Acoustic waveguide monitoring of the cure and structural integrity of composite materials

We have investigated the use of acoustic waveguide techniques to measure the cure and stress state of epoxy resins and composites. It is anticipated that the waveguides, which are incorporated within the structure, will also be valuable for nondestructive (NDE) evaluation of stress, strain, and damage caused to the structure during its life time. While dielectric techniques are commonly used to monitor the cure of composites, they suffer because metal electrodes intrude into the structure and poor signal‐to‐noise ratios may be encountered with graphite composites. Acoustic waveguides can be made compatible with the host structure and after the cure monitoring function is over, remain in the structure for NDE of flaws and voids, as well as to monitor other parameters. This paper presents results on experiments to monitor the cure and stress state of resins and composites with a 1.5 mm diameter polyester‐fiberglass acoustic waveguide in curing resin. A 1000 to 1 change in the peak value of a 71 kHz acoustic signal was found. Other parameters now being investigated are: frequency of transmission and phase change, transit time, waveguide size and shape, and acoustic wave attenuation characteristics.