16:45 〜 17:00
[SSS10-28] Failure Transition from Fracture to Plasticity in Nanoscale α-Quartz Asperity Collisions During Frictional Sliding
キーワード：Nanoscale asperities 、α-quartz、Molecular dynamics、Aspect ratio、Plasticity、Fracture
Fault surfaces consist of asperities spanning various scales, where the frictional interactions among them govern the initiation and propagation of rupture, ultimately influencing fault slip behavior and seismic energy release. Therefore, investigating the mechanical behavior of asperities during frictional contact is fundamental to understand the faulting mechanism and earthquake physics. In previous studies, macroscopically, significant progress has been made in understanding the frictional failure of asperities, where the failure is often assumed and observed to be dominated by brittle fracture (Wang et al., 2016). In contrast, studies at the nanoscale have also proposed that asperity failure mechanisms may be dominated by plastic deformation (Candela & Brodsky, 2016). However, recent studies in nanoscale or microscopic asperities of metal revealed a transition between plastic deformation and fracture (Hu et al., 2022). Therefore, these discrepancies and observations highlight the necessity of further investigating the failure type of rock asperities at small scales.
In this study, we investigated the failure mechanisms of asperity collisions using molecular dynamics simulations, focusing on a pair of nanoscale half-cosine-shaped 3D α-quartz asperities. The simulation setup mimics macroscopic friction experiments involving two sliders. Specifically, two identical asperities were positioned on substrate surfaces. In the boundary regions, the lower block was fixed, while the top of the upper block was constrained. Periodic boundary conditions were applied in the two horizontal directions. Building on previous macroscopic studies (Malekan, 2023), we explored the influence of aspect ratio (height-to-diameter ratio) on asperity failure modes, we considered four aspect ratios: 0.4, 0.6, 0.8, and 1.0. Additionally, we examined the size effect of asperities by varying their projected diameters across three scales: R = 4a, 8a, and 12a (a = 8.4903 Å). The sliding velocity was set to 20 m/s.
We found that the failure of nanoscale alpha-quartz asperities exhibited a transition from plasticity to fracture, influenced by both asperity size and aspect ratio. Smaller asperities primarily exhibit plastic failure, while larger asperities and higher aspect ratios promote fracture. These effects are primarily governed by the dynamic flow of contact stress and pressure-induced phase transitions as well as fracture toughness, which collectively influence the direction of the amorphization zone during dynamic contact. When the amorphization zone evolution is parallel to the contact surface, plastic deformation dominates. However, when the evolution of the amorphization zone forms a certain angle with the contact surface, it creates a weak zone inside the asperity that leads to the co-existence of crystalline phase. Thereby, the asperity fracture can occur when the fracture toughness is achieved. Moreover, we also found this failure transition can be largely mapped by the size and aspect ratio of asperities. These findings offer critical insights into the mechanisms controlling microscale gouge generation, contributing to a better understanding of earthquake energy dissipation, as well as experimental earthquake nucleation and propagation.
In this study, we investigated the failure mechanisms of asperity collisions using molecular dynamics simulations, focusing on a pair of nanoscale half-cosine-shaped 3D α-quartz asperities. The simulation setup mimics macroscopic friction experiments involving two sliders. Specifically, two identical asperities were positioned on substrate surfaces. In the boundary regions, the lower block was fixed, while the top of the upper block was constrained. Periodic boundary conditions were applied in the two horizontal directions. Building on previous macroscopic studies (Malekan, 2023), we explored the influence of aspect ratio (height-to-diameter ratio) on asperity failure modes, we considered four aspect ratios: 0.4, 0.6, 0.8, and 1.0. Additionally, we examined the size effect of asperities by varying their projected diameters across three scales: R = 4a, 8a, and 12a (a = 8.4903 Å). The sliding velocity was set to 20 m/s.
We found that the failure of nanoscale alpha-quartz asperities exhibited a transition from plasticity to fracture, influenced by both asperity size and aspect ratio. Smaller asperities primarily exhibit plastic failure, while larger asperities and higher aspect ratios promote fracture. These effects are primarily governed by the dynamic flow of contact stress and pressure-induced phase transitions as well as fracture toughness, which collectively influence the direction of the amorphization zone during dynamic contact. When the amorphization zone evolution is parallel to the contact surface, plastic deformation dominates. However, when the evolution of the amorphization zone forms a certain angle with the contact surface, it creates a weak zone inside the asperity that leads to the co-existence of crystalline phase. Thereby, the asperity fracture can occur when the fracture toughness is achieved. Moreover, we also found this failure transition can be largely mapped by the size and aspect ratio of asperities. These findings offer critical insights into the mechanisms controlling microscale gouge generation, contributing to a better understanding of earthquake energy dissipation, as well as experimental earthquake nucleation and propagation.