  In this paper, three different fiber polyester composites (glass fiber,
carbon fiber and hemp fiber) were separately prepared via pultrusion method.
Then, a Non Destructive Acoustic Test (NDAT) based on longitudinal free
vibration was utilized to measure viscoelastic properties of resultant
composites. Also, the way to obtain composites' elastic modulus was discussed
in details. The results showed a tremendous agreement between destructive
mechanical methods and NDAT method based on longitudinal free vibration.
Furthermore, this method revealed an excellent repeatability
