Local Failure Calculating according to ASME in negative hydrostatic stress

[BicMech]

Hello
I am writing because I have a question while reviewing local failure as follows.

I am reviewing local failure of the steel structure.
Design code : ASME Sec.8 Div.2 clause 5.3.3
Design Method : Elastic-Plastic Analysis
FEA Program : ANSYS
Material : Carbon Steel


I calculate 'Limiting triaxial strain, ɛ[sub]L[/sub]', as follows.
(ASME Sec.8 Div.2 Formula 5.6)

where,
Hydrostatic stress, (σ[sub]1[/sub]+σ[sub]2[/sub]+σ[sub]3[/sub])/3 = -150 MPa (from FEA) (A negative value is calculated because it is a compressed part)
Equivalent stress, σ[sub]e[/sub] = 250 MPa (from FEA)
α[sub]sl[/sub] = 2.2 (from ASME Sec.8 Div.2 Table 5.7)
ɛ[sub]Lu[/sub] = m[sub]2[/sub] = 0.27 (from ASME Sec.8 Div.2 Table 5.7)

Limiting triaxial strain, ɛ[sub]L[/sub] = 1.36

Here are the questions.
1. In general, the elongation of carbon steel is about 0.2 (20%), but the calculated 'Limiting triaxial strain' seems to be too large compared to this. I wonder if it was calculated well.

2. If 'Limiting triaxial strain' is well obtained, I do not know how such a large value is possible. Please let me know what material to study.

 

[DriveMeNuts]

Your triaxial stress is very compressive, making the material more ductile which means it can experience greater equivalent plastic strain before rupture.

 

[BicMech]

Is the graph below relevant?

 

[NRP99]

BicMech

Check this link for more information on the local failure criteria.

The local failure is possibility when you have hydrostatic tension +tensile stress condition (σ1>σ2>σ3>0) at the point of evaluation (generally notches/sudden geometry changes). Yes. The graph shows exactly what happens when the tri-axiality ratio changes from positive to negative. The required strain for failure is higher when components are having the hydrostatic compression stress state. This is why, in metal forming/drawing operations calculated amount of compression is applied to "draw" or plastically deform the billet without fracturing.

By the way any reference to this graph? Which material is this?