Isa Bar-On

Fracture toughness of advanced ceramics at room temperature

This report presents the results obtained by the five U.S. participating laboratories in the Versailles Advanced Materials and Standards (VAMAS) round-robin for fracture toughness of advanced ceramics. Three test methods were used: indentation fracture, indentation strength, and single-edge pre-cracked beam. Two materials were tested: a gas-pressure sintered silicon nitride and a zirconia toughened alumina. Consistent results were obtained with the latter two test methods. Interpretation of fracture toughness in the zirconia alumina composite was complicated by R-curve and environmentally-assisted crack growth phenomena.

A Performance Based, Multi-Process Cost Model for Solid Oxide Fuel Cells

A PERFORMANCE BASED, MULTIPROCESS COST MODEL FOR SOLID OXIDE FUEL CELLS

Stress intensity factor calculation for a modified round bend bar by 3-D finite element analysis

Stress intensity factors were calculated for a modified round bend bar (MRBB) using 3-D finite element analysis. The results were compared with published solutions for a rectangular and round bend bars. The stress intensity values for the MRBB are between those for the two other geometries as expected. The stress intensity solutions were non-dimensionalized utilizing limit solution for short and long cracks.

Quantitative Edge Cross-Section Angle Impact on Conductor Loss

This paper describes a method of forming different conductor edge cross-section angles by using thick-film screen printing on substrates of different surface roughness. The edge angle for each surface type is compared visually and measured using an interferometer microscope. The conductor edge angle is correlated to the respective transmission loss sample. The impact of different edge angles on conductor loss is observed.

The Effects of Residual Stress, Loading, and Crack Geometry on SEPB Fracture Toughness Test Results

Use of the single-edge precracked beam (SEPB) specimen with the “bridge indentation” method is one of the new ASTM standard test methods for the determination of fracture toughness of advanced ceramics. In support of this effort, the effects of several test parameters on the fracture toughness values were established. The effects of the residual stress due to the indent, the alignment in three- and four-point bending, and the crack front straightness on the fracture toughness values were determined using aluminum nitride specimens. The results show that indent loads should be kept below 100 N (10 kg), and the crack plane angle should be less than 5° in order to prevent artificially high fracture toughness values. Crack front straightness requirements can be less stringent than generally accepted.

Processing and Mechanical Properties of SiC-Metal Interpenetrating Composites

In this paper the authors demonstrate a liquid exchange process to introduce a ductile metal reinforcement phase in the amount of 10--30 vol. % into reaction-bonded silicon carbides (RBSCs). Immersion of RBSC in pure Al or Al-Si melts enables diffusional replacement of secondary phase silicon with metal. The Al and Al-Si exchanged composites show improvement in fracture toughness (single edge precracked beam technique) to 6--7 MPa{center_dot}m{sup 1/2} as compared to 3--4 MPa{center_dot}m{sup 1/2} in otherwise similar siliconized silicon carbide. Increased fracture strength (four point flexure) was also observed after the liquid exchange process.

Effect of Residual Stress on the J-R Curve of HY-100 Steel

Fabrication processes in structural components can result in intended or inadvertent material behavior changes due to plastic deformation and residual stresses. In this study, the effect of a residual stress field caused by prior plastic deformation of a beam in four-point bending on the J-R curve of 12.7 mm (0.5 in.) thick specimens of HY-100 steel was investigated. Specimen blanks were deformed in four-point bending to two different levels of deflection prior to machining the starter notch. The resulting residual stress field in a companion test specimen was measured on side surfaces and through-the-thickness by a multiple exposure X-ray diffraction technique. J-testing was performed in three-point bending on undeformed specimens and the two deformed specimen sets using a modified multiple-specimen technique. A decrease in J Q and tearing modulus, T, with increasing specimen deflection was observed. The decrease in J Q was comparable to that observed previously in uniformly prestrained specimens.

Modeling the Manufacturing Cost of Solid Oxide Fuel Cells

During the past years solid oxide fuel cells have been developed with improved power densities at reduced operating temperatures. These cells consist of additional layers beyond the traditional two electrodes and the electrolyte layer. Some of these layers are only several microns thin posing a manufacturing challenge for traditional high volume manufacturing processes like tape casting or screen printing. Our previously developed process based cost model has been updated to include the additional layers, to account for yield improvements for thinner layers, and to anticipate the effect of equipment and process development. These latter improvements are assumed to improve automation and process control. Our results indicate that for improved processes with high yields these cells can readily meet manufacturing cost targets.© 2008 ASME