Preface

Forensic methods have improved dramatically in recent times, increasing the chances of catching criminals, resolving disputes and enhancing product quality. It is common knowledge that forensic science has enabled many old, cold cases to be solved, especially unsolved murders committed years ago, provided the evidence was preserved at the time for modern analysis. But there has been similar, although less well known, progress in forensic engineering, the subject that deals with accidents, disasters and product failure of all kinds. Modern techniques have shed much light on the Tay and Dee bridge disasters, for example; disasters from a different era of technology (1, 2). Re-examination of the remaining evidence from old railway accidents such as that at Shipton-on-Cherwell in 1874, have revealed the nature of the fracture which derailed an entire train, causing 34 deaths among the passengers (3). Metal fatigue was an important failure mode in these Victorian disasters, but was for long unrecognized and failures continued without respite. Despite the advance in understanding in the 20th century, the problem continues down to the present in all engineering fields.

While case studies of metal product failure are well published today, those of other materials remain neglected, especially of non-metals such as glass, ceramics and polymers. Failures of plastic and elastomeric products are poorly published, perhaps a not unexpected problem given the reluctance by companies to advertise their failures, academic disdain of practical subjects, and their relatively recent introduction as engineering materials. However, some recent compilations have added much new and useful information of direct use to product designers. They include the pioneering books by Meyer Ezrin (4), David Wright (5) and John Scheirs (6) as well as our own previous work which presented a wider view of both metal and polymer product failures (7). In that book, we presented our cases as a narrative from failure to cause of the problem, with details that are often ignored, such as:

Many failure modes, for example, are common to many different types of material, especially fatigue from repeated loading below the nominal failure loads, corrosion or changes in a material as a result of interaction with its environment, creep rupture, wear and other mechanisms. So knowledge in one discipline can provide clues as to how failure occurred in other areas. Knowledge of several subjects is, indeed, often essential when products made of several different materials fail, such as the bridge bearing discussed in Chapter 2. The track record of parallel product failures is another topic of often vital interest because much will already have been determined and causes established, providing a context for a current investigation. Thus the information from the USA about an ongoing court case involving thermoplastic pipes was crucial in resolving a case involving a pipe junction failure in the UK (Chapter 6). While such information was frequently obscure in the past, the world wide web is exposing many such cases to public view and easy access. It can not only help resolve disputes, but also aid designers in selecting materials knowing the environment in which products have to perform reliably.

A third area we have emphasised both here and in our previous book is the role of alternative failure explanations. More often than not, complete information is rarely available to the investigator, so assumptions about the loads and environments must be made in order to pursue the failure causes. Litigation cases frequently restrict important information from one side or another, at least until the disclosure phase, requiring the investigator to keep an open mind about the failure or failures. But some investigators jump to conclusions which are often not justified by the evidence, and that opinion often coincides with the views of the client who is funding the action. Client bias is in fact very common, but must be resisted when performing an investigation. It is in that client’s own long-term interests to know just how a product failed, which is the function of an independent investigation. If bias creeps into a report, then costs mount as litigation proceeds to an inevitable and unfavourable conclusion. It is far better to know the bad news early rather than later, a seemingly obvious comment, but one frequently missed during litigation.

So we have included clear evidence of misleading or mistaken reports from other investigators, such as that from a study of radiator washer cracking (Chapter 7), where a report made incorrect deductions from the failed washers, and reached the wrong conclusions. Much extra work was then needed to find the real cause of the problem. Another example is given in Chapter 10. It involved cracked transformer plugs which could electrocute the user, a problem that was raised by the supplier in the UK. They imported the plugs from Japan, and the cases were in turn made in China. A Japanese group suggested a cause which we could not confirm, and they used a single method rather than relying on several independent methods. We suggested a quite different source of the problem, faulty moulding in China. The Chinese produced moulding records which confirmed our diagnosis, and the problem was solved for the affected batch of plugs.

Missing evidence is another problem often faced by the investigator. It is prevalent of course in fires, the key initiation point frequently destroyed by the fire itself. However, traces which do survive can hold the key to the solution of the problem, as discussed in more detail in Chapter 9 dealing with vehicle accidents. The material evidence in medical failures is also sometimes lost, especially if the product is disposable, such as with sutures used to stitch wounds (Chapter 3). Other agencies may lose samples, and failed samples may be discarded after inspection by the manufacturer, as in other cases discussed in Chapter 3. The extra uncertainties introduced make investigation yet more difficult, and explains why many legal cases take so long to resolve. Poor reporting on failures is not endemic to litigation but extends into the domain of the designer and manufacturer, where failed products should be studied in depth so as to prevent future failures. It is one hope that this and other failure compilations will help reverse that problem, by making failure case studies much more widely available to the specialist engineer. One way it can be achieved is by publication in learned and technical journals, and one such journal that has established a firm foundation is Engineering Failure Analysis edited by Dr DRH Jones. An increasing number of specialist papers dealing with non-metals are to be found there, helping to widen access to the study of product failure and ways to circumvent the many problems that ensue. Some of the cases published in this book are also published in that journal.

The theoretical basis for the study of polymeric product failures is established and laid down in Chapter 1, along with the special terminology needed with long chain materials. Polymer science is a relatively new subject, dating back to the 1920s, although materials like gutta-percha (insulation in electrical and communication cables), natural rubber, celluloid and Bakelite were well exploited in the Victorian period. New polymers are still being synthesized, and an understanding of the basics is normally needed, even when examining well-known polymers such as polyethylene, which displaced gutta-percha for cable insulation in the 1930s. The analytical tools used for examining failed products are discussed in Chapter 2 with some background to their utility, publication and limitations. A compilation of both common and specialist terms of use throughout this book is also available elsewhere (8).

The case studies proper begin at Chapter 3 with an examination of failed medical products, both transitory and permanent implants with a large polymer component. It is one of the most active areas of interest, and unlike many other areas, reasonably well published in the specialist medical literature. Chapters 4 and 5 encompass large and small containers, where polymers are well established as materials of construction. Both small and much larger failures can lead to extensive collateral damage when the fluid contents are released by cracking of the container walls. Pipes are discussed in Chapter 6, where polymers have revolutionized practice, especially for utility transportation. But mistakes in using polymers have occurred, and one such problem was so widespread in North America that it resulted in one of the largest, most expensive and long running class actions ever. Polymers have long been used for sealing pipe systems, and they are the subject of case studies in Chapter 7, including an example of a very expensive problem in a pneumatic system controlling a semi-conductor fabrication factory in Japan. Rubber seals failed and shut down several machines, not just once but several times, leading to loss of production.

Tools and related products follow in Chapter 8, and include products such as knife handles, power tools and ladders as well as all-plastic furniture. When such products suddenly fail, the safety of the user is immediately at risk. Modern cars contain many hidden safety-critical components such as fuel lines, as well as visible products such as tyres. Failure can have devastating consequences in driven vehicles, including both motorbikes and trucks. Road traffic accidents are the subject of Chapter 9, which on investigation proved to be traceable to the failure of polymer components. Polymers are ubiquitous in consumer products such as electrical insulation in plugs and other electrical equipment, where failure can result in electrocution, so great care is needed to prevent failure. They are also used for key anchors in luggage and baby cots, for example, where failure can result in serious personal injury (Chapter 10).

We believe that it is only by publicizing case studies of product failure that designers and producers will change their practices and procedures to eliminate risks to users, improving not just product safety but also their own reputations as manufacturers. And it is not as if many of the design changes needed are costly or difficult to make. A single example will suffice among the many discussed in the main text. The strength of many products could be increased easily by ameliorating stress concentrations, especially sharp corners on the inner sides of enclosures. It can be achieved by rounding out sharp corners on tool edges and corners, an operation taking only a few minutes depending on tool complexity. Many other examples are described in the text.