What is composite material?

The term composite materials represent a type of material that cannot be obtained naturally. It can form according to our requirements without dissolving and by the addition of two or more materials with different properties. The mixing of the different material constituents would not be at the atomic level. Nowadays, composite materials have a vast scope and application due to their various distinct properties. The example of composite material is- fiberglass, concrete, etc.

What is mechanical behavior of composite material?

The mechanical behavior means how the composite material behaves when it is under mechanical loading or action. As the composite materials form according to our required mechanical loading and thermal conditions, they have good strength and durability. The composite materials have good thermal resistance properties so that they can use in higher thermal conditions.

Types of composite materials

The composite materials are classified in different categories on the basis of base material and reinforced material, mechanical properties, and application given in the following steps.

Metal matrix composites

It is a type of composite in which base material is of metal type. Due to metal as base material, this type of composite is referred to as metal matrix composite. It has high tensile strength, more toughness, also more stiffness. It can provide better resistance against the corrosive environment and withstand in a high-temperature environment. Generally, the base metal material is steel, aluminum, titanium, copper, etc.

Metal matrix composites can also differentiate into various types: particle reinforced type, short fibers/whiskers reinforced, and continuous or sheet reinforced metal matrix composites.

Ceramic matrix composites

It is also a ceramic material in which base material forms from ceramic material, with very strong ionic bonding. Due to this, the ceramic matrix composites have higher melting points, excellent corrosion resistance, and very high compressive strength. Ceramic matrix composites are useful at a higher temperature than metal matrix composites.

Polymer reinforced ceramics

In this type of composite material, the base material forms from a polymer material. The polymer material fully covers the reinforced material. The polymer reinforced composites have lower mechanical properties compared to metal and ceramic matrix composites.

Function of matrix

Matrix holds all the reinforced fibers together and protects them from the corrosive environment, and it also helps distribute the total load to every reinforced fiber equally. The base matrix material improves impact and fractures resistance.

Function of reinforced materials

Reinforced material posses tensile and compressive strength to the composite material. Reinforced materials are generally stronger than the strength of base matrix material. The load-carrying capacity is higher for reinforced material, whereas load-carrying capacity of base matrix material is low.

Different mechanical properties of composite material

The mechanical behavior of composite material can be assessed with the help of mechanical properties of composites under different loading conditions. Composite materials have different types of mechanical properties, which is represented in the following steps:

Tensile strength

In composite materials, the tensile strength depends on the volume of the fiber-reinforced material and the types of reinforced material. The composite material can withstand high tensile loading if the fiber-reinforced material is of high tensile strength. For fiber-reinforced composites, the unidirectional tensile strength has good value. The tensile strength of unidirectional continuous fiber-reinforced composites can obtain by using the given formula.

${\sigma }_c={\sigma }_m{V}_m+{\sigma }_f{V}_f$

Here, ${\sigma }_c$ represents the tensile strength of composite, ${\sigma }_m$ represents the tensile strength of base matrix material, ${\sigma }_f$  represents the tensile strength of fiber reinforced,$V_m$ represents the volume fraction of matrix material and $V_f$ represents the volume fraction of fiber reinforced. 

The value of tensile strength depends on the structural fabrication like laminate type/particle reinforced type, and the deformation characteristics of composite material depend on the loading direction, etc.

Elastic modulus

The structure of the composite materials in the form of laminate can be obtained with the help of fiber-reinforced and metal matrix material. The value of the Young modulus of complete composite material can get with the help of the given expression.

$\frac{1}{E_c}=\frac{V_m}{E_m}+\frac{V_f}{E_f}$

Here, $E_c$ represents the elastic modulus of composite material, $E_m$ represents the elastic modulus of matrix material and $E_f$ represents the elastic modulus of fiber reinforced.

Compressive strength

The compressive strength of unidirectional fiber-reinforced composites where fibers respond as the elastic column under compression and failure of the composite material would take place by buckling phenomenon of the fiber-reinforced can obtain by using the given formula.

${\sigma }_c=\frac{{\pi }^{2}E}{16}{\left(\frac{d}{L}\right)}^2$

Here, ${\sigma }_c$ represents the compressive strength of composite, $d$ represents the diameter of fiber and $L$ represents the length of fiber.

The buckling of fiber-reinforced in a composite under compressive loading can see in the image given below.

Stiffness

The fibers in a composite material are made up of an elastic material with good stiffness properties and high stiffness value. The stiffness of matrix material is less than the stiffness of fibers. So, the composite materials have good stiffness.

Temperature resistance

The resistance of a composite material against high temperature is also good. The composite material can be used to make equipment that is required to operate in higher temperature zone. So, the durability of composite material in a high-temperature environment is good.

Corrosion resistance

The composite materials have high resistance against the corrosive environment where they can be readily used. Pure metallic or alloy can not be durable in a highly corrosive environment. The matrix material (base material) prevents the composite from the corrosive environment.

Factors affecting properties of composite materials

The mechanical behavior of composite material can be affected by various factors given in the following steps:

Crack propagation

Whenever cyclic loading acts on composite material, then there would be the possibility of fatigue failure. Due to this, the initiation and propagation of crack occurs in the direction perpendicular to the loading axis. Further, the crack propagates, and failure of the composite takes place.

Stress concentration

Whenever defects are present in a composite material, stress concentration starts taking place at the point of the defect. Due to this, the mechanical properties of composite materials get hampered. Furthermore, the failure of the composite material takes place.

High temperature environment

When a composite material is placed in a high-temperature zone, elongation in the composite fibers takes place, and internal stress develops. Due to this, the mechanical properties of composite material get reduced.

Deformation in composite material

The fiber-reinforced have the highest strength in comparison to the base matrix material. The matrix-based material has compressive strength and protects the composite material. The different types of deformation in a composite material under tensile loading can be observed in the figure below.

Common Mistakes

• Students sometimes get confused between composite material and ceramic material. However, composite materials are made up of two or more different materials, whereas ceramic is inorganic nonmetallic in nature. Ceramic materials get hardened with the application of temperature.
• Some times student also confuse the enhanced properties of composite materials with that of pure metallic material. However, composite materials are designed according to requirements, so they possess good durability.
• Students ge confused while selection of better composite materials. However, before selecting types of composite material, we should go through the physical, chemical, and thermal requirements of the place.

Contexts and Applications

The mechanical behavior of composite materials is very significant in the several professional exams and courses for undergraduate, Diploma level, graduate, postgraduate. For example:

• Bachelor of Technology in Mechanical Engineering
• Bachelor of Technology in Civil Engineering
• Master of Technology in Mechanical Engineering
• Doctor of Philosophy in Mechanical Engineering
• Diploma in Mechanical and Civil Engineering

Related concepts

• Mechanical behavior of composites
• Tensile test
• Impact Load
• Fatigue test
• Fabrication process
• Strain of composites
• Different loading conditions
• Weight to strength ratio

Practice problems

Q1. What is the softer constituent in a composite material?

a. Reinforment

b. Matrix

c. Both

d. None of these

Correct option: (b)

Q 2. What is the stronger constituent in a composite material?

a. Reinforment

b. Matrix

c. Both

d. None of these

Correct option: (a)

Q 3. Mechanical properties of fiber-reinforced composites vary with

a. Interface strength

b. Properties of constituents

c. Fiber length, volume and orientation

d. All of these

Correct option: (d)

Q 4. The main factor on which the longitudinal strength of fiber-reinforced composite depends on

a. Fiber length

b. Fiber orientation

c. Fiber strength

d. Volume fraction of fiber

Correct option: (c)

Q 5.In the sandwich structure of a composite material, which of the following materials can fill?

a. Wood

b. Cement

c. Polymer

d. All of them

Correct option: (d)