Introduction to forensic engineering
Introduction to forensic engineering

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Introduction to forensic engineering

7.3 Fuel pipes

The next case study involves extruded tubing used for fuel lines under the bonnet of a saloon car in the late 1970s. The car, the Fiat Mirafiori, was brand new at the time, but several were damaged from fires that were not the result of an accident. This raised issues of consumer safety, quite apart from the warranty damage involved.

One fire occurred when a new car owned by the late Sir John Gielgud, an eminent English actor, was travelling along the Embankment in London. Smoke came from the engine compartment, and although the fire was extinguished, substantial damage was caused (Figure 91). Examination of other fire-damaged cars by alert insurance inspectors showed the fires were probably caused by a leaking fuel pipe near the engine.

The matter was apparently solved with a recall of the cars and the replacement of the pipes. But a fire-damaged Mirafiori re-appeared in the early 1990s in a case before the High Court in Dublin. It raised the problem of the dissemination of failure studies, especially in an international market where identical or very similar products are supplied to different national markets. It also focused attention on the importance of quality control during product design and development, especially where consumer safety is paramount.

Figure 91: The engine compartment of Gielgud's car showing the smooth rubber fuel intake pipe at T (below centre), and the nylon return pipe at ‘O’ (above centre), where it passes under a U-clip. The fire probably started near O

7.3.1 Cracking and abrasion

Examination of the material of one of the pipes from the fire-damaged cars showed it to be nitrile butadiene rubber (NBR), which is petrol resistant, but an intact sample of pipe showed ozone cracks (Figure 92). Ozone gas is extremely reactive and is produced in the atmosphere by the action of sunlight on car exhaust fumes – the gas levels rise in the spring and summer. However, the gas is also produced near electrical equipment, especially during sparking, so concentrations can be high under the bonnet of petrol engines.

Figure 92: Ozone cracking in NBR rubber fuel pipe taken by insurers from an undamaged car. The cracks are deep and severe, and occurred when the tube was bent into position

Ozone cracking is easily started at bends in susceptible tubing (Box 21). Crack growth occurs laterally, and penetrates from the crack tip into the interior of tubing.

Box 21 Ozone cracking

One of the most serious ways many rubbers can be attacked by their environment is from ozone gas, normally present to a small extent in the atmosphere in 1–10 parts per hundred million (pphm). It is a reactive form of oxygen gas. Oxygen gas is diatomic with the formula O2 In ozone, an extra oxygen atom becomes attached by an input of energy to form a triatomic molecule, O3.

When air is bombarded with energetic electrons or sunlight, ozone can be formed. In the upper atmosphere, it forms a protective shield for life on earth by absorbing harmful short-wave ultraviolet radiation, but at ground level it is a polluting irritant. The most serious pollution incidents occur on bright, sunny days in the presence of car fumes.

Ozone is a powerful oxidizing agent, and is used for water purification, for example. However, it will attack and degrade many rubbers by combining with the double bonds present, and splitting the polymer chains. The net effect is that the polymer chains become smaller and the material loses its strength. The way it loses strength is by way of cracking from the surface at right angles to the applied strain (Figure 93).

Only small strains of about 1 per cent are needed to initiate cracks. Strains can occur simply by bending pipe or tubes slightly. Attack is concentrated at the crack tip where fresh polymer is exposed to the gas. The rubbers most at risk of attack are those with double-bonded chains, such as natural rubber, poly butadiene, and any copolymers, such as NBR.

Ozone cracking was commonly seen in old tyres, being easily distinguished from oxygen cracking by the deep and highly oriented pattern of cracks. However, it is rarely seen today, not only because tyres are discarded earlier in their lives, but also because anti-ozonants are added to rubber compounds to give protection against the gas.

Figure 93: Loss of strength by way of cracking from the surface at right angles to the applied strain

With the Mirafiori engine design, the material cracked, eventually punctured, and leaked petrol on to the manifold. When the petrol hit the hot engine manifold, it vaporised, and a small spark from an electrical circuit could initiate a fire.

In addition, a problem with another part of the fuel system revealed a thermoplastic material had been used for the return pipe that passed over the engine, returning unused fuel to the petrol tank. Owing to its poor abrasion resistance, this pipe could also leak if any part contacted the engine manifold, where rubbing caused a hole to develop (Figure 94). The pipe was an extrusion of nylon 11, and possessed a melting point of about 176° C. The fire in Gielgud's car had probably been caused by abrasion through the wall of the return pipe rather than ozone cracking of the feed pipe.

Figure 94: Abrasion marks on the nylon return pipe where it came into contact with the hot metal edge of the U-clip

As several cars had been damaged by such fires, it seemed clear that both unprotected NBR fuel pipes and nylon return pipes had been fitted widely, and so a recall of the model was carried out (Figure 95).

Figure 95: Report of the recall of the Fiat Mirafiori (Daily Telegraph, 12 July 1979)

7.3.2 Car fires resurface in Ireland

The recall was carried out in the early 1980s, so it came as a surprise when the original investigator was approached by lawyers involved in a High Court claim for damages in the early 1990s. A serious fire in the Irish Republic in the late 1980s in a rather old Fiat Mirafiori had caused serious injuries to two young children when in the rear of their mother's car. She had been shopping, and when arriving back home, had removed the ignition keys to open her front door. As she turned back to remove the children from their seat belts, the car interior burst into flames. Fortunately, the children were rescued by neighbours, but not before they had suffered serious burns. The resulting court action claimed substantial damages for their continuing care, and the pain and suffering.

The accident was unusual in that the fire had occurred in the passenger compartment of the car rather than the engine compartment – where the original problem had been discovered. What was the likely cause? Unfortunately, most of the evidence had been lost in the fire, the car being a burnt-out shell when examined later.

There were two possible causes: a faulty heater near the dashboard, and the plastic return pipe to the petrol tank at the rear of the vehicle. The seats were filled with inflammable polymer foam, but the rapidity of the fire suggested a petrol leak and ignition from a spark. Ignition could have been from the faulty heater, although it is well known that static sparks can easily ignite gas or fumes (Box 22). The fuel return pipe passed through the interior of the car, and could have leaked petrol just before the fire. It would then only need a small spark, perhaps when the woman left the car or closed the door, to ignite the fumes. A small fire could then become an inferno as the liquid petrol caught fire, and the fire grew into the seats. Such a scenario seemed feasible, but lacked corroboration in detail, mainly because most of the evidence had been destroyed in the fire.

Box 22 Static electricity

Although a common phenomenon, static electricity can be a safety hazards because of the energy released during discharge. It is caused by contact between dissimilar materials, where friction and wear break chemical bonds and so release electrons. The materials must be non-conductors, so allowing build-up of charge on the surfaces. An old example was said to be rubbing an ebonite rod on a cat's fur, but a more familiar example would be rubbing an inflated rubber balloon on fabric. The balloon will then stick to a vertical wall as if by magic.

Static electricity is of course the source of lightning during thunderstorms, and although the mechanism is not known in detail, is thought to be created by friction of raindrops against one another – if true, it breaks the dissimilar material rule.

Static electricity is used commercially in many different processes, for instance paint spraying where the paint particles are charged and are attracted to the surfaces to be painted.

Certain semi-conductors such as selenium will hold a static charge, except when exposed to light. So if an image is shone onto a statically charged plate of selenium, a replica of the image is formed on the plate. If a polymer toner dust is added to the plate, it is attracted to the image and can be developed by simply heating the particles.

The forensic method known as ESDA works on a similar principle, but detects the faint impressions left on paper when the sheets above are written upon. The method has been useful in revealing changes in statements or notebooks and has been crucial in exposing numerous unsafe convictions in the criminal courts.

The build-up of static can be a serious problem in atmospheres that contain inflammable gases. There were a series of disastrous explosions in the early 1950s in operating theatres, when ether-laden atmospheres were ignited by static sparks suddenly leaking to earth. But why should this have occurred when ether has been used for years in hospitals? The build-up of static was probably caused by the introduction of many new synthetic fibre textiles into clothing and hospital apparel under dry conditions. Nylon fabric, in particular, is known to create static. When a nurse or surgeon came close to a metal earth, such as a window frame, the static build-up created a spark that ignited the atmosphere. The solution to the problem was to provide conducting floor materials to allow any charge build-up to leak away harmlessly to earth. The addition of cotton to some synthetics also created safer fabrics.

There can also be problems in dust-laden atmospheres, especially if the dust is fine enough and flammable, as is the case with flour and resins. Conveyor belts are ideal static electricity generators, especially if the rubber belt rubs against another material. The sudden discharge creates a spark, which then ignites the atmosphere. Two remedies can help prevent the problem: increasing the humidity of the atmosphere allows slow leakage of static – conversely, dry atmospheres help cause build-up – and using conductive materials. Rubber can be filled heavily with carbon black to provide a conduction network that provides a leakage path for the electrons.

7.3.3 Murphy infants v. Fiat S.p.A.

By the time the investigator's original report was requested, the case had reached an impasse. In personal injury actions, the small claimant often faces the problem of discovering details of manufacturing processes, quality control procedures, and previous incidents, from the defendant companies. The problem of discovery from Fiat S.p.A. in the Republic of Ireland had led to a so-called striking-out claim against the company, because they either could not or would not release the required information to the claimants’ solicitors. The latter pursued the information independently, and came across the public information of the recall in the early 1980s. The trail led to the insurance company, which had unfortunately disposed of the original independent report on which the recall was based. However, they did have a record that the report had been commissioned, so they were able to make contact with the original investigator. Fortunately, he had retained all samples and reports from the first accidents, and was able to supply them direct to the legal team in Ireland. The report was supplied as an affidavit to the court, and the defendant witnesses – managers for litigation and engineering from Fiat, Italy – were cross-examined on the evidence of the fuel line problem.

The claimants succeeded in their action for striking out (Figure 96 shows a report made during the case), but the case went to appeal on the legal principle of striking-out the defence. Striking-out effectively meant Fiat could not succeed when and if the main claims went to full trial. Although Fiat won the appeal, the claim was settled before trial with a substantial award to the injured children.

Figure 96: Report of the striking-out motion in the Irish High Court (Irish Times, 12 October 1994)

7.3.4 Afterword

Owing to the extensive publicity in the Republic of Ireland, the case revealed a series of fires in this model, and further claims were lodged after the successful settlement of the Murphy children. In hindsight, it seems remarkable that greater care was not taken during testing of the new model.

The radiator case in Section 5 showed how new cars are now driven in realistic conditions before launch to reveal any latent defects to the design team. This is why FMEA methods are so important during development, and why it is important to know what previous problems have occurred in such simple and low cost items as fuel pipes. The same investigator followed up the original study by examining fires in new Ford Cortinas, for example.

Car fires continue to plague many new models, perhaps because cars are so vulnerable within the engine compartment. However, reinforced fuel pipe is readily available to designers, made from resistant and strong materials, and there is no reason why such pipe should not be specified during the design phase. Ozone-resistant elastomeric pipe is also widely available, so this particular problem is not insuperable. Many synthetic rubbers, which do not possess vulnerable double bonds, have been made available commercially within the past couple of decades, for example.

Car fires are particularly distressing in their effects, and the wider problem was recently (1999) highlighted in the USA, when the largest ever damages were awarded by a jury in Los Angeles. The car fire was caused by a rear collision that damaged the tank, and was not dissimilar to an earlier cause célèbre, fires in the Ford Pinto.

The case study of the Fiat Mirafiori illustrates the importance of cooperation between regulatory bodies in different countries, so that they are all aware of design problems with specific products, and can take appropriate action. It also illustrates why it is so important the basic engineering message of choosing components fit for their function is taken seriously, so that problems do not recur, and the consumer does not suffer as a consequence of ignorance or incompetence by the design team and the regulatory bodies.

An incidental issue is one of preservation of records and samples, so that the lessons of particular investigations are not lost. Indeed, publication achieves this purpose, because then the information is recorded permanently for public consultation.

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