Internal Friction in Solids

It is shown from tests on eighteen different solids, including several metals, glass, celluloid, rubber and maple wood, that the internal friction for strains below the elastic limit does not obey the liquid viscosity law, as is usually assumed, according to which the frictional force depends upon the velocity of strain, but that the internal friction is entirely independent of strain velocity, so far as can be observed. It was found to depend upon the amplitude of strain during the strain cycles and approximately to obey the law: Energy loss per cycle per unit volume equals {f₌}^2. In this expression f₌ is the maximum value of the stress during the stress cycle and a proportionality factor, which may be called the internal friction constant. The method used was to measure the transverse deflections of the end of a rod, about a meter long, of the material being studied, which transverse deflections were produced during rotation of the rod when its end was deflected downwards by suitable loads on it. The experiments differ from most previous work in that relatively large masses of material were employed, tending to reduce surface effects, which are likely to enter in the case of vibration decrement experiments on wires and on thin strips. A table of the internal friction constants obtained is given, and also a table of similar internal friction constants calculated from data of previous investigators. A reasonable agreement is found.