Characterization of visco-hyperelastic behavior of open cell polyurethane foam through transient shear testing

Soft materials find ever-growing use in load-bearing applications, and hence, there is an imminent need for accurate testing and modeling of their mechanical properties. Nonlinearities and rate-dependence are inherent in the mechanical response of such materials, thanks to their multiphasic flexible constituents. Fragility of soft materials and rapid changes in their constituents depending on the environment add further challenges in mechanical characterization. This study introduces an efficient methodology to measure nonlinear viscoelastic properties of soft materials from transient shear response and validates the methodology experimentally on flexible polyurethane foams. Direct estimation of friction forces due to the Poynting effect and independent uniaxial compression tests of the same foam are also documented, validating the characterization framework. Both methods deliver comparable results with the transient-shear-based framework. Simple sample preparation and installation further complements the reliable and quick estimation capability, and thus offers a suitable characterization methodology for fragile and time-sensitive samples such as biological tissues. While validating the proposed framework on foams, fast relaxations and sensitivity to peak strains and strain rates are observed. Buckling and failure of the foam in the vicinity of the inertial plate could be possible explanations of those observations. Besides estimating the visco-hyperelastic properties of the foams, the tester dissipation stemming from material and frictional damping is also estimated in the framework. Increases in frictional dissipation due to compromised lubrication in guide rails over a period of three days are observed to shorten transients. Proper lubrication and control of friction before each test suppresses stochastic dry friction and improves repeatability and accuracy of the material characterization methodology.