Bitumen fatigue resistance is critical to determine the overall fatigue performance and service life of asphalt pavements. However, the mechanisms responsible for fatigue damage of bitumen have previously not been well understood. Molecular dynamics (MD) simulation has recently emerged as a powerful computer-aided numerical technique to model the microscopic failure behaviours in materials. This study aims to use the MD method to investigate the molecular origin of bitumen fatigue damage. The molecular models of the virgin and aged PEN70/100 bitumen were firstly constructed based on their saturate, aromatic, resin and asphaltene (SARA) four fractions. An MD equilibrium was run on the developed bitumen models with the assigned interatomic potentials. Following an MD-based tensile simulation, a strain-controlled fatigue simulation was performed to study the nanostructure and damage behaviours of the virgin and aged bitumen under fatigue loading by calculating the stress-strain response, potential energy, molecular structure and nanovoid volumes. Furthermore, a rheometer measurement was also conducted to characterise the fatigue damage of the bitumen directly by a crack length at the macroscale. Results indicate that the bitumen molecules become unfolded and tend to align along the loading direction when fatigue loading was applied. The change in the molecular configuration helped the molecular chains move closer together and thus contributed to the reduction of the intermolecular interactions including the van der Waals and Coulombic energies. With the increasing load cycles, nanovoids were formed and grew in the bitumen through molecular rearrangement and movement, leading to microscopic fatigue damage of the bitumen. It was found that the aged bitumen produced more severe fatigue damage than the virgin bitumen, which was indicated by the MD-based nanovoid volume at the molecular scale and the DSR-based crack length at the macroscale. The findings from MD simulation provide a fundamental understanding of the molecular origin of fatigue damage, that cannot be experimentally detected for bitumen materials.

中文翻译：

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基于分子模拟和流变测量的低周应变控制疲劳载荷下沥青的纳米结构和损伤表征

沥青抗疲劳性能对于确定沥青路面的整体疲劳性能和使用寿命至关重要。然而，此前人们对沥青疲劳损伤的机制尚不清楚。分子动力学 (MD) 模拟最近已成为一种强大的计算机辅助数值技术，用于模拟材料的微观失效行为。本研究旨在利用MD方法研究沥青疲劳损伤的分子起源。首先根据饱和沥青、芳香烃、树脂和沥青质 (SARA) 四个组分构建了原始和老化 PEN70/100 沥青的分子模型。在具有指定原子间势的开发的沥青模型上运行 MD 平衡。在基于 MD 的拉伸模拟之后，进行应变控制疲劳模拟，通过计算应力应变响应、势能、分子结构和纳米空隙体积，研究疲劳载荷下原始和老化沥青的纳米结构和损伤行为。此外，还进行了流变仪测量，以直接通过宏观尺度的裂纹长度来表征沥青的疲劳损伤。结果表明，当施加疲劳载荷时，沥青分子展开并倾向于沿载荷方向排列。分子构型的变化有助于分子链靠得更近，从而有助于减少分子间相互作用，包括范德华能和库仑能。随着载荷循环的增加，通过分子重排和运动，纳米空隙在沥青中形成和生长，导致沥青的微观疲劳损伤。研究发现，老化沥青比原始沥青产生更严重的疲劳损伤，这可以通过分子尺度上基于 MD 的纳米空隙体积和宏观尺度上基于 DSR 的裂纹长度来表明。MD 模拟的结果提供了对疲劳损伤分子起源的基本了解，而沥青材料无法通过实验检测到这一点。