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NDT Used to Evaluate
Potential Damage to the Liberty Bell
Contribution made by Steve James and Lou Truckley, The Boeing Company.

Due to an unfortunate incident that occurred on April 6, 2001, Boeing employees in Philadelphia were given the privilege of evaluating the Liberty Bell for damage using NDT techniques. The Liberty Bell is over 200 years old, and is one of the USA's most treasured symbols of liberty. In the early days of the American colonies, the Liberty Bell was rung to call people together for special announcements and events. The Liberty Bell rang out from the tower of Independence Hall on July 8, 1776, summoning citizens to hear the first public reading of the Declaration of Independence. The Bell has a very interesting history which includes cracking on its test ring in 1752, being remelted and recast only to have a poor tone, being remelted and recast for a third time in June of 1753. Sometime before 1846, the Bell developed a crack and on during its ringing on George Washington's Birthday of that year, the crack grew to a length that rendered the Liberty Bell unringable.

The Liberty Bell now rests in the Liberty Bell Pavilion on Independence Mall in Philadelphia. Apparently during a regular tour, a man suddenly began to attack the Liberty Bell with a hammer. A park service ranger had to tackle the man to end the assault. It was reported that there was minimal damage to the bell which weighs over 2000 pounds. However, there was enough concern about damage for the National Park Service to call on experts to perform an evaluation. Boeing was asked to help with the repair because of their expertise in metal structures and nondestructive evaluation.

What kinds of nondestructive tests were conducted?

Boeing NDT specialists performed a series of nondestructive tests to evaluate the possible damage to the Bell and also to document the overall structural status of the Bell. These tests included:

  • Radiographs of the bell were taken that were compared to radiographs taken more than 25 years ago.
  • Fluorescent penetrant inspection was performed on a small area of the bell to check for possible cracks from the attack. (See picture)
  • Ultrasonic inspection was performed to attempt to locate any internal flaws within the Bell.
  • An eddy current inspection was conducted to map the surface cracks on the bell. The data obtained from this inspection was used as a baseline to compare and crack propagation that may occur in the future.
  • Eddy current methods were used to measure the electrical conductivity of the Bell's bronze casing at a various points to evaluate its uniformity. In the picture below, the white dots on the Bell indicate spots where conductivity measurements were taken.

 

More information on the Liberty Bell can be found at the following sites

http://www.ushistory.org/libertybell/

http://www.libertybellmemorial.com/