Structure, Scaling, and Performance of Natural Micro- and Nanocomposites

Abstract

Natural materials boast remarkable mechanical performances in some cases unmatched by their synthetic counterparts, and for this reason, they have become an inspiration for the development of new materials. In highperformance natural materials such as nacre, bone, or teeth, stiffness and toughness are achieved with the staggered microstructure, where stiff inclusions of high aspect ratio are embedded in a softer matrix. While the modulus and strength of the staggered structure is well understood, fracture toughness and scaling remains unclear. In this work, a fracture model based on the fundamental micromechanics of the staggered structure is presented. The model captures crack bridging and process zone toughening, and explicitly shows how these toughening processes are the most efficient with high concentrations of small tablets of high aspect ratio. In particular, a desirable non-steady cracking regime can be achieved with specific requirements for structure and interface properties, which are presented in detail. These attractive toughening mechanisms are only possible if the tablets themselves do not fracture. The benefits of small size have been explored in the past, but here, we show for the first time how the effects of a stress singularity generated by the junctions between the tablets can be alleviated by the softer interfaces, provided that a "soft wrap" condition is met. The models provide new insights into the optimization and scaling of natural and biomimetic composites.

An improved failure criterion for biological and engineered staggered composites

“…Within the tablets, the stresses are more uniformly distributed for large overlap ratio L o /t, approaching a uniform distribution for L o /t ¼ þ1. These results are identical to those calculated in previous work using finite elements and cohesive elements to simulate the interface [22]. Predicting the failure of the inclusion in this case is difficult because of the stress singularities.…”

“…Typically, only the average of the tensile stress over the cross section of the inclusions is considered, and compared directly with the tensile strength of the material to predict failure [11,16,17,21,22]. This simplified criterion may lead to large errors because the stress profile across the inclusion is far from being uniform [23].…”

Discontinuous crack-bridging model for fracture toughness analysis of nacre

“…On the other hand, the yield strength of the aragonite platelet is measured by nanoindentation tests to be of the order of 1 GPa (Bruet et al, 2005;Barthelat et al, 2006). Under the concept of maximal utilization of the structural elements Fleischli et al, 2008), the normal stress in the platelet and its strength should be of the same order when there is no high stress concentration (Bekah et al, 2011). This indicates that K 0 I can be of the order of 1 MPa m 1/2 .…”

“…In general, the spatial arrangement and the aspect ratio of mineral platelets vary for the nacres of diverse mollusk species, which exhibit a span of mechanical properties (Fleischli et al, 2008;Rabiei et al, 2010). Some previous experimental efforts have also specifically addressed the effect of the aspect ratio of platelets (Jackson et al, 1988;Fleischli et al, 2008;Bekah et al, 2011). It is believed that the special composition and hierarchical microstructure of nacre, especially its deceptively simple stacking arrangement of hard and soft components, is of great importance to its excellent mechanical properties (Currey, 1977).…”

Increasing carbon fiber composite strength with a nanostructured “brick-and-mortar” interphase

“…33,34 The working assumption, based on existing models, is that nacre-like properties can be replicated independently of absolute scale, as long as the aspect ratio of the reinforcing platelets is maintained (at about 10) and the relative thickness of the soft organic layers remains about 5 to 10% of the platelet thickness. 35,36 The thickness of a typical polymer layer, defined by the dimensions of polymer molecules, is around 1-2 nm, and thus platelets should ideally be approximately 10 to 20 nm thick and about 100-200 nm wide. At this scale, the curvature of the fiber is insignificant, and hence a conformal layered structure can, in principle, be accommodated.…”