Effects of international trade of food and feed and human diet shifts on food security and environmental safety: integrating scales

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Extracting material response from simple mechanical tests on hardening-softening-hardening viscoplastic solids

Compliant foams are usually characterized by a wide range of desirable mechanical properties. These properties include viscoelasticity at different temperatures, energy absorption, recoverability under cyclic loading, impact resistance, and thermal, electrical, acoustic and radiation-resistance. Some foams contain nano-sized features and are used in small-scale devices. This implies that the characteristic dimensions of foams span multiple length scales, rendering modeling their mechanical properties difficult. Continuum mechanics-based models capture some salient experimental features like the linear elastic regime, followed by non-linear plateau stress regime. However, they lack mesostructural physical details. This makes them incapable of accurately predicting local peaks in stress and strain distributions, which significantly affect the deformation paths. Atomistic methods are capable of capturing the physical origins of deformation at smaller scales, but suffer from impractical computational intensity. Capturing deformation at the so-called meso-scale, which is capable of describing the phenomenon at a continuum level, but with some physical insights, requires developing new theoretical approaches.

A fundamental question that motivates the modeling of foams is ‘how to extract the intrinsic material response from simple mechanical test data, such as stress vs. strain response?’ A 3D model was developed to simulate the mechanical response of foam-type materials. The novelty of this model includes unique features such as the hardening-softening-hardening material response, strain rate-dependence, and plastically compressible solids with plastic non-normality. Suggestive links from atomistic simulations of foams were borrowed to formulate a physically informed hardening material input function. Motivated by a model that qualitatively captured the response of foam-type vertically aligned carbon nanotube (VACNT) pillars under uniaxial compression [2011,“Analysis of Uniaxial Compression of Vertically Aligned Carbon Nanotubes,” J. Mech.Phys. Solids, 59, pp. 2227–2237, Erratum 60, 1753–1756 (2012)], the property space exploration was advanced to three types of simple mechanical tests: 1) uniaxial compression, 2) uniaxial tension, and 3) nanoindentation with a conical and a flat-punch tip. The simulations attempt to explain some of the salient features in experimental data, like
1) The initial linear elastic response.
2) One or more nonlinear instabilities, yielding, and hardening.

The model-inherent relationships between the material properties and the overall stress-strain behavior were validated against the available experimental data. The material properties include the gradient in stiffness along the height, plastic and elastic compressibility, and hardening. Each of these tests was evaluated in terms of their efficiency in extracting material properties. The uniaxial simulation results proved to be a combination of structural and material influences. Out of all deformation paths, flat-punch indentation proved to be superior since it is the most sensitive in capturing the material properties.

Questionário sobre o consumo de álcool e drogas entre profissionais de saúde: um estudo exploratório

A droga na atualidade é considerada uma ameaça para a humanidade. Nos países em desenvolvimento, o álcool é o principal fator de risco, dentre as demais substâncias psicoativas. Existem poucos estudos sobre a prevalência do uso de drogas nos locais de trabalho no Brasil, e sobre os meios de enfrentamento das instituições empregadoras frente ao consumo de drogas por seus trabalhadores e as condições que levam a tal uso. O estudo foi estruturado em duas etapas: 1) revisão bibliográfica de instrumentos auto-aplicáveis sobre drogas entre trabalhadores e 2) elaboração e aplicação de um questionário auto-aplicável sobre o consumo de drogas entre trabalhadores. Foi traçado os seguintes objetivos: 1 etapa - Levantar os estudos publicados, que apresentam como objeto o uso de álcool e drogas por trabalhadores, entre os anos de 1998 e 2008; Identificar e analisar os instrumentos auto-aplicáveis, que mensuram a prevalência e o padrão de consumo de drogas em trabalhadores, utilizados pelos estudos; e Subsidiar o desenvolvimento de um questionário auto-aplicável sobre o padrão de consumo de álcool e drogas entre trabalhadores; 2 etapa - Desenvolver um questionário auto-aplicável que permite identificar a prevalência e padrão de consumo de álcool e drogas entre profissionais de saúde, assim como, as formas de enfrentamento por parte do trabalhador e das instituições empregadoras; Realizar análise descritiva do questionário desenvolvido e de seus principais resultados; e Avaliar a compreensão das perguntas do questionário desenvolvido, a partir das sugestões e respostas marcadas pelos sujeitos do estudo. Trata-se de uma pesquisa quantitativa, descritiva e exploratória realizada com 111 alunos de pós-graduação latu sensu de uma Faculdade Pública de Enfermagem situada na Cidade do Rio de Janeiro. Através da revisão bibliográfica verificamos que existem poucos instrumentos auto-aplicáveis sobre o padrão de consumo de álcool e drogas entre trabalhadores. Foi construído um questionário visando identificar informações sócio-demográficas, a história profissional, informações sobre o consumo de álcool e outras drogas, informações sobre o estresse laboral, e informações sobre as formas de enfrentamento por parte do trabalhador e das instituições empregadoras sobre o consumo de drogas. Pela análise do questionário aplicado, observou-se que algumas questões foram de difícil compreensão e precisam ser reformuladas, a fim de melhorar a compreensão dos respondentes, já que um questionário auto-aplicável deve ser auto-explicativo. As escalas AUDIT e Job Stress Scale se mostraram importantes para identificar problemas relacionados ao álcool e o estresse laboral. O álcool foi a droga mais utilizada pelos profissionais de saúde, seguido pelas substâncias psicoativas. Portanto, deve-se dar um enfoque sobressalente para a questão do fenômeno das drogas no ambiente de trabalho, promovendo programas de prevenção e de qualidade de vida ao trabalhador. Ressalta-se, também, a importância de abordar as questões sobre drogas nas graduações da área da saúde, promovendo o conhecimento do futuro profissional quanto aos riscos e danos decorrentes do uso e abuso de drogas.

Stress and load calculation for unknown building

Not available.

Temperature and stress tuning characteristics of long-period gratings imprinted in Panda fiber

Temperature and stress tunabilities of long-period Bragg gratings imprinted in Panda fiber are presented in this letter. It is shown that the temperature and strain response of the resonance peaks for fast and slow axes are different not only in their magnitudes but also in the signs of the slope. Furthermore, the characteristics for different order modes are different both in magnitudes and signs. The complicated phenomena are discussed by using a simplified model.

Dynamic characterization of micro-particle systems

Ordered granular systems have been a subject of active research for decades. Due to their rich dynamic response and nonlinearity, ordered granular systems have been suggested for several applications, such as solitary wave focusing, acoustic signals manipulation, and vibration absorption. Most of the fundamental research performed on ordered granular systems has focused on macro-scale examples. However, most engineering applications require these systems to operate at much smaller scales. Very little is known about the response of micro-scale granular systems, primarily because of the difficulties in realizing reliable and quantitative experiments, which originate from the discrete nature of granular materials and their highly nonlinear inter-particle contact forces.

In this work, we investigate the physics of ordered micro-granular systems by designing an innovative experimental platform that allows us to assemble, excite, and characterize ordered micro-granular systems. This new experimental platform employs a laser system to deliver impulses with controlled momentum and incorporates non-contact measurement apparatuses to detect the particles’ displacement and velocity. We demonstrated the capability of the laser system to excite systems of dry (stainless steel particles of radius 150 micrometers) and wet (silica particles of radius 3.69 micrometers, immersed in fluid) micro-particles, after which we analyzed the stress propagation through these systems.

We derived the equations of motion governing the dynamic response of dry and wet particles on a substrate, which we then validated in experiments. We then measured the losses in these systems and characterized the collision and friction between two micro-particles. We studied wave propagation in one-dimensional dry chains of micro-particles as well as in two-dimensional colloidal systems immersed in fluid. We investigated the influence of defects to wave propagation in the one-dimensional systems. Finally, we characterized the wave-attenuation and its relation to the viscosity of the surrounding fluid and performed computer simulations to establish a model that captures the observed response.

The findings of the study offer the first systematic experimental and numerical analysis of wave propagation through ordered systems of micro-particles. The experimental system designed in this work provides the necessary tools for further fundamental studies of wave propagation in both granular and colloidal systems.

Fabrication, Characterization, And Deformation of 3D Structural Meta-Materials

Current technological advances in fabrication methods have provided pathways to creating architected structural meta-materials similar to those found in natural organisms that are structurally robust and lightweight, such as diatoms. Structural meta-materials are materials with mechanical properties that are determined by material properties at various length scales, which range from the material microstructure (nm) to the macro-scale architecture (μm – mm). It is now possible to exploit material size effect, which emerge at the nanometer length scale, as well as structural effects to tune the material properties and failure mechanisms of small-scale cellular solids, such as nanolattices. This work demonstrates the fabrication and mechanical properties of 3-dimensional hollow nanolattices in both tension and compression. Hollow gold nanolattices loaded in uniaxial compression demonstrate that strength and stiffness vary as a function of geometry and tube wall thickness. Structural effects were explored by increasing the unit cell angle from 30° to 60° while keeping all other parameters constant; material size effects were probed by varying the tube wall thickness, t, from 200nm to 635nm, at a constant relative density and grain size. In-situ uniaxial compression experiments reveal an order-of-magnitude increase in yield stress and modulus in nanolattices with greater lattice angles, and a 150% increase in the yield strength without a concomitant change in modulus in thicker-walled nanolattices for fixed lattice angles. These results imply that independent control of structural and material size effects enables tunability of mechanical properties of 3-dimensional architected meta-materials and highlight the importance of material, geometric, and microstructural effects in small-scale mechanics. This work also explores the flaw tolerance of 3D hollow-tube alumina kagome nanolattices with and without pre-fabricated notches, both in experiment and simulation. Experiments demonstrate that the hollow kagome nanolattices in uniaxial tension always fail at the same load when the ratio of notch length (a) to sample width (w) is no greater than 1/3, with no correlation between failure occurring at or away from the notch. For notches with (a/w) > 1/3, the samples fail at lower peak loads and this is attributed to the increased compliance as fewer unit cells span the un-notched region. Finite element simulations of the kagome tension samples show that the failure is governed by tensile loading for (a/w) < 1/3 but as (a/w) increases, bending begins to play a significant role in the failure. This work explores the flaw sensitivity of hollow alumina kagome nanolattices in tension, using experiments and simulations, and demonstrates that the discrete-continuum duality of architected structural meta-materials gives rise to their flaw insensitivity even when made entirely of intrinsically brittle materials.