  In many applications, viscoelastic properties of reinforced composites need
to be determined prior to their real service life. Such properties can be
assured by destructive and non-destructive tests. In this paper, a novel
non-destructive test (NDT) method based on flexural free vibration is
introduced to investigate the viscoelastic properties of fiber-reinforced
composites. Three different types of industrial fibers (carbon, glass, and
hemp) and an unsaturated polyester resin were selected to produce bar-shaped
composites via pultrusion technique. These composite bars were used in a simple
NDT method which just required a wooden hammer, two elastic jaws, a microphone
and a recorder software program to perform the experiment. The composite bars
were mounted on elastic jaws and hit by a wooden hammer at one end of the
specimen as a perpendicular impulse. The produced sound at the other side of
the bars was recorded. By analyzing the recorded sounds by means of fast
Fourier transform (FFT), viscoelastic properties such as flexural elastic
modulus and the loss parameter (Tan {\delta}) were calculated for the
fiber-reinforced composites. For determination of flexural elastic modulus, the
first three modes of vibration in FFT graphs were analyzed using Temuschenco
theory. Also, Tan {\delta} was determined by analyzing the drop in the
intensity of vibrational energy as a function of time. Although there was a
slight discrepancy between the calculated values from the NDT method and the
actual data from dynamic mechanical thermal analysis (DMTA) approach, a good
agreement was achieved between NDT and DMTA results.
