Young's modulus of uniform density thermoplastic foam

A generalized Ogden model for the compressibility of rubber-like solids

The aim of this paper is to further demonstrate the advantages and effectiveness of the constitutive formulation proposed by Huang (Huang 2014 J. Appl. Mech. 59, 902-908 (doi:10.1115/1.2894059)). In this formulation, any strain-energy function for an incompressible material can be easily generalized to include the effect of material compressibility, in which only a few material parameters and material functions to be fitted with the experimental data are required. To this end, the Ogden model for incompressible rubber-like solids is chosen as the starting point. By means of this formulation, the generalized Ogden strain-energy function, which takes into account material compressibility, can conveniently be constructed so long as its incompressible counterpart is given. The obvious advantage shown in this paper is that only a few material parameters and material functions are needed, i.e. in addition to the material parameters used in the original Ogden model for incompressible solids, only one material function depending on the volume ratio is involved to characterize the effect of compressibility. Both the material parameters in the original Ogden model and the material function suggested in this paper can be determined by fitting the experimental data for uniaxially tensile test and hydrostatic deformation test of rubbers, respectively. The present model considering compressibility is general since it can be applied to predict the stress-strain responses of rubber-like materials and porous rubbers in various loading conditions. With the present formulation, the applicable range of the celebrated Ogden model can be further broadened, which should be of practical importance for accurately describing the mechanical behaviour of rubber-like solids. This article is part of the theme issue 'The Ogden model of rubber mechanics: Fifty years of impact on nonlinear elasticity'.

Flexural Properties of Mica Filled Polyurethane Foams

Integral skin foams consist of a cellular core and a near solid skin of the same material surrounding the core. A study of the effect of mica on the flexural properties of free rise and integral skin polyurethane foams is presented. Addition of a small amount of mica (5% v/v) is found to significantly increase the flexural modulus of free rise foams; however, the modulus remains nearly constant with further increase in mica concentration when compared at equal densities. The flexural strength of free rise foams is found to decrease with an increase in mica concentration. Similar trends with mica concentration are obtained for the integral skin foams of a fixed overall density. In this case, the properties of the foams are also dependent on structural parameters such as the skin and core thicknesses and the corresponding densities which are measured.

A three region model for the integral skin foam structure (core and two skins) is developed to obtain the flexural modulus and strength for foams with dissimilar upper and lower skins in terms of the thickness and the properties of the regions. Empirical equations, based on the experimental data for free rise foams with different mica loadings, relating the flexural modulus and flexural strength to the density, are used to calculate the properties of each region in the model. The predicted values obtained from the model are found to be in reasonable agreement with experimentally measured values of flexural modulus and flexural strength. The mode of failure (tensile rupture as opposed to wrinkling) is also correctly predicted by the model.

Theoretical evaluation of tensile and flexural moduli of structural polymer foams

In this paper, the tensile and flexural behaviors of structural polymer foams with integrated skin layer are studied. Several models are reviewed to show the dependence of Young’s modulus of cellular solids on void fraction. Effective flexural modulus of structural foam is computed based on the Euler–Bernoulli beam theory and results are compared with experimental data available for uniform density core. The effects of skin thickness, core relative density, and density transition profile on the elastic modulus are investigated for symmetrical structural foams. Lower and upper bounds of effective moduli are obtained to evaluate the role of structural foams in increasing specific mechanical strength of polymeric structures. The optimum thickness of solid skin is then estimated in terms of relative density of cellular core to get maximum specific tensile or flexural modulus. Density profile of structural foams with graded density core is also optimized to find the most possible value of specific flexural modulus. The present study is expected to provide some useful information for evaluation and designing of structural foams with the aim of material saving and improvement in mechanical properties.

Ultra Low Density and Highly Crosslinked Biocompatible Shape Memory Polyurethane Foams

We report the development of highly chemically crosslinked, ultra low density (~0.015 g/cc) polyurethane shape memory foams synthesized from symmetrical, low molecular weight and branched hydroxyl monomers. Sharp single glass transitions (Tg) customizable in the functional range of 45-70 °C were achieved. Thermomechanical testing confirmed shape memory behavior with 97-98% shape recovery over repeated cycles, a glassy storage modulus of 200-300 kPa and recovery stresses of 5-15 kPa. Shape holding tests under constrained storage above the Tg showed stable shape memory. A high volume expansion of up to 70 times was seen on actuation of these foams from a fully compressed state. Low in-vitro cell activation induced by the foam compared to controls demonstrates low acute bio-reactivity. We believe these porous polymeric scaffolds constitute an important class of novel smart biomaterials with multiple potential applications.

Prediction of the Shear Modulus of Polymer Structural Foams

In this work, a model is proposed to predict the shear modulus of structural polymer foams. The calculations include morphological parameters such as skin thickness, core density, and a transition zone of variable density between the skin and core sections. A comparison between several models is made with experimental data taken on polyethylene structural foams.

Tensile Modulus Prediction of Structural Foams using Density Profiles

In this study, the tensile moduli of LDPE structural foams are related to their density profiles measured by X-ray densitometry. A review of the most important mechanical models for cellular materials are presented and adapted to include the complete density profile. Using experimental data, predicted values with this new approach are much closer (4% deviation) when compared with other models for the range of parameters studied.