Ray tracing method for ultrasonic array imaging of CFRP corner part using homogenization method

Abstract

A reliable nondestructive evaluation technique that can effectively detect and quantify the internal defects of corner parts in complex-shaped carbon fiber reinforced plastic (CFRP) structures is highly demanded for assuring safety and reliability of safety-critical structures. The conventional methods generally utilize normal incidence ultrasound and suffer from poor image quality due to the unfocused beam. To produce a fully focused image of the CFRP corner part, the delay-and-sum total focusing method (TFM) is preferred. The prerequisite for TFM imaging is to calculate ultrasonic ray paths, which is challenged by the curved shape, elastic anisotropy and multilayered microstructure. In this paper, the complex ultrasonic ray paths transmitting through the corner parts of CFRP structures from the concave side are calculated for ultrasonic array imaging. A ray theory-based homogenization method is firstly proposed to describe the wave velocity distribution of corner parts, which is critically important to the subsequent ray tracing calculation. A ray tracing method based on Dijkstra's algorithm is further developed for rapid calculation of multiple ray paths between two specified points when the ultrasound is incident from the corner's concave surface. To demonstrate the validity of ray tracing method, the TFM images of an CFRP corner part with side drilled holes (SDH) at different depths obtained using three different types of ray paths, are compared and analyzed. Results show that TFM images using ray paths with the minimum or maximum ultrasonic travel time present unsatisfactory image quality due to the obvious structural noises from ply reflections. In contrast, the signal to noise ratio of the image is significantly improved when TFM uses the ray paths with the weakest refraction, and all SDHs are detected and accurately positioned. This paper proves that the proposed ray tracing method is an effective method for imaging the corner parts of CFRP structures.