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I’ve written repeatedly on this blog about nature’s
masons. Nature is the ultimate inspiration for design; evolution showcases many masonry techniques.
Mollusks have recently been investigated by two researchers
at MIT, graduate student Ling Li and Professor Christine Ortiz. Their research findings were published in the
journal ‘Nature Materials’ (March, 2014) and focused on the mollusk Placuna
placenta.
This mollusk’s shell exhibits very tough qualities (resistant
to crack propagation) while simultaneously remaining optically
transparent. When subject to extreme
focused stress -such as may be encountered by its predators- the calcite
material of Placuna placenta’s shell demonstrated very efficient energy
dissipation and the ability to localize deformation, limiting damage to the
area directly impacted and preventing crack propagation.
The mollusk’s shell is comprised of around 99% calcite and
around 1% organic material which bind the calcite crystals together. This is somewhat similar to the sharp defensive
spikes found in sea urchins (as discussed here) which are also made primarily
of calcite with small amount of organic binder material present. Pure calcite (without organic binder) is a
brittle crystalline material which easily cracks.
The mechanism wherein the type of deformation in Placuna
placenta shell occurs was studied by using an indentation apparatus consisting
of a diamond tip which is forced into the mollusk shell. The resulting damage to the indent region was
then visually recorded using electron microscopy and diffraction techniques to
characterize the resulting damage.
This research cleverly showed that the deformation (or
strain) of the mollusk shell was a crystallographic ‘twinning’ response to the
applied stress. Crystal twinning occurs when
two separate crystals share some of the same crystal lattice points in a symmetrical manner. The
result is an intergrowth of two separate crystals in a variety of specific
configurations. A twin boundary or composition surface separates the two crystals.
Part of the crystal shifts its position in
a predictable way, leaving two regions with the same orientation as before, but
with one portion shifted relative to the other. This twinning process occurs
all around the stressed region, helping to form a kind of boundary that keeps
the damage from spreading outward (preventing crack propagation).
This twinning mechanism provides for conjugate shearing. The conjugate shearing mechanism has significance in terms of a toughened structure and is better than a conventional masonry arch structural response to
an applied stress of voussoirs forming hinges.
Conjugate shearing was initially employed by geologists as a
term to describe shear fractures in rocks subject to compressive stress. The context and scale of this geologic
feature have kept it from being analyzed, utilized or realized in the context
of microscopic analysis or in the context of masonry design and modular
structural systems. Similarly, it is apparent that biologists and
engineers have failed to fully appreciate the conjugate shearing mechanism
demonstrated by the Placuna placenta’s calcite shell structure in response to
applied stresses such as the indentation tests done by researchers at MIT.
The force required to cause conjugate shearing to occur (in
an architectural arch or in a mollusk shell) is much higher than the force required
to create a hinging mechanism as occurs in a conventional masonry arch
comprised of wedge-shaped voussoirs. For
example, Thor’s hero shrew’s spine is configured in such a manner that it is
disposed to conjugate shearing instead of creating a hinging mechanism which
leads to buckling and collapse of the spine. An adult human can stand upon and be supported
by the tiny Thor’s hero shrew’s spine without breaking the poor animal’s
back. Conversely, a common shrew does
not have the interlocking triangular design of Thor’s hero shrew’s vertebrae;
its spine would buckle and collapse in the hinging mechanism of a conventional
masonry arch if an adult human stood on top of it: the back would simply and
easily be broken (poor regular shrew).
The calcite shell of Placuna placenta and its unique crystallographic
twinning response to applied stress is another of Nature’s exemplars of
exquisite design which incorporates the structural response of conjugate
shearing to create a toughened structure which will blunt and stop crack propagation
in an otherwise brittle material.
