Contact Nonlinearity in Indenter–Foam Dampers

Abstract

In this paper, the nonlinear response of indenter-foam dampers is characterized. Those dampers consist of indenters pressed on open-cell foams swollen with wetting liquids. Recently, the authors identified the dominant mechanism of damping in those dampers as poro-viscoelastic (PVE) relaxations as in articular cartilage, one of nature's best solutions to vibration attenuation. Those previous works by the authors included dynamic mechanical analyses of the indenter-foam dampers under small vibrations, i.e., linear regime. The current study features the dynamic response of similar dampers under larger strains to investigate the nonlinear regime. In particular, the indenter-foam dampers tested in this paper consist of an open-cell polyurethane foam swollen with castor oil. Harmonic displacements are applied on the swollen and pre-compressed foam using a flat-ended cylindrical indenter. Measured forces and corresponding hysteresis (force-displacement) loops are then analyzed to quantify damping performance (via specific damping capacity) and nonlinearities (via harmonic ratio). The effects of strain and strain rates on the damping capacity and harmonic ratio are investigated experimentally. The dominant source of the non-linearity is identified as peeling at the indenter-foam interface (and quantified via peeling index). A representative model consisting of a linear viscoelastic foam and rate-dependent adhesive interface (slider element with limiting adhesive strength) explains the observed trends in peeling and thus nonlinear dynamic response. Possible remedies to suppress those nonlinearities in future designs of indenter-foam dampers are also discussed.

Characterization of drying-induced changes in moduli and internal stresses in a constrained gel using laser vibrometry

“…Our group also observed high adhesion when testing oil-imbibed open cell foams with indentation-based DMA. 23 One needs to suppress adhesion via surface treatment and/or lubrication in those techniques to re-use the samples. However, that would also trigger nonlinear interfacial mechanics such as slip and make the identification harder.…”

“…Vibration and wave-based characterization offers a viable alternative for soft materials. [20][21][22][23][24][25][26] For instance, laser vibrometry 21 is shown to characterize dynamic and viscoelastic properties of multiphasic biological tissues such as cartilage considerably faster than conventional techniques. 21 Studies utilized vibration techniques to understand parametric resonances in soft materials and water droplets; 27 develop variable stiffness artificial muscles for vibration absorption; 28 and achieve broadband vibration attenuation using soft materials.…”

Poroviscoelastic relaxations and rate-dependent adhesion in gelatin

“…A similar adhesion mechanism was proposed by Lai et al 16 where the amount of polymer chains adhering to the counter surface and thus pull-off forces increase with contact time. Moreover, adhesion measured on PVE materials depends on unloading rates as observed in cartilage, 15 insect feet, 17 oil-swollen foams, 18 and epithelial cells. 19 For explaining the rate-dependence, fracture and adhesion mechanics of viscoelastic solids has been promising.…”