Page 348 - Biomimetics : Biologically Inspired Technologies

Bar-Cohen : Biomimetics: Biologically Inspired Technologies DK3163_c012 Final Proof page 334 21.9.2005 11:55pm

334 Biomimetics: Biologically Inspired Technologies

Figure 12.29 (See color insert following page 302) Illustration of a sensor embedded in a composite braid.

commingles with the other reinforcing fibers of the material. However, fiber optic sensors do not
offer the diverse sensing potential of MEMS and other microsensors. More importantly, they do not
present a networking option within the structure itself, which is an essential ingredient of a truly
sensing composite, as will be discussed in greater detail below.
To address the issues associated with structural integrity, we envisage a multicomponent braid,
which consists of fibers for mechanical reinforcement, metallic (e.g., copper) wires for power and
communications, and polymer matrix material impregnated around periodically spaced sensor or
electronics packages, that can be integrated into the composite as a single fibrous braided element
(see Figure 12.29). The multicomponent braid also acts to isolate the sensor or electronics elements
within a protective environment that is commensurate with the composite structure as a whole.
Rather than acting as an inclusion, the sensor network is integrated directly into the fiber phase of the
composite. Braiding these sensors into and along with reinforcing fibers forms a protective casing
around the discontinuity that mitigates the flaws and related failure modes of embedded designs.
The resulting sensor braid is suitable for inclusion in the composite panel in one of two ways.
First, it may be directly laid into the composite layup, forming an integral vein within the composite
structure, the woven fiber sheets forming the basis of the material in the usual manner of laminated
composites. The spacing of these sensor braids can be such that they are isolated from each other
while providing the desired degree of sensing within the material. Alternatively, the braided
elements may be used in creating a woven fabric that commingles the braids with further reinfor-
cing fibers, similar to the woven fabric used in traditional laminated composites. Depending upon
the desired sensor density, the electronic sublattice braid can be included at appropriate spacing in
one or multiple directions as needed.
A critical issue to be addressed in forming a multicomponent braid with sensing elements is the
behavior of the sensor interconnects during normal composite processing. Both the high tempera-
ture and pressures can contribute to loss of interconnects during the processing; also, if not properly
managed, stresses can build during the thermal equilibration that can result in both interconnect
failure and incipient failure sites.
Three-dimensional braiding has been employed in the past as a method to integrate fiber
optic sensors into 3-D woven preforms of composites (El-Sherif and Ko, 1993). Our method,
however, uses two-dimensional tubular braiding to create continuous fibrous braids that protect
the sensor and wiring within the core of the braid. In our tubular braiding process, carriers containing
spools of reinforcing fiber weave in and out of each other in a radial pattern to form a protective
sheath around the sensor and wiring that feeds from a central carrier. As mentioned previously, these