|Safe and Sound
Some things are just born fragile. Others are created tough as nails, but time and constant use wear them out. One of my favorite icons of history here in the Philadelphia area is the Liberty Bell, and this baby was born fragile over 250 years ago. It cracked the first day it was hung and was melted down and recast, not once, but twice before a final tolling resulted in the crack that left it a silent symbol of American independence.
Around the time this issue of SN gets to your door, on Oct. 9, the bell is scheduled to be moved from its home of 27 years in the Liberty Bell Pavilion to the newly built Liberty Bell Center on Independence Mall. Over the years, the bell has undergone its share of nondestructive and mechanical tests, and more have been necessary recently in anticipation of this move. One of the Liberty Bells more highly publicized X-rays was taken not long after a vandal struck the bell repeatedly in 2001 with a sledgehammer. This was followed up by electrical conductivity testing, which is a way of showing the flow of electrons through a metals lattice structure. More recently, the Liberty Bell was submitted to a stress test wherein wireless strain gauges calculated the amount of stress that the bell and most importantly its vulnerable crack and hairline fracture would undergo upon movement. So far, so good, say the test results; the imperfect bell is looking otherwise intact, not too stressed by unaccustomed movement, and ready to roll.
Nondestructive and mechanical forms of testing do more than tell us the condition of fragile historic artifacts. They keep us safe and sound in an environment in which weve come to rely on the material integrity of complex and critical structures bridges and skyscrapers that test the bounds of engineering know-how, vehicles that take us into the sky and beyond, even the boilers and pressure vessels that hum away in the back rooms of our office buildings and schools. This months feature section will update you on some of the latest techniques being developed by scientists and standardized by ASTM Committees E07 on Nondestructive Testing and E28 on Mechanical Testing.
Editor in Chief
Copyright 2003, ASTM