This scenario must first be categorized in terms of its underlying failure mechanism. An underlying question to answer when categorizing a failure mechanism as either time independent or time dependent is ‘if there have been no changes in any forces, then why did it fail today instead of yesterday?’.
When a fitting or appurtenance has performed satisfactorily for some period of time and then fails even though no operational or environmental changes occurred, it can be considered to have failed from some time-dependent phenomenon. When vibration is believed to be a separate failure mechanism from fatigue, it can perhaps be most logically treated as increased PoF from cracking.