Introduction to forensic engineering
Introduction to forensic engineering

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Introduction to forensic engineering

5.4 Materials analysis

Further information on the reasons for the weld line became evident when the material of the reservoir was analysed. Although detailed chemical analysis was not needed, because it was known from the product specification, it was important to see how the weld line had formed and whether there were any more structural irregularities. Several methods were used to examine the failed sample.

  • Dusting of exposed surfaces.

  • Sectioning, polishing and etching.

  • X-ray inspection.

  • Microscopic examination.

Dusting surfaces with a fine white powder called whiting could reveal surface features more clearly, and in fact such an elementary method did just that, as Figure 6, Paper 3 shows. The reason is that moulded composites often show surface roughness due to the orientation of the fibres. The picture shows several prominent flow lines – precursors to weld lines – denoted by the arrow at left, and a totally unexpected feature, a fragment of an original granule from the moulding process (arrowed at right in Figure 6, Paper 3).

Secondly, sectioning a new reservoir showed features – potential defects – inside the reservoir. Typical sections are shown in Figures 7 and 8, Paper 3. They have been polished and then etched to remove superficial debris and reveal the resistant glass fibre structure more clearly. Not only did the method reveal flow lines in the moulding, but it also showed up internal voids in the structure. However, as the method is necessarily partially destructive, it was not felt appropriate to apply it to the failure specimen, especially as there were other non-destructive methods available.

Because the material was a composite of short glass fibres and thermoplastic nylon, it was thought X-ray radiography could yield useful information on the pattern of fibre orientation in the failed sample. The method is well established as a non-destructive technique (Box 18) for many different materials, and so could be used directly on the failed specimen. It was useful in two ways, first, for showing internal clumping of fibres, seen rather dimly in the upper radiograph of Figure 9, Paper 3, and secondly, for showing the crack in outline (lower radiograph of Figure 9). Although medical, or soft, X-rays were used, the contrast in the radiographs was low, and the method turned out to be of limited benefit.

Box 18 Non-destructive testing

While both DSC and spectroscopic methods generally rely on partial destruction of the sample, there are other routes to determining chemical composition. A non-destructive test (NDT) does not destroy any of the critical sample, so it would be favoured because it allows further examination. An example of such a technique was given in Box 17, it involved the use of polarised light to reveal frozen-in strain. Most macroscopic and optical microscopic inspection methods are also non-destructive, provided the sample is not dissected for viewing.

There is another reason why failed samples should be preserved intact as far as is possible: where litigation is started, other experts will want to examine the specimen independently.

Many methods have been developed over the years, especially with fatigue sensitive materials such as aluminium alloys in air frames. Some can also be used on polymeric materials. Soft X-rays are useful for showing the internal structure of composite materials, and even thermoplastics. They also reveal internal cracks.

Soft X-rays were used in a dispute concerning gas injection moulding, a new technology used to make the arms of office chairs. A contract was based on several sample arms moulded with modified tools at the contractor's factory abroad. Unfortunately, the technology proved inadequate for making strong arms. In particular, the exact position of the internal gas pocket could not be controlled accurately and reproducibly. The arm needed to be very strong where it was attached to the chair shell, but often the gas pocket penetrated this region, weakening the product drastically. As the trial approached, the original demonstration samples were discovered and produced in evidence. The defendants would not allow the samples to be sectioned physically, so the expert for the claimants suggested a trip to the local hospital to radiograph the arms non-destructively. One such radiograph is shown in Figure 68 where it can be seen that the gas pocket – the light grey – has penetrated the attachment zone where the two moulded holes are.

The other arms showed a similar problem. A key metal part of the machine that operated the gas injection phase had also failed by fatigue (and been mended twice, on the instructions of the defendants) indicating a serious problem with the method. The dispute went close to trial. The judge visited the factory to see the machine for himself, but the evidence of faulty arms was overwhelming, and the dispute was settled just before trial.

The settlement was a good example of a fair agreement that satisfied both parties. The defendants had many new conventional machines, which could not be sold owing to depression in the market, and the moulder needed to re-equip his factory. The deal involved the moulder taking enough machines at a discount equivalent to the damages he was requesting, so goodwill between the companies was preserved. The technology of gas injection moulding has advanced, but the problems of appropriate design and reproducible manufacture should not be forgotten.

Figure 68: Radiograph of chair arm

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