Designing and Preparing for Failure
To Engineer is Human by
Henry Petrosky concerns itself with the impact of
failure in
design. The book asserts that not only is it the
responsibility of the designer to make sure things do not break, but also to be able to spot when they are vulnerable to
failure and correct the problem before it becomes
comprehensive and
impossible to
fix. "Failure Analysis of Composite Dielectric of Power Capacitors Used in
Distribution Systems" by A. Farag is an article about how to predict when
capacitors will fail, and also how to make them so they last longer. This supports much of Petroski’s
argument.
Throughout To Engineer Is Human, the idea that failure is ok, and in many cases, actually desired, is emphasized. "The engineer," states Petrosky, "learns more from his mistakes and those of others than he does from all the masterpieces created..."(82) Failure is helpful, as long as we learn from it and do not make the same mistakes again. Often failure is due to unforeseen conditions that adversely effect the performance of the structure. Sometimes, a project can fail because the original idea is flawed or the project becomes prohibitively expensive and must be modified to a form that no longer satisfies the original requirements.
Capacitors are devices that utilize conductors and insulators to hold a charge. They are similar to any other electronic device in that times at which they are most likely to fail are very soon after installation and a long time after installation. These "Early Failures" and "Wearout failure" are fairly predictable and little can be done to lessen their existence. The other types of failures that exist are knows as "random failures." They can be caused by shoddy workmanship or inferior materials, as well as interaction with outside devices. Random failures have shown to be the easiest to prevent because there is a direct correlation between quality of parts and number of random failures.
Although they may seem random, random failures can be predicted based in empirical evidence. Petrosky stresses the need to be able to predict failures before they occur and therefor be able to take preemptive steps to minimize service interruptions. One example he gives of a failure that could have been predicted was the collapse of the Hyatt Regency skywalks, "an accident just waiting to happen"(88). In the capacitor article, several mathematical expressions are given to assist in predicting when a component will fail. These include F(t)= 1-e-t^m/t0 where f(t) is number of failures, and m is a constant determined by the type and quality of capacitor. (558)
Another assertion that Petrosky makes throughout the book is that there are steps that one can take to help prevent failure. Using stronger materials than seem necessary, and trying to anticipate additional forces that will be present in the real world, so called "unanticipated traumas"(80), are also efficient ways to lessen the likelihood of failure. Similarly, the article discusses that the use of better materials and cleaner clean rooms do a great deal to prevent failure, especially during the first few months. Finally, the operating environment plays a factor. Capacitors that are used in hot environments or in systems susceptible to momentary surges in power are also more likely to fail.
In addition, a point brought up by both Petroski and the article is the comparison between a design limitation and a design failure. Most all devices, if exposed to forces that they are not designed to withstand because they are being used in a way that is different than they were intended to be used, will fail. Petrosky asserts that a designer should try to predict how a device could be put in place, and either design it not to work in that way (by making it asymmetrical if it should not be plugged in backwards for example), or design it to withstand that treatment.
Every structure that is man made will fail given sufficient time and exposure to elements. The challenge of the engineer is to design in such a way that failure is kept from occurring for a reasonable lifetime of the product. Some products such as bridges have lifetimes of up to 100 years while capacitors are not expected to last more than 20. Engineers must consider what external forces will be acting on the design that will lessen its life and what can be done to make the structure resist these forces without significantly raising the price. Although it is impossible to predict all external forces or prevent all fatal manufacturing inconsistencies, it is possible to lessen their effects. This applies to all designs from the biggest bridge to the smallest capacitor.
Works Cited
Farag, A. "Failure Analysis of Composite dielectric of power capacitors used in distribution systems." Electrical Insulation Conference, 1997, and Electrical Manufacturing & Coil Winding Conference (557-564), ISBN: 0-7803-3959-2
Petrosky, H., "To Engineer is Human: The Role of Failure in Successful Design". New York: St. Martin's Press.
Note: I wrote this for HUM108 at Drexel University in the year 2000. Please do not steal it. Thanks.