Metal Fatigue

Fatigue is an oft studied problem. In the million cycle regions, theory and practice fit together so that fabricators build structures that avoid this issue. Notches are smoothed or reduced. Stresses are calculated and lowered etc.

However in the lower cycle region where both crack initiation and crack propagation take up significant portions life, this expert has dealt with some interesting cases. One was a large piece of emergency electrical switching apparatus designed for a limited number of cycles. Qualification of a protective stainless bellows was done by test loading the bellows to a bit higher cyclic stress than usual and a bit longer life than required. This arbitrary standard had been met for years. One order of steel failed this test prompting concerns that there was something substandard. Investigation found that the fracture was in-fact fatigue, but no substandard attribute was found in the metal or fabrication. Extending the examination to a previous "passed" sample found an internal crack which had been initiated but had not yet progressed to leak status. The test was modified.

To see the resume of the expert associated with this case study, see the link below.