Can you see what really had happened?

The preliminary step of visual examination is an important part of a failure analysis. An analyst needs to look carefully at the failed product or component with his naked eyes, a magnifying glass, and a stereo microscope (at a magnification up to ~50X). He takes photos to document that examination process. He asks himself where failure began - the origin or origins, and how it grew - in what direction(s) by what mechanism(s). 

Over two decades ago I worked on a consulting engineering case that already had turned into litigation. Fuel oil had wound up under the slab foundation to a home. There were two conflicting stories about where it all had come from. The owner alleged that his fuel oil supplier has overfilled the outdoor storage tank in the fall, and was responsible for spilling oil on the ground. The supplier admitted he had overfilled that tank, but said it was by just a few gallons. Then he looked at the other components. There was a fuel oil filter assembly similar to this one on the supply tubing leading from an outdoor storage tank to the furnace. It had a pinhole in the housing wall at the bottom. 

The owner claimed that the supplier must have drilled that hole to confuse the situation and cast doubt on his negligence. His attorney had an expert look at the housing, and he said the hole looked drilled. The attorney for the fuel oil supplier brought me the filter housing. When I looked at the hole from the outside it seemed nearly round. But what did it look like from the inside? The usual 1X lens on a stereo microscope had too short of a working distance to let me see in there.

Getting a close look inside took a slightly unusual setup, as is shown above. A 0.5X objective lens was used to double the working distance for the stereo microscope. I used cyanoacrylate instant glue to mount a small front-surface mirror on a wooden block at a 45-degree angle. (You still can find surplus mirrors for less than $10 here or here). A forceps was used to place that block on the bottom surface of the coffee-cup sized housing. The housing was put beneath a stereo microscope mounted on a boom stand. A fiber-optic light source was used to illuminate the inner wall of the housing. Now it could be examined and photographed with the attached 35-mm camera.

There were clusters of pits on the interior at the bottom. That pinhole was at the center of the cluster with the deepest pit. What had happened? Moisture in the tank air had condensed to produce water, which later dropped to the bottom of the housing (the lowest point in the system) and corroded the bare steel interior. Oil might have been leaking for months. 

The attorney for the fuel oil supplier eventually deposed the expert hired by the owner’s attorney. When he asked the expert why he hadn’t looked inside, the expert replied that his stereo microscope had too short of a working distance for him to see in there. The attorney for the fuel oil supplier said well, our expert had looked inside. He laid out my photographs of the interior like the cards from a winning poker hand. Then he asked that expert if he’d like to revise his opinion, and the expert said yes. It may have been one of his worst moments. Oops! 

In legal cases the attorneys and experts often get around to agreeing to a protocol for doing some destructive testing. The filter housing might eventually have been cut open so it could be more easily examined. Getting a look inside before then was very useful.