Introduction to forensic engineering
3 Forensic methods
So, how do forensic engineers approach failure problems? What methods are used, and what are the objectives?
The aim of all investigations is to determine what actually happened during an incident or accident, based on the material evidence. There is usually direct evidence in the form of witness statements, if litigation has already started, or descriptions from personnel involved. The failed object or objects must be inspected and examined, an objective that is usually achieved by photography. In addition, the investigator may wish to examine the scene of the incident personally to corroborate statements from others. The examination of the material evidence will be considered here, but as later case studies will show, documentary evidence is usually important for building up a comprehensive picture of events.
The methods described in this section are mainly, but not exclusively, used for examination of failed polymeric materials or products. However, some methods are of such generality they are used for any failed product, irrespective of the material of construction. Both macro-photography and micro-photography are therefore of universal application. On the other hand, chroma-tography is only of use in analysing polymers, because they are long-chain molecules. Metals and ceramics do not have such structures, so the method is inapplicable.
Many commercial disputes involve not just one failed product but several, in which case a representative sample, if not all the samples, need to be examined. Large numbers of failed products present their own problems, most important is determining how representative the sample is of the larger number alleged to be faulty (Box 8). Single failures present different problems. A sample, whether one of many or the only one, is amenable to the analytical tools that are now available in many laboratories. Care is needed to ensure as little as possible of that sample is affected by analysis.
Box 8 Inductive and deductive reasoning
There are broadly two ways of examining evidence: inductively or deductively. Inductive arguments rely on examination of a representative sample of failed products so that any conclusions are firmly based on the evidence available.
I was recently involved in litigation that alleged a medical product had been poorly made, did not conform to the dimensional requirements specified in an engineering drawing, and apparently did not function properly. The component involved was a transparent sight tube for breathing apparatus (Figure 18). The engineering drawing specified polysulphone plastic for the moulding.
Although over 100 had been made, the several different moulding companies had experienced problems with the tool supplied to them, and blamed it for their problems in trying to make acceptable mouldings. An independent expert appeared to suggest the tool was faulty, but that it could be rectified at extra cost. The manufacturer of the medical apparatus sued the toolmaker claiming the tool was faulty.
Another independent expert was engaged by the solicitor for the defendant toolmaker, and naturally wanted to examine the mouldings directly. It proved difficult, but after many months delay a few were passed over for inspection. They showed mainly moulding defects, and possibly some damage that could be attributed to the tool itself. Later, the expert was able to view a large sample of around 100 mouldings not released initially. A number were taken at random from the boxes in which they were packed. Inspection showed that the largest batch – made by one moulder – showed variable moulding conditions indicating they had problems with handling the polymer. It is well known that polysulphone is a difficult polymer to mould easily, for example, a hot tool heated to more than 100° C must be used. Initially, the moulders used water cooling and could not mould at all, but then managed to fit an oil unit for hot moulding. The sample of the large batch showed that all of the defects were caused by variable moulding conditions (Figure 19), with the exception of some marks from a blunt tool such as a screwdriver levering against the precision tool surfaces (Figure 20). This examination of a single piece of evidence is an example of deductive reasoning.
The expert for the claimant refused to consider the sample mouldings, and repeated his original view. The case collapsed after the claimant was cross-examined in the witness box on the third day of the trial.