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Question Related to Mechanical Design Fundamentals:- Part–2

mechanical fundamentals

1. How much design temperature shall be considered for the structural parts which are heated by steam, thermic fluid etc?
Design temperature shall be the highest expected temperature of the heating media or highest expected body part temperature plus 10C. Here, 10 C is the safety margin.
2. Define proof stress?
It is also called 0.2% proof stress. It is the stress for 0.2% strain. In simpler terms, it is the stress value for strain value of 0.002 on the stress strain curve.
3. What are the types of failures encountered in Piping?
1. Catastrophic failure. 2. Fatigue failure.
4. Define catastrophic failure?
The failures which occurs suddenly as soon as the load crosses the threshold (ultimate tensile strength). These failures take place on the first occurrence of the loads in excess of yield stress.
5. What is fatigue failure?
The failure which occurs due to damaging of the grain structure of the specimen subjected to prolonged application of sustained load and or tensile–compressive load cycle.

6. What are the types of fatigue failure?
Types of fatigue failure are:-
A. Static fatigue.
B. Cyclic fatigue.
7. What is static fatigue?
The specimen which fails under sustained load subjected to considerable length of time. The total time for which the load was applied is important. Whether it is applied continuously or in installments is not important.
8. What is cyclic fatigue?
The specimen which fails under a load cycle subjected to considerable numbers of times. The total numbers of cycles for which the load was applied is important. Whether the cycle was frequent or infrequent is not important.
9. Explain the failure of the specimen subjected to cyclic load?
Consider a specimen subjected to a tensile load increasing from zero to X and then this load is gradually withdrawn till it is zero. Now compressive load applied gradually till it is X and then it is drawn till the load is zero. This comprises a cycle and is repeated again and again. With each cycle, the grains in the material get displaced relative to each other and get more and more interlocked. With each cycle, the material loses its ductility in small increments. A time comes when the grains are so badly interlocked that they cannot allow deformation to withstand load and a small crack develops. This crack grows with further cycles and failure occurs.
10. What is creep failure?
Occurrence of static fatigue failure when the material is under prolonged sustained load coupled with high temperature is called creep failure.
11. Write the sequence of stress values at failures for solid rod, sphere and cylinders?
A typical sequence of stress values at failure for solid rods and two most important shapes in process industry namely, sphere and cylinder is Rod > Sphere > Cylinder.

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