RiverDave2:
Cumulative fatigue is a stress analysis method (also known as Miners Law) based upon a known (or estimated) fatigue spectra with the intent of determining how close something is to failure. Every metal has what is known as an endurance limit, which is the stress level where infinite fatigue life exists. Therefore, cumulative fatigue damage can be applied to aluminum, carbon steels, alloy steels, titanium (comm. pure and alloy), nickel-based super alloys, etc. The technique has no dependency on the metal.
The hinge pin arrangement shown sets up a complex load and stress distribution in the attaching structure. That is what I am trying to get across. This arrangement also forms a weak link. Think of the trailer as a series of springs. The tab/pin arrangement acts as a spring. When trailering, high bending loads exist due to road accelerations (up/down motions). These bending loads are carried thru these "springs". Loads accumulate with time yielding fatigue. It may well be that the stress levels that result are below the endurance limit of plain carbon steel. If so, great. If not, sooner or later we'll see this in a courtroom as a product liability action.
Sorry, not on my trailer, carrying my boat with $$$ invested, attached to my tow rig ($$$) carrying my grandkids (priceless).