A “Toolbox” for Forensic Engineers

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Fluid Transport 209 Figure 6.33 Air vent plug with cracked washer, showing traces of paint on the seal. Evidence of extrusion on the inner side is shown by the arrow. Figure 6.34 Section of fractured washer with origin at upper right corner. gradually increasing heats of fusion, from 32.3 mJ/mg for raw granules, 36.2 mJ/mg for new washers and 43.9 mJ/mg for failed washers. Since the failed washers were all much stiffer than new washers, trial experiments were organized by an expert working for RAPRA in the U.K., to measure the set induced by exposure to hot water. 10 Five stacks of four new washers were compressed between steel plates, and washers fitted to
210 Forensic Materials Engineering: Case Studies Figure 6.35 Close-up of lower right corner showing fatigue striations, probably produced by thermal cycling. plugs using a torque of 27 Nm as recommended by the manufacturers. All were then exposed to 85°C for 3 weeks. After the experiment, permanent set of 90% was measured on the stack of washers. A torque of about 7 Nm was needed to unscrew the plugs, compared with about 2 Nm for a failed radiator plug. The hardness of the material had also increased by about 50% on average. The material possessed little sealing ability due to the lowered elasticity, the effect being comparable to that seen on failed washers exposed to central heating temperatures in service. Finally, the compression stress-strain behavior of new and exposed washers was compared. The new washers possessed an initial slope of 6 N/mm, while the failed washers showed a comparable result of 11.8 N/mm. The exposed washers were thus twice as stiff as new seals. 6.7.3 Discussion Since the change in material properties observed on the failed washers could be reproduced experimentally, it was concluded that exposure to heat caused a critical loss of sealing ability of the material used in the injection molded washers. The temperatures to which they were exposed in service were not measured directly, but probably lay within the range 75 to 85°C, the higher temperatures being primarily used in old people’s homes. CHS in those locations also tend to have longer hot periods, so the time to failure will have been lower, assuming a linear rate of material hardening. The loss of sealing capacity thus could be explained by material changes, probably by enhanced crystallization of the material at high water temperatures. 11 But how could
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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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Intellectual Property Cases 13.1 In
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