The mollusk, the arch and conjugate shearing

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.