No abstract

No abstract

This study demonstrates a new and sustainable methodology for recycling continuous carbon fibers from end-of-life thermoset composite parts using Joule heating. This process addresses the longstanding challenge of efficiently recovering carbon fibers from composite scrap and reusing them to make fresh composites. The conductive carbon fibers volumetrically heat up when an electric current is passed through them, which in turn rapidly heats up the surrounding matrix sufficiently to degrade it. Fibers can be easily separated from the degraded matrix after the direct current (DC) heating process. Fibers reclaimed using this method were characterized to determine their tensile properties and surface chemistry, and compared against both as-received fibers and fibers recycled using conventional oven pyrolysis. The DC-and oven-recycled fibers yielded similar elastic modulus when compared against asreceived fibers; however, an around 10-15 % drop was observed in the tensile strength of fibers recycled using either method. Surface characterization showed that DC-recycled fibers and asreceived fibers had similar types of functional groups. To demonstrate the reusability of recycled fibers, composites were fabricated by impregnation with epoxy resin and curing. The mechanical properties of these recycled carbon fiber composites (rCFRCs) were compared against conventional recycling methods, and similar modulus and tensile strength values were obtained. This study establishes DC heating as a scalable out-ofoven approach for recycling carbon fibers.

Nanoscale reinforcements have the potential to improve mechanical properties of fiber reinforced composite. Here, effect of nanofiller morphology and dispersion in augmenting mode I fracture toughness of unidirectional carbon fiber reinforced composite materials is studied. The nanofillers used is electrospun carbon nanofibers (CNFs). Unlike most nanofillers which are in particulate form, CNFs exist in both continuous nanofiber mat and particulate forms. This trait allowed us to compare the effect of particulate nanofillers (CNFs dispersed in B-staged epoxy) vs. dry mats on fracture toughness of composites while all other parameters are kept constant. To enhance CNFs-matrix interactions, a novel approach was utilized to functionalize CNFs surface with melamine, so that epoxy functional groups can form strong bonds to matrix. The improvement in mode I initiation fracture toughness with CNF mats was statistically significant, while in B-staged samples, statistical analysis revealed insignificant improvement. In addition, in both CNFs reinforced samples, crack propagation fracture toughness decreased with crack growth and approached that of the composites with no CNFs. The decline was steeper in samples with B-staged CNFs. This behavior was explained by evaluating fracture path via SEM imaging. It was concluded that while CNFs bridge crack tip initially and delay crack initiation, crack deflects towards a lower resistance path by tearing CNFs mat and propagating along resin-rich interface between CNFs and microfibers. These alternative and weaker fracture planes are more readily available in B-staged samples due to poor integration of the B-staged epoxy with the rest of the composite.

Humidity-responsive soft actuators can be driven by external stimuli and provide biomimetic environmental adaptations. Here, we report a humidity-responsive axial soft actuator of sulfonated polyether ether ketone which shows greatly tailorable actuation performance upon embedding graphene nanoplatelets (GNPs). Analysis of the experimental data shows that adding only 0.5 wt % GNP increases the actuation by 50% and provides a maximum actuation stroke of 24% and work capacity of 230 J/kg. In addition, 0.5 wt % GNP facilitates faster actuation, with significantly enhanced rates of both contraction and expansion. However, the addition of 1 wt % GNP slightly decreases the actuation magnitude. The nonmonotonic actuation performance was correlated with ion exchange capacity, water uptake, and GNP dispersion. By utilizing actuation magnitude tunability via GNP, the axial actuators were converted into a walking robot stacked of two active layers consisting of fibers of the same material system. The bilayer robot demonstrated self-crawling and locomotion ability in response to humidity changes. This study shows a uniquely tunable humidity actuator that demonstrates linear and bending actuation.