Dear all

Allowable stress = 1.25 (Sc+Sh) as per B31.3

Please let me know on what basis factor 1.25 appear in above equation.

I am looking explanation on this


Regards

More789

regards

more789

Your equation is not complete but it does focus on one of the components of the fatigue check - low cycle fatigue. Yielding is permitted for displacement-based loads but you cannot have a strain range that causes yielding on both ends of the cycle as that would result in fatigue failure after "a few" cycles.

Example: Let yield be 30ksi in cold condition and 20ksi in hot condition. That summed together is 50ksi. If a point starts cold at 0ksi and has a stress range due to thermal strain of 40ksi, that point will yield in the hot position so that, eventually, the hot stress will settle down to 20ksi. So when the hot stress is 20ksi, then when the system cycles to cold and goes through the 40ksi stress range, the cold stress will be -20ksi. This system will now operate between these two states with no additional yielding - the system finished the (acceptable) shakedown. But what if the stress range due to strain is greater than 50ksi? Say 60ksi? Now, no matter how much yielding occurs, the system will yield at every half cycle and fatigue failure will occur. (We ignore the fact that we are working with centerline moments and the actual yield point will shift around the circumference between the hot yield and cold yield.)

So hot yield plus cold yield is the low cycle fatigue limit. The basic allowable stress can be set on material yield; B31.3 offers 2/3 yield. So 1.5Sc could be yield cold and 1.5Sh could be yield hot. Hot yield plus cold yield could be 1.5(Sc+Sh). The Code allows 1.25(Sc+Sh) to build in some factor for safety. Note that f can be up to 1.2 so f[1.25(Sc+Sh)-SL] (this is the complete equation) can get you back up to 1.5.

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