Three dimensional total focusing imaging of volumetric defects using laser diffuse ultrasonic phased arrays

Laser-induced phased arrays (LIPAs) have been used for defect detection, offering the advantages of being non-contact and capable of coping with complex geometry and harsh environments. However, the detection speed is constrained by slow data acquisition. This is due to the extensive averaging required to suppress noise and because the number of reception sampling points at which data can be recorded in parallel is limited by the number of laser interferometers, and is typically one. Consequently, significant time is needed for data acquisition to construct a full matrix capture (FMC) array dataset, compared to using piezoelectric ultrasonic arrays. This limitation becomes particularly prominent when using high element-count two-dimensional (2D) sampling layouts for three-dimensional (3D) volumetric imaging. In this paper, we propose the laser diffuse ultrasonic phased array (LDUPA) technique as a means of acquiring FMC data from a 2D array configuration more rapidly. 3D images are then formed from the FMC data using the total focusing method. In the proposed inspection configuration, a single conventional piezoelectric transducer is employed to transmit ultrasound into a specimen at a single location, while a laser interferometer is utilized to collect data at each point in a predetermined 2D sampling pattern. The performance of the proposed inspection method, using 2D reception square sampling patterns and a transmission transducer with a central operating frequency of 10 MHz is demonstrated for 3D imaging of flat bottom holes with diameters of 1 mm and 3 mm in an aluminum specimen.