The Sandia Fracture Challenge: blind round robin predictions of ductile tearing

Existing and emerging methods in computational mechanics are rarely validated against problems with an unknown outcome. For this reason, Sandia National Laboratories, in partnership with US National Science Foundation and Naval Surface Warfare Center Carderock Division, launched a computational challenge in mid-summer, 2012. Researchers and engineers were invited to predict crack initiation and propagation in a simple but novel geometry fabricated from a common off-the-shelf commercial engineering alloy. The goal of this international Sandia Fracture Challenge was to benchmark the capabilities for the prediction of deformation and damage evolution associated with ductile tearing in structural metals, including physics models, computational methods, and numerical implementations currently available in the computational fracture community. Thirteen teams participated, reporting blind predictions for the outcome of the Challenge. The simulations and experiments were performed independently and kept confidential. The methods for fracture prediction taken by the thirteen teams ranged from very simple engineering calculations to complicated multiscale simulations. The wide variation in modeling results showed a striking lack of consistency across research groups in addressing problems of ductile fracture. While some methods were more successful than others, it is clear that the problem of ductile fracture prediction continues to be challenging. Specific areas of deficiency have been identified through this effort. Also, the effort has underscored the need for additional blind prediction-based assessments

Overview paper: New insights into aerosol and climate in the Arctic

Motivated by the need to predict how the Arctic atmosphere will change in a warming world, this article summarizes recent advances made by the research consortium NETCARE (Network on Climate and Aerosols: Addressing Key Uncertainties in Remote Canadian Environments) that contribute to our fundamental understanding of Arctic aerosol particles as they relate to climate forcing. The overall goal of NETCARE research has been to use an interdisciplinary approach encompassing extensive field observations and a range of chemical transport, earth system, and biogeochemical models. Several major findings and advances have emerged from NETCARE since its formation in 2013. (1) Unexpectedly high summertime dimethyl sulfide (DMS) levels were identified in ocean water (up to 75&thinsp;nM) and the overlying atmosphere (up to 1&thinsp;ppbv) in the Canadian Arctic Archipelago (CAA). Furthermore, melt ponds, which are widely prevalent, were identified as an important DMS source (with DMS concentrations of up to 6&thinsp;nM and a potential contribution to atmospheric DMS of 20&thinsp;% in the study area). (2) Evidence of widespread particle nucleation and growth in the marine boundary layer was found in the CAA in the summertime, with these events observed on 41&thinsp;% of days in a 2016 cruise. As well, at Alert, Nunavut, particles that are newly formed and grown under conditions of minimal anthropogenic influence during the months of July and August are estimated to contribute 20&thinsp;% to 80&thinsp;% of the 30–50&thinsp;nm particle number density. DMS-oxidation-driven nucleation is facilitated by the presence of atmospheric ammonia arising from seabird-colony emissions, and potentially also from coastal regions, tundra, and biomass burning. Via accumulation of secondary organic aerosol (SOA), a significant fraction of the new particles grow to sizes that are active in cloud droplet formation. Although the gaseous precursors to Arctic marine SOA remain poorly defined, the measured levels of common continental SOA precursors (isoprene and monoterpenes) were low, whereas elevated mixing ratios of oxygenated volatile organic compounds (OVOCs) were inferred to arise via processes involving the sea surface microlayer. (3) The variability in the vertical distribution of black carbon (BC) under both springtime Arctic haze and more pristine summertime aerosol conditions was observed. Measured particle size distributions and mixing states were used to constrain, for the first time, calculations of aerosol–climate interactions under Arctic conditions. Aircraft- and ground-based measurements were used to better establish the BC source regions that supply the Arctic via long-range transport mechanisms, with evidence for a dominant springtime contribution from eastern and southern Asia to the middle troposphere, and a major contribution from northern Asia to the surface. (4) Measurements of ice nucleating particles (INPs) in the Arctic indicate that a major source of these particles is mineral dust, likely derived from local sources in the summer and long-range transport in the spring. In addition, INPs are abundant in the sea surface microlayer in the Arctic, and possibly play a role in ice nucleation in the atmosphere when mineral dust concentrations are low. (5) Amongst multiple aerosol components, BC was observed to have the smallest effective deposition velocities to high Arctic snow (0.03&thinsp;cm&thinsp;s−1).</p

On the Interaction between Superabsorbent Hydrogels and Blended Mixtures with Supplementary Cementitious Materials

This article studies the interactions between hydrogels with two distinct chemical compositions and hydrating blended mixtures containing supplementary cementitious materials (SCMs), including Class F fly ash, slag, and silica fume. The effect of SCMs on the absorption behavior and chemical characteristics of hydrogels was investigated. The desorption of hydrogels in hydrating blended pastes was examined at various times using scanning electron microscopy. A reduction in the absorption of hydrogels was observed in the blended slurry with fly ash compared to other slurries. Hydrogels showed faster desorption in the paste with silica fume and delayed desorption in the paste with fly ash. This was attributed to faster densification of the microstructure and development of the capillary suction in the paste with silica fume and delayed densification in the paste with fly ash at early ages

Desorption of superabsorbent hydrogels with varied chemical compositions in cementitious materials

This paper examines the desorption of hydrogels synthesized with varied chemical compositions in cementitious materials. The absorption, chemical structure and mechanical response of hydrogels swollen in a cement mixture were studied. The effect of the capillary forces on the desorption of hydrogels was investigated in relation to the chemical composition of the hydrogels. In the second set of experiments, the behavior of the hydrogels in a hydrating cement paste was monitored by tracking the size and morphology evolution of hydrogels interacting with the cement paste matrix. It was shown that the changes on the surface characteristics of hydrogels as a result of interactions with the pore solution and cement particles can affect the desorption rate of hydrogels in contact with porous cementitious materials. Scanning electron microscopic examination demonstrated two different desorption modes with distinct morphologies of hydrogels depending on the chemical composition of hydrogels. The effect of the interfacial bonding between the hydrogels and the cementitious matrix and its relation to the desorption is illustrated

Ductile failure in polycrystalline OFHC copper

AbstractDuctile failure in polycrystalline oxygen-free, high-conductivity copper is explored through uniaxial tension experiments. Specimens obtained through tests interrupted at various stages of deformation and failure evolution are examined through quantitative microscopy to discern the mechanisms of failure and the local strain evolution. The formation of a single rectangular prismatic channel-like cavity, and its subsequent growth as a rectangular cavity, are demonstrated. Fractographic observations are used to suggest that self-similar expansion of the cavity is through an alternating slip mechanism. Local strain levels are estimated from measurements of the change in grain size with deformation and used to indicate that the local values of failure strains are likely to be much larger than that estimated from strains averaged over characteristic specimen dimensions such as the gage length or the specimen diameter