For the newly built Town Center Tower you are asked to choos

For the newly built Town Center Tower, you are asked to choose a suitable material for a bell. A bell rings because it has a low loss (or damping) coefficient, n; a high damping gives a dead sound. Use the loss coefficient-modulus chart to identify different class of material that should make good bells. Comment on each of the class of selected materials.

Solution

Traditionally, Bells are made of bronze. They can be made of glass, silicon carbide. Metals, glasses, polymers and ceramics under right proportions have low intrinsic damping or internal friction, an important property when structures vibrate. Intrinsic damping is measured by the loss coefficient.

There are many mechanisms of intrinsic damping and hysteresis. Some are associated with a process that has a specific time constant, then the energy loss is centred about a characteristic frequency. Others the hysteresis mechanisms are associated with time independent mechanisms; they absorb energy at all frequencies.

Different classes of materials used for Bells depending on Loss Coefficient Chart:

Metals: A large part of the loss is hysteretic, caused by dislocation movement. It is high in soft metals like lead and pure aluminium. Heavily alloyed metals like bronze and high-carbon steels have low loss because the solute pins the dislocations these are the materials for bells. Exceptionally high loss is found in the Mn-Cu alloys, because of a strain-induced martensite transformation and in magnesium perhaps because of reversible twining.

Polymers: In polymers, chain segments rub against each other when loaded; the relative motion dissipates energy. The ease with which they slide depends on the ratio of ambient temperature, in the case room temperature, to the glass temperature, of the polymer. At the temperatures when this ratio is less than 1, the secondary bonds are frozen, the modulus is high and the damping is relatively low.

Ceramics: Ceramics have low damping because the enormous lattice resistance pins dislocations in place at room temperature. Porous ceramics are filled with cracks, the surfaces of which rub dissipating energy when the material is loaded.