dielectric AND DC RESISTANCE Composite CURE MONITORING
Dielectric cure monitors measure the electrical properties of a thermoset with AC signals across a range of frequencies. Measurement of resistance with DC methods can also reveal information about a material, but has disadvantages and limitations that the user must consider.
AC measurements of thermosets can obtain the full range of information about cure state. Simpler DC methods provide data that are limited but still useful. Resistance monitors are essentially highly sensitive ohmmeters that use a DC voltage source to drive current through the material between a pair of electrodes. Resistivity differs from resistance by only a scaling factor that depends on sensor geometry, so issues about resistivity apply equally to resistance.
Instruments using DC measurements achieve simplicity while sacrificing flexibility, and possibly accuracy, because of the following disadvantages:
1) DC measurements can only obtain DC resistance. AC methods measure capacitance, frequency independent resistance and frequency dependent resistance. Frequency independent resistance is the more accurate term that describes DC resistance and can be measured across a range of frequencies that includes DC.
2) DC measurements may have systematic errors such as offset voltage drifts, thermal drifts and leakage currents in circuits that cannot be distinguished from the true DC signal.
3) DC measurements are not possible through release layers or vacuum bags
4) DC measurements may produce distorted data caused by electrode polarization and the boundary layer effect.
While DC measurements of resistance are simple to make, they have disadvantages compared to AC measurements of dielectric properties. For thermoset cure monitoring, electrode polarization may distort DC data and cause misinterpretation of cure state. Although electrode polarization can also affect low frequency AC measurements, the additional information gained from dielectric properties allows correction of the distorted AC data. In contrast, correction of DC measurements is not possible.