Ship Shape : How NDE Techniques Saved History

Although debated by scholars, American folklore dictates that George Washington, the general that led the Continental Army to victory over the British (and the first president of our nation), had wooden teeth. Although the honor of preserving these presidential dentures has eluded the Forest Products Laboratory (FPL), researchers here have had a hand in ensuring the longevity of other cultural artifacts from American history.

The U.S.S. Constitution, also known as "Old Ironsides."

The U.S.S. Constitution, also known as “Old Ironsides.”

The USS Constitution is the oldest floating commissioned ship in the U.S. Navy. In the 1990s, FPL employees were involved in developing an inspection methodology for the use of Navy personnel responsible for maintaining the ship. Several nondestructive evaluation (NDE) techniques were used to assess the condition of the vessel, including, radiography, ultrasounds, and stress waves.

NDE techniques are exactly that—they test and evaluate a material without destroying it. In most cases, these tests can be accomplished without even bringing the material into the lab.

Radiographic and ultrasonic techniques were used to assess the condition of the copper pins used as fasteners for the different wooden components. More importantly, however, the stress wave NDE technique was used to locate areas of degradation within the wood itself.

The test goes as follows: a stress wave is induced by striking a piece of wood, in this case, a structural member of Old Ironsides, with an impact device. The timer begins. Fractions of a second later, a second accelerometer, which is coupled to another part of the wood being tested, sends a stop signal to the timer when it detects the stress wave.

The speed at which the wave travels through the material gives researchers an idea of the condition of the wood beneath the surface.

Stress waves travel significantly slower through deteriorated wood when compared with sound wood. This technique had already proven to be an effective method for locating large, degraded areas in timbers, but previous efforts were aimed at locating degradation in softwood only. No information was found on the use of these techniques for assessing hardwood timbers—which is where FPL stepped in.

Much of the wood used in the construction (and countless repairs) of the USS Constitution is live oak, a hardwood species. After first establishing baseline information using structurally sound live oak in the lab, FPL researchers headed to the field. All of the deck beams (four decks of approximately 32 beams each), various knees, the stern post, and the keel, underwent the NDE procedures.

If the transmission times were significantly longer than the lab-established baselines, deterioration was deemed to be present, and those components were removed from the ship. After the removal, inspecting these members confirmed what the scientists knew all along: the severity of degradation did indeed corresponded to increased transmission times.

The information was published in 1997, and today, with as little as $3,500 dollars in equipment and minimally trained personnel, manufacturers and other preservation experts can replicate the test for other projects, using the established baselines. Even if Washington’s teeth are lost to history, the hardwood structures of the world still stand a fighting chance to see another day, thanks to FPL researchers and the pioneers of hardwood NDE testing.

For more information, and the original publication, click here.