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In this study, viscoelastic properties of polymeric composites wereinvestigated through a non-destructive test (NDT) method based on longitudinalfree vibration. First, three different polyester composites reinforcedseparately with carbon, glass, and hemp fibers, as well as, one polyestercomposite sample reinforced with poplar wood powder were manufactured viapultrusion and hand lay-up methods, respectively. In the proposed resonant freevibration non-destructive method, each rod-shaped test specimen was hit by ahammer at one end of specimen and immediately the acoustic response of thespecimen was recorded by a microphone at the other end of the sample.Subsequently, the recorded sounds were analysed using fast Fourier transformtechnique. Then, frequency of the first mode of vibration for each compositespecimen was utilised to calculate modulus of elasticity. Further, decrement invibration energy as a function of time was examined to obtain loss parameter(tan {\delta}) of the provided composites. Moreover, parameters (i.e., elasticmodulus and tan {\delta}) were also compared with those determined by dynamicmechanical thermal analysis (DMTA). It is found that the results obtained fromthe examined non-destructive test method are in consistent with those measuredby DMTA approach, providing reliable, accurate and quick responses.

iranian polymer journal.

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