durability of duct?

[johnab]

I have one of the supplier supplying plastic duct having bellows for flexibility purpose & running Durability testing to check the flexing of duct. But, it did fail/brake before meet the requirement.(one of the convolute get teared) Did try different material like soft nylon, different bellows shape like square/triangular but didn't work. (duct vibrates +/- 20 mm at one end & fixed at other end under different temperature conditions & failing near by the same convolute section each time)

(did confirm the wall thickness around the failure area but was OK)

Any suggestions would be appreciated!