Huh, there's actually a Wikipedia article on it:
https://en.wikipedia.org/wiki/Solder_fatigueonly discusses the models though, no example parameters for alloys.
This seems interesting,
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4121147/just scanning it, I don't see a discussion of fatigue limit (stress for number of cycles)...
Or this,
https://www.osti.gov/servlets/purl/1818047preliminary, so presumably there's a follow-up reporting high-cycle data, hmm...
But yeah, needless to say, solder doesn't do well with static loads, or heavy vibration. A certain amount of static strain, particularly at elevated temperature, can be relieved by creepage, but constant or repeated creepage eventually leads to crack formation. High frequencies are better (more strength for given fatigue cycles), as suggested by the time dependency. Which is again why single numbers don't mean a thing, sure it might be 15KSI, at what rate? Is that a shock load or what? Against what materials, what surface prep, what joint geometry? All of these matter and the results can vary wildly. And if you're trying to apply that number to some other geometry, how confident are you that that number will even apply? A solder joint is not simply a solder joint!
Also, how much and in what ways strain relief happens, probably varies substantially with and without lead (at all, a few percent, ~half..). And pure tin is known to relieve stress by whiskering; I haven't seen mention of that alongside this [bulk stress, fatigue] yet, but I wouldn't be surprised if that's part of it. Annealed matte tin is the best to deal with whiskering, IIRC, which is to say, relieved stresses.
Like I said, if you need to fix something in place, using epoxy is likely to fare better; it is free from creepage AFAIK, and apparently can have exceptional fatigue limits (particularly composites like carbon fiber?). Even if you need to make the joint bigger to handle a lower tensile or adhesion strength (say 1 or 2 KSI peak repetitive loading), as long as you have the area available, sure, slather it up eh?
Tim