These are SmartBolts, they tell you when they’ve been tightened sufficiently and when they haven’t. An indicator disk shows you the actual tightness of a bolt. The indicator disk works by compressing a small fluid filled area.
Changes color depending on how tight the bolt is.
http://www.smartbolts.com/tech.htm
Smartbolts can be individually tailored to meet the customer’s needs.
A wide variety of bolt sizes and finishes are available.