I am getting confused with a definition of nonhardened Material as I read the book Fracture Mechanics written by T.L. Anderson.
I got two understandings and am not sure which one is right.
1)    Material with lower yield strenth, which was not work-hardened.
2)    d(stress)/d(strain) = 0 after yielding.

Triangular elements with 2 coincident but untied nodes produce a 1/r variation of stress/strain field.
But the stress/strain field has a 1/√r singularity.
Can you tell me why or recommend me some papers taking about it.

I followed this blog with interest for several weeks before I decide to ask my first question. As a newcomer in fracture mechanics, I got confused with a FE-Analysis several days ago, which calcultes the J-Integral of a 3D-crack. Ich transferred the J-Integral calculated using FEM (ADINA) to K (stress intensity factor) and found that the calculated FE-Value is 50% greater than the value acording to the IWM-Methode, which is developed to calculate the K-factors für crack in cylinders.