Deformation behavior of structures exposed at very high temperatures to cyclic thermal and mechanical loadings

At very high temperatures, such as the operating temperature of the heat exchangers of the Helium Cooled High Temperature Reactor, enhanced creep deformations may be produced by the superposition of load controlled stresses and cyclic thermal stresses (creep ratcheting). Thick walled tubes made from ALLOY 800 were loaded by an axial force at temperatures around 900 C. The inductive heating facility was cyclically operated in such a manner that in the tube walls temperature differences of 40 C occurred. The axial elongation was measured. For comparison the elongation was also measured under constant temperatures (at the mean temperature of the cycle). Under cyclic conditions a larger elongation rate was observed. The ratcheting effect was clearly seen. The inelastic stress analysis using simple constitutive creep equations predicted a lower effect