Modeling with line spring elements to find failure pressure of steam generator tube

Hello, All
I am trying to model steam generator tube of length 110 , radius 11mm, thickness 1.27mm. It has 2 axial cracks of length 6.35mm. length between 2 cracks is 0.254mm. 

As line spring elements cannot be created using Graphical user interface I first need input file. So I modelled the tube with shell elements S8R without any crack (once this model will run I will get input file and can modify it). As per paper which I am following the fracture coalescence pressure is 60Mpa means at this pressure J integral exceed the critical JIc. 

**Material Property**
Material is Alloy 600.
Material property (Entered in ABAQUS): Elastic Modulus : 200 Gpa, Poission ratio : 0.3 
Plastic property : Stress(MPa) Strain(mm/mm)
300 0
512 0.09274
687 0.178865
840 0.2582
946 0.33177
1125 0.5
1700 1.05
When I tried to run model WITHOUT CRACK (just simple cylindrical tube, length 55mm with end cap, quarter symmetry). At 30 MPa I didn’t get any warning. But at 60 Mpa I am getting warning that strain increment has exceeded 50 times the strain to cause first yield at xxxx points. Moreover I looked in job diagnostic so it shows that one node have numerical singularity.

What should I do to troubleshoot this warnings?