Abstract
Molten marks identified at a fire site can aid in determining the cause of a fire. Quantitative analysis of such molten marks has not yet been reported, despite several identification methods for molten marks being proposed based on optical micrographs. Herein, we propose a new methodology to establish the quantitative microstructure parameters of the molten marks using electron backscatter diffraction (EBSD). The globules were generated by heating the copper conductor to 1100°C in a non-energized state. Then, the arc beads were prepared by shorting the copper wire at 25°C and 900°C in an energized state. The globules did not show evident demarcation lines and indicated that the microstructure consisted of globular or dendritic grains; this demonstrated that their characteristics were distinctly different from those of arc beads. However, the primary arc beads (PABs) shorted at 25°C exhibited a strong (001) texture perpendicular to the demarcation line and comprised large fractions of columnar grains with a small grain aspect ratio (GAR). The microstructure of secondary arc beads (SABs) shorted at 900°C presented a mixture of elongated and equiaxed grains with a large GAR and no specific development of texture. The GAR and (001) fraction perpendicular to the demarcation line could be discriminant parameters of PABs and SABs. Consequently, EBSD methods can be newly applied for the classification of globules, PABs, and SABs based on the quantitative microstructure information and orientation distribution.
Highlights
Molten marks refer to rounded masses of melted copper
primary arc beads (PABs) are molten marks resulting from the initial short circuit that acts as a source of ignition
The globules exhibited gradual necking in appearance and indistinct demarcation lines in the electron backscatter diffraction (EBSD) results
Summary
Molten marks refer to rounded masses of melted copper. Molten marks are classified into two categories [1, 2]: globules melted by fire and the arc beads melted by electrical energy. Globules are molten marks formed at high temperatures above the melting point (1083°C) of copper in a non-energized state. Arc beads are molten marks formed when a significant short-circuit current surges through an electrical wire. PABs are molten marks resulting from the initial short circuit that acts as a source of ignition. SABs are molten marks formed due to a short circuit that occurred in an energized state after the wire insulation had been burned off by the flames. The molten marks identified at a fire site can aid in determining the cause of the fire
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