RESUMEN
The hole expansion ratio (HER) test is used to determine the stretch-flangeability of materials. Standard HER tests are performed on specimens sized a few tens of centimeters, termed macro-HER tests. This leads to significant material wastage due to the destructive nature of the tests. No information at the microstructure length scale is obtained, and the results suffer from user uncertainty in the identification of the through-thickness crack. This paper presents a novel miniature HER setup (termed micro-HER test), in which miniature specimens are tested inside a scanning electron microscope (SEM). The deformation is imaged from the top using the secondary electron detector installed in the SEM, coupled with the digital image correlation (DIC) technique, allowing for measurement of full field strains at the microstructural scale and identifying their deformation/fracture mechanisms. As a case study, six different steel grades were tested to measure their micro-HER values and compare them with the corresponding macro-HER values. The latter were found to be higher for the more ductile grades of steel. Late detection of through-thickness cracks and thicker samples leading to a higher volume of plastic deformation could contribute to this overestimation of values in macro-HER tests. DIC results from micro-HER tests on a ferrite-martensite (10% volume fraction) dual-phase steel showed high magnitudes of strain localization at the ferrite-martensite interfacial regions, indicating that such interfaces might be hotspots for failure under triaxial stress states. The challenges and errors associated with the measurements are also discussed.
RESUMEN
Pigments can retain their color for many centuries and can withstand the effects of light and weather. The paint industry suffers from issues like aggressive moisture, corrosion, and further environmental contamination of the pigment materials. Low-cost, long-lasting, and large-scale pigments are highly desirable to protect against the challenges of contamination that exist in the paint industry. This exploratory study reinforces the color and thermal stability of industrial-grade (IG) magnetite (Fe3O4). IG Fe3O4 pigments were further considered for surface treatment with sodium hexametaphosphate (SHMP). This metaphosphate hexamer sequestrant provides good dispersion ability and a high surface energy giving thermal and dust protection to the pigment. Various physicochemical characterizations were employed to understand the effectiveness of this treatment across various temperatures (180-300 °C). The X-ray diffraction, Raman, and X-ray photoelectron spectroscopy techniques signify that the SHMP-treated Fe3O4 acquired magnetite phase stability up to 300 °C. In addition, the delta-E color difference method was also adopted to measure the effective pigment properties, where the delta-E value significantly decreased from 8.77 to 0.84 once treated with SHMP at 300 °C. The distinct color retention at 300 °C and the improved dispersion properties of surface-treated Fe3O4 positions this pigment as a robust candidate for high-temperature paint and coating applications. This study further encompasses an effort to design low-cost, large-scale, and thermally stable pigments that can protect against UV-rays, dust, corrosion, and other color contaminants that are endured by building paints.