A “Toolbox” for Forensic Engineers

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Criminal Cases 367 Figure 12.2 (A) Etched microsection of legally minted coin made from wrought copper nickel alloy showing recrystallized grains with directional coring from original cast billet from which the coinage strip was rolled (magnification: 75¥). (B) Etched microsection of counterfeit coin made by casting, revealed by large irregular grains having nickel rich cores merging into copper rich matrix due to nonequilibrium cooling from the liquid state (magnification: 75¥). so sharply defined as a coin made in a coining press from solid metal, but they were practically impossible to detect visually in normal transactions. There were, however, physical ways of detecting the counterfeits. Legally minted coins start off as cast slabs that are rolled down to thin strip and the coins then blanked out and stamped in power presses with machined dies. This is sometimes followed by milling the edges. The counterfeit coins were as-cast so they had a different microstructure, as illustrated in Figure 12.2A and Figure 12.2B, which are etched sections of the wrought and the cast cupro-nickel alloy, respectively. The cast structure (Figure 12.2B) exhibits large grains that are cored (that is a compositional segregation due to the first solid to appear being higher in nickel than that formed near the end of the solidification process), which is especially pronounced in copper-nickel alloys. Figure 12.2A shows a longitudinal section of a minted coin that has
368 Forensic Materials Engineering: Case Studies been rolled from a 125-mm billet to a 3-mm-thick strip. This exhibits smaller equiaxed grains that are drawn out in the direction of working and the coring segregation is oriented in the direction of rolling. A microstructural examination requires a section to be cut from the suspect coin and to be polished and etched. Once it had been established that the counterfeits were being made by casting, then physical tests that do not require such preparation could be devised to quickly sort them. However, as the value of the currency fell and the size of the coins was later reduced, the risk and surrepticious effort were unjustified, so the quantity of forged coins in circulation fell. 12.2 Shotgun Pellets A drug dealer was murdered, shot with a double-barrelled shotgun. Two cartridges had been discharged at close range, the pellets from the first shot hitting him in the chest and those from the second in the lower abdomen. A week or so before the murder he had received a threatening letter about defaulting on payments to which had been taped 10 shotgun pellets, found to be the same size as those of the 48 recovered from the chest wound. Pellets recovered from the lower wound were larger. The prime suspect arrested was found to own a double-barrelled shotgun and there was evidence to suggest this person was the sender of the letter, although this was vehemently denied. The 10 pellets on the letter were individually weighed and analyzed very accurately for 12 elements, including impurities usually present at low levels in lead shot as well as the principal alloy elements, antimony and arsenic. Both harden the shot but the arsenic also increases surface tension of the molten metal so that it will form spherical droplets in the shot-making process. These results were compared statistically with similar analyses of 20 pellets from the chest wound. Mean weights of these two batches were found to be 0.1107 and 0.1088 g with standard deviations of 0.0077 and 0.0083 g, respectively. The forensic science laboratory that conducted this investigation had built up a database of over 600 analyses of shotgun pellets covering the full range of commercially produced shot sizes. The laboratory reported there was a closer match between the pellets from the letter and the ones recovered from the chest wound than could be found when matching either group of these pellets with its database. On this basis the laboratory concluded that the murder pellets were from the same cartridge as those on the letter. At trial this conclusion was challenged by three different experts. A metallurgist pointed out that the various alloys are prepared and their compositions adjusted in a furnace holding between 50 and 100 t of metal. This is cast into 25-kg ingots and sent to various shotmakers. The shotmakers
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Library of Congress Cataloging-in-P
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Preface Case studies of product fai
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could not be simpler in shape, they
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Acknowledgments We thank The Open U
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The Authors Peter R. Lewis, Ph.D.,
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Laboratories of Imperial College of
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3 Establishing the Load Transfer Pa
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6.5.2 A Problem in Eire 197 6.5.3 M
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9.4.2 Failure in Femoral Stem of To
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13.3.7 Design Development Sequence
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Materials in Distress 2.1 Introduct
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Materials in Distress 27 It is usef
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Materials in Distress 29 (A) Unload
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Materials in Distress 31 1. Flowing
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Materials in Distress 33 to compens
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Materials in Distress 35 Fracture o
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Materials in Distress 43 Figure 2.8
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Accidents in the Workplace 255 8 De
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Accidents in the Workplace 257 ensu
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Accidents in the Workplace 259 4. F
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A “Toolbox” for Forensic Engineers

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