PL	critical internal pressure	σr	radial stress
E 0	energy released by wrinkles	σθ	circumferential stress
E 1	strain energy in unwrinkled tube	εr	radial strain
E 2	strain energy in wrinkled tube	εθ	circumferential strain
A	area where wrinkles appear	f	friction coefficient
$\phi$	the height of wrinkles	t	thickness of the tube
S	the length of wrinkles	L	length of the tube
y	the shape of the wrinkled part	Y	correction coefficient when upsetting
a	the frequency of the mode	Pcr 1	critical wrinkling pressure
r	internal diameter	Pcr 2	critical rupturing pressure
K	strength coefficient	M	correction coefficient when bulging
n	strain hardening exponent	Ps	critical yield pressure
R	anisotropy coefficient	Pcr	selectable internal pressure
ε 0	pre-strain