Grange's law of visual examination also is from about thirty years ago. It states that you should: "First take a look at the failure at 10Xwith a magnifier, and then look again and think harder".
A lot can be learned from most mechanical failures by careful visual examination aided by a magnifier, or preferably a stereo microscope.
A couple times I have seen people claim that a failure had not occurred progressively via fatigue cracking because they could not find striations at high magnifications (~8000X) in the scanning electron microscope (SEM). However, there were concentric crack arrest markings ("beach marks") visible at low (~8X) magnifications.
Ray Grange was a metallurgist at the US Steel Research Lab. You can still find some of Ray’s 1956 work on tempering of carbon steel referenced in the Heat Treating volume 4 of the current ASM Handbook.
Wednesday, May 30, 2007
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
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
Subscribe to:
Posts (Atom)