Behavior of bellows

Becht, C. (2000) Behavior of bellows. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

While analysis of bellows, and in particular unreinforced bellows, has been investigated over the course of the last several decades, there remain a significant number of unanswered questions with respect to their behavior. The present research addresses their behavior under cyclic displacement loading; in particular, strain due to displacement. Prediction of strain range due to displacement is important in the fatigue design of bellows. In addition, a number of other areas are clarified by ancillary research. -- Design of bellows for cyclic displacement loading primarily relies on data obtained from bellows fatigue tests. Further, fatigue data on bellows fabricated from one material is not considered a reliable indication of the fatigue performance of bellows fabricated from another material. This is because there has not been a good correlation between the fatigue performance of bellows based on calculated stress versus cycles to failure with that for the material of construction, as represented by polished barfatigue curves. -- This research shows that the differences in fatigue behavior of reinforced and unreinforced bellows, and the differences between bellows fatigue data and polished bar fatigue data is due to plastic strain concentration that occurs in bellows. -- The existing, widely used charts and equations for evaluation of unreinforced bellows were found in this research to have sufficient accuracy for calculation of the significant stresses in the elastic regime for most bellows geometries. Some specific observations for further improvement are made. However, it was found that the elastic prediction of stresses in unreinforced bellows is not sufficient to accurately evaluate the displacement strains in bellows. Significant plastic strain concentration due to displacement loading occurs in the highly stressed regions of bellows. Regimes of behavior, depending on geometry parameters, were found. Prediction of fatigue performance of bellows is significantly improved by proper consideration of the effect of convolution geometry on plastic strain concentration. -- Consideration of strain concentration effects eliminated the apparent disparity between the fatigue behavior of reinforced and unreinforced bellows. Further, it was found that polished bar fatigue data could be used to predict bellows fatigue life for a range of geometries.

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