Failure Analysis of a Weld on a Bellows

IMR received a pair of bellows assemblies with cracks in the welds for failure analysis. 

After mounting, polishing and metallographic analysis of four sections from each assembly, it was determined that all 8 locations failed to meet the customer-supplied fillet angle requirements and six of eight failed to meet weld size requirements. Samples taken from areas where there were no visual crack indications also contained cracks originating at the weld root.

One crack from each assembly was opened to examine the fracture surface using an optical stereoscope and a scanning electron microscope (SEM). The overall morphology of the crack surfaces is consistent with multiple origin fatigue originating at the root of the weld.

It was determined that undersized welds are the likely cause of the cracking and that there was no evidence of corrosion, weld defects or loose bolted connections. 

IMAGE CAPTIONS (LEFT TO RIGHT)

  1. Typical crack appearance on the exterior of the bellows assembly
  2. Fracture Surface through the weld material
  3. Scanning Electron Microscope image of fracture surface showing multiple origins at the root of the weld
  4. Mounted and etched sample showing cracking originating at the weld root. Similar cracks progressed to the outside surface and were detected by the customer.