The influence of viscosity on the response of open-cell polymeric foams in uniaxial compression: experiments and theoretical model

Abstract

In a preceding paper (Pampolini and Del Piero in J Mech Mater Struct 3:969-981, 2008), the response of blocks made of a polymeric open-cell foam subject to uniaxial compression was analyzed. The hysteretic behavior exhibited in cyclic tests was interpreted within the context of nonlinear elasticity, as an effect of strain localization due to the non-convexity of the energy. Here we study some inelastic aspects of the response, such as rate dependence, strength decay after repeated loading, and memory effects. The analysis of some key experiments led us to the conclusion that viscous effects prevail over plasticity and damage. Consequently, we propose here a visco-elastic model, obtained by adding linear visco-elastic elements to our previous chain of nonlinear elastic springs. The paper is completed by the description of a series of experiments and of numerical simulations.

Nonlinear elasticity, viscosity and damage in open-cell polymeric foams

“…The viscous properties of the foam play a significant role even in the response at small amplitude cyclic tests. In the work by Del Piero and Pampolini [3] small amplitude loading-unloading cycles with intermittent holding periods were carried out on open-cell polyurethane foam. The occurrence of stress relaxation effects corresponding to the holding periods confirmed the viscous behavior of the foam.…”

“…The second contribution of the present study is the construction of an identification process that combines qualitative analysis with optimization methods. The identification process here developed have permitted the numerical simulations carried out simultaneously with the present work in [3]. The validity of the model is evaluated by comparing numerical simulation results and experimental data for a set of complex loading cycles.…”

“…These configurations can be associated to different phases of the material [6]. Extensions of these models including the rate-dependent properties of foams have been presented in [3,27]. In particular, in the former work the foam was represented as a chain, each element of which consisted of a non-linear elastic spring connected in parallel to a linear visco-elastic element.…”

“…This interpretation is used here for open cell foams, and the stress softening is attributed to damage of the foam cell walls during first compression. The first contribution of the present study is to refine the rheological model proposed by Del Piero and Pampolini [3] by introducing damage. The non linear elastic springs are considered as damageable elements and extra damage variables and their work-conjugated forces are introduced.…”

“…The model based on the use of non linear elastic springs introduced in [24] is first briefly recalled. The viscoelastic elements developed by Del Piero and Pampolini [3] are added, and the damage is then included. Section 4 deals with the numerical implementation of the model, Section 5 describes the procedure developed to identify the material constants, and the numerical simulation results are discussed and compared with various experiments in Section 6.…”