## 4 Conclusions  
Crack prevention and control is a long-term research field of road engineering. A correct understanding of the internal crack propagation mechanism of asphalt pavement will help road workers to evaluate road working conditions more comprehensively, and make better decisions in design, construction, and maintenance. In this study, the FEM software ABAQUS was used to establish a three-dimensional asphalt pavement layered model. Fracture mechanics theory and the extended finite element method were used to investigate the expansion behavior of preset cracks in the pavement base layer, and the following conclusions were obtained. * A feasible three-dimensional pavement layered structure model was established, and numerical analysis was conducted using fracture mechanics theory and finite element software ABAQUS. The normal stress S33, shear stress S23, J-integral, and stress intensity factor K1 and K2 of the crack tip under different working conditions were calculated. Due to the existence of cracks, the difference between the positive and negative stresses between the base layer reached 2.209 \(\times\) 10\({}^{5}\) Pa, and the difference between shear stress reached 4.018 \(\times\) 10\({}^{4}\) Pa, indicating that the cracks showed a strong development trend both in the vertical and horizontal directions. The mechanical response of the crack tip is approximately a straight line when the vehicle load changes, and it is approximately an inverse proportional function when the modulus of each layer change. The R values of the fitted curves all reached over 0.99. In addition, the mechanical response of the stress and the stress intensity factor in some particular areas is high-order polynomial, generally second or third. * Cracks will not strictly follow the preset path but will deviate from the preset vertical direction due to various factors. Therefore, in the actual monitoring of crack behavior, the normal stress and shear stress changes in the pavement structure should be monitored simultaneously in a larger area, and should not be limited to the crack tip. * The load has a significant impact on the stress in the crack area. Under other conditions, the value of the stress change in the crack area is much smaller than under the load. In the process of increasing the load by 2.8 times, the principal stress increased by about 181.29%, and the shear stress increased by about 180.82%. In the process of increasing the modulus of each layer by 4500 MPa, the maximum reduction of normal stress is 11.93% of the surface layer, and the maximum shear stress is 34.13% of the ground layer. From a numerical point of view, the effect of increasing the modulus on crack propagation is relatively small compared with the effect of increasing load on cracking. Therefore, the vehicle load should be strictly limited to prevent road cracks. * The entropy method was used to analyze the primary and secondary effects of various indicators in the process of reflecting crack propagation in asphalt pavement. The most important factor is the modulus of each layer, and the modulus of the bottom surface layer is the largest, followed by vehicle load. This shows that it is feasible to increase the modulus of each layer, especially the modulus of the bottom surface layer, to suppress the development of reflection cracks, but the impact of increasing the modulus on other road performance should not be ignored. Conceptualization, J.Y., Y.W., H.W. ([PERSON]) and H.W. ([PERSON]); methodology, J.Y., Y.W., H.W. ([PERSON]) and H.W. ([PERSON]); validation, Y.W. and H.W. ([PERSON]); formal analysis, Y.W. and H.W. ([PERSON]); investigation, Y.W. and H.W. ([PERSON]); data curation, [PERSON] and [PERSON]. ([PERSON]). ; writing--original draft preparation, [PERSON] and [PERSON]. ([PERSON]); writing--review and editing, [PERSON], [PERSON], [PERSON] ([PERSON]) and [PERSON] ([PERSON]); visualization, [PERSON] and [PERSON]. ([PERSON]); supervision, [PERSON] and [PERSON]. ([PERSON]); project administration, [PERSON] and [PERSON].