206-report7 - MIDDLE EAST TECHNICAL UNIVERSITY METE 206...

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MIDDLE EAST TECHNICAL UNIVERSITY METE 206 MATERIALS LABORATORY Report 7 Impact Test Mustafa COŞKUN 1394071
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INTRODUCTION Many of the solid materials are able break in either ductile or brittle manner. Classification is based on the material’s ability to undergo plastic deformation. In ductile fracture, materials show extensive plastic deformation, absorbing high energy, before void are formed inside and combined resulting in the final failure. On the other hand, materials exhibit very low or no plastic deformation with very low energy absorption in brittle fracture. As the resistance to crack propagation is very low, energy to break the material is also very low. a) highly ductile fracture in which the specimen necks down to a point b) moderately ductile fracture after some necking c) brittle fracture without any plastic deformation Stress-strain diagram, which is obtained from tensile test, gives us the toughness of the material as the area under the curve in the diagram. From tensile test;
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If we want to compare some materials; Toughness, which is the area under stress-strain curve, can be affected by several factors. Notch effect is one of these. When a material of a certain depth of notch is machined around a round bar, the stress-strain diagram may be affected a little or drastically. If the affect is little, material is notch-tough. However, if the material is very sensitive to notch and it is affected drastically, then it is notch-brittle. Railroad rails are shown as example since the high-carbon steel in the rail displays an exceptional performance under heavy loads unless there is a flaw or crack present, which causes brittle failure under the same load. Therefore, railroads should be inspected regularly) On the other hand, during the World War II, brittle failure of ships focused great attention to brittle failure of mild steel. Three basic factors contribute to a brittle-cleavage type of fracture. They are * a triaxial state of stress, * a low temperature, and * a high strain rate or rapid rate of loading.
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