Resonance frequencies of a spherical aluminum shell subject to prestress from internal fluid pressure

Abstract

Vibration measurements of a spherical aluminum shell (6 in. diameter) filled with water show that the resonance frequencies of the shell shift higher or lower with increasing water pressure, depending on the specific mode of vibration. For a given mode, the rate of frequency shift with pressure change Δf/Δp is approximately linear for gauge pressures up to 100 psi. Frequency shifts were detected for pressure changes as small as 0.2 psi or 10 mm Hg. Observations of positive frequency shifts are consistent with previous studies (from the 1950s) involving submerged cylindrical shells subject to much larger pressures. Analysis from this era suggests that the phenomenon is due to geometric nonlinearity; however, the negative frequency shift observed with low order modes is not predicted by this theory. The feasibility of developing a noninvasive method for monitoring intracranial pressure using shifts in skull resonance frequencies will also be discussed.Vibration measurements of a spherical aluminum shell (6 in. diameter) filled with water show that the resonance frequencies of the shell shift higher or lower with increasing water pressure, depending on the specific mode of vibration. For a given mode, the rate of frequency shift with pressure change Δf/Δp is approximately linear for gauge pressures up to 100 psi. Frequency shifts were detected for pressure changes as small as 0.2 psi or 10 mm Hg. Observations of positive frequency shifts are consistent with previous studies (from the 1950s) involving submerged cylindrical shells subject to much larger pressures. Analysis from this era suggests that the phenomenon is due to geometric nonlinearity; however, the negative frequency shift observed with low order modes is not predicted by this theory. The feasibility of developing a noninvasive method for monitoring intracranial pressure using shifts in skull resonance frequencies will also be discussed.