Unprecedented Capacitance-Based Nondestructive Evaluation of Steels

This work reports for the first time capacitance-based nondestructive evaluation (NDE) of a monolithic metal, namely steel. This means that the structural material is self-sensing, without the need for embedded or attached sensors. The capacitance (2 kHz) can be through-thickness (involving sandwiching electrodes) or in-plane (involving coplanar electrodes), with the latter being superior for NDE, partly due to the relative permittivity being higher in-plane (2 × 10⁶) than through-thickness (28), and partly due to the more effective spreading of the in-plane electric field lines between the coplanar electrodes than the spreading of the through-thickness field lines between the sandwiching electrodes. The fringing field serves as the probe for the flaws at various distances from the electrodes, which are much smaller in area than the specimen. Due to the electrical conductivity of the specimen and the fact that an LCR meter is not designed for measuring the capacitance of a conductive material, each electrode is separated from the specimen by a dielectric film. The conductivity of the specimen accentuates the fringing field. The flaws cause the apparent capacitance to increase, due to the increase in the effective area reached by the fringing field. The capacitance increases monotonically with increasing damage, except that it decreases when the damage in the form of through holes is severe enough to cause the permittivity to decrease. The defect sensing effectiveness is superior for low carbon steel than stainless steel, due to the higher electrical conductivity of low carbon steel and the consequent greater fringing field (current spreading).