A “Toolbox” for Forensic Engineers

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Failure of Storage Vessels 233 Figure 7.17 EDAX spectrum showing presence of aluminum and silicon with explanatory diagram of cause of fire below (schematic of probable failure mode from aluminum stud that impacted against battery base). been used when installing large and heavy traction batteries to ensure that their containers were clear of any debris. 7.3.3 UV Degradation Although polypropylene has been almost universally adopted for many different sizes of lead-acid battery, it can suffer degradation if exposed for long periods to sunlight. When plastic garden furniture was first introduced, users complained of surface cracking and crazing, which in some cases led to accidents when the largest cracks became critical under imposed loads (especially tables and chairs, Figure 7.18). Such deterioration is caused by the ultraviolet component, the most energetic part of the light spectrum, interacting with polymer chains at or near the exposed surfaces of products. The same UV light causes sunburn on exposed skin, which flakes away after continued exposure. Similar damage occurs with many polymeric materials, as many consumers are aware. Curtains, for example, will suffer by a process known as “phototendering” after long exposure to direct sunlight if the fibers are unprotected. The UV light is absorbed by the polymer chains and split, decreasing the strength of the product and encouraging brittle cracks to grow.
234 Forensic Materials Engineering: Case Studies Figure 7.18 Network of cracks formed in flat surface of polymer table by ultraviolet degradation. The problem discussed here occurred during one of the many military campaigns by the Israeli Army during the 1970s and 1980s. Fork lift trucks used during vehicle maintenance are often electrically powered, but the large lead-acid traction batteries must be recharged regularly to maintain their capability in service. Normally such charging would occur at night, but circumstances were such that charging occurred during the day, leaving the batteries exposed to intense sunlight. The tops of many such batteries suffered severe degradation as a direct result, creating much embarrassment to the battery supplier (Figure 7.19). It was observed that the heat-welded tops were most severely affected in the weld zone, the material becoming bleached by the suns rays (an effect known as “chalking”). The effect on the molecular mass of the polymer was confirmed using a special analytical method known as gel permeation chromatography (GPC), as shown in Figure 7.20. Polymer from the inside or hidden surface of a battery showed a much higher molecular mass than that on the upper exposed surface. FTIR spectroscopy showed the presence of oxidized chains (carbonyl groups) in the exposed material, precursors of chain cleavage. The fact that degradation was more severe in the thermally affected weld zone was caused by excessive temperatures used
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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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Materials in Distress 49 Stress Amp
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Failure of Medical Implements 9.1 I
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Intellectual Property Cases 13.1 In
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A “Toolbox” for Forensic Engineers

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