We show that composite materials can exhibit a viscoelastic modulus (Young's modulus) that is far greater than that of either constituent. The modulus, but not the strength, of the composite was observed to be substantially greater than that of diamond. These composites contain bariumtitanate inclusions, which undergo a volume-change phase transformation if they are not constrained. In the composite, the inclusions are partially constrained by the surrounding metal matrix. The constraint stabilizes the negative bulk modulus (inverse compressibility) of the inclusions. This negative modulus arises from stored elastic energy in the inclusions, in contrast to periodic composite metamaterials that exhibit negative refraction by inertial resonant effects. Conventional composites with positive-stiffness constituents have aggregate properties bounded by a weighted average of constituent properties; their modulus cannot exceed that of the stiffest constituent.
Poisson's ratio, shear modulus, and damping of polycrystalline indium-tin (In-Sn) alloys in the vicinity of the morphotropic gamma(c)-gamma þ beta(b) phase boundary were measured with resonant ultrasound spectroscopy. Negative Poisson's ratios were observed from 24 C to 67 C for alloys near the phase boundary. Properties were unaffected by annealing at 100 C for 2 days. This isotropic fully dense negative Poisson's ratio material is temperature insensitive, in contrast to other materials that undergo phase transformation. V
Bulk properties of open cell polyurethane foam are studied in a hydrostatic compression experiment under strain control. A linear region of behaviour is observed in the stress-strain curve, followed by a non-monotonic region corresponding to a negative incremental bulk modulus. The bulk modulus in the linear region is in reasonable agreement with the value calculated from compressional Young's modulus and Poisson's ratio. The linear region of behaviour in hydrostatic compression corresponds to less than half the axial strain range observed in axial compression.
Composites with VO 2 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 . Material instability occurs at high inclusion volume fraction in harmony with predictions from composite theory.
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