Mechanical Behavior Of Foams Research Articles

Effect of strain rate and load orientation on cyclic response of anisotropic polyurethane foam

Anisotropic cellular materials, such as polymeric foams, play an important role in structures subjected to cyclic loadings. The present paper provides an experimental investigation of the mechanical behavior of an anisotropic polyurethane foam subjected to cyclic compressive loadings under two perpendicular orientations: the rising and perpendicular directions. The foam samples are loaded under three different strain rates and various deformations. The experimental results are presented in terms of elasticity modulus, maximal compressive stress, effective energy absorption capacity, and residual strain. It is proved that the investigated polyurethane foam presents a macroscopic mechanical anisotropy caused by microscopic cell elongation in the foaming direction. Moreover, it is demonstrated that the mechanical behavior of the foam is fully influenced by both deformation rates and imposed strains. The experimental stress–strain curves are modelized using an empirical model considering an adjustable modulus of elasticity. The analytical results show a good agreement with the experiments.

Mechanical behavior and microstructure of compressed Ti foams synthesized via freeze casting

Pure Ti and Ti–5%W foams were prepared via freeze casting. The porosity and grain size of both the materials were 32–33% and 15–17µm, respectively. The mechanical behavior of the foams was investigated by uniaxial compression up to a plastic strain of ~0.26. The Young׳s moduli of both foams were ~23GPa, which was in good agreement with the value expected from their porosity. The Young׳s moduli of the foams were similar to the elastic modulus of cortical bones, thereby eliminating the osteoporosis-causing stress-shielding effect. The addition of W increased the yield strength from ~196MPa to ~235MPa. The microstructure evolution in the grains during compression was studied using electron backscatter diffraction (EBSD) and X-ray line profile analysis (XLPA). After compression up to a plastic strain of ~0.26, the average dislocation densities increased to ~3.4×1014m−2 and ~5.9×1014m−2 in the Ti and Ti–W foams, respectively. The higher dislocation density in the Ti–W foam can be attributed to the pinning effect of the solute tungsten atoms on dislocations. The experimentally measured yield strength was in good agreement with the strength calculated from the dislocation density and porosity. This study demonstrated that the addition of W to Ti foam is beneficial for biomedical applications, because the compressive yield strength increased while its Young׳s modulus remained similar to that of cortical bones.

Study on Factors to Improve Comfort of Stab-Resistant Vests Taking into Account Wearing Pressure and Movement Restriction

To investigate the compressive efiects through microscopic features of ∞exible low-density open-cell PU foam during moulding process, FE simulation of deformation based on a regular dodecahedron cell unit with uniaxial compression behavior is undertaken to examine the mechanical properties of the vertical loading direction, and to facilitate the incorporation of solid PU material properties and variations in cell geometry. By the use of a factorial design, the mean strut thickness of solid struts has a positive non-linear efiect on the maximum reaction force as compared against cell size with a negative non-linear efiect. The Young’s modulus of solid polymers had a positive linear efiect on the response while the Poisson’s ratio had the least impact on the response. Development of a simplifled cell unit can model the behavior of open-cell PU foam under uniaxial compression which is an efiective way to understand the mechanical behavior of foam during moulding process.

Digital Image Correlation technique applied to mechanical characterisation of aluminium foam

In this paper the possibility to employ the Digital Image Correlation (DIC) technique for the mechanical behaviour analysis of metallic foam was investigated. Image Correlation allowed to measure displacement and strain fields on closed and open cells aluminium foam specimens under two different loading conditions (compression and shear) and to characterise, with opportune calculation, information on the mechanical behaviour of foams. The adopted technique is suitable to conduct a deep analysis and also to appreciate the local heterogeneities that appear in a specimen during test. The parameters obtained with DIC analysis are confirmed by the global data obtained by testing machine proving the adopted methodology represents a valid tool for the study of these new materials.

Investigation on the influence of cell shape anisotropy on the mechanical performance of closed cell aluminium foams using micro-computed tomography

The mechanical behaviour of closed-cell aluminium foams made by both powder metallurgy (LKR) and liquid state (Hydro) processes is investigated. Hydro foams exhibit a significant anisotropy in their mechanical behaviour. The transverse direction stands out as the most favourable one in terms of strength. In contrast, LKR foams show an almost isotropic compressive behaviour. Both foams perform at a level far below the theoretical predictions. The reduced values are a result of imperfections and defects in the cellular microstructure. X-ray microfocus computed tomography (μCT) is therefore used for internal investigation of the foam cell structure. 2D and 3D quantitative image analyses have been performed on μCT images to characterise the morphometric parameters of the foams. The main parameters of interest are cell size, cell size distribution and cell features information. A preferred cell orientation in Hydro foams is observed along the normal and the transverse directions of the specimen. This cell shape anisotropy is quantified using the dimensions of the three axes of the equivalent ellipsoids. The orientation of the cells is well characterised by pole figures of the three axes of equivalent ellipsoids. The influence of this geometrical anisotropy on the mechanical behaviour of the foam is discussed.

Taylor Averaging on Heterogeneous Foams

In this article we present a formulation to account for the effect of cell size distribution on the mechanical behavior of foams. The present approach averages the response of unit cells with different sizes via a Taylor averaging approach. This technique can be applied to different unit cell models where the cell size is explicitly given. In order to keep the averaging formulation analytically traceable, we select a unit cell model, bubble model (Ortiz, M., Gioia, G. and Cuitino,A.M. Manuscript in preparation), that considers the behavior of an homogeneous array of thin-walled spherical cells undergoing uniform finite deformation. This model relays on the computation of the strain energy density for the deformed configuration due to membrane stretching and bending. For this unit cell model, the effect of the cell size distribution is described only by the first three moments of the distribution, or the mean radius, variance and skeweness. The effect of these parameters on the foam behavior subjected to uniaxial compression is analyzed for two different values of the relative density of the foam. The model is compared against a unixial compression test of a ductile foam system with nearly spherical cells of different sizes. The cell size distribution is measured independenly and introduced to the model as a input. The prediction of load displacement response is in general good agreement with the experimental test.

Localized Impact on Sandwich Structures With Laminated Facings

Currently, there is strong interest in the effects of impact induced damage to composite structures as evidenced by the large number of articles on this topic that have appeared in the literature in recent years. With laminated composite structures, foreign object impacts, that are expected to occur during the life of the structure, can introduce damage that is often difficult to detect and can significantly reduce the strength of the structure. Research efforts concentrate on monolithic laminates while sandwich structures with laminated facings, which are also used extensively in aerospace and other applications, received less attention. A comprehensive review of the literature dealing with impact on sandwich structures is presented in this review article. The mechanics of contact between a rigid indentor and a sandwich structure is discussed in detail since it is very important to account for the local deformation in the contact zone in developing a model for predicting the contact force history. The mechanical behavior of foam and honeycomb core materials is reviewed along with experimental results and models for predicting the contact law between a smooth indentor and a sandwich structure. The development of mathematical models for predicting the contact force history and the overall response of the structure is also discussed. Finally we review the failure mechanisms involved in impact damage development, the parameters affecting damage size, and the residual properties of sandwich structures. This review article has 85 references.

The Mechanical Behavior of Foams and Emulsions

Using the simulation program of Kermode and Weaire, we have calculated stress‐strain curves for a number of samples of two‐dimensional foam, under extensional shear (planar extension). The elastic constant takes a value close to that of the ideal hexagonal foam (with the same mean cell area) for all the samples studied. Similarly the plastic behavior shows only slight variation among reasonable disordered structures. Stress increases monotonically with strain, with an asymptotic yield stress of the same order as the values calculated by Kermode and Weaire for the hexagonal structure. The stress‐strain relationship is therefore roughly represented by a hyperbolic tangent function.

Microwave dielectric and mechanical behavior of foams from polystyrene and poly(methyl methacrylate) blends

AbstractFoams have been generated from blends of polystyrene and poly(methyl methacrylate) under identical conditions of solvent–nonsolvent ratio, impregnation time, and heating period. Scanning electron micrographs of copper‐coated fractured surfaces of the samples reveal a distinct transition in their structures from a more or less homogeneous cell‐size distribution with prominent grain boundaries to complete lack of foaming and cellularity through random dispersions of spherical poly(methyl methacrylate) inclusions in polystyrene‐foam matrix. Dielectric measurements have been made on test specimens at 9.375 GHz X‐band microwave frequency by the method of Smith and Hippel modified by Dakin and Works. Percent porosity is the least at 20 wt% of polystyrene while foam density linearly decreases with percent porosity. Dielectric constant is linear in foam density on direct and semilog scales (passing through unity and zero, respectively), in blend composition and also in percent porosity. Dielectric constant increases with increase of foam density or weight percent of polymethyl‐methacrylate and decreases with increase of percent porosity. The dielectric constant is, however, nonlinear in volume fraction of polystyrene on both direct and semilog scales, obeying Weiner's inequalities. The logarithmic law of Lichtenecker and Rother with an empirical factor 0.8276 in the index fits the data best. Specific polarization is minimal at 60 wt% of polymethyl methacrylate. The calculated Bottcher‐Bruggleman plot based on the interacting spherical particle model is found to be of limited applicability. This is explained on the basis of long‐range intra‐ and intermolecular dipole–dipole interactions. The pattern of the change of dielectric loss tangent (tan δ), lying in the range 0.150–0.045 and attaining a minimum at 20 wt% polystyrene, has similarly been explained in terms of α and β relaxations due to conformational rearrangements and steric hindrances of rotations. The compressive strength measured as a function of composition shows that the reinforcement depends on an optimization between the degree of cellularity and packing.