Model of the acoustic path of a separate-combined optical-acoustic transducer

Ultrasonic testing methods occupy one of the key positions in flaw detection, structurescopy, in assessing the strength characteristics of materials and the stress-strain state of products. The method is based on the phenomenon of acoustoelasticity and makes it possible to control the stress-strain state of products by changing the propagation velocity of a longitudinal subsurface ultrasonic wave. To excite acoustic waves, a separate-combined optical-acoustic transducer and a laser-ultrasonic flaw detector are used. The design of a separate-combined optical-acoustic transducer should ensure the measurements accuracy of the time it takes for a longitudinal subsurface wave to reach the receiver of acoustic oscillations. To analyze the recorded acoustic signals and extract from them the signal of a longitudinal subsurface wave, in this work, a finite element model of the acoustic path of a dual-coupled optical-acoustic transducer is proposed and developed. The finite element model was implemented in the COMSOL Multiphysics software package using an explicit solver based on the discontinuous Galerkin method. The developed finite element model makes it possible to visualize the displacement fields of acoustic oscillations, obtain A-scans, and calculate the time of arrival of a longitudinal subsurface wave at the receiver of the optical-acoustic transducer. The calculated values of the arrival time of a longitudinal subsurface wave at the receiver of an optical-acoustic transducer are compared with the results of a full-scale experiment. Calculations and full-scale experiments were performed for steel plates of various thicknesses. The adequacy of the model was confirmed using the Fisher criterion (F-measure). The A-scans obtained as a result of the simulation made it possible to identify the signals recorded by the optical-acoustic transducer: the signal of the longitudinal subsurface wave, the signals of the head and reflected transverse waves, and the intrinsic noise of the optoacoustic transducer. The developed model makes it possible to single out the signal of the longitudinal subsurface wave among the recorded signals of the optical-acoustic transducer. The proposed model can be used in the design of new optical-acoustic transducers, as well as in non-destructive testing (NDT) and materials science.