Introduction to forensic engineering
Introduction to forensic engineering

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Introduction to forensic engineering

3.2 Macroscopic inspection

Photography at the failure site is often invaluable immediately after a failure, either because the sample is too large to be easily removed, or because the extra evidence is crucial to the investigation. However, this is not always available, and the investigator must frequently rely on reports from loss adjusters or others who are first on the scene, before removal of the key material evidence.

The product itself usually reveals more about the failure than is apparent from a superficial inspection. If the product is paired with another, the second product can also be of value when examined closely. This is what happened when a failed crutch and an intact crutch were examined, as Box 9 reveals. Traces of wear on the pair of crutches corroborated the statement of the injured person, and helped to build up a picture of the circumstances surrounding the accident.

Macro-photography of the failed product is always the first task, before any disassembly or further handling of the sample. The aim is simply to make a permanent record of the sample in the as-received state, even if the features shown on the specimen make no sense. Subsequent evaluation will aim to explain all unusual or unexpected features.

Box 9 Failed crutch

An elderly woman suffered trauma and injury when one of the pair of crutches she was using to support herself suddenly broke at the junction of the aluminium handle and the main shaft. She was recovering from an operation to remove the lower part of one of her legs, and fell onto her stump.

Each crutch consists of an aluminium tube to which is fitted another aluminium telescopic arm to enable their height to be adjusted for different users. The crutch in question had fractured at the junction of the two tubes, where they were connected by a plastic insert (arrowed in Figure 21). The fracture surface on the insert was not especially revealing, but did show brittle-like behaviour, which was unusual for the material, a polypropylene copolymer.

Careful inspection of the interior of the polymer insert, with a stereomicroscope, showed that there were several subcritical cracks or crazes close to the edge of the fracture surface. They showed that the material had been loaded to exceed the tensile strength of the material. There was no evidence for excessive loads, and the material showed unusual discolouration inside the insert, but lower down than the subcritical cracks. The latter are incomplete cracks separate from the critical crack path. Such discolouration indicated some degradation had occurred and the part was thus defective. The wear patterns on the aluminium tubes confirmed that the failed crutch had been more heavily used than could be accounted for in supporting the woman (Figure 22).

After receipt of the report on the defective crutch, the injured woman received compensation for the trauma and injury.

Figure 21: Plastic insert on crutch
Figure 22: Wear patterns on the aluminium tubes

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