Kenneth W. White

Fracture toughness behavior of the YBa2Cu3O7-x superconducting ceramic with silver oxide additions

The influence of silver oxide additions on the fracture behavior of YBa2Cu3O7−x superconducting ceramic was studied. The KIC value has been found to increase from about 1 MPa√m to as much as 2.4 MPa√m with the blending of 30 wt.% Ag2O with the 1‐2‐3 powder. All compositions exhibited increasing resistance to fracture with crack extension. The addition of silver to the monolithic sintered structure not only raises the level of the KR curve, consistent with the rise in KIC, but also increases dKR/dΔa, indicative of an improvement in the cumulative toughening efficiency. Microstructural studies implicated crack face bridging by both the 1‐2‐3 grains, as well as the Ag component, as a major contribution to the observed improvement in toughening. The superconducting properties of these composites were not adversely affected by additions of up to 20% Ag2O.

FRACTURE PROCESS ZONE MODELING OF MONOLITHIC AL2O3

Abstract The crack tip displacement fields obtained from moire interferometry were used interactively to drive a finite element (FE) model of double cantilever beam (DCB), alumina specimens subjected to stable crack growth. Through this inverse analysis, the crack-closure stress (CCS) distribution which provided the best fit between the measured and computed crack-opening displacement (COD) along the fracture-process zones (FPZ) trailing the crack tip of two alumina microstructures were determined. The dissipated energy, which was obtained from this FE analysis, at the FPZ was found to be the major energy sink in the larger as well as the smaller grain-size alumina.

Influence of specimen geometry and relative crack size on the R-curve

Abstract The material property responsible for increasing crack growth resistance in monolithic ceramics is the relation between the bridging stresses in the wake of a crack and the actual crack face separation. By analytical and numerical computations, it is shown how crack resistance curves ( R -curves) are strongly influenced by test specimen geometry and the relative crack length. As a result of these considerations, it is concluded that the R -curve for small crack extensions may be considered a true material property. However, this is not true for large crack lengths where the influence of geometric data becomes important.

Fatigue degradation of the crack wake zone in monolithic alumina

Abstract This study examines the microstructural role of crack-face bridging mechanisms in a monolithic ceramic, subjected to cyclic loading conditions. By utilizing a unique post-fracture-tensile experiment, the fatigue properties of a commercially available alumina are examined. Based on the current results, one will conclude that a frictional wear mechanism is activated when the crack-bridging grains are permitted to slide in their sockets. When the peak loads and displacements are held below values which cause grain sliding, no fatigue damage is evident. Above the threshold values, however, notable damage may be observed. This cumulative wear process reduces the effectiveness of the bridging stress and lowers the toughness of the material. For the quasistatic bridging problem, accumulation of damage has been related to grain-size features, but apparent from data in this study, fatigue-related damage depends upon damage to sub-grain-size features.

Bridging stress relation from a combined evaluation of the R-curve and post-fracture tensile tests

Although the R-curve has provided a convenient description of the crack growth resistance behavior for ceramic materials, its shape depends not only on intrinsic material properties, but also on the selected specimen geometry. Therefore, it is of general interest to isolate the geometry-independent material-specific effects. In the case of alumina, bridging stresses in the wake of a crack are responsible for the observed R-curve. In the present study, a procedure is demonstrated which enables one to determine the bridging stress relation from the initial part of the R-curve and post-fracture tensile (PFT) tests. Based on the bridging stresses, the complete R-curve, including large crack extensions, can be computed. The calculated R-curve agrees very well with experimental results.

Strength of HVOF coating-substrate interfaces

Abstract A spherical indentation technique has been developed to determine the interfacial adhesion between thick film coatings and substrates. The threshold stress for interfacial delamination is found through a hybrid experimental–numerical approach. The threshold load for delamination is found experimentally, and goes together with the materials elastic–plastic descriptors and the coating thickness into a finite element model that calculates the corresponding threshold stress for delamination.

Crack growth in alumina at high temperature

Abstract The trailing fracture process zone associated with stable crack growth at room and elevated temperature in high density polycrystalline alumina was analyzed by a hybrid experimental-numerical procedure. The experimental and numerical procedures involved moire interferometry and finite element analyses, respectfully. The crack bridging stress and the dissipated energy in the fracture process zone were determined at room temperature, 600°C, 800°C and 1000°C. Both quantities decreased at elevated temperature due to grain boundary sliding.

Bridging stress determination by evaluation of the R-curve

A procedure is proposed which allows for bridging stress determination from a measured R-curve. An application of the method is illustrated for a magnesium aluminate spinel ceramic that exhibits an extended linear R-curve behavior. The analytical determination of small displacement behavior and the treatment for large displacements using cubic splines is outlined in detail. After an initial increase, bridging stress values are found to decrease for larger crack opening displacements.

Thermoelectric and mechanical properties on misch metal filled p-type skutterudites Mm0.9Fe4−xCoxSb12

Most of the recent work focused on improving the dimensionless figure-of-merit, ZT, of p-type skutterudites uses one or two fillers to tune the electrical and thermal properties. Considering the fact that the different fillers with varying atomic mass and ionic radii can vibrate with different amplitudes to scatter phonons of different mean free paths, we synthesized misch metal filled p-type skutterudites Mm0.9Fe4−xCoxSb12 (where Mm is La0.25Ce0.5Pr0.05Nd0.15Fe0.03, called misch metal). The samples were synthesized by hot pressing nano-powder made by ball milling the annealed ingot of Mm0.9Fe4−xCoxSb12 with varying concentration of cobalt, x. By tuning the Fe/Co ratio, we achieved a thermal conductivity of ∼2 W m−1 K−1 at room temperature and ∼2.3 W m−1 K−1 at about 530 °C and a power factor of ∼30 μW cm−1 K−2 at about 425 °C in Mm0.9Fe3.1Co0.9Sb12, leading to a peak ZT ∼1.1 at about 425 °C. The nano-indentation experiment reveals that hardness and elastic modulus of the material is about 4.2 GPa and 116...

Elastic constants determined by nanoindentation for p-type thermoelectric half-Heusler

This paper presents a study of the elastic properties of the p-type thermoelectric half-Heusler material, Hf0.44Zr0.44Ti0.12CoSb0.8Sn0.2, using nanoindentation. Large grain-sized polycrystalline specimens were fabricated for these measurements, providing sufficient indentation targets within single grains. Electron Backscatter Diffraction methods indexed the target grains for the correlation needed for our elastic analysis of individual single crystals for this cubic thermoelectric material. Elastic properties, including the Zener ratio and the Poisson ratio, obtained from the elasticity tensor are also reported.