High Performance Composites with Active Stiffness Control

High performance carbon fiber reinforced
composites with controllable
stiffness could revolutionize the use of composite materials in structural
applications. Here we describe a structural material, which has a
stiffness that can be actively controlled on demand. Such a material
could have applications in morphing wings or deployable structures.
A carbon fiber reinforced–epoxy composite is described that
can undergo an 88% reduction in flexural stiffness at elevated temperatures
and fully recover when cooled, with no discernible damage or loss
in properties. Once the stiffness has been reduced, the required deformations
can be achieved at much lower actuation forces. For this proof-of-concept
study a thin polyacrylamide (PAAm) layer was electrocoated onto carbon
fibers that were then embedded into an epoxy matrix via resin infusion.
Heating the PAAm coating above its glass transition temperature caused
it to soften and allowed the fibers to slide within the matrix. To
produce the stiffness change the carbon fibers were used as resistance
heating elements by passing a current through them. When the PAAm
coating had softened, the ability of the interphase to transfer load
to the fibers was significantly reduced, greatly lowering the flexural
stiffness of the composite. By changing the moisture content in PAAm
fiber coating, the temperature at which the PAAm softens and the composites
undergo a reduction in stiffness can be tuned.