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Griffith's theory of fracture - Coursework Example

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Griffith's theory of fracture

Ceramics and Metals have different atomic structure and ionic bonding. Ceramic materials have both ionic and covalent bonds. Ionic bond is the one that keeps them brittle. Ionic bond is lose and no definite direction.The positive and negative ions are arranged and are held together by attraction created by each other of different charge. When stress is applied, ions slide past each other then goes to different directions; ions of the same charge bumps and trying to repel each other; causing the atoms to spread. The material then breaks. In metals, the atomic structure consist more positively charged ions on a flow of negatively charge electrons. The electrons allows the positive charge ions to slide and roll past each other without breaking the bond when a certain stress is applied. This causes the metal to be tough. 2) Glass in general has a low tensile strength. This lies entirely with the fact that glass is a solid lacking crystalline structure or namely amorphous solid. Amorphous solid has a moving structure of molecules and are not compact, this results to having a low tensile strength than of other solids. However, there are methods of somehow improving glass’s tensile strength, strengthening them. One is by soaking the glass into a bath of potassium nitrate solution on an about 400 to 450 degrees Celsius having. Potassium nitrate has larger molecule structure than of a common sodium nitrate molecule in the surface of the glass. The process will now replace the sodium nitrate on a glass by the potassium nitrate in the bath solution, leaving the glass having a thick layer and a hard wedge. This is called “chemical strengthening”. Another one is called “heat strengthening” where a glass surface is heated for about 1100-1500 degrees Fahrenheit then suddenly cooled. After suddenly cooling, the glass molecules compresses, thus having a thick and compressed layer than before. 3) Griffith’s equation steps directly not to the strength of a material but the fracture in it and how hard can it withstand a force before it collapses. Fracture is the propagation of a crack or a flaw across loaded or force-applied segment. Toughness is the property of a material to withstand fracture. Now glass has literally low resistance to fracture than steel has. In Griffith’s equation, you would get stress over strain or Young’s modulus of a material first then that you will determine its toughness. A glass generally has only 50-90 GPa. Its ability to resist fracture is very low compared of steel which has a stress over strain ratio of around 190 to 210 GPa. This makes steel as a more preferable material than glass. 4) Elastic deformation by definition is reversible. Meaning, the material will return to its original state when stressed and bending stress is no longer applied. Uniform plastic deformation is an irreversible deformation where the material strains uniformly upon stress. The material will not return to its original state rather it cracks or ruptures when certain stress is strong enough for the material to withstand. Local plastic deformation or also known as “necking” differs from uniform plastic deformation by means of the strain applied. It is a mode of tensile deformation where relatively large amounts of strain localize disproportionately in a small region of the material. The localised portion of the material where there is a decrease in local area became the basis for “neck”. 5) In rubber elasticity the polymer chains of an elastomer coils at random at a relaxed state. On extension, the chains are stretched out, and their conformational entropy is reduced. This is the reason why a rubber material, when suddenly relaxed from extension, snaps back. This is the basis in an elasticity modulus of a material. The material is highly elastic when its stiffness and extension are balanced together. Stiffness is an extensive property of a material to resist deformation. The high the stiffness can be the high the material’s extensions. 6) Thermosetting resin has a composition ...Show more

Summary

Ceramics and Metals have different atomic structure and ionic bonding.Ceramic materials have both ionic and covalent bonds.Ionic bond is the one that keeps them brittle.Ionic bond is lose and no definite direction.The positive and negative ions are arranged and are held together by attraction created by each other of different charge…
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Griffiths theory of fracture essay example
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