Justify, briefly, the following statement. (1) Dislocation movement is difficult
ID: 1766572 • Letter: J
Question
Justify, briefly, the following statement.
(1) Dislocation movement is difficult in ceramics/ionic solids
(2) The engineering stress- engineering strain diagram is easier to determine than the true stress-true strain diagram
(3) The tensile strength of coarse pearlite is grater than that of fine pearlite
(4) The presence of crack in a material leads to stress cooncentration and causes the material to fail at a much lower value than the fracture strength
(5) The sharp corners of machined parts must be rounded out to prevent fatigue failure
(6) A martensitic structure is very hard and brittle
Explanation / Answer
1. Ceramics and ionic solids are made up of either covalent or ionic bonds which are much stronger than metallic bonds resulting in lesser number of slip systems and hence is the dislocation movement very difficult.
2. Engineering stress and strain are funtions of the original cross section area of work piece which is a constant value. However in case of true stress and strain, they are functions of actual area of cross section at that particular instant which varies and hence is difficult to determine.
3. The more finer are the grains in pearlite the less are the slip planes formed also the ductilty decreases causing a decrease in tensile strength
4. A material is strongest if stress is uniformly distributed. Prescence of cracks can lead to changes in area of cross section and we know that stress is inversely proportional to area of cross section and hence at low area of cross section caused by cracks, stress increases.
5. The roundness distributes the stress which would have been concentrated to a single point to a wider area and thus prevents fatigue failure. The idea is to increase the area of cross section to reduce the stress.
6. As martensite is producted by rapid cooling of austenite (peferably quenching), this rapid cooling causes the bcc structure changes to body centered tetragonal structure thus causing a large increase in volume and a higly stressed structure making it hard and brittle.
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