Characterization of fatigue damage mechanism of asphalt mixtures with acoustic emission

Understanding the fatigue damage mechanism of asphalt mixtures is essential to prolong the service life of asphalt pavements. Acoustic emission (AE) technique can effectively detect minor damage and evaluate material mechanical properties. The objective of this paper was to investigate the temporal-spatial variation of AE parameters associated with fatigue damage behavior of asphalt mixtures and characterize the irreversibility of damage evolution utilizing the Kaiser effect. Firstly, the semi-circular bending (SCB) tests with different rest time and AE tests were carried out simultaneously to discuss the AE characteristics of fatigue damage behavior of asphalt mixtures. Secondly, the spatial evolution of AE events was analyzed to correlate with the crack propagation path of asphalt mixtures. Finally, the Felicity ratio (RF) and wavelet scalogram were utilized to reveal the irreversible characteristics of the AE process of asphalt mixtures. The results show that the "blank area" in AE energy temporal, representing the intensity of AE activities in asphalt mixtures during the process of microcracks concentration, gradually disappears with the increase of rest time. The inflection point of the decline of AE amplitude b-value could be considered as a precursor to the rupture of asphalt mixtures. The spatial distribution of AE events from disorder to order reflects the influence of the microstructures of asphalt mixtures and loading mode on crack propagation. The change of RF could be divided into the three stages, corresponding to the deformation and compaction stage where 1 ≤ RF, microcracks initiation and concentration stage where 0.4 ≤ RF < 1, and macrocracks extension stage where RF < 0.4. The Kaiser effect mainly exists at the stage of deformation and compaction of asphalt mixtures. After the occurrence of macrocracks, the AE process would lose irreversible characteristics. The damage mechanisms related to the Kaiser effect and the Felicity effect could be identified by the energy distribution of AE waveforms.