Bioinspired Compliance Grading Motif of Mortar in Nacreous Materials

The impressive toughness and strength of natural nacre, attributed to its multi-scale and -material hierarchical architecture, has inspired biomimicry and bioinspired materials development, and here we show that material compliance gradients are a motif that can help explain their advantaged mechanical performance. We present experiments enabled via additive manufacturing that allow direct evaluation of a compliance grading motif of the mortar between the relatively stiff bricks of the nacreous material. Spatial grading of the mortar compliance redistributes stresses away from critical regions (at, and around, brick corners), resulting in overall increases of ∼60% in strength, ∼ 70% in toughness, and ∼30% in strain-to-break, while maintaining macroscopic stiffness. Mechanistically, failure initiation threshold is delayed due to enhanced strain-tolerance and strain-localization as revealed in prefailure experimental strain maps, and in agreement with numerical analyses. We further demonstrate that this modulus grading motif, beyond the stiffness mismatch between the brick and mortar periodic architecture, is a significant contributor to the performance of the much-studied nacreous systems and is suggested as a natural but overlooked mechanism in such systems.