Engineering Materials

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Assignment No.: 4
Unit Name: Properties and Applications of Engineering Material
Unit No.: 19

Task 1: Describe the principles of the modes of failure known as ductile/brittle fracture, fatigue and creep.
a) Ductile/ brittle fractures: brittle material breaks easily when heavy force is applied on it. Ceramics and cements are the best examples of brittle fracture. Ductile fracture is better than brittle fracture, because ductile fracture occurs over a period of time, while brittle fracture is fast, and can take place at lower stress levels than a ductile fracture. Ductile fracture involves plastic deformation in the area of an advancing crack, and is a slow process. b) Fatigue: It is defined as
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Degradation process: A change in the chemical structure, physical properties or presence of a material from natural or artificial contact is known as degradation.
a) Degradation of Metals: Degradation of Metals defines the effects of atmospheric exposure, high-temperature gases, soil water, weak and strong chemicals, liquid metals, and nuclear radiation. Details the high and low temperature effects of oxidizing agents, such as oxygen, sulphur and water vapour, the halogens, and carbon dioxide. It shows the effect of hydrogen on metal, including the loss of ductility and internal cracking, roasting, fissuring and cracking. It shows how improvements in component design can reduce corrosion.

Metals are degraded by Oxidation, Erosion and stress corrosion. 1) Oxidation: Oxidation is defined as the loss of electrons or an increase in oxidation state by a particle, atom, or ion. Oxidation reactions are normally related with the formation of oxides from oxygen particles. In oxidation the oppositely charged ions are then involved to each other. For example: Ferrous [Iron (Fe)] combines with oxygen (O2) combines with and forms ferrous
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For example if copper(cu) comes in contact with oxygen (O2) it forms copperoxide (Cu O2). Oxidation reactions are commonly associated with the formation of oxides from oxygen molecules. If a material is forming an oxide at its surface then the underlying material is being expended and the engineering part becomes weaker Most oxide films are brittle and have poorer mechanical properties than the underlying materials

b) Thermal shock: In thermal shock the material first begins to crack and then finally degrades. Thermal shock occurs when a thermal incline causes different parts of an object to enlarge by different amounts. Thermal shock defines the way in which some materials are proved to damage if they are in contact to an unexpected change in temperature. If nothing stops this crack from propagating through the material, it will cause the object's structure to fail. Borosilicate glass is made to withstand thermal shock better than most other glass through a combination of reduced expansion coefficient and greater strength, though fused quartz outperforms it in both these respects. Some glass-ceramic materials include a controlled proportion of material with a negative expansion coefficient, so that the overall coefficient can be reduced to almost exactly zero over a reasonably wide range of temperatures. Reinforced carbon-carbon is

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