Engineered structures that experience fatigue loading surround us every day, sometimes in places we’d never think of.
Churches and bell towers have long been the handiwork of both architects and engineers. Breakthroughs in structural engineering are evident in the evolving architecture of cathedrals over millennia. Blocky towers were replaced with soaring buttresses as construction techniques and structural analysis progressed. But even through these design changes, the most significant structural element in a church or bell tower continues to be the bell itself!
Bells have long served as both musical and cultural mainstays. You can probably think back to some historical event that was dignified and immortalized by the tolling of bells. A bell’s natural ringing characteristic is both its blessing and its curse: they exist to ring, but the ringing leads to their eventual failure!
Bells are designed to produce certain desirable acoustic characteristics by vibrating in certain modes at specified frequencies. You might recognize these vibration modes as stresses that need to be managed in order to prevent failure over time. The striking action by the clapper excites these frequencies, causing both sound and stresses. The resulting tones will sustain or ring due to low damping, but let’s not forget, that ringing is an indicator of fatigue cycling.
There are many historic examples of bell fatigue. America’s Liberty Bell suffered a famously low cycle fatigue problem, having failed on its first ring. And even those that don’t fail immediately have the challenge of a long design life. For example, the Gloriosa Bell in Erfurt, Germany was produced in 1497 and supposed to ring forever. It tolled admirably for hundreds of years before cracking in 1985. Considerable effort was spent to repair it, only to have it crack again 15 years later. But let’s not consider that bell a failure: 488 years is quite a long life for a metal part cast at the end of the Dark Ages!
To avoid these failures it’s critical for an engineer to recognize that vibration and fatigue are two linked phenomena that need to be understood and managed together. Learn more about the role that vibration plays in fatigue by watching this recorded webinar or attend a free seminar, The Role of Vibration in Fatigue hosted by HBM-nCode this fall. Visit www.ncode.com/vibes for dates, locations, and more information.
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