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# Tangent Modulus Elastic-Plastic Method ASME VIII Div.2

## Tangent Modulus Elastic-Plastic Method ASME VIII Div.2

(OP)
Dear All

I am running an elastic-plastic analysis for a pressure vessel according to ASME VIII Div.2 - item 5.2.4 and I pretend to use a bilinear material model.
But I am in troubling about the value of the Tangent Modulus that I should use. I plotted the stressxstrain curve of the SA-516 70 according to Annex 3-D and took the values of the tangent modulus (Et). (See attached in my calculations)
To illustrate my doubt I have separated 3 values of Et.

1 - A value near of yield point (Et =15193 MPa)
https://files.engineering.com/getfile.aspx?folder=...

2 - A value that is near of the curve (Et = 2020 MPa)
https://files.engineering.com/getfile.aspx?folder=...

3 - The value taken on ultimate stress (Et = 1315 MPa)
https://files.engineering.com/getfile.aspx?folder=...

Which value is more suitable?

Regards

### RE: Tangent Modulus Elastic-Plastic Method ASME VIII Div.2

Hi,

I think you should use the value from Section II, Part D. (as stated on ANNEX 3-E.1)

Victor
from Brazil

### RE: Tangent Modulus Elastic-Plastic Method ASME VIII Div.2

Don't use a bi-linear stress-strain curve. Use the full curve as provided in Annex 3D. Why would you degrade your analysis when you actually have the full curve at your disposal?

### RE: Tangent Modulus Elastic-Plastic Method ASME VIII Div.2

(OP)
Thank you Victor and TGS4

@TGS4
I was thinking of bi-linear to minimize the computing time.
But I will continue with the full curve

Best regards

### RE: Tangent Modulus Elastic-Plastic Method ASME VIII Div.2

It won't minimize the computational time. And the answer may be wrong. Good call to return to the full curve.

### RE: Tangent Modulus Elastic-Plastic Method ASME VIII Div.2

When going from linear to limit load (Bilinear) analysis, the maths needs to jump from the elastic modulus to the Tangent modulus.
This requires a time consuming multi step analysis.
Doesn't adding additional modulus curves (to achieve and elastic plastic analysis) increase the likelihood of needing more analysis steps and therefore overall taking more time to complete the entire multi step analysis?

### RE: Tangent Modulus Elastic-Plastic Method ASME VIII Div.2

Generally no. Some of the biggest issues arise when the stress-strain curve is dis-continuous. A smooth continuous curve will assist with the number of convergence steps. The other bigger issue with Elastic-Plastic vs Limit Load with respect to time is that at the fully-factored load, a large portion of the structure is near the true ultimate, where the effective tangent modulus is low.

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