Microstructural characterization of pipe bomb fragments
Introduction
In the United States, pipe bombs are a common improvised explosive device. Construction is straightforward, and both metal and plastic pipes are readily available. Many of the pipe fillers are propellants which may include black powder, smokeless powder, photo and fireworks powder, and even match heads which can be purchased without special permit [1]. Over the years, law enforcement has developed robust protocols for processing bomb scenes and analytical procedures for identifying explosive residue, but chemical residue is not always recovered [2], [3]. Generally, the recovered fragments are intermixed with a host of metal shards and debris characteristic of the chaotic scene of an explosion. A wealth of forensic information remains unexploited in terms of “metallurgical evidence” found in each pipe fragment. Such evidence is embodied in changes in the microstructure and microhardness of the pipe fragments. Metallurgical changes in armor steels have been extensively studied in order to estimate explosive loadings [4]. The degree of shock loading in plain carbon steel is reflected by pearlite and ferrite grain deformation, twinning, and slip band formation [5]. This study attempts to correlate such microstructural features found in recovered steel pipe bomb fragments with the power and quantity of the propellant used, as well as other characteristics of the initial device [6]. The characterization included the application of quantitative stereology techniques to study the microstructural changes that occur in plain carbon-steel pipes due to high strain rate deformation and variations in microhardness obtained under different loading conditions. A correlation between pipe fragment microstructure, microhardness, and wall thinning with the pipes propellant filler may provide clues concerning the relative nature of the propellant in cases where chemical residue is not recovered.
Section snippets
Materials/Methods
Commercially available, continuously welded, schedule 40 steel pipes [2 in. I.D. by 12 in. long (5 cm × 30.5 cm)] were used in this investigation. These galvanized AISI 1030 steel pipes nominally contain 0.3% carbon by weight and trace amounts of other impurities. Standard end caps were used at both ends of the threaded pipes and the pipes were usually initiated in a vertical orientation, with the detonator threaded through a hole drilled in the top end cap. To contain the fragments, pipes were
Thinning
Considerable thinning of the pipe wall was usually observed. Average fragment wall thickness is shown in Fig. 2, along with the associated plastic strain. Reduction in wall thickness was caused by plastic deformation that occurred prior to the rupture of the pipe material during explosion [10]. As the pressure wave propagated through the pipe, the material at the inner wall was placed in compression. This caused the material in the outer wall to expand (deform) biaxially and, as a result, the
Conclusions
More powerful, faster burning, propellants produced more plastic deformation in steel pipe bombs prior to fracture than lower power propellants. This can be shown by quantitative stereology and microhardness analysis of the pipe fragments recovered after the explosion. As the power of the propellant filler increased, the extent of work hardening in the pipe fragments also increased, as measured by microhardness. A radial variation in microhardness was observed, with larger Knoop Hardness values
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