Introduction to forensic engineering
Introduction to forensic engineering

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Introduction to forensic engineering

5.2 Failure investigation

A nylon radiator reservoir failed after a small number of journeys in a new model of car, causing engine seizure owing to the sudden loss of cooling water. The car had not been introduced into the market, and was being tested by the manufacturers. As in so many such investigations, the critical failed part was the focus of initial attention, and was inspected macroscopically first. It enables the investigator to stand back from the actual failure zone, and place the whole component in its context. This in itself can prompt key questions that may lead on to the solution to the problem.

The reservoir, which failed catastrophically, was fitted to a flat part of the engine, and comprised just a half-shell (Figures 1–3, in Paper 3). The reservoir was 41 cm long and 11 cm wide. Comparison of a failed product with an intact equivalent is always useful, and in this case showed the failed part was highly distorted (Figure 3, Paper 3). The centre part of the reservoir had contracted significantly when compared with a new reservoir. The same photograph also shows the way the part was made, by injection moulding from a central gate, so leaving a remnant sprue in the centre of the inside of the component. The sprue is circular and rather large because a wide gate is needed to allow the highly viscous molten polymer to penetrate all parts of the metal tool during moulding (Figures 3 and 6, Paper 3).

Figure 64: DSC melting curve of glass-filled nylon 6,6 showing a single sharp melting point at about 265° C

Why should the distortion be important? It clearly distinguishes the failed reservoir from a new reservoir, so it might be critical in determining the cause or causes of the failure. What then could cause such distortion? A factor common to all shaping processes is frozen-in stress or strain. When material is cooled from its molten state into the final product, such stresses and strains can be caused by over-fast removal of heat from the material. In polymers, the problem is one of frozen-in strain, and is discussed in greater detail in Box 17. Frozen-in strain can provide a driving force for crack growth, as well as causing distortion and therefore mismatch with mating parts. In this case, the distortion had probably been revealed by exposure to the hot water of the cooling system, at a temperature not far from the melting point of the material of about 265° C (see the DSC trace for the material shown in Figure 64).

Although there was no obvious connection between the distortion and the crack, it was an observation to be kept in mind as the investigation progressed.

Box 17 Frozen-in strain in polymers

Because polymers are poor conductors of heat, they require long cooling cycles during injection moulding. However, if the cooling rate is increased by chilling the tool, polymer chains can be frozen into an unstable state. The usual motive for chilling the tool is to lower the cycle time and hence increase return on the high capital investment of machine and tools. Although the product appears normal, any rise in temperature over periods of time can allow the chains to relax to a more stable state, producing distortion in the product. In addition, frozen-in strain can provide energy for crack growth.

The problem came to particular prominence with the development and application of so-called engineering plastics in the 1970s, when materials like polycarbonate were used in demanding safety products like miners’ lamps. The cases for the lead-acid batteries suspended from the miner's waist were found to have cracked, spilling acid onto the miner, and led to loss of charge. The situation was serious, with whole collieries out of action because no lamps were available for the work force. Investigation of the situation at one colliery showed lamps had an average life much less than the specification of three years before being withdrawn, as Figure 65 shows.

Figure 65: Failure rates of miner's lamps (Failure record of red T-type polycarbonate cases at Sutton Manor Colliery, Lancashire. October 1974–August 1976)

What was the cause? It was a combination of chill moulding by a supplier together with solvent welding – which initiated crazes, followed by cracks. And the design exhibited numerous sharp inner corners that lowered the impact strength substantially. Crack growth was driven by the frozen-in strain, and occurred quickly under the demanding working conditions in the collieries concerned.

The solution to the problem involved tool modifications to remove stress concentrations – by chamfering cores – and the use of hot oil circuits so that the polymer was cooled slowly during moulding. The failing material was filled, so a transparent grade was substituted. Unfilled grades are frequently stronger owing to the lack of stress-raising filler particles. The levels of frozen-in strain were reduced by the slow cooling, and a quick monitoring method enabled faulty mouldings to be identified easily and rejected. The method involved viewing the new transparent cases using polarised light (Figure 66).

Figure 66: Polarised light to monitor polycarbonate mouldings (good moulding at left)

The technique is completely non-destructive and easy to use once quality standards have been established. The modifications increased the life of lamps substantially (Figure 67).

Figure 67: Modifications increased life of batteries
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