CYCLIC PLASTIC DEFORMATIONS IN THIN TUBES UNDER. BILI-NEAR RADIAL THERMAL GRADIENT

The cyclic plastic behaviour of thin walled tubes subjected to internal pressu: and cyclic radial thermal gradient is investigated. Such structural problems find many applications in high temperature .nuclear plants, conventional power plants and chemical industr:.es where the integrity of mechanical components is governe•: by growth of inelastic strains due to plastic ratchetting and time dependent creep as well as the interaction between creep damage and low cycle fatigue damage.
The tube problem is treated analytically with the aid of an approximate uniaxial model which compensates the effects of axial stress by magnifying the thermal stress in the hoop di-rection. The model is shown to provide conservative estimates' of observed strains. Linear kinematic hardening material model is used in the plastic analysis and an approximate tech-nique is employed to account for the cyclic hardening pheno-mena. Emphasis is placed upon the effects of the non-linear temperature distribution since ratchetting tests on thin tubes have shown that the observed temperature distribution is far from linear. The results of the analysis show that the bi-linear form of radial temperatures introduces new modes of cyclic deformation in which cyclic plasticity is confined to the inner skin of the tube wall.
The results are compared with test data on AISI 304 stainless steel tubes under internal pressure and cyclic thermal shocks between 1100 °F and 800 °F with hold periods of 160 hr/cycle at 1100 °F. The stress fields obtained from rapid cycle solu-tions are used to estimate upper bounds for creep deformations which accumulate during the hold periods. The predictions of plastic and creep strains are seen to provide moderate conser-vative estimates of test results.