Mapping Blue and Red Color-Coated Steel Sheet Roof Buildings over China Using Sentinel-2A/B MSIL2A Images

Accurate and efficiently updated information on color-coated steel sheet (CCSS) roof materials in urban areas is of great significance for understanding the potential impact, challenges, and issues of these materials on urban sustainable development, human health, and the environment. Thanks to the development of Earth observation technologies, remote sensing (RS) provides abundant data to identify and map CCSS materials with different colors in urban areas. However, existing studies are still quite challenging with regards to the data collection and processing costs, particularly in wide geographical areas. Combining free access high-resolution RS data and a cloud computing platform, i.e., Sentinel-2A/B data sets and Google Earth Engine (GEE), this study aims at CCSS material identification and mapping. Specifically, six novel spectral indexes that use Sentinel-2A/B MSIL2A data are proposed for blue and red CCSS material identification, namely the normalized difference blue building index (NDBBI), the normalized difference red building index NDRBI, the enhanced blue building index (EBBI), the enhanced red building index (ERBI), the logical blue building index (LBBI) and the logical red building index (LRBI). These indexes are qualitatively and quantitatively evaluated on a very large number of urban sites all over the P.R. China and compared with the state-of-the-art redness and blueness indexes (RI and BI, respectively). The results demonstrate that the proposed indexes, specifically the LRBI and LBBI, are highly effective in visual evaluation, clearly detecting and discriminating blue and red CCSS covers from other urban materials. Results show that urban areas from the northern parts of P.R. China have larger proportions of blue and red CCSS materials, and areas of blue and red CCSS material buildings are positively correlated with population and urban size at the provincial level across China.

Numerical Study of the Effect of Workpiece Thickness on the Interfacial Behavior of a Molten Aluminum Droplet on a Zinc-coated Steel Sheet

In brazing, the interfacial conditions between the molten filler material and the solid workpiece are important, yet they cannot be observed experimentally. A two-dimensional axisymmetric simulation was conducted to analyze the behavior of a single droplet of molten aluminum on zinc-coated steel sheet as a simplified brazing process. The simulation models were verified through a comparison with experimental results in terms of bead shape, zinc distribution, and molten metal behavior. The results show that Young’s equation was not valid in explaining the wetting behavior because of the instant solidification. In this respect, the effects of the workpiece thickness and wetting angle on the bead width were negligible. Two periods of time, namely the times for the temperature difference and solidification, and their ratio (interface number) were defined to analyze the temperature behavior at the interface over time as well as the effects of workpiece thickness. The interfacial temperature behaviors tended to be divided into three regions: linear (or inversely proportional), singular, and convergence. The interface number converged to a value of one with the increase in the thickness.

Friction-Stir Spot Mechanical Joining between Hot-Dip 55% Aluminum–Zinc Alloy-Coated Steel Sheet and A5083 Aluminum Alloy Plate Using Conventional Punching

In this study, we fabricated a mechanical joining with spot friction-stirring between an aluminum alloy plate and an aluminum–zinc alloy-coated steel sheet; this was achieved by utilizing a conventional press-punching preprocess. In the preprocessing, a hot-dip 55% aluminum–zinc alloy-coated steel sheet was punched using a press and an inclined surface was generated at the wall of a punched hole due to shear droop. Subsequently, an A5083P-O aluminum alloy plate was positioned against it, and friction stirring was conducted on its rear surface. The hole was filled with the aluminum alloy to generate a mechanical interlock at the tilt of the wall. Further, we evaluated its cross tensile strength (CTS) and tensile shear strength (TSS) and obtained an average CTS and TSS of 368 kN and 1470 kN, respectively.