In each case, the effective length of the bellows increases, lowering the material available to withstand the pressure, thereby increasing the hoop stresses. As the length increases, the tendency to squirm increases and the stresses become higher and higher until catastrophic failure occurs.
A simple way to visualize this phenomena is to remember that the bellows is a cylinder of given volume. Internal pressure tries to increase a vessel's volume. Since a bellows is flexible in the axial direction, it can increase its volume by increasing the length of its centerline. With the ends fixed, it does so by simulating the appearance of a buckling column.
We perform standard pressure tests on all our bellows to ensure the unit will not squirm in service. Tests are not mandatory by EJMA, but it is highly recommended to perform a pressure or hydrostatic test prior to service.