8.4 Catastrophic failures
The cases described so far in this block have had drastic effects on those concerned, which is why disputes arose over their causes. If they were design failures, they would be widespread, although individual faults or misuse can affect just one product, as the Titanic disaster illustrates (Table 8). As decided in the High Court a few years after 1912, the sinking was caused by travelling too fast into a known ice field.
The Senghenydd disaster of 1913 caused the worst casualties the UK has ever seen in a coal mine – 439 died. This was a disaster caused by a methane explosion that then ignited a coal dust explosion, which then travelled throughout the workings. Other more recent colliery disasters include that at Cresswell Crags in Derbyshire, when a rubber conveyor belt caught fire. The result was a change to regulations so that PVC belts were designed for colliery use. It is more resistant to fire than natural rubber. Other disasters involving polymeric materials are highlighted in Table 8.
Such disasters are, of course, thoroughly and exhaustively investigated to determine the cause or causes. Just like most of the case studies examined so far, important lessons for designers will be the normal outcome. With major disasters, there is naturally greater public interest, and changes to existing legislation will often be the result.
Table 8: Classification of failures by severity
|disasters||marine||SS Titanic sinking (1912)||travelling too fast into known ice field; possibly low quality steel|
|mining||Senghenydd explosion (1913)||gas and coal dust explosion|
|Cresswell Crags (1954)||fire from natural rubber conveyor belt|
|Markham colliery (1973)||fatigue of steel brake rod|
|structural||Tay bridge (1879)||poor design; low quality cast iron with many casting defects|
|Summerland fire (1965)||poor design of building; use of PMMA cladding|
|Alexander Kielland oil rig (1980)||fatigue of support structure|
|aerospace||Comet crashes (1955)||fatigue from porthole corners|
|Challenger space shuttle (1988)||design of rocket casing; use of low resilience Viton o-ring|
|nuclear||Chernobyl (1984)||systems failure|
|business||Kodak infringement (1986)||copying of instant photography patent|
|BP infringement (1998)||copying of acetic acid resin patent|
|serious accidents||automotive||Ford Pinto fires (1967)||poor design of petrol tank|
|Fiat Mirafiori fires (1978)||polymer fuel lines and electrical defects|
|widespread failures||structural||miners’ lamps (1974)||poor design, manufacture of product in polycarbonate|
|plumbing failures (1988)||polybutene, acetal in hot water plumbing pipes and fittings|
|accidents||structural||storage tanks (1995) welded tanks (1998)||poor design of polypropylene tanks|
Summarise the main design or quality lessons of the major case studies considered in Sections 4, 5, 6 and 7 of this block. Include prime materials producers, manufacturers, designers, and consumers or users in your discussion.
The main lessons of the case studies of Sections 4, 5, 6 and 7 are as follows.
Ladder accident. The case highlighted poor consumer knowledge in the use of ladders, despite written advice on the product. One design lesson might be to improve ladder design so that ladder stability is not dependent on a single angle of repose. It was suggested that rotating feet are one design option that ameliorates the single angle restriction. Devices that can be attached to the lower part of a ladder are also available, but still require accurate location to be effective. Another lesson for consumers might be that they need some education in ladder use – this might come from government agencies.
Radiator reservoir. The defective radiator reservoir was a maverick product, the lesson being primarily one of improved quality control for the manufacturer and moulder. Machine operators require on-the-job training to spot non-specification products, so they can prevent mavericks being used in final products.
Storage tank. Storage tanks made from thermoplastic material must be designed according to best practice, as defined in this case by a German standard, DVS 2205. Welding should also be improved by pre-forming walls to eliminate frozen-in stress. An additional lesson would be to improve bund wall design so that high leaks are contained safely.
Acetal fitting. The main lesson of the acetal fitting failure must be to reduce the use of acetal with water that might contain chlorine. The advice could be given by the prime material manufacturers in their technical brochures. In addition, injection moulders should tighten quality procedures to eliminate the supply of maverick products into the market. A subsidiary lesson of the USA experience is that manufacturers must listen to their laboratory staff, and make sensible decisions about the widespread use of specific materials in an environment known to be harmful to the material in question.
Car fuel pipes. The use of materials appropriate for their intended purpose is the message for car designers. Fuel pipes are one of the simplest of components but are safety-critical, so the component requires care in choice and specification. The consequences of a poor choice or poor fitment could be catastrophic for the users. Car mechanics should also be aware of the two failure modes possible in NBR and nylon 11.
Catheter. Materials for medical use must be tested and monitored to a higher level than those used in non-medical products because there must be no leaching of chemicals into the body. At the same time, greater care is needed by suppliers and assemblers to ensure sensitive materials are not exposed to sunlight at any time – further exposure to gamma radiation could continue a degradation process started by UV radiation.