Introduction to forensic engineering
Introduction to forensic engineering

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Introduction to forensic engineering

5.5 Microscopic examination

Of greater use where fracture surfaces are concerned is microscopy, initially at low magnification with a simple optical microscope, followed by scanning electron microscopy. It is sensible to be reluctant to deliberately break a unique specimen, but as all other methods had been exhausted, valuable evidence should be shown by the fracture surfaces. This turned out to be the case, in fact.

Figure 10 in Paper 3 presents the optical micrographs of one side of the fracture surface, which at first sight seemed to show rather little in the way of a clear origin or other features that can often enable a picture of crack development to be reconstructed. Careful inspection however, revealed tide marks left on the outer buttress. As there were several such features, it was reasonable to suggest several cracks had existed before the final failure. The evidence for that event lay in a much larger tide mark below, showing cooling water debris had collected there following a major loss of water.

The second interesting feature seen at low magnification was a line of smooth material on the inner side of the fracture. This could represent unfused polymer present within the weld line already detected. Such indeed proved to be the case when the sample was examined in the SEM (Figure 11, Paper 3). Owing to the greater resolution of the method, vague or ill defined features from the optical microscope can be seen with greater clarity. The top picture already shows substantial detail in the fracture, with a three-dimensional view unequalled by optical microscopy. The sequence of shots is of increasing magnification from (a) to (c) as shown by the captions. Shots at greater magnification were taken, but revealed nothing new that couldn't be seen at lower magnification. The resolution is in fact so good that individual fibres can be seen. The picture at (c) shows the flat region at the base, or inner side of the fracture, to be quite smooth with no fibres.

One hidden bonus of SEM, at least in this case, came from the need to coat the sample with gold to give a conducting surface, which is essential with non-metals to bleed away electrons from the main beam. When the sample was re-examined optically, the contrast and definition were much improved, as the panoramic sequence of Figure 12, Paper 3 shows. The weld line surface was now shown to be running for a large distance along the fracture surface. But more significantly, there were visible numerous cold slugs of partly melted granules embedded in the surface – shown by the longer arrows in the photograph. Their presence seemed to confirm a problem of cold moulding in the sample. The problem occurs when the moulding machine is started up ready for production. The barrel heaters are still warming up, so the polymer granules are not fully melted and homogenised before injection into the tool.

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