Sensing the stress in steel by capacitance measurement

This paper provides the first report of the sensing of stress in a metal (namely steel and stainless steel) by capacitance measurement. The method involves the measurement of the in-plane capacitance (2 kHz) between two coplanar electrodes that are on the same surface of the specimen. The capacitance decreases with increasing normal compressive stress. The fractional decrease in capacitance (up to 20%) is higher than the compressive strain calculated based on the elastic modulus by orders of magnitude. The capacitance decrease is essentially reversible and is attributed to the direct piezoelectric effect. Because an LCR meter is not designed to measure the capacitance of an electrical conductor, it is essential for a dielectric film to be positioned between the electrode and specimen. Two configurations are used for the electrodes. Configuration I involves aluminum foil as the electrode and a stack of 3 layers of double-sided adhesive tape as the dielectric film; no pressure is used. Configuration II involves copper sheet as the electrode and a glass fiber polymer-matrix composite film as the dielectric film, with normal pressure applied to the stack. The fractional decrease in capacitance per unit stress is much greater for configuration I than configuration II. Configuration I is better for sensing low stresses, whereas configuration II is better for sensing high stresses. Configuration I is less expensive and more practical than configuration II.