Piezoelectret-based and piezoresistivity-based stress self-sensing in steel beams under flexure

This work reports for the first time the structural self-sensing of stress in steel beams under flexure (elastic regime, ≤200 MPa) through measurement of the electric field (piezoelectret), capacitance (dielectricity) and resistance (piezoresistivity). Coplanar electrodes on the top or bottom surface are used. Under flexure, the electric field and resistance increase at the top surface, but decrease at the bottom surface, while the capacitance decreases at both surfaces. Under normal compression, the electric field, capacitance and resistance decrease at both surfaces. For flexure and normal compression, the resistance and capacitance changes are essentially completely reversible, whereas the electric field change is not. Longitudinal tension (whether in flexure or normal compression) causes the electric field to decrease, whereas longitudinal compression (in flexure) causes the electric field to increase. Longitudinal tension (whether in flexure or normal compression) causes the resistance to decrease, whereas longitudinal compression (in flexure) causes the resistance to increase (negative piezoresistivity). The capacitance (2 kHz) decreases due to the stress (whether flexure or normal compression and whether at the top or bottom surface) and is attributed to the permittivity decrease, except that the capacitance decrease under longitudinal compression (in flexure) is attributed to the AC voltage used to measure the capacitance penetrating the full thickness and the effect of longitudinal tension at the bottom surface dominating over that of longitudinal compression at the top surface. In contrast, the DC voltage used to measure the resistance is much smaller and does not penetrate the full thickness, so that the resistances at the top and bottom surfaces respond to flexure differently. The magnitude of the fractional change in electric field, capacitance or resistance increases with decreasing beam width.