Microstructural characterization of pipe bomb fragments

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Abstract

Recovered pipe bomb fragments, exploded under controlled conditions, have been characterized using scanning electron microscopy, optical microscopy and microhardness. Specifically, this paper examines the microstructural changes in plain carbon-steel fragments collected after the controlled explosion of galvanized, schedule 40, continuously welded, steel pipes filled with various smokeless powders. A number of microstructural changes were observed in the recovered pipe fragments: deformation of the soft alpha-ferrite grains, deformation of pearlite colonies, twin formation, bands of distorted pearlite colonies, slip bands, and cross-slip bands. These microstructural changes were correlated with the relative energy of the smokeless powder fillers. The energy of the smokeless powder was reflected in a reduction in thickness of the pipe fragments (due to plastic strain prior to fracture) and an increase in microhardness. Moreover, within fragments from a single pipe, there was a radial variation in microhardness, with the microhardness at the outer wall being greater than that at the inner wall. These findings were consistent with the premise that, with the high energy fillers, extensive plastic deformation and wall thinning occurred prior to pipe fracture. Ultimately, the information collected from this investigation will be used to develop a database, where the fragment microstructure and microhardness will be correlated with type of explosive filler and bomb design. Some analyses, specifically wall thinning and microhardness, may aid in field characterization of explosive devices.

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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