Navies recognized metal-hull fatigue early

When did navies discover that fatigue cracking was a problem for steel-hulled ships?

Quite early, because iron hulls had already exposed similar metal-fatigue problems and navies designed around them.

The problem did not begin with steel because iron-hulled ships also suffered cracking and degradation. Navies learned early that salt water, repeated loading, and a moving hull damaged metal structures over time. An adequately sized steel member can operate below the stress range at which ordinary cycling produces fatigue failure. Overbuilding does not remove corrosion, shock, or impact damage, but it changes which structural threat is most likely to govern service life.