## 3 Results and Discussion
### Fracture Failure Process Analysis Subjected to F-T Conditions  
#### 3.2.1 Fracture Failure Evaluation by AE Index  
(1) Compressive fracture failure analysis under F-T cycles  
To facilitate comparative analysis, the load is usually standardized. In the high-temperature uniaxial compression test, the failure process of asphalt mixture could be divided into three parts on account of compressive loading versus loading time [29]. These three stages are described as: (1) the loading level is stable at a low level for a long time; (2) the loading level has an obvious inflection point and increases rapidly; (3) the loading level has an obvious inflection point again and gradually tends to be gentle, reaching the maximum value. By comparison, AE signal indexes are adopted to describe fracture process growth, particularly in identifying damage stages. Their difference is in the second stage according to the compressive load curve vs. loading time and could be further divided into another two stages, which is a new finding in comparison with previous studies. Figure 5 shows the AE signal's direct parameters (including amplitude and count) versus compressive load level for the asphalt mixture subjected to F-T conditions. As illustrated, the results of signal amplitude and the count would have more significant and detailed fracture stages accompanied by the load level with loading time. The variation trends of the AE signal's direct parameters (including amplitude and count) for the asphalt mixture show a similar change trend for different F-T cycles. Combined with the intensity change in the AE signal's direct parameters and load level change, the fracture failure process can be divided into several stages (I-IV), which is in accordance with the present results [30]. Figure 5: _Cont._Figure 5: The AE direct parameters (amplitude and count) versus compressive load level for asphalt mixture subjected to F-T condition: (**a**) AE amplitude/count vs. load level under 0 cycle; (**b**) AE amplitude/count vs. load level under 3 cycles; (**c**) AE amplitude/count vs. load level under 9 cycles; (**d**) AE amplitude/count vs. load level under 15 cycles; (**e**) AE amplitude/count vs. load level under 21 cycles. Stage I--micro-crack initiation:  
Firstly, the smaller signal values of amplitude and count are observed, in which the amplitude values at Stage I are mainly distributed in the range of 40-50 dB, and the count values at this stage are mainly distributed in the range of 0-10 times. The reasons for the change in the AE signal's direct parameters are mainly because of micro-crack formation inside the specimens under the action of compressive loading. * Stage II--micro-crack accumulation:  
Then, the AE signal values of amplitude and count change obviously, in which the amplitude values at Stage II are mainly distributed in the range of 50-60 dB, and the count values at this stage are mainly distributed around 20 or 30 times. At this time, the load level is at the first inflection point (i.e., the boundary between red and blue areas) and then increases sharply. The AE signal is mainly caused by the number of micro-cracks in the specimen gradually increasing and the micro-damage continuing to accumulate. * Stage III--damage development:  
Subsequently, the AE signal values of amplitude and count decrease a little, in which the amplitude values at Stage II are mainly distributed around 50 dB, and the count values at this stage are mainly distributed around 10 or 20 times. At this time, the second inflection point (i.e., the boundary between yellow and green areas) occurs in the load level of this stage. The AE signal is mainly caused by the load continuing to load, and the gradual accumulation of micro-crack leads to the rapid growth of interior damage. * Stage IV--macro-crack generation:  
Finally, the AE signal values of amplitude and count change only slightly, but the compressive load level gradually becomes gentle and reaches the maximum value. At this stage, obvious macroscopic cracks appear on the asphalt mixture specimens.