 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
can be substituted as a result of this.

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

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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

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. 