Study On Damage And Healing Of Asphalt And Asphalt Mastic Under Different Load Levels

Asphalt materials,as a major component of asphalt pavement,fatigue damage is one of the main forms of failure.Although they may not be damaged due to a single load transfer,asphalt materials will experience fatigue damage and failure as the number of load repetitions increases.Due to the complexity of fatigue behavior,pavement researchers and engineers have been widely concerned with correctly understanding and simulating the fatigue damage behavior of asphalt materials under traffic loads.This study revealed the variation patterns of various performance index parameters of asphalt materials through time sweep(TS)fatigue tests of asphalt and asphalt mastic under different loading methods.A study on the nonlinear fatigue damage and life of asphalt materials related to loading history and test conditions based on modulus attenuation was conducted,which has a significant scientific reference value for improving the existing fatigue design of asphalt materials.The main research work carried out in the thesis is as follows:(1)The effect of single-stage load on the fatigue performance of asphalt materials.The TS test using strain control mode was used to evaluate the evolution of various fatigue parameters under single-stage cyclic loading.The modulus reduction criterion can serve as a simple and reasonable fatigue criterion,reflecting the transition process from microcrack initiation to propagation.Based on the tests,a new fatigue performance index(FI)based on pseudo strain energy and a new fatigue damage accumulation index(DR)based on dissipated energy history is proposed.It provides significant assistance in quantitatively comparing fatigue performance and characterizing the nonlinear accumulation process of fatigue damage.Finally,a fatigue damage accumulation and fatigue life prediction model for asphalt materials under single-stage loading was established to understand the fatigue damage development of asphalt materials.(2)The evolution law of various fatigue performance indicators was studied through multi-level constant amplitude load TS tests,while also considering the influence of test temperature on these indicators.The fatigue test results indicate that the accumulation of fatigue damage is not only influenced by the test temperature and load level,but also by the comprehensive effect of loading sequence,load level number,and loading time,indicating the limitations of the current linear accumulation theory.Subsequently,based on the M-H model,a theoretical calculation formula for multi-level nonlinear damage accumulation considering loading order and load level number was derived.Finally,based on the damage accumulation model,a fatigue life prediction equation for asphalt materials under multi-level loads was established.Introducing loading history and loading sequence can better predict the fatigue life of asphalt materials.(3)In order to consider the difference in fatigue damage performance of asphalt materials under linear viscoelastic(LVE)and nonlinear viscoelastic(NLVE)characteristics,a multi-stage variable amplitude combined load TS test was proposed to study the influence of NLVE characteristics on asphalt fatigue behavior and the cumulative damage behavior of asphalt under NLVE-LVE combined characteristics.The experimental results confirm the nonlinear accumulation characteristics of fatigue damage,indicating that the conventional Miner Law cannot accurately characterize the fatigue damage accumulation process.A nonlinear damage accumulation model under NLVE-LVE combination characteristics was established to fill this gap.(4)Through observation of multi-level load fatigue tests,it was found that there is a special situation in the complex shear modulus G*and fatigue damage accumulation process of asphalt materials.When the load transitions from high to low strain levels,the G*of asphalt materials has a certain recovery,and the evolution rate of D shows a decrease or stagnation phenomenon within a certain range,which is called the fatigue damage delay phenomenon.This phenomenon is very similar to the damage recovery of asphalt materials after rest.Given this,t fatigue shift factor SF,comprehensive evaluation index HF,and rapid recovery zone were proposed based on TS and fatigue healing fatigue(F-H-F)experiments.To characterize the fatigue damage delay phenomenon of asphalt materials under different load levels and the self-healing performance considering rest time.Furthermore,the effects of different load levels,loading sequences,and rest healing performance on the accumulation of fatigue damage and fatigue life of asphalt materials were studied.(5)Finally,the fatigue damage and failure mechanism of asphalt materials under shear load were studied.Firstly,evaluate the fracture mode through finite element simulation.Subsequently,a new indicator CI for crack evaluation was constructed based on the image method.CI achieved high consistency with the crack results calculated based on the DSR-C model.The constructed CI master curve can be used to characterize the development of cracks in specimens over a larger range of load levels.The fatigue crack propagation equation based on the CI coupling fatigue process,strain level,and test temperature can accurately predict the crack propagation process in cylindrical specimens.The use of CI and crack propagation indicators under different experimental conditions is of great significance for predicting the development of specimen cracks during shear fatigue testing.The changes of nano microstructure under different fatigue tests were studied by atomic force microscope(AFM).According to the changes of three AFM factors,namely root mean square roughness Rq,average roughness Ra,and image surface area difference,it can be inferred that cyclic fatigue load has a negative correlation with the degree of "roughness" in the microstructure of the "bee structure".This proves that fatigue loading has somewhat destroyed the microstructure of asphalt materials,providing a new method to explain their fatigue performance from a nano-micro perspective.The results of this study help practitioners better understand the fatigue damage accumulation laws of asphalt materials under different load levels and provide new research ideas and evaluation indicators for accurately evaluating the service safety of asphalt materials.