Research On The Methodology For High Temperature Creep Based On Energy And Dimensional Analysis

Creep fracture is one of main reasons for high temperature structures damage. As a key and basic problem in the field of strength design for high temperature structures, the creep crack behavior has been investigated for more than half a century. However, it remains very challenging to reasonably describe the creep crack behavior in terms of time, temperature and stress given limited testing time and data. The difficulties mainly root in the varieties of micro-mechanism and time-dependent characteristic for the creep crack failure as well as the complexity for the creep crack constraint.High chromium ferritic steel 25Cr2NiMol V is the material developed for the steam turbine rotor of ultra super-critical power stations. In this thesis, the steel is selected to investigate its creep properties, crack behavior and failure mechanism by the experiment. By using dimensional analysis technique and energy methods, creep crack regulations, similarity of uniaxial tension creep specimens and prediction method of creep crack life are put forth effort to study. All research work and results for the thesis are generalized as follows:(1) Dimensional analysis and self-similar principles have been applied in the study of crack behavior of materials under creep loading conditions. The correlations between creep crack growth grate da/dt and C*, stress tension factor K, net section stressσnet, crack open displacement rate dd/dt and Q* have been established. Parameters in the above correlations were proved not to correspond to material characteristics based on the concept of incomplete similarity. On the contrary, they are dependent on dimensional characteristics.(2) The similarity rules and scaling relationships of the loads, the stress and the life of uniaxial tension specimens are derived by using dimensional analysis. The relationship curves have been gained between the dimensionless forces, stresses and specimens diameters. Thus, the results of the paper contributes researches of creep mechanics and fracture mechanics. (3) A new model for life estimation of creep derived from the law of energy conservation principle was proposed with damage mechanics. By mathematical technique, the expressions of the model is transformed into a simple function, which can describe between Wc the mechanic work per unit by applied loads and time t power function that it accumulates for the constant at uniform temperature. To check the validity of the new model proposed in this paper, the published data of 2.25Cr-1Mo, 1Cr-0.5Mo,18Cr-12Ni-Mo and 1Cr-1Mo-0.25V steels have been studied and analyzed. A well agreement is noted between the model and the results. A new model for life estimation of creep was developed for long term creep life from short experimental data based on energy process.(4) A new model for creep life prediction is proposed based on power processes. The dissipated power of applied loads is used to describe creep processes. Based on comparisons with the published data for 2.25Cr1.0Mo steel and TiAl-base intermetallics, the model of the relationship between stress and time to rupture is a better representation of the creep process compared with the Larson-Miller method, represents the published data well and produces extreme extrapolation results that behave as would be expected. the model developed in the present study more accurately predicts creep life.