A microstructurally informed dynamic ductile failure model

Dynamic spall failure of ductile metals is a complex multiscale, multirate process. On the macroscale the process involves a period of shock compression followed by dynamic tension set up by the stress wave interactions. During the shock compression, the material undergoes shock-dependent microscopic processes that may include dislocation multiplication, nucleation, trapping, pile-up, annihilation, recovery, cell evolution, as well as vacancy generation and clustering. In addition to shock hardening the material, this new shock-induced defect structure seeds the material with potential void nucleation sites that may be activated during the proceeding period of dynamic tensile loading. In addition to these shock-induced void nucleation sites, the material also possesses pre-existing nucleation sites, e.g., triple junctions, grain boundaries, and second-phase particles. Upon nucleation, these voids undergo dynamic growth to coalescence, constrained by inertia and viscoplastic resistance to deformation. A multiscale predictive model is developed to analyze the role of these time-dependent processes in the experimentally observed spall strength dependence on grain size, impurity content, tensile loading rate, and shock stress magnitude

Multiscale Mechanics of Failure in Extreme Environments

Over the past five decades there has been an intense effort to understand and control the thermomechanical response of materials in extreme environments. A number of technologies and applications critical to our safety and well-being stand to benefit from such understanding, including inertial confinement fusion, nuclear stockpile reliability, defense systems, spacecraft and hypersonic aircraft shielding, as well as vehicular crashworthiness. Materials in such extreme environments often exhibit complex, somewhat non-intuitive behavior that is difficult to predict with empirical or phenomenological models. As such, there has been an increasing effort to understand the microscale processes that govern the macroscale response. Here we provide a contribution to this effort through the development of a number of multiscale mechanism-based models that explore the fundamental nature of various microscale processes governing the macroscale thermomechanical response of materials in extreme environments. The extreme environments of interest here may include pressures on the order of the bulk modulus, shear stresses near the ideal strength, temperatures approaching melting, and timescales ranging from nanoseconds to the age of our Solar System (~5 billion years). We focus on materials in two particular extreme environments in this thesis. First and foremost, we explore the behavior of metals subject to very high rate deformation. Second, we study the behavior of planetary materials subject to the extreme thermomechanical environments in our Solar System. One of the main themes presented in the thesis is that the time-dependent failure of materials is governed, in part, by the kinetics of a hierarchy of microscopic material defects. Furthermore, the kinetics of one particular defect are often governed by lower length-scale defects. Examples of this are provided for twin boundary propagation at high loading rates, dynamic void growth in ductile materials, and fatigue crack growth in quasi-brittle asteroidal materials. A second theme is that simple mechanism-based models are powerful and instructive, particularly when it comes to building an intuition for dynamic failure processes. We make use of such simple scaling laws to help establish a deeper understanding of dynamic ductile failure of metals. We particularly focus on understanding how the rate-sensitivity of spall strength depends on a competition between the pre-existing material microstructure (e.g. second-phase particles and grain boundaries) and the shock-induced microstructure

Educating Law Enforcement Agencies on Nutrition and Fitness

Safety plays a major role in the job of a police officer and being in physical condition and being healthy allows the officer to perform so that they are able to stay safe. Police officers need to be in the best physical condition in order to perform their job to best of their ability. Officers need to be physically fit so that they are able to defend themselves against those trying to harm them or others, chase after criminals who chose to run, and many other scenarios that require a physically fit person to fulfill the task at hand. Departments all over the country are looking into adding a nutritional program and a physical fitness program into their curriculum to help officers achieve being in physical shape. A good nutritional and physical fitness program will help officers maintain being in physical shape and provide the steps to achieve it. Not only will these programs help officers at work but also help officers in their daily lives. Officers have to be willing to accept change and change for many people means trying something new. Adding a nutritional and physical condition program might cause hesitation to participate from officers. Adding programs like that to a department means taking time to do physical activity, spending money, and finding time on the job to maintain healthy habits. These are all factors that add to the challenge of being a police officer. Trying to convince officer’s to try these programs might create negativity in the department and cause even more trouble for departments and officers. There are ways to encourage officers to want to participate in programs that will help them maintain healthy and physical shape, and provide examples of how other departments such as the Los Angeles Police Department have implemented their health and nutritional program into their department

Fatigue Enhancement of a Carbon Fiber Reinforced Nanocomposite

The primary objective of the present investigation is to study the fatigue characteristics of a woven carbon fiber reinforced polymer which has been modified with either amine or fluorine functionalized carbon nanotubes on the fiber-matrix interface. Multi-wall functionalized carbon nanotubes are sprayed onto both sides of each fiber at 0.2-wt % with respect to the fibers. The composites are fabricated using high temperature vacuum assisted resin transfer molding with four-harness satin weave fabric and EPON 862/Epi-Kure W epoxy. Due to the heterogeneous nature of carbon fiber composites, under dynamic loading the composites undergoes a series of complex failure mechanisms: matrix cracking, fiber-matrix debonding, fiber fracture, and buckling. It is believed that debonding of the fiber-matrix interface is the most crucial of these failure mechanisms. Debonding of the fiber-matrix interface critically hinders the matrix’s ability to transfer loads to the fibers, leading to a poor distribution of load. Due to this distribution, one of three failures occurs: individual yarns of fibers are overloaded and fracture, the matrix losses strength and buckles, or a mixture of the two occurs. It will be shown that functionalized multi-wall carbon nanotubes can strengthen the fiber-matrix interface, resulting in fatigue life improvement. The research investigates this behavior for both tension-tension and tension-compression fatiguing. It is believed that improvements will be best at negative R-ratios and high cycle regimes, because the damage is almost entirely matrix dominated occurs under these conditions. Results have shown improvements in static tensile properties of about twenty percent and an order of magnitude improvement in the fatigue life. Fractographic analysis reveals that the nanocomposites can withstand far greater matrix damage prior to final failure. In addition, both optical and scanning electron microscopy indicates that the nanocomposite exhibits reduced fiber-matrix debonding

African American and disabled youth are overrepresented in behavior-focused and academic remediation schools

School districts in the US often have many different types of high schools - some are traditional neighborhood schools, while others are more innovative or focused on behavior or academic remediation. In new research Aaron B. Perzigian, Kemal Afacan, Whitney Justin, and Kimber L. Wilkerson examine the characteristics of students across these school types. They find that African American students, ..

Advancing Green Purchasing in Italian Municipalities

Italy was the first European country to create a full mandatory plan known as the National Action Plan on Green Public Procurement. This plan established a framework through which green purchasing polices can diffuse throughout Italian municipalities. A primary reason for this action is that green purchasing policies have the potential to significantly reduce carbon impacts across the globe and can help Italy achieve its carbon emissions goals. However, at the local level, many municipal governments have struggled to implement green purchasing policies. Consequently, green purchasing has not reached its full potential to help municipalities mitigate their environmental impacts. These are significant concerns that the United Nations Environmental Programme, the Organization for Economic Co-operation and Development, the Sustainable Purchasing Leadership Council (SPLC), and others suggest must be resolved if Italy is to move toward an environmentally sustainable economy. Researchers at Sant’ Anna School of Advance Studies’ Institute of Management and Arizona State University’s (ASU’s) Sustainable Purchasing Research Initiative have sought to address these issues. Our three broad objectives are to: 1) Determine the facilitators and barriers to adoption and implementation of green purchasing policies in Italian municipalities 2) Recommend actions for advancing green purchasing practices more effectively 3) Encourage Italian municipalities that lack green purchasing policies to adopt and implement them within their jurisdictions. To accomplish these objectives, we conducted a national survey of finance, environmental, and municipal engineering directors in Italian municipalities. The survey generated 152 individual responses from 395 municipalities with 25,000 residents or more. These municipalities were representative based on their population size, income, and geographic dispersion by prefecture

Characteristics of Students in Traditional Versus Alternative High Schools: A Cross-Sectional Analysis of Enrollment in One Urban District

Urban school districts are comprised of many diverse high school environments including comprehensive neighborhood schools as well as a variety of smaller alternative models that focus on innovative practices, behavior remediation, or academic recovery. In terms of enrollment distribution, urban school districts are increasingly offering nontraditional school placement options for students presenting academic and behavioral difficulty or for students seeking specific curricular emphasis or pedagogy, including—but not limited to—use of school choice voucher programs. In this study, we examined student distribution across school types in one large urban district to investigate enrollment patterns with regard to gender, race, socioeconomic status, and disability status. The results of this cross-sectional analysis indicated significant disproportionality in student demographics within different school types, including overrepresentation of African American students, male students, and students with disabilities in restrictive and segregated alternative schools; overrepresentation of White students and female students in self-selected and innovative alternative schools; and underrepresentation of Hispanic and Asian students in remedial alternative schools. Implications of this disproportionality for policy and practice are discussed

Rapid Quantification of Dynamic and Spall Strength of Metals Across Strain Rates

The response of metals and their microstructures under extreme dynamic conditions can be markedly different from that under quasistatic conditions. Traditionally, high strain rates and shock stresses are measured using cumbersome and expensive methods such as the Kolsky bar or large spall experiments. These methods are low throughput and do not facilitate high-fidelity microstructure-property linkages. In this work, we combine two powerful small-scale testing methods, custom nanoindentation, and laser-driven micro-flyer shock, to measure the dynamic and spall strength of metals. The nanoindentation system is configured to test samples from quasistatic to dynamic strain rate regimes (10$^{-3}$ s$^{-1}$ to 10$^{+4}$ s$^{-1}$). The laser-driven micro-flyer shock system can test samples through impact loading between 10$^{+5}$ s$^{-1}$ to 10$^{+7}$ s$^{-1}$ strain rates, triggering spall failure. The model material used for testing is Magnesium alloys, which are lightweight, possess high-specific strengths and have historically been challenging to design and strengthen due to their mechanical anisotropy. Here, we modulate their microstructure by adding or removing precipitates to demonstrate interesting upticks in strain rate sensitivity and evolution of dynamic strength. At high shock loading rates, we unravel an interesting paradigm where the spall strength of these materials converges, but the failure mechanisms are markedly different. Peak aging, considered to be a standard method to strengthen metallic alloys, causes catastrophic failure, faring much worse than solutionized alloys. Our high throughput testing framework not only quantifies strength but also teases out unexplored failure mechanisms at extreme strain rates, providing valuable insights for the rapid design and improvement of metals for extreme environments

Thermal fatigue as the origin of regolith on small asteroids

Space missions and thermal infrared observations3 have shown that small asteroids (kilometre-sized or smaller) are covered by a layer of centimetre-sized or smaller particles, which constitute the regolith. Regolith generation has traditionally been attributed to the fall back of impact ejecta and by the break-up of boulders bymicrometeoroid impact. Laboratory experiments6 and impact models, however, show that crater ejecta velocities are typically greater than several tens of centimetres per second,which corresponds to the gravitational escape velocity of kilometre-sized asteroids.Therefore, impact debris cannot be the main source of regolith on small asteroids. Here we report that thermal fatigue, a mechanism of rock weathering and fragmentation with no subsequent ejection, is the dominant process governing regolith generation on small asteroids.We find that thermal fragmentation induced by the diurnal temperature variations breaks up rocks larger than a few centimetres more quickly than do micrometeoroid impacts. Because thermal fragmentation is independent of asteroid size, this process can also contribute to regolith production on larger asteroids. Production of fresh regolith originatingin thermal fatigue fragmentationmay be an important process for the rejuvenation of the surfaces of near-Earth asteroids, and may explain the observed lack of low-perihelion, carbonaceous, near-Earth asteroids

Clinical, genomic, and metagenomic characterization of oral tongue squamous cell carcinoma in patients who do not smoke: Clinical and genomic study of nonsmokers with oral tongue cancer

Evidence suggests the incidence of oral tongue squamous cell carcinoma is increasing in young patients, many who have no history of tobacco use