Engineering failure is useful: Page 3 of 5

February 19, 2016 //By Peter Clarke
Engineering failure is useful
Dennis Feucht discusses the works of the engineering author Henry Petroski and what he had to say about failure.
even more dramatic. Power-circuits designers are generally more caught up with avoidance of catastrophic failure than, say, test-instrument or consumer-electronics designers. Failure for most of electronics is parametric, a matter of not meeting the specified claims, and therefore contributes to more of a moral than a physical hazard.

The ultimate failure

So engineering is living with failure. We try to learn from it and minimize it, but only to an extent. The removal of all weakness from design is an ideal of a good design engineer, but not a practical goal. What would happen if we pushed elimination of failure too far in an imperfect world?

Petroski illustrates the reductio ad absurdum conclusion by a poem written long ago by Oliver Wendell Holmes. "The Deacon's Masterpiece" tells of a carriage builder intent upon not having any component of a "one-hoss shay" weaker than any other. In the poem, he succeeds. The shay lasts for a century and more, yet it cannot last forever, and one day it fails. But when it fails, with no component of it being weaker than any other, it fails spectacularly, disintegrating all at once.

An electrical engineer might imagine this as a plot of failure versus time, and like a perfect low-pass filter, it has wide bandwidth followed by a vertical slope. Imagine your DMM or desktop computer lasting 50 years and then failing ultimately beyond repair.

Some components are designed for controlled failure. Large electrolytic capacitors often have a crease indented into the case or a rubber-plugged hole in the bottom to provide a weak point for easing what otherwise would be an explosive situation. The plethora of protective devices, such as fuses, circuit-breakers, varistors, and spark gaps, have the sole function of being weak links to allow for controlled failure and minimal subsequent circuit repair. Experienced engineers know with some accuracy where the failure limits of a design are, and can manipulate failure


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