes them especially suitable for use in aeronautical, space and marine applications .

Sandwich panels are composite structural elements, consisting of two thin, stiff, strong faces

separated by a relatively thick layer of low-density and stiff material. The faces are commonly

made of steel, aluminium, composite and the core material may be foam, honeycomb and balsa

wood. The faces and the core material are bonded together with an adhesive to facilitate the load

transfer mechanisms between the components. This particular layered composition creates a

structural element with both high bending stiffness - weight and bending strength – weight ratio .

Figure1.1:Structure of Sandwich composite

\

3

Figure 1.2 : Typical sandwich construction

COMPONENTS IN SANDWICH COMPOSITES

Sandwich composites primarily have two components namely, skin and core as

shown in Figure1. If an adhesive is used to bind skins with the core, the adhesive layer can also

be considered as an additional component in the structure. The thickness of the adhesive layer is

generally neglected because it is much smaller than the thickness of skins or the core. The

properties of sandwich composites depend upon properties of the core and skins, their relative

thickness and the bonding characteristics between them.

Core:

Based on the performance requirements, large numbers of materials are used as core. Popular

core materials can be divided into three classes as described below.

1. Low density solid materials: open and closed cell structured foams, balsa and other types of

wood.

2. Expanded high-density materials in cellular form: honeycomb, web core.

3. Expanded high-density materials in corrugated form: truss, corrugated sheets.

4

High-density materials used for the purpose of making expanded core include aluminum,

titanium and various polymers. The structure of the core material affects the interfacial contact

area between skins and the core. Expanded high density materials normally provide much

smaller contact area compared to the solid low density materials. The choice of appropriate

structure for core provides additional parameter to design a sandwich composite as per given

specifications or service conditions. The use of cores like closed cell structured foam gives some

distinct advantages over open cell structured foams and cores. The specific compressive strength

of close cell structured foams is much higher. They also absorb less moisture than open cell

structured foam.

Skins:

A wide variety of materials are available for use as skins. Sheets of metals like aluminum,

titanium and steel and fibre reinforced plastics are some of the common examples of skin

materials. In case of fibre reinforced skins, the material properties can be controlled directionally

in order to tailor the properties of the sandwich composite. Fibre reinforced polymers are used

widely as skins due to their low density and high specific strength. Another advantage offered by

the use of polymer composites in skins is that the same polymer can be used to make the skin

and the core. Cross-linking of polymer between core and skin would provide adhesion strength

level equal to the strength of the polymer. This provides possibility of making the skin an

integral part of the structure eliminating the requirement of the adhesive. When an adhesive is

used to bond the skin and the core together, selection of adhesives becomes very important, as

they should be compatible with both the skin and the core materials. The adhesion must have

desired strength level and should remain unaffected by the working environment. In case of

metallic components, welding or brazing is used as a means of binding the core and skins

together. Use of adhesives is also possible but is limited to such cases where one or more of the

components cannot withstand heat. Choice of skins is important from the point of view of the

work environment as this part of the structure comes in direct contact with the environment.

Corrosion, heat transfer characteristics, thermal expansion characteristics, moisture absorption

and other properties of the whole sandwich composite can be controlled by proper choice of skin

material. In most cases both skins of the sandwich are of the same type, but could be of different

type depending up on specific requirements. Difference may be in terms of materials, thickness,

fibre orientation, fibre volume fraction or in any other possible form.

5

Why to use sandwich composites:

Sandwich composites are becoming more and more popular in structural design, mainly for

their ability to substantially decrease weight while maintaining mechanical performance. This

weight reduction results in a number of benefits, including increased range, higher payloads and

decreased fuel consumption. All have a positive impact on cost as well as a decreased impact on

the environment. These benefits are possible because, as has long been known, separating two

materials with a lightweight material in between increases the structure’s stiffness and strength.

This distinction, along with many other material characteristics available through strategic choice

of core material – such as thermal insulation, low water absorption, sound and dielectric

properties, among others – benefit a wide range of industries and applications, including wind,

marine, aerospace, transportation and industry.

Decrease weight Reduce the impact on the environment One of the most driving reasons

to use sandwich composites is that they provide mechanical properties to much lower weight

than traditional monolithic materials (for example, steel). It is not only the sandwich principle

itself that makes this possible. Sandwich composite materials also enable designers to engineer

with extreme precision to their loading requirements. Core is one of the variables in a sandwich

composite that enables this due to the wide range of mechanical properties it provides. In other

words, a sandwich solution will prevent over engineering, save weight and increase performance

compared to many designs that use conventional materials such as wood and steel. The

combination of sandwich principle and core material saves energy and enables faster and more

effective solutions in many areas. For example, sandwich composite design is an absolute

necessity to reach competitive cost per megawatt from wind energy. In transportation, lower

weight in container or vessel construction enables higher payloads resulting in reduced

emissions. Sandwich composites provide vital strength and speed for the sports equipment

segment. In industry, a lightweight solution can result in faster and smaller robots. In

construction, a bridge or façade designed with sandwich composites facilitates fast, effective

installation. Clearly, the benefits of lightweight solutions go on and on. Obviously, anything that

moves consumes energy. The heavier it is, the more energy consumed. Since using sandwich

composites makes structural designs lighter, sandwich solutions are extremely environmentally

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friendly. With a sandwich solution, less material is consumed in the construction. This saves

resources as well as weight in the final construction, making the construction less energy-

consuming over its lifetime. To illustrate the impact weight has on the environment, follow any

kind of vessel – whether airplane, bus, train or car – through its lifetime (25 years). Every kilo

saved in its construction results in less energy needed to move people or materials around the

world. Less energy expended every day for 25 years saves the environment from enormous

amounts of pollutants. A simplified comparison between a steel panel and a composite panel is

shown under the heading “Basics of sandwich composite,” demonstrating and explaining the

potential weight savings from using sandwich composites. Due to increasing fuel costs, many

industries are also realizing it is not only good for the environment, but it also costs less to

design with lightweight solutions. The environmental impact of material choice in the beginning

(energy) and in the end (recycling) of a vessel’s life cycle is minor (as long as the material choice

saves weight) in comparison with the vessel’s fuel savings over its lifetime.

benefits with sandwich composites :

Those described above are the main benefits and reasons many industries use sandwich

composites. However, with diverse structural core materials to choose from – each with its own

set of material characteristics – you can obtain additional benefits. (By structural core material

we mean a core material that has a specification and tolerances, as well as significant mechanical

performance.) Here we mention a few of the most important benefits.

Fire, Smoke & Toxicity (FST) FST regulations are tough in applications involving public

transportation like buses, trains and aircraft. In order to “harvest” the benefits of sandwich

composites, some structural core materials have specific raw materials making them self-

extinguishable as well as nontoxic when burning, qualifying them for use in public

transportation. Thermal insulation Polymer core materials are built up by a cell structure. These

cells are filled with air. Due to this, some core materials do not transfer heat or cold well. This

could be of great benefit in, for instance, the building industry, subsea applications or

applications where insulation is important.

Sound insulation Based on the same principle of cell structure, some core materials

(particularly those with closed cell structures) have a good ability to insulate/absorb sound. This

is useful in many applications, such as speakers, but first and foremost in aircraft interiors, where

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good sound insulation improves the interior environment in commercial airliners and private

aircraft.

Corrosion resistance Removing the risk for corrosion has a significant impact on ensuring long

operational lifetime for an application. With their non-corrosive properties, using polymer core

materials and polymer skins eliminates the risk for corrosion damages on a structure. This makes

sandwich composites ideal for marine applications as well as subsea structures.

Very low water absorption For applications used in marine environments or in places

with moisture or condensation, polymer core materials are excellent. The reason for this is, once

again, the closed cell structure. This prevents water or moisture from entering the core and

increasing weight or ruining mechanical performance. This is also important for aircraft interiors,

where traditional materials like honeycombs trap water in the cell structure, adding weight

during its lifetime. In comparison, most closed cell polymer materials have extremely low water

vapor permeability or water absorption over their lifetime. Ease of repair Sandwich composites

are easy to repair.

Cracks and slamming damage can be repaired relatively easily without reducing the structure’s

mechanical performance. Compared to steel, for which a large part of the structure must be cut

out and replaced, professionals can repair a sandwich composite locally without reducing the

performance or the design.

Dielectric properties some core materials have excellent dielectric properties. This means they

do not interfere with radio waves – useful when designing and building radomes, spherical

housing for radar equipment or x-ray equipment.

PROPERTIES OF SANDWICH COMPOSITES:

ACOUSTICAL PROPERTIES OF SANDWICH COMPOSITE MATERIALS:

In applications where the use of lightweight structures is important viscoelastic core layer,

which has high inherent damping, between two face sheets, can produce a sandwich structure

with high damping. Composite sandwich structures have several advantages, such as their high

strength-to-weight ratio, excellent thermal insulation, and good performance as water and vapor

barriers. So in recent years, such structures have become used increasingly in transportation

vehicles. However their fatigue, vibration and acoustic properties are known less. This is a

problem since such composite materials tend to be more brittle than metals because of the

8

possibility of delamination and fibre breakage. Structures excited into resonant vibration exhibit

very high amplitude displacements which are inversely proportional to their passive damping.

The transmission loss of such composite panels is also poor at coincidence. Their passive

damping properties and attempts to improve their damping at the design stage are important,

because the damping properties affect their sound transmission loss, especially in the critical

frequency range, and also their vibration response to excitation. The research objects in this

dissertation are polyurethane foam-filled honeycomb sandwich structures. The foam-filled

honeycomb cores demonstrate advantages of mechanical properties over pure honeycomb and

pure foam cores. Previous work including theoretical models, finite element models, and

experimental techniques for passive damping in composite sandwich structures was reviewed.

The general dynamic behavior of sandwich structures was discussed. The effects of thickness

and delamination on damping in sandwich structures were analyzed. Measurements on foam-

filled honeycomb sandwich beams with different configurations and thicknesses have been

performed and the results were compared with the theoretical predictions. A new modal testing

method using the Gabor analysis was proposed. A wavelet analysis-based noise reduction

technique is presented for frequency response function analysis. Sound transmission through

sandwich panels was studied using the statistical energy analysis (SEA). Modal density, critical

frequency, and the radiation efficiency of sandwich panels were analyzed.

THERMAL PROPERTIES OF SANDWICH COMPOSITES:

Heat conductive materials, which are widely used in the fields like electronic

information, electrical engineering and aerospace, are required high thermal conductivity,

excellent electrical insulation, corrosive resistance, chemical stability .

ELECTRICAL PROPERTIES OF SANDWICH COMPOSITES:

Electrical properties of adherent, low stressed aluminum nitride films prepared by magnetron

sputtering have been studied. Arrhenius plots in the 100°-170°C range of temperature and

transient current curves with different bias steps (at room temperature and 150°C) have been

performed. The conduction activation energy has been found and its dependence on subsequent

annealings has been studied. A phenomenological conduction model at the ITO-AlN interface

based on the shape of the transient curves has been elaborated. The conditions of behaviour of

9

AlN as a highly insulating layer (sputtering conditions, thicknesses, annealing temperatures)

have been established

MECHANICAL PROPERTIES OF SANDWICH COMPOSITES:

The strength of the composite material is dependent largely on two factors:

The outer skins: If the sandwich is supported on both sides, and then stressed by

means of a force in the middle of the beam, then the bending moment will

introduce shear forces in the material. The shear forces result in the bottom skin

in tension and the top skin in compression. The core material spaces these two

skins apart. The thicker the core material the stronger the composite.

The interface between the core and the skin: Because the shear stresses in the

composite material change rapidly between the core and the skin, the adhesive

layer also sees some degree of shear force. If the adhesive bond between the two

layers is too weak, the most probable result will be delamination.

The sandwich composites with 0° GF orientation possessed relatively much higher mechanical

properties as compared with those with 45° and 90° GF orientations, especially for the impact

strength. Low mechanical properties of the sandwich composites with 45° and 90° GF

orientation angles could be overcome by incorporation of DOP plasticizer into the GF/PVC core

layer with the recommended DOP loadings of 5–10 parts per hundred by weight of PVC

components.

STRUCTURAL PROPERTIES: There are many core and face sheet materials that can be selected for the sandwich construction.

The components of the sandwich are bonded together using adhesives or mechanical fastenings

such that they can act as a composite load bearing unit. The basic underlying concept of

sandwich is that face sheets carry the bending stress and the cores carry the shear stress. The

bending stiffness of the sandwich is very much higher than a solid structure having the same total

weight and the same material as the facings.

10

Structural Considerations:

As properties of honeycomb cores and face sheet materials are directional, it is vital to make sure

that the materials are oriented along the optimum axis to take the best advantage. These

structures are used to maximize stiffness at very low weights. The face sheets should be thick

enough to withstand tensile and compressive stresses induced by mechanical loads. The overall

structure should have high flexural and shear rigidity to avoid high deflections under heavy

loads. The face sheets should have sufficient stiffness to provide higher fundamental frequency.

The cores should have sufficient shear modulus to prevent buckling of the sandwich under load.

Fibre and matrix in composites

I. Role of fibre:

a. Depending upon the volume fraction, 70-90% of the load is carried by fibres.

b. They provide strength, stiffness, thermal stability and other structural properties in the

composites.

c. They provide electrical conductivity or insulation depending on the material.

II. Role of matrix:

a. To provide a good surface finish and aid in the protection of net shape or near shape parts.

b. To provide protection to reinforcing fibres against chemical attack and mechanical damage.

c. Matrix isolates individual fibres so that they can act separately and slow the propagation of the

crack.

d. To provide rigidity and shape to the structure.

e. Bind the fibres and transfer the load to the fibres.

Types of resins Resins are used to transfer stress between the reinforcing fibres. They act as a

glue to hold the fibres together and protect the fibres from mechanical and environmental

damage. There are two major groups into which resins are divided. They are

I. Thermoplastic resins: They become soft when heated, and maybe shaped or moulded

while in a heated semi-fluid state and become rigid when cooled. Example:

Polypropylene, Polyethylene.

II. Thermo-set resins: They are usually liquids or low melting point solids in their initial

form which solidify irreversibly when heated. Example: Epoxies, Vinyl esters.

11

Properties of Sandwich Composites:

Main advantage of any type of composite material is the possibility of tailoring their

properties according to the application. The same advantage also applies to sandwich

composites. Proper choice of core and skins makes sandwich composites adaptive to a large

number of applications and environmental conditions. Some general characteristics of sandwich

composites are described below:

1. Low density: Choice of lightweight core or expanded structures of high-density materials

decrease the overall density of the sandwich composite. Volume of core is considerably higher in

the sandwich composite compared to the volume of skins so any decrease in the density of the

core material has significant effect on the overall sandwich density.

2. Bending stiffness: This property comes from the skin part of the sandwich. Due to a higher

specific stiffness sandwich composites result in lower lateral deformation, higher buckling

resistance and higher natural frequencies compared to other structures.

3. Tensile and compressive strength: The z-direction properties are controlled by the properties

of core and x and y directions properties are controlled by properties of skins.

4. Damage tolerance: Use of flexible foam or crushable material as core makes sandwich

material highly damage tolerant structure. For this reason foam core or corrugated core sandwich

structured materials are popular materials in packaging applications.

ADVANTAGES OF SANDWICH COMPOSITES

Some of the advantages of sandwich composites are:

• Tailoring of properties according to requirements.

• Large available choice of constituents for core and skins.

• Low density leading to saving of weight.

• High bending stiffness.

• Higher damage tolerance.

• In-situ fabrication.

• Good vibration damping capacity.

LIMITATIONS OF SANDWICH COMPOSITES

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There are current limitations that can be overcome through the development of new material

sand manufacturing methods. Some of these are:

• Higher thickness of the sandwich composites.

• Higher cost of sandwich composites compared to conventional materials.

• Processing is expensive.

• Difficult to join.

• Difficult to repair, if damaged.

APPLICATIONS OF SANDWICH COMPOSITES

There are several applications that require materials of low density, high strength and high

damage tolerance. Due to their lightweight, sandwich composites are widely used in various

kinds of vehicles used for air, ground or sea transportation. Some of the main areas of

applications of sandwich composites are listed below.

1. Structural applications: aircraft, spacecraft, submarine, ships and boats, surface transport

vehicles, building materials etc.

2. Packaging materials.

3. Thermal and electrical insulation.

4. Storage tanks.

Innovativeness is essential in finding new combinations of core and skin materials and new

waysto use them in various applications where conventional materials have already reached

theirperformance limits.

13

LITERATURE REVIEW

Rodrigo Silva et al (1998) (2) studied the sandwich composite plate effects of the

transverse shear deformation are often significant. For this reason, the results of the first-order

shear deformation theory (FSDT) can be severely affected by the choice of the shear correction

factor (k), especially in sandwich plates with low shear modulus core. The objective of this

research work was to validate the first-order shear deformation theory (FSDT) in the elastic

analysis of sandwich plate structures. The FSDT was compared with a solid finite element

model which approximates the three-dimensional elasticity solution. The static response

(deflection and axial strain) under transverse uniform load and first natural frequency of

vibration were investigated. Three values of k were considered: 1) Based on shear strain energy

equivalence, 2) Based on a parabolic transverse shear strain distribution 3) Isotropic plates

(k=5/6). In addition, a simplified model for sandwich plates was evaluated. The effect of the

span/depth ratio and transverse shear modulus of the core material was studied as well. The

analyzed sandwich plates were symmetric cross-ply laminates with simply supported boundary

conditions. The face sheets are made of woven E-glass/Vinyl ester composite. Two core

materials were considered: balsa wood and foam. It was found that a good correlation between

the FSDT using ‘k’, based on strain energy and the simplified model for sandwich plates with the

solid finite element model in the prediction of deflection, strain and natural frequencies.

Therefore, the FSDT can be confidently applied to the linear elastic analysis of sandwich plates.

R.Vijayalakshmi Rao made an attempt[3] to study the flexural and fatigue

behavior of E- glass/Vinyl ester/Polyurethane foam sandwich composites. Three types of

sandwich composites were synthesized with E-glass fabric and polyurethane foam densities

having 65:35 ratio of fibre to resin weight fraction. The sandwich specimens were prepared by

hand lay-up method followed by compression at room temperature. The specimens are then

tested mechanically to ensure flexural and fatigue behavior. The objective of the present work is

to investigate the integrity of sandwich composites with E-glass reinforcements over the

variation of foam density. The effect of different test frequencies such as 1Hz, 3Hz, 5Hz, 7Hz

14

and 9Hz respectively are applied during the test. The experimental study reveals that the cyclic

load and test frequency play a critical role in determining fatigue strength and change in core

density attributes to the failure through foam-face sheet delamination, and at higher frequencies

early stiffness degradation leading to foam crushing and failure dictated by the shear strength of

the sandwich composites.

C. Borsellino [4] studied experimental and numerical evaluation of sandwich

composite structures. The main problem working with sandwich composite structures is their

intrinsic anisotropy and non-homogeneity that does not allow their correct modelling. Nowadays,

the available data on mechanical properties of complex structures, necessary to allow a

correct and reliable design, are not sufficient. The aim of the present work is to extend the

knowledge of mechanical properties bothon single components and on complete structures,

focusing on the effects induced by different kind of skin arrangements (Kevlar,glass and carbon

fibres). Compressive, shear and flexural tests were performed for a complete static mechanical

characterisation of the sandwich structure both on each single component and on the complex

structures in order to acquire important comparisonparameters. The mechanical results of each

component were used as input data in order to implement the FEM analysis by the commercial

ANSYS code. A simplified model is proposed to simulate the compressive and flexural tests of a

glass fibre sandwich structure. In addition their mechanical behaviour has been compared with

experimental data by the aforesaid static tests of complex sandwich structure.

Xing-jiang MA [5] had done three-point bending of sandwich beams with aluminum

foam-filled corrugated cores. Static three-point bending tests of aluminum foam sandwiches with

glued steel panel were performed. The deformation and failure of sandwich structure with

different thicknesses of panel and foam core were investigated. The results indicate that the

maximum bending load increases with the thickness of both steel panel and foam core. The

failure of sandwich can be ascribed to the crush and shear damage of foam core and the

delamination of glued interface at a large bending load. The crack on the foam wall developed in

the melting foam procedure is the major factor for the failure of foam core. The sandwich

structure with thick foam core and thin steel panel has the optimal specific bending strength. The

maximum bending load of that with 8 mm panel and 50 mm foam core is 66.06 kN.

Chava Uday et al [6] had studies on double lap shear and peel properties of sandwich

composites-The focus of the investigation is on adhesively bonded joints of glass/epoxy skin-

15

rigid unfilled thermoset foam core material sandwich composite structures to study their shear

failure properties. Rigid foam cores of Polyurethane (PUF) or Polyisocyanurate (PIR) of four

different densities – 64,125,250 and 500 kg/m3 were used with uniform thicknesses. Plain weave

glass fabric and a room temperature epoxy GY 257 with A140 hardener were used for the skin

design. The lap shear and peel test specimens were prepared by vacuum bagging technique. The

double lap shear properties were compared with the single lap shear properties, evaluated and a

detailed comparative analysis was made on the influence of different foam densities and their

adhesion to the skin on the failure behaviour of otherwise identical sandwich composite

samples[5]. The peel properties of the skin were also evaluated in Mode I cantilever set up

against foams of different densities. The highlight of his work was, comparison of porosity levels

of foams, their influence on adhesive and cohesive fracture, failure mode property correlation

and the usefulness of the obtained data in the design of sandwich joints.

Vishakh Vijayan [7] investigated on the influence of foam densities and span to depth

ratios on the Flexural properties of rigid polyisocyanurate foam-glass fabric/epoxy Sandwich

Composites The experimental investigation here focuses on the study of flexural and shear

properties of sandwich composites. Rigid unfilled thermoset Polyisocyanurate (PIR) foam is

used as the core material and glass fabric reinforced epoxy based laminates are used as the outer

skin layers of the sandwich. PIR foams with densities of 125kg/m3 and 250kg/m3 are used for

the study. Skin to core weight ratios of 3:1 and 4:1 were maintained for the panels fabricated

separately with the vacuum bagging technique. Specimens with varying span to depth ratios of

16:1, 12:1 and 6:1 were tested using an Instron UTM machine. The investigation follows a three-

point bending test method carried out for finding bending strength, flexure rigidity, shear

deflection, shear stress, shear strain etc. The test results were compared and analyzed in depth to

find the influence of the core density and span to depth ratios on the flexural and shear properties

of the sandwich composite. Useful conclusions have been drawn based on the fracture behaviour

of these sandwich composites.

Surya Teja Varma[8] - The experimental studies for determining the flexural properties of

thermo set rigid polyurethane unfilled foam core – glass/epoxy skin sandwich composites, are

presented here. Sandwich composites were fabricated in the shape of panels by using glass

fabric/epoxy as the skin material and rigid polyurethane foam (PUF) as the core. PUF materials

16

of 125 & 250 kg/m3 foam densities with 3:1 & 4:1 skin to core weight ratios were fabricated

separately using the vacuum bagging technique. The sandwich panels were tested at different

span to depth ratios. The flexural properties like the bending strength, flexural rigidity, shear

stress, shear deflection and shear strain were evaluated and a detailed analysis made on the

influence of foam densities and different span to depth ratios on the fracture behaviour of these

sandwich composites in flexure. Due comparisons have been made on the flexural behaviour

with other foams also.

L. J. GIBSON[9] A new method for maximizing stiffness per unit weight in sandwich

beams with foam cores is described. Optimum values of core thickness, face thickness and core

density are obtained from the analysis. Measurements of the stiffness per unit weight have been

made on sandwiches with foamed polyurethane cores. The theoretical analysis is in good

agreement with the results of these tests.

Hemnath T and Padmanabhan K, [10]Study of the strength of polyurethane foam core/

fabric reinforced-epoxy based sandwich composites with and without design optimization using

the finite element software ANSYS, an experimental hand lay-up technique and mechanical

testing in a Universal testing machine

Fredrik Edgrenet [11] plastic micro buckling approach is investigated in order to see

whether it can be used to analytically predict the residual strength of carbon fiber sandwich

structures. A parametric study on impact damage resistance and residual strength of sandwich

panels with carbon fiber-vinylester faces and PVC foam core is conducted. Two sandwich

configurations are studied. The first configuration consists of thin faces and an intermediate

density core, representative of a panel from a superstructure. The second configuration consists

of thick faces and a high density core, representative of a panel from a hull. Two different

impactor geometries are used. One spherical impactor and one pyramid shaped impactor are used

in a drop weight rig to inflict low velocity impact damage of different energy levels in the face of

the sandwich. The damages achieved ranges from barely visible damages to penetration of one

face. Residual strength is tested using in-plane compression of the sandwich plates either

instrumented with strain gauges or monitored with digital speckle photography.

17

L.L. Yan a et al [12)]“Three-point bending of sandwich beams with aluminum foam-

filled corrugated cores”. Sandwich panels having metallic corrugated cores had distinctly

different attributes from those having metal foam cores, the former with high specific

stiffness/strength and the latter with superior specific energy absorption capacity. To explore the

attribute diversity, all-metallic hybrid-cored sandwich constructions with aluminum foam blocks

inserted into the interstices of steel corrugated plates were fabricated and tested under three-point

bending. Analytical predictions of the bending stiffness, initial failure load, peak load, and

failure modes were obtained and compared with those measured. Good agreementbetween

analysis and experiment was achieved. Failure maps were also constructed to reveal

themechanisms of initial failure. Foam insertions altered not only the failure mode of the

corrugated sandwich but also increased dramatically its bending resistance. All-metallic

sandwich constructions with foam-filled corrugated cores hold great potential as novel

lightweight structural materials for a wide range of structural and crushing/impulsive loading

applications.

K.H. Leong [13] Tensile tests were performed on glass reinforced polymer (GRP) composites

with three-dimensional (3D) orthogonal, normal layered interlock, and offset layered interlock

woven fibre architectures. The mechanical properties and failure mechanisms under tensile

loading were similar for the three composites. Cracks formed at low strains within the resin-rich

channels between the fibre tows and around the through-thickness binder yarns in the

composites, although this damage did not alter the tensile properties. At higher applied tensile

stresses the elastic modulus was reduced by 20–30% due to inelastic tow straightening and

cracking around the most heavily crimped in-plane tows. Further softening occurred at higher

strains by inelastic straightening of all the tows. Composite failure occurred within a localised

region and the discrete tow rupture events that have caused tow lock-up and pullout mechanisms

in other 3D woven composites were not observed.

Brahmananda Pramanik, P. Raju Manten [14]This paper presents an investigation on energy

absorption characteristics of nano-reinforced panels, laminated face sheets and sandwich

composites in high velocity ballistic and low velocity punch-shear experiments. The vinyl ester

panels were reinforced with 1.25 and 2.5 wt. percent nanoclay and exfoliated graphite platelets.

18

Three different face sheets were manufactured with E-glass, Owens Corning HP ShieldStrand®

glass and T-700 Carbon woven fabric in vinyl ester; and one with the E-glass and graphite

platelets impregnated vinyl ester matrix. The sandwich composites were fabricated with balsa,

PVC foam, 3-D fibre reinforced Tycor® and fire resistant fly-ash based Eco-Core® cores in

between E-glass/vinyl ester face sheets. Ballistic tests were conducted according to NIJ level III

using a universal re- ceiver equipped with a barrel to launch 0.308 caliber M80 ball round

projectile at about 890 m/s. Low velocity punch-shear tests were performed at around 3 m/s

according to ASTM D3763 Standard using a drop-weight impact test system. The tortuosity of

the fractured surface in nanocomposite specimens has been investigated using digital micro-

scope. In ballistic tests, the 3-D fibre reinforced Tycor® core provided the most resistance when

projectile strikes at the web-flange interface region. The 2.5 wt. pct. graphite platelet reinforced

nanocomposite, HP ShieldStrand® glass vinyl ester face sheets, and E-glass/Eco-Core®

sandwich composite showed the best energy absorption under low velocity punch-shear

19

CHAPTER-3

METHODOLOGY& APPROACH

The objective of this paper is fabrication of tensile test specimens in dog bone shape for

125 kg/m3 density foam, with thickness of 10mm, using vacuum bagging technique. Glass fabric

s used as skin material and PUF and PIR used as core materials. Sandwich panels with varying

thickness 10mm, 25mm, 50mm are considered for the present investigation. The flexural

specimens tested at different span to depth ratios, mainly 16:1, skin to core weight ratio of 4:1 is

considered to understand the shear flow and shift in neutral axis in the flexural specimens. The

mechanical properties for the sandwich panels are to be calculated for the above conditions.

Selection of Required Materials:

Glass fabric: 280 & 100 GSM, plain weave

Epoxy: GY257

Hardener: ARADUR 140

Room Temperature Cured System

Foam: Thermo-set Unfilled Rigid Polyurethane, rigid poly –isocynuarate.

MATERIALS USED

POLYURETHANE FOAM: Polyurethane (PUR and PU) is a polymer composed of a chain

of organic units joined by carbonate (urethane) links. While most polyurethanes

are thermosetting polymers that do not melt when heated, thermoplastic polyurethanes are also

available. Polyurethane polymers are traditionally and most commonly formed by reacting a di-

or polyisocyanate with a polyol. Both the isocyanates and polyols used to make polyurethanes

contain on average two or more functional groups per molecule. The urethane polymer is formed

through the reaction between isocyanate and polyol, and tri-color-flour-methane or carbon

dioxide which is used as a blowing agent and is vaporized by the heat released by the exothermic

reaction. PUR foams are produced in many variations from soft foam with more or less open

cells to rigid types with predominantly closed cells and in densities of 30 to 500 kg/m3

20

Figure3.1:Polyurethanefoamboard

Polyurethane products often are simply called “urethanes”, but should not be confused

with ethyl carbonate, which is also called urethane. Polyurethanes neither contain nor are

produced from ethyl carbonate. Polyurethanes are used in the manufacture of flexible, high-

resilience foam seating; rigid foam insulation panels; microcellular foam seals and gaskets;

durable elastomeric wheels and tires (such as roller coaster  and escalator wheels); automotive

suspension bushings; electrical potting compounds; high performance adhesives; surface

coatings and surface sealants; synthetic fibres (e.g., Spandex); carpet underlay; hard-plastic parts

(e.g., for electronic instruments); hoses and skateboard wheels.

POLY-ISOCYNURATE FOAM:

Polyisocyanurate, also referred to as PIR, polyiso, or ISO, is a thermoset plastic typically

produced as a foam and used as rigid thermal insulation. Its chemistry is similar to

polyurethane (PUR) except that the proportion of methylene dipheny disocyanate (MDI) is

higher and a polyester-derived polyol is used in the reaction instead of a polyetherpolyol.

Catalysts and additives used in PIR formulations also differ from those used in PUR. PIR is

typically produced as foam and used as rigid thermal insulation. Its thermal conductivity has a

typical value of 0.16 BTU*in/hr*ft2*°F (0.023 W/(m·K)) depending on the perimeter: area ratio.

PIR foam panels laminated with pure embossed aluminum foil are used for fabrication of pre-

insulated duct that is used for heating, ventilation and air conditioning systems. Prefabricated

PIR sandwich panels are manufactured with corrosion-protected, corrugated steel facings bonded

to a core of PIR foam and used extensively as roofing insulation and vertical walls (e.g. for

21

warehousing, factories, office buildings etc.). Other typical uses for PIR foams include industrial

and commercial pipe insulation, and carving/machining media (competing with expanded

polystyrene and rigid polyurethane foams).

Figure3.2 : Polyisocynurate foam

1.4.3 GLASS FABRIC:

Woven glass fabric is a lightweight, strong, and robust material. Although strength properties are

somewhat lower than carbon fibre and it is less stiff, the material is typically far less brittle, and

the raw materials are much less expensive. Note that woven fibreglass is much finer than woven

roving, which is a heavier glass cloth, often used for boat building in conjunction with CSM as a

simple means of adding bulk to increase stiffness. E-glass is the common, all-purpose

reinforcement while S2-glass (high-Strength), is stronger, stiffer and far more expensive. Glass

fibre is commonly used as an insulating material. It is also used as a reinforcing agent for

many polymer products; to form a very strong and light fibre reinforced

polymer (FRP) composite material called glass reinforced plastic (GRP), popularly known as

"fibreglass". Glass fibre has roughly comparable properties to other fibres such as polymers

and carbon fibre . Although not as strong or as rigid as carbon fibre, it is much cheaper and

significantly less brittle.

22

Figure3.3: Glass fabric (280 gsm) Figure 3.4: Glass fabric (100 gsm)

For this project, we have used epoxy resin GY257 and hardener AD140. GY257 resin it is a

Liquid, Low-Viscosity, Biphenyl-A based Epoxy Resin, modified with reactive diluents. Its

properties are

Excellent mechanical properties and chemical resistance

Variable within wide limits by using changing hardeners and fillers

Very good processing properties

Good tendency to crystallize.

Its applications are that it is cured with polyamides, polyamidoamines or theiradducts for solvent

free coatings, flooring screeds, trowelling compounds etc.

AD 140 HARDENER:

A hardener is a substance or mixture added to a plastic composition to take part in and

promote or control the curing action, Also a substance added to control the degree of hardness of

the cured film. AD 140 hardener is generally used for reactive adhesives, castings, heat-resistant

mortars.

23

GLASS LAMINATE PROPERTIES:

The CADEC Software available in the Composite lab is used to find the various

micromechanical properties of laminate skin of sandwich composite inputs like modulus of

elasticity of, Fabric – Ef, Matrix – Em and the volume - Vf are given to attain values by Rule of

Mixtures, Halpin-Tsai Equation, Cylindrical Assemblage model and Stress Partitioning

equations.

Density for E-Glass Fabric = 2.52 gms/cc.

Volume Fraction of fabric in the composite = 0.3

Young’s Modulus of Elasticity, E-Glass Fabric = 35 GPa

Poisson's ratio of the E glass fibre = 0.25 + / - 0.01

RESIN PROPERTIES:

Epoxy GY 257 & Aradur 140 –

Density of epoxy is considered = 1.2 g /cc. Young’s Modulus of Elasticity,

Epoxy Resin mixture = 1.5 GPa ;

Poisons ratio of the resin =0.35

FOAM PROPERTIES : The two different types of foams used here are Polyurethane and Polyisocyanurate. The properties considered are :

PUF :Density 64 kg/m3 125 kg/m3

Young’s Modulus of Elasticity, E1, MPa

10.7 11

Poisson’s Ratio 0.3 0.312In Plane Shear Modulus, G12, MPa

4.11 4.192

24

PIR : Density 64 kg/m3 125 kg/m3

Young’s Modulus of Elasticity, E1, MPa

12 24

Poisson’s Ratio 0.32 0.332In plane Shear

Modulus, G12, MPa4.54 9

Table3.1 : Calculation for Fabrication Process of 125 kg/m3 density foam at4:1 panels with

10mm,25mm,50mm.

Foam type PIR(125kg/m3 ) PUF(125kg/m3 )

Foam thickness(mm) 10 25 50 10 25 50

Volume Fraction 0.3 0.3 0.3 0.3 0.3 0.3

Dimension of foam

(mm)

500*480

*10

1000*5

00*25

1000*52

0*50

500*480

*10

1000*50

0*25

1000*520

*50

No. of layers

8(280

Gsm), 4

(100

gsm)

24(280

gsm)

22(540

gsm)

9.67≈10

(280

Gsm)

18(280

gsm),

2(100

gsm)

24(600gs

m)

Volume of resin used

(cc)454.24 1649.41 3666.5 481.33 1116 4073.8

Volume of hardener

used (cc)227.12 824.7 1833.26 240.86 435.6 2036.9

3.VACUUM BAGGING PROCESS:

25

Vacuum Bag molding is a process using a two-sided mould set that shapes both surfaces

of the panel. On the lower side is a rigid mould and on the upper side is a flexible me.mbrane or

vacuum bag. The flexible membrane can be a reusable silicone material or an extruded polymer

film. Then, vacuum is applied to the mould cavity. This process can be performed at either

ambient or elevated temperature with ambient atmospheric pressure acting upon the vacuum bag.

Most economical way is using a venturi vacuum and air compressor or a vacuum pump. A

vacuum bag is a bag made of strong rubber-coated fabric or a polymer film used to bond or

laminate materials. In some applications the bag encloses the entire material, or in other

applications a mold is used to form one face of the laminate with the bag being single sided to

seal the outer face of the laminate to the mold. The open end is sealed and the air is drawn out of

the bag through a nipple using a vacuum pump. As a result, uniform pressure approaching one

atmosphere is applied to the surfaces of the object inside the bag, holding parts together while

the adhesive cures.

Fig 3.5 Cross section of vacuum bagging process

Vacuum bagging is widely used in the composites industry as well. Carbon fibre fabric

and fibreglass, along with resins and epoxies are common materials laminated together with a

vacuum bag operation. Typically, polyurethane or vinyl materials are used to make the bag,

which is commonly open at both ends. This gives access to the piece, or pieces to be glued. A

plastic rod is laid onto the bag, which is then folded over the rod. A plastic sleeve with an

opening in it, is then snapped over the rod. This procedure forms a seal at both ends of the bag,

26

when the vacuum is ready to be drawn. A “platen” is used inside the bag for the piece being

glued to lay on. The platen has a series of small slots cut into it, to allow the air under it to be

evacuated. The platen must have rounded edges and corners to prevent the vacuum from tearing

the bag.

VACCUM BAGGING PROCESS FOR SANDWICH PANELS:

For 4:1 skin to core weight ratios of 125 kg/m3 foams of varying thicknesses

10mm ,25mm,50mm., we have calculated the number of layers required on each side. Therefore,

we need to cut out 280 gsm & 100 gsm glass fabric with required dimensions. A volume

fraction of 0.3 was taken between the resin and glass fabric. The mixture of resin and hardener in

the appropriate ratio is applied on each glass fabric layer and the layers arranged accordingly. To

have proper adhesion between the layers, rolling is done alongside on each layer. The whole

sample is then inserted into a vacuum bag which is sealed at both ends. The vacuum pump is

switched on for half an hour. The atmospheric air pressure then acts on the panel as the bag is

evacuated. The composite is kept within the bag for a day for complete curing of the resin.

Figure3.6 :PIR -125 ,4:1(10mm) Sandwich panels kept in sealed vacuum bags for curing

27

Figure3.7:PUF 125 ,4:1. (50mm)Vaccum bagging technique for fabrication of sandwich

composites

FABRICATION OF TENSILE TEST SPECIMENS:

The tensile specimens are fabricated from two types of foam PIR AND PUF with 125

kg/m3 density with 10 mm thickness. The dimensions of the tensile specimen standards are

considered from ASTM 3039 and also ASTM D638. As the foams would get crushed between

the grips of the tensile fixture, the design is modified to suit the requirements by taking long

specimens with end grooves provided (as shown in pic),

FIG.3.8: FABRICTION OF TENSILE SPECIMEN AFTER HANDMILL GRINDING

28

which are inserted with glass laminates. It was seen by Chava Uday [6 ] in the study conducted

on foams of various densities, for double lap shear, that by adhesively bonding glass epoxy

laminates, exhibit high load bearing capacity. Hence the glass epoxy laminates were used to hold

the specimens between the grips of the tensile fixture in the UTM. The dimensions include

length of foam specimen 300 mm, width 25mm, thickness 10mm(approx. 10 – 12mm),centre

width 12.5mm and radius of curvature 8.1mm(refer fig.) .

FIG3.9 : grooves made at the end of specimens ,glass laminates inserted into grooves

The grooves made were of 25 mm depth and 3mm thickness to give a tight fit to the glass

laminate with epoxy bonding. Epoxy resin used for bonding was GY 257 and hardener Aradur

140. The joints were cured for 24 hours under vacuum bagging, and age cured for another 48

hours. The grooves that showed gaps in resin fills , were later filled with resin and cured again.

29

CHAPTER-4

4 .TESTING

INSTRON 8801 Universal Testing Machine:

The 8801 servo hydraulic testing system meets the challenging demands of a varied range of

both dynamic static and static testing requirements. The 8801 provides complete testing solutions

to satisfy the needs of advanced materials and component testing, and is ideally suited for high

and low-cycle fatigue testing, thermo-mechanical fatigue testing, and fracture mechanics. The

higher capacity of up to 100 kN, a larger working space, a high stiffness, and the precision of

alignment all make the 8801 an exceptionally versatile and reliable testing system. The precision

mechanical systems above combined with the advanced features of the 8800 digital controller

and Dynacell™ enable Instron® to supply fully-integrated turnkey solutions to meet the most

demanding applications. Console Software provides full system control from a PC: including

waveform generation, calibration, limit set up, and status monitoring. Figure shows the

INSTRON 8801 testing machine.

Features of the INSTRON 8801:

Up to ±100 kN (22,500 lbf) axial force capacity

Patented Dynacell load cell features compensation for inertial loads caused by heavy

grips and fixtures.

Standard or extra-height frame options.

Wide range of grips, fixtures, and accessories.

30

Figure 4.1: INSTRON 8801

In this project tensile test is conducted on each specimen PUF-PIR havingdensity125Kg/m3

with thickness (10-12mm). INSTRON 8801 UTM is used for testing the specimens.The ends of

specimens glass laminates is inserted to hold the grips by fixture . telescope micrometre is used

to measure the readings of each specimen fracture and failure modes are observed while doing

the test. load v/s deflection graphs and max load values ,linear extension of specimen after the

specimen breaks shown in computer .the values are used for calculating poisons ratio, youngs

modulus . Failures occurs different points in each specimen as shown In the fig.

31

Fig4.2:PUF 125 SAMPLE before tensile test

Fig 4.3 Failure occur below the cente line of PIR 125kg/m3

32

FLEXURAL TEST: flexural test specimens of 10mm,25mm,50mm of PUF&PIR125 with

weight ratio 4:1 is considered .the test conducted on instron 8801 as shown in fig.

Fig4.4 : Testing of PIR-125-4:1-16:1 specimen fig4.5:Testing of PUF -125-4:1-16:1

specimen

COMPRESSION TEST :

Compression tests of foam samples having density 125kg/m3 is considered for finding

r ratio ,shift in neutral axis .compression test of few samples done as shown in fig

Fig 4.6:PUF125 Foam Compression Test

33

Fig4.7 : Experimental Set Up Of PUF& PIR Foams In Compression Test

THEORITICAL CALCULATIONS:

Figure 4.8: Foam core sandwich construction showing the architecture

34

c = Thickness of the core, mm

b = Width of the Laminate , mm

t = Thickness of Skin, mm

d = Distance between centrodial axis of two skins, mm

h = Height of the Laminate, mm

(i) Flexural rigidity

+ Es (2bt)( ) + _______________(1)

Es=Elastic Modulus of skin in N/mm2

Ec=Elastic Modulus of core in N/mm2

d =distance between the centroidal axis of the two skins in mm

(ii) Shear Deflection

δshear = (W.l.C)/(4.b.d2.Gc) _________________(2)

W = Central load applied in Newton

l = Span length of the support in mm

Gc = Shear modulus of the core in N/mm2

C = Thickness of core in mm

(iii) Bending stress

__________________(3)

Bending moment, M = W.l/4 in N-m

(iv) Shear stress in core

35

τ = [(Es.t.d/2)+((Ec/2) ((c2 /4)-y2 ))] __________________(4)

Shear Force, Q=W/2 in Newton

y= any distance from the neutral axis

D=Flexural Rigidity in N-mm2

If Ec = 0, then τ = Q/(b.d)

(v) Shear Strain

γ = _______________(5)

(vi) Normal Stress

𝜎x = _______________(6)

Second moment of area, If = mm4

(vii) Bending Strength

M= N-mm ________________(7)

P=load of the specimen in Newton

a=Span length0

THEORITICAL CALCULATION

36

r =

=

Stresses in sandwich beams:

Shear stress occurs in core region

Bending stresses occurs in skin

The position of the neutral axis ,flexural rigidity, and second moment of area is

calculated based on shift in neutral axis.

Tc=Q/b.d

σ = Mh/btd2

SHEAR FLOW DIAGRAM

COMPARISON BETWEEN COMPRESSION AND TENSILE TEST OF FOAMS PUF

&PIR:

We see a shift in neutral axis during the test toward the tensile side of the specimen.

Understanding the tensile stresses acting on the foam core of a foam core glass epoxy sandwich

laminate of a particular weight ratio, helps understand the shear flow nature in the sandwich

37

composites. No shift in neutral axis is find while doing compression test Given below are the

details of the compressive stress and taken by a 125 kg/cum foam in

PIR foam density observed for Compression Tests. Poisson’s ratio for 125 kg/m3 is seen to be

high. Compressive stress is 1.16 MPa for PIR foam

PUF foam density observed for tensile tests .poison’s ratio for 125 kg/m3 is seen to be

high .tensile stress is 1.7 MPa for PUF foam

Compressive strength of PIR 125 is more compare to PUF 125&Tensile strength of PUF125 is

more compare to PIR125 vice versa.

Table 4.1:poisons ratio and compressive stress values for compression test

Foam Poisson’s Ratio Compressive Stress

PIR 125 0.3207 1.0376 MPa

PUF 125 0.295 0.631MPa

Table 4.2: TENSILE TEST

Foam Poissons ratio Tensile stress

PIR 125 .06 1.32

PUF 125 0.205 1.7

The poisons ratio of PUF125&PIR125 of compression test is more than that of tensile

test

Failure Modes of Sandwich Structures:

Sandwich, despite its high stiffness, should also possess high strength. There are four

different modes of failure of sandwich composites when loading in bending. The structure will

fail at the mode that occurs at the lowest load. The failure modes are:

I. Yielding or fracture of the tensile face

This type of failure occurs when the normal tensile stresses due to the tensile loading exceeds the

yield strength of the face sheet materials.

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Fig4.11 : Tensile fracture of face sheets

II. Buckling or Wrinkling of the face

This method of failure occurs due to the excessive compressive stress, which causes instability in

the face sheets.

Fig 4.12 : Face sheet wrinkling

III. Failure of the core in shear

Generally the failure occurs when the shear stress in the core exceeds the shear strength. The

shear strength of the core depends on the foam density, pore size and the heat treatment

temperature.

Fig4.12 : Shear failure in core

IV. Failure of the bond between face and core

This failure occurs only when stresses at the interface are high enough to cause delamination.

Fig4.13 : Delamination

CALCULATIONS FOR R RATIO AND C/D RATIO:

39

PIR-125 :

r = =1.32/1.0376 = 1..272

Substitue r value in c/d equation

= = 0.007174

PUF-125:

r = =1.7/0.631 =2.694

Substitue r value in c/d equation

= = 0.1051

CHAPTER-5

5.RESULTS AND DISCUSSION:

Table 5.1 : mechanical properties of PUF &PIR125 at 4:1skin to weight ratio with thickness

10mm,25mm,50mm

Type of foam

PIR 125 PUF 125

Foam thickness(mm) Foam thickness(mm)

10 25 50 10 25 50

40

Max bending stress 40.21 47.92 56.8 21.88 23.96 033.19

Flexural Rigidity

( N-mm2/mm) per

unit width*106

6.38 20.81 77.7 5.20 20.79 139

Shear Strength in

Core(N/mm2)

0.286

0.3940.39 0.29 0.196 0.176

Normal

Stress(N/mm2)

20.39867

73747.68 56.69 21.45 23.84 18.78

Bending Shape

Factor for Stiffness

3.815 7.183 11.42 3.97 7.183 7.99

Shear deflection 3.62 13.14 20.82 10.29 13.11 21.69

Shear strain at

max load 0.0383 0.053 0.04845 0.11 0.0579 0.0050

COMPARISON PLOTS FOR VARIOUS FOAM THICKNESSES (PIR-125&PUF-125)

WITH VARIOUS MECHANICAL PROPERTIES:

41

Fig.no5.1. Foam thickness Vs Maximum bending stress

Fig. no5.2. Foam thickness vs. Normal stress (N/mm2)

42

Fig. no5.3. Foam thickness vs. shear strength in core

Fig. no5.4. Foam thickness vs. bending shape factor for stiffness

43

Fig no5.5:Foam thickness Vs flexural rigidity per unit width

Fig. no5.6. Foam thickness Vs Shear deflection

44

Fig. no5.7. Foam thickness Vs Shear strain at max load

45

TENSILE BEHAVIOUR OF FOAMS UNDER INSTRON:

LOAD Vs EXTENSION PLOTS:

fig.no5.8. ;load vs extension plot for PIR -125 sample-1

fig.no5.9. ;load vs extension plot for PIR -125 sample-2

46

fig.no5.10. ;load vs extension plot for PIR -125 sample-3

fig.no5.11. ;load vs extension plot for PUF -125 sample-1

47

fig.no5.12. ;load vs extension plot for PUF -125 sample-2

fig.no5.13. ;load vs extension plot for PUF -125 sample-3

48

Table 5.2 Tensile Test Data:

Specimen Load Modulus

(Automatic

Young's)

(Mpa)

Tensile

extension at

Yield (Zero

Slope)

(mm)

Comment

PIR -125

Sample-1

196.87414 108.838 6.26259 Failure Above

Centre Line

PIR -125

Sample-2

139.16492 78.089 3.90598 Failure Below

2nd Line From

Centre

PIR -125

Sample-3

191.81967 49.902 12.22291 Failure At

Centreline

PUF -125

Sample-1

212.27598 163.287 3.66258 Failure Below

Centreline

PUF-125

Sample-2

271.10577 141.547 7.00860 Failure 1 Inch

Above Centre

Line

PUF -125

Sample-3

202.08359 77.308 11.31756 Failed Above

Centreline

OBSERVATIONS:

It is observed that while doing the tensile test failure patterns occurs at different points

from centre line the load vs extension graph shows max load and yielding takes place at point of

extension

In PIR-125 SAMPLE -1 the max load taken by the specimen 196.87 N . The failure of

specimen Is observed above the centre line at extension of 6.262 mm

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In PIR-125 SAMPLE-2 the max load taken by the specimen 139.16N. The failure of

specimen is observed below 2ND line from centre line at extension of 3.905mm

In PIR -125 SAMPLE-3 the max load taken by the specimen 191.81 N. The failure of

specimen Is observed failure at centre .this is the perfect sample at extension of

12.22291mm

In PUF-125 SAMPLE1 the max load taken by the specimen 212.275N. The failure of

specimen Is observed below the centre at extension of 3.6625 mm.

InPUF -125 SAMPLE 2 the max load taken by the specimen 271.105N. The failure of

specimen is observed 1INCH above the centre line at extension of 7.00860mm

In PUF-125 SAMPLE 3 the max load taken by the specimen 203.3N.The failure of

specimen is observed above centre line at extension of 11.31756mm

By observing all the specimens PUF 125 more elastic compared to PIR125 because

PUF125 having high load bearing capacity.

Tensile test of PUF125 & PIR125 foams performed for finding poisons ratio,

young’s modulus ,tensile stresses based on these values were found r ratio and c/d (shift in

neutral axis)

Table 5.3 Comparison For C/D Ratio In Tensile Foam Specimen:

RATIO TYPE PIR-125 PUF-125

R ratio 1.272 2.694

C/D 0.007174 0.1051

PIR-125:

This concludes that the shift in neutral axis from the centroidal axis is 0.71 percent of

foam thickness &r ratio is 1.272

50

PUF-125:

This concludes that the shift in neutral axis from the centroidal axis is 10.6 percent of

foam thickness& r ratio is 2.694

Table 5.4:Shear Flow In PIR -125 And PUF -125(4:1skin To Weight Ratio)

Sample Thickness(mm) Shear Flow

PIR-125 PUF-125

10 2.86 2.9

25 9.75 4.9

50 19.5 8.8

SHEAR FLOW: If the shearing stress fv is multiplied by thickness t, we obtain a quantity q

known as the shear flow, which represents the longitudinal force per unit length transmitted

across a section at a level y1 from the neutral axis.

Q = fv*t

The shear flow values is calculated based on max shear stress in core multiplied by thickness

PLOTS FOR SHEAR FLOW WITH VARIOUS MECHANICAL PROPERTIES:

PIR-125:

51

Fig no5.14: Shear flow Vs Maximum Bending Stress (N/mm2)

Fig no: Thickness Vs Shear flow (N/mm)

Fig No5.14: Shear Flow Vs Normal stress

52

Fig no5.15: Shear Flow Vs Bending Shape Factor

Fig No5.16: Shear Flow Vs Shear Deflection

53

Fig No5.17: Shear Flow Vs Shear Strain

Fig No5.18: Shear Flow Vs flexural rigidity per unit width

PUF 125

54

Fig No5.19: Shear Flow Vs Max Bending Stress

Fig .no5.20: Shear Flow Vs Normal Stress

55

Fig .No5.21: Shear Flow Vs Flexural Rigidity per Unit Width

Fig .no5.22: Shear Flow Vs bending shape factor

56

Fig No5.23: Shear Flow Vs Shear Deflection

Fig No5.24: Shear Flow Vs Shear Strain

57

CO –RELATION OF SHEAR FLOW WITH SHIFT IN NEUTRAL AXIS:

PIR-125:

Fig no5.25:shift in neutral axisVs Shear Flow

58

PUF-125:

Fig no5.26: shift in neutral axis Vs Shear flow

CONCLUSIONS:

High flexural rigidity is one of the major factor in sandwich composites for its aerospace

applications. For this purpose, high modulus skins are combined with thick and light core

resulting in sandwich composites with maximum bending strength and stiffness and minimum

weight of core and facing material combinations.

In this project we are considering PUF 125&PIR125 having varying thickness

10mm,25mm,50mmwith skin to weight ratio 4:1 at particularly 16:1 span to depth ratio .

From the results we conclude that the flexural properties for PIR 125, such as max bending

stress, flexural rigidity ,bending shape factor , normal stress proportional to thickness by

increasing the thickness shear flow also increasing close to linearity . The flexural properties

show proportionality to shear flow.

In case of, flexural properties of PUF125, flexural rigidity is increasing with thickness but in

normal stress, increasing up to some extent and then decreasing .by increasing the thickness the

59

shear flow also increasing close to linearity and but slight variation in shear flow vs normal

stress

PIR foam density observed for Compression Tests. Poisson’s ratio for 125 kg/m3 is seen

to be high. Compressive stress is 1.16 MPa for PIR foam

PUF foam density observed for tensile tests poisson’s ratio for 125 kg/m3 is seen to be

high .tensile stress is 1.7 Mpa for PUF foam

In PIR -125 SAMPLE-3 the max load taken by the specimen 191.81 N. The failure of

specimen is observed at centre with extension of 12.22291mm

In PUF-125 SAMPLE 3 the max load taken by the specimen 203.3N.The failure of

specimen s observed above centre line at extension of 11.31756mm

PIR-125:

This concludes that the shift in neutral axis from the centroidal axis is 0.71 percent of

foam thickness & r ratio is 1.272

which can be understood in case of flexure in foam core sandwich beams.

PUF-125:

The shift in neutral axis from the centriodal axis is 10.6 percent of foam thickness& r

ratio is 2.694 in PUF 125 samples.

By observing all the specimens PUF 125 is seen to be more elastic compared to PIR 125

because PUF 125 having high load bearing capacity

From shear flow Vs shift in neutral axis plot we observed that by varying

thickness10mm,25mm,50mm. as shear flow increases the shift in neutral axis also

increases with different thickness in both PUF125 & PIR125 is observed. If the thickness

of sample is high shear flow is high and the shear properties of PUF are seen to show

better performance than PIR which is considered more stiff than PUF.

It is observed that while testing sandwich specimens failure occurs mainly through face

sheet shear and compressive core crushing in 125 kg/m3 density foam sandwich panels

s

60

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