Publications

Modification of the Gurson Model for shear failure

Recent experimental evidence points to limitations in characterizing the critical strain in ductile fracture solely on the basis of stress triaxiality. A second measure of stress state, such as the Lode parameter, is required to discriminate between axisymmetric and shear-dominated stress states. This is brought into the sharpest relief by the fact that many structural metals have a fracture strain in shear, at zero stress triaxiality, that can be well below fracture strains under axisymmetric stressing at significantly higher triaxiality. Moreover, recent theoretical studies of void growth reveal that triaxiality alone is insufficient to characterize important growth and coalescence features. As currently formulated, the Gurson Model of metal plasticity predicts no damage change with strain under zero mean stress, except when voids are nucleated. Consequently, the model excludes shear softening due to void distortion and inter-void linking. As it stands, the model effectively excludes the possibility of shear localization and fracture under conditions of low triaxiality if void nucleation is not invoked. In this paper, an extension of the Gurson model is proposed that incorporates damage growth under low triaxiality straining for shear-dominated states. The extension retains the isotropy of the original Gurson Model by making use of the third invariant of stress to distinguish shear dominated states. The importance of the extension is illustrated by a study of shear localization over the complete range of applied stress states, clarifying recently reported experimental trends. The extension opens the possibility for computational fracture approaches based on the Gurson Model to be extended to shear-dominated failures such as projectile penetration and shear-off phenomena under impulsive loadings.

Hindered rotation in substituted paracyclophanes

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTHindered rotation in substituted paracyclophanesGeorge M. Whitesides, Beverly A. Pawson, and Arthur C. CopeCite this: J. Am. Chem. Soc. 1968, 90, 3, 639–644Publication Date (Print):January 1, 1968Publication History Published online1 May 2002Published inissue 1 January 1968https://pubs.acs.org/doi/10.1021/ja01005a015https://doi.org/10.1021/ja01005a015research-articleACS PublicationsRequest reuse permissionsArticle Views72Altmetric-Citations33LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose Get e-Alerts

Red and Processed Meats and Health Risks: How Strong Is the Evidence?

Prevailing dietary guidelines have widely recommended diets relatively low in red and processed meats and high in minimally processed plant foods for the prevention of chronic diseases. However, an ad hoc research group called the Nutritional Recommendations (NutriRECS) consortium recently issued “new dietary guidelines” encouraging individuals to continue their current meat consumption habits due to “low certainty” of the evidence, difficulty of altering meat eaters’ habits and preferences, and the lack of need to consider environmental impacts of red meat consumption. These recommendations are not justified, in large part because of the flawed methodologies used to review and grade nutritional evidence. The evidence evaluation was largely based on the Grading of Recommendations, Assessment, Development, and Evaluation (GRADE) criteria, which are primarily designed to grade the strength of evidence for clinical interventions especially pharmacotherapy. However, the infeasibility for conducting large, long-term randomized clinical trials on most dietary, lifestyle, and environmental exposures makes the criteria inappropriate in these areas. A separate research group proposed a modified and validated system for rating the meta-evidence on nutritional studies (NutriGRADE) to address several limitations of the GRADE criteria. Applying NutriGRADE, the evidence on the positive association between red and processed meats and type 2 diabetes was rated to be of “high quality,” while the evidence on the association between red and processed meats and mortality was rated to be of “moderate quality.” Another important limitation is that inadequate attention was paid to what might be replacing red meat, be it plant-based proteins, refined carbohydrates, or other foods. In summary, the red/processed meat recommendations by NutriRECS suffer from important methodological limitations and involve misinterpretations of nutritional evidence. To improve human and planetary health, dietary guidelines should continue to emphasize dietary patterns low in red and processed meats and high in minimally processed plant foods such as fruits and vegetables, whole grains, nuts, and legumes.

Kondo and Superconducting Proximity Effect in Semiconductor Nanostructures

No abstract is provided for this article.

Abstract 16886: Metabolomic Profiling of Alcohol Consumption in 10,000 Young Adults

Introduction: High alcohol consumption is a major cause of morbidity, yet moderate alcohol intake is favorably associated with cardiovascular disease and many risk factors. We aimed to characterize the associations of regular alcohol consumption with a comprehensive systemic metabolic profile. Methods: 86 circulating lipids, fatty acids and metabolites were quantified by high-throughput NMR metabolomics. Cross-sectional associations of alcohol intake with the metabolomic biomarkers were assessed for ≈10,000 individuals from three population-based cohorts from Finland aged 24-45 years. Metabolic changes associated with change in alcohol intake during 6-year follow-up were examined for 1466 individuals. Results: Increased alcohol intake was associated with circulating biomarkers across multiple metabolic pathways, including increased levels of monounsaturated fatty acids and decreased levels of omega-6 fatty acids, lower concentrations of glutamine and citrate as well as higher lipid concentrations in HDL subclasses and smaller LDL particle size (P Conclusions: Alcohol consumption is associated with a complex metabolic signature, including aberrations in multiple biomarkers for elevated cardiometabolic risk. The metabolic signature tracks with long-term changes in alcohol consumption. These results elucidate the double-edged effects of alcohol on cardiovascular risk and may help to explain mediation of cardiovascular risk related to alcohol intake.

Arthrobots

This article describes a class of robots-"arthrobots"-inspired, in part, by the musculoskeletal system of arthropods (spiders and insects, inter alia). Arthrobots combine mechanical compliance, lightweight and simple construction, and inexpensive yet scalable design. An exoskeleton, constructed from thin organic polymeric tubes, provides lightweight structural support. Pneumatic joints modeled after the hydrostatic joints of spiders provide actuation and inherent mechanical compliance to external forces. An inflatable elastomeric tube (a "balloon") enables active extension of a limb; an opposing elastic tendon enables passive retraction. A variety of robots constructed from these structural elements demonstrate (i) crawling with one or two limbs, (ii) walking with four or six limbs (including an insect-like triangular gait), (iii) walking with eight limbs, or (iv) floating and rowing on the surface of water. Arthrobots are simple to fabricate and are able to operate safely in contact with humans.

The interaction of proteins and cells with self-assembled monolayers of alkanethiolates on gold and silver

Scanning tunneling microscopy investigations of the local structure of Tl2Ba2CaCu2O8 single crystals

The in-plane structure of single-crystal samples of Tl2Ba2CaCu2O8 has been imaged at room temperature using a scanning tunneling microscope. Atomic-resolution surface images exhibit areas in which the structure has tetragonal symmetry (peak spacing 2.5±0.2 Å) and regions in which the structure is distorted from tetragonal symmetry. The lattice spacing indicates that the observed structure corresponds to the in-plane thallium and oxygen positions. The observation of both sites also suggests that the Tl-O band makes a significant contribution to the density of states near the Fermi level. In addition, a weak one-dimensional superlattice (period 10±0.5 Å) which shows short-range order has been observed.

Comparison of Viable and Non-Viable Indoor Airborne Fungal Spore Collections

Particle reinforcement of ductile matrices against plastic flow and creep

A theoretical investigation is made of the role of non-deforming particles in reinforcing ductile matrix materials against plastic flow and creep. The study is carried out within the framework of continuum plasticity theory using cell models to implement most of the calculations. Systematic results are given for the influence of particle volume fraction and shape on the overall behavior of composites with uniformly distributed, aligned reinforcement. The stress-strain behavior of the matrix material is characterized by elastic-perfectly plastic behavior or by power-law hardening behavior of the Ramberg-Osgood type. A relatively simple connection is noted between the asymptotic reference stress for the composite with the power-law hardening matrix and the limit flow stress of the corresponding composite with the elastic-perfectly plastic matrix. The asymptotic reference stress for the composite with the power-law matrix is applicable to steady-state creep. A limited study is reported on the overall limit flow stress for composites with randomly orientated disc-like or needle-like particles when the particles are arranged in a packet-like morphology.

Black–White Differences in the Relationship Between Alcohol Drinking Patterns and Mortality Among US Men and Women

Objectives. We investigated Black–White differences in the association between average alcohol drinking patterns and all-cause mortality. Methods. We pooled nationally representative samples of 152 180 adults in the National Health Interview Survey from 1997 to 2002 with mortality follow-up through 2006. Usual drinking days per week and level of alcohol consumed per day were based on self-report. We used race- and gender-specific Cox proportional hazards regression analyses to adjust for physical activity, smoking status, and other potential confounders. Results. Over 9 years, 13 366 deaths occurred from all causes. For men, the lowest multivariable-adjusted hazard ratio (HR) for total mortality among drinkers was 0.81 among White men who consumed 1 to 2 drinks 3 to 7 days per week (compared with abstainers) and Black men who abstained. For women, the lowest mortality risk was among White women (HR = 0.71) consuming 1 drink per day 3 to 7 days per week and Black women (HR = 0.72) consuming 1 drink on 2 or fewer days per week. Conclusions. Risks and benefits of alcohol consumption in relation to mortality risk were dependent on race- and gender-specific drinking patterns.

Poroelastic Bimaterial Effects in Rupture Dynamics

Numerical Methods, Software and Analysis

Numerical Methods, Software, and Analysis, Second Edition introduces the methods, tools, and ideas of numerical computation.

Self-Organization

Assessing diet in epidemiologic studies

Variants in MTNR1B influence fasting glucose levels

Overview no. 2

Existing models for the diffusive growth of voids on grain interfaces, at elevated temperature, are for the most part based on quasi-equilibrium assumptions: surface diffusion is assumed to be sufficiently rapid that the cavity has a rounded, equilibrium shape, and hence cavity growth is assumed to be rate-limited only by grain boundary diffusion. However, creep rupture cavities sometimes have narrow, crack-like shapes and it is appropriate to investigate non-equlibrium models for diffusive rupture. We do so here by comparing the quasi-equilibrium model to another limiting case based on a narrow, crack-like cavity shape. Criteria for choosing between the models are given on the basis of representative relaxation times for the surface diffusion process, and also by examining the properties of a ‘self-similar’ solution for cavity shape. By a suitable choice of parameters which measure the growth rate, this solution can be made to give results corresponding to either limiting case, and aids the interpolation between them. The results suggest that if s is the ratio of the applied stress to that which just equilibrates cavities against sintering, then for circular cavities on a grain boundary with diameter equal to a quarter of their average center-to-center spacing, the quasi-equilibrium mode applies when s < 1 + 6Δ and the crack-like mode when s > 2 + 9Δ. Here αD is the ratio of surface to grain boundary diffusivity. Also, the stress dependence of the growth rate and rupture lifetime is established in each case, and the results are discussed in relation to the interpretation of experimental data.

Preparation of Fullerenes and Fullerene-Based Materials

This chapter discusses the preparation, isolation, and characterization of fullerene clusters, and solids based on these clusters. Several techniques are used to prepare crude mixtures of fullerenes, and isolate and characterize pure C. Simple resistive vaporization of graphite rods could produce fullerenes in substantial yield. This procedure is often termed the Kratschmer–Huffman method and is a straightforward and low cost method for generating large quantities of fullerene-containing carbon soot. There are several experimental factors that can be varied to maximize the yield of fullerene clusters in the soot produced from the vaporization of graphite electrodes. These factors include the vaporization current density and helium partial pressure. A second, very useful and powerful technique for producing fullerene clusters involves laser ablation of graphite in a helium atmosphere. Several other techniques, including hydrocarbon combustion, low-pressure helium sputtering, electron beam evaporation, and inductively coupled RF evaporation of graphite targets have been used to prepare fullerene containing carbon products. The chapter also discusses several approaches for preparing isotopically substituted clusters. Finally, the chapter discusses the status and prospects of several new fullerene building blocks, including endohedral metal clusters and carbon nanotubes.