A methodology of fitness-for-service evaluation (FFSE) for explosive containment vessels (ECVs) is introduced that utilizes change-in-thickness measurements pre- and post-test to determine the propensity of the structure to ratchet or to shake down. The method focuses on ductile failure and complements previously developed brittle failure methodologies associated with fatigue-fracture of flaws introduced during manufacture or subsequent service. The methodology is illustrated using measured thickness changes on a spherical vessel and is intended to eliminate or diminish the need for detailed, challenging finite element calculations of ratcheting and shakedown. An example is presented, based upon measured thickness changes in an explosively loaded containment vessel. Current limitations of the procedure are discussed. Applicable consensus code requirements and issues with the numerical modeling of ratcheting are briefly presented.