Biomimetic Hard Materials - Engineered Biomimicry - page 66

stress-intensity factor and setting the condition
K I < K IC gives the soft-wrap condition for pre-
venting the fracture of tablets:

interfaces ( Figure 3.7 a) displays stiffness and
strength values comparable to those of mono-
lithic silicon carbide and toughness values four
times higher. Following the same idea of fabri-
cation of biomimetic materials at macro scales,
Mayer [39] developed a segmented nacre-like
composite made of almost 90 %v/v of alumina
tablets that are bonded with adhesive tapes
( Figure 3.7 b). This material shows toughness
six times higher than that of monolithic
alumina.
The effects of various arrangements of tablets,
different glues at the interface, and different min-
eral concentrations were researched by Mayer,
whose conclusions are summarized as follows:
(i) using weaker glues at the interfaces improves
the toughness of the structure, which agrees well
with Eq. (3.5) ; (ii) composites made of continu-
ous or segmented layers with 82 %v/v ceramic
undergo a catastrophic failure, whereas the com-
posite made of segmented layers with 89 %v/v
ceramic undergo significant deformation before
failure, as shown in Figure 3.7 b; and (iii) the
toughness amplification of these macroscale arti-
ficial nacres is a result of considerable crack
deflection observed when the material is loaded
in bending [39] .
Espinosa et al . [40] utilized a rapid prototyping
method to fabricate artificial nacres composed of
acrylonitrile butadiene styrene (ABS, a common
prototyping material) inclusions as the stiffer
phase and BGEBA (diglycidyl ether of bisphe-
nol-A) epoxy as the softer phase. The tablets
were made wavy by incorporating dovetails at
both edges in order to generate hardening in the
materials; see Figure 3.8 a. This study showed
that the incorporation of wavy inclusions results
in the spread of the deformation through the
whole material so that catastrophic failure is
prevented. The section labeled C-D in Figure
3.8 a shows that the presence of dovetail-like fea-
tures in the tablets results in interlocking and
hardening in the material.
Barthelat and Zhu utilized traditional
machining techniques to fabricate wavy tablets

K IC
τ S √ T

> 0. 58.

(3.8)

The soft-wrap condition also shows that
decreasing the thickness of the tablets improves
the fracture resistance of the structure. Rather
than improving the fracture resistance of the
tablets, reducing the size of building blocks also
increases the number of interfaces and therefore
increases the energy dissipation during loading
and unloading of organic material, improving
fracture resistance. These recent developments
in the understanding of design aspects of these
materials can greatly assist the optimization of
biomimetic hard and stiff materials.

3.3 BIOMIMETIC HARD
MAT ERIALS AT THE MACROSC ALE

3.3.1 Fabrication
The fabrication of nacre-like synthetic compos-
ites that duplicate its structure, mechanisms,
and properties has been and remains a formida-
ble challenge. To this day there is no fabrication
technology that can duplicate the highly regular
brick-and-mortar structure of nacre at the micro-
scale. One possible approach is to relax the con-
straints on small-length scales to fabricate
millimeter-size structures. Although the advan-
tage of small-length scales on the strength of the
inclusion is lost, working at larger scale means
that highly accurate fabrication techniques can
be used. The structure and composition of nacre
in term of organic/inorganic materials ratios
can be duplicated, and the resulting materials
show interesting mechanisms that are similar to
natural nacre.
Clegg et al . [38] showed that a laminated
composite material made of silicon-carbide
(20 mm × 20 mm × 200 μ m) sheets with graphite