Ultrasonic signal analysis to monitor damage development in short fiber-reinforced polymers

This paper demonstrates that signal processing of digitized ultrasonic B-scan data can be applied to quantify damage in composites, The studies were carried out on polypropylene samples with short glass-fiber reinforcement. The specimens were characterized with B-scans before and after being subjected to various levels of applied strain in tension tests. The B-scans were analyzed with respect to velocity, damping and statistical properties, with the following findings. (i) The velocity decreases with the amount of damage due to the loss of stiffness in the material. The decrease is independent of frequency. (ii) The damping of the ultrasonic beam results mainly from intrinsic effects (alpha similar to f) which can be attributed to the high mechanical loss factor of the viscoelastic polymer matrix. In the frequency range from 2 to 7 MHz Rayleigh scattering at fibers or cracks does not contribute significantly. (iii) The statistical analysis based on the 'meanfield theory' evaluates the coherence (or incoherence) of the wavefront. Regions with a higher degree of damage display more incoherence which can be attributed to stronger fluctuations of the elastic constants in this case. Results obtained with light microscopy are presented to better illustrate specific properties of the material under investigation. (C) 1998 Elsevier Science B.V.