Anelastic instability in composites with negative stiffness inclusions

Composites with VO2 particulate inclusions as a negative stiffness phase were fabricated through powder metallurgy. The composites are predicted to exhibit enhanced anelastic damping by virtue of the partially constrained negative stiffness of the inclusions in the vicinity of a ferroelastic phase transformation, and are predicted to become unstable for sufficiently high concentration (5 vol%) of inclusions. Composite specimens with 5 vol% inclusions studied in subresonant dynamic torsion displayed various manifestations of mechanical instability during cooling in a temperature range including the inclusion transformation temperature. Instability was manifested as macroscopic specimen undulations (slow thrashing) and fluctuation of the damping tan δ. Material instability occurs at high inclusion volume fraction in harmony with predictions from composite theory.