Atomic insights into shock-induced spalling of polyurea by molecular dynamics simulation

KL Yao and ZL Liu and Z Zhuang, EXTREME MECHANICS LETTERS, 55, 101805 (2022).

DOI: 10.1016/j.eml.2022.101805

Some block-copolymer elastomers are regarded as competitors to metal for lightweight impact protection design. However, the microscopic spalling mechanism of polymer is still unclear owing to the complex macromolecule structure. Molecular dynamics (MD) can overcome the drawback in the experiment and reveal the nature of dynamic damage at the microscale. In this work, the spalling of polyurea under different impact velocity is analyzed using MD method and coarse-grained molecular dynamics model. The simulation results show that spalling failure originates from the nucleation and development of void-damage. The flow stress has a plateau in the early and middle stages of void flow. The Hugoniot relation is obtained. Furthermore, the free surface velocity is calculated, which shows the typical characteristics observed in the experiment. The spalling strength is calculated by the direct and indirect methods, respectively. The non-monotonic variation of spalling strength with impact velocity is the result of the competition between the strain-rate strengthening and temperature weakening. The molecular mechanism is further analyzed, which indicates that the molecules are oriented during void flow, and the hard segments are more rigid and hysteretic. Besides, microphase-separated block copolymers have a special fibril toughening mechanism. The relationship between the flow stress and deformation of a fibril is quantitatively analyzed. Finally, the unit cell simulation, which represents one point in macroscopic, is compared with the spalling simulation. (c) 2022 Elsevier Ltd. All rights reserved.

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