1.2 Forensic engineering
What is forensic engineering? To answer the question, we need to look at the meaning of the words. Consulting Chambers English Dictionary gives the following definitions:
belonging to courts of law;
appropriate or adapted to argument;
of or relating to sciences or scientists connected with legal investigations.
engineering noun the art or profession of an engineer.
engineer noun someone who designs or makes, or puts to practical use, engines or machinery of any type.
So forensic engineering could mean of or relating to engineers connected with legal investigations. But what legal investigations is a forensic engineer concerned with? They could include:
product failure – such as the breakage of a critical part of a product, as in the ladder case just mentioned;
process failure – such as a manufacturing process failing to achieve the intended effect;
design failure – such as the premature failure of all products in the marketplace;
business failure – such as the infringement of intellectual property rights.
Not all such failures lead to litigation. Product failure occurs frequently with consumer products, for example, but resorting to the law is rarely used to alleviate the problem. A new car may develop an unexpected defect, such as a leaking radiator. If covered by a warranty, the seller will repair the damage at no cost to the owner. One common condition of a warranty is that the car is maintained according to the maker's instructions; if not, the warranty may be void.
Failure is reasonably common with a range of products, and there are well-established routes to replacing failed products without resorting to litigation. Components that have a limited life are therefore routinely replaced as a precautionary measure, and others are changed when wear becomes visible, such as car tyres. However, some failures are more critical than others because they may involve safety-critical parts, for example. A safety-critical part is essential to the correct functioning of a product, because if it fails, people could be put at risk.
It is the primary thesis of this unit that all unexpected failures, however trivial they might appear, should be investigated to determine the cause. The more serious the failure, such as with safety-critical components, the greater the need to investigate. If a faulty car radiator causes an engine to seize, the repair cost escalates; but if the seizure of the engine led to a car accident, the safety issue becomes paramount.
It is the techniques of forensic engineering which, when applied objectively to failures, can isolate the cause or causes. The engineer analyses and isolates, then reaches conclusions that lead to improvements in processing, design, or material. Perhaps the manufacturer needs better quality methods; perhaps there was a design defect; perhaps the problem was caused by the user (by not topping-up a car radiator, for instance). With the problem isolated, action can be taken to eliminate it, or warnings can be issued to other customers.
The case studies in this unit deal with polymer products, and show how forensic methods can be used to determine causation. As a relatively new class of materials, polymers have found widespread application in many consumer and industrial products, often in a safety-critical role. New polymeric or composite materials, such as polycarbonate or polysulphone, offer many advantages in a variety of ways, be it in greater design freedom or lighter weight and so on. But there are some disadvantages, especially as experience of long-term use is often absent. Many new polymers were invented within the last 50 years, so their long-term behaviour is difficult to assess.
Knowledge of the failure modes of polymers and composites is a vital part of their safe application, and case studies are an important way of gaining more precise knowledge of their properties. Textbooks tend to ignore many of the realities of product failure, which is unfortunate given the importance of the subject to designers. Textbooks also tend to emphasise theoretical aspects of particular materials, without mention of the practical problems of using those materials for safety-critical components.
Metals and alloys, on the other hand, have a long lineage, so more is known about their safe application, both in the short-term and long-term. Even so, it doesn't prevent failures in products made using those materials. On the other hand, many failure modes are well established, can be recognised with confidence, and appropriate action taken by the design team.
Analyse and discuss the possible cause or causes of the following failures. Indicate any possible causal connections. Indicate which involve a safety-critical component.
A car fails to start on a cold, wet morning.
The front door on a washing machine opens during a wash, spilling the contents into the kitchen.
An electric kettle boils dry and melts the casing.
A petrol pipe near the fuel pump of a car with the engine on leaks petrol, and the car is destroyed by the ensuing fire.
(a) A car that fails to start on a cold, wet morning could be caused by several factors: if petrol fuelled (rather than diesel), the electrical system may be the cause. It is well known that moisture deposited on cold metal or ceramic surfaces can allow leakage of charge from the high-voltage circuit supplying the plugs. Alternatively, the petrol tank may be empty, or the car may need servicing. The battery charge may be too low to turn the engine. There are clearly a number of possible causes.
(b) The failure of the front door of an automatic washing machine during a washing cycle suggests the door catch may have failed – if the door is still attached by its hinges. Such latches are usually strong, because they are safety-critical, and are locked at the start of the washing cycle. If the complete door fell away, both hinges and latch have failed to fulfil their design function. Hinges are also strong and safety-critical, and there are many possible reasons for failure, including fracture – the complete or partial separation of a key component. Alternatively, the door may not have been firmly closed by the user of the machine. Another possibility is that the door appeared latched and locked, but was not, due to some fault with the control mechanism of the door.
(c) An electric kettle is a familiar enough device in the home, and it relies for its safe functioning on a thermostatic switch that will turn the current off when the water boils. The switch clicks audibly when this occurs during boiling. If the water boils dry, however, it is reasonably clear the switch failed in its primary function of controlling the electricity supply.
(d) A fuel pipe conveys petrol from the fuel pump to the carburettor or injectors, when the engine is turning. If it fails and fractures petrol will leak, and owing to its proximity to a hot engine with numerous electrical components, can easily cause a flame or even an explosion. Fracture can occur by many mechanisms, but one obvious failure mode involves overloading. It is clearly important to determine the material of construction because different materials behave in quite different ways when stressed. Only a tough material that is crack-resistant should be used, but a mistake could allow a brittle material to be used, or the originally tough material may have become brittle in the engine compartment. The engine compartment is hot and there are numerous chemicals that can escape, such as battery acid, water, antifreeze, petrol itself and various oils and hydraulic fluids. They can react with fuel pipe materials if circumstances permit.
All the above examples of product failure can be safety-critical, depending on the circumstances surrounding the failure. If the car was parked in a heavily used road, failure to start could be safety-critical. Likewise, failure of the door of a washing machine could be safety-critical if the ensuing flood reached electrical circuitry causing a short circuit and hence a fire. Failure of the kettle control switch could also be safety-critical if the casing melted and caught fire, and the fire spread. Live wires could be exposed during the failure, creating another hazard. The car fire is clearly a danger to the occupants.