Low-cycle fatigue in a convolution bellow

A special machine has been designed and constructed in order to carry out the indicated tests. This apparatus consists essentially of a central flange on whose sides two bellows are arranged; at the other extremity the bellows end with two blank flanges. The central flange is set in motion by an electric motor and kinematic motion which allows to carry out the designed excursion; the end flanges are connected to one another by a screw mechanism. A hydraulic system allows the provision of the desired pressure inside the bellows. The two bellows are connected by means of flexible pipes in order to maintain the pressure constant inside them during the displacement of the central flange. The bellows have been constructed from a 0.3mm sheet bended and welded with an automatic continuous apparatus (TIG) helium protected. The bellows have been subjected at 10000 fatigue tests cycles for three flange displacements: 25mm, 52mm, 75mm. These three displacements have been realized in tension and in compression. After these fatigue tests the maximum pressure the bellows can support has been measured. The rupture of the bellows depends also on the constraint conditions; in the prototype there is a gap of 3mm between the internal bellow surface and the external surface of the internal tube. This gap has been simulated in the tests. In conclusion, the results show that the burst pressure is strongly affected by the displacement amplitude. The energy absorbed by the bellows at the rupture is very low; before the rupture (due to internal pressure) there is a shape deformation and a convolution removal. The new shape assumed depends on the bellow instability and therefore on the internal pipe constraints; a different gap between the internal pipe and the bellow changes the results