Dear all

I have some qeuries on pipe stress.
I am beginer for pipe stress
a few days ago, my colleague asked to me what is phisycal mean of stress range.
For primary stress, if primary stress is over than allowable stress of material (2/3 yield stress or 1/3 tensile stress...), physically, this this pipe can yield(in other words, can be fractured)
If so, for secondary stress, this value is higher than allowable stress range...physically, what can be happened?

additionally,
why do we have to check "exp" case as full displacement?

Thanks in advanced