maxwelg2,
The MRGB housing casting cracks on the S-92 are a problem that should probably not have occurred if all of the FAA regulations were followed. FAR 29.621 defines a structural MRGB casting as "critical", which means it should have an additional "casting factor" of 1.25 applied on top of the standard 1.5 FoS used for structural analysis. So a proper structural analysis of this part should have had plenty of room for error.
FAR Part 29 Sec. 29.621 effective as of 11/28/1997
Additionally, "critical" class castings are required to undergo 100% radiographic and dye penetrant inspection under FAR 29.621, so there should be no castings put in service with any existing surface/subsurface structural flaws.
Good manufacturing practice also would dictate that critical areas in a structural casting (like the attachment feet in the S-92 housing) would be areas where tensile test coupons are periodically cut from sample castings and checked for mechanical properties. This ensures that foundry processes used to produce the casting are being properly controlled.
As for in-service cracking, some high strength aluminum alloys can experience stress corrosion cracking, especially under conditions of sustained load and exposure to marine atmospheres. But most MRGB housings are cast from 356 or 357 aluminum alloys, which have good corrosion resistance and thus are normally resistant to stress corrosion cracking.
Regards,
riff_raff