Numerical Modeling and Experimental Behavior of Closed-Cell Aluminum Foam Fabricated by the Gas Blowing Method under Compressive Loading.

This paper deals with the experimental and numerical study of closed-cell aluminum-based foam under compressive loading. Experimental samples were produced by the gas blowing method. Foam samples had an average cell size of around 1 mm, with sizes in the range 0.5-5 mm, and foam density of 0.6 g/cm3. Foam samples were subjected to a uniaxial compression test, at a displacement rate of 0.001 mm/s. Load and stress were monitored as the functions of extension and strain, respectively. For numerical modeling, CT scan images of experimental samples were used to create a volume model. Solid 3D quadratic tetrahedron mesh with TETRA 10-node elements was applied, with isotropic material behavior. A nonlinear static test with an elasto-plastic model was used in the numerical simulation, with von Mises criteria, and strain was kept below 10% by the software. Uniform compressive loading was set up over the top sample surface, in the y-axis direction only. Experimental tests showed that a 90 kN load produced complete failure of the sample, and three zones were exhibited: an elastic region, a rather uniform plateau region (around 23 MPa) and a densification region that started around 35 MPa. Yielding, or collapse stress, was achieved around 20 MPa. The densification region and a rapid rise in stress began at around 52% of sample deformation. The numerical model showed both compressive and tensile stresses within the complex stress field, indicating that shear also had a prominent role. Mainly compressive stresses were exhibited in the zones of the larger cells, whereas tensile stresses occurred in zones with an increased number of small cells and thin cell walls.

Development of a Hopkinson Bar Apparatus for Testing Soft Materials: Application to a Closed-Cell Aluminum Foam.

An increasing interest in lightweight metallic foams for automotive, aerospace, and other applications has been observed in recent years. This is mainly due to the weight reduction that can be achieved using foams and for their mechanical energy absorption and acoustic damping capabilities. An accurate knowledge of the mechanical behavior of these materials, especially under dynamic loadings, is thus necessary. Unfortunately, metal foams and in general "soft" materials exhibit a series of peculiarities that make difficult the adoption of standard testing techniques for their high strain-rate characterization. This paper presents an innovative apparatus, where high strain-rate tests of metal foams or other soft materials can be performed by exploiting the operating principle of the Hopkinson bar methods. Using the pre-stress method to generate directly a long compression pulse (compared with traditional SHPB), a displacement of about 20 mm can be applied to the specimen with a single propagating wave, suitable for evaluating the whole stress-strain curve of medium-sized cell foams (pores of about 1-2 mm). The potential of this testing rig is shown in the characterization of a closed-cell aluminum foam, where all the above features are amply demonstrated.

PCR-DGGE analysis of the bacterial composition of a kaolin slurry showing altered rheology.

Kaolin is an important industrial raw material and a basis of a range of different products. Microbial spoilage is a detrimental process observed especially in kaolin slurries, leading to low quality products and economic loss. Although the alteration of kaolin slurries in ceramic industry was observed, the process and the microbial background have not been analyzed in details. This study provides the first data using a cultivation independent molecular biological approach (PCR-DGGE) regarding the bacterial composition of an altered kaolin slurry. The results show that potential exopolymer (EPS) producer bacteria (e.g. Acinetobacter, Pseudomonas) appear in the altered kaolin slurry, which may have an important role in the modification of kaolin slurries.