Fatigue Cracking in a Stainless Steel Welded Exhaust Hose Assembly

A failure analysis was conducted to determine the cause of recurring failure of flexible bellows in an exhaust hose assembly. The bellows were made of type 321 stainless steel. Visual examination showed that cracks followed a path along the seam weld in the bellows. Most of the cracks followed a multidirectional/circular pattern, occasionally chipping off the convolutions, an indication of high-resonance fatigue-type cracking. Scanning electron fractography showed fatigue striations throughout the fracture surface. The microstructure consisted of relatively large grains and an abnormal degree of titanium-base stringers. Wall thickness was about 0.15 mm (0.006 in.) underside. It was concluded that the high vane pass frequency excited the natural vibration of the bellows to a higher resonance and cracked the bellows after a relatively short service period. The assembly was redesigned, and no further cracking occurred.