Numerical Analysis Of Fatigue Crack Growth In Asphalt Pavement Based On Extended Finite Element Model

Fatigue cracking is an important factor causing damage to the service level and structural capacity of asphalt pavement. Because of the importance and complexity of fatigue cracking, it has always been a great concern to researchers. Traditionally, fatigue failure process was always artificially divided into two stages, which are crack initiation stage and crack propagation stage. Then these two stages would be studied separately. There exists much difficulty in considering fatigue failure as a continuous and natural process and simulating free propagation of fatigue crack. However, the application of Extended Finite Element Method (XFEM) has brought a great improvement to research in this field. In this paper, an effective approach was proposed to simulate fatigue damage accumulation and fatigue crack propagation as a continuous process based on fatigue damage mechanics and XFEM. The main research contents of this paper are listed as follows.Fracture behavior of asphalt concrete under monotonic loading was investigated first, which provides the basis for the study of fatigue crack growth. Using XFEM, fracture processes in single-edge notched beam specimen under pure mode I and mixed-mode Ⅰ-Ⅱ were simulated. The fracture of indirect tensile specimen was analyzed as well. On the basis of the study of mechanical response and fracture shape, a thorough explanation to the fracture mechanism of asphalt concrete was made. The SEB specimen has already been under damage accumulation stage when the load reaches the peak value and crack initiates after peak load. Fracture behavior of the SEB can be considered as a process during which tensile stress decreases in the damaged zone and increases in the undamaged zone. Meanwhile, the length of the damaged zone is increasing. Based on the comparison of numerical results and experimental records, it can be concluded that XFEM can serve as an efficient tool for simulating crack path and calculating mechanical response in asphalt concrete.Secondly, a close-form solution for the fatigue damage life in the beam specimen was provided according to the fatigue damage mechanics theory. The numerical method of studying fatigue damage in ABAQUS was presented by use of user subroutine UMAT. The traditional fatigue damage model cannot effectively describe the stiffness reduction in asphalt concrete. In addition, there is much difficulty in defining the fatigue failure states under different controlled modes. In order to solve these problems, the characteristics and mechanism of stiffness reduction were investigated so that a unified definition of failure states was proposed. The moment when macroscopic fatigue crack initiation occurs was considered as the failure state. In order to accurately characterize the stiffness reduction and incorporate the concept of fatigue endurance limit, a revised model of fatigue damage evolution was proposed. The determination of fatigue endurance limit was also investigated through regression analysis of three-parameter fatigue equation. A reliable result of fatigue endurance limit can be obtained when the correlation of three-parameter fatigue equation is much better than that of two-parameter fatigue equation and regression result can meet the requirement of t-test. Once such a reliable result is found, no more fatigue tests under lower strain level were needed. For the application of the revised model, how to determine critical value of fatigue damage was discussed. The reduction of residual strength during the fatigue damage accumulation was investigated and a corresponding reduction model was built, which paved the way for simulating the fatigue crack initiation and propagation. Finally, flexural beam fatigue test was simulated to improve the practicability of revised model. Some issues relating to the fatigue process, such as fatigue lives, stiffness reduction, damage evolution and mechanical responses, were all discussed. The numerical analysis shows that strain of beam under strain-controlled mode is actually increasing and stress of beam under stress-controlled mode is actually decreasing.Thirdly, after discussion about the variation of material property during the fatigue process, the fatigue process is divided into three stages, which are micro-crack initiation stage, macro-crack initiation stage and macro-crack propagation stage. Moreover, fatigue damage accumulation exists in the whole fatigue process and cannot be separated from crack propagation. A constitutive relation of asphalt concrete was presented to describe fatigue cracking based on the fatigue damage model and dynamic XFEM failure mechanism. As a result, a theoretical model for simulating the whole process of fatigue damage accumulation and fatigue crack propagation was built. Based on the programming of user defined damage initiation criterion (UDMGINI), the model was applied in the ABAQUS. Then the whole failure processes of semi-circular specimen and beam specimen with/without original notch were simulated. On the basis of comparison of numerical results and experimental records, the proposed model was proved to be effective. Moreover, characteristics of fatigue crack propagation and influence of fatigue cracking on mechanical responses and damage evolution were discussed. Abrupt and rapid fatigue crack propagation is found in beam spencimen with original notch under both mode I and mixed-mode Ⅰ-Ⅱ. Fatigue life of macro-crack propagation stage in beam without notch accounts for only 13.5 percent of total fatigue life, which is in consistence with experimental result. The reduction of beam stiffness obtained through numerical analysis shows good agreement with laboratory result. It is indicated that initiation of macro-crack is the turning point of stiffness reduction.Finally, numerical model of asphalt pavement structure was built and the whole process of fatigue failure in pavement structure was studied based on the proposed model. The whole process consists of the fatigue damage accumulation stage, macro-crack initiation stage and fatigue crack propagation stage. The fatigue crack propagation of reflective crack initiated in the subbase layer was also investigated. On the basis of thorough analysis about the fatigue failure features, a better understanding of the fatigue cracking behavior in asphalt conrete was achieved. It can be seen that fatigue life of macro-crack propagation stage in asphalt pavement accounts for 38 percent of total life and 60 percent of fatigue life in micro-crack initiation stage. When an initial crack exists in the subbase layer, reflective cracking in the base layer has a small fatigue life of 210000. The speed of reflective cracking is 49 times larger than that of macro-crack propagation in regular pavement, which indicates that reflective cracking can be regarded as a premature failure.In conclusion, the numerical analyses can serve as an effective tool for fatigue life prediction and therefore provide guidance for performance evaluation and structural anti-cracking design of asphalt pavement.