API 579: Plasticity Interaction Factor

[EduardoT]

Hello,

I am new at this forum and it is the first time that I write a post, so I apologize beforehand for the mistakes that I might make.

I want to calculate the Failure Assessment Diagram level 2 for an autofrettaged cylinder using the API 579-1/ASME FFS-1 2007 Fitness-For-Service.

In order to obtain the plasticity interaction factor (eq 9.22), the Residual Reference Stress should be calculated (eq D.74 for this configuration).

If I am not mistaken, in an autofrettaged cylinder configuration, the residual membrane stress distribution (P_m) is always zero, and the bending stress distribution (P_b) is a negative value, so the Residual Reference Stress is zero. According to the tables 9.4 and 9.6 (parameters φ and ψ), both are zero too, and the equation 9.22 (plasticity interaction factor Φ) is an indetermination zero/zero.

Could someone help me?



Thank you.

 

[EduardoT]

Hello again,
Regarding to the previous post, I would like to ask if the bending stress distribution (Pb) could be negative, or I should consider this stress as a positive value.
Regards.