Research of Surface Adaptive Ultrasonic Testing on Defects in Curved Composite Structures Based on Beam Directivity

A weak resolution in the circumferential direction of curved composite structures based on the surface adaptive ultrasonic (SAUL) combined with phased array ultrasonic brings challenges to the quantitative testing of defects. To improve the circumferential resolution of SAUL testing, an elastic anisotropic finite element model based on a T stringer manufactured with T700/epoxy resin by hot compression curing is established. Compared with the elastic isotropic model, the wave propagation behavior is analyzed. It is found that the noise of ribs on both sides is mainly caused the strong interfacial reflection echoes of layers and the anisotropic nature of the material. On this basis, the beam directivity of the array elements is studied. Results show that with the introduction of a correction coefficient of directivity for each element and reduction of the probe frequency, the amplitude of pulse signal is optimized, and the emitted ultrasonic field is more suitable for the curved structure. Accordingly, the reflection of the ribs on both sides is weakened, and the imaging quality is improved. Then a delamination with a 4.5 mm length and 1.5 mm depth in circumference is quantitatively detected. In the simulation and experimental results, the errors of circumferential length are reduced by 7.4% and 13.1%, respectively. The results show that the circumferential resolution of SAUL could be effectively improved by optimizing the directivity of array elements. The method would be valuable and promising to improve the detection quality in curved composite structures. © 2022 Editorial Office of Chinese Journal of Mechanical Engineering. All rights reserved.