Properties - Steady State Oscillation Testing

Dynamic Properties - Steady State Oscillation Testing 

Viscoelastic properties are often determined with steady state oscillation or vibratory tests using small tensile (compressive) bars, thin cylinders or flat strips in torsion, beams in bending, etc. The approach is usually referred to as dynamic mechanical analysis (DMA) testing or sometimes dynamic mechanical thermal analysis (DMTA). The latter term is more appropriate as properties are often determined and expressed in terms of temperature 

What is now known as the correspondence principle for converting viscoelastic problems in the time domain into elastic problems in the transform domain was first discussed by Turner Alfrey in 1944. As a result, the principle is sometimes referred to as Alfrey s correspondence principle. Later in 1950 and in 1955 the principle was generalized and discussed by W.T. Read and E. H. Lee respectively. (See bibliography for references.) 

Assume a small uniaxial sample is loaded with a strain input, 

Here E (iw) is defined as the complex modulus and can be decomposed into real and imaginary parts as 

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A constant strain rate test may be used to determine the relaxation modulus and a constant stress-rate test may be used to find the creep compliance. Steady state oscillation tests may also be used to determine the viscoelastic properties of polymers. These details and the interrelation between various test approaches are given in Chapter 5. 

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