In

order to select a composite material, a combination of such properties is being

sought that is optimum, rather than one particular property. For example, the

fuselage and wings of an aircraft must be light weight as well as strong, stiff,

and tough. Therefore, finding such a material that contents these requirements

is a difficult task. Many fiber-reinforced polymers are present nowadays that

can be substituted as a result of this.

There

are three factors that can determine the properties of composite materials:

1. The

materials that can be used as component in the composite material

2. The

geometric shapes of the constituents and resulting structure of the composite

system

3. The

manner in which the phases interact with one another.

9.1. Rule of Mixtures

The

properties of a composite material are a component of the beginning materials.

Certain properties of a composite material can be registered by methods for a

control of blends, which includes computing a weighted normal of the

constituent material properties. Thickness is a case of this averaging

standard. The mass of a composite material is the entirety of the majority of

the framework and fortifying stages:

mc = mm + mr

where

m = mass, kg (lb); and the subscripts c, m, and r indicate composite, matrix,

and reinforcing phases, respectively.

In

the same way, the volume of composite materials is the sum of its constituents:

Vc = Vm + Vr

+ V?

Where

V = volume, cm3 (in3). V? is the volume of any

voids in the composite (e.g. Pores).

Now

density of composite can be achieved dividing by mass by volume.

?c =

=

=

=

+

= ?m fm + ?r fr

Where fr and fm are the volume fractions of the

reinforcement and matrix phases.

9.2. Fiber – Reinforced Composites

Determining

mechanical properties of composites from constituent properties is usually more

involved. The rule of mixtures can sometimes be used to estimate the modulus of

elasticity of a fiber-reinforced composite made of continuous fibers where Ec is measured in

the longitudinal

Figure

9.2(a). Model of fiber-reinforced composite material

direction. The situation is depicted in Figure 9.2(a);

we assume that the fiber material is much stiffer than the matrix and that the

bonding between the two phases is secure. Under this model, the modulus of the

composite can be predicted as follows:

Ec

= Em

fm + Er fr

Where

Ec, Em, Er are the elastic moduli of the composite

and its constituents, MPa (lb/ in2); and fm and fr

are again the volume fractions of the matrix and reinforcing phase. The effect

of this equation can be seen in Figure 9.2(b). Right angle to the longitudinal direction,

fibers contribute very less to the overall stiffness excluding their filling

effect.

The composite modulus can be estimated in this direction

using the following:

E’c

=

Figure

9.1(c).

Variation of Elastic modulus and tensile strength as a function of fiber

angle

Figure 9.2 (b). Stress-Strain

relationship for composite material and its constituents

Where E’c = Elastic modulus perpendicular to the

fiber direction. Both above equations for Ec

demonstrate important anisotropy of fiber-reinforced composites. This directional

effect can be seen in Figure 9.2(c) for a fiber-reinforced polymer composite, in

which both elastic modulus and tensile strength are measured relative to fiber

direction. Most of the composites have tensile strengths few times greater in a

fibrous form than in bulk form. Yet, the applications of fibers are limited by

surface flaws, when subjected to compression. By imbedding the fibers in a

polymer matrix, such a composites can be obtained that avoids the problems of

fibers but utilizes their strengths. As a whole, matrix provides bulk shape to

protect the fiber surfaces and at the same time resist buckling; while the fibers

provides high strength to the composite. When load is applied, the low-strength

matrix deforms and distributes the stress to the high-strength fibers, which can

then carry the load. If individual fibers break, the load is redistributed through

the matrix to other fibers which is also referred as the phenomenon of

filleting.

9.3. Mechanical properties comparison

As

per a research conducted in India 2 in 2016, a sandwich structure having skin

material of Aluminum and core material of polyethylene (hexagonal honeycomb and

rhombus honeycomb) as shown in Figure. 9.3, the following results were

achieved:

Sr.#

Mechanical

Properties

Hexagonal

core

Rhombus

core

1

Ultimate load

24.96 kN

26.64 kN

2

Total

deformation

0.45 mm

0.2 mm

3

Maximum

deflection

13.5 mm at 200

N

10.8 mm at 300

N

4

Elastic limit

2000 N

10,000 N

Figure 9.2. Core structures

From the above results it is

therefore concluded that the composite sandwich panel of Aluminum having

rhombus honeycomb core structure has more tensile strength and stiffness than

the one having hexagonal honeycomb core structure.

Hence sandwich panel composite material (with rhombus structure) is

acceptable in Automobile, Aerospace, and High speed trains.