Final answer:
Darkening, localized cracking, and protrusion of the fasteners are all warning signs of fastener imperfections that need to be assessed for structural safety.
Step-by-step explanation:
Darkening, localized cracking, and protrusion of the fasteners are all warning signs of fastener imperfections. When examining fasteners for potential issues, it is important to look for visual indicators that suggest a problem. Darkening can indicate overheating, localized cracking might suggest material fatigue or stress, and protrusion could signify incorrect installation or material expansion. These signs warn of possible fastener failure, which can have serious consequences in structural applications.