Dynamic mechanical analysis : theoretical considerations on mechanical properties

English summary Dynamic mechanical analysis (DMA) is a technique used to study and characterize materials. In the test method, small test specimens, or beams, are typically subjected to harmonic, three-point bending loads. During the test programme, the temperature is varied, for a given frequency of the external load, and a set of mechanical properties and the glass transition temperature are determined by the DMA instrument. The main purpose of the present report is to gain more insight into some of the various material parameters included in DMA and the relation between them. The technical manual for the DMA 2980 instrument from TA Instruments contains a set of expressions for certain stiffness parameters of the materials tested. In the present report, some of the terms from these relations have been investigated carefully. Focus has also been put on increasing the basic knowledge and physical understanding of the storage modulus and the loss modulus obtained through DMA. In this report, a careful load-displacement analysis of a beam subjected to three-point bending has been conducted. Based on the analysis, it is found that the specific terms in the stiffness relations from the manual represent the effect of deflection due to bending moments and shear forces. Moreover, from a fundamental viscoelastic analysis of the beam, it has been shown that the storage modulus and the loss modulus, both determined through DMA, equal the Young’s modulus and the viscosity coefficient, respectively, for isotropic materials.