Functionality and durability prediction of the thin shell edge welded metal bellows

Metal bellows are indispensable components needed in aerospace fluid systems and mechanical seal systems. Edge welded metal bellows (EWMB) are an elegant way to manufacture bellows structures out of simple doughnut-like shape disks. Compared to its counterpart, the formed bellows, higher tolerance control and uniformity in convolutions can be achieved in the EWMB. However, there are side effects for using welded bellows that have tormented bellows design engineers and prevented the improvements in the performance and reliability of edge welded metal bellows.; The first problem is that the force-deflection response of welded metal bellows is unpredictable when it is subjected to combined stroke and pressure loading. This information is especially important for the welded metal bellows used in mechanical face seals. The sudden reaction force increase associated with the higher operating pressure (also known as the effective diameter shift problem) will usually destroy a balanced seal design. This study has developed an analytical procedure and demonstrated that the finite element method can be used to solve the mystery of effective diameter shift.; The second problem is that the fatigue life prediction of welded metal bellows is difficult caused by its built-in crack-like geometry which creates stress risers in the structures and singularities in the mathematical models. This study first developed a crack propagation model using the fracture mechanics approach to predict the fatigue life and demonstrated that the fatigue life of welded metal bellows associated with crack propagation is only a small portion of the total life.; The model was then modified to include the life associated with crack initiation by using the local stress-strain approach. The local strain has been calculated using finite element procedure, and the calculated local strains were translated to cycle life using Coffin-Manson equation. The complete procedure has been used to analyze a 316L stainless steel welded bellows and shown good correlation with test data. A critical part of the analytical procedure is to calculate the similitude parameter for local stress-strain approach which allows the notch behavior in a complex structure to be represented by a simple laboratory shape.