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Neu, Richard W.

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    Understanding the Role of Glaze Layer With Multiple Surface Characterization Techniques Aligned by Computer Vision Algorithms
    (Georgia Institute of Technology, 2021-02-10) Zhang, Chuchu ; Neu, Richard W.
    A glaze layer that significantly reduces friction and wear has been found on the surface of many Fe-, Cr-, and Ni-based material systems undergoing fretting/sliding at elevated temperature. In this work we proposed a novel way to understand the role of glaze layer using computer vision algorithms. Two workflows, one for quantitative glaze layer identification and the other for image alignment, have been developed. For glaze layer identification, we used computer vision concepts that considers the color and reflectiveness of glaze layer under optical microscope (OM). For image alignment, we developed a strategy to conduct pixel-to-pixel alignment of images acquired by multiple techniques (e.g., OM, scanning electron microscopy, 3D optical profilers) with sub-pixel error. As such, the correlation between the height map and locations of the glaze layer within the wear scar can be readily determined. These methods are used to evaluate wear scars and quantify glaze layer coverage on 310S stainless steel under like-on-like, cylinder-on-flat fretting conditions from 20°C to 700°C. The glaze layer is found to always occupy relatively high locations within wear scar, and the height difference between glaze layer and none-glaze layer is statistically significant. The results provide evidence that severe-to-mild wear transition resulted from spreading of glaze layer coverage, and glaze layer may reduce friction and wear by reducing real contact area. The open-source workflows we developed are powerful tools that enable multi-spectrum analysis without upgrading existing characterization tools, which can be easily transferable to all other applications in academia and industry.
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    Remotely Measuring Mechanical Properties of Frozen Surfaces Using Indentation Methods
    (Georgia Institute of Technology, 2020-04) Bhat, Anirudh ; Neu, Richard W.
    The uniaxial stress-strain curve of a material is an important mechanical property, which is required to model the response of material and structures. To get this property, a tensile test is usually conducted, which requires a large load frame and a specific test specimen. However, in certain cases this is not feasible because of the lack of material to fabricate a specimen or the inability to use a load frame. Spherical indentation has been shown to capture the complete elastic-plastic response of polycrystalline alloys from the force-displacement data. This technique has an advantage of being compact and able to test small volumes of the material compared to conventional uniaxial testing. However, due the higher hydrostatic pressure associated with the constraint of the surrounding material under the indenter tip, yielding occurs at a higher stress than it would for the same material under uniaxial load, hence the two are stress-strain curves are not the same. We have developed a model that is able to extract uniaxial stress-strain curves from indentation data and validated it on Al 7050 samples with a large variation of mechanical properties as shown in Figure 1. Europa’s surface is postulated to be composed of hydrated Mg- and Na-sulfate hydrate salts mixed with water ice. The analysis of terrestrial ice has shown that their microstructures are very reminiscent of polycrystalline alloys. For this reason, the analysis procedure developed here could easily be applied to measure the uniaxial mechanical properties of Europa’s icy surface. The advantage of the indentation techniques lies in the simple data collection (indentation force and depth) and specimen preparation. Furthermore, a testing fixture can be easily adapted to measure other mechanical properties such as, creep properties, fracture and impact toughness by simply changing the indenter tip, like a Swiss army knife of mechanical characterization tools.