A “Toolbox” for Forensic Engineers

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A “Toolbox” for Forensic Engineers 99 Figure 4.8 Sketches of the components and their respective compositions. metal eye splinter will illustrate just how valuable metallography can be as a tool. A workman in a machine shop was removing a large drill, with a Morse taper shank, from a radial arm drilling machine. He said that he had used a soft (steel) drift to remove the drill from the machine, striking the drift with an ordinary engineer’s hammer. This method was a normal everyday (and correct) procedure for drill removal. However, while undertaking the routine, the workman was hit in the eye by a flying metal splinter. The eye splinter was submitted to ascertain which of the tools being used was the likely source. The components and their respective compositions are shown in Figure 4.8. Because all four components in this case were made from different materials, it was decided to examine their respective microstructures and compare them with those of the splinter, in order to establish the source of the splinter. The microstructures are shown in Figure 4.9. Direct microstructural comparison clearly reveals the eye splinter is a small piece of HSS, broken from one of the outer flutes of the drill, not the hardened steel hammer head, the medium carbon steel drift or the soft iron Morse taper shank. It was therefore a straightforward task to arrive at a sequence of events that caused the metal splinter to pierce the workman’s eye. The simple answer was that the workman must have struck the drill flutes with the hammer when extracting the drill from the machine, and NOT used the drift as claimed. A trained workshop engineer would know full well that drill flutes are exceptionally hard, with little or no temper — therefore prone to splinter or shatter under impact loading — and the hardened steel face of a hammer is perhaps the worst tool he could use to knock it free. The resultant eye injury was therefore a direct consequence of the workman’s irresponsible mistake or poor working practice, as well as lack of eye protection.
100 Forensic Materials Engineering: Case Studies Figure 4.9 Micrographic montage showing the structures of each component and the eye splinter. (A) Martensitic structure of the hammer head. (B) Annealed 0.4% carbon steel drift. (C) 0.2% carbon steel tapered shank of the drill. (D) Splinter fragment removed from eye. (E) HSS steel drill tip fragment transverse section (longitudinal flutes). (F) HSS steel drill tip fragment longitudinal section (transverse flutes). 4.4.6 Scanning Electron Microscopy (SEM) Advances in electronics and instrumentation have brought microanalytical techniques out of the realms of research and into the hands of everyday failure analysis. One of the best-known instruments is the scanning electron microscope (SEM). Fractography (observing the fracture surface of materials) is one of the most popular uses of the SEM. To illustrate the potential of SEM for forensic examination, Figure 4.10 typifies a fatigue failure of a fracture toughness sample. The fracture surface is shown at two different locations
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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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13.3.7 Design Development Sequence
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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 43 Figure 2.8
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Accidents in the Workplace 255 8 De
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A “Toolbox” for Forensic Engineers

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