## 1 Background and Introduction  
Asphalt pavement is one of the main highway forms throughout the world. With the continuous development of the world economy, there have been tremendous numbers of vehicles passing on the pavements every day, causing significant irreversible damage to pavement. It is therefore necessary to study asphalt material performance, since it is directly relevant to its service life, where structure is one of the most important factors that affects its mechanical performance. Recent studies show that the asphalt material microstructure may affect its properties at the macro-scale [1, 2].  
There have been many experimental approaches on the asphalt material microstructure due to the development of modern testing instruments. [PERSON] et al. [3], used scanning electron microscopy (SEM) to observe the microstructure of asphalt materials. Fourier transform infrared spectroscopy (FTIR) can be used to study the distribution and contents of asphalt components [4, 5, 6]. Gel permeation chromatography (GPC) can determine the molecular weight and distribution of asphalt rapidly and reliably [7]. Differential scanning calorimetry (DSC) can be used to analyze the difference of the thermal stability of asphalt [8, 9]. Atomic force microscope (AFM) can be used to observe three-dimensional (3D) spatial images of asphalt [10, 11, 12]. These new experimental methods can be used to further investigate the effect of modifiers on the modification of asphalt materials at nanometer scales [13, 14]. For example, SEM can be used to observe and compare the microstructures of different nanometer modified asphalts [15], and AFM, FTIR, GPC and DSC can be used to further analyze the modify mechanism of nano-modifier materials.  
There also have been many studies on asphalt material properties using the simulation, where molecular dynamics (MD) simulation is one of the effective methods to investigate the microstructure and micro-properties of asphalt materials [16; 17; 18]. In the aspect of asphalt oxidation aging, MD simulation can be employed to investigate internal chemical and molecular mechanical property changes [19]. Molecular Dynamics simulation can also be used to determine the thermodynamic properties of asphaltene before and after oxidative aging [20]. In other aspects, molecular dynamics simulation can also be used to study molecular movements and microstructure changes of epoxy modified asphalt in the process of healing [21]. [PERSON] et al. [22], conducted AFM experiments and MD simulation to study the adhesion between asphalt and aggregate. The mechanical properties of asphalt and aggregate can be further investigated by MD simulation at the nano-scale [23], and by investigating mechanical properties of the asphalt-aggregate interface [24].  
Although there has been significant progress in asphalt material microstructure and micro-properties, research on the fundamental mechanism of asphalt microstructure cracking using simulations and experiment approaches are very few. In this study, MD simulation was conducted to study micro-crack initiation, and propagation of typical asphalt molecular structures. The ultimate goal was to try to understand the fundamental mechanism of asphalt molecular structure cracking.