Thursday, May 3, 2007

Laws of failure analysis II: Sloter’s law (Have a plethora of pictures)

Sloter’s law of photography is from about thirty years ago and says that the difference between an amateur photographer and a professional is that the pro uses a lot more film (and thus makes sure that he gets all the relevant pictures).

In the initial visual examination of a failure it is tempting to take just a few pictures of the "as-received" product or component, and then to rush into cleaning or cutting. This temptation should be resisted.

It is far better to have an excess of pictures showing all orientations than to later realize (perhaps even at the report writing stage) that something relevant may have been missed.

For a litigation case I once photographed a trailer hitch coupler that allegedly had a manufacturing defect leading to an accident. Examination of the top surface revealed impressions of the heads of vertically oriented carriage bolts used to attach the coupler to the trailer tongue. These impressions were carefully photographed using oblique lighting.
An important witness for one party claimed that the coupler had only been used on one trailer, where it had been attached via horizontally oriented bolts. I never testified, but my enlarged photos raised considerable doubt in court regarding the credibility of that witness and his version of the service history of the coupler.

Lewis Sloter currently is Associate Director for Materials and Structures in the Office of the Deputy Under Secretary of Defense for Science and Technology. He was recently interviewed:
http://www.dodcorrosionexchange.org/CorrDefense/Spring-2006/other_news1.htm

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