Fractal cracking: An emergent failure mode of nacreous materials with microstructural randomness

Nacreous biological materials, such as mother-of-pearl and bone, exhibit emergent mechanical properties that surpass those of their individual constituents, largely due to their complex, hierarchical “brick-and-mortar” architectures. These structures are inherently non-uniform, exhibiting random fluctuations in both constituent properties and microstructural geometry. The impact of such microstructural randomness on the macroscopic mechanical behavior of nacreous materials remains an open question. In this study, we investigate the fracture behavior of nacreous structures with randomness in both constituent properties and geometry by employing a multiscale tension-shear-chain (TSC) network model. We reveal a complete causal chain accounting for how randomness gives rise to critical change in fracture modes. Initially, randomness triggers the emergence of complex crack morphologies, including independent crack nucleation, branching, and coalescence, which are captured by changes in fractal dimension. These morphological changes subsequently lead to variations in measurable fracture properties, such as the shape of the crack resistance curve, the intrinsic and extrinsic fracture toughness, and the scaling laws governing crack sensitivity. These changes in both morphology and property together indicate a key mechanistic transition: as microstructural randomness increases, fracture behavior gradually shifts from crack-tip-dominated fracture (localized failure) to failure in the bulk (dispersed failure), reflecting a fundamental change in the underlying fracture mechanism. These results demonstrate that intrinsic architectural randomness cannot be eliminated by statistical averaging; instead, it plays a decisive role in shaping fracture behavior. This insight establishes a new paradigm for designing bio-inspired materials with tailored flaw tolerance.