P.G. Allison

Tungsten Toxicity, Bioaccumulation, and Compartmentalization into Organisms Representing Two Trophic Levels

Metallic tungsten has civil and military applications and was considered a green alternative to lead. Recent reports of contamination in drinking water and soil have raised scrutiny and suspended some applications. This investigation employed the cabbage Brassica oleracae and snail Otala lactea as models to determine the toxicological implications of sodium tungstate and an aged tungsten powder-spiked soil containing monomeric and polymeric tungstates. Aged soil bioassays indicated cabbage growth was impaired at 436 mg of W/kg, while snail survival was not impacted up to 3793 mg of W/kg. In a dermal exposure, sodium tungstate was more toxic to the snail, with a lethal median concentration of 859 mg of W/kg. While the snail significantly bioaccumulated tungsten, predominately in the hepatopancreas, cabbage leaves bioaccumulated much higher concentrations. Synchrotron-based mapping indicated the highest levels of W were in the veins of cabbage leaves. Our results suggest snails consuming contaminated cabbage accumulated higher tungsten concentrations relative to the concentrations directly bioaccumulated from soil, indicating the importance of robust trophic transfer investigations. Finally, synchrotron mapping provided evidence of tungsten in the inner layer of the snail shell, suggesting potential use of snail shells as a biomonitoring tool for metal contamination.

Finite element modeling of multilayered structures of fish scales

The interlinked fish scales of Atractosteus spatula (alligator gar) and Polypterus senegalus (gray and albino bichir) are effective multilayered armor systems for protecting fish from threats such as aggressive conspecific interactions or predation. Both types of fish scales have multi-layered structures with a harder and stiffer outer layer, and softer and more compliant inner layers. However, there are differences in relative layer thickness, property mismatch between layers, the property gradations and nanostructures in each layer. The fracture paths and patterns of both scales under microindentation loads were different. In this work, finite element models of fish scales of A. spatula and P. senegalus were built to investigate the mechanics of their multi-layered structures under penetration loads. The models simulate a rigid microindenter penetrating the fish scales quasi-statically to understand the observed experimental results. Study results indicate that the different fracture patterns and crack paths observed in the experiments were related to the different stress fields caused by the differences in layer thickness, and spatial distribution of the elastic and plastic properties in the layers, and the differences in interface properties. The parametric studies and experimental results suggest that smaller fish such as P. senegalus may have adopted a thinner outer layer for light-weighting and improved mobility, and meanwhile adopted higher strength and higher modulus at the outer layer, and stronger interface properties to prevent ring cracking and interface cracking, and larger fish such as A. spatula and Arapaima gigas have lower strength and lower modulus at the outer layers and weaker interface properties, but have adopted thicker outer layers to provide adequate protection against ring cracking and interface cracking, possibly because weight is less of a concern relative to the smaller fish such as P. senegalus.

Strain-Controlled Low-Cycle Fatigue Properties of Extruded 6061-T6 Aluminum Alloy

One of the most commonly extruded aluminum alloys is the 6061 alloy largely because of its good formability and high specific strength (Ref 1). The ability of the 6061 aluminum alloy to be extruded or otherwise formed into complex geometries at relatively low cost is the reason for its widespread use in aerospace, construction, transportation, and many other industries. In fact, 6061 aluminum alloy has been produced in various forms including plate, extrusion, foil, sheets, pipes, forgings, and structural forms (Ref 2). Since most load bearing components fail due to cyclic loading, analytical prediction and finite element modeling of fatigue damage of such a prevalent alloy is critical for safe designs. A review of literature regarding fatigue in 6061 aluminum alloys reveals most experimental studies characterized fatigue behavior based on load-control cyclic tests. However, for applications where low-cycle fatigue (LCF) is dominant with variable-amplitude histories and sequence effects, strain-controlled fatigue tests better characterize the fatigue behavior compared to stress-controlled tests (Ref 3). However, only a few data sets on strain-controlled fatigue of 6061 aluminum alloy exist and are not easily located, with the one data set published in an obscure journal (Ref 4), and the other published in a conference proceeding (Ref 5). The purpose of this paper is to provide readily available LCF properties of an extruded 6061-T6 extruded aluminum alloy and to validate the results of Ref 5. Furthermore, the cyclic stress-strain behavior, including initial and stabilized hysteresis loops, is not reported in literature and thus will be presented here. The material used in this study was an extruded 6061-T6 aluminum alloy. Taber Extrusion, LLC (Russellville, AR) provided the extruded panels. The nominal composition in atomic weight of the 6061 aluminum alloy is given in Table 1 (Ref 6). For comparison purposes, Table 2 lists the monotonic properties of the extruded 6061-T6 aluminum alloy. Cylindrical specimens were machined from the extruded alloy so that the loading axis was parallel to the extrusion direction (ED). The fatigue specimens were designed following ASTM Standard E.606 specifications (Ref 7) with a nominal 27-mm gage length and diameter of 6.35 mm. Prior to testing, all specimens were hand ground in the loading directionwith 800-grit silicon carbide paper to remove residual stresses and any machining marks. Fully reversed cyclic (R = 1) tests were performed using a servo-hydraulic MTS load frame at room temperature with relative humidity level of 45%. The fatigue specimens were tested to failure under strain-controlled conditions at 5 Hz frequency at the following strain amplitudes: 0.002, 0.003, 0.004, 0.005, 0.006, and 0.007. The specimen tested at 0.002 strain amplitude was fatigued at 5 Hz to 20,000 cycles, and then the test was stopped and resumed in load control at 30 Hz to failure. A fatigue-rated MTS extensometer with a 25.4-mm length gage was used to directly measure the axial strain induced in the gage length. Final failure of the specimen was defined as a 50% drop in peak load during the test, as recommended by ASTM Standard E.606 (Ref 7). Figure 1 shows the results of the strain-controlled fatigue tests conducted on the extruded 6061-T6 aluminum alloy. Note that the 0.002 amplitude shown in Fig. 1 did not fail but is plotted here as a run-out. Also shown in Fig. 1 is the strain-life equation (Ref 3), including the elastic and plastic strain amplitudes, where the total strain-life is the summation of the elastic and plastic strain components. In strain-controlled cyclic deformation, the elastic strain amplitude is more dominant in small strains or longer lives, and the plastic strain amplitude is particularly dominant in large strains or short lives (Ref 3). The total fatigue life is shown in Eq 1

Mechanical, thermal, and microstructural analysis of polyvinyl alcohol/montmorillonite nanocomposites

Structural biomaterials such as nacre, bone, and fish scales possess unique structures that have hierarchical spatial configurations, which provide excellent mechanical properties when compared to their individual constituents. These observations have been the motivation for designing and characterizing bioinspired materials with high strength, high stiffness, and corrosion-resistant properties while at the same time being environmentally friendly. It has been demonstrated that polymer-clay nanocomposites can simulate the behavior of nacreous biomaterials such as abalone shell. Mechanical, thermal, and microstructural analyses characterized solution-cast polyvinyl alcohol (PVA)/montmorillonite (MMT) nanocomposite properties over compositions ranging from the neat polymer to 25% volume fraction of MMT nanoclay. Uniaxial tensile experiments were performed at displacement rates of 1 mm/min and 50 mm/min. Strength values are similar to those shown by nacre and represent a homogeneous dispersion of the MMT in the polymer matrix. Strength-to-weight ratios are similar to many structural metals.

Characterization of multi-layered fish scales (Atractosteus spatula) using nanoindentation, X-ray CT, FTIR, and SEM.

The hierarchical architecture of protective biological materials such as mineralized fish scales, gastropod shells, ram’s horn, antlers, and turtle shells provides unique design principles with potentials for guiding the design of protective materials and systems in the future. Understanding the structure-property relationships for these material systems at the microscale and nanoscale where failure initiates is essential. Currently, experimental techniques such as nanoindentation, X-ray CT, and SEM provide researchers with a way to correlate the mechanical behavior with hierarchical microstructures of these material systems1-6. However, a well-defined standard procedure for specimen preparation of mineralized biomaterials is not currently available. In this study, the methods for probing spatially correlated chemical, structural, and mechanical properties of the multilayered scale of A. spatula using nanoindentation, FTIR, SEM, with energy-dispersive X-ray (EDX) microanalysis, and X-ray CT are presented.

Gastropod (Otala lactea) shell nanomechanical and structural characterization as a biomonitoring tool for dermal and dietary exposure to a model metal

Metallic tungsten (W) was initially assumed to be environmentally benign and a green alternative to lead. However, subsequent investigations showed that fishing weights and munitions containing elemental W can fragment and oxidize into complex monomeric and polymeric tungstate (WO4) species in the environment; this led to increased solubility and mobility in soils and increased bioaccumulation potential in plant and animal tissues. Here we expand on the results of our previous research, which examined tungsten toxicity, bioaccumulation, and compartmentalization into organisms, and present in this research that the bioaccumulation of W was related to greater than 50% reduction in the mechanical properties of the snail (Otala lactea), based on depth-sensing nanoindentation. Synchrotron-based X-ray fluorescence maps and X-ray diffraction measurements confirm the integration of W in newly formed layers of the shell matrix with the observed changes in shell biomechanical properties, mineralogical composition, and crystal orientation. With further development, this technology could be employed as a biomonitoring tool for historic metals contamination since unlike the more heavily studied bioaccumulation into soft tissue, shell tissue does not actively eliminate contaminants.

Ultrasonic Processing of 6061-Based Nanocomposites for High Performance Applications

Previously studies show that microstructure and mechanical properties of a cast component can be considerably improved when ceramic nanoparticles are used as a reinforcement to form a metal-matrix-nano-composite material.

Microstructure, mechanical properties and fracture behavior of 6061 aluminium alloy-based nanocomposite castings fabricated by ultrasonic processing

It has been revealed that microstructure and mechanical properties of aluminium castings can be significantly improved by adding nanoparticles as reinforcement to fabricate aluminium-based metal matrix nanocomposites (MMNCs). One of the common problems in fabricating MMNCs is the agglomeration of reinforcement nanoparticles. In the present study, ultrasonic stirring technology (UST) is deployed to assist 6061 nanocomposite casting process by promoting the dispersion and deagglomeration. Al2O3/SiC nanoparticles are used as reinforcement materials. Nanoparticles are added into the molten alloy and dispersed by ultrasonic cavitation and acoustic streaming caused by UST. The microstructure, fracture behaviour and mechanical properties of 6061-based MMNC samples have been investigated. Tensile strength and yield strength of MMNC samples remain at the same level while the elongation increases significantly compared to reference samples.

Strain Rate and Stress-State Dependence of Gray Cast Iron

An investigation of the mechanical strain rate, inelastic behavior, and microstructural evolution under deformation for an as-cast pearlitic gray cast iron (GCI) is presented. A complex network of graphite, pearlite, steadite, and particle inclusions was stereologically quantified using standard techniques to identify the potential constituents that define the structure–property relationships, with the primary focus being strain rate sensitivity (SRS) of the stress–strain behavior. Volume fractions for pearlite, graphite, steadite, and particles were determined as 74%, 16%, 9%, and 1%, respectively. Secondary dendrite arm spacing (SDAS) was quantified as 22.50 lm 6 6.07 lm. Graphite flake lengths and widths were averaged as 199 lm 6 175 lm and 4.9 lm 6 2.3 lm, respectively. Particle inclusions comprised of manganese and sulfur with an average size of 13.5 lm 6 9.9 lm. The experimental data showed that as the strain rate increased from 10 3 to 10 s , the averaged strength increased 15–20%. As the stress state changed from torsion to tension to compression at a strain of 0.003 mm/mm, the stress asymmetry increased 470% and 670% for strain rates of 10 3 and 10 s , respectively. As the strain increased, the stress asymmetry differences increased further. Coalescence of cracks emanating from the graphite flake tips exacerbated the stress asymmetry differences. An internal state variable (ISV) plasticity-damage model that separately accounts for damage nucleation, growth, and coalescence was calibrated and used to give insight into the damage and work hardening relationship. [DOI: 10.1115/1.4035616]

Laboratory Characterization of Fatigue Performance of AM2 Aluminum Airfield Matting

AbstractAM2, an airfield matting system made from extruded 6061-T6 aluminum alloy, is used to construct temporary aircraft operating surfaces. This matting system can support heavy aircraft loads even when placed directly over graded in situ soils. This paper presents the development of a test protocol and corresponding relationships that can be used to predict fatigue failure of AM2’s mechanical joints over any subgrade California bearing ratio (CBR) when subjected to high tire pressure single-wheel aircraft loading. First, full-scale simulated aircraft traffic experiments were conducted over sections of AM2 installed on subgrades with CBRs of 6, 10, 15, 25, and 100% to monitor subgrade deformation and fatigue failure. An increasing amplitude displacement function developed from a subgrade deformation model was then used to create a new laboratory procedure to simulate fatigue experienced by the matting system’s complex mechanical connectors under moving aircraft loads. Laboratory test results had strong...