Vibration measurement on transparent pipe via refraction-based panoramic electronic speckle pattern interferometry

A refractive amplitude-fluctuation electronic speckle pattern interferometry (AF-ESPI) was proposed for measuring the panoramic vibration characteristics of transparent pipes. This pioneering method involves the introduction of refracted light paths into transparent pipelines for panoramic measurement. The employment of a conical lens facilitates the acquisition of 360° illumination and imaging, thereby circumventing the measurement blind spots inherent in traditional optical methods, which are caused by object obstruction. The imaging principle is elaborated in detail, taking into account the effects of light refraction within the transparent tube walls on the optical path and light intensity. Experiments were conducted to determine the vibration characteristics of transparent pipes, with measurements consistent with the results derived via ANSYS simulations. To investigate the influence of defects, the proposed refractive panoramic AF-ESPI was further applied to measure the mode shapes and resonance frequencies of transparent pipes with axial or circumferential defects. Experimental results demonstrated that, for defects of equivalent length and size, axial defects exerted a more significant influence on modal shapes and frequencies than circumferential defects. From the perspective of defect-induced stiffness changes, the underlying mechanisms fringe pattern alterations in defective pipes were analyzed. This study proposes a novel full-field optical measurement technique for non-contact, non-destructive testing of transparent piping systems, demonstrating significant potential for application in the field of pipeline health monitoring.