Self-assembled nanotubular arrays on Ti alloys could be used for more effective implantable devices in various medical approaches. In the present work, the adhesion of TiO2 nanotubes (TiO2 NTs) on Ti-6Al-4V (Ti64) was investigated by laser spallation and scratch test techniques. At first, electrochemical anodization was performed in an ammonium fluoride solution dissolved in a 90:10 ethane-1,2-diol (ethylene glycol) and water solvent mixture. This process was performed at room temperature (23?C) at a steady potential of 60 V for 1 h. Next, the TiO2 nanotubes layer was heat-treated to improve the adhesion of the coating. The formation of selforganized TiO2 nanotubes as well as the microstructural evolution, are strongly dependent on the processing parameters and subsequent annealing. From microscopic analysis, highly oriented arrays of TiO2 nanotubes were grown by thermal treatment for 90min at 500?C. Further heat treatment above 500?C led to the detachment of the nanotubes and the complete destruction of the nanotubes occurred at temperature above 700?C. Scratch test analysis over a constant scratch length (1000 ?m) indicated that the failure point was shifted from 247.4 to 557.9 ?m while the adhesion strength was increased from ~862 to ~1814mN after annealing at 500?C. The adhesion measurement determined by laser spallation technique provided an intrinsic adhesion strength of 51.4MPa for the TiO2 nanotubes on the Ti64 substrate.
The plateau stress and energy absorption of low density (≤300 kg/m3) polyurea (PU) foams and expanded polystyrene (EPS) were measured at deformation rates ranging from 0.004 s−1 to 5000 s−1. Low (≤10−1 s−1) strain rate testing was performed using an Instron load frame, intermediate (101–102 s−1) strain rates using a drop-weight impact tower, and high (≥103 s−1) strain rate conditions using a modified split-Hopkinson pressure bar. The plateau stress and energy absorption of low density PU foams exhibit a strong rate dependence across all deformation rates. This result has been previously unreported for low density polymer foams under low and intermediate strain rates. The strain rate sensitivity of PU foams was found to be strongly dependent on cell size for low strain rates and cell wall aperture size for intermediate and high strain rates. EPS type foam, however, remained nearly insensitive to strain rate. At low and intermediate strain rates, the plastic crushing in the EPS and the high plateau stress yield a much higher energy absorption capability than the viscoelastic dissipation in the PU foams. However, PU foams were found to display similar energy absorption properties as EPS based foams under high strain rates. Thus, controlling the strain rate sensitivity of PU foams through aperture diameter can lead to an increase in energy absorption properties at high strain rates, while simultaneously maintaining the peak stress below certain injury thresholds. Additionally, unlike EPS, which undergo plastic crushing after first impact, flexible polyurea foams will recover fully after each impact and thus will have multiple hit capabilities. This will allow these materials to have a wide range of applications, in advance body armors and protective headgears to use in low-cost protection systems for a wide range of military platforms, civilian, and space applications.
Understanding the particle-scale dynamics of granular materials during rapid compaction and flow is of fundamental importance for manufacturing, planetary science, geology, and defense applications. Time-resolved 2D radiography and static 3D x-ray tomography are powerful in situ tools for studying particle-scale dynamics but provide detail only in 2D or with significant time-scale limitations, respectively. Here, we introduce a new method that uses 2D in situ x-ray imaging for determining time-resolved 3D particle-scale dynamics in rapidly compressed granular materials. The method employs initial particle packing structures obtained from x-ray tomography, a 2D x-ray image generation algorithm, and an optimization algorithm. We first describe and validate the method using finite element simulations. We then apply the technique to x-ray phase-contrast images obtained during rapid compaction of granular materials with varying particle morphology and sample thickness. The depth-resolved particle-scale dynamics reveal complex velocity and porosity fields evolving heterogeneously along and perpendicular to the compaction direction. We characterize these features, their fluctuations near the compaction front, and the compaction front thickness. Our technique can be applied to understanding granular dynamics during rapid compaction events, and rearrangements during slower, but non-quasi-static, flows.
An outstanding challenge in developing a complete equation of state for materials at elevated pressure and temperature is a robust method of determining the bulk temperature state under dynamic conditions. In metals, the determination of bulk temperature states by optical pyrometry is complicated by the small optical depth and thermal conduction effects. These effects lead to observed temperatures differing by 20% or more from the bulk temperature state. In this work, we show the presence of thermal conduction effects in temperature measurements of tin and iron coatings during dynamic compression experiments. We demonstrate that tin, in contrast to iron, coatings can fail to converge to a bulk temperature source over the time scale of the experiment, requiring the experimenter to modify assumptions, design, or analysis. This work bounds thermal transport at shocked conditions.
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