Full definition
Failure mode where seal material is forced into the clearance between mating metal surfaces by fluid pressure, permanently damaging the seal and causing leakage. Prevention: reduce diametral clearance (<0.1 mm at 350 bar), increase hardness to 90 Shore A, install backup rings (PTFE or PEEK) on the low-pressure side of the O-ring. Extrusion risk increases sharply above 7 MPa without backup rings.