Local plastic flattening of microscopic high spots at every loaded interface: under the head, under the nut, on the thread flanks and between joint plates. As they settle, the bolt springs back slightly and loses part of its preload, often 2 to 10 percent, and more in joints with many interfaces, paint, plating or soft materials. Short, stiff bolts suffer most because a small settlement is a large share of their stretch.
Breakaway audit came in eight percent low, that's embedment in the painted flanges, the nuts haven't moved.
Flange crews do a final check pass after a wait because it recovers some of the preload lost to embedment and gasket creep.
Also heard as
- seating loss
- settling
- short-term relaxation
Do not mix it up with
- Self-loosening: Embedment loses preload without the nut turning; self-loosening is the nut actually rotating off.
Related terms
Preload
The tension created in a bolt by tightening it, which clamps the joint and keeps it from slipping or opening.
Belleville washer
A dished, conical spring washer that flattens under load and keeps clamp force on a joint as it settles or expands.
Breakaway torque
The torque needed to start a tightened fastener turning again, used to audit joints and rate locking features.
Clamp load
The compressive force a tightened fastener puts on the joint members, holding them together in service.