Experimental and theoretical investigation of shear-stress-induced energy harvesting using a piezoelectric element coupled to viscoelastic media

Abstract This work investigates the generation of electrical energy from a piezoelectric material predominantly loaded in shear. A small, constant normal preload is maintained solely to guarantee stable contact, as pure shear cannot be produced in its absence. An experimental setup was designed applying moderate normal pressure systematically to an NCE40 piezoelectric element that was coupled with five different polymeric foams, each tested under oscillatory shear at 1, 3, and 7 Hz, under two normal-pressure regimes: a low preload P0 = 2000 Pa, and a higher preload to enhance interfacial coupling P1 = 8000 Pa. The results demonstrated that the foam stiffness and interfacial pressure play a crucial role in energy conversion as does the excitation frequency used in the experiment. Among the three frequencies tested (1, 3 ,7 Hz), 7 Hz produced the highest energy output, yielding peak-to-peak voltages ranging from 23 to 143 mV. To interpret the experimental data, an analytical model based on Bleustein–Gulyaev (B–G) shear waves in viscoelastic media was employed, incorporating both Maxwell and Kelvin–Voigt rheological representations. The dimensionless analysis—through the Womersley number ( α ), Deborah number ( D e ), electromechanical coupling factor ( β ), and charge–discharge parameter ( σ )—enabled the comparison between theoretical predictions and experimental trends. The Kelvin–Voigt model, describing elastic solids with moderate viscous damping, exhibited better agreement with the experimental voltages, while the Maxwell model captured the behavior of more deformable foams. Theoretical results ( ϕ a , pp = 112.7672 mV, ϕ b , pp = 88.64 mV) were consistent with the measured range, confirming that shear-induced piezoelectric generation can be effectively modeled using a viscoelastic–piezoelectric coupling framework. These findings validate shear-based energy harvesting as a viable low-frequency mechanism. The approach provides a foundation for the design of compact, self-powered devices and sensors that exploit mechanical shear interactions in viscoelastic materials.