High damping and high stiffness (HDHS) viscoelastic materials, which exhibit desired energy‐dissipating capability and structural integrity, may be achieved by using negative‐stiffness inclusions, such as ferroelastic particles, in composite materials. However, stability has long been questioned for such composites to be realistic. Wang and Ko (pp. http://doi.wiley.com/10.1002/pssb.201384231) examine a two‐dimensional, two‐phase, negative‐stiffness viscoelastic composite under partial displacement loading by time–domain analysis with the finite element method. It was found that force responses may be stable or unstable, depending on the amount of negative stiffness. A small amount of negative stiffness in the viscoelastic composites may not deteriorate their stability, but is insufficient to trigger HDHS. Through examining the existing stability criteria of three‐dimensional composites, the softening anomaly in the overall viscoelastic stiffness may be in the stability range, indicating stable damping enhancement. However, simultaneously enhancing damping and stiffness violates ellipticity requirements, hence the system is unstable/metastable. The cover image shows stress distributions around the inclusion of the two‐dimensional composite in the background and stiffness/damping anomalies, as well as divergent force responses, deviating from expected sinusoidal responses.
Measurements of time-dependent material properties in the context of linear viscoelasticity, at a given frequency and temperature, require accurate determination of both loading and deformation that are subjected to the testing materials. A pendulum-type viscoelastic spectroscopy is developed to experimentally measure loss tangent and the magnitude of dynamic modulus of solid materials. The mechanical system of the device is based on the behavior of the cantilever beam, and torsion and pure bending moment are generated from the interaction between a permanent magnet and the Helmholtz coils. The strength of the magnetic interactions may be determined with a material with known mechanical properties, such as aluminum 6061T4 alloy. The sensitivity of the torque measurement is on the order of one micro N-m level. With the high accurate torque measurement and deformation detection from a laser-based displacement measurement system, viscoelastic properties of materials can be experimentally measured in different frequency regimes. Sinusoidal driving signals are adopted for measuring complex modulus in the sub-resonant regime, and dc bias driving for creep tests in the low frequency limit. At structural resonant frequencies, the full-width-at-half-maximum (FWHM) method or Lorentzian curve fitting method is adopted to extract material properties. The completion of determining material properties in the wide frequency spectrum may help to identify the deformation mechanisms of the material and to create better models for simulation work.
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