Analysis of the mechanical properties of polymer materials considering lateral confinement effects

The mechanical properties of polyurethane foam (PUF) and glass fiber-reinforced PUF (R-PUF), which are widely used in various fields, were investigated considering the influence of lateral confinement. PUF and R-PUF – representative polymeric foams with a porous structure – are suitable for severe environments (e.g. the cryogenic field) because they have an excellent adiabatic function and load-bearing capability. However, when PUF and R-PUF are used in actual structures, the mechanical performance of the foam materials may change depending on the conditions. Assuming that a compressive load acts on these polymeric foams in a confined space, analysis of the material properties resulting from the confining pressure of the adjacent structures is necessary. Therefore, in this study, a comparison and evaluation of the mechanical behavior of PUF and R-PUF were performed with regard to the constraint. The results suggest that it is possible to provide an experimental basis for measuring the strength performance for the application of insulation in a confined space.

Image Analysis and the Characterization of Cellular Plastics

An evaluation of sprayed polyurethane foams manufactured with alternative blowing agents is being performed under a joint research program be tween the Society of the Plastics Industry of Canada and the Institute for Research in Construction. Image analysis, or the computer-aided quantification of the cell structure of the foams is a part of this evaluation.

Samples of foams prepared using the same polymer with different blowing agents were examined under a low-power microscope. The blowing agents used in prepar ing the foam samples were: CFC-11, CFC-11 + 1% water, HCFC-123, HCFC- 141b, and a blend of the two HCFCs with 0.5% water. With the aid of a computer, histograms were obtained of the distribution of areas and of aspect ratios for win dows undamaged in the sectioning process as well as for cells that were opened during the sectioning of the foams. A comparison of these histograms allows one to differentiate the foams. In addition, reasonable estimates of average cell size can be made from measurements of cross-sectional areas of cell windows and of opened cells. Thickness of struts, i.e., the edges where two cell windows intersect, were esti mated by the use of a thinning technique on strut images, and estimates of cell wall thickness were made using an optical interference technique.

On the Linear Elastic Properties of Regular and Random Open-Cell Foam Models

Foams can be created from coagulation of gas bubbles in liquid. After removal of cell faces, an open-cell foam remains consisting of a strut framework. In the past, mechanical properties were estimated by a small unit cell consisting of only a few struts. However, the random geometry of the foam can be of importance for the linear elastic properties. Here, large foam unit cells are created using Voronoi techniques. A smooth transition from regular to random geometries is made, showing the strong sensitivity of the mechanical properties from the geometry of the microstructure. Uniaxial global loads are transmitted through chains of highly loaded struts. The deformation of the struts in the foam is a mixture of bending and normal deformation, the ratio of which shown here to be dependent on the magnitude of the density.

Influence of cell shape on mechanical properties of Ti-6Al-4V meshes fabricated by electron beam melting method

Ti-6Al-4V reticulated meshes with different elements (cubic, G7 and rhombic dodecahedron) in Materialise software were fabricated by additive manufacturing using the electron beam melting (EBM) method, and the effects of cell shape on the mechanical properties of these samples were studied. The results showed that these cellular structures with porosities of 88-58% had compressive strength and elastic modulus in the range 10-300MPa and 0.5-15GPa, respectively. The compressive strength and deformation behavior of these meshes were determined by the coupling of the buckling and bending deformation of struts. Meshes that were dominated by buckling deformation showed relatively high collapse strength and were prone to exhibit brittle characteristics in their stress-strain curves. For meshes dominated by bending deformation, the elastic deformation corresponded well to the Gibson-Ashby model. By enhancing the effect of bending deformation, the stress-strain curve characteristics can change from brittle to ductile (the smooth plateau area). Therefore, Ti-6Al-4V cellular solids with high strength, low modulus and desirable deformation behavior could be fabricated through the cell shape design using the EBM technique.

Finite element modeling of compression behavior of extruded polystyrene foam using X-ray tomography

Extruded polystyrene rigid foams have attracted a great attention as a superior load-bearing thermal insulation material since their implementation in building construction. One of the most common application areas of this type of thermal insulation material is under raft foundations, where the foam normally undergoes high levels of compression loads. The purpose of this research is to determine how to simulate and optimize the structural response of extruded polystyrene under compression stresses. The optimization has been achieved through investigating the relation between the foam microstructure and the global mechanical properties. The foam was first examined using X-ray tomography imaging technique to acquire some morphological information about the microstructure. The obtained morphological data of extruded polystyrene boards were then utilized to develop microstructure-based finite element models based on the so-called idealized realistic Kelvin cell. The finite element simulation was accomplished with the help of the nanoindentation technology to explore the mechanical properties of the cell wall material. The finite element models were validated by comparisons between the simulated and the experimental results. It has been found that the mechanical properties of the foam in different loading directions can be adequately simulated using the approach of the idealized realistic Kelvin cell. With the help of these models, a parameter study was carried out. This study included the effect of cell size and cell anisotropy on the mechanical response of extruded polystyrene boards under compression stresses. Charts relating between the foam microstructure characteristics and the compression behavior were generated based on the parametric study.