Damage and discontinuity detection in planar composites using millimeter wave permittivity measurement

A free space measurement of permittivity within the millimeter wave band is employed to detect and locate discontinuities within fiberglass composite samples. This technique offers a noncontacting and nondestructive technique of locating a variety of discontinuities. The millimetric wave experimental technique is described and its theoretical basis is supplied. Experiments have been conducted on 12 samples of composite materials, some of which were nominally ideal control samples while others had damage induced within them deliberately. The discontinuity categories were low level impact damage, incomplete cure and vacuum release during cure. Ultrasonic C-scan imaging was used to confirm the presence or absence of discontinuities in each sample. The permittivity of a large planar sample was mapped and found to be reasonably consistent. The measured permittivity data are then studied to determine the feasibility of identifying the discontinuities. The results demonstrate the ability to resolve discontinuities from nominally ideal material and to identify the nature of the discontinuity. The experimental technique can also detect low level impact damage with a superior sensitivity to that of the ultrasonic imaging technique. The spatial resolution of the millimetric technique is around 40 mm (1.6 in.) or better. This paper also discusses the potential of millimeter wave imaging techniques as a nondestructive test tool for the rapid scanning of structures for discontinuity detection.