Shell elements have 6 DOFs(ux,uy,uz,rotx,roty,rotz) whereas solid elements have 3 DOFs.(ux,uy,uz)
Therefore using shell elements over the solid elements will give you more accurate results.
PW, YOU ARE RIGTH ABOUT FATIGUE PROBLEM. IF YOU DO NOT HAVE AN CYCLIC LOADING FATIGUE IS NOT AN ISSUE, CREEP MIGTH BE AN ISSUE THOUGH IF THE TEMPERATURE IS TOO HIGH.
GREGLOCOCK IS RIGTH ABOUT HYDROSTATIC LOADING CAUSE NO SHEAR
BUT IT DOES NOT MEAN NO MECHANISM FOR CRACK GROWTH. AS LONG AS YOU...
In fatigue analysis usually shear stress criteria(Not Von mises stress criteria ) is used. For shear stress criteria
maximum principal and minimum principal stresses are used. Therefore you need to look at the both of the principal stress , not to one of them(as long as you are not using maximum...