Composite densification was studied by performing slip casting and sintering experiments on an Al2O3 matrix and Si3N4 whisker system. Even though all the slip-cast powder compacts exhibited high green densities (up to...
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Composite densification was studied by performing slip casting and sintering experiments on an Al2O3 matrix and Si3N4 whisker system. Even though all the slip-cast powder compacts exhibited high green densities (up to 70% of the theoretical) and narrow pore-size distribution (pore radius around 15 to 30 nm), significant differential densification on a microscopic scale was found due to the existence of local whisker agglomeration. The inhomogeneous whisker distribution resulted in a binary mixture of large and small pores in the sintered composites, in which whisker-associated flaws remained stable even after prolonged sintering. The sintered microstructures showed that the spatial distribution as well as the volume fraction of the Si3N4 affect composite densification. Inhomogeneous whisker distribution dominated the complete densification of the composites.
Gruneisen's constant, gamma, is ideally temperature-independent on constant-volume lines;a similar constant, lambda, which is ideally temperature-independent on isobaric lines, has been found. The comparison betwe...
Gruneisen's constant, gamma, is ideally temperature-independent on constant-volume lines;a similar constant, lambda, which is ideally temperature-independent on isobaric lines, has been found. The comparison between gamma and lambda shows that both expressions contain the ratio of the volumetric thermal expansion coefficient to the specific heat capacity;gamma has an additional temperature dependence due to the thermal part of the compressibility, whereas lambda has only a slight thermal component due to the volume per unit mass. Lambda has been chosen so that lambda = gamma at the point where the temperature and pressure are both zero. For most materials it is expected that lambda greater-than-or-equal-to gamma with a different temperature dependence. It is shown empirically for a selection of ceramics that lambda is nearly temperature-independent and has a value between 0.7 and 1.4.
Water infiltrated nanoporous glasses have a damping peak at about 1 Hz (4.6 nm pore size) and 6 Hz (8.8 nm pore size) at room temperature; their relaxation strengths are consistent with the damping mechanism in which ...
Water infiltrated nanoporous glasses have a damping peak at about 1 Hz (4.6 nm pore size) and 6 Hz (8.8 nm pore size) at room temperature; their relaxation strengths are consistent with the damping mechanism in which water flows from the regions of compression to those of tension. However, the damping peaks disappear when water content is below 60% for the 4.6 nm pore size glass and 40% for the 8.8 nm pore size glass. These water contents correspond to about a 0.85 and 1.0 nm thick interfacial water layer on the internal surfaces of the nanoporous glasses. The adsorbed layer contributes only to the damping background. Finally, dependencies of relaxation time on sample thickness, bulk modulus and viscosity agree well with the predictions of the model.
The stress field of an edge dislocation near a finite length crack depends on where the dislocation is originated. If the dislocation is emitted from the crack which is originally stress-free, the image force increase...
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The stress field of an edge dislocation near a finite length crack depends on where the dislocation is originated. If the dislocation is emitted from the crack which is originally stress-free, the image force increases with decreasing crack length. But the opposite is true if the dislocation is originated from elsewhere. At the same distance from the crack tip, the image force for the former is always larger than that for the latter. They become equal only if the crack is semi-infinitely long. If the dislocation is emitted from one of the crack tips, it generally will shield this crack tip but antishield the other tip. However the dislocation will shield or antishield both crack tips if it is originated from elsewhere.
The interaction between a screw dislocation and an interface cylindrical hole is found to depend on the source and location of the screw dislocation. When a screw dislocation is inside one of the media it has one imag...
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The interaction between a screw dislocation and an interface cylindrical hole is found to depend on the source and location of the screw dislocation. When a screw dislocation is inside one of the media it has one image in the other medium. If the real dislocation is emitted from the hole which is originally stress free, it has only two images inside the hole. However, if the real dislocation is emitted from elsewhere and moves to the vicinity of the hole, there is a third image at the center of the hole to keep the total Burgers vector of the images in the hole zero. These considerations were used to study the interaction of a screw dislocation with an interface crack (or elliptical hole) by means of conformal mapping. The effect of the screw dislocation on the stress intensity factor at the crack tip is continuous when the screw dislocation moves from one medium across the interface to the other medium. The image force can be divided into two parts. One part is due to the interface and the other is due to the interface crack. The composite solid behaves like a homogenous solid with an effective shear modulus when the dislocation is in the interface. The interaction between two or more screw dislocations in the presence of an interface crack is studied also. The latter result can be used to study the emission of an array of screw dislocations from an interface crack.
A new approach is proposed to analyze the fracture behavior with crack tip plasticity by the use of three independent measurements: E0 is the energy required per unit length of crack front to prepare the crack just be...
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A new approach is proposed to analyze the fracture behavior with crack tip plasticity by the use of three independent measurements: E0 is the energy required per unit length of crack front to prepare the crack just before propagation;the E(f)-curve is a plot of the energy required to produce a unit area of fractured surface versus the crack length;and the K(a)-curve is a plot of the applied stress intensity factor K(Iapp) versus the crack length. For ABS polymers, the prefracture energy E0 is 33 Jm-1;the E(f)-curve is a horizontal line of 5.2 x 10(4) Jm-2;and the K(a) curve has three different regions which depend on the development of the plastic zone during fracture.
The B-H loops of ferromagnetic amorphous alloys can be modified with the apparent coercivity reduced by superposing a suitable ac magnetic field. For as-cast Fe78B13Si9 (Allied 2605S2) with the superposed field, the w...
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The B-H loops of ferromagnetic amorphous alloys can be modified with the apparent coercivity reduced by superposing a suitable ac magnetic field. For as-cast Fe78B13Si9 (Allied 2605S2) with the superposed field, the width of the dc B-H loops could be reduced to a third of the dc value without the ac field. For dc or low-frequency B-H loops, it seems that an ac magnetic field of proper frequency and magnitude can vibrate the domain walls sufficiently so that a small additional field can move them. When the frequency for B-H loop measurement was higher than 10 Hz, the effect of superposing an ac magnetic field appeared to be just the superposition of the two applied fields.
The blunt tip geometry of the so-called nanoindenter is modeled by a spherical cap of various radii. The relation between area and penetration depth of the indenter is comparable with experimental results if the radiu...
The blunt tip geometry of the so-called nanoindenter is modeled by a spherical cap of various radii. The relation between area and penetration depth of the indenter is comparable with experimental results if the radius of the tip is about 1.0-mu-m. Simulation of indentation tests was carried out using the finite element method by incremental loading and unloading based on a continuum model. Good agreement with the experimental results for nickel is obtained also for a tip radius of 1.0-mu-m.
Models of pressure-sintering and consolidation (such as that found in the HIP process) often assume a uniform spherical powder. However, it is known that use of mixtures, containing particles of differing composition ...
Models of pressure-sintering and consolidation (such as that found in the HIP process) often assume a uniform spherical powder. However, it is known that use of mixtures, containing particles of differing composition and/or size, alters the initial powder packing and subseguent HIPing behavior. We have experimentally investigated these effects using the simplest form of mixture; a bi-modal powder with various size ratios and particle fractions. Packing experiments confirm the existence of “optimum” packing fractions for which the density is maximized over both monosized and the as-received (continuously distributed) powders. During HIPing these powder mixes remain relatively more dense than monosize packings. The consolidation of composite powder mixtures as well as microstructural evolution during HIP are also discussed. Results are compared with those predicted by modeling.
Both energy and load required to fracture the material are important quantities and they are not necessarily related to each other as suggested by the fracture mechanics of brittle materials. For example, the differen...
Both energy and load required to fracture the material are important quantities and they are not necessarily related to each other as suggested by the fracture mechanics of brittle materials. For example, the difference of the failure behavior between the semi-brittle ABS polymer and the ductile Al 6061-TO can be well characterized by a pre-fracture energy E0 (Joule/m) and a fracture energy Ef (Joule/m2 ). Al 6061-TO has much higher values of E0 and EF than the ABS polymer. This accounts for the ductility of Al 6061-TO. However, the applied stress intensity factor KIapp for both materials are similar. The reason is that their plastic zones have similar shielding effects, even though Aluminum has dislocations and the ABS polymer has cavities and crazes.
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