When investigating the physical nature of the macroscopic mechanical behavior of aerogels—such as compressive strength, brittle fracture, and elastic modulus—relying solely on macroscopic continuum mechanics often fails to reveal their complex microscopic failure mechanisms. With the rapid advancement of high-performance computing, molecular dynamics (MD) simulations and multiscale mechanical modeling have become pivotal tools for exploring the deformation and fracture toughness evolution of aerogel 3D nano-networks at atomic and molecular scales. By constructing virtual nano-skeletons of amorphous silica or polymer aerogels—comprising hundreds of thousands to millions of atoms—within a computational environment, scientists can "visualize" the processes of atomic-level chemical bond rupture and reorganization as the material undergoes mechanical loading.
Molecular dynamics simulations reveal that the macroscopic brittle fracture of aerogel networks stems from an atomic-scale "cascade effect of stress concentration." When an aerogel is subjected to external tensile or shear loads, stress is not distributed uniformly throughout the amorphous network; instead, it concentrates heavily at the "neck junctions" between nanoparticles and at the high-curvature corners of microscopic pore walls.
At these sites of microscopic stress concentration, local interatomic distances stretch, distorting the potential energy of the chemical bonds. When the applied load exceeds the critical bond energy threshold, the initial atomic-level chemical bond ruptures.
The rupture of a single chemical bond is not an isolated event; it instantly redistributes the mechanical stress previously borne by that bond to neighboring bonds, causing them to become "overloaded" and triggering a chain reaction of ruptures. This dynamic cascade propagation of microscopic bond ruptures rapidly coalesces into micro-cracks on a nanosecond timescale.
However, molecular dynamics simulations also confirm the positive role that the topological structure of the microscopic network plays in inhibiting crack propagation. When micro-cracks propagate and encounter regions characterized by spatial anisotropy or a high density of flexible cross-linking bonds, the crack tips undergo significant blunting and deflection. This forces the cracks to follow a tortuous path along the nanopore walls, thereby dissipating substantial fracture surface energy and markedly enhancing the material's effective fracture toughness. By integrating molecular dynamics with finite element multi-scale coupling algorithms, materials scientists can quantitatively predict how variations in cross-linking density and nanoparticle size distribution influence the fracture toughness of aerogels, providing atomic-level theoretical guidance for the design of novel, high-strength, and high-toughness aerogels.