Hoop stress, EJMA “S2”, runs circumferentially around the bellows resulting from pressure differential between the inside and outside diameter of the bellows.
Bulge stress, EJMA “S4”, runs longitudinal to the bellows centerline acting on the sidewall of the bellows convolutions.
Deflection Bending Stress
Deflection Bending Stress, EJMA ”S6”, is stress resulting from deformation of the sidewalls of the convolutions caused by the movement of the bellows as it compresses, extends, or angulates. The stress is highest in the crest and root of the convolutions.
Knowledge Base Handbook for Metal Bellows Expansion Joints