Bronze Fatigue— Why Bells Crack
© Fraunhofer LBF. Finite-element methods were used to determine the characteristic overtones produced when a bell oscillates at one of its own resonant frequencies. Click on the picture to see a larger version. |
Church bells may be subject over time to structural fatigue. But they are such an important part of our liturgical and cultural heritage, and we want them to last for centuries. Researchers have been investigating the damage caused to bells when they are rung.
Nowadays, many people only hear the ringing of bells on important days in the Christian church calendar, such as Christmas. And yet for centuries the bells have rung not only to call the faithful to prayer but also to warn the inhabitants of the town of impending danger, to tell them the time and to announce happy events such as a birth or marriage. Each bell had its own special sound, and even its own name. And even though ringing bells is not as common as it used to be, bells are still just as special today.
Bells will crack if they are poorly made— particularly, if impurities or air bubbles are allowed to remain in the bronze when it is poured. There's no substitute for a foundry that knows what it's doing! But like everything else, church bells can also suffer fatigue over the years, and eventually fail. Thus on behalf of the Verein Deutscher Gießereifachleute (VDG)— the German federation of foundry experts, researchers at the Fraunhofer Institute for Structural Durability LBF in Darmstadt studied the damage caused to bells when they are rung, both experimentally and using digital simulation.
The greatest risk
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The aim of the project was to analyze the effect of various factors on the wear and tear of a bell, and to find ways of reducing damage that may be attributable to the shape and weight of the clapper, for example, or to the angle of the bell when it is struck and the ambient temperature,.
The researchers analyzed samples of bell bronze to determine their metallic composition. They also ascertained the strength of the material in fatigue tests at different temperatures.
Other experiments were carried out on a moving bell. The scientists used strain gauges and accelerometers to identify the places where tension and expansion forces act, which may cause the bell to crack.
In addition, the phases of the ringing cycle were simulated digitally on a computer. "Our investigations proved the effectiveness of a practice that has been in common use for some time. Turning the bell through an angle of about 30 degrees at longish intervals can increase its useful life by up to a third", reports Dr Dietrich Flade of the LBF.
"Parameters such as the shape and weight of the clapper don't have any significant effect on damage. Contrary to expectations, the ambient temperature is not of great significance either."
The greatest risk to church bells is over-enthusiasm on the part of the bell ringers. For example, attempts to make the bells heard over the noise of local traffic by ringing them even harder will result in reduced life span.
Also, for swinging bells, the higher the swing, the greater the force and speed with which the clapper strikes the bell. Increasing the angle of the bell when it is struck by just one degree reduces its useful life by an average of 14 per cent.
These findings will help to understand how to ensure that the final hour will not strike for a long time for your bells.