Hierarchical Interfaces as Fracture Propagation Traps in Natural Layered Composites

Abstract

Compared with their monolithic version, layered structures are known to be beneficial in the design of materials, especially ceramics, providing enhanced fracture toughness, mechanical strength, and overall reliability. This was proposed in recent decades and extensively studied in the engineering literature. The source of the property enhancement is the ability of layered structures to deflect and often arrest propagating cracks along internal interfaces between layers. Similar crack-stopping abilities are found in nature for a broad range of fibrillary layered biological structures. Such abilities are largely governed by complex architectural design solutions and geometries, which all appear to involve the presence of various types of internal interfaces at different structural levels. The simultaneous occurrence at several scales of different types of interfaces, designated here as hierarchical interfaces, within judiciously designed layered composite materials, is a powerful approach that constrains cracks to bifurcate and stop. This is concisely described here using selected biological examples, potentially serving as inspiration for alternative designs of engineering composites.

Crack deflection in laminates with graded stiffness—lessons from biology

“…To better understand the difference between these structural types, we observe two examples from nature: (i) a fibrous laminate, such as that of the scorpion cuticle [15,16], which consists of soft matrix reinforced by strong chitin fibers. Such laminates have a hierarchical structure ranging from nanoscale to microscale, with matching hierarchical interfaces, and both their layer thickness and modulus vary with their location in the laminate [1]. Crack deflection can occur in any of the interfaces, even between two neighboring fibers.…”

“…The scorpion chela's endocuticle (the inner part of the cuticle) is composed of interleaved parallel Bouligand layers and interlayers (figure 5(a), see section 5). The Bouligand layers thickness ranges from about 2.5 µm at the inner side to about 7 µm at the outer side (figure 5(b)) [1,15,16]. The Bouligand consists of progressively twisted and tilted laminae of unidirectional chitin-protein fibers, embedded in a proteinaceous matrix [15].…”

“…The Bouligand layers are separated by thin interlayers of unidirectional chitin fibers oriented along the y axis, embedded in a proteinaceous matrix. The interlayers have variable thickness as well, ranging from about 1.14 µm at the inner side of the endocuticle to about 2.08 µm at the outer side (figure 5(b)) [1,15,16]. The interlayers moduli calculated by laminate analysis, assuming perfect fiber alignment with the y axis, are 41 GPa in the y direction (chitin fibers direction) and 3.3 GPa in the x direction [15].…”

“…For example, a crack starting at the surface of a structure and propagating deeply into it, can be forced to deflect perpendicularly in order to avoid catastrophic failure. In composite laminates, a deflected surface crack causes delamination between laminae, absorbing energy by deforming and tearing the interlaminar adhesive [1].…”

“…Nature excels in optimizing structures for resilience and self-healing, given the limited available resources and the high costs involved in building structural tissues. Our recent communication reviews the various architectural strategies used in nature, most notably layered composite structures and complex geometries, and introduces the notion of hierarchical interfaces [1]. Elaborate micro-structural mechanisms are implemented in biology to combine strength and toughness, often conflicting properties, and may inspire engineering design [2].…”