electrochemical modeling of AC and dc cure monitoring

            Both AC and DC techniques can be used to study thermoset and composite cure. AC dielectric cure monitoring, also called Dielectric Analysis (DEA) excites a sensor with a sinusoidal signal of chosen frequency, f; amplitude, VAC; and a DC bias of zero volts. The measured frequency independent resistance, or ion viscosity, is consistent with the behavior of an ideal electronic component and correlates with cure state. DC resistance cure monitoring similarly probes a material but instead uses a constant excitation voltage VDC.

            Ideally, AC frequency independent resistance should be the same as DC resistance. Thermoset resins, however, do not act as ideal electronic components—during early through mid-cure, electrochemical reactions can affect DC results. In contrast, without a DC bias, AC measurements have zero average current and do not experience electrochemical effects. Consequently, at properly chosen frequencies, AC measurements are accurate through the entire cure while DC measurements have limitations in accuracy through mid-cure.

AC and DC measurements converge to the same value at the end of cure and both may be used with equal accuracy during this time. Although the nature of resin electrochemistry is unknown, a DC electrochemical resistance added to the model of a curing thermoset can reproduce the behavior of DC resistance measurements.