Abstract
The inherent error involved in the measurement of dynamic mechanical properties of viscoelastic materials by a conventional forced-vibration method is theoretically evaluated by considering propagation of strain and stress waves in a sample. The frequency f and the sample length l must satisfy the criterion fl<0.12(|E*|/ρ)1/2, when the dynamic modulus and loss angle have to be determined within 10% error. Here, E* is the complex modulus and ρ is the sample density. For an ordinary polymer sample of a few centimeter length, the criterion sets the limit of applicable frequency below a few hundred Hz. To avoid these difficulties, we propose a new method of viscoelastic spectroscopy hopefully applicable in the audio frequency range, employing a technique of the mechanical impedance measurement.
The new method involves the following procedures: Two equivalent vibraters are attatched to the both ends of a specimen. The vibraters are operated simultaneously with the phase difference θ of 0, π/2, π, and 3π/2, or only one vibrater is operated with the other end fixed (without operating the second vibrater). The impedance Z(θ) or Zfix is measured by an impedance meter attached to the end of the specimen. Then, the complex modulus E* can be estimated by one of several simple algebraic relations among the Z. One of such relations, for example, is E*=Z(0)Z(π)/ρA2, where ρ and A are the density and the cross-sectional area of the specimen, respectively.
