Cohesive analysis of a 3D benchmark for delamination growth under quasi-static and fatigue loading conditions

This paper evaluates the capabilities of the recently developed CF20 cohesive fatigue model, which can predict crack initiation as well as the rates of crack propagation by relying on intrinsic relationships between a stress-life diagram and its corresponding Paris law. The model is validated here using a partially reinforced double cantilever beam (R-DCB) benchmark proposed in literature. The two parameters needed for the CF20 cohesive fatigue model were obtained by performing preliminary analyses of a conventional DCB. The analysis results indicate that the CF20 cohesive fatigue model can accurately reproduce the complex evolution of the delamination observed in the R-DCB

The importance of accounting for large deformation in continuum damage models in predicting matrix failure of composites

The work presented in this paper investigates the ability of continuum damage models to accurately predict matrix failure and ply splitting. Two continuum damage model approaches are implemented that use different stress–strain measures. The first approach is based on small-strain increments and the Cauchy stress, while the second approach account for large deformation kinematics through the use of the Green–Lagrange strain and the 2nd Piola–Kirchhoff stress. The investigation consists of numerical benchmarks at three different levels: (1) single element; (2) unidirectional single ply open-hole specimen and (3) open-hole composite laminate coupon. Finally, the numerically predicted failure modes are compared to experimental failure modes at the coupon level. It is shown that it is important to account for large deformation kinematics in the constitutive model, especially when predicting matrix splitting failure modes. It is also shown that continuum damage models that do not account for large deformation kinematics can easily be adapted to ensure that the damage modes and failure strength are predicted accurately

Participatory mapping for transformation: multiple visual representation of foodscapes and environment in informal settlements in Nairobi

Although branded as ‘obstructionists’ and major agents of ‘disease and filth’ by city authorities, food vendors remain the pivotal node in the local food system in most informal settlements; therefore, their interaction with the environment and infrastructure services, and challenges they face to keep the food safe to eat, requires further grounded exploration. Food vendors from informal settlements in Nairobi, Kenya, who are acting as mappers and change agents, are building multi-layered views of places through the deliberative process of knowledge coproduction by participatory sensing, which lead to opportunities and challenges to improve those places

“It Ruined My Life”: The effects of the War on Drugs on people who inject drugs (PWID) in rural Puerto Rico

Background—The War on Drugs has raised the incarceration rates of racial minorities for non-violent drug-related crimes, profoundly stigmatized drug users, and redirected resources from drug prevention and treatment to militarizing federal and local law enforcement. Yet, while some states consider shifting their punitive approach to drug use, to one based on drug treatment and rehabilitation, nothing suggests that these policy shifts are being replicated in Puerto Rico. Methods—This paper utilizes data from 360 PWID residing in four rural towns in the mountainous area of central Puerto Rico. We initially recruited 315 PWID using respondent-driven sampling (RDS) and collected data about risk practices and conducted HIV and HCV testing. During a second phase, we conducted 34 micro-ethnographic assays, in which we randomly recruited 34 participants from the first phase and included their ego networks in this phase. Our ethnographic inquiry produced significant data regarding the effects of the war on drugs on the local drug trade, drug availability, and injectors’ social networks. Results—Findings suggest that repressive policing has been ineffective in preventing drug distribution and use among those in our study. This type of law enforcement approach has resulted in the disproportionate incarceration of poor drug users in rural Puerto Rico, and mainly for nonviolent drug-related crimes. In addition, incarceration exposes PWID to a form of a cruel and unusual punishment: having to quit heroin “cold turkey” while the prison environment also represents a HIV/HCV risk. In turn, the war on drugs not only diverts resources from treatment but also shapes treatment ideologies, punishing non-compliant patients. Conclusion—Shifting the emphasis from repression to treatment and rehabilitation is likely to have a positive impact on the health and overall quality of life of PWID and their communities

The Icebreaker Mission to Search for Life on Mars

The search for evidence of life on Mars is the ultimate motivation for its scientific exploration. The results from the Phoenix mission indicate that the high N. latitude ice-rich regolith at low elevations is likely to be a recently habitable place on Mars [Stoker et al., 2010]. The near-surface ice likely provided adequate water activity during periods of high obliquity, 3 to 10 Myr ago. Carbon dioxide and nitrogen are present in the atmosphere, and nitrates may be present in the soil. Together with iron in basaltic rocks and perchlorate in the soil they provide carbon and energy sources, and oxidative power to drive metabolism. Furthermore, the presence of organics is possible, as thermally reactive perchlorate would have prevented their detection by Viking and Phoenix. The Mars Icebreaker Life mission [McKay et al., 2013] focuses on the following science goals: (1) Search for biomolecular evidence of life; (2) Search for organic matter from either exogeneous or endogeneous sources using methods that are not effected by the presence of perchlorate; (3) Characterize oxidative species that produced reactivity of soils seen by Viking; and 4) Assess the habitability of the ice bearing soils. The Icebreaker Life payload (Figure 1) includes a 1-m rotary percussive drill that brings cuttings samples to the surface where they are delivered to three instruments (Fig. 1), the Signs of Life Detector (SOLID) [Parro et al., 2011] for biomolecular analysis, Laser Desorption Mass Spectrometer (LDMS) [??? 2015]) for broad spectrum organic analysis, and Wet Chemistry Laboratory (WCL) [Hecht et al., 2009] for detecting soluble species of nutrients and reactive oxidants. The Icebreaker payload fits on the Phoenix spacecraft and can land at the well-characterized Phoe-nix landing site in 2020 in a Discovery-class mission

The Signs of Life Detector (SOLID): An Instrument to Detect Molecular Biosignatures on Mars

The case for life on Mars grows stronger. Investigations at Gale Crater by Curiosity have revealed fine-grained sedimentary rocks inferred to represent an ancient lake environment suited to support life. In addition, Curiosity tentatively found a heterogeneous distribution of organic carbon within these sediments, consistent with the detection of native organic C in Mars meteorites. Furthermore, modern potentially habitable environments have been recognized on Mars including the N. Polar region visited by Phoenix, gully features suggesting modern water flows, and RSLs that occur seasonally suggest liquid processes. The time is ripe for missions to Mars incorporating a search for biochemical evidence of life

Theory of the "honeycomb chain-channel" reconstruction of Si(111)3x1

First-principles electronic-structure methods are used to study a structural model for Ag/Si(111)3x1 recently proposed on the basis of transmission electron diffraction data. The fully relaxed geometry for this model is far more energetically favorable than any previously proposed, partly due to the unusual formation of a Si double bond in the surface layer. The calculated electronic properties of this model are in complete agreement with data from angle-resolved photoemission and scanning tunneling microscopy.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Lett (the ugly postscript error on page 4 has now been repaired

Crater Morphology in the Phoenix Landing Ellipse: Insights Into Net Erosion and Ice Table Depth

Icebreaker [1] is a Discovery class mission being developed for future flight opportunities. Under this mission concept, the Icebreaker payload is carried on a stationary lander, and lands in the same landing ellipse as Phoenix. Samples are acquired from the subsurface using a drilling system that penetrates into materials which may include loose or cemented soil, icy soil, pure ice, rocks, or mixtures of these. To avoid the complexity of mating additional strings, the drill is single-string, limiting it to a total length of 1 m