On improving the fatigue life of U-form bellows

Abstract

Fatigue failure invariably occurs at the root in hydroformed bellows, as the hydroforming process does not cold work the root region and raise yield point there, as it does in other regions. Both elastic and plastic analyses confirm this by predicting that the yield criterion is first satisfied in the root region during deflection. On the other hand, the roll-forming process imparts a substantial amount of cold work to the root of the bellows. A judicious combination of hydroforming and roll-forming processes is examined here therefore as a viable alternative (as opposed to individual routes), to enhance the fatigue life of austenitic steel bellows for low-pressure, low-temperature applications. This combination process has the added merit that bellows with extra-deep convolutions can be formed as replacements to costlier diaphragm-type bellows and for applications having space restrictions. Whilst testing the bellows for fatigue, it is noted that corrugations do not share the total movement equally, this phenomenon appearing to be a major cause for the scatter in values obtained in life tests. Investigation reveals that a correlation exists between this unequal sharing of movement and fatigue life. Based on these observations a new non-destructive stretch test is proposed with which original life estimates can be refined.