SDH detection of CFRP without pre-knowledge of anisotropic group velocity

Ultrasonic phased array has been widely used for the nondestructive detection of carbon fiber-reinforced plastic (CFRP). The accurate anisotropic group velocity must be obtained beforehand for the detection imaging. It's a great challenge because of the anisotropy of CFRP. In this paper, a novel method is presented for the Side-Drilled Hole (SDH) detection in CFRP, in which the pre-knowledge of anisotropic group velocity is not needed. To begin with, the detection signal of CFRP with SDH was gained by the mode of FMC (full matrix capture). Then a limited angle range of the anisotropic group velocity is obtained by the back-wall reflection method (BRM). The angle range of velocity is extended by matching the delay time of back-wall and SDH reflection and analyzing the relation between the reflection of back-wall and SDH. Although the acquired angle range can't cover all the directions, it's still sufficient to image SDH. Finally, the total focusing method (TFM) is used to image CFRP. Furthermore, the weak defect located between SDH and back-wall may also be detected. An experiment was conducted on a sample of CFRP with SDH. The SDH can be seen clearly in the image.