[BillA]

no, it is not that simple
what is required is a strain analysis in the 3 directions
and the 3 principal sources will be the applied load, thermal (slight I agree), and any due to swelling due to absorption of whatever

- this last was Mota's quite valid point

my experience in this area is largely with epoxy/glass fiber composites which DO always fail by brittle fracture

understanding that the strains are cumulative, and just how little is required to enable propagation, thermal should never be ignored with any plastics as the coefficient of thermal expansion is much higher than metals while the moduli are lower
- little things can result in big strains (= energy at the crack tip)

still thinking "NOT" ?
a retraction would be nice

never apologize for 'talking technical', unknown terms will serve as a stimulus to those with an interest
(others use the Back button)

EDIT: just saw your edit to your post
are we talking about the same thing ? I think not as I am not even slightly concerned with the change in length of the top
I am looking at the combined strain at the thread root diameter with a hoop tensile stress of "X" imposed by the barb's pin end
to which is then added . . . . (as above)

out of curiosity, how are you quantifying the 'needed' crack tip energy for polycarbonate ?
I don't have a clue as to that value, but I know from experience that it is notorious for 'stress cracking' (what we are dealing with here)