The ratchetting behavior of advanced 9-12% chromium ferrite steel was investigated by cyclic loading tests with various hold times and stress ratios at elevated temperature of 873 K. Particular attention was paid to the effect of hold time on the whole-life ratchetting deformation and failure mechanism. Results indicate that the total ratchetting strains under creep-fatigue loading can be decomposed into two parts, i.e., cyclic accumulated creep strain produced during the peak stress hold time ( ), cyclic accumulated inelastic strain produced during the stress change process ( ). A transition in ratchetting components and rupture behavior with the increase of hold time was observed. In the long hold time domain, a quick shakedown of ratchetting strain occurs after the very first few cycles and the rupture behavior is fully controlled by the time-dependent creep damage. In the short hold time domain, ratchetting strain increases till the specimens fails and a mixed damage mode is responsible for the failure. An attempt is made to explain the existence of these two domains in terms of the evolutions of three internal stress components (back stress, isotropic stress and viscous stress) measured at the end of the holding period.

An analytical model is presented for the effect of the interactions of dislocations with precipitate coherency strains on the generation of second-harmonic of Lamb waves in metallic alloys. The cumulative second-harmonic of Lamb wave propagation is shown to depend dominantly on the dislocation density, pinning dislocation length, internal stress due to the coherency strain, volume fraction of the precipitates, and the phase matching degree between the primary Lamb wave and the double frequency Lamb wave (DFLW). Experiments were carried out to introduce controlled levels of creep-induced damage to determine the nonlinear response of Lamb waves in titanium alloy Ti60 plates. A like mountain-shape change in the normalized acoustic nonlinearity of Lamb wave versus the creep loading time has been observed. Microscopic image analyses were performed to interpret the variation of the measured acoustic nonlinearity and to obtain the microstructure parameters of the Ti60 specimens with different creep damages. The analytical model was applied to these creep damaged Ti60 specimens, which revealed a good accordance with the measured results of the nonlinear Lamb waves. These results indicate that the acoustic nonlinearity of Lamb wave increases due to the rising of the precipitation volume fraction and the dislocation density in the early stage, and it decreases as a combined result of the reduction of the precipitation volume fraction and the dislocation density and the increasing mismatch of the phase velocity between the primary Lamb wave and the DFLW after a further creep loading