My issue is regarding these two problems. Now I\'m finding it difficult to under

Question

My issue is regarding these two problems. Now I'm finding it difficult to understand for instance in the first question, it say "the Maximum Allowable stress in Tension" ... How do I know if that stress is Circumferential Stress or Axial Stress or Radial Stress?

A cylindrical water storage tank fabricated from steel and butt-welded together is to be 10m in diameter and 5m high. What must be the minimum thickness for the bottom side plates ? The allowable stress in tension for the selected steel is 130 MPa. Assume that the weld efficiency is 80% and neglect local stresses at the junction between the vertical wall and the bottom. (2.36 mm) 2. A mild steel shaft of 50 mm diameter carries a bending moment of 2000 Nm. If the yield poi nt stress in simple tension is 200 MN/m, find the maximum torque that can be applied n addition to the bending moment without causing yielding, using each of the three theories of elastic failure: maximum principal stress, Tresca's criterion, von Mises criterion. (2092 Nm, 1410 Nm, 1623 Nm)

Explanation / Answer

In case of a pressure vessel (the cylindrical tank), one must understand that, when there is pressure build up within the vessel it "bugles out". Meaning, the fabric of the vessel is under tension in all directions. As such, Circumferential Stress or Axial Stress or Radial Stress are merely the direction of this "Tensile" stress along the three axis.

Hence, when solving this problem, we can calculate the axial stress in the tank and compare it to the given "allowable stress in tension"; which will be the same in all directions.

Now, in case of a shaft under bending, the upper fibers of the shaft will stretch and hence will be under tension. Whereas, the bottom fibers will be under compression. Both these forces are equal in magnitude but opposite in direction. Thus, the calculated bending stress of the shaft can be simply compared to the given yield stress (tensile or compressive).

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