Flexural free vibration as a non-destructive test for evaluation of viscoelastic properties of polymeric composites in bending direction

摘要

In many applications, viscoelastic properties of reinforced composites needto be determined prior to their real service life. Such properties can beassured by destructive and non-destructive tests. In this paper, a novelnon-destructive test (NDT) method based on flexural free vibration isintroduced to investigate the viscoelastic properties of fiber-reinforcedcomposites. Three different types of industrial fibers (carbon, glass, andhemp) and an unsaturated polyester resin were selected to produce bar-shapedcomposites via pultrusion technique. These composite bars were used in a simpleNDT method which just required a wooden hammer, two elastic jaws, a microphoneand a recorder software program to perform the experiment. The composite barswere mounted on elastic jaws and hit by a wooden hammer at one end of thespecimen as a perpendicular impulse. The produced sound at the other side ofthe bars was recorded. By analyzing the recorded sounds by means of fastFourier transform (FFT), viscoelastic properties such as flexural elasticmodulus and the loss parameter (Tan {\delta}) were calculated for thefiber-reinforced composites. For determination of flexural elastic modulus, thefirst three modes of vibration in FFT graphs were analyzed using Temuschencotheory. Also, Tan {\delta} was determined by analyzing the drop in theintensity of vibrational energy as a function of time. Although there was aslight discrepancy between the calculated values from the NDT method and theactual data from dynamic mechanical thermal analysis (DMTA) approach, a goodagreement was achieved between NDT and DMTA results.