Structural Qs
Did a double take on that one. 'q' is the aeronautical symbol for "half rho V squared". This has relevance to IAS so that maximum structural q would likely mean max IAS. Exceeding this runs the risk of structual failure resulting from local overpressures.
Genghis
The F-111 structural problems were/are in the high strength D6AC steel heavily loaded elements. Cracking can readily emanate from any 'crack starter' with the crack growing as a result of 'stress corrosion cracking'. Hydrogen ions migrate to the tip of the crack when under load to cause corrosion which then is the growth mechanism for the crack.
This type of cracking is not uncommon in the steel oleos and other steel parts of many undercarriages which obviously are under continual load when not airborne.