A comparative study on viscoelastic properties of polymeric composites measured by a longitudinal free vibration non-destructive test and dynamic mechanical thermal analysis

In this study, viscoelastic properties of polymeric composites were investigated through a non-destructive test (NDT) method based on longitudinal free vibration. First, three different polyester composites reinforced separately with carbon, glass, and hemp fibers, as well as, one polyester composite sample reinforced with poplar wood powder were manufactured via pultrusion and hand lay-up methods, respectively. In the proposed resonant free vibration non-destructive method, each rod-shaped test specimen was hit by a hammer at one end of specimen and immediately the acoustic response of the specimen was recorded by a microphone at the other end of the sample. Subsequently, the recorded sounds were analysed using fast Fourier transform technique. Then, frequency of the first mode of vibration for each composite specimen was utilised to calculate modulus of elasticity. Further, decrement in vibration energy as a function of time was examined to obtain loss parameter (tan ??) of the provided composites. Moreover, parameters (i.e., elastic modulus and tan ??) were also compared with those determined by dynamic mechanical thermal analysis (DMTA). It is found that the results obtained from the examined non-destructive test method are in consistent with those measured by DMTA approach, providing reliable, accurate and quick responses.