Strain-Based Damage Mechanics Model And Its Application To Numerical Simulations Of Creep Crack Growth

The creep crack growth is a critical issue in the life prediction of structures containing flaws operating at elevated temperatures, which is dependent on various factors, including the temperature, time, environment, material, geometry, and multiaxial stress state. Numerical simulations can analyze in detail the intricate variations of the stress, strain, energy and damage with time in an engineering structure and the zone ahead of a crack tip. Hence, this approach is gaining increasing attention in both academia and industry.Creep constitutive equations and damage evolution models serve as the fundamental basis for numerical simulations of creep crack growth. However, the existing theories are far from satisfaction. For instance, existing stress-based models contain a great many material parameters needing to be calibrated very carefully; multiaxial creep ductility factors employed in strain-based models either lose the physical significance or only work in a narrow range of multiaxial stresses. In addition, due to the complexity of three-dimensional crack models, there have been very few successful examples of simulations of the creep growth of surface cracks. Therefore, the current dissertation applies the high temperature fracture mechanics to simulations of the creep growth of three-dimensional surface cracks. On the other hand, it aims to provide a novel creep constitutive equation and a multiaxial creep ductility model by theoretical analysis, and to develop the numerical simulation methods based on the creep damage mechanics. The proposed models are subsequently applied to the analyses of crack growth rates and progressive crack profiles for various specimens or components, whose results are compared with available test data. The main tasks and findings of the dissertation research are summarized below:(1) The theory of high temperature fracture mechanics is combined with the finite element method. By using a step-by-step analysis procedure, numerical simulations of surface crack growth are conducted on tension-loaded thumbnail crack specimens of316stainless steel tested at600℃. The comparisons of the simulation results with available experimental data from literature show that the fracture mechanics-based finite element method can predict the evolution of crack profiles fairly well. Nevertheless, the predicted propagation times are2-3times as long as the actual test durations.(2) A new damage-coupled creep constitutive model is proposed from the micromechanics viewpoint. Based on the new model, ABAQUS user subroutines are written. Then, simulations of creep crack growth in various specimens of316stainless steel tested at600℃and T91steel tested at550℃are conducted. It is found that, compared with the widely-accepted Kachanov-Rabotnov model, the newly proposed model can give a more accurate description of the effect of damage on creep rate; compared with the Liu-Murakami model, it can better describe the influence of multiaxial stress state and can produce less significance dependence on the mesh size. Both of the crack profile and propagation time predicted by the developed numerical simulation method match the experimental results very well, which proves the predictive capability of the proposed creep-damage model.(3) On the basis of the power-law creep controlled cavity growth theory, a novel multiaxial ductility model is developed. Compared with the widely-used Cocks-Ashby approximate model, the newly proposed model gives cavity growth rates that are more consistent with the theoretical values, predicts multiaxial creep fracture strains in better agreement with the average values of experimental data at high stress triaxiality, and reasonably estimates the effect of load biaxiality on creep ductility. In addition, corresponding subroutines are written. Then, analyses of creep crack growth in compact tension and C-shaped tension specimens of316H stainless steel test at550℃are conducted. The predicted results are compared with available test data and existing simulation results, which again confirms the feasibility of the newly developed creep-damage model and the numerical simulation method.(4) By using the fracture mechanics-based and the damage mechanics-based finite element methods, respectively, the interaction, propagation, and coalescence processes of two semi-elliptical surface creep cracks in a tension-loaded plate are investigated. Further, the existing combination rules for two interacting defects in the creep regime are examined. It is observed that an individual crack grows almost independently when the minimum distance is greater than the crack depth. When the two cracks are closer to each other, more significant crack propagation is observed in the neighboring zone than that in other regions. After contact, the concave positions of the newly combined crack front show a relatively high growth rate. Ultimately, the crack shape saturates with the re-entrant section being smooth in a short time. In addition, for the material constants in the study, the combination rule suggested by API579and GB/T19624is believed to be overly conservative for defects under creep conditions.