INVESTIGATIONS OF CREEP RATCHETING ON THICK-WALLED TUBES

The superposition of load-controlled stresses and cyclic thermal stres ses at very high temperatures can produce remarkably enhanced creep de formations (creep ratcheting). During an experimental project thick-wa lled tubes made from ALLOY 800 were loaded by an axial force at temper atures around 900-degrees-C. The inductive heating facility was cyclic ally operated in such a manner that in the tube walls temperature diff erences of 40-degrees-C occurred. Thermal stresses were so induced. Th e axial elongation was measured. For comparison at the same specimen t he elongation was also measured under constant temperatures (at the me an temperature of the cycle). Under cyclic conditions a larger elongat ion rate was observed. The ratcheting effect was clearly seen. Inelast ic stress analysis using simple constitutive creep equations predicted a lower effect.