16 Search Results

We demonstrate a methodology for diagnosing the multiscale dynamics and energy transfer in complex HED flows with realistic driving and boundary conditions. The approach separates incompressible, compressible, and baropycnal contributions to energy scale-transfer and quantifies the direction of these transfers in (generalized) wavenumber space. We use this to compare the kinetic energy (KE) transfer across scales in simulations of 2D axisymmetric vs fully 3D laser-driven plasma jets. Using the FLASH code, we model a turbulent jet ablated from an aluminum cone target in the configuration outlined. We show that, in addition to its well known bias for underestimating hydrodynamic instability growth, 2D modeling suffers from significant spurious energization of the bulk flow by a turbulent upscale cascade. In 2D, this arises as vorticity and strain from instabilities near the jet's leading edge transfer KE upscale, sustaining a coherent circulation that helps propel the axisymmetric jet farther (⁠ ≈25% by 3.5 ns) and helps keep it collimated. In 3D, the coherent circulation and upscale KE transfer are absent. Here, the methodology presented here may also help with inter-model comparison and validation, including future modeling efforts to alleviate some of the 2D hydrodynamic artifacts highlighted in this study.

An {ital in situ} high temperature heat treatment was used to investigate the crystallization and growth behavior of AlB{sub 2} flakes in aluminum. Aluminum samples containing 1.8{percent} boron were heated above the liquidus and then rapidly cooled through the Al{sub (L)}+AlB{sub 12} region to avoid the formation of AlB{sub 12} crystals. Subsequently, a homogeneous distribution of high aspect ratio AlB{sub 2} flakes crystallized upon holding below the peritectic transition temperature. Growth rate in the (a) and (c) dimensions increased during elevated hold temperatures below the peritectic transition temperature. Surprisingly, faster cooling rates from above the liquidus to room temperature resulted in thinner, wider flakes. Similar to graphite this phenomenon is believed to result from a need to accommodate a changing misfit strain energy between the solidifying aluminum and the growing AlB{sub 2} flakes. {copyright} {ital 1998 Materials Research Society.}

Fracture resistance of two model Al{sub 2}O{sub 3}/Al interfaces was examined under pure shear and mixed Mode-I and Mode-II loading conditions, to study the effect of interfacial microstructure on the mechanical property of the interface. Interface specimens were made by bonding high-purity Al2O3 with molten Al-Cu alloy under pressure. The interface was subsequently modified by inducing interfacial precipitation of Al-Cu compounds as the Al-Cu alloy was heat-treated to peak-aged and extended-overaged conditions. The fracture resistance of the interface was characterized in terms of the shear strength and the fracture resistance curve. The shear strength of the metal/ceramic interface was measured from the torsion of a rectangular beam, and was notably higher for the peak-aged specimen. The fracture resistance curve of the interface was established by a flexural peel interface-crack growth technique. While the peak-toughness of the interface was found to scale with the yield strength of the metal, the initiation toughness of the two interfaces differed by a factor of 8. The differences in the fracture properties between the two interfaces are related to the disparity in the interfacial microstructure.

Behavior of fatigue crack growth was examined in an Al{sub 2}O{sub 3} particulate reinforced Al-Cu matrix composite and its matrix alloy, heat-treated to peak-aged and overaged conditions. The fatigue crack growth resistance of the composite was much better, especially at low growth rates, and depended on aging condition. Measurements of crack opening profiles have shown that crack tip in the composite remained closed at fairly high stress intensities and was opened to a much smaller displacement compared to the crack tip opening in the matrix alloy at the same stress intensity. Correlation of the fatigue crack growth rate with crack tip opening displacement (CTOD) revealed that both the composite and the matrix alloy obey the same fundamental relationship that the fatigue crack growth rate scales with the square of the CTOD.

Fatigue crack growth along an Al/epoxy interface was examined under different combinations of mode-I and mode-II loadings using the flexural peel technique. Fatigue crack growth rates were obtained as a function of the total strain energy rate for G{sub II}/G{sub I} ratios of 0.3 to 1.4, achieved by varying the relative thickness of the outerlayers for the flexural peel specimen. Fatigue crack growth resistance of the interface was found to increase with increasing G{sub II}/G{sub I} ratio. Such a shear-enhanced crack growth resistance of the interface resulted in a gradual transition of crack growth mechanism from interfacial at the low G{sub II}/G{sub I} ratio to cohesive at the high G{sub II}/G{sub I} ratio. Under predominantly mode-I loading, the damage in the polymer took the form of crazing and cavitation. In contrast, laminar shear occurred under highly shear loading, resulting in a larger amount of plastic dissipation at the crack tip and improved fatigue crack growth resistance.

The flexural peel technique was used to study the fracture resistance of two model Al{sub 2}O{sub 3}/Al interfaces. The bimaterial interface was formed by bonding high-purity Al{sub 2}O{sub 3} with molten Al-5 pct Cu alloy under pressure. The specimens were then heat treated so that the Al-Cu alloy reached peak-aged and extended-overaged conditions. The fracture resistance curve was established for two interfaces with either the peak-aged or overaged Al alloy. The fracture resistance of the interface with the peak-aged Al-Cu alloy was higher in terms of both the initiation and peak toughness. While the peak toughness of the interface scaled with the yield strength of the metal, the initiation toughness differed by a factor of 8. The difference in the initiation toughness is discussed in terms of the disparity in the interfacial microstructure.

A flexural peel (FP) technique was developed to study the crack growth behavior along a bimaterial interface. The technique was based on a sandwich specimen where one arm of the specimen was peeled away from the interface under a combined tensile and shear mode. An approximate linear elastic fracture mechanics solution was derived analytically from the J integral formulation. The solution was compared to finite element calculations based on the crack closure method and experimental measurements. From the finite element analysis, ratios of the mode-I and mode-II components of the crack tip field were determined for a wide range of modulus and thickness ratios.

High-temperature crack growth behavior of a polycrystalline alumina was examined under Mode-I tension-tension cyclic loading. Locally at the crack tip, the fatigue crack was found to advance in shear by frictional sliding of grains on alternating sets of planes of the maximum shear. Evidence of a shear-driven crack growth was given in terms of topological and morphological analyses of the fatigue crack growth kinetics. Based on experimental observations, a ne model of fatigue crack growth by alternating shear was proposed.

Fatigue crack growth behavior of a peak-aged Al[sub 2]O[sub 3]/Al-Cu composite was examined at 150 C and compared to the behavior at room temperature (RT). At 150 C, fatigue crack growth rates showed strong dependence on loading time. At short loading time, when stress-intensity range was decreased to approach fatigue threshold, crack growth rates at 150 C were comparable to those measured at RT. Prolonged fatigue testing at near-threshold crack growth rates resulted in oscillations of crack growth rate until the fatigue crack growth behavior was stabilized to become similar to that in an overaged composite. Measurement of the matrix hardness at different distances from the crack plane and transmission electron microscopy examination of the fatigue specimen have shown that the matrix microstructure at the tip of the fatigue crack underwent overaging during prolonged testing in the near-threshold regime. Consequently, the fatigue fracture mechanism was modified, a lower crack closure developed, and the fatigue threshold reduced to that of the overaged composite.

In this research, the surface topography of fatigue cracks developed during high temperature fatigue in two high-purity polycrystalline aluminas was examined by a scanning laser confocal microscopic. Based on the topographical analysis, a new model of alternating shear is suggested for the high temperature fatigue crack growth in polycrystalline ceramics. The model postulates that crack grows under cyclic loading by grain sliding first along one of the maximum-shear planes in the loading half of the fatigue cycle and then on the alternate maximum-shear plane upon unloading. The postulate is supported by the evidence of crack-surface sliding under pure Mode-I far-field loading and frictional debris on the fatigue crack surface.