SIFs & SSIs background in ASME B31 Codes are strictly correlated with METALLIC PIPING limit and failure states.
Such limit states (yielding, fatigue failure etc.) ARE NOT APPLICABLE TO NON-METALLIC PIPING.

For Non-metallic piping, the SIFs are actually pure Stress Concentration Factors, mostly dependent on fitting/component geometry.

For your HDPE piping, consult the dedicated literature - such as Plastics Pipe Institute.


Regards,