A “Toolbox” for Forensic Engineers

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A “Toolbox” for Forensic Engineers 87 Such visual evidence will need to be recorded in photographs. “A simple picture is worth more than a thousand words” is literally true, as well as more convincing. Visual observations of an incident or a failed product must be recorded by camera or video film at the scene of an incident or at the time the product has failed, or during the dismantling of a device. The value of recording evidence is shown by a case that involved contamination of an industrial site. Scene of Incident Inspection. A sheet metal fabricator wanted to pinpoint the source of contamination of material and finished goods held in the company stockyard. The nature of work being undertaken at the site demanded the use of galvanized, clad or prepainted stock material to generate a specific surface finish on the end product. Contamination from the work yard of an adjacent company had caused areas of staining and corrosion over the stock material, rendering it worthless for the manufacturing process. A visit to the site was made in order to take photographs and select samples of the contaminating material, for analysis and identification of the contaminant source. The samples were taken from selected areas around the work yard of the adjacent company, and a photographic record was made of each site of contamination. However, the following day it was decided to return to the adjacent site to take additional photographs because the lighting levels were better (the sun had come out). On entering the yard, all evidence of contamination had been removed by a bulldozer, only hours after the initial visit. Sample evidence and related photography, gathered from the first visit, were sufficient to prove the source of the contamination. It is impossible to know when circumstance may intervene. 4.2.1 Significance of Initial Observations An example that relied on understanding the circumstances that surrounded an event concerned the failure of a hoist carrier. This example will also highlight the use of the human ear as a prime investigative tool. In a factory making upholstery, slices were taken off a large block of polyurethane, rolled up and then carried over the slicing machine to a dispatch area. The carrier was a bar with chains and hooks at both ends, and a shackle attached to a wire rope hoist at the center. The carrier bar broke at its center and one half of the transported material fell onto a man’s head, causing serious injury. A graphic representation of the relevant components is shown in Figure 4.1. The bar appeared to be far stronger than it needed to be to carry the few kilograms weight of the roll. Photographs taken by a factory inspector were available and calculation showed that the bar was only lightly loaded during the lift and traverse to the dispatch area.
88 Forensic Materials Engineering: Case Studies Frame (in Roof) Carrier Bar Chain ? Fractured Surface Hoist motor Cable Hooks to Carry Rolls of Foam Figure 4.1 Graphic sketches of hoist, lifting bar, foam roll and fracture surface. A visit to observe the replacement bar in operation revealed the cause. On the return from the dispatch area the bar had to be lifted up until it came up against the stop for the hoist pulley. It swung wildly from side to side. The operator raised it at full speed, traversed back to the slicing machine and then lowered it so that the chains could be attached to the next roll. Every time it reached the top of the return lift, the swinging end of the bar slammed against the frame on which the hoist was mounted. Eventually the bar broke at the center as a result of metal fatigue. The clue to the cause of the fatigue was the loud clank as the swinging bar came up against the hoist limit switch. Without hearing this in action, it would have been difficult to discover where the fatigue stresses originated. They had nothing to do with loads carried by the hoist. It was the repeated violent slams on the return journey, and when the hoist was not carrying any load, that caused the failure. What can be found by simple observations may turn out to be highly significant and pre-empt the need for further or detailed laboratory examination. In the case described, it was prior events that revealed how failure ? Fracture
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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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Accidents in the Workplace 255 8 De
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A “Toolbox” for Forensic Engineers

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