Gees why do I feel like I am being baited.
The answer to that question would be the result of several thousand doctorial thessis ( thesi ?).
Con rod makers got their act together very early re quality control.
It is very rare for one to fail from the inside out they usually fail from the outside in.
When I was actively doing ultra sound we could not do castings as the coarsest wave we could generate either got reflected or refracted by the rather large grain boundries. Then again these were the days of a trace on a screen or if you had the research grade gear ( and I did ) you could get 3 traces on the screen.
Metallurgy would have to be one of the most conservative fields that I have ever come across.
I was banned from tendering to one government department because my CRT had a "revolutionary" square screen.
As for x-raying yes done thousands of them, tiny little buggers from refrigeration compressors a right royal PIA and very expensive ( 4 exposures each one), Then again so is a container full of rancid meat or cooked vaccines.
Never done a motorcycle one.
The grade of aluminium used for rods ( it is an alloy specifically designed for this application) is very fracture tough and extreamly notch insensitive so they will take a lot of abuse before they give up the ghost. Design wise the stuff we play with is also about twice as heavy as it needs be .( stretch excluded).
Steels otoh tend to fracture from the inside out.
The actual mechanism for this is migration of dissolved hydrogen atoms to the points of greatest internal stress . When you get a few of the buggers there they form a hydrogen atom and this causes the crack to start and travel through the body of the metal.
My old prof (Hugh Muir ) got an Acta Metalurgica ( metallurgy's Nobel prize) for his work on this and as an undergraduate I did about 0.0000000000000000001 % of the work on the project.
This is why double vacuum melted steels are so strong ( no dissolved hydrogen) they are also just a tad dear for fitting onto the average A10.
There are tables of the fatigue limits of just about every material known to man and without rummaging around to find my ASTM handbooks & data sheets Aluminium casting alloys go around 10,000,000 to 100,000,000 cycles to failure. The best alloy for fatigue resistance is the one used for wheels AP601 or BP601 and this one is good for 1,000,000,000 cycles.
Note that is not how many stress cycles that your rods can go through, that is the number of cycles a prepared bar can go through on a test rig and is purely for comparing one alloy to another.
Heat treating can vary these results and elevated operating temperatures tend also to increase the cycles to failure. OTOH creep ( or stretch) is increased with temperature so as always it is a balancing act.
Working life of component is used to be determined by running one in a rig till it fails, Then doing the same till you have enough data to fulfill the percentage certency required.
If you want to be 100% sure, tough luck.
99,99% is around 1473 determinations
90% is around 1200.
If I was making bits for an aeroplane or rocket ship then I could justify destroying 1500 of them first but for a BSA it might be just a tad dear.
However now days you an do some really clever computer modeling because so much more data is available and we know so much more than we did in the days when Hele & Hopwwod were designing motorcycles.
So what exactly did you want to know ?