Material Science: Short answer type questions from AMIE exams

Define anelasticity.

The time-dependent elastic behaviour is called anelasticity. In most engineering materials, there exists a time-dependent elastic component. That is elastic deformation will continue after the stress application and upon load release, some finite time is required for complete recovery. The anelasticity is due to microscopic and atomistic processes that are attendant to the deformation.


Define viscoelasticity.

It is a type of deformation exhibiting the mechanical characteristics of viscous flow and elastic deformation. The phenomenon of viscoelastic behaviour is found in a polymeric material.


What is the fatigue limit of a material?

For fatigue, the maximum stress amplitude level below which a material can endure an essentially infinite number of stress cycles and not fail is called the fatigue limit. in other words: 

The maximum stress to which a material can be subjected without fatigue, regardless of the number of cycles is called the fatigue limit.


What is corrosion fatigue?

Corrosion fatigue is fatigue in a corrosive environment. It is the mechanical degradation of a material under the joint action of corrosion and cyclic loading. Nearly all engineering structures experience some form of alternating stress and are exposed to harmful environments during their service life. The environment plays a significant role in the fatigue of high-strength structural materials like steel, aluminium alloys and titanium alloys. Materials with high specific strength are being developed to meet the requirements of advancing technology. However, their usefulness depends to a large extent on the extent to which they resist corrosion fatigue. The phenomenon should not be confused with stress corrosion cracking, where corrosion (such as pitting) leads to the development of brittle cracks, growth and failure. The only requirement for corrosion fatigue is that the sample is under tensile stress.


What is cermet? Give examples.

A cermet is a composite material composed of ceramic (cer) and metallic (met) materials. A cermet is ideally designed to have the optimal properties of both a ceramic, such as high-temperature resistance and hardness and those of a metal, such as the ability to undergo plastic deformation. The metal is used as a binder for an oxide, boride, or carbide. Generally, the metallic elements used are nickel, molybdenum, and cobalt. Depending on the physical structure of the material, cermets can also be metal matrix composites, but cermets are usually less than 20% metal by volume. Cermets are used in the manufacture of resistors (especially potentiometers), capacitors, and other electronic components which may experience high temperature.

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