Ultrasonic Phased Array Imaging for Defects in Angle Blind Spots Based on the Solid Directivity Function

The FMC-TFM is currently a popular method for ultrasonic phased array imaging. In the FMC-TFM, ultrasonic echo energy is mainly used for imaging, but the directional nature of ultrasound phased array elements leads to differences in the energy of ultrasonic waves in different propagation directions, resulting in uneven imaging amplitudes of defects in different directions. When the beam pointing angle gradually approaches -90° and 90°, the beam directivity will slowly degenerate and the acoustic energy will progressively weaken, forming an angle blind spot for imaging. When the detection space is limited and the ultrasonic phased array transducer cannot be moved, defects within the angle blind spot will not be detected. Therefore, the paper analyzes the causes of and factors that influence the formation of ultrasonic phased array imaging angle blind spots, describes the distribution characteristics of the acoustic field radiation angle of the array element by using the solid directivity compensation factor, and constructs an ultrasonic phased array TFM algorithm based on the solid directivity compensation factor. The numerical simulation and experimental results show that when the array element width is 0.5 ( \(a = 0.5\lambda\) $a=0.5\lambda$ , which is commonly used in industrial detection for phased array transducers), the solid directivity compensation TFM algorithm has a better ability to compensate for the imaging amplitudes of defects in blind spots than the conventional directivity compensation TFM algorithm. When the angle blind spot is small (i.e., \(\theta_{0} = 72.3^\circ\) ${\theta }_0=72.3^{\circ }$ ), the clarity of the defect imaging of the solid directivity compensation TFM algorithm is better than that of both the TFM algorithm and the conventional directivity compensation TFM algorithm. When the angle blind spot is large (i.e., \(\theta_{0} = 76.5^\circ\) ${\theta }_0=76.5^{\circ }$ ), defect imaging in the angle blind spot cannot be achieved by using the TFM algorithm and the conventional directivity compensation TFM algorithm, but the solid directivity compensation TFM algorithm can achieve accurate imaging, effectively suppressing the influence of angle blind spots and expanding the detection range of ultrasonic phased arrays.