Identification of material parameters of thin curvilinear viscoelastic solid layers in ships and ocean structures by sensing the bulk acoustic signals

Ships and other ocean structures have components, which are thin planar or curvilinear viscoelastic solid layers surrounded by air or water. The present work deals with the identification of material parameters of these layers to extend the scope of the real-time structural health monitoring. The work proposes the approach to the parameter identification from passive sensing of acoustic signals resulting from the operational load. The identification is based on the partial integro-differential equation (PIDE) for the non-equilibrium part of the average normal stress. The PIDE is derived in the work. It includes the Boltzmann superposition integral associated with the stress-relaxation function. It is shown that, in the exponential approximation for this function, the PIDE expresses the steady-state solution (with respect to a certain variable) of the corresponding third-order partial differential equation (PDE) of the Zener type. The operators of both the equations are identical. The equations are applicable at all values of the stress-relaxation time. The roots of the characteristic equation of this operator are consistently analyzed, and the acoustic attenuation coefficient for arbitrary high frequencies is indicated. The approach is exemplified with the identification of the layer-material stress-relaxation time and ratio of the bulk-wave speed to the layer thickness. This identification can be carried out from the acoustic acceleration normal to the layer measured by an acoustic accelerometer attached to the layer surface and is applicable to both planar and curvilinear layers. The identification method presumes the finite-difference calculation of the time derivatives of the measured acoustic acceleration up to the third order and can be computationally efficient.