Damage characterization and modeling of a 7075-T651 aluminum plate

In this paper, the damage-induced anisotropy arising from material microstructure heterogeneities at two different length scales was characterized and modeled for a wrought aluminum alloy. Experiments were performed on a 7075-T651 aluminum alloy plate using sub-standard tensile specimens in three different orientations with respect to the rolling direction. Scanning electron microscopy was employed to characterize the stereology of the final damage state in terms of cracked and or debonded particles. A physically motivated internal state variable continuum model was used to predict fracture by incorporating material microstructural features. The continuum model showed good comparisons to the experimental data by capturing the damage-induced anisotropic material response. Estimations of the mechanical stress–strain response, material damage histories, and final failure were numerically calculated and experimentally validated thus demonstrating that the final failure state was strongly dependent on the constituent particle morphology.     

SPOC-a stored program optical computer

The Digital Optical Computing (DOC) group at the University of Colorado at Boulder have built the world's first stored program optical computer (SPOC). Several features that distinguish this computer from traditional electronic computers are: the use of optical fibers and pulses of light instead of wires and electronic signals; the use of space and time to store information instead of flip-flops; synchronization based on the predictable propagation time of signals; and return to zero signal encoding. One goal was to show that optics could be used to build a general purpose stored program computer. Another goal was to demonstrate that predictable signal propagation time could replace flip flops for synchronization. The SPOC is a functionally complete computer. The constant speed of light was used to synchronize the signals in the SPOC. We adjusted the length of the input fibers to the logic gates so the signals would arrive at the same time. This synchronization technique is called time-of-flight design. The group developed a design tool, called XHatch, to help design time-of-flight circuits. We used XHatch to determine the lengths of the optical fibers for the SPOC based on its circuit design. The fibers act as registers by storing the signals in space and time until they are fed into a logic element     

Atomistic simulations of fatigue crack growth and the associated fatigue crack tip stress evolution in magnesium single crystals

Using Large-scale Atomic Molecular Massively Parallel Simulator (LAMMPS), a classical molecular dynamics code, atomistic simulations were performed to investigate the fatigue crack growth rate and the evolution of the associated atomic stress fields near the crack tip during fatigue crack growth in magnesium single crystals. The interatomic bonds of atoms were described using the EAM potential. The specimens with initial edge cracks were subjected to uniaxial Mode I cyclic loading. For the sake of revealing the influence of the initial cracks’ crystal orientations, three different orientations were considered. The fatigue growth rate can be expressed by da/dN = cCTOD, where the values of constant c are determined by the atomistic simulations. Notably, the values of the constant c are much larger for magnesium single crystals than for FCC single crystals and vary widely from one orientation to another. The simulation results show that the evolution of atomic stress fields was highly dependent on the crystal orientations due to anisotropy and magnesium single crystals’ HCP structure. Interestingly, the von Mises stress or normal stress around the crack tip controlled the fatigue crack growth behaviors.     

Optimal scale factors for Gaussian field expansions for a circular aperture

Gaussian field expansions for circular aperture antennas are reviewed, and a method for finding optimal scales in these expansions is given, paralleling the method for rectangular antennas presented in an earlier paper. This procedure is shown to lead to approximations structured in the sense that the number of lobes in the approximation corresponds to the number of terms retained in the expansion. An example is given of the development of a cosine distribution from the aperture, through the Fresnel region, to the far field.     

High cycle fatigue mechanisms of aluminum self‐piercing riveted joints

A study examining the fatigue failure mechanism of self‐piercing riveted (SPR) joints between aluminum alloy 6111‐T4 and 5754‐O is presented in this paper. In particular, the high‐cycle fatigue behavior of the SPR joints in the lap‐shear configuration is characterized. Experimental fatigue testing revealed that failure of SPR joints occurred because of cracks propagating through the sheet thickness at locations away from the rivet. In‐depth postmortem analysis showed that significant fretting wear occurred at the location of the fatigue crack initiation. Energy dispersive X‐ray of the fretting debris revealed the presence of aluminum oxide that is consistent with fretting initiated fatigue damage. High‐fidelity finite element analysis of the SPR process revealed high surface contact pressure at the location of fretting‐initiated fatigue determined by postmortem analysis of failed coupons. Furthermore, fatigue modeling predictions of the number of cycles to failure based on linear elastic fracture mechanics supports the conclusion that fretting‐initiated fatigue occurred at regions of high surface contact pressure and not at locations of nominal high‐stress concentration at the rivet.     

Fatigue Behavior of Solid-State Additive Manufactured Inconel 625

A transformative hybrid solid-state additive manufacturing process provides a new path to fabricate or repair components with wrought-like performance. In this work, the fatigue behavior of Inconel 625 (IN625) manufactured via a high-shear deposition process is quantified for the first time. In this unique process, feedstock is deposited via a hollow non-consumable rotating cylindrical tool, thereby generating heat and plastically deforming the feedstock through controlled pressure as consecutive layers are metallurgically bonded upon a substrate. To quantify the fatigue behavior of the as-deposited IN625, stress-life experiments were conducted, where improved fatigue resistance was observed compared with the feedstock. Post-mortem analysis of the as-deposited IN625 revealed a similar fatigue nucleation and growth mechanism to the feedstock for a majority of the specimens tested in this study. Last, a microstructure-sensitive fatigue life model was utilized to elucidate structure–property fatigue mechanism relations of the as-deposited and feedstock IN625 materials.     

Interpreting relationships between age and promotion in age-discrimination cases.

Addresses the logic behind the negative relationship between age and promotions and presents a simulation model that assesses this relationship. Expected negative relationships with a job level between age and promotion assumes that people in the same job level vary in how much potential they have to do the next job up the hierarchy and that there is some validity to the organization's promotional policy. The model simulates the promotion process using 1,000 employees in a target level job by considering the validity of the process, by calculating the distribution of replacement, by allowing for changes in promotability over time as a result of learning and other factors, by considering retirement, by continuing the process until age and promotion stabilizes, and by eliminating age discrimination from the model by random assignment. The model empirically demonstrates the expected negative relationship between age and promotions and provides a means for predicting the strength of the relationship that would be expected in the absence of age discrimination. (8 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Mapping And Analysis of Aerial Conductivity Measurements from INPUT System over Geothermal Areas

Remotely sensed electromagnetic data taken with an INPUT system for the U.S. Geological Survey in aerial surveys over a known geothermal resource area have been reduced and plotted in a gray scale format for two-and three-dimensional projections of the apparent subsurface conductance. The apparent conductance is calculated using both the channel ratio method and the theoretical two-layer model of the earth. Matching the survey data to the two-layer model gave good results consistent with the ratio method of apparent conductance calculation and permitted the construction of maps of horizontal slices of apparent conductance at different depths into the earth, from one to fifty meters. The channel ratio and horizontal slice maps of apparent conductance give a picture of the apparent conductance which is consistent with known topographical features of the regions.