Nondestructive test instrument for determining fiber volume, ply count and thickness of fiber-containing electrically conductive composite materials

摘要

A probe (10) inductively couples radio-frequency electromagnetic energy into a test piece that is made of a multi-ply graphite-epoxy composite material whose fiber volume, ply count and/or thickness are to be measured. The probe (10) and the test piece form a system characterized by an electrical impedance vector, whose magnitude and phase are determined by the composition of the test piece. A frequency synthesizer (16) drives a compensating circuit (15), which applies the output of the frequency synthesizer (16) to the probe (10). The waveform and frequency of the frequency synthesizer (16) are selected to optimize the amount of energy coupled from the probe (10) into the test piece. A measurement of voltage at point A in one leg of the compensating circuit (15), and a measurement of phase difference between point A in one leg and point B in the other leg of the compensating circuit (15), are converted to a resistance measurement (corresponding to the real component of the impedance) and a reactance measurement (corresponding to the imaginary component of the impedance) of the system. The resistance and reactance measurements are plotted as coordinates on a rectangular Cartesian coordinate system. A templet comprising resistance and reactance coordinates for reference pieces whose fiber volume, ply count and thickness are known is superimposed on the same coordinate system, and lines are drawn connecting the coordinates for reference pieces of constant ply count. The position of the resistance and reactance coordinates for a test piece along a line connecting the resistance and reactance coordinates for two reference pieces indicates the fiber volume, ply count and thickness of the test piece.