Q1. How are materials classified?

Depending upon their respective molecular bond configurations, engineering materials can be divided into four classes:

1.Metals (e.g., Aluminum and Steel)

2.Ceramics (e.g., Glass)

3.Polymers (e.g., Plastics and Wood)


Composite materials are created by combining two or more materials at the macroscopic scale. Unlike metal alloys, it is possible to distinguish the individual constituents in a composite material. Composite materials are not new. An early form of composite materials could be traced back to ancient Persia in the form of straw reinforced bricks. At a much later time, cement, gravel, and steel bars were combined to form what is known as reinforced concrete. Development of thin fibers and thermoset polymers led to the creation of fiber-reinforced materials such as glass-epoxy. A much more recent examples are carbon-polymer and metal matrix composite materials with applications in aeronautics, automotive, space, sports, and biomedical fields.

In study of composites we separate materials into five different types based on their mechanical properties. This separation also identifies the number of independent elastic constants needed to form the constitutive relations between stress and strain. The five types of materials are:

  1. Anisotropic - A material whose mechanical properties depend on orientation at a point in the body. For example, such a material has no unique Young's modulus and tensile strength. In this case there is no plane of symmetry for material properties. These materials are also known as Triclinic materials.

  2. Monoclinic - Similar to anisotropic material with slightly less non-uniformity, it has one plane of symmetry for material properties.

  3. Orthotropic - Similar to anisotropic material but with three planes of symmetry for material properties.

  4. Transversely Isotropic - At every point of this material there is one plane in which the mechanical properties are the same in all directions.

  5. Isotropic - At every point of this material the mechanical properties are the same in all directions. Such materials have infinite planes of symmetry for material properties.

Other Frequently Asked Questions

A homogeneous material is one whose physical properties, such as density, do not change from one point to another. A non-homogeneous material has non-uniform physical properties. For example, its density at one location could be different from another. Non-homogeneous materials are also known as heterogeneous materials.

Alloying of a basic metal with one or several other metals is a method by which a desired characteristic of the basic metal can be enhanced. For example, to improve the strength of aluminum, it is alloyed with zinc.

Yes. It is an unfortunate fact that as an alloy is created some characteristics of the new material become degraded as others are enhanced. For example, 7075 aluminum alloy which uses zinc as the main alloying material is much stronger than pure aluminum but has a rather poor fatigue characteristic.

As a metal is cooled, its strength and stiffness increase. However, the main drawback is that its ductility is also reduced making it very brittle and susceptible to fracture.

Yes! Specimen temperature can significantly alter the shape of the stress-strain curve for a given material. In addition, the time of exposure has a significant effect on the yield and ultimate strength values of most metals like aluminum alloys.

Proportional limit is referred to a point on the stress-strain diagram where the curve becomes non-linear. Beyond this point the linear stress-strain variation described by Hooke's law ceases to be valid. The yield point, on the other hand, is a point at which a permanent deformation or slip occurs and upon unloading, the specimen will not go back to its original dimensions. The proportional limit and yield points are usually very close to each other. In the absence of accurate data, the two points are assumed to coincide.

No! Elongation (e) is a measure of material ductility whereas modulus of elasticity (E) is a measure of material stiffness. The two parameters are independent of each other. It is possible for a material to have a very small e but a very large E like 17-4 PH stainless steel (e=6% for gage length = 2 in. and E=27.5x106 psi) or to have a large eand E like INCONEL-X steel (e=20% for gage length = 2 in. and E=31x106 psi).

Heat treatment is a way of increasing the strength properties of a metal alloy. Young's modulus, however, is not significantly affected by this process.

Formability is a function of material type, material form, and temperature. It is very difficult to bend a sheet of high strength aluminum alloy into a multi-corner section without breaking it. In such a case, the sheet may be heated prior to forming.

Secant modulus and tangent modulus are used to model a mathematical relationship between stress and strain in the inelastic range for a given material. Each can have different values depending on the location considered along the stress-strain curve.

In most cases, the young's modulus of a material in tension is no different from that in compression. However, the strength of the material can be significantly different in compression and in tension. This is in fact the reason for creating steel reinforced concrete beams. While concrete has a very high strength in compression, it is rather weak in tension. Therefore, steel bars are introduced in order to improve the tensile strength of the concrete beam.