I think the idler studs fail due to fatigue as they're constantly being bounced around under the continuous load of the tensioner. Which material handles fatigue better? I recently had a timing belt spacer bolt snap on the re-install while I was torquing it to spec and I think they're made of the same material (turned into a real nightmare --I gave up and RTV'ed that section of the timing belt cover back on).
I suspect the OEM stud material is stronger but more brittle than standard bolt steel, whatever that is. Which one would you buy as a replacement for the original? Any thoughts of doing a shear test?