Multi-Material Ultrasonic Consolidation

Ultrasonic consolidation (UC) is a recently developed direct metal solid freeform fabrication process. While the process has been well-demonstrated for part fabrication in Al alloy 3003 H18, including with intricate cooling channels, some of the potential strengths of the process have not been fully exploited. One of them is its flexibility with build materials and the other is its suitability for fabrication of multi-material and functionally graded material parts with enhanced functional or mechanical properties. Capitalizing on these capabilities is critical for broadening the application range and commercial utilization of the process. In the current work, UC was used to investigate ultrasonic bonding of a broad range of engineering materials, which included stainless steels, Ni-base alloys, brass, Al alloys, and Al alloy composites. UC multimaterial part fabrication was examined using Al alloy 3003 as the bulk part material and the above mentioned materials as performance enhancement materials. Studies were focused on microstructural aspects to evaluate interface characteristics between dissimilar material layers. The results showed that most of these materials can be successfully bonded to Al alloy 3003 and vice versa using the ultrasonic consolidation process. Bond formation and interface characteristics between various material combinations are discussed based on oxide layer characteristics, material properties, and others.Mechanical Engineerin

Unitarity constraints on the ratio of shear viscosity to entropy density in higher derivative gravity

We discuss corrections to the ratio of shear viscosity to entropy density $\eta/s$ in higher-derivative gravity theories. Generically, these theories contain ghost modes with Planck-scale masses. Motivated by general considerations about unitarity, we propose new boundary conditions for the equations of motion of the graviton perturbations that force the amplitude of the ghosts modes to vanish. We analyze explicitly four-derivative perturbative corrections to Einstein gravity which generically lead to four-derivative equations of motion, compare our choice of boundary conditions to previous proposals and show that, with our new prescription, the ratio $\eta/s$ remains at the Einstein-gravity value of $1/4\pi$ to leading order in the corrections. It is argued that, when the new boundary conditions are imposed on six and higher-derivative equations of motion, $\eta/s$ can only increase from the Einstein-gravity value. We also recall some general arguments that support the validity of our results to all orders in the strength of the corrections to Einstein gravity. We then discuss the particular case of Gauss-Bonnet gravity, for which the equations of motion are only of two-derivative order and the value of $\eta/s$ can decrease below $1/4\pi$ when treated in a nonperturbative way. Our findings provide further evidence for the validity of the KSS bound for theories that can be viewed as perturbative corrections to Einstein Gravity.Comment: Sign error in the equations of motion corrected, leading to several numerical changes. Clarifications added, references added. Main results and cnclusions essentially unchanged. V3 published version. Clarifications added, discussion of Gauss-Bonnet moved to main tex

Hysterisis in Food Safety Investments

Concerns regarding the safety and integrity of the fresh produce supply chain are becoming all too common in the media. In 2006, an outbreak of E. coli O157:H7 from farms in Central California sickened almost two hundred people and lead to the deaths of three. Estimated costs to the industry ranged from $100 per month to$200 million until spinach sales returned to normal. By some accounts, the spinach industry has yet to recover and may not for years to come. The incident, however, has lead to a host of initiatives from industry officials, legislators and fresh produce retailers to ensure the safety of fresh produce. The necessary technology and best practices knowledge exists, yet some growers have not made the investment required to ensure that such outbreaks do not happen again in the future.Agribusiness, Agricultural and Food Policy, Farm Management, Food Consumption/Nutrition/Food Safety, Industrial Organization,

Analyses of Generic Dairy Advertising, 1984-97

Generic advertising raised fluid milk sales about 6.0 percent, or 18.1 billion pounds, between September 1984 and September 1997. Sales of cheese rose by about 6.8 billion pounds (milk equivalent) in the same period because of increased generic advertising. An assessment of 15 cents per hundredweight of milk sold commercially, mandated by the Dairy and Tobacco Adjustment Act of 1983, funded the advertising. Activities of the National Fluid Milk Processor Promotion Board also contributed to increased milk sales over the past year. Gross returns to dairy farmers between September 1984 and September 1997 were estimated to increase by $3.44 for each dollar spent on generic advertising.cheese, fluid milk, advertising, demand, entry, exit, distributed lag, econometrics, simulation, elasticities, Milk Processor Education Program, Livestock Production/Industries, Marketing,