High temperature components, such as aero engine turbine disks, are usually subjected to the random multiaxial thermo-mechanical fatigue loading during service process. Therefore, it is of great theoretical and practical significance to investigate the damage mechanism and fatigue life prediction method for high temperature structure durability design. Based on the investigations of damage mechanism and deformation behavior of superalloys under the multiaxial thermo-mechanical fatigue loading, the variable amplitude multiaxial thermo-mechanical fatigue life prediction method of the material and structural level was proposed in this paper.
Firstly, through the constant/variable amplitude uniaxial/multiaxial thermo-mechanical fatigue tests of Ni-based superalloy GH4169, the damage mechanism of the material was revealed under multiaxial thermo-mechanical fatigue loading. It is found that the tensile stress can cause creep voids between grains at high temperature, and shear stress can increase creep damage by tearing the voids. The continuous evolution process of creep voids can induce intergranular fracture, which can lead to the sharp decrease of failure life. It is also found that non-proportional additional hardening behavior can increase the stress response, which can increase the fatigue, creep and oxidation damages, and result in the failure life to be dramatically decreased.
Secondly, based on the microstructure observation, the deformation behavior of the material is further investigated under multiaxial thermo-mechanical cyclic loading. Due to the temperature dependence of mechanical properties, the stress response of the material at low temperature is larger than that at high temperature, resulting in a mean stress biased towards low temperature under symmetrical strain loading. It is also found that the strengthening phases were elongated under dynamic strain aging, which can increase the pinning of dislocation motion and cause cyclic hardening.
Thirdly, the path dependent ration factor and the non-proportional hardening coefficient were introduced in the kinematic hardening rule to consider the effect of non-proportional additional hardening on cyclic mechanical behavior, in which the rotation factor was used to characterize the path dependence of hardening degree. At the same time, the dynamic strain aging influence factor was introduced to consider the caused cyclic hardening behavior. Based on the above modification, a viscoplastic constitutive model considering non-proportional additional hardening and dynamic strain aging was proposed. The stress-strain data of multiaxial thermo-mechanical fatigue tests were used to verify the proposed model, and the prediction error is between -1.51% and 7.54%.
Forthly, the relationship between pseudo stress increment and actual stress increment was proposed by analyzing stress-strain curves of the material and structure. Then, combined with the proposed viscoplastic constitutive model, the notch stress-strain evaluation method was proposed, which can consider the influence of temperature change on notch correction. The results of thermal-structural non-linear finite element analysis for fir tree structural specimen were used to verify the proposed method, and the prediction error ranges from -4.98% to 6.44%.
Fifthly, a multiaxial thermo-mechanical cycle counting method considering temperature history was proposed to process loading history in real time. At the same time, based on the mechanism study, the multiaxial fatigue-oxidation-creep damage was characterized, especially considered the effect of non-proportional additional hardening on damage. Then, combined with the notch stress-strain evaluation method, the variable amplitude multiaxial thermo-mechanical fatigue life prediction method was proposed, which is suitable for the actual structure, and a multiaxial thermo-mechanical fatigue life prediction system was developed. The life results of constant/variable amplitude multiaxial thermo-mechanical fatigue tests were used to verify the proposed method, and the prediction errors were within a factor of 2.
Finally, the turbine disk structure of an aero engine was analyzed by the proposed method. The turbine disk structure was simulated by thermal-structural non-linear finite element analysis based on secondary development, and the location of the dangerous point was determined. Then, the stress-strain history and temperature history of the dangerous point were extracted to evaluate the multiaxial fatigue-oxidation-creep damage at this point. It was found that the reason for the low life of the point may be that the larger tensile stress to cause more creep damage.