Ultrasonic inspection of delaminations in thick composite structures with variable fiber angles using optimal total focusing method : simulation and experiment

Abstract

Given the challenges associated with delamination inspection of thick composite structures, such as ultrasonic wave deflection, attenuation, and structural noise due to the complex microstructure, an optimized total focusing method (TFM) has been proposed for inspecting delaminations in both uniformed-thickness and ply-drop composite structures. Simulations were conducted using OnScale® to examine wave propagation and its interactions with delaminations, allowing for the tailoring of time-delays for ultrasound beam manipulation. In the uniformed-thickness composite structures, TFM at 5 MHz provided a more accurate inspection of delaminations compared to 2.25 MHz. However, in the ply-drop composite structures, significant wave attenuation and deflection at 5 MHz resulted in the failure of traditional ultrasonic testing (UT) to detect deep-sited delaminations. Using the developed segmented-scanning TFM strategy with an optimal aperture size of 10 elements at 2.25 MHz, all delaminations of 2 mm in length and at varying depths, were precisely imaged and located. Time corrected gain (TCG) and background noise normalization were employed to enhance delamination imaging performance, achieving substantial signal-to-noise ratios (SNRs) and overcoming the severe limitations of traditional UT techniques, particularly in deep positions. The proposed segmented-scanning TFM strategy, with its optimized parameters, establishes a solid foundation for ultrasonic inspection of thick composite structures with varying fiber angles and demonstrates robustness against anisotropic wave deflection and strong attenuation.